Terahertz frequency multiplier devices and terahertz sources based on dielectric wageguides

Abstract

Terahertz (THz) frequency multiplier devices and THz sources based on nonlinear circuit elements, such as diodes, coupled to dielectric waveguides are disclosed. Embodiments of the frequency-multiplier devices can include an input dielectric waveguide configured to support an input electromagnetic wave having an input frequency, a frequency-multiplying circuit electromagnetically coupled to the input dielectric waveguide to induce an input electrical signal oscillating at the input frequency in the circuit, the frequency-multiplying circuit including a nonlinear circuit element configured to generate an output electrical signal containing higher harmonics of the input electrical signal, and an output dielectric waveguide electromagnetically coupled to the frequency-multiplying circuit to excite an output electromagnetic wave in the output dielectric waveguide at an output frequency lying in the THz frequency range and corresponding to one of the higher harmonics of the input electrical signal. The output dielectric waveguide includes an output port for outputting the output electromagnetic wave.

Description

TERAHERTZ FREQUENCY MULTIPLIER DEVICES AND TERAHERTZ SOURCES BASED ON DIELECTRIC WAVEGUIDESRELATED PATENT APPLICATION

[0001] The present application claims priority to U.S. Provisional Patent Application No. 63 / 582,565 filed on September 14, 2023, the disclosure of which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The technical field generally relates to sources of terahertz (THz) electromagnetic radiation, and more particularly to THz frequency multiplier devices and sources based on dielectric waveguides.BACKGROUND

[0003] Numerous types of THz radiation sources and frequency multiplier devices exist, their operation being based on various emission mechanisms, technologies, and processes. Terahertz radiation typically refers to electromagnetic radiation with frequency ranging from about 0.1 THz to about 10 THz, corresponding to a wavelength ranging from about 30 pm to about 3000 pm. THz radiation can penetrate materials that are opaque to visible light while remaining safe for humans and animals, unlike X-ray radiation. Optical sensing and imaging systems that employ THz radiation can be used in various commercial and industrial applications including security screening and inspection, industrial quality control, medicine, communications, and spectroscopy. However, the development of these applications has been impeded by the lack of reliable, compact, and affordable THz sources and detectors, a deficiency often referred to as the “THz gap”. Despite ongoing efforts to develop THz radiation generation and detection systems, various challenges remain in this field.SUMMARY

[0004] The present description generally relates to THz frequency multiplier devices and THz sources based on nonlinear circuit elements, such as diodes, coupled to dielectric waveguides.

[0005] In accordance with an aspect, there is provided a THz frequency multiplier device, including: an input dielectric waveguide configured to receive and support an input electromagnetic wave oscillating at an input frequency;a frequency-multiplying circuit electromagnetically coupled to the input dielectric waveguide, wherein the coupling is configured for the input electromagnetic wave to induce an input electrical signal oscillating at the input frequency in the frequency-multiplying circuit, the frequency-multiplying circuit including a nonlinear circuit element configured to generate an output electrical signal including higher harmonics of the input electrical signal; and an output dielectric waveguide electromagnetically coupled to the frequency-multiplying circuit, wherein the coupling is configured for the output electrical signal to excite an output electromagnetic wave in the output dielectric waveguide at an output frequency in the THz frequency range, the output frequency corresponding to one of the higher harmonics of the input electrical signal, the output dielectric waveguide including an output port for outputting the output electromagnetic wave.

[0006] In some embodiments, the output frequency corresponds to the second or third harmonic of the input electrical signal. In some embodiments, the output frequency ranges from about 100 GHz to about 1000 GHz.

[0007] In some embodiments, the input dielectric waveguide and the output dielectric waveguide are made of silicon. In some embodiments, the input dielectric waveguide and the output dielectric waveguide have rectangular cross-sections. In some embodiments, the input dielectric waveguide and the output dielectric waveguide have respective cross-sectional areas, and the ratio of the cross-sectional area of the input dielectric waveguide to the cross-sectional area of the output dielectric waveguide substantially matches the square of the ratio of the output frequency to the input frequency.

[0008] In some embodiments, the input dielectric waveguide is configured to receive the input electromagnetic wave via waveguide coupling from an input radiation emitter. In other embodiments, the input dielectric waveguide is configured to receive the input electromagnetic wave via free-space optics coupling from an input radiation emitter.

[0009] In some embodiments, the output port of the output dielectric waveguide is designed as a THz antenna configured to radiate the output electromagnetic wave.

[0010] In some embodiments, the nonlinear circuit element includes a Schottky diode.

[0011] In some embodiments, the frequency-multiplying circuit further includes a loop arrangement electrically connected to the nonlinear circuit element and enclosing the input dielectric waveguide and the output dielectric waveguide.

[0012] In some embodiments, the loop arrangement includes a first circuit loop enclosing the input dielectric waveguide and a second circuit loop enclosing the output dielectric waveguide, with the first and second circuit loops arranged in a figure-of-eight configuration and sharing a common circuit path along respective portions thereof, and the nonlinear circuit element is disposed along the common circuit path. In some embodiments, the first circuit loop is configured as a first resonant sub-circuit designed to resonate at a first frequency substantially matching the input frequency; and / or the second circuit loop is configured as a second resonant-sub-circuit designed to resonate at a second frequency substantially matching the output frequency.

[0013] In some embodiments, the loop arrangement includes a single circuit loop enclosing both the input and output dielectric waveguides, with the nonlinear circuit element disposed along the single circuit loop.

[0014] In some embodiments, the frequency-multiplying circuit includes a first electric-dipole antenna disposed on or near a side surface of the input dielectric waveguide, and a second electric-dipole antenna disposed on or near a side surface of the output dielectric waveguide, with the first and second electric-dipole antennas interconnected by the nonlinear circuit element.

[0015] In some embodiments, the frequency-multiplying circuit is one of a plurality of frequencymultiplying circuits located at different longitudinal positions along the input and output dielectric waveguides.

[0016] In some embodiments, the output dielectric waveguide is one of a plurality of output dielectric waveguides located at different longitudinal positions along the input dielectric waveguides, each output dielectric waveguide including a respective output port for outputting a respective output electromagnetic wave at a respective output frequency in the THz frequency range; and the frequency-multiplying circuit is one of a plurality of frequency-multiplying circuits, each frequency-multiplying circuit being electromagnetically coupled to the input dielectric waveguide and to a respective one of the output dielectric waveguides. In some embodiments, at least some of the output frequencies of the plurality of output dielectric waveguides differ from another. In other embodiments, the output frequencies of the plurality of output dielectric waveguides are all the same.

[0017] In accordance with another aspect, there is provided a source of THz electromagnetic radiation, including: an oscillator circuit including a nonlinear circuit element having a current-voltage characteristic exhibiting a region of negative differential conductance, wherein the oscillator circuit is configured to generate therein an electrical signal oscillating at a source frequency in the THz frequency range upon electrical biasing of the nonlinear circuit element in the region of negative differential conductance; and a dielectric waveguide electromagnetically coupled to the oscillator circuit, wherein the coupling is configured for the oscillating electrical signal generated by the oscillator circuit to excite THz electromagnetic radiation at the source frequency inside the dielectric waveguide, the dielectric waveguide including an output port for outputting the THz electromagnetic radiation.

[0018] In some embodiments, the source frequency ranges from about 100 GHz to about 1000 GHz.

[0019] In some embodiments, the nonlinear circuit element is electrically biased with a direct current (DC) power supply connected to the oscillator circuit. In some embodiments, the nonlinear circuit element is electrically biased with a bias voltage ranging from about 0.1 V to about 1 V.

[0020] In some embodiments, the dielectric waveguide is made of silicon. In some embodiments, the dielectric waveguide has a rectangular cross-section.

[0021] In some embodiments, the output port of the dielectric waveguide is designed as a THz antenna configured to radiate the THz electromagnetic radiation. In some embodiments, the output port has a tapered configuration.

[0022] In some embodiments, the dielectric waveguide extends between a first end and a second end, with the first end serving as the output port and the second end terminating in a reflective termination.

[0023] In some embodiments, the nonlinear circuit element includes a resonant-tunneling diode.

[0024] In some embodiments, the oscillator circuit further includes a circuit loop enclosing the dielectric waveguide, with the nonlinear circuit element disposed along the circuit loop. In someembodiments, the circuit loop is configured to resonate at a frequency substantially matching the source frequency.

[0025] In some embodiments, the oscillator circuit further includes an electric-dipole antenna disposed on or near a side surface of the dielectric waveguide, with the nonlinear circuit element positioned along the electric-dipole antenna.

[0026] In some embodiments, the oscillator circuit is one of a plurality of oscillator circuits disposed at a plurality of locations along the dielectric waveguide. In some embodiments, consecutive ones of the oscillator circuits are spaced by an integer multiple of the wavelength of the THz electromagnetic radiation excited within the dielectric waveguide.

[0027] In accordance with another aspect, there is provided a THz frequency multiplier device, including: an input dielectric waveguide configured to receive and support therein an input electromagnetic wave having an input frequency; a frequency-multiplying circuit electromagnetically coupled to the input dielectric waveguide, wherein the coupling between the input dielectric waveguide and the frequency-multiplying circuit is configured for the input electromagnetic wave propagating in the input dielectric waveguide to induce an input electrical signal oscillating at the input frequency in the frequency-multiplying circuit, the frequency-multiplying circuit including a nonlinear circuit element, for example, a diode, configured to generate an output electrical signal including harmonics of the input electrical signal; and an output dielectric waveguide electromagnetically coupled to the frequency-multiplying circuit, wherein the coupling between the frequency-multiplying circuit and the output dielectric waveguide is configured for the output electrical signal to excite an output electromagnetic wave inside the output dielectric waveguide, the output electromagnetic wave having an output frequency in the THz frequency range corresponding to one of the harmonics of the input electrical signal, the output dielectric waveguide including an output port for outputting the output electromagnetic wave.

[0028] In accordance with another aspect, there is provided a source of THz electromagnetic radiation, including:an oscillator circuit including a nonlinear circuit element, for example, a resonant-tunneling diode, having a current-voltage characteristic exhibiting a region of negative differential conductance, wherein the oscillator circuit is configured to generate therein an oscillating signal at a source frequency in the THz frequency range upon electrical biasing of the nonlinear circuit element in the region of negative differential conductance; and a dielectric waveguide electromagnetically coupled to the oscillator circuit, wherein the coupling between the oscillator circuit and the dielectric waveguide is configured for the oscillating signal generated by the oscillator circuit to induce excitation of THz electromagnetic radiation at the source frequency inside the dielectric waveguide, the dielectric waveguide including an output port for outputting the THz electromagnetic radiation.

[0029] Other features and advantages of the present description will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the appended drawings. Although specific features described in the above summary and in the detailed description below may be described with respect to specific embodiments or aspects, it should be noted that these specific features can be combined with one another, unless stated otherwise.BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Fig. 1 is a schematic perspective view of an embodiment of a THz frequency multiplier device.

[0031] Fig. 2 is a schematic side elevation view of the THz frequency multiplier device of Fig. 1.

[0032] Fig. 3 is a schematic perspective view of another embodiment of a THz frequency multiplier device.

[0033] Fig. 4 is a schematic perspective view of another embodiment of a THz frequency multiplier device.

[0034] Fig. 5 is a schematic perspective view of another embodiment of a THz frequency multiplier device.

[0035] Fig. 6 is a schematic perspective view of another embodiment of a THz frequency multiplier device.

[0036] Fig. 7 illustrates a simplified equivalent circuit representation of the frequency-multiplying circuit of a THz frequency multiplier device, in accordance with an embodiment.

[0037] Fig. 8 is a schematic representation of the magnetic field coupling between the input dielectric waveguide and the first circuit loop of the frequency-multiplying circuit, in accordance with an embodiment.

[0038] Fig. 9 is a schematic representation of the magnetic field coupling between the input dielectric waveguide and the first electric-dipole antenna of the frequency-multiplying circuit, in accordance with an embodiment.

[0039] Fig. 10 is a schematic perspective view of another embodiment of a THz frequency multiplier device.

[0040] Fig. 11 is a schematic perspective view of an embodiment of a THz frequency multiplier device.

[0041] Fig. 12 is a schematic side elevation view of the THz frequency multiplier device of Fig. 11.

[0042] Fig. 13 is a schematic perspective view of an embodiment of a THz frequency multiplier device.

[0043] Fig. 14 is a schematic side elevation view of the THz frequency multiplier device of Fig. 13.

[0044] Fig. 15 is a schematic side elevation view of another embodiment of a THz frequency multiplier device.

[0045] Fig. 16 is a schematic perspective view of an embodiment of a THz radiation source.

[0046] Fig. 17 is a schematic perspective view of another embodiment of a THz radiation source.

[0047] Fig. 18 is a schematic perspective view of another embodiment of a THz radiation source.

[0048] Fig. 19 is a schematic perspective view of another embodiment of a THz radiation source.

[0049] Fig. 20 is a schematic perspective view of another embodiment of a THz radiation source.DETAILED DESCRIPTION

[0050] Throughout the present description, similar features in drawings have been given similar reference numerals. To avoid cluttering certain figures, some elements may not be indicated ifpreviously identified in preceding figures. Elements in drawings are not necessarily depicted to scale, emphasis being on clearly illustrating elements and structures of disclosed embodiments. Positional descriptors indicating the location or orientation of one element relative to another are used for ease and clarity of description. Unless indicated otherwise, these descriptors should be understood in the context of the figures and should not be considered limiting. Such spatially relative terms are intended to encompass different orientations in use or operation of disclosed embodiments, in addition to orientations exemplified in the figures. Furthermore, when a first element is referred to as being “on”, “above”, “below”, “over”, or “under” a second element, the first element can be directly or indirectly on, above, below, over, or under the second element, respectively, such that one or multiple intervening elements may be disposed between the first element and the second element.

[0051] The terms “a”, “an”, and “one” are defined herein to mean “at least one”, and do not exclude a plural number of elements, unless stated otherwise.

[0052] The term “or” is defined herein to mean “and / or”, unless stated otherwise.

[0053] Terms such as “substantially”, “generally”, and “about”, which modify a value, condition, or characteristic of a feature of an exemplary embodiment, should be understood to mean that the value, condition, or characteristic is defined within tolerances that are acceptable for the proper operation of this exemplary embodiment for its intended application, or that fall within an acceptable range of experimental error. In particular, the term “about” generally refers to a range of numbers that one skilled in the art would consider equivalent to the stated value (e.g., having the same or nearly the same function or result). In some instances, the term “about” means a variation of ±10% of the stated value. It is noted that all numerical values used herein are assumed to be modified by the term “about”, unless stated otherwise.

[0054] The term “based on” as used herein is intended to mean “based at least in part on”, whether directly or indirectly, and to encompass both “based solely on” and “based partly on”. In particular, the term “based on” may also be understood as meaning “depending on”, “representative of”, “indicative of”, “associated with”, “relating to”, and the like.

[0055] The terms “match”, “matching”, and “matched” refer herein to a condition in which two or more elements are either the same or within some predetermined tolerance of one another. These terms are meant to encompass not only “exactly” or “identically” matching the two or moreelements, but also “substantially”, “approximately”, or “subjectively” matching the elements, as well as providing a higher or best match among a plurality of matching possibilities.

[0056] The terms “connected” and “coupled”, and derivatives and variants thereof, refer herein to any connection or coupling, either direct or indirect, between two or more elements, unless stated otherwise.

[0057] The terms “light” and “optical”, and variants and derivatives thereof, refer herein to radiation in any appropriate region of the electromagnetic spectrum. These terms are not limited to visible light but may also include, without being limited to, the infrared, terahertz, millimeter, and centimeter wave regions.

[0058] The term “terahertz radiation” generally refers herein to electromagnetic radiation having a center frequency ranging from about 0.1 THz to about 10 THz, corresponding to a center wavelength ranging from about 30 pm to about 3000 pm. The definition of “terahertz radiation” with reference to a particular spectral range may vary depending on the technical field or standard under consideration and is not meant to limit the scope of application of the present techniques. Although various embodiments described herein operate with THz radiation, other embodiments could additionally or alternatively operate with radiation lying in other regions of the electromagnetic spectrum. Examples of such regions are the millimeter- wave region (i.e., electromagnetic radiation with a center frequency ranging from about 30 GHz to about 300 GHz, corresponding to a center wavelength ranging from about 1 mm to about 10 mm) and the centimeter- wave region (i.e., electromagnetic radiation with a center frequency ranging from about 3 GHz to about 30 GHz, corresponding to a center wavelength ranging from about 1 cm to about 10 cm). According to the above definitions, the upper-frequency end of the millimeter- wave region overlaps with the lower-frequency end of the THz region.

[0059] The present description generally relates to THz frequency multipliers and radiation sources that use nonlinear circuit elements, such as diodes, coupled to dielectric waveguides.

[0060] Certain conventional THz frequency multipliers and sources rely on interconnected metal circuits, cavities, and waveguides. While these metal-based devices may offer advantages, they also come with drawbacks and limitations, particularly at operation wavelengths shorter than about 1 mm (i.e., for frequencies higher than about 0.3 THz). The challenges encountered include increased ohmic losses in metal components at higher THz frequencies and difficulties in scaling down the manufacturing of metal components to submillimeter size with high precision, low cost,and high yield. Various embodiments of the present techniques aim to address or mitigate these challenges by providing compact, higher-power, and lower-cost devices for conversion or generation of THz radiation.

[0061] The present techniques have potential use in various commercial and industrial applications that may benefit from enhanced or improved sources of THz radiation. Examples include quality control (e.g., non-destructive inspection), security screening (e.g., concealed object detection), medicine (e.g., health diagnosis), imaging, spectroscopy, sensing, communications, and fundamental research.

[0062] Various aspects, features, and implementations of the present techniques are described below with reference to the figures.Implementations of the THz Frequency Multiplier

[0063] Referring to Figs. 1 and 2, schematic views of an embodiment of a THz frequency multiplier device 100 are depicted. The term “frequency multiplier device”, or simply “frequency multiplier”, refers herein to a device configured to receive an input signal and, using a nonlinear circuit element such as a diode, generate an output signal that is a higher harmonic of the input signal. Specifically, the output signal’s frequency n is an integer multiple of the input signal’s frequency f0, where n is a positive integer greater than one. In this context, the input signal corresponds to the first harmonic (also referred to as the fundamental) of the harmonic series.

[0064] The THz frequency multiplier device 100 includes an input dielectric waveguide 102, a frequency-multiplying circuit 104, and an output dielectric waveguide 106. The THz frequency multiplier device 100 is used in combination with an input radiation emitter 108 coupled to the input dielectric waveguide 102.

[0065] The input dielectric waveguide 102 is configured to support an input electromagnetic wave 110 received from the input radiation emitter 108. The input electromagnetic wave 110 oscillates at an input frequency fin(e.g., a center frequency) within a certain spectral band.

[0066] The frequency-multiplying circuit 104 is electromagnetically coupled to the input dielectric waveguide 102 in a manner such that the input electromagnetic wave 110 induces an input electrical signal oscillating at the input frequency finwithin the frequency-multiplying circuit 104. The term “electromagnetic coupling” refers herein to the transfer of signals between two or more elements through electromagnetic fields. In some embodiments, the coupling may be primarily mediated through, or described in terms of, a magnetic field, while in others, it may be primarilymediated through, or described in terms of, an electric field. Incorporating at least one nonlinear circuit element 112, such as a Schottky diode, the frequency-multiplying circuit 104 is configured to generate an output electrical signal that includes higher harmonics of the input electrical signal. The term “nonlinear circuit element” refers herein to an electronic component that exhibits a nonlinear current-voltage (l-V) characteristic (i.e. , a nonlinear equivalent resistance) within at least a portion of its operational range. The nonlinear response of the nonlinear circuit element 112 produces an output signal that scales nonlinearly with the input signal, resulting in the generation of harmonics.

[0067] Through electromagnetic coupling of the frequency-multiplying circuit 104 with the output dielectric waveguide 106, the output electrical signal excites an output electromagnetic wave 114 inside the output dielectric waveguide 106. The output electromagnetic wave 114 has an output frequency fOut (e.g., a center frequency) in the THz frequency range, corresponding to one of the higher harmonics of the input electrical signal, that is, fOut = nfn, where n = 2, 3, 4, 5, ... is a positive integer greater than one. For example, when fOut is equal 2finor 3fin, the THz frequency multiplier device may be referred to as a frequency doubler or tripler, respectively. In some embodiments, the output frequency fOut can range from about 100 GHz to about 1000 GHz.

[0068] Further details on the structure, configuration, and operation of these and other possible components associated with the THz frequency multiplier device 100 are provided in the following description. The schematic representations illustrated in Figs. 1 and 2, as well as in subsequent figures, aim to illustrate several components and features associated with the THz frequency multiplier device 100. Additional components and features useful for practical operation of the device 100 may not be specifically depicted.

[0069] The input dielectric waveguide 102 extends longitudinally along an input waveguide axis, between a first end and a second end. The first end serves as an input port 116 of the input dielectric waveguide 102 and is optically coupled to an output port 118 of the input radiation emitter 108 via appropriate optical coupling means, as described in greater detail below. The second end of the input dielectric waveguide 102 terminates in a reflective termination. In the illustrated embodiment, the reflective termination is a short-circuit termination embodied by a piece or film of electrically conducting material 120, providing a reflective wall surface at the second end of the input dielectric waveguide 102.

[0070] The output dielectric waveguide 106 extends longitudinally along an output waveguide axis, between a first end and a second end. The first end serves as an output port 122, throughwhich the output electromagnetic wave 114 exits the output dielectric waveguide 106. In some embodiments, the output port 122 functions as a THz antenna configured to radiate the output electromagnetic wave 114 in free space. As illustrated in Figs. 1 and 2, the output port 122 of the output dielectric waveguide 106 has a tapered configuration. The antenna’s geometry, including tapering angle, cross-sectional shape, and size of the tapered tip, can be tailored to achieve performance targets for some specific characteristics such as the emission frequency, radiation pattern, polarization state, and direction. Alternatively, the output port 122 may be coupled to another waveguide or optical component configured to receive the output electromagnetic wave 114 from the output dielectric waveguide 106. The second end of the output dielectric waveguide 106 terminates in a reflective termination. In this embodiment, the reflective termination is a short-circuit termination that shares the same piece of electrically conducting material 120 that is coupled to the second end of the input dielectric waveguide 102. However, as illustrated in Fig. 3, the short-circuit terminations of the two waveguides 102, 106 may also be provided by two different pieces of electrically conducting material 120i , 1202.

[0071] Returning to Figs. 1 and 2, the input dielectric waveguide 102 and the output dielectric waveguide 106 can be of any suitable type. The term “dielectric waveguide” refers herein to a waveguiding structure made primarily, but not necessarily exclusively, of a dielectric material and designed to confine and support electromagnetic waves in one or multiple guided modes. Various dielectric materials are suitable for operation at or near THz frequencies, including crystalline materials (e.g., silicon, germanium, sapphire, quartz), glass materials (e.g., fused silica), and polymer materials (e.g., polytetrafluoroethylene (PTFE), high-density polyethylene (HDPE)). The dielectric waveguide materials may be selected for their high refractive indices and low losses at THz frequencies. For example, in some embodiments, both the input dielectric waveguide 102 and the output dielectric waveguide 106 can be made of silicon, specifically single-crystal, high- purity, high-resistivity silicon (HR-Si), which has a refractive index of about 3.4, favorable thermomechanical properties, and low ohmic losses at THz frequencies.

[0072] In the embodiment illustrated in Figs. 1 and 2, the input dielectric waveguide 102 and the output dielectric waveguide 106 are each provided as an elongated solid piece of dielectric material with a rectangular cross-section. Dielectric waveguides shaped this way can be referred to as “dielectric ribbon waveguides” (DRWs). Various other waveguide configurations can be used in other embodiments. However, different configurations can be used in other embodiments, including waveguides with non-rectangular cross-sections (e.g., circular), waveguides with cladding, effective-medium-cladding waveguides, hollow-core waveguides, ridge waveguides, ribwaveguides, optical fibers, and the like. Providing the input and output dielectric waveguides 102, 106 as DRWs, specifically HR-Si ribbon waveguides, offers advantages such as manufacturing at low-cost using microfabrication techniques of common use in the monolithic microwave integrated circuit (MMIC) industry. In some embodiments, each of the input dielectric waveguide 102 and output dielectric waveguide 106 can have a cross-sectional diagonal dimension of the order of half of the guided wavelength. In the illustrated embodiment, the cross- sectional area of the input dielectric waveguide 102 is larger than that of the output dielectric waveguide 106. The ratio between these cross-sectional areas is selected based on the square of the ratio of the output frequency fOut to the input frequency f\nof the waves propagating in the THz frequency multiplier device 100.

[0073] In the embodiment depicted in Figs.1 and 2, the input dielectric waveguide 102 has a longitudinally curved configuration including two straight portions connected by a 90° elbow portion while the output dielectric waveguide 106 has a straight longitudinal configuration. However, other configurations for the two waveguides 102, 106 are also possible. For example, as shown in Fig. 4, another embodiment of the THz frequency multiplier device 100 features both waveguides 102, 106 with straight configurations.

[0074] Returning to Figs. 1 and 2, the input and output dielectric waveguides 102, 106 can each support various waveguide modes, depending on their configuration, in either single-mode or multimode operation. In some embodiments, the dielectric waveguides 102, 106 can support a quasi-transverse electromagnetic (TEM) fundamental mode, where the longitudinal electric and magnetic field components (i.e. , along the waveguide axis) are both negligible compared to their transverse counterparts (i.e., within the waveguide cross-section). In other embodiments, the dielectric waveguides 102, 106 can support a quasi-transverse electric (TE) fundamental mode, characterized by a negligible longitudinal electrical field component but a non-negligible longitudinal magnetic field component. In further embodiments, the dielectric waveguides 102, 106 can support a quasi-transverse magnetic (TM) fundamental mode, with a negligible longitudinal magnetic field component but a non-negligible longitudinal electric field component. In yet other embodiments, the dielectric waveguides 102, 106 can support different types of modes, such as quasi-TEM modes in the input dielectric waveguide 102 and quasi-TE modes in the output dielectric waveguide 106. For simplicity, the prefix “quasi” may be omitted in further references to waveguide modes, as is common practice in the art.

[0075] The input radiation emitter 108 can be embodied by any suitable device or combination of devices configured to generate the input electromagnetic wave 110 oscillating at the input frequency fin. Examples include vacuum electronic sources and solid-state electronic sources. The theory, configuration, implementation, and operation of such electromagnetic energy sources are well established in the art and need not be detailed herein beyond facilitating an understanding of the present techniques. In some embodiments, the input frequency fincan range from about 40 GHz to about 150 GHz. Notably, this exemplary frequency range extends partly below the typical THz frequency range of 0.1 to 10 THz referenced earlier. This indicates that, in certain embodiments, the THz frequency multiplier device 100 disclosed herein can generate a THz output signal having an output frequency fOut > 0.1 THz from a sub-THz input signal having an input frequency fin< 0.1 THz.

[0076] In some embodiments, the output port 118 of the input radiation emitter 108 is coupled to the input port 116 of the input dielectric waveguide 102 via free-space optics coupling. For example, the input radiation emitter 108 may radiate directly into free space using an antenna. Various antenna designs can be used, including horn antennas (e.g., diagonal horn antennas, conical horn antennas, wide-angle scalar feed horn antennas, corrugated horn antennas), bowtie antennas, dipole antennas, log-periodic antennas, and Vivaldi antennas. For example, the output port 118 of the input radiation emitter 108 depicted in Figs. 1 to 4 is provided as a current-loop (or magnetic-dipole) antenna, configured to couple the input electromagnetic wave 110 into a TE mode inside the input dielectric waveguide 102.

[0077] In other embodiments, the output port 118 of the input radiation emitter 108 is coupled to the input port 116 of the input dielectric waveguide 102 via waveguide coupling. For example, the output port 118 can be configured as a waveguide port featuring a waveguide flange (e.g., a WR- 10 rectangular metal waveguide with a UG-387 / U-M circular flange) configured for coupling to the input port 116. To reduce or prevent parasitic signal reflections at the interface between the input radiation emitter 108 and the input dielectric waveguide 102, the input port 116 of the input dielectric waveguide 102 may be provided with a tapered configuration. In this case, the tapered input port 116 may be inserted into the waveguide-terminated output port 118 of the input radiation emitter 108, as illustrated in Fig. 5. In some embodiments, such a configuration can be used to couple the input electromagnetic wave 110 into a TEM mode inside the input dielectric waveguide 102.

[0078] Returning to Figs. 1 and 2, the frequency-multiplying circuit 104 includes, in addition to the nonlinear circuit element 112 mentioned above, a first circuit loop 124i and a second circuit loop 1242. The first circuit loop 124i encloses the input dielectric waveguide 102 while the second circuit loop 1242 encloses the output dielectric waveguide 106. Depending on the application requirements, the spacing between each loop 124i , 1242 and its respective waveguide 102, 106 may be adjusted to prevent or mitigate unwanted parasitic coupling. The axis of the first circuit loop 124i , oriented normal to the plane of the loop, is parallel to the axis of the input dielectric waveguide 102, and the axis of the second circuit loop 1242 is parallel to the axis of the output dielectric waveguide 106. However, neither arrangement is a requirement. In the illustrated embodiment, both circuit loops 124i, 1242 are rectangular, but other shapes (e.g., circular or oval) and arrangements may be used in other embodiments. Depending on the implementation, the shape of the circuit loops 124i, 1242 may or may not match the cross-sectional shape of their corresponding waveguides 102, 106.

[0079] The first circuit loop 124i and a second circuit loop 1242 can be made of electrically conducting wire of any suitable length, cross-sectional size and shape, and material composition. In the illustrated embodiment, the first circuit loop 124i and the second circuit loop 1242 are arranged in a figure-of-eight configuration, sharing a common circuit path along respective portions thereof. The nonlinear circuit element 112 is disposed along this common circuit path. Various other loop arrangements enclosing the input and output dielectric waveguides 102, 106 can be incorporated within the frequency-multiplying circuit 104. For example, as shown in Fig. 6, the frequency-multiplying circuit 104 of another embodiment of the THz frequency multiplier device 100 includes a single circuit loop 124 that encloses both the input and output dielectric waveguides 102, 106, with the nonlinear circuit element 112 disposed along this loop 124. Other embodiments may include more complex or otherwise different loop arrangements, such as multiple-turn loop arrangements or open-loop arrangements.

[0080] Returning to Figs. 1 and 2, in the illustrated embodiment the nonlinear circuit element 112 is implemented as a diode, such as a Schottky diode. Such diodes are commonly used in THz frequency multipliers due to their ease of fabrication and high efficiency. However, other types of currently available (e.g., Gunn or IMPATT diodes) or forthcoming nonlinear semiconductor devices with suitable current-voltage characteristics can also be considered for use in other embodiments. It should be noted that while the nonlinear circuit element 112 is depicted in Figs. 1 and 2 as a single component for illustrative purposes, the term “nonlinear circuit element” encompasses both a single and multiple nonlinear circuit elements.

[0081] Fig. 7 illustrates a simplified equivalent circuit representation of the frequency-multiplying circuit 104 shown in Figs. 1 and 2, where C is the intrinsic capacitance of the Schottky diode 112, Li is the inductance of the first circuit loop 124i, and L.2 is the inductance of the second circuit loop 1242. In this simplified representation, the first circuit loop 124i can be modeled as a first LC resonant sub-circuit, and the second circuit loop 1242 can be modeled as a second LC resonant sub-circuit. The resonance frequenciesof the two sub-circuits can be expressed as follows:

[0082] While the expressions given in Equations (1) and (2) are approximate, they offer a useful analytical framework for understanding the operation of the frequency-multiplying circuit 104 and its coupling with the input and output dielectric waveguides 102, 106.

[0083] The resonance frequencyof the first LC resonant sub-circuit can be set to match the input frequency fmof the input electromagnetic wave 110 to optimize or at least enhance the electromagnetic coupling efficiency between the input dielectric waveguide 102 and the first circuit loop 124i . Similarly, the resonance frequency of the second LC resonant sub-circuit can be set to match the output frequency fOut = nf\nof the output electromagnetic wave 114 to optimize or at least enhance the electromagnetic coupling efficiency between the second circuit loop 1242 and the output dielectric waveguide 106. In such a case, the relationship between the two inductances can be written as L1 / L2 = n2> 1. The values of the inductances Li and L.2 can be set by adjusting the geometry of the first and second circuit loops 124i, 1242 (e.g., loop area, wire diameter), respectively. In some embodiments, the condition L1 / L2 > 1 can be achieved by setting the loop area of the first circuit loop 124i greater than that of the second circuit loop 1242, as illustrated in Figs. 1 and 2.

[0084] The operation of the THz frequency multiplier device 100 shown Figs. 1 and 2 is detailed below.

[0085] The input radiation emitter 108 is configured to emit the input electromagnetic wave 110 oscillating at the input frequency f\n, and to couple this wave 110 into the input dielectric waveguide 102 at its input port 116. The input dielectric waveguide 102 is configured to supporta quasi-TE fundamental mode oscillating at fin, which can be achieved by proper selection of its cross-sectional dimensions. The output port 118 of the input radiation emitter 108 is configured as a magnetic-dipole antenna, as noted above, which can favor the coupling of the input electromagnetic wave 110 in a TE mode inside the input dielectric waveguide 102. This is due to the magnetic-dipole antenna creating a longitudinal magnetic field whose configuration (e.g., orientation and intensity distribution) substantially matches or conforms with the configuration of the longitudinal magnetic field component of the TE mode in the input dielectric waveguide 102.

[0086] Due to the short-circuit termination at its second end, the input dielectric waveguide 102 is configured to support a standing-wave pattern from the electromagnetic wave 110 excited therein along at least a portion thereof. This standing wave is configured to induce an input electrical signal (e.g., an electrical current or voltage) oscillating at the input frequency f\nin the first circuit loop 124i of the frequency-multiplying circuit 104, in accordance with Lenz’s and Faraday’s laws of electromagnetic induction. Specifically, the longitudinal magnetic field component of the electromagnetic wave 110 within the input dielectric waveguide 102 creates a time-varying magnetic flux through the first circuit loop 124i . This change in magnetic flux induces a time-varying voltage oscillating at frequency f\nacross the first circuit loop 124i, generating a time-varying electrical current whose magnetic field opposes the magnetic flux change. The coupling of electromagnetic energy between the input dielectric waveguide 102 and the first circuit loop 124i is favored in this arrangement since the configuration (e.g., orientation and intensity distribution) of the longitudinal magnetic field component of the electromagnetic wave 110 supported in the input dielectric waveguide 102 substantially matches or conforms with the configuration of the longitudinal magnetic field induced in the first circuit loop 124i . This coupling between the input dielectric waveguide 102 and the first circuit loop 124i is illustrated schematically in Fig. 8, where the longitudinal magnetic field component of the quasi-TE wave supported in the input dielectric waveguide 102 is denoted by / 7TE, and the axial magnetic field and circumferential current induced in the first circuit loop 124i are denoted by H\oopand Jioop, respectively. Although not depicted in Fig. 8, a similar match exists with respect to the coupling between electric fields.

[0087] Returning to Figs. 1 and 2, the electromagnetic coupling strength between the input dielectric waveguide 102 and the first circuit loop 124i can be enhanced when the first circuit loop 124i is resonant at the input frequency ^ of the input electromagnetic wave 110. The coupling strength can also be improved by positioning the first circuit loop 124i at a location along the input dielectric waveguide 102 that corresponds to an antinode of the standing-wave patternof the input electromagnetic wave 110. However, neither of these conditions is a requirement, allowing for embodiments where the input frequency f\nof the input electromagnetic wave 110 does not correspond to a resonance frequency of the first circuit loop 124i and / or where the first circuit loop 124i is not positioned at an antinode of the standing-wave pattern of the input electromagnetic wave 110.

[0088] In response to the input electrical signal at frequency fingenerated in the first circuit loop 124i, the nonlinear circuit element 112, due to its nonlinear current-voltage characteristic, generates an output electrical signal (e.g., an electrical current or voltage) including a set of harmonics of the input electrical signal. The second circuit loop 1242, which is resonant at frequency nfn, is configured to couple more effectively with the nthharmonic of the output electrical signal produced by the nonlinear circuit element 112. In some embodiments, the nonlinear circuit element 112 (e.g., a diode) may be biased (e.g., by a constant-voltage source) to a particular operating point of its current-voltage characteristic. However, this is not a requirement, as the output electrical signal can be generated within the frequency-multiplying circuit 104 solely from the energy received from the input electromagnetic wave 110.

[0089] The coupling arrangement between the second circuit loop 1242 and the output dielectric waveguide 106 ensures that the nthharmonic generated in the second circuit loop 1242 excites, through electromagnetic induction, the output electromagnetic wave 114 at the output frequency fOut = nf within the output dielectric waveguide 106, where fOut lies in the THz frequency range. In the illustrated embodiment, the output dielectric waveguide 106 is configured to support a quasi-TE fundamental mode oscillating at fOut, which can be achieved by proper selection of its cross-sectional dimensions. The coupling between the second circuit loop 1242 and the output dielectric waveguide 106 is favored in this arrangement since the configuration (e.g., orientation and intensity distribution) of the longitudinal magnetic field produced by the electric current flowing in the second circuit loop 1242 substantially matches or conforms with the configuration of the longitudinal magnetic field component of the quasi-TE fundamental mode supported in the output dielectric waveguide 106.

[0090] Due to the short-circuit termination at its second end, the output dielectric waveguide 106 is configured to support a standing wave between the second end and the position of the second circuit loop 1242, and a traveling wave propagating from the position of the second circuit loop 1242 toward the first end, where it is outputted as the output electromagnetic wave 114 via the output port 122. As noted above with respect to the first circuit loop 124i, while it may beadvantageous for the second circuit loop 1242 to be resonant at the frequency fOut, this is not a requirement. This allows for embodiments where the output frequency fOut of the output electromagnetic wave 114 does not correspond to a resonance frequency of the second circuit loop 1242. Furthermore, it may be advantageous for the position of the second circuit loop 1242 along the output dielectric waveguide 106 to correspond to an antinode of the standing-wave pattern established between the second end and the second circuit loop 1242.

[0091] Table I below presents typical values for some key design parameters of a frequency multiplier device configured to operate as a frequency doubler with a 250-GHz output port, based on the embodiment depicted in Figs. 1 and 2. These parameter values are representative of the chosen configuration, and the cross-sectional dimensions of the waveguides and circuit loops can be adjusted accordingly to achieve the desired ratio of the output and input frequencies. For example, a frequency multiplier device configured to operate as a frequency tripler with a 300- GHz output port, based on a similar design, could be realized using an input dielectric waveguide with a cross-section of 0.75x0.75 mm2and an output dielectric waveguide with a cross-section of 0.25x0.25 mm2.Table I: Representative parameter values for a frequency multiplier device configured to operate as a frequency doubler with a 250-GHz output port.

[0092] The frequency-multiplying circuit 104 illustrated in the embodiments of Figs. 1 to 4 and 6 is based on circuit loops 124i , 1242 for coupling electromagnetic energy from and to the input and output dielectric waveguides 102, 106, respectively. However, alternative embodiments may employ different coupling approaches and configurations. Returning to Fig. 5, in this embodiment the frequency-multiplying circuit 104 includes a pair of electric-dipole antennas, namely a firstelectric-dipole antenna 126i disposed on or near the outer peripheral surface of the input dielectric waveguide 102, and a second electric-dipole antenna 1262 disposed on or near the outer peripheral surface of the output dielectric waveguide 106. The dipole axis of the first electricdipole antenna 126i is parallel to the axis of the input dielectric waveguide 102, and the dipole axis of the second electric-dipole antenna 1262 is parallel to the axis of the output dielectric waveguide 106. The first electric-dipole antenna 126i and the second electric-dipole antenna 1262 are interconnected by the nonlinear circuit element 112.

[0093] The first and second electric-dipole antennas 126i, 1262 can be formed from elongated metal or electrically conducting materials, such as wires, rods, or films, which are deposited, patterned, or otherwise arranged on or near a side surface of the input and output dielectric waveguides 102, 106, respectively. It is known that electric-dipole antennas disposed on the surface of a dielectric medium tend to radiate toward the dielectric medium rather than into air, which can be advantageous for use in certain embodiments disclosed herein.

[0094] The frequency-multiplying circuit 104 based on electric-dipole antennas 126i, 1262 depicted in Fig. 5 can be used in implementations where the input and output dielectric waveguides 102, 106 are configured to support quasi-TEM modes. One reason is that the electric and magnetic fields generated by the electric-dipole antennas 126i, 1262 have a configuration (e.g., orientation and intensity distribution) that substantially matches or conforms with the configuration of the electric and magnetic fields of the TEM mode supported by the input and output dielectric waveguides 102, 106. Fig. 9 schematically illustrates the coupling between the input dielectric waveguide 102 and the first electric-dipole antenna 126i for magnetic field components. In this figure, the transverse magnetic field component of the quasi-TEM wave in the input dielectric waveguide 102 is denoted by / 7TEM, and the circumferential magnetic field and longitudinal electric current induced in the first electric-dipole antenna 126i are denoted by H<nPand Jdip, respectively.

[0095] Returning to Fig. 5, in some embodiments the resonance frequency of the first electricdipole antenna 126i can be set to match the input frequency finof the input electromagnetic wave 110 to optimize or at least enhance the coupling efficiency between the input dielectric waveguide 102 and the first electric-dipole antenna 126i. Similarly, the resonance frequency of the second electric-dipole antenna 1262 can be set to match the output frequency fOut = nfm of the output electromagnetic wave 114 to optimize or at least enhance the coupling efficiency between the second electric-dipole antenna 1262 and the output dielectric waveguide 106. The resonancefrequencies of the first and second electric-dipole antennas 126i, 1262 can be changed by adjusting their geometry, such as their length. However, as mentioned above with respect to loopbased coupling schemes, while having the first electric-dipole antenna 126i resonant at f\nand / or the second electric-dipole antenna 1262 resonant at fOut may be beneficial, it is not required, and non-resonant operation is possible in some embodiments. Furthermore, positioning the first and second electric-dipole antennas 126i, 1262 with their centers positioned at or near antinodes of the standing-wave patterns established by the input and output electromagnetic waves 110, 114 can further enhance the coupling efficiency with the input and output dielectric waveguides 102, 106, respectively.

[0096] In operation, the quasi-TEM wave 110 supported by the input dielectric waveguide 102 is configured to induce an input electrical signal oscillating at the input frequency f\nin the first electric-dipole antenna 126i. In response, the nonlinear circuit element 112 produces, due to its nonlinear current-voltage characteristic, an output electrical signal containing harmonics of the input electrical signal. Because the second electric-dipole antenna 1262 is resonant at fOut = nf , it is configured to couple more effectively with the nthharmonic of the output electrical signal. Depending on the application, the nonlinear circuit element 112 (e.g., a diode) may or may not be biased to a particular operating point of its current-voltage characteristic. The coupling arrangement between the second electric-dipole loop 1262 and the output dielectric waveguide 106 enables the nthharmonic of the output electrical signal to excite, by electromagnetic induction, a quasi-TEM wave 114 oscillating at the output frequency fOut inside the output dielectric waveguide 106.

[0097] Fig. 10 illustrates another embodiment of a THz frequency multiplier device 100 that use an electric-dipole-based coupling scheme. The embodiment of Fig. 10 differs from that of Fig. 5 in that the first and second electric-dipole antennas 126i, 1262 are disposed on different side surfaces of the input and output dielectric waveguides 102, 106, respectively, to account for the fact that the fields of the quasi-TEM modes supported by the waveguides 102, 106 in Fig. 10 are orthogonal to those in Fig. 5.

[0098] Both loop-based and electric-dipole-based coupling schemes can be tailored to specific input and output dielectric waveguides 102, 106 based on various parameters. Examples of such parameters include the field patterns of the modes supported by the waveguides 102, 106, the values of the input and output frequencies fin, fOut, and the cross-sectional dimensions and shapes of the dielectric waveguides 102, 106. In addition to the loop-based and electric-dipole-basedcoupling schemes described above, other electromagnetic coupling approaches are possible. Such methods can employ capacitive coupling, such as using metal plates disposed on the side surface of the input and output dielectric waveguides 102, 106.

[0099] THz frequency multiplier devices based on dielectric waveguides can provide various advantages over conventional metal-waveguide-based approaches. Examples of such advantages include: (i) ease of manufacturing, since the diodes are coupled to the dielectric waveguides by conductors (e.g., loops and dipoles) located outside the waveguides; and (ii) reduced ohmic losses associated with the generated THz signal.

[0100] Conventional THz frequency multipliers using a single Schottky diode are limited in power mainly due to heat damage of the diode. Conversely, the present techniques using loop-based and electric-dipole-based coupling schemes can be readily implemented in configurations with multiple diodes (or multiple nonlinear circuit elements). In addition, since silicon DRWs are relatively easy to fabricate (e.g., by silicon wafer laser cutting or by bulk or surface micromachining), they can support various multiple-diode configurations, potentially enabling higher-power and higher-efficiency THz frequency multipliers.

[0101] Figs. 11 and 12 illustrate another embodiment of a THz frequency multiplier device 100. This embodiment shares several features with previously described embodiments, which will not be detailed again except to highlight differences between them. In the embodiment of Figs. 11 and 12, the THz frequency multiplier device 100 includes a plurality of frequency-multiplying circuits 104 / , where / = {1 , 2, 3, 4}, located at different longitudinal positions along the input and output dielectric waveguides 102, 106. Each frequency-multiplying circuit 104, includes a pair of circuit loops 124 / , 1, 124 / , 2 and a nonlinear circuit element 112, (e.g., a diode, such as a Schottky diode). The input dielectric waveguide 102 is configured to support a quasi-TE mode oscillating at fm, and the output dielectric waveguide 106 is configured to support a quasi-TE mode oscillating at fout. The operation of each frequency-multiplying circuit 104, to provide frequency multiplication from fm to fout between the input and output dielectric waveguides 102, 106 can be similar to the operation of the frequency-multiplying circuit 104 described above for the embodiment of Figs. 1 and 2. Since the THz frequency multiplier device 100 includes a plurality of frequency-multiplying circuits 104,, its conversion efficiency can be increased compared to single-circuit embodiments. For example, in some designs, the operation of each frequency-multiplying circuit 104, can be enhanced by setting its first and second circuit loops 124,,i, 124, ,2 to be resonant at fnand fout, respectively. In such embodiments, the multiple frequency-multiplying circuits 104, may all beoptimally or ideally designed to receive the input signal at fmfrom the input dielectric waveguide 102, generate the multiple output signals at foutfrom the input signal, and couple the multiple output signals into the output dielectric waveguide 106.

[0102] In some implementations, the coupling efficiency provided by the THz frequency multiplier device 100 depicted in Figs. 11 and 12 can be further enhanced by positioning each frequencymultiplying circuit 104, at a location that corresponds to an antinode of the standing-wave pattern of the input electromagnetic wave 110 within the input dielectric waveguide 102. However, this is not a requirement, and some embodiments may operate effectively without this alignment. It is noted that while the THz frequency multiplier device 100 depicted in Figs. 11 and 12 includes four frequency-multiplying circuits 104 / , other configurations may include any suitable number of frequency-multiplying circuits 104 / , for example, between two and a few hundred.

[0103] Figs. 13 and 14 depict another embodiment of a THz frequency multiplier device 100 including multiple frequency-multiplying circuits 104,. This embodiment shares several features with previously described embodiments, with differences highlighted below. The THz frequency multiplier device 100 in Figs. 13 and 14 includes not only a plurality of frequency-multiplying circuits 104 / , as in the embodiment of Figs. 11 and 12, but also a respective plurality of output dielectric waveguides 106 / , where / = {1 , 2, 3, 4}. Each output dielectric waveguide 106, extends along an output waveguide axis between a first end defining an output port 122, and a second end (e.g., a reflective termination) located at a respective one of a plurality of longitudinally distributed positions along the input dielectric waveguide 102. In the illustrated embodiment, the axis of each output dielectric waveguide 106, is perpendicular to the axis of the input dielectric waveguide 102, but other relative orientations are possible. It should be noted that while Figs. 13 and 14 show physical contact between the second end of each output dielectric waveguide 106, and the input dielectric waveguide 102, such contact is not a requirement.

[0104] The input dielectric waveguide 102 is configured to support a quasi-TE mode oscillating at frequency f\n, and each output dielectric waveguide 106, is configured to support and output an output electromagnetic wave 114, in a quasi-TE mode oscillating at frequency fout, / = n,fn, where the fout, / are not necessarily the same. Consequently, the THz frequency multiplier device 100 can provide simultaneous emissions at multiple output frequencies fout, / = n,fnbased on a single input signal at fin. In this embodiment, the second circuit loop 124, ,2 of each frequency-multiplying circuit 104, is designed to resonate at fOut, / , and the cross-sectional dimensions of each output dielectric waveguide 106, are designed to support a quasi-TE mode oscillating at fOut, / . Such THzmultiple-frequency multiplier devices can be useful, for example, in hyperspectral and other advanced imaging applications.

[0105] In some implementations, the coupling efficiency of the THz frequency multiplier device 100 depicted in Figs. 13 and 14 can be enhanced by positioning the first circuit loops 124 / , 1 at longitudinal positions at or near antinodes of the standing-wave pattern of the input electromagnetic wave 110 within the input dielectric waveguide 102. However, this positioning is not a requirement. Although the THz frequency multiplier device 100 depicted in Figs. 13 and 14 includes four frequency-multiplying circuits 104 / , any suitable number of frequency-multiplying circuits 104, can be used in other embodiments, for example, between two and a few hundred. It should also be noted that if the output frequencies fOut, / of the output dielectric waveguides 106, are all the same, the THz frequency multiplier device 100 can be operated as an array of singlefrequency THz sources. Such a configuration may be advantageous in certain applications, for example, in conveyor belt imaging systems.

[0106] Fig. 15 illustrates another embodiment of a THz multiple-frequency multiplier device 100 including multiple frequency-multiplying circuits 104, and multiple output dielectric waveguides 106, configured to emit at different output frequencies fOut, / . This embodiment shares several features with the embodiment of Figs. 13 and 14, with differences highlighted below. In the embodiment depicted in Fig. 15, each frequency-multiplying circuit 104, includes a first electric-dipole antenna 126 / , 1 disposed on the side surface of the input dielectric waveguide 102, and a second electric-dipole antenna 126, ,2 disposed on the side surface of the corresponding output dielectric waveguide 106,. The input dielectric waveguide 102 is configured to support a quasi-TEM mode oscillating at frequency fm, and each output dielectric waveguide 106, is configured to support a quasi-TEM mode oscillating at frequency fOut, / = n,fin. Each frequencymultiplying circuit 104, can be designed so that its first and second electric-dipole antennas 126 / , 1 , 126, ,2 are resonant at frequencies finand fOut, / , respectively. The operation of each frequency-multiplying circuit 104, to provide frequency multiplication from f\nto fOut,, can be similar to the operation of the frequency-multiplying circuit 104 described above with respect to the embodiment of Fig. 5.

[0107] In some embodiments, the coupling efficiency provided by the THz frequency multiplier device 100 can be further enhanced by positioning each first electric-dipole antenna 126,,i at a longitudinal position that coincides with an antinode of the standing-wave pattern of the input electromagnetic wave 110 within the input dielectric waveguide 102.

[0108] In the embodiment illustrated in Fig. 15, the THz frequency multiplier device 100 is packaged in a housing 128, which may be made of HDPE or another suitable material. The housing 128 extends along the axis of the input dielectric waveguide 102 between a first end 130i and a second end 1302. In the illustrated embodiment, both ends 130i, 1302 terminate in circular flanges (e.g., UG-387 / U-M circular flanges), with the first end 130i configured for connection with the waveguide output port 118 of an input radiation emitter 108 configured to supply the input electromagnetic wave oscillating at f\ninto the input dielectric waveguide 102. In other similar embodiments, the provision of a housing 128 may not be necessary.Implementations of the THz Radiation Source

[0109] Referring to Fig. 16, there is illustrated a schematic view of a possible embodiment of a source 200 of THz electromagnetic radiation. The THz radiation source 200 generally includes an oscillator circuit 202 and a dielectric waveguide 204 electromagnetically coupled to the oscillator circuit 202. The THz radiation source 200 operates in combination with a direct current (DC) or low-frequency alternating current (AC) power supply 206 connected to the oscillator circuit 202. In this context, the expression “low-frequency AC signal” refers to an AC signal with a frequency substantially below the emission frequency range of the THz radiation source 200.

[0110] The oscillator circuit 202 generally includes a nonlinear circuit element 208 having a current-voltage characteristic (or l-V curve) exhibiting a region of negative differential conductance / resistance (NDC / NDR), where the current flowing through the element 208 decreases with increasing bias voltage. In the embodiment of Fig. 16, the nonlinear circuit element 208 is a resonant-tunneling diode (RTD). However, other existing and forthcoming nonlinear circuit elements exhibiting NDC could be employed in other embodiments. It should be noted that while the nonlinear circuit element 208 is depicted in Fig. 16 as a single component for illustrative purposes, the term “nonlinear circuit element” encompasses both a single and multiple nonlinear circuit elements.

[0111] The term “oscillator circuit” refers broadly herein to an electronic circuit configured to convert an input signal from a DC or low-frequency AC power supply into an AC output signal. In the case of a low-frequency AC power supply, the frequency of the AC output signal is significantly higher than that of the input signal. The AC output signal can vary in frequency and waveform and its spectrum may contain either a single or multiple frequencies. In the present description, the oscillator circuit 202 generally includes a resonant circuit in addition to the nonlinear circuit element 208. In its simplest form, the resonant circuit can include an inductor and a capacitorforming an LC resonant circuit, but more complex resonant circuits can be used in other embodiments. Examples of possible resonant circuits include loop-based, electric-dipole-based, capacitor-based circuits, such as those described above with respect to the frequency multiplier implementations. The oscillator circuit 202 depicted in Fig. 16 is configured to generate a signal oscillating at a source frequency fS0Urce in the THz frequency range when the nonlinear circuit element 208 is electrically biased within its NDC region.

[0112] The dielectric waveguide 204 is electromagnetically coupled to the oscillator circuit 202, enabling the oscillating signal generated by the oscillator circuit 202 to excite THz electromagnetic radiation 210 at the source frequency fS0Urce inside the dielectric waveguide 204. The dielectric waveguide 204 includes an output port 212 for emitting the THz radiation 210. In some embodiments, the source frequency fS0Urce can range from about 100 GHz to about 1000 GHz.

[0113] The structure, configuration, and operation of these and other possible components of, or coupled to, the THz radiation source 200 are described in greater detail below. It should be noted that Fig. 16 (as well as other figures referenced below) is a schematic representation intended to illustrate a number of components and features of the THz radiation source 200. As such, additional components and features that may be useful for practical operation may not be specifically depicted.

[0114] As mentioned earlier, the NDC nonlinear circuit element 208 is an RTD in the embodiment shown in Fig. 16. An RTD is a diode with a resonant tunneling structure including multiple thin layers of specifically doped semiconductor materials configured to provide the RTD with a current-voltage characteristic exhibiting one or more NDC regions. Because of the presence of these NDC regions, an RTD connected to an LC resonant circuit and biased with a constant or low-frequency voltage can generate an electrical signal with a frequency corresponding to that of electromagnetic radiation in a given range, such as THz radiation. The theory, implementation, and operation of RTDs, including their use in devices for generating THz radiation, are generally known in the art and need not be described in detail herein except to facilitate an understanding of the present techniques.

[0115] RTDs are recognized as promising semiconductor devices for use in THz radiation sources, particularly for emission at frequencies above 500 GHz. Because single RTDs have limited power (e.g., output power levels in the microwatt range), THz radiation sources based on RTD arrays have been proposed. Many conventional THz radiation sources based on RTD arrays incorporate metal components such as microstrip line connections and antennas, similar to thoseused in monolithic microwave integrated circuits (MMICs). As noted above, while such metalbased devices may offer certain advantages, they also have drawbacks and limitations, notably ohmic losses, which fundamentally restrict or limit their applicability to higher THz frequencies. Another drawback of existing RTD arrays is the significant portion of THz radiation generated by on-wafer metal antennas that is radiated into the substrate and therefore lost. Furthermore, RTD arrays often suffer from low brightness, as they generally emit THz radiation from a relatively large surface area. Low-brightness THz sources can be a disadvantage in certain applications such as THz inspection and other imaging applications. Various embodiments of the present techniques aim to address or mitigate at least some of these challenges by providing more compact, higher- power, and lower-cost devices for generating THz radiation.

[0116] The dielectric waveguide 204 extends longitudinally along a waveguide axis between a first end and a second end. The first end defines the output port 212 through which the THz radiation 210 is emitted. In some embodiments, the output port 212 acts as a THz antenna configured to radiate the THz radiation 210 in free space, as illustrated in Fig. 16, where the output port 212 has a tapered configuration. The geometry of the output port 212, including tapering angle, cross-sectional shape, and size of the tapered tip, can be tailored to achieve performance targets for some specific characteristics such as the emission frequency, radiation pattern, polarization state, and direction. In other embodiments, the output port 212 may be coupled to another waveguide or optical component configured to receive the THz radiation 210 emitted by the THz radiation source 200. The second end of the dielectric waveguide 204 terminates in a reflective termination 214. In the illustrated embodiment, the reflective termination is a short-circuit termination embodied by a piece or film of electrically conducting material providing a reflective wall surface at the second end of the dielectric waveguide 204.

[0117] The dielectric waveguide 204 can be made of various dielectric materials suitable for use at or near THz frequencies. Examples of such materials include crystalline materials (e.g., silicon, germanium, sapphire, quartz), glass materials (e.g., fused silica), and polymer materials (e.g., PTFE, HDPE). In some embodiments, the dielectric waveguide 204 can be made of silicon, specifically HR-Si. In the illustrated embodiment, the dielectric waveguide 204 is provided in the form of an elongated piece of dielectric material with a rectangular cross-section, which, as noted above, can be referred to as a dielectric ribbon waveguide (DRW). However, different waveguide configurations can be used in other embodiments, including waveguides with non-rectangular cross-sections (e.g., circular), waveguides with cladding, effective-medium-cladding waveguides, hollow-core waveguides, and the like. In some embodiments, the dielectric waveguide 204 mayhave transverse dimensions (e.g., width and height) ranging from about 125 pm for operation at higher THz frequencies (e.g., about 600 GHz) to about 250 pm for operation at lower THz frequencies (e.g., about 300 GHz). In the illustrated embodiment, the dielectric waveguide 204 has a longitudinally straight configuration, but angled or curved configurations can be used in other embodiments. Depending on its configuration, the dielectric waveguide 204 can support propagation of various waveguide modes, in either single-mode or multimode operation. In some embodiments, the dielectric waveguide 204 can support a quasi-TEM or quasi-TE fundamental mode, as noted above with respect to the frequency multiplier implementations.

[0118] Referring still to Fig. 16, the oscillator circuit 202 includes, in addition to the RTD 208 introduced above, a circuit loop 216 that encloses the dielectric waveguide 204. The circuit loop 216 has a rectangular shape, but other shapes (e.g., circular or oval) and arrangements (e.g., multiple-turn loop arrangements, open-loop arrangements) are possible in other embodiments. Depending on the implementation, the shape of the circuit loop 216 may or may not match the cross-sectional shape of the dielectric waveguide 204. In the illustrated embodiment, the axis of the circuit loop 216 is parallel to the axis of the dielectric waveguide 204, but this is not a requirement. The circuit loop 216 can be made of an electrically conducting wire of any suitable length, cross-sectional size and shape, and material composition.

[0119] The RTD 208 is positioned along the circuit loop 216, with its terminals connected to the power supply 206 via appropriate electrical connections 218 (e.g., electrodes). In some embodiments, the power supply 206 may be implemented as an AC-DC converter, a chemical battery, or another suitable type of power sources. The power supply 206 is configured to apply a DC or low-frequency bias voltage to the RTD 208, allowing it to operate within the NDC region of its current-voltage characteristic. The bias voltage may range from about 0.1 V to about 1 V, although other bias voltage values may be used in other embodiments, depending on factors such as the number of RTDs and the THz output power requirements.

[0120] In the illustrated embodiment, the oscillator circuit 202 can be modeled as a resonant LC circuit with a resonance frequency frexpressed as:where C is the intrinsic capacitance of the RTD 208, and L is the inductance of the circuit loop 216. Although the expression for frgiven in Equation (3) is approximate, it offers a useful analyticalframework for understanding the operation of the oscillator circuit 202 and its coupling with the dielectric waveguide 204. The resonance frequency frcan be set to match the intended emission frequency fS0Urce of the THz radiation source 200. In particular, the value of the loop inductance L can be set by adjusting the geometry of the circuit loop 216 (e.g., loop area, wire diameter, number of turns).

[0121] The operation of the THz radiation source 200 shown in Fig. 16 will be described in greater detail below.

[0122] The oscillator circuit 202 generates an electrical signal oscillating at a frequency fS0Urce in the THz region, which can be approximated using Equation (3). In some embodiments, the electrical connections 218 may be designed to reflect the THz signal back into the circuit loop 216. This can be achieved by proper selection of the width of the electrical connections 218, their spacing, and their connection locations to the circuit loop 216. In certain cases, the lower part of the loop 216 (i.e., the part of the circuit loop 216 located on the side surface of the dielectric waveguide 204 opposite to the side surface facing the RTD 208) may act as an unwanted short- circuit path if its resistance is too low. This can be prevented by providing another circuit component in the lower part of the loop 216 (e.g., a resistor, a capacitor, or another RTD).

[0123] Due to the coupling arrangement between the circuit loop 216 and the dielectric waveguide 204, and by proper selection of the cross-sectional dimensions of the dielectric waveguide 204, the electrical signal oscillating at frequency fS0Urce generated in the oscillator circuit 202 is configured to excite a quasi-TE electromagnetic wave 210 at frequency fS0Urce inside the dielectric waveguide 204. The coupling between the circuit loop 216 and the dielectric waveguide 204 is favored in this arrangement since the configuration (e.g., orientation and intensity distribution) of the longitudinal magnetic field produced by the electric current flowing in the circuit loop 216 substantially matches or conforms with the configuration of the longitudinal magnetic field component of the quasi-TE mode of the dielectric waveguide 204. The THz radiation 210 travels along the dielectric waveguide 204 until it reaches the output port 212.

[0124] The output port 212 acts as an antenna, radiating the THz radiation 210 outwardly (e.g., into the air) according to a radiation pattern defined, for example, by the tapering angle of the output port 212. The reflective termination 214 at the second end of the dielectric waveguide 204 is configured to reflect the THz radiation 210 toward the output port 212. It is appreciated that the position of the circuit loop 216 along the waveguide with respect to the reflective termination 214 can be adjusted to optimize or at least enhance the electromagnetic coupling efficiency betweenthe circuit loop 216 and the dielectric waveguide 204. This can be achieved by positioning the circuit loop 216 at a position along the dielectric waveguide 204 that corresponds to a region where the magnitude of the longitudinal magnetic field component is maximum or sufficiently large.

[0125] Table II below presents typical values for some key design parameters of a 300-GHz radiation source based on the embodiment depicted in Fig. 16. A waveguide output port with a tapering angle of 10° can provide nearly reflectionless conditions; however, other tapering angles may also be used. In other implementations, the output port may be tapered along one or both transverse dimensions. The antenna characteristics of the output port (e.g., emission frequency, radiation pattern, polarization) can be determined by the specific design of the tip geometry. The cross-sectional dimensions of the waveguide and circuit loop can be adjusted according to the desired output frequency of the THz radiation source. For example, a THz radiation source configured to emit at 600 GHz based on a similar design could be realized using a dielectric waveguide with a cross-sectional area of 0.125x0.150 mm2.Table II: Representative parameter values for a 300-GHz radiation source.

[0126] It is appreciated that while the oscillator circuit 202 illustrated in Fig. 16 uses a circuit loop 216 to couple electromagnetic energy to the dielectric waveguide 204, other embodiments may employ different coupling approaches and configurations. For instance, Fig. 17 depicts an embodiment of a THz radiation source 200 where the oscillator circuit 202 includes an electricdipole antenna 220 disposed on or near one of the side surfaces of the dielectric waveguide 204. The dipole axis of the electric-dipole antenna 220 is parallel to the axis of the dielectric waveguide 204. The RTD 208 is disposed along the electric-dipole antenna 220, for example, at its midpoint. The electric-dipole antenna 220 can be embodied by an elongated metal structure deposited, patterned, or otherwise arranged on or near the side surface of the dielectric waveguide 204.

[0127] As discussed above for the frequency multiplier implementations, the electric-dipole- based oscillator circuit 202 depicted in Fig. 17 can be used in devices in which the dielectric waveguide 204 is configured to support quasi-TEM modes. One reason is that the electric and magnetic fields generated by the electric-dipole antenna 220 have a configuration (e.g., orientation and intensity distribution) that substantially matches or conforms with the configuration of the electric and magnetic fields of the TEM mode excited in the dielectric waveguide 204. The resonance frequency of the electric-dipole-based oscillator circuit 202 can be set to match the desired output frequency fS0Urce of the THz radiation source 200. The resonance frequency of the oscillator circuit 202 can be changed by adjusting the geometry of the electric-dipole antenna 220, such as its length. Furthermore, as in loop-based designs, positioning the electric-dipole antenna 220 at a location along the dielectric waveguide 204 where the field intensity is maximum or sufficiently large can enhance the coupling efficiency with the dielectric waveguide 204.

[0128] Referring to Figs. 18 to 20, alternative embodiments of a THz radiation source 200 are illustrated. These embodiments share several features with previously described embodiments, and only key differences will be highlighted. In these alternative embodiments, the THz radiation source 200 includes a plurality of oscillator circuits 202, disposed at a plurality of locations along the dielectric waveguide 204, with each oscillator circuit 202, including an RTD 208, and a circuit loop 216,. The terminals of each RTD 208, are connected to a power supply (not shown) via appropriate electrical connections 218 (not shown), as described above for the embodiment of Fig. 16. The operation of each individual oscillator circuit 202, shown in Figs. 18 to 20 may be similar to the operation of the oscillator circuit 202 depicted in Fig. 16. In multiple-RTD implementations the positioning of the different oscillator circuits 202, along the longitudinal profile of the electromagnetic wave excited within the dielectric waveguide 204 can be adjusted to enhance performance. In some embodiments, coherent summation of the electromagnetic fields excited within the dielectric waveguide 204 by the various oscillator circuits 202, can be achieved when consecutive oscillator circuits 202, are separated by a distance equal to an integer multiple of the wavelength of the excited waveguide mode. However, other embodiments may not implement such coherent power summation. Multiple-RTD-based THz radiation sources 200, such as those shown in Figs. 18 to 20, can achieve high brightness because the total power generated by all the oscillator circuits 202, is radiated by the same antenna, namely the output port 212 of the dielectric waveguide 204.

[0129] In Fig. 18, the dielectric waveguide 204 is configured with a straight longitudinal design, whereas in Fig. 19, the dielectric waveguide 204 includes a ring-shaped portion 222 and astraight, power-combining portion 224. The ring-shaped portion 222 includes two branches 226i, 2262, each of which accommodating two RTD-based oscillator circuits 202,. The configuration depicted in Fig. 19 provides an efficient way to integrate a large number of RTD- based oscillator circuits 202 / within a compact arrangement. This configuration may also simplify fabrication by eliminating the need for metal short-circuit terminations. The embodiment of Fig. 20 has a similar design to Fig. 19, featuring a dielectric waveguide 204 with a curved, serpentine configuration designed to accommodate a large number of RTD-based oscillator circuits 202, in a compact arrangement. It is appreciated that the non-straight waveguide designs depicted in Figs. 19 and 20 are illustrative examples only. Other configurations can be employed in different embodiments. Additionally, these embodiments can include any suitable number of RTD-based oscillator circuits, for example, between two and a few hundred, to generate THz radiation over broad ranges of wavelengths and output power levels.

[0130] The THz radiation sources disclosed herein can provide various advantages over conventional metal-waveguide-based devices. Examples of such advantages include: (i) reduced losses in the higher THz frequency range due to the use of dielectric waveguides; (ii) precise and cost-effective manufacturing facilitated by Si-based MEMS fabrication technology; (iii) higher output power levels achieved by coherent summation of electromagnetic fields generated by multiple RTD-based oscillator circuits; and (iv) higher brightness achieved by compact integration of multiple RTD-based oscillator circuits.

[0131] In some applications, the THz radiation source 200 described with reference to Figs. 16 to 20 may function as the input radiation emitter 108 configured for use with the THz frequency multiplier device 100 described with reference to Figs. 1 to 15.

[0132] Numerous modifications could be made to the embodiments described above without departing from the scope of the appended claims.

Claims

CLAIMS1. A THz frequency multiplier device, comprising: an input dielectric waveguide configured to receive and support an input electromagnetic wave oscillating at an input frequency; a frequency-multiplying circuit electromagnetically coupled to the input dielectric waveguide, wherein the coupling is configured for the input electromagnetic wave to induce an input electrical signal oscillating at the input frequency in the frequency-multiplying circuit, the frequency-multiplying circuit comprising a nonlinear circuit element configured to generate an output electrical signal comprising higher harmonics of the input electrical signal; and an output dielectric waveguide electromagnetically coupled to the frequency-multiplying circuit, wherein the coupling is configured for the output electrical signal to excite an output electromagnetic wave in the output dielectric waveguide at an output frequency in the THz frequency range, the output frequency corresponding to one of the higher harmonics of the input electrical signal, the output dielectric waveguide comprising an output port for outputting the output electromagnetic wave.

2. The THz frequency multiplier device of claim 1 wherein the output frequency corresponds to the second or third harmonic of the input electrical signal.

3. The THz frequency multiplier device of claim 1 or 2, wherein the output frequency ranges from about 100 GHz to about 1000 GHz.

4. The THz frequency multiplier device of any one of claims 1 to 3, wherein the input dielectric waveguide and the output dielectric waveguide are made of silicon.

5. The THz frequency multiplier device of any one of claims 1 to 4, wherein the input dielectric waveguide and the output dielectric waveguide have rectangular cross-sections.

6. The THz frequency multiplier device of any one of claims 1 to 5, wherein the input dielectric waveguide and the output dielectric waveguide have respective cross-sectional areas, and wherein the ratio of the cross-sectional area of the input dielectric waveguide to the cross- sectional area of the output dielectric waveguide substantially matches the square of the ratio of the output frequency to the input frequency.

7. The THz frequency multiplier device of any one of claims 1 to 6, wherein the input dielectric waveguide is configured to receive the input electromagnetic wave via waveguide coupling from an input radiation emitter.

8. The THz frequency multiplier device of any one of claims 1 to 6, wherein the input dielectric waveguide is configured to receive the input electromagnetic wave via free-space optics coupling from an input radiation emitter.

9. The THz frequency multiplier device of any one of claims 1 to 8, wherein the output port of the output dielectric waveguide is designed as a THz antenna configured to radiate the output electromagnetic wave.

10. The THz frequency multiplier device of any one of claims 1 to 9, wherein the nonlinear circuit element comprises a Schottky diode.

11. The THz frequency multiplier device of any one of claims 1 to 10, wherein the frequencymultiplying circuit further comprises a loop arrangement electrically connected to the nonlinear circuit element and enclosing the input dielectric waveguide and the output dielectric waveguide.

12. The THz frequency multiplier device of claim 11 , wherein the loop arrangement comprises a first circuit loop enclosing the input dielectric waveguide and a second circuit loop enclosing the output dielectric waveguide, with the first and second circuit loops arranged in a figure-of-eight configuration and sharing a common circuit path along respective portions thereof, wherein the nonlinear circuit element is disposed along the common circuit path.

13. The THz frequency multiplier device of claim 12, wherein: the first circuit loop is configured as a first resonant sub-circuit designed to resonate at a first frequency substantially matching the input frequency; and / or the second circuit loop is configured as a second resonant-sub-circuit designed to resonate at a second frequency substantially matching the output frequency.

14. The THz frequency multiplier device of claim 11 , wherein the loop arrangement comprises a single circuit loop enclosing both the input and output dielectric waveguides, with the nonlinear circuit element disposed along the single circuit loop.

15. The THz frequency multiplier device of any one of claims 1 to 10, wherein the frequencymultiplying circuit comprises a first electric-dipole antenna disposed on or near a side surface ofthe input dielectric waveguide, and a second electric-dipole antenna disposed on or near a side surface of the output dielectric waveguide, with the first and second electric-dipole antennas interconnected by the nonlinear circuit element.

16. The THz frequency multiplier device of any one of claims 1 to 15, wherein the frequencymultiplying circuit is one of a plurality of frequency-multiplying circuits located at different longitudinal positions along the input and output dielectric waveguides.

17. The THz frequency multiplier device of any one of claims 1 to 15, wherein: the output dielectric waveguide is one of a plurality of output dielectric waveguides located at different longitudinal positions along the input dielectric waveguides, each output dielectric waveguide comprising a respective output port for outputting a respective output electromagnetic wave at a respective output frequency in the THz frequency range; and the frequency-multiplying circuit is one of a plurality of frequency-multiplying circuits, each frequency-multiplying circuit being electromagnetically coupled to the input dielectric waveguide and to a respective one of the output dielectric waveguides.

18. The THz frequency multiplier device of claim 17, wherein at least some of the output frequencies of the plurality of output dielectric waveguides differ from another.

19. The THz frequency multiplier device of claim 17, wherein the output frequencies of the plurality of output dielectric waveguides are all the same.

20. A source of THz electromagnetic radiation, comprising: an oscillator circuit comprising a nonlinear circuit element having a current-voltage characteristic exhibiting a region of negative differential conductance, wherein the oscillator circuit is configured to generate therein an electrical signal oscillating at a source frequency in the THz frequency range upon electrical biasing of the nonlinear circuit element in the region of negative differential conductance; and a dielectric waveguide electromagnetically coupled to the oscillator circuit, wherein the coupling is configured for the oscillating electrical signal generated by the oscillator circuit to excite THz electromagnetic radiation at the source frequency inside the dielectric waveguide, the dielectric waveguide comprising an output port for outputting the THz electromagnetic radiation.

21. The source of THz electromagnetic radiation of claim 20, wherein the source frequency ranges from about 100 GHz to about 1000 GHz.

22. The source of THz electromagnetic radiation of claim 20 or 21 , wherein the nonlinear circuit element is electrically biased with a direct current (DC) power supply connected to the oscillator circuit.

23. The source of THz electromagnetic radiation of any one of claims 20 to 22, wherein the nonlinear circuit element is electrically biased with a bias voltage ranging from about 0.1 V to about 1 V.

24. The source of THz electromagnetic radiation of any one of claims 20 to 23, wherein the dielectric waveguide is made of silicon.

25. The source of THz electromagnetic radiation of any one of claims 20 to 24, wherein the dielectric waveguide has a rectangular cross-section.

26. The source of THz electromagnetic radiation of any one of claims 20 to 25, wherein the output port of the dielectric waveguide is designed as a THz antenna configured to radiate the THz electromagnetic radiation.

27. The source of THz electromagnetic radiation of claim 26, wherein the output port has a tapered configuration.

28. The source of THz electromagnetic radiation of any one of claims 20 to 27, wherein the dielectric waveguide extends between a first end and a second end, with the first end serving as the output port and the second end terminating in a reflective termination.

29. The source of THz electromagnetic radiation of any one of claims 20 to 28, wherein the nonlinear circuit element comprises a resonant-tunneling diode.

30. The source of THz electromagnetic radiation of any one of claims 20 to 29, wherein the oscillator circuit further comprises a circuit loop enclosing the dielectric waveguide, with the nonlinear circuit element disposed along the circuit loop.

31. The source of THz electromagnetic radiation of claim 30, wherein the circuit loop is configured to resonate at a frequency substantially matching the source frequency.

32. The source of THz electromagnetic radiation of any one of claims 20 to 29, wherein the oscillator circuit further comprises an electric-dipole antenna disposed on or near a side surface of the dielectric waveguide, with the nonlinear circuit element positioned along the electric-dipole antenna.

33. The source of THz electromagnetic radiation of any one of claims 20 to 32, wherein the oscillator circuit is one of a plurality of oscillator circuits disposed at a plurality of locations along the dielectric waveguide.

34. The source of THz electromagnetic radiation of claim 33, wherein consecutive ones of the oscillator circuits are spaced by an integer multiple of the wavelength of the THz electromagnetic radiation excited within the dielectric waveguide.

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