High-energy terahertz radiation source based on semiconductors

Abstract

This invention relates to a method for generating terahertz radiation and a terahertz radiation source. In the method according to the invention, a pump beam (14) emitted from a pump beam source is incident on a block of a semiconductor nonlinear optical medium (10) having a planar parallel front boundary surface and a rear boundary surface (11, 12), wherein the pump beam (14) is decomposed into a set of sub-pump beams upon refraction on a periodic structure (13) formed by pairs of planar surfaces in the front boundary surface of the block. The sub-pump beams travel in angular directions required to satisfy velocity matching conditions. The envelope of the pump pulse leading edge segment propagating in the sub-pump beam segment propagates in a direction perpendicular to the rear boundary surface (12) (i.e., the exit surface of the optical medium (10)) at the propagation speed of terahertz radiation, and generates terahertz radiation in the semiconductor optical medium (10) by means of optical rectification to satisfy the velocity matching condition; here, is the group velocity of the pump beam (14), is the phase velocity of the generated terahertz radiation, and y is the angle between the envelope of the pulse leading edge of the pump beam (14) and the phase leading edge.

Description

Technical Field

[0001] This invention relates to methods for generating terahertz (THz) radiation and to terahertz radiation sources. More specifically, this invention relates to novel methods for generating terahertz pulses and to terahertz radiation sources for terahertz pulses with improved beam characteristics, efficiency, and energy scalability. The terahertz radiation source according to the invention requires no imaging optics or gratings, and in a preferred embodiment, the medium used to generate terahertz radiation is a semiconductor material with nonlinear optical properties. Background Technology

[0002] As is well known, in order to efficiently generate terahertz radiation through nonlinear optical processes, a so-called velocity matching condition must be met. According to this condition, the group velocity of the pump pulse used for excitation must match the phase velocity of the terahertz pulse being generated.

[0003] Furthermore, efficient terahertz radiation generation requires crystals with high (typically exceeding tens of picometers per volt) second-order nonlinear optical coefficients to be used for the generation. These materials include semiconductors such as gallium phosphide (GaP), zinc telluride (ZnTe), and gallium arsenide (GaAs), as well as lithium niobate (LN) and lithium tantalate (LT). A drawback of these materials is that the difference between the group refractive index at the pump frequency and the phase refractive index in the terahertz range makes it difficult to achieve the aforementioned velocity matching. The tilted pulse front technique provides a solution to this problem (see J. Hebling et al., “Velocity matching by pulse front tilting for large-area THz-pulse generation,” OpticsExpress, Vol. 10, No. 21, pp. 1161-1166 (2002)), which generates terahertz radiation through optical pulses, where the pulse front (intensity front) is tilted at an angle ( ) to the wavefront to the desired size for wavefront formation. Since the generated terahertz beam propagates perpendicular to the leading edge of the tilted pulse, the group velocity of the pump in the direction of terahertz radiation propagation, as a consequence of velocity matching requirements, is... The projection must be equal to the terahertz phase velocity. That is, the following conditions must be met:

[0004] (1)

[0005] Specifically, for the pump wavelength in the near-infrared band, for LN, For LT, For semiconductor materials, the required angle The value should ideally be much smaller than this, typically less than about 30º.

[0006] Currently, the tilted pulse front excitation technique can generate pulses with the highest energy, typically in the frequency range of 0.1-1 terahertz (THz) (see JA Fülöp et al., “Efficient generation of THz pulses with 0.4 mJ energy”; Optics Express, Vol. 22, No. 17, pp. 20155-20163 (2014)). The high-energy terahertz sources described in this paper, providing 0.43 mJ pulse energy, all use prism-type LN crystals as nonlinear optical crystals. One reason for this is that, to minimize reflection losses, the pump beam must enter the crystal perpendicularly, and the resulting terahertz beam must also exit perpendicularly. Furthermore, the perpendicular coupling of the terahertz beam ensures that the generated terahertz beam is free of angular dispersion, a crucial requirement in applications. Therefore, to satisfy the velocity matching condition (1), the exit surface of the crystal should form a wedge angle with the incident surface of the nonlinear optical crystal, the size of which is exactly the same as the angle γ.

[0007] Using a prism-type terahertz source medium for high-energy terahertz excitation is extremely detrimental to the quality of the terahertz beam. With a wide pump beam (crucial for generating high-energy terahertz pulses), the terahertz beams generated on opposite sides of the pump beam cross-section are excited at significantly different lengths, resulting in varying degrees of absorption and dispersion in the LN crystal used. Furthermore, the nonlinear effects at the excitation sites also differ. Consequently, the induced terahertz pulse intensities and the temporal progression of the electric field within the pulses differ significantly at symmetrical locations on opposite sides of the pump beam, leading to a highly asymmetrical and poor-quality terahertz beam. Therefore, the terahertz beam is highly unfocusable (i.e., consistent with the diffraction limit), a serious drawback for many applications.

[0008] In conventional tilted-pulse-front terahertz sources, the tilt of the pulse front in the pump beam is typically generated by diffraction using a (reflective or transmissive) grating placed in the optical path. After imaging through a lens or telescope, the beam is then coupled into a crystal with nonlinear optical properties to generate terahertz radiation: an image of the light spot on the grating surface is formed in the crystal. Therefore, in conventional tilted-pulse-front terahertz sources, imaging errors lead to pump pulse distortion, i.e., a local increase in pump pulse length (see L. Pálfalvi et al., “Novel setups for extremely high power single-cycle terahertz pulse generation by optical rectification”; Applied Physics Letters, Vol. 92, No. 1, pp. 171107-171109 (2008)). This effect is extremely detrimental to the efficiency of terahertz pulse generation in pump beams with large cross-sections (i.e., wide pump beams).

[0009] Mitigating or even eliminating the adverse effects of these limiting factors (prism nonlinear materials and imaging optics) has recently become an important task in the development of terahertz radiation sources.

[0010] One known solution to this problem is the so-called contact grating arrangement, which eliminates the need for imaging optics and thus avoids imaging errors (see L. Pálfalvi et al., “Novel setups for extremely highpower single-cycle terahertz pulse generation by optical rectification”; Applied Physics Letters, Vol. 92, No. 1, pp. 171107-171109 (2008)). In this device, the pulse leading edge tilt is generated by diffraction of a transmission grating formed directly in the surface of the nonlinear crystal. The period of the grating to be formed (in the micrometer or submicrometer range) is determined by the material of the nonlinear crystal and the pump wavelength. For example, for lithium niobate and assuming a pump wavelength of approximately 1 micrometer, the contact grating required to couple into the crystal typically needs to be designed with a grating density of at least 2500-3000 lines / mm (see Nagashima et al., “Design of Rectangular Transmission Gratings Fabricated in LiNbO3 for High-Power Terahertz-WaveGeneration”; Japanese Journal of Applied Physics, Vol. 49, pp. 122504-1-122504-5, p. 122504-1, p. 122504-1 (2010); and its revised communication “Errata: Design of Rectangular Transmission Gratings Fabricated in LiNbO3 for High-Power Terahertz-WaveGeneration”; Japanese Journal of Applied Physics, Vol. 51, p. 122504-1 (2012), and Ollmann et al., “Design of a contact grating setup for mJ-energy”). THz pulse generation by optical rectification; Applied Physics B 108, Vol. 4, pp. 821-826 (2012). However, currently, it is technically not feasible, or even impossible, to manufacture gratings with such a grating density.Furthermore, experiments show that if the grating line density exceeds a threshold (e.g., for LN, the threshold is approximately 2000 lines / mm), the profile of the generated grating becomes blurred. Consequently, the diffraction efficiency of the resulting grating is far lower than theoretically predicted, leading to a sharp decrease in terahertz radiation efficiency.

[0011] Another significant drawback of terahertz sources with contact gratings is that they cannot efficiently generate terahertz radiation with a planar parallel structure; the inclination of the incident and exit surfaces relative to each other (approximately 30° in the case of LN) and therefore the use of a terahertz radiation medium as a prism element are unavoidable (see the communication by Ollmann et al., 2012, cited above).

[0012] L. Pálfalvi et al., in their paper “Numerical investigation of a scalable setup for efficient terahertz generation using a segmented tilted-pulse-front excitation” (see Optics Express, Vol. 25, No. 24, pp. 29560-29573 (2017), and U.S. Patent No. 10,481,468 B2), proposed a terahertz pulse source with a planar parallel structure (primarily based on LN or LT, or further composed of other nonlinear optical media). This scheme can generate symmetrical terahertz beam modes even with a wide pump beam. In an arrangement that satisfies the velocity matching condition, a first optical element with angular dispersion characteristics, an imaging optics, and a medium with nonlinear optical properties for generating terahertz radiation are arranged sequentially along the beam propagation direction in the propagation path of the pump beam emitted by the pump beam source. The medium with nonlinear optical properties is a planar parallel crystal defined by parallel incident and exit surfaces, wherein the incident surface is formed into a stepped structure. This arrangement allows for the primary goal of obtaining a perfectly symmetrical terahertz beam. However, in this arrangement, the increase in achievable terahertz energy is limited because it also includes a conventional tilted pulse front arrangement (i.e., optical elements used for angular dispersion and imaging). In tilted pulse front terahertz sources, imaging errors cause pump pulse distortion, i.e., a local increase in pump pulse length. In this example, although the required (pre)tilt of the generated pulse front is smaller than that of a conventional tilted pulse front arrangement, and therefore the pump pulse distortion is smaller, this distortion may still be unacceptable for large beam sizes.

[0013] To address the limitations imposed by imaging, U.S. Patent No. 10,747,086 B2 discloses a component for use as a terahertz pulse source, comprising a grating and a wedge-shaped structure with a periodically processed incident surface. Besides being less compact (as it consists of two main components), another drawback is that, due to the wedge design, it cannot produce a uniform beam pattern.

[0014] In addition to the terahertz sources discussed above, another promising arrangement is a terahertz source employing a so-called back-reflective echelle grating (see Gy. Tóth et al., “Single-cycle scalable terahertz pulse source in reflection geometry”; Optics Express, Vol. 27, No. 21, pp. 30681-30691 (2019), and International Patent Publication No. WO2020 / 188307 A2 and U.S. Patent No. 11,474,414 B2). This scheme is based on a highly compact planar parallel-shaped nonlinear medium that eliminates the need for imaging optics and aims to eliminate errors caused by imaging and prism shape. The incident surface of the medium is very flat, and the back side has a periodically undulating structure on which the pump beam incident perpendicularly to the crystal is reflected / diffracted. Thus, the (average) pulse leading edge tilt required to generate velocity-matched terahertz radiation can be provided in the reflected / diffracted beam.

[0015] Besides terahertz (LN), semiconductor materials and some organic salt crystals also play important roles in media capable of generating terahertz radiation. These materials are not substitutes but complements; LN is suitable for the 0.1–1 THz range, while semiconductors and organic salt crystals are suitable for high-energy terahertz radiation in the 1–5 THz and 1–10 THz frequency ranges, respectively, because their absorption coefficients are much lower in the terahertz range.

[0016] A major advantage of semiconductor materials and organic salt crystals over LN is that the former requires a much smaller pulse-lead-edge tilt angle, typically below 30º, compared to 62-63º required for LN. Therefore, the aforementioned technical difficulties regarding the practical feasibility of contact gratings do not arise, because the smaller pulse-lead-edge tilt here requires a larger lattice period (lower frequency scribe lines, lower scribe line density) compared to the lattice period required for LN. Furthermore, semiconductor materials are readily available in everyday practice, especially gallium arsenide, and their machinability is significantly superior to that of LN.

[0017] The paper "Highly efficient scalable monolithic semiconductor terahertz pulse source" by Fülöp et al. (Optica, Vol. 3, No. 10, pp. 1075-1078 (2016)) explores the generation of terahertz pulses using ZnTe contact gratings. The authors used a contact grating with a line density of 780 lines / mm to generate terahertz pulses with an energy of 3.9 microjoules and a generation efficiency of 0.3%. The periodic structure of the contact grating used for this purpose was fabricated by combining electron beam lithography and dry etching methods.

[0018] One of the key factors affecting the generation efficiency of terahertz radiation in nonlinear media with periodic structures is the diffraction efficiency through the periodic structure. Studies have shown that for pump polarization associated with terahertz (THz) generation, an inherently high diffraction efficiency of 78% (total, for + / -1 diffraction orders) can be achieved using ZnTe contact gratings, while structures operating at high diffraction orders can surpass this efficiency.

[0019] Given the above, it seems reasonable to realize a terahertz (THz) radiation source by employing a back-surface reflection step and using semiconductors as the optical medium with nonlinear properties. However, this presents a significant obstacle. Due to the relatively small band gap of semiconductors, multiphoton absorption occurs even at low diffraction orders for pump wavelengths in the visible or near-infrared domains, leading to the generation of free charge carriers in the optical medium used. This indirectly results in significant terahertz absorption in the material, which is a major limiting factor affecting the generation efficiency of terahertz radiation, as this absorption significantly reduces the generation efficiency. Minimizing this effect is a major technical problem that needs to be solved when using semiconductor materials as the optical medium. In a terahertz radiation source with a back-surface reflection step, the generation of free charge carriers in the optical medium made of semiconductor material begins while the pump beam is still propagating toward the back-surface structure, and no terahertz radiation is generated without velocity matching. Therefore, when using semiconductors, terahertz sources with back surface reflection steps are not a practical alternative to LN sources (where these problems are less significant due to the larger bandgap value) because, as mentioned above, such terahertz sources are extremely inefficient. Summary of the Invention

[0020] In view of the above, the present invention aims to develop a method and a terahertz radiation source, collectively referred to as a terahertz generation scheme, for generating terahertz radiation for practical applications. This scheme is capable of generating terahertz pulses with excellent beam characteristics (most importantly, a substantially symmetrical beam profile) in a more scalable and technically easier / simpler manner than previously described solutions, and in a compact embodiment limited to semiconductor and organic nonlinear optical materials. The term "scalability" here refers to the fact that the radius of the pump beam used in the terahertz beam source according to the invention (whose square is proportional to the desired terahertz pulse energy) in its cross-section can be varied virtually within any range while maintaining the excellent beam characteristics of the generated terahertz radiation. Preferably, the radius of the associated beam can vary from millimeters to several centimeters.

[0021] Another objective of the present invention is, in particular, to provide a terahertz radiation generation scheme that preferably operates in the frequency range of 1-5 THz, which improves the currently available terahertz pulse energy and terahertz generation efficiency.

[0022] Another object of the present invention is to provide a terahertz radiation generation scheme, preferably operating in the frequency range of 1-5 THz, which requires the use of as few optical components as possible. Thus, a compact terahertz (pulse) source is realized.

[0023] In our study, we conclude that the above objective can be achieved through a terahertz radiation generation technique based on the velocity matching condition (1), wherein a transparent medium with nonlinear optical properties suitable for terahertz radiation generation (i.e., transparent to the pump beam) and having parallel front and rear surfaces is arranged in the propagation path of the pump beam emitted by the pump beam source. The rear surface is planar, and the front surface is provided in the form of a periodic structure. This periodic structure is constructed such that its period consists of a pair of symmetrically arranged planar portions with a given width, wherein the members of the pair of planar portions intersect on a common line. Each member of the pair of planar portions forms an angle with the midplane (or in other words, its mean plane) of the front surface relative to an imaginary plane orthogonal to the midplane of the intersection line of the members of the pair of planar portions. Considering both positive and negative meanings, where the angle is... satisfy The relationship, among which and These are the incident angle when the pump beam is incident perpendicularly to the average plane and the refraction angle when the beam is refracted in a plane of width w according to the Snellius-Descartes law, respectively. It satisfies the speed matching condition (1). The required tilt angle of the pump beam.

[0024] By developing the method according to claim 1, the objective of implementing a method for generating terahertz radiation for practical applications is achieved. Possible further preferred exemplary embodiments of the method according to the invention are set forth in claims 2 to 4. By using the component according to claim 5, the objective of implementing a radiation source for generating terahertz radiation for practical applications is achieved. Possible preferred exemplary embodiments of the component according to the invention are defined by claims 6 to 10. Attached Figure Description

[0025] The invention will now be described in more detail with reference to the accompanying drawings, wherein... Figure 1 This is a schematic diagram of a preferred exemplary embodiment of a semiconductor-based terahertz radiation source according to the present invention; Figure 1 The optical medium is shown in the form of a vertical cross-sectional view. Detailed Implementation

[0026] like Figure 1 As shown in the schematic diagram, the pulsed terahertz radiation source according to the present invention includes a pump beam source that emits a pump beam 14. Figure 1 (Not shown in the image) and an optical medium 10 having the ability to generate terahertz pulse characteristics. The optical medium 10 is provided in the form of a material block of a given volume and shape, wherein the material has nonlinear optical properties and is transparent at the wavelength of the pump beam 14, i.e., transparent to the pump beam 14. The optical medium 10 is preferably made of a semiconductor material, particularly preferably one of GaP, ZnTe, GaAs, and GaSe. Those skilled in the art will appreciate that the optical medium 10 can also be made of any other suitable semiconductor material or (one or more) organic materials. The organic material is preferably an organic salt crystal, specifically one of the following: 4-N,N-dimethylamino-4'-N'-methylstyryl-2,4,6-trimethylbenzenesulfonate (DSTMS), 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohexyl-2-enyl)malonitrile (OH1), and diethylaminosulfur trifluoride (DAST). The optical medium 10 is arranged in the propagation path of the pump beam 14.

[0027] The block of optical medium 10 has a parallel front boundary surface 11 and a rear boundary surface 12. The rear boundary surface 12 is formed by a plane. The front boundary surface 11 is formed by a periodic structure 13. The periodic structure 13 is formed by machining (micromachining), preferably by surface milling. The machining of this structure is carried out on the front boundary surface 11 of the block of optical medium 10, which is parallel to the plane of the rear boundary surface 12 before the machining begins.

[0028] like Figure 1As shown, the periodic structure 13 is constructed such that its 2w-width period is formed by symmetrically arranged pairs of surface members 13a and 13b, which are planar portions with a fixed width, wherein the pairs of surface members 13a and 13b intersect along a common line E. Each surface member 13a and 13b forms an angle with the midplane S (or its mean plane) of the front boundary surface 11. It is considered to be orthogonal to an imaginary plane that is orthogonal to the midplane S of the intersection line E of the surface members 13a and 13b perpendicular to the surface pair, wherein the angle is... Satisfying Relationships And it is assumed that its direction alternates between positive and negative (clockwise); here and These are the incident angles of the pump beam 14, which is perpendicular to the mid-plane S, and the refraction angles of the refracted beam 15, which is refracted on the surface members 13a and 13b according to the Snell-Descartes law, respectively. It is the tilt angle of the pulse leading edge of pump beam 14, and its magnitude must satisfy the velocity matching condition (1). .

[0029] The spatial period of the periodic structure with a width of 2w is at least one order of magnitude larger, preferably two orders of magnitude larger, than the wavelength of the pump beam 14 emitted by the pump beam source, while this spatial period is at least half the wavelength of the terahertz radiation to be generated in the nonlinear optical medium (i.e., the wavelength of the terahertz radiation considered in the block of optical medium 10). When precisely designing the width w, it is important to ensure that the structure 13 is at an angle of... This produces good diffraction efficiency in the diffraction direction. For a width w several orders of magnitude larger than the pump wavelength, it is advantageous not only with… Angle-dependent diffraction orders (usually higher-order diffraction orders), and their neighboring diffraction directions are close. Angular diffraction orders can also promote terahertz radiation generation by improving generation efficiency because these neighboring diffraction orders satisfy the velocity matching condition under good approximation. Therefore, good diffraction efficiency can be achieved for some neighboring, almost overlapping (higher-order) diffraction orders. This avoids the problem of interference between diffraction orders moving in different directions, a problem inherent in contact gratings with low diffraction orders that negatively impacts terahertz propagation.

[0030] In the operation of the terahertz beam source according to the present invention, the pump beam 14 emitted by the pump beam source enters the nonlinear optical medium 10 through the periodic structure 13 forming the front boundary surface 11. It enters the optical medium 10 through each surface member 13a, 13b of the surface pair and continues its path as split sub-beams 16a, 16b, one set of beams propagating clockwise (in a positive sense) relative to the incident pump beam 14, and the other set of beams propagating counterclockwise (in a negative sense). These sub-beams 16a, 16b ( Figure 1 The example shown in the image is each along a path that is close to the incident pump beam 14 at an angle. The direction of propagation (clockwise and counterclockwise), at this angle This corresponds to the speed matching condition.

[0031] The intensity leading edges of sub-beams 16a and 16b are typically 17a and 17b (considered rectangular in the geometric optics approximation, perpendicular to...). Figure 1 The planes of symmetry of the planes together define a plane, but they are not tilted relative to their respective phase leading edges. However, in sub-beams 16a and 16b, the average values ​​of the intensity leading edges 17a and 17b define a plane that is parallel to the front (average) boundary surface 11 and the rear (average) boundary surface 12 of the nonlinear optical medium 10, and extends towards the rear boundary surface 12 of the nonlinear optical medium 10 according to the velocity fitting (1). Wavelength. Based on the velocity fitting condition (1), with velocity... The moving average intensity front generates terahertz radiation in the nonlinear optical medium 10. In this example, the intensity magnitude measured along the intensity front is constant and equal to the peak intensity value. If the intensity fronts of sub-beams 16a and 16b do not form a continuous surface but a segmented surface, the average value of the intensity front is considered as its envelope, which can be external, internal, or an average of both. However, considering typical size scales, these three average values ​​essentially coincide in practice. The generated terahertz radiation (for clarity, Figure 1 (Not shown) propagates in the direction of the rear boundary surface 12 of the nonlinear optical medium 10, perpendicular to it, and then leaves the nonlinear optical medium 10 without changing direction, and can be used for various applications after leaving.

[0032] The pump source used in this invention is a visible light, near-infrared, or mid-infrared laser, or an optical parametric amplifier capable of emitting laser pulses with a pulse length of at least 5 femtoseconds but not exceeding a few picoseconds.

[0033] Compared to semiconductor contact grating solutions, one of the advantages of this inventive solution is that the periodic structure can be formed using simple techniques and it is more advantageous in terms of energy scalability.

[0034] Abstract: A novel device or terahertz source for generating high-energy terahertz radiation can be obtained by forming a symmetrical periodic structure in a semiconductor nonlinear optical medium defined by a planar parallel front (input) surface and a rear surface, with a period length (w) that is tens to hundreds of times the wavelength of the pump beam emitted from the pump beam source onto the optical medium. The main advantage of this device is that the optical medium, composed of a semiconductor nonlinear optical crystal, can be used as a unit with a planar parallel interface in the device. Therefore, terahertz beams with excellent beam quality and physical symmetry can be generated with high generation efficiency. Since the optical medium is designed as a semiconductor material, the frequency of the generated terahertz beam is preferably in the range of 1-5 terahertz. Because the device contains neither imaging optics nor a grating, the cross-sectional size of the pump beam can be substantially arbitrarily large. Therefore, the energy of the terahertz pulses generated by this method can also be arbitrarily large. The terahertz radiation source and method of the present invention, based on the device described herein, are particularly preferred for generating high-energy terahertz radiation requiring a wide pump beam.

Claims

1. A method for generating terahertz radiation, wherein, The pump beam (14) passes through the front boundary surface (11) of the planar parallel nonlinear optical medium (10) and is coupled into the planar parallel nonlinear optical medium (10) by refraction through the periodic structure (13) forming the front boundary surface (11). The periodic structure (13) is composed of symmetrically arranged planar surface members (13a, 13b) with a certain width, and the surface members (13a, 13b) form an angle with the midplane (S) of the front boundary surface (11). , wherein the angle satisfy The relationship is considered to alternate between positive and negative in terms of orientation, among which, It meets the speed matching condition. Required pulse leading edge tilt angle It is the group velocity of the pump beam (14). It is the phase velocity of the terahertz radiation. and The angles of incidence and refraction are respectively the angles of incidence and the angles of refraction of the beam (15) incident perpendicularly to the midplane (S) and then refracted on the planar surface members (13a, 13b) according to the Snell-Descartes law, wherein, after refraction, the pump beam (14) consists of a set of sub-beams that are oriented in a direction relative to the incident pump beam (14). The beam propagates in the direction of the angle, where the average of the intensity leading edges of each sub-beam is an imaginary plane, which propagates at a velocity... The light travels toward the exit boundary surface (12) of the nonlinear optical medium (10) and generates terahertz radiation within the nonlinear optical medium (10) through a nonlinear optical process, particularly optical rectification, wherein the terahertz radiation thereby generated is decoupled from the nonlinear medium (10) through the exit boundary surface (12).

2. The method according to claim 1, wherein, The pump beam (14) is a laser pulse in the visible, near-infrared or mid-infrared range, with a length of at least 5 femtoseconds and at most a few picoseconds.

3. The method according to claim 1 or 2, wherein, The nonlinear optical medium (10) is a semiconductor material.

4. The method according to claim 1 or 2, wherein, The nonlinear optical medium is an organic material.

5. A terahertz radiation source (10) comprising a pump beam source for emitting a pump beam (14) and a nonlinear optical medium (10) for generating terahertz pulses, said optical medium (10) being defined by at least two planar parallel boundary surfaces (11, 12), wherein, The pump beam source and the nonlinear optical medium (10) together define the optical path, wherein the front and rear boundary surfaces (11, 12) of the nonlinear optical medium (10) are substantially perpendicular to the optical path, and wherein the front boundary surface (11) is composed of a periodic structure (13), the periodic structure (13) being composed of pairs of surface members (13a, 13b) connected to each other along the intersection line (E), wherein each surface member (13a, 13b) is a planar surface, the planar surface forming alternating positive and negative angles of the same magnitude with an imaginary orthogonal plane disposed at the intersection line (E) to the front boundary surface (11). ), wherein the angle formed by the surface members (13a, 13b) and the rear boundary surface (12) is such that the direction of the pump beam (14) incident on and refracted onto the surface members (13a, 13b) changes by an angle ( ) is equal to the angle at which the pulse leading edge propagates in the nonlinear optical medium (10) and satisfies the velocity matching condition (1). ).

6. The terahertz radiation source according to claim 5, wherein, The width (w) of the half-cycle of the periodic structure (13) of the front boundary surface (11) of the planar parallel (10) nonlinear optical medium (10) is at least 10 micrometers and at most several hundred micrometers.

7. The terahertz radiation source according to claim 5 or 6, wherein, The nonlinear optical medium (10) is made of semiconductor material.

8. The terahertz radiation source according to claim 5 or 6, wherein, The nonlinear optical medium (10) is made of organic materials.

9. The terahertz radiation source according to any one of claims 5 to 8, wherein, The pump source is configured to emit laser pulses in the visible, near-infrared, or mid-infrared range, wherein the pulse length of the laser pulse is at least 5 femtoseconds and at most a few picoseconds.

10. The terahertz radiation source according to claim 8, wherein, The organic material is an organic salt crystal, particularly one of the following: 4-N,N-dimethylamino-4'-N'-methylstyryl-2,4,6-trimethylbenzenesulfonate (DSTMS), 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohexyl-2-enyl)malonitrile (OH1), and diethylaminosulfur trifluoride (DAST).

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