Semiconductor-based high-energy terahertz radiation source

Abstract

In the method according to the present invention, a pump beam (14) emitted by a pump beam source is incident on a block of a semiconductor nonlinear optical medium (10) having a plane-parallel front boundary surface and a rear boundary surface (11, 12), and the pump beam (14) is reconstituted into a set of sub-pump beams by refraction on a periodic structure (13) formed by a pair of planar surfaces formed on the front boundary surface of the block. The sub-pump beams are subjected to velocity matching conditions [Math 1] It moves in the direction of angle γ necessary to satisfy JPEG2026522507000006.jpg15170. The envelope of the pump pulse front segment propagating within the subpump beam segment is the propagation velocity v of the terahertz radiation. THz,f Then, it propagates in a direction perpendicular to the rear boundary surface (12), which is the exit surface of the optical medium (10), and the velocity matching conditions [Math 2] THz radiation is generated within the semiconductor optical medium (10) by optical rectification to satisfy JPEG2026522507000007.jpg15170, where v p,cs This is the group velocity of the pump beam (14), and v THz,f γ is the phase velocity of the generated THz radiation, and γ is the angle between the pulse front envelope and the phase front of the pump beam (14).

Description

Technical Field

[0001] The present invention relates to a method for generating terahertz (THz) radiation and a THz radiation source. In particular, the present invention relates to a novel method for generating terahertz pulses, and a THz radiation source with improved beam characteristics, efficiency, and energy scalability of terahertz pulses. The THz radiation source according to the present invention has neither an imaging optical system nor an optical grating. In a preferred embodiment, the medium used for generating THz radiation is a semiconductor material having nonlinear optical properties.

Background Art

[0002] It is known that so-called velocity matching must be satisfied for efficient terahertz radiation generation by a nonlinear optical process. Therefore, the group velocity of the pump pulse used for excitation needs to match the phase velocity of the generated THz pulse.

[0003] In addition, efficient terahertz radiation generation requires that the crystal having nonlinear optical properties used for generation has a large (usually exceeding several tens of pm / V) second-order nonlinear optical coefficient. Such 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). The drawback of these materials is that the difference between the group refractive index at the pumping frequency and the phase refractive index in the THz range makes it difficult to achieve the above-mentioned velocity matching. A solution to this problem is provided by the tilted pulse front technique (see Non-Patent Document 1). According to this technique, terahertz radiation is generated by an optical pulse whose pulse front (intensity front) forms an angle (γ) of a desired magnitude with respect to the wavefront. The generated THz beam propagates perpendicular to the tilted pulse front as a result of the velocity matching requirement. Therefore, the projection of the group velocity v p,cs of pumping in the propagation direction of the THz radiation must be equal to the THz phase velocity v THz,f , that is, the following condition,

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[0004] Typically, pulses with the highest pulse energy and within the frequency range of 0.1–1 THz can be generated using a tilted pulse-front excitation method (see Non-Patent Literature 2). The high-energy THz source described in this paper supplies a pulse energy of 0.43 mJ and always uses a prism-shaped LN crystal as the nonlinear optical crystal. One reason for this is that, in order to minimize reflection losses, the pump beam must be incident perpendicularly to the crystal, and the generated THz beam must exit perpendicularly. Furthermore, the right-angle coupling of the THz beam output ensures that there is no angular dispersion in the generated THz beam, which is a very important requirement for the application. Therefore, in order 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, and the magnitude of this wedge angle precisely matches the angle γ.

[0005] When prism-shaped terahertz radiation sources are used for high-energy THz excitation, the quality of the THz beam is significantly degraded. In the case of a broad pump beam, essential for generating high-energy THz pulses, the THz beams generated on either side of the pump beam cross-section are excited over significantly different lengths and therefore undergo different degrees of absorption and dispersion in the LN crystal used. Furthermore, nonlinear effects at the excitation sites also differ. As a result, the intensity of the THz pulses induced in symmetrically positioned portions of the pump beam and the time course of the electric field within the pulses differ significantly, leading to a highly asymmetrical and poor-quality THz beam. Consequently, the THz beam becomes extremely difficult to focus (i.e., it becomes uncoordinated with the diffraction limit), which is a significant drawback for many applications.

[0006] In conventional tilted pulse-front THz sources, the pulse-front tilt of the pump beam is typically caused by diffraction through an optical grating (reflective or transmissive) placed in the beam path. By imaging through a lens or telescope, the beam is coupled to a crystal with nonlinear optical properties used for terahertz emission generation, and an image of the beam spot on the grating plane is formed within the crystal. Therefore, in conventional tilted pulse-front THz sources, imaging errors cause distortion of the pump pulse, i.e., a local increase in the pump pulse length (see Non-Patent Literature 3). This effect is very detrimental to the efficiency of terahertz pulse generation in pump beams with large cross-sections (i.e., wide pump beams).

[0007] Mitigating or completely eliminating the adverse effects of these limiting factors (prism-like nonlinear materials and imaging optical systems) has become an important effort in the field of terahertz radiation source development in recent years.

[0008] One known solution to this problem is the so-called contact grating configuration, which does not involve an imaging optical system and therefore does not produce imaging errors (see Non-Patent Literature 3). In this configuration, the pulse front tilt is created by diffraction through a transmission optical grating directly formed on the surface of the nonlinear crystal. The period of the formed grating (in the range of micrometers or submicrons) is determined by the material of the nonlinear crystal and the pumping wavelength. For example, in the case of LN, assuming a pumping wavelength typically up to about 1 μm, the contact grating required to achieve bonding to the crystal usually needs to be designed with a groove density of at least 2500-3000 grooves / mm (1 / mm) (see Non-Patent Literature 4, and its modified version, Non-Patent Literature 5, and Non-Patent Literature 6). However, at present, it is technically impossible, and even questionable, to fabricate an optical grating with such a groove density. In addition, trial experiments have shown that when the groove density of the grating exceeds a threshold (e.g., about 2000 grooves / mm in the case of LN), the profile of the generated grating becomes blurred. As a result, the diffraction efficiency of the obtained lattice was significantly lower than the theoretically predicted value, leading to a dramatic reduction in the efficiency of THz radiation.

[0009] Another significant drawback of THz sources with contact gratings is that terahertz radiation cannot be efficiently generated using a plane-parallel structure, meaning that the incident and exit surfaces must be tilted relative to each other (approximately 30° in the case of LNs), and this necessitates the use of a prism-like element in the terahertz radiation medium (see Non-Patent Literature 6).

[0010] Terahertz pulse sources with a plane-parallel structure (mainly based on LN or LT, or further using other nonlinear optical media) have been proposed in Non-Patent Document 7 and Patent Document 1. This solution allows for a symmetrical terahertz beam pattern even with a wide pump beam. In a configuration that satisfies the velocity matching condition, a first optical element having angular dispersion properties, an imaging optical system, and a medium having nonlinear optical properties for generating terahertz radiation are sequentially arranged in the direction of beam propagation along the propagation path of the pump beam emitted by the pump beam source. The medium having nonlinear optical properties is a plane-parallel crystal surrounded by parallel incident and exit surfaces, with the incident surface formed as a stepped structure. This configuration achieves the main objective of realizing a perfectly symmetrical terahertz beam. However, in this configuration, the increase in the obtained terahertz energy is hindered by the fact that it also includes a conventional pulse-front tilt configuration (i.e., optical elements for angular dispersion generation and imaging). In a tilted pulse-front THz source, distortion of the pump pulse occurs due to imaging errors, i.e., the pump pulse length increases locally. In this case, the pulse front (pre) slope to be generated is smaller than in conventional sloped pulse front configurations, and therefore the distortion of the pump pulse is smaller. However, this distortion may still be unacceptable when the beam size is large.

[0011] To address the limitations arising from imaging, Patent Document 2 discloses an assembly as a terahertz pulse source comprising an optical grating and a wedge-shaped structure having a periodically machined incident surface. Further drawbacks of this solution, besides the fact that it is not sufficiently compact (as it consists of two main elements), are that a uniform beam pattern cannot be obtained due to the wedge-shaped design.

[0012] In addition to the THz sources described above, another promising configuration is a TH source with a so-called rear-reflecting echelle (see Non-Patent Document 8 and Patent Documents 3 and 4). This solution is based on a very compact, plane-parallel nonlinear medium without an imaging optical system, and is designed to eliminate errors caused by both imaging and prism shapes. The incident surface of the medium is perfectly flat, and the rear surface has a periodic relief structure on which a pump beam incident perpendicularly to the crystal is reflected / diffracted. In this way, the (n-average) pulse-front tilt angle required for velocity-matched terahertz emission can be achieved in the reflected / diffracted beam.

[0013] In addition to LN, semiconductor materials and some organic salt crystals play an important role among the media that can generate terahertz radiation. These are complementary materials, not substitutes; while LN is suitable for the 0.1–1 THz range, semiconductors and organic salt crystals are generally suitable for high-energy terahertz radiation in the frequency ranges of 1–5 THz and 1–10 THz, respectively, because they have much lower absorption coefficients in the terahertz region.

[0014] A major advantage of semiconductor materials and organic salt crystals over LN is that they require a much smaller pulse-front tilt angle, typically less than 30°, compared to 62-63° for LN. As a result, the aforementioned technical difficulties regarding the practical feasibility of contact lattices do not arise, because the smaller pulse-front tilt angle required here necessitates a much larger lattice period (fewer groove patterns and lower groove density) compared to that required for LN. In addition, semiconductor materials are common materials used daily, readily available, and GaAs in particular has far superior machinability compared to LN.

[0015] Non-patent document 9 discusses THz pulse generation using a ZnTe contact grating. The authors generated terahertz pulses with an energy of 3.9 μJ and a generation efficiency of 0.3% using a contact grating with a groove density of 780 grooves / mm. The periodic structure of the contact grating used for this purpose was fabricated by combining electron beam lithography and dry etching.

[0016] One of the key factors influencing the efficiency of terahertz emission generation in nonlinear media with periodic structures is the diffraction efficiency through the periodic structure. The inherently high diffraction efficiency of 78% (total, ±1st order diffraction) achievable with ZnTe contact gratings has been shown to be surpassed by using structures that operate at high diffraction orders for the pump polarization associated with THz generation. [Prior art documents] [Patent Documents]

[0017] [Patent Document 1] U.S. Patent No. 10,481,468B2 [Patent Document 2] U.S. Patent No. 10,747,086B2 [Patent Document 3] International Publication No. 2020 / 188307A2 Pamphlet [Patent Document 4] U.S. Patent No. 11,474,414B2 [Non-patent literature]

[0018] [Non-Patent Document 1] J. Hebling et al., "Velocity matching by pulse front tilting for large-area THz-pulse generation," Optics Express, Vol. 10, No. 21, pp. 1161-1166 (2002). [Non-Patent Document 2] J.A. Fue (Uumurauto) loe (Oumurauto) p et al., "Efficient generation of THz pulses with 0.4 mJ energy", Optics Express, Vol. 22, No. 17, pp. 20155 - 20163 (2014)

Non - Patent Document 3

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Non - Patent Document 7

Non - Patent Document 8

Non - Patent Document 9

Summary of the Invention

Problems to be Solved by the Invention

[0019] Based on the above, it seems reasonable to apply semiconductors as optical media with nonlinear properties and implement a THz radiation source equipped with a rear-reflection echelon. However, there is a serious obstacle. Due to the relatively small band gap of semiconductors, multiphoton absorption already occurs at low diffraction orders for pumping wavelengths in the visible or near-infrared region, leading to the generation of free charge carriers within the optical media used. This indirectly leads to significant THz absorption within the material, which is a major limiting factor for THz radiation generation efficiency because this absorption greatly reduces THz radiation generation efficiency. Minimizing this effect when using semiconductor materials as optical media is a major technical challenge that must be addressed. In a THz radiation source equipped with a rear-reflection echelon, the generation of free charge carriers within the optical medium made of semiconductor material begins while the pump beam is still propagating toward the rear structure, and if there is no velocity matching, terahertz radiation is not generated. Therefore, when using semiconductors, a THz source with a rear-reflecting echelon is not a practical technical alternative to an LN source (these problems are not so significant due to the large bandgap value) because the efficiency of such a THz source is, as mentioned above, very low.

[0020] In view of the above, the present invention aims to develop a method and a terahertz radiation source for generating terahertz radiation for practical applications, collectively referred to as a terahertz radiation generation scheme, thereby enabling the generation of terahertz pulses with excellent beam properties (most importantly having a substantially symmetric beam profile) in a more scalable and technically easier / simpler manner than the aforementioned solutions, and in a compact embodiment when limited to semiconductors and organic nonlinear optical materials. Here, the term "scalable" means that the radius in the cross-section of the beam spot of the pump beam used in the terahertz beam source according to the present invention is proportional to the desired terahertz pulse energy, and that the radius can be changed within a substantially arbitrary range while maintaining the excellent beam properties of the generated terahertz radiation. Preferably, the radius of the beam spot can be changed in the range from a few millimeters to a few centimeters.

[0021] A further object of the present invention is to provide a terahertz emission generation method that operates particularly preferably in the frequency range of 1 to 5 THz, while increasing the currently available THz pulse energy and THz generation efficiency.

[0022] A further object of the present invention is to provide a terahertz emission generation method that preferably operates in a frequency range of 1 to 5 THz while minimizing the use of optical elements. This makes it possible to use a compact THz (pulse) source. [Means for solving the problem]

[0023] The study concluded that the above objective can be achieved by a technical solution for terahertz emission generation based on velocity matching condition (1), in which a light-transmitting medium having nonlinear optical properties suitable for terahertz emission generation (i.e., transmission to the pump beam) and parallel front and rear surfaces is placed 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. The periodic structure is configured such that its period is formed by pairs of symmetrically arranged planar portions of a predetermined width, and the members of each pair are joined at a common line. With respect to a virtual plane perpendicular to the central plane of the joint line of the pair of members, each pair of members in the planar portion is considered to make an angle α with the central plane (or its mean plane) on the front, and this angle is thought to alternate between positive and negative, where the angle α satisfies the relationship α-β=γ, where α and β are the angle of incidence of the pump beam incident perpendicular to the mean plane and the angle of refraction of the light beam refracted in a planar portion of width w according to the Snelius-Descartes law, respectively, where γ is the velocity matching condition (1)

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[0024] The objective of implementing a method for generating THz radiation for use in practical applications was achieved by developing the method according to claim 1. Possible more preferred exemplary embodiments of the method according to the present invention are described in claims 2 to 4. The objective of implementing a radiation source for generating THz radiation for use in practical applications was achieved by the assembly according to claim 5. Possible more preferred exemplary embodiments of the assembly according to the present invention are defined by claims 6 to 10. [Brief explanation of the drawing]

[0025] The present invention will be described in further detail with reference to the accompanying drawings. [Figure 1] This is a schematic diagram of a preferred embodiment of a semiconductor-based THz radiation source according to the present invention, with the optical medium shown in a vertical cross-sectional view. [Modes for carrying out the invention]

[0026] As can be seen from the schematic diagram shown in Figure 1, the pulsed THz radiation source according to the present invention includes a pump beam source (not shown in Figure 1) that emits a pump beam 14, and an optical medium 10 having properties capable of generating THz pulses. The optical medium 10 is supplied as a block of material having a predetermined volume and shape, and this material has nonlinear optical properties and is transparent at the wavelength of the pump beam 14, i.e., transmits to the pump beam 14. The optical medium 10 is preferably composed of a semiconductor material, and in particular, the material is preferably one of GaP, ZnTe, GaAs, and GaSe. It will be obvious to those skilled in the art that the optical medium 10 may be made from other suitable semiconductor materials or organic materials (multiple may be available). The organic material is preferably an organic salt crystal, and in particular is one of the following: 4-N,N-dimethylamino-4'-N'-methylstilbazolium-2,4,6-trimethylbenzenesulfonate (DSTMS), 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohexa-2-enilidene)malononitrile (OH1), and diethylaminosulfur trifluoride (DAST). The optical medium 10 is placed within the propagation path of the pump beam 14.

[0027] The block of the optical medium 10 has a parallel front interface 11 and a rear interface 12. The rear interface 12 is formed as a plane. The front interface 11 is formed as a periodic structure 13. The periodic structure 13 is formed by machining (micromachining), preferably by surface milling. The formation of the structure by machining is performed prior to the start of machining on the front interface 11 of the block of the optical medium 10 that is parallel to the rear interface 12.

[0028] As shown in Figure 1, the periodic structure 13 is configured such that its 2w-width period is formed by pairs of symmetrically arranged surface elements 13a and 13b, each consisting of a fixed-width planar portion, where the paired surface elements 13a and 13b are tangent along a common line E. Each surface element 13a and 13b forms an angle α with the central plane S (or its mean plane) of the front interface surface 11, which is considered to be orthogonal to a virtual plane, and the virtual plane is orthogonal to the plane perpendicular to the central plane S of the junction line E of the paired surface elements 13a and 13b, where the angle α satisfies the relation α-β=γ and is considered to alternate between positive and negative with respect to direction (clockwise), where α and β are the angle of incidence of the pump beam 14 incident perpendicular to the central plane S and the angle of refraction of the refracted beam 15 refracted on the surface elements 13a and 13b according to the Snelius-Cartes law, respectively, where γ is the velocity matching condition (1)

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[0029] The spatial period of a periodic structure with width 2w is at least one order of magnitude, preferably two orders of magnitude, larger than the wavelength of the pump beam 14 emitted from the pump beam source, but smaller than at least half the wavelength of the terahertz radiation produced in the nonlinear optical medium, i.e., smaller than the wavelength of the terahertz radiation considered in the block of the optical medium 10. When designing the width w precisely, it is important to ensure that the structure 13 produces good diffraction efficiency in the diffraction direction of angles ±γ. When the width w is orders of magnitude larger than the pump wavelength, not only the diffraction orders (typically higher orders) associated with angles ±γ, but also their neighboring orders diffracting in directions close to angles ±γ have the advantage of contributing to THz generation by increased production efficiency, because the velocity matching condition is satisfied by these neighboring orders with good approximation. Thus, good diffraction efficiency can be achieved for adjacent, nearly overlapping (high) diffraction orders. As a result, the problem of interference of orders moving in different directions, which is a challenge for low diffraction order contact gratings and adversely affects THz propagation, can be avoided.

[0030] During the operation of the THz beam source according to the present invention, the pump beam 14 emitted from the pump beam source penetrates the periodic structure 13 that forms the forward interface surface 11 and enters the nonlinear optical medium 10. After passing through the surface elements 13a and 13b of the paired surfaces and entering the optical medium 10, the beams continue their paths as divided sub-beams 16a and 16b. One beam group moves clockwise (inclined in the positive direction) relative to the incident pump beam 14, and the other beam group moves counterclockwise (inclined in the negative direction). Each of these sub-beams 16a and 16b (one of which is illustrated in Figure 1) moves in a direction that closes the angle γ corresponding to velocity matching with the incident pump beam 14 (clockwise and counterclockwise).

[0031] The intensity fronts of subbeams 16a and 16b are typically 17a and 17b (considered rectangular in the geometric optical approximation, with an axis of symmetry perpendicular to the plane in Figure 1 defining the plane together), but are not tilted with respect to the corresponding phase front. However, for subbeams 16a and 16b, the average of intensity fronts 17a and 17b is parallel to both the forward (average) interface 11 and the backward (average) interface 12 of the nonlinear optical medium 10, and v toward the backward interface 12 of the nonlinear optical medium 10 according to velocity matching condition (1). THz,f Define a plane that extends along the wavelength. Velocity v THz,f This moving average intensity front generates THz radiation within the nonlinear optical medium 10 according to velocity matching condition (1). In this case, the magnitude of the measured intensity along the intensity front is constant and equal to the peak intensity value. If the intensity fronts of subbeams 16a and 16b form segmented surfaces rather than continuous surfaces, the average of the intensity fronts is considered to be their envelope surface, which may be external, internal, or an average of both. However, considering typical size scales, these three averages practically coincide. The generated terahertz radiation (not shown in Figure 1 for clarity) propagates perpendicularly to the direction of the rear interface 12 of the nonlinear optical medium 10, and then exits the nonlinear optical medium 10 without changing direction, becoming available for use in post-exit applications.

[0032] The pump source used in this invention is an optical parametric amplifier capable of emitting visible light, near-infrared light, mid-infrared laser, or laser pulses, wherein the laser pulse has a pulse length of at least 5 fs but less than or equal to a few ps.

[0033] One of the advantages of the present invention over semiconductor contact grid solutions is that the periodic structure can be formed using only simple techniques and is also more suitable in terms of energy scalability.

[0034] Abstract A new generation configuration for high-energy terahertz radiation, namely a THz source, can be realized by forming a symmetric periodic structure in a semiconductor nonlinear optical medium divided into a plane-parallel front (input) and rear surface, and setting its period length (w) to a range of 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 configuration is that the optical medium, made of a semiconductor nonlinear optical crystal, can be used as a unit having a plane-parallel interface within the configuration. As a result, a THz beam with excellent beam quality and physically symmetrical properties can be generated with high generation efficiency. By designing the optical medium as a semiconductor material, the frequency of the generated THz beam is preferably in the range of 1 to 5 THz. Since this configuration does not include an imaging optical system or an optical grating, the size of the pump beam cross-section can be substantially arbitrary. Therefore, the energy of the terahertz pulse generated by this method can also be arbitrary. The terahertz radiation source and method of the present invention based on the configuration discussed herein are particularly preferred for the generation of high-energy THz radiation, and it requires the use of a wide range of pump beams.

Claims

1. A method for generating terahertz radiation, The pump beam (14) is coupled to the plane-parallel nonlinear optical medium (10) by refraction through its front interface (11) by a periodic structure (13) that forms the front interface (11), and the periodic structure (13) is formed by symmetrically arranged planar surface elements (13a, 13b) of a certain width. The surface elements (13a, 13b) form an angle α with the central plane (S) of the front boundary surface (11), and the angle α satisfies the relationship α - β = γ and is considered to be alternately positive and negative with respect to direction, where γ is the velocity matching condition [Math 1] The pulse front tilt angle required to satisfy this condition is v ｐ，ｃｓ v is the group velocity of the pump beam (14), ＴＨｚ，ｆ is the phase velocity of the THz radiation, and α and β are the angle of incidence and angle of refraction of the light beam (15) that is incident perpendicularly to the central plane (S) and then refracted on the planar surface elements (13a, 13b) according to the Snerius-Cartes law, respectively. After refraction, the pump beam (14) is formed by a set of sub-beams propagating in a direction that makes an angle ±γ with the direction of the incident pump beam (14). The average of the intensity fronts of the individual subbeams is a virtual plane. The aforementioned virtual plane has a velocity v ＴＨｚ，ｆ The THz radiation is then moved toward the exit interface (12) of the nonlinear optical medium (10), and THz radiation is generated within the nonlinear optical medium (10) by a nonlinear optical process, particularly optical rectification, and the THz radiation generated is separated from the nonlinear medium (10) via the exit interface (12). A method characterized by the following:

2. The pump beam (14) is a laser pulse in the visible light, near-infrared, or mid-infrared range, with a duration of at least 5 femtoseconds and a maximum of several picoseconds. The method according to feature 1.

3. The aforementioned nonlinear optical medium (10) is a semiconductor material. The method according to 1 or 2, characterized by the above.

4. The aforementioned nonlinear optical medium is an organic material. The method according to 1 or 2, characterized by the above.

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, The optical medium (10) is separated by at least two plane-parallel interface surfaces (11, 12), The pump beam source and the nonlinear optical medium (10) together define the optical path. The front and rear interface surfaces (11, 12) of the nonlinear optical medium (10) are substantially perpendicular to the optical path. The aforementioned front boundary surface (11) is formed by a periodic structure (13), The periodic structure (13) is formed by a pair of surface elements (13a, 13b) connected to each other along a junction line (E), Each of the aforementioned surface elements (13a, 13b) is a plane that alternately forms positive and negative angles (α) of the same magnitude with respect to a virtual plane that passes through the joint line (E) and is perpendicular to the front boundary surface (11), and the surface elements (13a, 13b) form an angle with the rear boundary surface (12), The angle (β) of the change in direction of the pump beam (14) incident on the surface elements (13a, 13b) and refracted is configured such that the pulse front propagation is equal to the angle (γ) that satisfies the velocity matching condition (1) in the nonlinear optical medium (10). A terahertz radiation source (10) characterized by the following:

6. The width (w) of a half-period of the periodic structure (13) that forms the front interface (11) of the plane-parallel nonlinear optical medium (10) is at least 10 micrometers and at most several hundred micrometers. The terahertz radiation source according to feature 5.

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

8. The aforementioned nonlinear optical medium (10) is made of an organic material. The terahertz radiation source according to feature 5 or 6.

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

10. The organic material is an organic salt crystal, and in particular is one of the following: 4-N,N-dimethylamino-4'-N'-methylstilbazolium-2,4,6-trimethylbenzenesulfonate (DSTMS), 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohexa-2-enilidene)malononitrile (OH1), and diethylaminosulfur trifluoride (DAST). The terahertz radiation source according to feature 8.

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