Semiconductor-based high-energy terahertz radiation source

Abstract

The invention relates to a method for generating terahertz radiation and a THz radiation source. In the method according to the invention, a pump beam (14) emitted by a pump beam source is incident on a block of semiconductor nonlinear optical medium (10) having plane-parallel front and rear boundary surfaces (11, 12), wherein the pump beam (14) is decomposed into a set of sub pump beams by refraction on a periodic structure (13) formed by pairs of planar surfaces formed in the front boundary surface of the block. The sub pump beams travel in the direction of the angle y required to satisfy the velocity matching condition v p, cs cos(γ) = V THz,f . The envelope of the pump pulse front segments propagating in the sub pump beam segments propagates at the propagation velocity V THz,f of the terahertz radiation in a direction perpendicular to the rear boundary surface (12), which is the exit plane of the optical medium (10), and generates THz radiation in the semiconductor optical medium (10) by optical rectification in a manner satisfying the velocity matching condition v p,cs cos(γ) = V THz,f ; here v p,cs is the group velocity of the pump beam (14), V THz,f is the phase velocity of the THz radiation generated, and y is the angle between the envelope of the pulse front and phase front of the pump beam (14).

Description

[0001] P138488-13773D SZT

[0002] SEMICONDUCTOR-BASED HIGH-ENERGY TERAHERTZ RADIATION SOURCE

[0003] The present invention relates to a method for generating terahertz (THz) radiation and a THz radiation source. More specifically, the present invention relates to a new method for generating terahertz pulses and a THz radiation source with improved beam characteristics, efficiency and energy scalability of the terahertz pulses. The THz radiation source according to the invention is free of both imaging optics and optical grating, and in a preferred embodiment, the medium for THz radiation generation used therein is a semiconductor material with non-linear optical properties.

[0004] It is known that for efficient terahertz radiation generation by nonlinear optical processes, a so-called velocity matching must be fulfilled. According to this, the group velocity of the pump pulse used for the excitation must match the phase velocity of the THz pulse being generated.

[0005] In addition, efficient terahertz radiation generation requires that the crystal with the nonlinear optical properties used for the generation has a high (typically exceeding several times ten 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 tan- talate (LT). A disadvantage 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 is such that it makes it difficult to fulfill the above-mentioned velocity matching. A solution to this problem is provided by the tilted pulse front technique (see 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)), whereby terahertz radiation is generated by a light pulse in which the pulse front (intensity front) makes an angle (y) of the desired magnitude with the wavefront. Since the generated THz beam propagates perpendicular to the tilted pulse front, as a consequence of the velocity matching requirement, the projection of the group velocity vp,cs of the pumping in the propagation direction of the THz radiation must be equal to the THz phase velocity VTHZ , i.e. the condition ^P,cScos(r) =THzJ(1 ) must be fulfilled. Specifically, for pump wavelengths in the near-infrared domain, y « 62-63° for LN, y « 68-69° for LT, and the required value of angle y is preferably significantly less than this, typically less than about 30° for semiconductor materials.

[0006] Pulses typically falling in the frequency range 0.1 - 1 THz with the highest pulse energy can currently be generated using the tilted pulse front excitation technique (see J. A. Fuldp 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 THz sources described in this paper, which provide a pulse energy of 0.43 mJ, all use prismatic LN crystals as nonlinear optical crystals. One reason for this is that, in order to minimize reflection losses, the pump beam must enter the crystal perpendicularly and the generated THz beam must exit perpendicularly. Furthermore, the perpendicular coupling out of the THz beam ensures that the THz beam generated is free of angular dispersion, which is a very important requirement for applications. Accordingly, to fulfill the velocity matching condition (1 ), the exit plane of the crystal shall make a wedge angle with the entry plane of the nonlinear optical crystal, the magnitude of which is exactly the same as the magnitude of the angle y.

[0007] The use of a prismatic terahertz radiation source medium for high energy THz excitation is extremely detrimental to the quality of the THz beam. In the case of a wide pump beam, which is essential for generating high-energy THz pulses, the THz beams generated at opposite sides of the pump beam cross section are excited over significantly different lengths, and are therefore subject to different extents of absorption and dispersion in the LN crystal used. Moreover, the nonlinear effects at the sites of excitation are also different. For this reason, the intensity of the THz pulses induced on symmetrically located parts of the pump beam on its opposite sides, as well as the time course of the electric field in the pulses, differ significantly, that results in a highly asymmetric THz beam of poor quality. Consequently, the THz beam highly non-focusable (i.e., in harmony with the diffraction limit), which is a serious drawback for many applications.

[0008] In a conventional tilted pulse front THz source, the pulse front tilt in the pump beam is usually created by diffraction through an (reflection or transmission) optical grating arranged in the beam path. By means of being imaged through a lens or a telescope, the beam is then coupled into a crystal with nonlinear optical properties used for terahertz radiation generation: the image of the beam spot on the grating surface is formed in the crystal. Thus, in conventional tilted pulse front THz sources, imaging errors result in a distortion of the pump pulse, namely a local increase in the pump pulse length (see L. Palfalvi 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 highly disadvantageous to the efficiency of terahertz pulse generation in pump beams with large cross-section (i.e. in wide pump beams).

[0009] To mitigate or even completely eliminate the adverse effects of these limiting factors (prismatic nonlinear material and imaging optics) has recently become an important effort in the field of terahertz radiation source development.

[0010] One of the known solutions to this problem is the so-called contact-grating arrangement, which is free of imaging optics and thus also of imaging errors (see L. Palfalvi 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)). In this setup, the pulse front tilt is created by diffraction through a transmission optical grating formed directly in the surface of the nonlinear crystal. The period of the grating to be formed (which falls into the micrometer or sub-micron range) is determined by the material of the nonlinear crystal and the wavelength of the pumping. For example, for LN and typically assuming a pump wavelength of ~1 pm, the contact grating required to achieve the coupling into the crystal should typically be designed with a scratch density of at least 2500-3000 1 / mm (see Nagashima et al. "Design of Rectangular Transmission Gratings Fabricated in LiNbO3 for High-Power Terahertz-Wave Generation"-, Japanese Journal of Applied Physics, vol. 49, 122504-1 - 122504-5. Pages 122504-1 , 122504-1 (2010); and its revised communication "Erratum: Design of Rectangular Transmission Gratings Fabricated in LiNbO3 for High-Power Terahertz-Wave Generation"-, Japanese Journal of Applied Physics, vol. 51 , pp. 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, at the moment, fabricating an optical grating with such a scratch density is not technically feasible, if possible at all. In addition, trial experiments show that if the grating scratch density exceeds a threshold value (e.g. for LN, a value of about 2000 1 / mm), the profile of the produced grating becomes blurred. As a consequence, the diffraction efficiency of the resulting grating is far below the theoretically predicted value, which leads to a drastic decrease in the THz radiation efficiency.

[0011] Another important shortcoming of the THz source with contact grating is that terahertz radiation cannot be generated with a plane-parallel structure with good efficiency; the tilting of the entry and exit planes relative to each other (about 30°, in the case of LN) and thus the use of the terahertz radiation medium as a prismatic element is unavoidable (see the communication by Ollmann et al. in 2012, cited above).

[0012] A terahertz pulse source with a plane-parallel structure (based primarily on LN or LT, or further formed of other nonlinear optical media) is proposed in the paper by L. Palfalvi et al. "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 US Patent No. 10,481 ,468 B2). The solution results in a symmetric terahertz beam pattern even for wide pump beams. In the arrangement which satisfies the velocity matching condition, a first optical element with angular dispersion property, imaging optics and a medium with nonlinear optical property for generating terahertz radiation are arranged in succession in the propagation path of the pump beam emitted by the pump beam source in the direction of beam propagation. The medium with nonlinear optical properties is a plane-parallel crystal bounded by parallel entry and exit surfaces, wherein the entry surface is formed as a stepped structure. This arrangement allows the primary objective of achieving a perfectly symmetric terahertz beam to be achieved. However, in the case of this arrangement, the increase of the obtainable terahertz energy is hampered by the fact that it also contains a conventional pulse front tilted arrangement (i.e. optical elements for angular dispersion generation and imaging). In tilted pulse front THz sources, imaging errors result in distortion of the pump pulse, namely a local increase of the pump pulse length. In the present case, although a smaller pulse front (pre)ti Iting has to be generated than in a conventional tilted pulse front arrangement, and therefore the distortion of the pump pulse is smaller, said distortion may still be intolerable for large beam sizes.

[0013] To address the limitations arising from imaging, US Patent No. 10,747,086 B2 discloses an assembly as terahertz pulse source comprising an optical grating and a wedge-shaped structure with a periodically machined entry surface. A further shortcoming of this solution, apart from the fact that it is not sufficiently compact (as it consists of two main elements), is that it does not result in a homogeneous beam pattern due to the wedge-shaped design.

[0014] In addition to the THz sources discussed above, another promising arrangement is a THz source with a so-called back-surface reflection echelle (see. Gy. Toth et al. "Single-cycle scalable terahertz pulse source in reflection geometry"', Optics Express, vol. 27, no. 21 , pp. 30681 -30691 (2019), as well as international publication pamphlet No. W02020 / 188307 A2 and US Patent No. 11 ,474,414 B2. The solution, which is based on a highly compact non-linear medium of plane-parallel shape, free of imaging optics, is designed to eliminate errors due to both imaging and prism shapes. The entry surface of the medium is perfectly flat and the back surface has a periodic relief structure on which the pump beam incident perpendicular to the crystal suffers reflection / diffraction. In this way, in the reflected / diffracted beams, a (n average) pulse front tilt required for velocity-matched terahertz radiation generation can be provided.

[0015] In addition to LN, semiconductor materials and some organic salt crystals play an important role among the media capable of generating terahertz radiation. These are not alternatives but complementary; 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 1 -5 THz and 1 -10 THz frequency ranges, respectively, due to their much lower absorption coefficients in the terahertz range.

[0016] A major advantage of semiconductor materials and organic salt crystals over LN is that the former require a much smaller pulse front tilt angle, typically below 30°, compared to the 62-63° tilt angle required for LN. As a result, the technical difficulties mentioned above with regard to the practical feasibility of the contact grating do not arise, as the smaller pulse front tilt here requires a much larger lattice period (less frequent scratch pattern, lower scratch density) compared to the lattice period required for LN. In addition, semiconductor materials are common materials in everyday practice, are easy to obtain, especially GaAs, and their mechanical machinability is significantly better than those of LN.

[0017] The paper by Fuldp et al. "Highly efficient scalable monolithic semiconductor terahertz pulse source"', Optica, volume 3, number 10, pages 1075-1078 (2016)) discusses the THz pulse generation using a ZnTe contact grating. The authors produced terahertz pulses with an energy of 3.9 pJ and a generation efficiency of 0.3% using a contact grating with a scratch density of 780 1 / mm. The periodic structure of the contact grating used for this purpose was prepared by combining electron beam lithography and dry etching methods.

[0018] One of the key factors affecting the efficiency of terahertz radiation generation in a nonlinear medium with a periodic structure is the efficiency of diffraction through the periodic structure. It is shown that the inherently high diffraction efficiency of 78% (in total, for the + / -1 -th diffraction orders) achievable with a ZnTe contact grating can be exceeded with structures operated in high diffraction orders for the pump polarization relevant for THz generation.

[0019] In light of the above, it would therefore seem reasonable to implement the THz radiation source with a back surface reflection echelon with applying a semiconductor as the optical medium with nonlinear properties. However, there are serious obstacles to this. Due to the relatively small band gap of semiconductors, multiphoton absorption occurs already at low diffraction orders for pumping wavelengths in the visible or near-infrared domain, leading to the generation of free charge carriers in the optical medium used. This indirectly leads to a significant THz absorption in the material, which is a major limiting factor for the efficiency of THz radiation generation, as said THz absorption reduces it significantly. When using semiconductor materials as optical media, minimizing this effect is a major technical problem to be solved. In a THz radiation source with a back surface reflection echelon, the generation of free charge carriers in the optical medium made of a semiconductor material starts while the pump beam is still propagating towards the back surface structure and, in the absence of velocity matching, no terahertz radiation is generated. Therefore, when using a semiconductor, a THz source with a back surface reflection echelon is not a realistic technical alternative to an LN one (wherein these problems are much less significant due to the larger band gap values), since the efficiency of such a THz source is, as explained above, extremely low.

[0020] In view of the above, the present invention aims at developing a method and a THz radiation source, collectively a THz generation scheme, for generating terahertz radiation for practical applications, which enables the generation of terahertz pulses with excellent beam characteristics (having a substantially symmetrical beam profile as its most important characteristic), in a more scalable and technically eas- ier / simpler manner to implement than the previously described solutions, and in a compact embodiment when limited to semiconductor and organic nonlinear optical materials. The term "scalable" refers herein to the fact that the radius of the beam spot of the pump beam in cross-section used in the terahertz beam source according to the invention - the square of which is proportional to the desired terahertz pulse energy - can be varied practically within arbitrary limits while maintaining the excellent beam properties of the terahertz radiation generated. Preferably, the radius of the beam spot concerned can be varied from the order of mm up to a few cm.

[0021] A further objective of the present invention, particularly, is to provide a terahertz radiation generating scheme, operating preferably in the frequency range of 1 -5 THz, which increases the THz pulse energy and the THz generation efficiency available nowadays.

[0022] A yet further objective of the present invention is to also provide a terahertz radiation generating scheme, operating preferably in the frequency range of 1 -5 THz, which requires the use of as few optical elements as possible. Thus, a compact THz (pulse) source becomes available.

[0023] In our investigations, we concluded that the above objectives can be achieved by a technical solution for terahertz radiation generation based on the velocity matching condition (1 ), wherein a light transmissive medium with nonlinear optical properties suitable for terahertz radiation generation (i.e. transparent to the pump beam) and having parallel front and back surfaces is arranged in the propagation path of the pump beam emitted by a pump beam source. The back surface is planar and the front surface is provided in the form of a periodic structure. The periodic structure is constructed so that its periods are formed by pairs of planar portions of given width arranged symmetrically, wherein the members of the pairs meet in a common line. Relative to the imaginary plane orthogonal to a midplane of the line of meeting of said members of the pairs, each member of the pairs of planar parts makes an angle a with the midplane of the front surface (or, otherwise, the mean plane thereof), considered in a positive and negative sense, wherein said angle a satisfies the relation of a-0 = y, wherein a and 0 are the angle of incidence of the pump beam incident perpendicular to the mean plane and the angle of refraction of the light beams refracted on the plane portions of width w in accordance with the Snellius-Descartes law, respectively, and wherein y is the angle of tilting of the pump beam needed to satisfy the velocity matching condition (1 ) ofv^ cos(y) =THz f.

[0024] The objective of implementing a method for generating THz radiation to be used in practical applications has been achieved by developing a method according to claim 1. Possible further preferred exemplary embodiments of the method according to the invention are set forth in claims 2 to 4. The objective of implementing a radiation source for generating THz radiation to be used in practical applications has been achieved by the assembly according to claim 5. Possible preferred exemplary embodiments of the assembly according to the invention are defined by claims 6 to 10.

[0025] The invention will now be described in further detail with reference to the accompanying drawing, wherein Figure 1 is a schematic sketch of a preferred exemplary embodiment of a semiconductor-based THz radiation source according to the invention; Figure 1 shows the optical medium in a perpendicular cross-sectional view.

[0026] As can be seen in the schematic sketch illustrated in Figure 1 , the pulsed THz radiation source according to the invention comprises a pump beam sources (not shown in Figure 1 ) emitting pump beam 14 and an optical medium 10 with the property of being capable of generating THz pulse. The optical medium 10 is provided as a block of material of a given volume and shape, wherein the material has nonlinear optical properties, is transparent at the wavelength of the pump beam 14, i.e. transparent to the pump beams 14. The optical medium 10 is preferably made of a semiconductor material, in particular, the material is preferably one of GaP, ZnTe, GaAs and GaSe. It will be apparent to a person skilled in the art that the optical medium 10 may also be made of any other suitable semiconductor material or organic material(s). The organic material is preferably an organic salt crystal, specifically one of 4-N,N-dimethylamino-4'-N'-methylstilbazolium-2,4,6-trime- thylbenzene sulphonate (DSTMS), 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2- enylidene)malononitrile (OH1 ) and diethylamino-sulphur trifluoride (DAST). The optical medium 10 is arranged in the propagation path of the pump beam 14.

[0027] The block of the optical medium 10 has parallel front 11 and rear 12 boundary surfaces. 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 mechanical machining (micromachining), preferably by surface milling. The formation of the structure by machining is carried out in the front boundary surface 11 of the block of the optical medium 10, which is plane-parallel to the rear boundary surface 12, prior to the start of machining.

[0028] As shown in Fig. 1 , the periodic structure 13 is constructed such that its 2w- width periods are formed by symmetrically arranged pairs of surface elements 13a, 13b of planar portions having a fixed width, wherein the surface elements 13a, 13b of the said pairs meet along a common line E. Each of the surface elements 13a, 13b makes an angle a with a midplane S of the front boundary surface 11 (or a mean plane thereof), considered orthogonal to an imaginary plane orthogonal to the plane orthogonal to the midplane S of the meeting line E of said surface elements 13a, 13b of the surface pairs, wherein the angle a satisfies the relation a-p = y and considered to be alternately positive and negative (clockwise) as to the orientation; here a and p are the angle of incidence of the pump beam 14 incident perpendicular to the midplane S and the angle of refraction of the refracted beam 15 refracted in harmony with the Snellius-Descartes law on the surface elements 13a, 13b, respectively, and wherein y is the angle of tilting of the pulse front of the pump beam 14, the extent of which is required to satisfy the velocity matching condition

[0029] The spatial period of the periodic structure having a width of 2w is by at least one order of magnitude, preferably by two orders of magnitude, larger than the wavelength of the pump beam 14 emitted by the pump beam source, while it is smaller than at least the half of the wavelength of the terahertz radiation to be produced in the nonlinear optical medium, i.e. the wavelength of the terahertz radiation considered in the block of the optical medium 10. When accurately designing the width w, it is important to ensure that the structure 13 produces good diffraction efficiency in diffraction directions with angles ±y. For widths w larger by orders of magnitude than the pump wavelength, it is advantageous that not only the diffraction orders (typically of high order) associated with ±y angles, but also their neighbours, diffracting in directions close to ±y angles, contribute to the THz generation by increasing the generation efficiency, due to the fact that the velocity matching condition is satisfied by these neighbours with a good approximation. Thus, it is possible to achieve good diffraction efficiency for certain neighbouring, almost overlapping (high) diffraction orders. Consequently, the problem of interference of orders moving in different directions, which is a problem for a contact grating with low diffraction order, with a negative impact on THz propagation, can be avoided.

[0030] During the operation of the THz beam source according to the invention, the pump beam 14 emitted by the pump beam source enters the nonlinear optical 10 medium by breaking through the periodic 13 structure forming the front 11 boundary surface. Passing through each of the surface elements 13a, 13b of the surface pairs into the optical medium 10 and continuing their path as broken sub-beams 16a, 16b, one group of beams travels clockwise (positive sense tilt) and the other group of beams travels counter-clockwise (negative sense tilt) with respect to the incoming pump beam beam 14. Each of these 16a, 16b sub-beams (one of which is shown as an example in Figure 1 ) travels in the direction closing the angle y corresponding to the velocity match with the incoming 14 pump beam (clockwise and counterclockwise). The intensity fronts of the subbeams 16a, 16b are normally 17a, 17b (which are considered as rectangles in the geometrical optical approximation, whose axes of symmetry perpendicular to the plane of Figure 1 together define a plane), but are not inclined with respect to their corresponding phase fronts. However, in the subbeams 16a, 16b, the average of the intensity fronts 17a, 17b defines a plane which is parallel to both the front (average) 11 and back (average) 12 boundary surfaces of the nonlinear optical medium 10 and which extends VTHZ, / wavelengths towards the back 12 boundary surface of the nonlinear optical medium 10, according to the velocity fit (1 ). This average intensity front moving at a velocity VTHZ, / generates THz radiation in the nonlinear optical medium 10 in accordance with the velocity fitting condition (1 ). In the present 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 the sub-beams 16a, 16b do not form a continuous surface but a segmented surface, then the average of the intensity fronts is taken to be their envelope surface, which may be external, internal, or an average between the two. However, given the typical size scales, these three averages are in practice essentially coincident. The generated terahertz radiation (which is not shown in Figure 1 for clarity) travels in the direction of the rear 12 boundary surface of the nonlinear optical 10 medium, perpendicular to it, and then exits the nonlinear optical 10 medium without changing direction, and is available for use in applications after exit.

[0031] The pump source used in the invention is a visible, near- and mid-infrared laser or an optical parametric amplifier capable of emitting laser pulses with pulse lengths of at least 5 fs but not more than a few ps.

[0032] One of the advantages of the inventive solution over the semiconductor contact grating solution is that the periodic structure requires a simple technology to form and is more suitable in terms of energy scalability.

[0033] Summary: by forming a symmetric periodic structure in a semiconductor nonlinear optical medium delimited by plane-parallel front (input) and rear surfaces, with the period length (w) between a few tens and a few hundreds of times the wavelength of a pump beam emitted by a pumped beam source onto the optical medium, a new generation arrangement or THz source for high energy terahertz radiation can be obtained. The main advantage of the arrangement is that the optical medium consisting of a semiconducting nonlinear optical crystal can be used as a unit with plane-parallel interfaces in the arrangement. As a result, THz beams with excellent beam quality and physically symmetric properties can be produced with high gener- ating efficiency. Due to the design of the optical medium as a semiconductor material, the frequency of the THz beams produced is preferably in the frequency range of 1 -5 THz. Since the arrangement contains neither imaging optics nor an optical grating, the size of the pump beam in cross-section can be essentially arbitrarily large. Accordingly, the energy of the terahertz pulse generated by this method can also be arbitrarily large. The terahertz radiation source and method of the invention based on the arrangement discussed here are particularly preferred for the generation of high energy THz radiation, which requires the use of wide pump beams.

Claims

CLAIMS1. A method for producing terahertz radiation, wherein a pump beam (14) is coupled into a plane-parallel nonlinear optical medium (10) through a front boundary surface (11 ) thereof by being subjected to refraction through a periodic structure (13) forming said front boundary surface (11 ), the periodic structure (13) is formed by symmetrically arranged planar surface elements (13a, 13b) of certain width, said surface elements (13a, 13b) form an angle a with a midplane (S) of the front boundary surface (11 ), wherein said angle a satisfies the relation a-0 = y and considered to be alternately positive and negative as to the orientation, wherein y is the angle of the pulse front tilting required to satisfy the velocity matching condition-VP ,'CS is the group velocity of the pump beam (14), MTHZ- is the phase velocity of the THz radiation, and a and 0 are the angles of incidence and refraction, respectively, of a light beam (15) incident perpendicular to said midplane (S) and then refracted in harmony with the Snellius-Descartes law on the planar surface elements (13a, 13b), and wherein, after refraction, the pump beam (14) is formed by a set of sub-beams that propagate in a direction making the angle ±y with the direction of the incoming pump beam (14), wherein the average of the intensity fronts of the individual sub-beam is an imaginary planar surface, said imaginary plane travels towards an exit boundary surface (12) of the nonlinear optical medium (10) at a velocity MTHZ- and generates THz radiation within the nonlinear optical medium (10) by nonlinear optical processes, in particular optical rectification, and wherein the THz radiation generated thereby is uncoupled from the nonlinear medium (10) through the exit boundary surface (12) thereof.

2. The method according to claim 1 , wherein the pump beam (14) is a laser pulse in the visible, near- 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 THz pulse generation, the optical medium (10) is delimited by at least two plane-parallel boundary surfaces (11 , 12), wherein the pump beam source and the nonlinear optical medium (10) together define a light path, wherein the front and rear boundary surfaces (11 , 12) of the nonlinear optical medium (10) are substantially perpendicular to said light path, and wherein the front boundary surface (11 ) is formed by a periodic structure (13), the periodic structure (11 ) is formed by pairs of surface elements (13a, 13b) connected to each other along meeting lines (E), wherein the individual surface elements (13a, 13b) are plane surfaces forming alternately positive and negative angles (a) of the same magnitude with imaginary orthogonal planes set in the meeting lines (E) to the front boundary surface (11 ), wherein the surface elements (13a, 13b) form an angle with the rear boundary surface (12) in such a way that the angle of change of direction ( ) of the pump beam (14) incident and being refracted on the surface elements (13a, 13b) is equal to the angle (y) at which the pulse front propagation satisfies the velocity matching condition (1 ) in the nonlinear optical medium (10).

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

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

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

9. The terahertz radiation source according to any one of claims 5 to 8, wherein the pump source is a pump source configured to emit a laser pulse in the visible, near infrared or mid-infrared range with a pulse length of 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, in particular one of 4-N,N-dimethylamino-4'-N'-methylstilbazolium-2,4,6-trimethylbenzene sulfonate (DSTMS), 2-(3-(4-hy- droxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile (OH1 ) and diethyla- mino-sulfur trifluoride (DAST).

Images