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EP-4735954-A1 - SEMICONDUCTOR-BASED HIGH-ENERGY TERAHERTZ RADIATION SOURCE

EP4735954A1EP 4735954 A1EP4735954 A1EP 4735954A1EP-4735954-A1

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).

Inventors

  • ALMÁSI, Gábor
  • HEBLING, János
  • KRIZSÁN, Gerg
  • TIBAI, Zoltán
  • TÓTH, György

Assignees

  • Pécsi Tudományegyetem

Dates

Publication Date
20260506
Application Date
20240630

Claims (10)

  1. 1. 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 - V P ,'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. 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. 3. The method according to claim 1 or 2, wherein the nonlinear optical medium (10) is a semiconductor material.
  4. 4. The method according to claim 1 or 2, wherein the nonlinear optical medium is an organic material.
  5. 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. 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. 7. The terahertz radiation source according to claim 5 or 6, wherein the nonlinear optical medium (10) is made of a semiconductor material.
  8. 8. The terahertz radiation source according to claim 5 or 6, wherein the nonlinear optical medium (10) is made of an organic material.
  9. 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. 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).

Description

P138488-13773D SZT SEMICONDUCTOR-BASED HIGH-ENERGY TERAHERTZ RADIATION SOURCE 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. 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. 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,cS cos(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. 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. 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. 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 tera