Method to generate terahertz radiation and terahertz radiation source

Abstract

The present application relates to generating terahertz radiation, wherein a pump pulse is subjected to pulse front tilting, the thus obtained pump pulse having tilted pulse front is coupled into a nonlinear optical medium and THz pulse is generated by the optical medium in nonlinear optical processes, particularly by means of optical rectification by the pump pulse. The application also relates to a terahertz radiation source (100), comprising a pump source (10) for emitting a pump pulse and a nonlinear optical medium for generating THz pulse. The pump source (10) and the nonlinear optical medium define together a light path, said light path is arranged to guide the pump pulse from the pump source (10) to the nonlinear optical medium. A first optical element (20) having angular-dispersion-inducing property and imaging optics (30) are disposed in the light path after each other in the propagation direction of the pump pulse.

Claims

1. A method to generate terahertz radiation in a nonlinear optical medium, the method comprising: subjecting a pump pulse to pulse front tilting to obtain a tilted pump pulse having a tilted pulse front; coupling the tilted pulse front into the nonlinear optical medium; and generating a THz pulse using the optical medium in nonlinear optical processes, inducing said pulse front tilt of the pump pulse to satisfy a velocity matching condition of .sub.p;cscos()=.sub.THz;f as a sum of a plurality of the pulse front tilts, each of the plurality of the pulse front tilts induced separately as a partial pulse front tilt of a same sense of the pump pulse in subsequent steps, wherein v.sub.p;cs is a group velocity of the pump pulse, vTH.sub.z;f is a phase velocity of the THz pulse, and is an angle formed between the pulse front and the phase front of said pump pulse.

2. The method according to claim 1, further comprising inducing a first partial pulse front tilt of the pump pulse by guiding the pump pulse on a first optical element having an angular-dispersion-inducing property, and then by guiding the pump beam leaving the first optical element onto an imaging optics.

3. The method according to claim 2, further comprising inducing further partial pulse front tilts of the pump pulse by guiding the pump pulse having the partial pulse front tilt with the imaging optics via coupling into the nonlinear optical medium by a contact grating being in optical coupling with the nonlinear optical medium or formed on an entry plane of the optical medium.

4. The method according to claim 1, wherein the pump pulse is a laser pulse in the visible, near- or mid-infrared domain having a length of at most several hundred femtoseconds.

5. The method according to claim 1, wherein the first optical element is a diffraction based optical element or a refraction based optical element or an optical element that is a combination thereof.

6. The method of claim 1 wherein the generating the THz pulse by the pump pulse using the optical medium in nonlinear optical processes comprises using optical rectification.

7. A terahertz radiation source, comprising a pump source configured to emit a pump pulse; a nonlinear optical medium configured to generate a THz pulse, a first optical element having an angular-dispersion-inducing property; at least one further optical element with an angular-dispersion-inducing property; and an imaging optics, wherein the pump source and the nonlinear optical medium define together a light path, said light path being arranged to guide the pump pulse from the pump source to the nonlinear optical medium, and wherein the first optical element and the imaging optics are arranged in said light path after each other in a propagation direction of the pump pulse, wherein the at least one further optical element is provided within said light path, said at least one further optical element located after both the first optical element and the imaging optics in the propagation direction of said pump pulse.

8. The radiation source according to claim 7, wherein the at least one further optical element with angular-dispersion-inducing property comprises a single contact grating arranged at an entry plane of the nonlinear optical medium bordering the light path.

9. The radiation source according to claim 8, wherein contact grating is formed in a material of the nonlinear optical medium.

10. The radiation source according to claim 7, wherein the first optical element is selected from a group comprising diffraction based optical elements, refraction based optical elements, and optical elements constructed as a combination thereof.

11. The radiation source according to claim 10, wherein the first optical element is a transmissive optical grating.

12. The radiation source according to claim 7, wherein the imaging optics is selected from the group comprising imaging optics having any one of a lens, a refracting telescope, a mirror, a reflecting telescope, and concave mirrors.

13. The radiation source according to claim 7, wherein the nonlinear optical medium is formed by a material having a nonlinear optical coefficient of at least 1 pm/V and different refractive indices in a terahertz frequency range and a visible domain.

14. The radiation source according to claim 7, wherein the nonlinear optical medium is formed by a lithium niobate (LiNbO.sub.3) or a lithium tantalate (LiTaO.sub.3) crystal.

15. The radiation source according to claim 7, wherein the pump pulse is a laser pulse in a visible, nearor mid-infrared domain having a length of at most several hundred femtoseconds.

16. A terahertz radiation source, comprising: a pump source configured to emit a pump pulse; a nonlinear optical medium configured to generate a THz pulse, a first optical element having an angular-dispersion-inducing property; at least one further optical element with an angular-dispersion-inducing property; and an imaging optics, wherein the pump source and the nonlinear optical medium define together a light path, said light path being arranged to guide the pump pulse from the pump source to the nonlinear optical medium, and wherein the first optical element and the imaging optics are arranged in said light path after each other in a propagation direction of the pump pulse, wherein the at least one further optical element is provided within said light path, said at least one further optical element located after both the first optical element and the imaging optics in the propagation direction of said pump pulse, wherein the at least one further optical element comprises a single contact grating arranged at an entry plane of the nonlinear optical medium bordering the light path.

17. The radiation source according to claim 16, wherein the single contact grating is formed in a material of the nonlinear optical medium.

18. The radiation source according to claim 16, wherein the first optical element is selected from a group comprising diffraction based optical elements, refraction based optical elements, and optical elements constructed as a combination thereof.

19. The radiation source according to claim 16, wherein the first optical element is a transmissive optical grating.

20. The radiation source according to claim 16, wherein the imaging optics is selected from the group comprising imaging optics having any one of a lens, a refracting telescope, a mirror, a reflecting telescope, and concave mirrors.

21. The radiation source according to claim 16, wherein the nonlinear optical medium is formed by a material having a nonlinear optical coefficient of at least 1 pm/V and different refractive indices in a terahertz frequency range and a visible domain.

22. The radiation source according to claim 16, wherein the nonlinear optical medium is formed by a lithium niobate (LiNbO.sub.3) or a lithium tantalate (LiTaO.sub.3) crystal.

23. The radiation source according to claim 16, wherein the pump pulse is a laser pulse in a visible, nearor mid-infrared domain having a length of at most several hundred femtoseconds.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) In what follows, the invention is described in detail with reference to the accompanying drawings, wherein

(2) FIG. 1 illustratesin an enlarged viewthe characteristic parameters of the grating profile of a contact grating with a binary grating profile that represents a preferred embodiment of the contact grating that can be used as the optical element with angular-dispersion-inducing property in the hybrid generation arrangement according to the invention;

(3) FIG. 2 shows schematically a preferred embodiment of the THz radiation source implementing a hybrid generation arrangement according to the invention with an angular-dispersive optical element formed by a transmission optical grating disposed in the light path before (i.e. upstream of) the imaging optics and a contact grating arranged after (i.e. downstream of) the imaging optics resulting in pulse front tilt of the pump beam is more than one separate steps;

(4) FIG. 3 shows the diffraction efficiency of a contact grating used in the hybrid generation arrangement according to the invention formed in the surface, in particular, of an LN crystal with a grating profile as illustrated in FIG. 1 as a function of the incidence angle (.sub.2) of the pump beam having a tilted pulse front, with the geometric parameters summarized in Table 2;

(5) FIG. 4 shows the diffraction efficiency of a contact grating formed in the surface, in particular, of an LN crystal with a grating profile as illustrated in FIG. 1 used in the hybrid generation arrangement according to the invention as a function of the filling factor (f=w/d.sub.2) and the profile depth (h); and

(6) FIG. 5 shows changes in pulse length () in a transversal direction (y) perpendicular to the propagation of the pump beam due to imaging errors of a hybrid generation arrangement according to the invention implemented with a contact grating etched into the surface of, in particular, an LN crystal and of a corresponding conventional generation arrangement.

DETAILED DESCRIPTION

(7) The so-called hybrid tilted-pulse-front THz beam generation arrangement according to the invention, and a preferred embodiment of the THz radiation source 100 for generating THz radiation are illustrated in FIG. 2. The radiation source 100 comprises a pump source 10 for providing a pump beam 12, and an optical element 50 having nonlinear optical properties for generating THz radiation 60. The optical element 50 has an exit plane 52 and an opposing entry plane 51, which forms a predetermined angle with the exit plane 52; therefore the optical element 50 is preferably formed as a prism or optical wedge. The THz radiation 60 is generated as a result of the nonlinear optical interaction of the material of the optical element 50 and the pump beam 12, preferably by optical rectification, in the volume of the optical element 50 substantially at a distance L.sub.3 from the entry plane 51, then it leaves the optical element 50 through its exit plane 52 and then it may be subjected to further use. A contact grating 40 characterized by predetermined geometrical parameters is arranged on the entry plane 51 in perfect optical coupling with said entry plane 51 or formed in the entry plane 51. In practice, the contact grating 40 is formed by an optical grating having an optical axis O2 and a lattice constant dz, wherein the optical axis O2 is substantially perpendicular to the entry plane 51.

(8) The pump source 10 and the optical element 50as an initial element and a final elementtogether define a light path extending between the emission point of the pump source 10 and the entry plane 51. The radiation source 100 comprises in said light path, in the propagation direction of the pump beam 12, i.e. toward the entry plane 51 respectively a first optical element having angular dispersion inducing property, i.e. an angular-dispersive optical element 20 and an imaging optics 30 in a predetermined geometrical arrangement; the optical element 20 and the imaging optics 30 are located at a mutual distance L.sub.1 along the light path, while the imaging optics 30 and the contact grating 40 are arranged at a mutual distance L.sub.2. Determining the geometric parameters comprising the distances L.sub.1, L.sub.2 and L.sub.3 is described in detail in the following.

(9) The pump source 10 is preferably a laser source suitable to produce laser pulsesthe pump beam 12in the visible, near- or mid-infrared domain with a pulse length of several hundred fs, e.g. a diode pumped Yb laser with a central emission wavelength of 1030 nm. However, different lasers may also be used as pump source 10.

(10) The optical element 20 is formed as an optical element inducing the dispersion of the pump beam 12 incident thereon and transmitted therethrough, and thus changing the tilt of the initial pulse front 15 (characterized by preferably zero tilt) of the pump beam 12 by a predetermined amount. The angular-dispersive optical element 20 may be provided by a diffraction based optical element (preferably optical grating) or by a refraction based optical element (preferably a prism/prisms) or by a combination thereof (e.g. a prism combined with a diffraction grating, i.e. a so-called grism). In the embodiment of the radiation source 100 illustrated in FIG. 2, the optical element 20 is preferably a transmissive grating having an optical axis O1 and a lattice constant d.sub.1, howeveras it is obvious to a person skilled in the artit may also be provided by other angular-dispersive optical elements, e.g. a reflective grating, prism, etc. The advantage of constructing it as a transmission grating is the relatively high separation of the incident pump pulse 12 and the beam 22 diffracted e.g. in the first order. In this case, the pump beam 12 incident in an angle .sub.1 relative to the optical axis O1 on the optical element 20 provided in the form of a transmission grating is diffracted in an angle .sub.1 and transmitted in the light path as the beam 22, wherein the pulse front 25 of the beam 22 has a certain extent of tilt; the extent of tilt is explicitly determined by the wavelength of the pump beam 12, the lattice constant d.sub.1 and the angle of incidence .sub.1 through the following relations:

(11) $\begin{array}{cc}\sin \left({}_1\right)=\frac{}{d_1}-\sin \left({}_1\right)& \left(1\right)\\ \frac{d{}_1}{d}=-\frac{1}{d_1\cos \left({}_1\right)}& \left(2\right)\\ \tan \left({}_1\right)=-\frac{d{}_1}{d}& \left(3\right)\end{array}$

(12) As it is known to a person skilled in the art, the tilt of the pulse front 25 may be derived through similar relations when different types of angular-dispersive optical elements 20 are used.

(13) The imaging optics 30 may be provided by any imaging optics known from the field of conventional tilted-pulse-front THz generation arrangements having imaging optics. Thus, for example the imaging optics 30 may be provided in the form of a lens, refracting telescope, mirrors, reflecting telescope, or a concave mirror or in the case of a reflecting telescope, the telescope having concave mirrors may be formed by both spherical and cylindrical mirrors. Any of said optical elements used for the imaging optics 30, may be formed with spherical shape, cylindrical shape or a shape with minimized imaging error (bestform). In the embodiment of the radiation source 100 illustrated in FIG. 2, the imaging optics 30 is provided in the form of a focusing lens having a certain diameter and a focal length f.sub.L (e.g. f.sub.L=200 mm). The imaging optics 30 images the (in this case) diffracted beam 22 having a pre-tilted pulse front 25 into the optical element 50 as a beam 32 having a pulse front 35. In particular, in this embodiment of the radiation source 100, the beam 22 is guided by the imaging optics 30in the form of beam 32 at an angle .sub.2 relative to the optical axis O2to a further optical element with angular-dispersion-inducing property, in particular, to a contact grating 40 and after being diffracted it is introduced into the volume of the optical element 50 in the form of beam 42. The pulse front 45 of the beam 42 is tilted to the extent required by the phase matching condition. The tilted-pulse-front beam 42 propagates in the optical element 50 at angle .sub.2 to the normal of the crystal lattice. The phase fronts of the generated THz radiation 60 are parallel to the tilted pulse front 45, thus the propagation direction of the THz radiation 60 is necessarily perpendicular to these planes.

(14) The contact grating 40 is preferably provided as an optical grating having a grating profile illustrated in FIG. 1 i.e. a so-called binary grating profile. It is known to a person skilled in the art, that the contact grating may be implemented with different grating profiles (e.g. sinusoidal, sawtooth etc.), further preferred exemplary grating profiles may be known from the scientific publication of Ollmann Z. et al. entitled Design of a high-energy terahertz pulse source based on ZnTe contact grating [Optics Communication, 2014, issue 315, pages 159-163].

(15) The optical element 50 is made of a material having an outstandingly high nonlinear optical coefficient, practically at least 1 pm/V, and having different refractive indices in the terahertz and the visible domain, preferably LN or lithium-tantalate, preferably with a crystal-axis orientation that facilitates the occurrence of nonlinear optical phenomena, thus aiding e.g. the occurrence of optical rectification and thus the generation of THz radiation. Moreover, in order to minimize reflection losses, the exit plane 52 of the optical element 50 is formed to emit the THz radiation 60 generated in the optical element 50 through the exit plane 52 of the optical element 50 perpendicularly.

(16) As mentioned previously, the geometric parameters (see FIG. 2) characterizing the geometric arrangement of the elements forming parts of the radiation source 100 may be obtained by optimization. The optimization is based on the following conditions of efficient THz radiation generation: the summed tilt of the pulse front 45 of the beam 42 satisfies the velocity matching condition; in order to minimize the effect of the imaging errors on the efficiency of THz beam generation, the pulse length of the pump pulse along the pulse front 45 inside the optical element 50 is as close to the transformation-limited value as possible, i.e. the image of the beam spot of the beam 22 appearing on the angular-dispersive optical element 20 in the optical element 50 contacts the tilted-pulse-front surface along the optical axis.

(17) Without showing detailed mathematical deductions (which are obvious to a person skilled in the art), the general equations for the selection of the angular-dispersive optical element 20 and the imaging optics 30, and for determining the geometry of the radiation source 100 after fixing the grating profile of the contact grating 40 are summarized in the following. Accordingly:

(18) $\begin{array}{cc}\sin \left({}_1\right)=\frac{}{d_1}-\sin \left({}_1\right)& \left(4\right)\\ \sin \left({}_1\right)=a\frac{}{d_1{\mathrm{nn}}_g}k& \left(5\right)\\ L_1=f_L\left(\sqrt{a}+1\right)& \left(6\right)\\ L_2=\frac{f_L{L}_1}{L_1-f_L}-\frac{L_3}{n}\frac{{\cos }^2\left({}_2\right)}{{\cos }^2\left({}_2\right)},\mathrm{wherein}\mathrm{parameters}a\mathrm{and}k\mathrm{satisfy}\mathrm{the}& \left(7\right)\\ a=\sqrt{\frac{{}^2}{d_1^2{\tan }^4\left({}_0\right)}+\frac{4k^2}{n^2{n}_g^2}}.Math.\frac{d_1{n}^2{n}_g^2}{2k^2}-\frac{n^2{n}_g^2}{2{\tan }^2\left({}_0\right)k^2},& \left(8\right)\\ k=\frac{\cos \left({}_2\right)}{\cos \left({}_2\right)}\frac{\tan \left(\right)}{\tan \left({}_0\right)}{n}_g& \left(9\right)\end{array}$
relations, and wherein the parameters are summarized in the following Table 1.

(19) TABLE-US-00001 TABLE 1 description of parameters used in equations (1) to (10). central wavelength of the .sub.1 angle of incidence at the optical pump source 10 grating forming the angular- dispersive optical element 20 f.sub.L focal length of the lens .sub.1 diffraction angle at the optical forming the imaging grating forming the angular- optics 30 dispersive optical element 20 required extent of pulse .sub.2 angle of incidence at the contact front tilt inside the grating 40 optical element 50 .sub.0 extent of pulse front .sub.2 diffraction angle at the contact pre-tilting (initial/ grating 40 partial tilt of the pulse front) n refractive index of the L.sub.1 relative distance of the angular- optical element 50 dispersive optical element 20 and the imaging optics 30 n.sub.g group refractive index L.sub.2 distance of the imaging optics 30 of the optical element 50 and the contact grating 40 d.sub.1 lattice constant of the L.sub.3 distance of intensive THz optical grating forming radiation generation inside the the angular-dispersive optical element 50 from the entry element 20 plane 51 thereof

(20) Assuming real physical parameters, the results of (analytic) calculations carried out on the basis of a system of equations comprising the aforementioned equations (4) to (9) determine explicitly the geometric parameters of the hybrid generation arrangement according to the invention (i.e. of the radiation source 100).

(21) Parameters , f.sub.L, , n, n.sub.g, d.sub.1, L.sub.3 are determined and/or chosen and/or preset by the user through the selection of the pump source and/or the material of the nonlinear optical medium to be used for the THz radiation generation.

(22) The extent of pre-tilting of the pulse front of the pump beam 10 (i.e. the first part of pulse front tilting) is selected so that the tangents of the angles characterizing the subsequent (two) steps of inducing pulse front tilts are about the same, i.e.

(23) $\begin{array}{cc}\mathrm{tg}\left({}_0\right)=\frac{\tan \left(\right)}{2}.& \left(10\right)\end{array}$

(24) It is hereby noted, that the energy of the generated THz pulse is primarily determined (when usingamong othersa pump pulse with a given transformation-limited pulse length) by the changes in pulse length originating from the imaging errors of the imaging optics 30 and occurring in the optical element 50, and the diffraction efficiency on the contact grating 40 formed in the entry plane 52 of the optical element 50. Accordingly, as the result of the efficiency-optimization carried out for the contact grating 40, the parameters .sub.2, .sub.2, d.sub.2 become fixed.

(25) The remaining (free) parameters, i.e. .sub.1, .sub.1, L.sub.1, L.sub.2 characterize the geometric relations of the hybrid THz generation arrangement according to the invention.

EXAMPLE

(26) The tilted-pulse-front hybrid generation arrangement according to the invention is described in detail in the following with the exemplary use of LN crystal as the nonlinear optical medium. The use of LN crystal may be considered special, because this material has outstanding nonlinear properties, but it can only be machined without the deterioration of the profile quality of the grating to be formed in its surface for the widely used (about 1030 nm) pump beam wavelength to a line-density of at most about 2000/mm. Therefore, the use of the hybrid generation arrangement according to the invention is particularly advantageous in its case. However, the methodwith the necessary modifications obvious to a person skilled in the artmay naturally be used in the case of any other material, wherein tilted-pulse-front pumping is required and implementing a solution with merely a contact grating is problematic.

(27) In this case, the grating profile of the contact grating is the same as the binary grating profile shown in FIG. 1, a transmissive optical grating is used as the angular-dispersive optical element, and the imaging optics is formed in this case by a (preferably focusing) lens. The extent of pulse front tilt induced by the transmissive optical grating may be simply obtained on the basis of the aforementioned equations (1) to (3) and (10).

(28) In order to ensure efficient THz beam generation, the aforementioned conditions are satisfied, i.e. the summed pulse front tilt of the pump beam satisfies the velocity matching condition, i.e. it is about 63 in the case of LN; in order to minimize the effect of the imaging errors on the efficiency of THz beam generation, the pulse length of the pumping along the pulse front in LN crystal is as close to the transformation-limited value as possible, i.e. practically the image of the beam spot appearing on the transmissive optical grating having lattice constant d.sub.1 in the crystal is in contact with the tilted-pulse-front surface along the optical axis.

(29) Under these conditions, the geometric parameters obtained by analytic calculations for LN crystal through the aforementioned equations (4) to (9) may be explicitly derived after setting the values of the parameters to be fixed (LN crystal, 1030 nm pump wavelength, etc.).

(30) Accordingly, the diffraction efficiency for the contact grating with a lattice constant of d.sub.2 of the hybrid generation arrangement constructed in an optimal manner regarding the optical imaging is illustrated in FIG. 3 as a function of the incidence angle .sub.2 of the pump beam having a pre-tilted pulse front, without the use of RIML and with the LN contact grating cooled to 100K. The diffraction efficiency is shown as a function of the incidence angle .sub.2 with a lattice constant chosen for each value that ensures the required extent of pulse front tilt in the LN crystal, and with an equal extent of pre-tilting (65 in air). According to the figure, the diffraction efficiency exceeds 85% in the range between 21 and 26, the angle of incidence corresponding to the flat maximum is .sub.2=24. From now on, the hybrid generation arrangement with the geometry corresponding to this point is considered optimal and discussed further.

(31) For comparison purposes, Table 2 below summarizes the parameters of three different generation arrangements based on the tilted-pulse-front technique. That is, the characteristic parameters of the hybrid arrangement, the conventional generation arrangement optimized as described in the scientific publication of Flp J. et al. entitled Design of high-energy terahertz sources based on optical rectification [Optics Express, 2011, vol. 18, issue 12, pages 12311-12327], and the contact grating generation arrangement optimized as described in the scientific publication of Ollmann Z. et al. entitled Design of a contact grating setup for mJ-energy THz pulse generation by optical rectification [Applied Physics B, 2012, vol. 108, issue 4, pages 821-826].

(32) TABLE-US-00002 TABLE 2 Characteristic geometric parameters for different types of generation arrangements in the case of using an LN crystal, assuming an optimal incidence angle .sub.2. pump generation pump pulse lens arrangement wavelength length l/d.sub.1 l/d.sub.2 f.sub.L .sub.1 .sub.2 .sub.2 hybrid 1030 nm 200 fs 1400/mm 1563/mm 200 mm 46 24 34 conventional 1030 nm 200 fs 1400/mm 200 mm 37 contact 1030 nm 200 fs 2874/mm 70 46 grating

(33) At a given lattice constant, the diffraction efficiency depends on the parameters characterizing the structure of the grating profile; in the case of the selected square grating having binary grating profile these are the filling factor f=w/d.sub.2 and the profile depth h (see also FIG. 1)

(34) FIG. 4 illustrates the diffraction efficiency as a function of the filling factor f and the profile depth h on the entry plane of the LN crystal in the case of a square grating having binary grating profile in harmony with the parameters in the hybrid arrangement line of Table 2. As shown, diffraction efficiency above 85% may be achieved, and the range having similarly high diffraction efficiency is relatively wide; this provides a high tolerance regarding the machining (line formation) on the entry plane of the LN crystal. The profile depth h corresponding to the high efficiency range is smaller than the lattice constant d.sub.2, and the filling factor f is close to 50%, which values are also advantageous regarding the machining. Moreover, the lattice constant of 0.639 m (line-density of 1564/mm) of the grating to be formed in the LN crystal is also under the about 2000/mm threshold value, which also guarantees that the contact grating is practically realizable.

(35) According to the aforementioned, the efficiency of THz beam generation is significantly affected by the pulse length distortion of the pump pulse due to imaging errors. FIG. 5 illustrates this for an optimized conventional arrangement and a hybrid generation arrangement. The local pulse length change () caused by the imaging errors is shown in the plane of angular dispersion transversally (y) along the beam diameter. The two curves clearly show the advantage of the hybrid arrangement according to the invention versus the conventional one, as the extent of pulse length expansion is significantly lower in the case of the hybrid arrangement. It can also be seen from the curves that for the hybrid arrangement, the pulse length remains below 500 fs everywhere in the beam even for beam diameters exceeding 10 mm, while for the conventional arrangement, the pump beam becomes longer than 1000 fs at the edge of the beam.

(36) Due to inducing the tilt of the pulse front is carried out in two different steps, creating high quality grating profile with the lower line-density (at most 2000/mm) that can be used in the LN crystal does not cause a manufacturing problem. Furthermore the significantly reduced imaging errors due to the use of the hybrid generation arrangement improves the efficiency of THz beam generation, and the quality and focusability of the generated THz beam is better than what could be obtained by a conventional generation arrangement.

(37) Besides the more efficient THz beam generation, a further advantage of the hybrid generation arrangement is that in the case of LN crystal, the exit plane of the THz generating prism forms an angle with the entry plane that is significantly smaller (30) than in the case of a conventional solution (63). This is advantageous for the intensity distribution of the THz beam. A further advantage is that THz beam generation of high efficiency is possible in a generation arrangement which does not contain/require refractive index matching liquid.