TECHNICAL FIELD

[0001] The present disclosure is related in general to the technical field of generation of electromagnetic radiation and in particular to the generation of terahertz radiation by utilizing the off-diagonal or transverse thermoelectric ef fect.

BACKGROUND

[0002] The search for efficient sources for the THz fre quency range (~ 0.1— 100THz) (1 THz = 1 x 10 ¹² Hz) is current ly very actively pursued in both science and technology. The strong interest is driven by the significant benefits of THz radiation in fields such as spectroscopy, object imaging, fast data transfer, etc., which relate to fundamental science investigations, medicine, security, communications, and other specialized applications. However, the realization of THz sources is associated with significant challenges. Most cur rently available sources require specialized equipment and conditions such as expensive high-quality single crystals (some of them toxic), lithography for micro- or nano structuring, application of external fields, cryogenics, or cumbersome system adjustment procedures. In addition, the output power of the most commonly used THz sources is limited and difficult to enhance.

[0003] For these and other reasons there is a need for the present disclosure. SUMMARY

[0004] A first aspect of the present disclosure is related to an electromagnetic radiation source comprising a material having an anisotropic Seebeck coefficient and being config ured to generate a transverse current pulse by means of the off-diagonal or transverse thermoelectric effect upon laser pulse irradiation, wherein the electromagnetic radiation is generated by the change in time of the current.

[0005] A second aspect of the present disclosure is relat ed to a method of generating an electromagnetic radiation, the method comprising directing a short-time laser pulse on a material, the material having an anisotropic Seebeck coeffi cient and being configured to generate a transverse current pulse by means of the off-diagonal or transverse thermoelec tric effect upon laser pulse irradiation; and emitting elec tromagnetic radiation generated by a change in time of the transverse current.

[0006] A third aspect of the present disclosure is related to a material, in particular for use in an electromagnetic radiation source, the material having an anisotropic Seebeck coefficient and being configured to generate a transverse current pulse by means of the off-diagonal or transverse thermoelectric effect upon laser pulse irradiation, wherein the electromagnetic radiation is generated by the change in time of the current.

DESCRIPTION OF EMBODIMENTS [0007] It should be mentioned that the term "transverse current" does not imply a current which is perfectly trans verse to the temperature gradient. It can also mean a current which is only essentially transverse to the temperature gra dient.

[0008] According to an embodiment of the electromagnetic radiation source of the first aspect, the material has a two- dimensional crystallographic symmetry, wherein an axis normal to a crystallographic symmetry plane is tilted with respect to the direction of a temperature gradient produced by the laser pulse irradiation. Also a lower symmetry than the two- dimensional symmetry will work.

[0009] According to an embodiment of the electromagnetic radiation source of the first aspect, the material is a film or layer or sheet disposed on a surface of a substrate, or a free-standing layer. The thickness of the film can be equal to or less than 500 nm or 100 nm or 60 nm or the thickness of the film is optimized with respect to the penetration depth of the electromagnetic radiation (1 nm = 1 x 1CT ⁹ m). The surface of the substrate may comprise a step-terrace topogra phy.

[0010] According to an embodiment of the electromagnetic radiation source of the first aspect, the material is a bulk material.

[0011] According to an embodiment of the electromagnetic radiation source of the first aspect, the material has an an isotropic Seebeck coefficient by virtue of its intrinsic or artificial structure. The Seebeck coefficient of the material may have any polarity, i.e. negative, positive, or a combina tion of both.

[0012] According to an embodiment of the electromagnetic radiation source of the first aspect, the material is a mate rial with anisotropic charge carrier transport properties. In particular, the material can be a material having intrinsic anisotropic charge carrier transport properties such as e.g. layered transition metal oxides, or an artificial material, e.g. grown by epitaxy, such as a multi-layer heterostructures material, a superlattice heterostructure, e.g. a semiconduc tor superlattice heterostructure. The charge carriers may have any polarity, i.e. may be electrons or holes, or a com bination of both.

[0013] According to an embodiment of the electromagnetic radiation source of the first aspect, the material is a lay ered transition metal oxide, in particular PdCoCh, La2- _x Sr _x CuC>4, PtCoCh, Ca ₃ Co4C>9.

[0014] According to an embodiment of the electromagnetic radiation source of the first aspect, a laser spot diameter on the material can in general be in the nm or cm range. In particular, it can be equal to or greater than 100 nm or 1 mm or 1 cm, or is optimized with respect to the material surface area.

[0015] According to an embodiment of the electromagnetic radiation source of the first aspect, a micropattern is su perimposed onto the material and/or the substrate, e.g. the micropattern forming a grating. The grating may have a grat ing period corresponding to a wavelength of the emitted elec tromagnetic radiation (e.g. several few mm to 10 pm). In par- ticular, the micropattern can be formed by a surface topology of the material or by a surface topology of a substrate on which the material is disposed or by a pattern of a grating material applied to the material. Also possible is that the band-structure of the material is patterned (band-structure engineering), e.g. by epitaxial growth patterning or by do pant patterning, e.g. by forming a dopant profile in the ma terial. The grating can also be formed by removing of materi al, e.g. by etching.

[0016] According to an embodiment of the electromagnetic radiation source of the first aspect, the electromagnetic ra diation source further comprises a pulsed laser configured to generate the laser pulse irradiated on the material. The pulsed laser can be any conventional laser source, in partic ular one or more of a short pulse laser, a solid state laser, a (Ti-)sapphire laser, an erbium laser, a diode laser, a glass fiber laser, or a (tunable) OPA laser. The laser maybe tuned to high absorption in the material. The pulse energy per laser pulse can be equal to or greater than 1 pj or 10 pj or 100 pj or 1 mJ or 10 mJ (1 pj = lxl0 ⁶ J, 1 mJ = lxl0 ³ J). The duration of the laser pulse can be equal to or less than 1 ns or 1 ps or 100 fs or 10 fs or 1 fs. (1 ns = 1 x 10 ⁹ s,

1 ps = 1 x 10 ¹² s, 1 fs = 1 x 10 ¹⁵ s). The repetition rate of the laser can be equal to or larger than lHz or 1kHz or lGHz (1 kHz = lxl0 ³ Hz, 1 GHz = lxlO ⁶ Hz).

[0017] According to an embodiment of the electromagnetic radiation source of the first aspect, the generated radiation is independent of any possible polarization of the laser light. [0018] According to an embodiment of the electromagnetic radiation source of the first aspect, the electromagnetic ra diation is in the GHz or THz frequency range.

[0019] According to an embodiment of the electromagnetic radiation source of the first aspect, the emission of the electromagnetic radiation is in transmission direction and in reflection direction.

[0020] According to an embodiment of the electromagnetic radiation source of the first aspect, an external voltage and/or a magnetic field is applied to the electromagnetic ra diation source.

[0021] According to an embodiment of the electromagnetic radiation source of the first aspect, one or more of a coat ing, a protective layer, a reflective layer, an absorption layer is deposited onto the electromagnetic radiation source.

[0022] According to an embodiment of the electromagnetic radiation source of the first aspect, the pump laser wave length is tuned to optimize the absorption in the material, hence to optimize also the temperature gradient in the mate rial.

[0023] Embodiments of the method according to the second aspect can be formed together with any features or embodi ments as described in connection with the electromagnetic ra diation source of the first aspect.

[0024] Embodiments of the material according to the third aspect can be formed together with any features or embodi- merits as described in connection with the electromagnetic ra diation source of the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings are included to provide a further understanding of embodiments and are incorporated in and constitute a part of this specification. The drawings il lustrate embodiments and together with the description serve to explain principles of embodiments. Other embodiments and many of the intended advantages of embodiments will be readi ly appreciated as they become better understood by reference to the following detailed description.

[0026] The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals desig nate corresponding similar parts.

[0027] Fig. 1 shows a schematic illustration of a method for the generation of THz radiation based on the transverse thermoelectric effect.

[0028] Fig. 2 comprises Fig. 2a and 2b and shows perspec tive schematic views of a material exemplifying the conven tional Seebeck effect (a) and an anisotropic material exem plifying the transverse thermoelectric effect (b).

[0029] Fig. 3 comprises Fig. 3a to 3c and shows a thin film of an anisotropic material deposited on an offcut sub strate (a), a layered structure of PdCoCt (b), and the tem perature dependent Seebeck coefficient of PdCoCh along the main crystallographic axes (c). [0030] Fig. 4 comprises Fig. 4a and 4b and shows a side view of a laser-induced transverse voltage (LITV) setup (a) and a diagram of LITV versus time generated by an excimer la ser (b).

[0031] Fig. 5 comprises Fig. 5a and 5b and shows a side view of an experimental setup for the generation of terahertz radiation (a) and terahertz signals versus time for two dif ferent offcut angles and two THz gating directions probing the THz radiation polarization state via electrooptical sam pling (b).

[0032] Fig. 6 comprises Fig. 6a and 6b and shows the te rahertz signal from a thin film of PdCo02 compared with that from a ZnTe crystal (a) and the corresponding THz emission frequency spectra (b).

DETAILED DESCRIPTION

[0033] In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodi ments in which the disclosure may be practiced. In this re gard, directional terminology, such as "top", "bottom", "front", "back", "leading", "trailing", etc., is used with reference to the orientation of the Figure(s) being de scribed. Because components of embodiments can be positioned in a number of different orientations, the directional termi nology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present dis closure is defined by the appended claims.

[0034] It is to be understood that the features of the various exemplary embodiments described herein may be com bined with each other, unless specifically noted otherwise.

[0035] As employed in this specification, the terms "bond ed", "attached", "connected", "coupled" and/or "electrically connected/electrically coupled" are not meant to mean that the elements or layers must directly be contacted together; intervening elements or layers may be provided between the "bonded", "attached", "connected", "coupled" and/or "electri cally connected/electrically coupled" elements, respectively. However, in accordance with the disclosure, the above- mentioned terms may, optionally, also have the specific mean ing that the elements or layers are directly contacted to gether, i.e. that no intervening elements or layers are pro vided between the "bonded", "attached", "connected", "cou pled" and/or "electrically connected/electrically coupled" elements, respectively.

[0036] Further, the word "over" used with regard to a part, element or material layer formed or located "over" a surface may be used herein to mean that the part, element or material layer be located (e.g. placed, formed, deposited, etc.) "indirectly on" the implied surface with one or more additional parts, elements or layers being arranged between the implied surface and the part, element or material layer. However, the word "over" used with regard to a part, element or material layer formed or located "over" a surface may, op tionally, also have the specific meaning that the part, ele ment or material layer be located (e.g. placed, formed, de- posited, etc.) "directly on", e.g. in direct contact with, the implied surface.

[0037] In the following, a detailed description will be presented of a new method for the generation of THz radiation based on the transverse thermoelectric effect (TTE), which can be realized in a simple experimental scheme by using com ^¬ mercial pulsed lasers and, for example, thin films of materi ^¬ als that exhibit an anisotropic Seebeck coefficient.

[0038] Experimental data will be presented that demon ^¬ strate THz generation from thin films of PdCoCh deposited on AI ₂ O ₃ offcut substrates. The obtained THz emission is linear ^¬ ly polarized with a bandwidth of at least v » 0.1 — 2.6THz. A direct comparison of the emitted THz electric field from PdCoCh thin films with a commercially available THz source - ZnTe (optical rectification) crystal - shows that they are already comparable in magnitude.

[0039] Fig. 1 shows a schematic illustration of a method for the generation of THz radiation based on the transverse thermoelectric effect.

[0040] The method involves femtosecond laser pulses, which create an ultrafast transient temperature gradient V _Z T, and thin films of material 1 with anisotropic Seebeck coefficient (the Seebeck coefficient has different values along at least two of the crystallographic directions a,b,c), such as layered transition metal oxides PdCoCt, La _2-x Sr _x CuC> ₄ , PtCoCt, Ca ₃ Co ₄ C> ₉ , etc., where S _a = S _b = S _ab ¹ S _c . The orientation of the thin film 1 is such that the main crystallographic axes are non- collinear with respect to the temperature gradient, V _Z TZ c=

Q. The THz radiation is generated on the principle of the transverse (or off-diagonal) thermoelectric effect (TTE), as follows (Fig. 1):

(1) The femtosecond laser pulse creates an ultrafast transient temperature gradient V _Z T in the thin film.

(2 ) The temperature gradient triggers a quasi-instant directed diffusion of hot charge carriers (electrons or holes, or an asymmetric combination of both), equivalent to an electric current - an in-plane current along the projection of the tilted c axis onto the film surface, i.e. perpendicular to the temperature gradient j _x 1 V _Z T "transverse current". This process is driven by the trans verse thermoelectric effect, where the Seebeck coefficient anisotropy (S _ab S _c ) establishes a thermoelectric field component E _x = ^ (S _ab S _c )V _Z T perpendicular to the temper ature gradient E _x 1 V _Z T (3) The abrupt changes in the electric current density j _x with time (accelera tion/deceleration of charge carriers) induced by the femtosecond laser pulse (at its arrival, passing through, and leaving the sample) are sources of THz radiation dj _x /dt E _THZ , i.e. a dipole radiation.

[ 0041 ] The TTE THz generation is not encumbered by the typical limitations of available THz sources. Specifically, it does not require an external voltage or magnetic field, cryogenics, phase-matching conditions, or expensive high- quality crystals. Since the TTE method is based on thin films 1 of metallic compounds with anisotropic Seebeck coeffi cients, layered transition metal oxides can offer potentially a large choice. This should allow realization of simple and cost effective THz generation with a broad bandwidth, highly scalable (femtosecond laser power / THz radiation power) out put power, and a large number of parameters for further opti- mization and manipulation of the THz emission characteris tics.

[0042] Fig. 2 comprises Fig. 2a and 2a and illustrates the conventional Seebeck effect in a material exposed to a tem perature gradient (a), and the transverse thermoelectric ef fect in an anisotropic material (b).

[0043] The conventional thermoelectric Seebeck effect re sults in the buildup of a thermoelectric field E _Z =SV _Z T, where S is the Seebeck coefficient, parallel to the tempera ture gradientV _Z T (Fig. 2a). The effect can emerge in any ma terial, regardless of its symmetry, as long as it contains mobile electric charge carriers and a temperature difference across the sample is present. The TTE (or off-diagonal ther moelectric effect) is relatively less studied. It occurs in anisotropic materials, with S _a = S _b = S _ab ¹ S _c (or lower sym metry), in a non-collinear geometry with respect to the tem perature gradient V _Z TZ c= Q (Fig. 2b) [2,3,4]. In this case, the thermoelectric field has two components - one collinear with the temperature gradient due to the conventional Seebeck effect, and one transverse to the temperature gradient E _{x 1} V _Z T due to the non-zero off-diagonal elements in the See beck coefficient tensor.

[0044] Fig. 2a illustrates the conventional Seebeck effect in materials exposed to a temperature gradient. The thermoe lectric field arises due to the thermal diffusion of charge carries from the hot to the cold end, in direction parallel to the temperature gradient E _z ||V _Z T. The Seebeck tensor (shown below Fig. 2a) has only diagonal components. [ 0045 ] Fig. 2b illustrates the transverse thermoelectric effect in anisotropic materials (S _a = S _b = S _ab ¹ S _c) in a non- collinear geometry of the crystal c axis with respect to the temperature gradient (V _z T z c = Q ) . In this case, the Seebeck tensor (shown below Fig. 2b) has non-zero off-diagonal ele ments resulting in thermoelectric field with component per pendicular to the temperature gradient (E _x 1 V _Z T).

[ 0046 ] The field components in the case of the TTE are as follows:

[ 0047 ] Fig. 3 comprises Fig. 3a to 3c and shows a thin film of an anisotropic material (S _a = S _b = S _ab ¹ S _c) deposited on offcut substrate, with an offcut angle Q (a), a layered crys tal structure of PdCoCh with a highly anisotropic and ambipo- lar Seebeck coefficient (b), and the temperature dependent Seebeck coefficient of PdCoCh along the main crystallographic axes - in-plane (ab), and along the c axis (c) [5].

[ 0048 ] The TTE can be realized in thin films of aniso tropic materials deposited on offcut substrates, which forces the growth of the crystal c axis at an angle Q with respect to the surface normal. A temperature gradient applied along the surface normal then generates a thermoelectric voltage along the offcut (ab/c) direction (Fig. 3a). The material in vestigated here is PdCoCh (Fig. 3b) - a material with a strongly anisotropic and ambipolar Seebeck coefficient (Fig. 3c) [5].

[ 0046 ] Fig. 4 comprises Fig. 4a and 4b and shows a side view of a laser-induced transverse voltage (LITV) setup (a) and a diagram of LITV versus time generated by an excimer la ser (b).

[ 0050 ] When the temperature gradient in such a TTE thin film structure is generated by a laser pulse (Fig. 4a), the resulting laser-induced transverse voltage (LITV) can be sub stantial - tens or even hundreds of volts [2,4,5]. The volt age components are given by:

U _x =E _x l l (2) (the transverse component)

U _y = 0 (3) (by symmetry)

U _z = E _z d = (S _ab sin ² 0+ S _c cos ² 0)A _Z T (4) (the conventional See beck effect component)

[ 0051 ] Here V _Z T = _z T/d, and D _Z G is the temperature dif ference at the surface and the back side of the film, d is the film thickness, and l is the laser spot diameter on the film surface.

[ 0052 ] Fig. 4a shows a side view of a laser-induced transverse voltage (LITV) setup. Laser pulses are used to create the temperature gradient. The sample is a thin film (S _a = S _b = S _ab ¹ S _c ) deposited on an oftout substrate V _Z T Z c = Q . The result is the generation of a voltage in direction along the projection of the tilted c axis onto the film surface (denoted here by (x) , or ( ab/c ) in Fig. 3a), and perpendicu lar to the temperature gradient { U _x 1 V _Z T ) .

[ 0053 ] Fig. 4b shows a typical LITV generated by an excim- er laser (wavelength l = 248 nm, pulse duration LPD = 5 ns, rep etition rate RR = 1 Hz ) in PdCoCh thin film on AIO offcut (0 = 10°) substrate. The large voltage ([/= 40 V) , and the short response time (FWHM = 7 ns) , are due to the large ambi- polar Seebeck coefficient anisotropy and the high electrical conductivity/thermal diffusivity in this material. The re sponse time thus may be limited by the laser pulse duration. The voltage signal t/(t) is measured by using Pt electrical contacts deposited on the film surface and an oscilloscope.

[ 0054 ] Thus, there are several obvious and important im plications of the TTE: (1) Large voltages can be generated in the material with no need for an external source. (2 ) Since the generated voltage is in the direction perpendicular to the temperature gradient, thin films can be used, which elim inates the need for large and expensive single crystals. (3) The voltage is proportional to the Seebeck coefficient ani sotropy ( _. S _ab — S _c ) , the offcut angle Q , and the aspect ratio (Z/d) , which can lead to a signal enhancement of ~10 ⁴ — 10 ⁵ compared to the conventional Seebeck effect - i.e. the system is highly scalable.

[ 0055 ] Fig. 5 comprises Fig. 5a and 5b and shows a side view of an experimental setup for the generation of terahertz radiation (a) and terahertz signals versus time for two dif- ferent offcut angles and two different THz gating directions probing the polarization state of the THz radiation (b).

[0056] Considering the extremely fast response measured in PdCoCh (see Fig. 4b), with a reaction time comparable to that of the laser pulse duration, it can be concluded that the re action time in this material may not be limited by its funda mental physical properties but rather by the laser pulse du ration. In order to examine the fundamental limits of the re sponse time in PdCoCh, an experiment was performed by utiliz ing a femtosecond laser (Fig. 5a), which resulted in an unbi ased generation of radiation with a frequency in the range of at least v « 0.1— 2.6 THz (Fig. 5b and Fig. 6).

[0057] Fig. 5a shows the experimental setup for the gener ation of THz emission (transmission mode) based on TTE. Opti cal laser pulses with characteristics (LPD « 100— 200/s, l = 800nm, RR = 150 kHz, fluence « 0.5 mj/cm ² ) were used. It is to be noted that no external bias is applied to the samples.

[0058] Fig. 5b shows the THz signal (emitted field) versus time (relative units) of PdCo0 ₂ films on AI ₂ O ₃ offcut sub strates. Two different types of thin films were investigated - with offcut angles 0 = 5° and 10°, respectively. The emitted THz radiation shows linear polarization - results for two different THz field gating directions probing the polariza tion state of the THz radiation are shown - (x) corresponding to the offcut direction U _X , E _X , and (y), which is along the in-plane (ab ) and is forbidden by symmetry U _y = 0, E _y = 0 (Eqs. 1-4).

[0059] Fig. 5b shows the first experimental data set of the generation of THz radiation due to the TTE. The depend- ence of the signal strength on the offcut angle 0 = 5°, 10°, as well as the linear polarization state of the THz field con firm that the origin of the emission is indeed the TTE, ac cording to Eqs. 1-4. Furthermore, control experiments were performed with a PdCoCh film deposited on normal substrate = 0°), and with a thin film of pure Pd deposited on offcut substrate = 10°), which both yielded no emission in the en tire measured frequency range. The THz field strength showed only a negligible dependence on the laser pump polarization.

[0060] As will be seen in the following a direct compari son of the emitted terahertz field of the PdCoCh films with a commercially available THz source - ZnTe crystal (optical rectification source) (Fig. 6) showed that they are already comparable in magnitude in the initial testing phase.

[0061] Fig. 6 comprises Fig. 6a and 6b and shows the te rahertz signal from a thin film of PdCo02 compared with that from a ZnTe crystal (a) and the corresponding THz emission frequency spectra (b).

[0062] Fig. 6a illustrates a comparison of the emission field from a commercially available ZnTe optical rectifica tion crystal and a thin film of PdCoCh on offcut substrate. (The comparison is done by replacing the PdCoCh thin film with the ZnTe crystal, Fig. 5a). The signal strength from the PdCoCh in this data set is 8 times smaller compared to that of the ZnTe source. However, in the course of further work, data were collected from other PdCoCh thin film samples (sam ples are still in testing phase - not yet optimized) that show much larger signal levels, of just 4 times smaller com pared to ZnTe. It should further be noted that the PdCoCh films can be further optimized - reference is made to points 1-12 in the text below.

[0063] Fig. 6b shows the corresponding THz emission fre quency domain. It should be noted also that the detector (al so based on ZnTe) sensitivity (v » 0.1— 2.6THz) and the laser pulse duration may limit the obtained emission bandwidth of PdCo0 ₂ ·

[0064] A preliminary analysis indicates that the magnitude of the generated field E _THz and the fast thermoelectric field reaction are facilitated by the large Seebeck coefficient an isotropy and the large electron mobility and thermal diffu- sivity in PdCo0 ₂ . In a complementary proof-of-concept experi ment with thin films of La _2-x Sr _x CuC>4, THz emission was ob served with a smaller signal level likely due to the smaller (S _{ab —} S _c ) and carrier mobility/thermal diffusivity compared to PdCo0 ₂ .

[0065] Below is a list with important aspects and ad vantages over other existing methods for THz generation:

1. The TTE method is relatively simple. Since the key re quirement is a material with an anisotropic Seebeck coeffi cient, the relatively large number of available layered tran sition metal oxides can offer here a potentially large choice.

2. The specificity of the TTE method allows one to take the full advantage of thin film technology. By varying the film material, substrate material (epitaxial strain), chemical doping, and/or by engineering a combination of materials, the THz emission field characteristics can be optimized. 3. The variation of the experimental parameters: film thick ness d, offcut angle Q, laser fluence (laser power/laser spot diameter l), represent additional parameters to influence the THz emission.

4. As ( _. S _ab — S _c ) , and all other relevant quantities, such as the electric conductivity and thermal diffusivity, are usual ly temperature dependent (Fig. 3c), cooling or heating can be used to tune the emitted THz signal strength, and/or the THz bandwidth.

5 . As the anisotropy in the Seebeck coefficient creates an intrinsic field, the THz generation does not generally re quire an external voltage source. It is, however, still pos sible to apply a bias voltage (and/or magnetic field) to the films, which creates a further opportunity to manipulate the emission characteristics.

6. Other means to manipulate the signal strength include mi crostructuring or micropatterning, so that the outgoing emis sion is enhanced by interference. Similarly, micro-antenna structures can be used for more efficient THz emission.

7. Based on the specificity of TTE, the emission is expected to be highly linearly polarized. This expectation was con firmed in the experiments on PdCo02.

8. So far, the measurements on PdCo0 ₂ thin films indicate that the emission spectrum (0.1— 2.6THz) and THz field strength are comparable to the most commonly used table top source, ZnTe, but the method may even yield a broader emis sion bandwidth; this requires further experiments.

9. The power of the emitted THz pulse scales with the one of the optical laser pulse. Since PdCoCh is a highly metallic compound, it is expected to be stable against laser induced damage even at high fluence, thus presumably allowing a high THz output power. 10. The small dimension of the TTE THz source allows for the engineering of compact THz integrated (on-chip) scalable de vices.

11. The TTE method imposes no limitations or constraints on the laser spot position or the laser spot size on the surface of the material.

12. It is possible that a coating - a protective layer, and/or a reflective, and/or an absorption layer is deposited onto the material, and/or the pump laser wavelength is tuned to optimize the absorption in the material, hence also to op timize the resulting temperature gradient in the material.

[0066] Although the disclosure has been shown and de scribed with respect to one or more implementations, equiva lent alterations and modifications will occur to others skilled in the art based at least in part upon a reading and understanding of this specification and the annexed drawings. The disclosure includes all such modifications and altera tions and is limited only by the concept of the following claims. In particular regard to the various functions per formed by the above described components (e.g., elements, re sources, etc.), the terms used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illus trated exemplary implementations of the disclosure. In addi tion, while a particular feature of the disclosure may have been disclosed with respect to only one of several implemen tations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. [0067] References

[1] P. Shumyatsky, R. R. Alfano, J. Biomed. Opt. 16, 033001 (2011). [2] H. Lengfellner, S. Zeuner, W. Prettl, and K.F. Renk, Eu- rophys. Lett. 25, 375 (1994).

[3] L. R. Testardi, Appl. Phys. Lett. 64, 2347 (1994)

[4] Th. Zahner, R. Schreiner, R. Stierstorfer, 0. Kus, S. T. Li, R. Roessler, J. D. Pedarnik, D. Bauerle, and H. Lengfell- ner, Europhys. Lett. 40, 673 (1997).

[5] P. Yordanov, W. Sigle, P. Kaya, M. E. Gruner, R. Pentche- va, B. Keimer, and H.-U. Habermeier, Phys. Rev. Mater. 3, 085403 (2019).