Article Terahertz Optical Properties of KTiOPO4 in the Temperature Range of (−192)–150 ◦C

Alina Rybak 1,2, Valery Antsygin 2, Alexander Mamrashev 2 and Nazar Nikolaev 2,*

1 Laboratory of Functional Diagnostics of Low-Dimensional Structures for Nanoelectronics, Novosibirsk State University, 630090 Novosibirsk, Russia; [email protected] 2 Terahertz Photonics Group, Institute of Automation and Electrometry SB RAS, 630090 Novosibirsk, Russia; [email protected] (V.A.); [email protected] (A.M.) * Correspondence: [email protected]

Abstract: This paper presents the results of an experimental study of the optical properties of highly resistive monocrystals of potassium titanyl phosphate (KTiOPO4, KTP) in the frequency range of 0.2–1 THz and the temperature range of (−192)–150 ◦C. The of the refractive indices is approximated in the form of Sellmeier equations. The results show that the temperature dependence of the Sellmeier coefficients for all three principal optical axes is close to linear and, most likely, does not experience an extremum in the vicinity of the activation temperatures of the cationic conductivity of the KTP crystal at (−73)–(−23) ◦C. Weak frequency dependence of an optical

axis direction angle VZ in the range of 0.2–1 THz is confirmed. However, the change in VZ with temperature is three times higher than reported before.

Keywords: potassium titanyl phosphate; KTiOPO4; terahertz spectroscopy; dispersion; ; Sellmeier equations; temperature dependence  

Citation: Rybak, A.; Antsygin, V.; Mamrashev, A.; Nikolaev, N. 1. Introduction Terahertz Optical Properties of Potassium titanyl phosphate crystal (KTiOPO4, KTP) is a well-known nonlinear optical KTiOPO4 Crystal in the Temperature material for efficient frequency conversion of radiation in the near- and mid-infrared − ◦ Range of ( 192)–150 C. Crystals range. The popularity of KTP originates in its high laser-induced damage threshold of up to 2021 11 , , 125. https://doi.org/ 30 GW/cm2 (single-shot measurement with 8.5 ns pulses at the wavelength of 1.064 µm [1]), 10.3390/cryst11020125 wide transparency range of 0.35−4.5 µm, sufficiently high nonlinear optical coefficients of d = 2.2 pm/V, d = 3.7 pm/V and d = 14.6 pm/V [2], and possibilities of manufacturing Received: 6 November 2020 31 32 33 periodically poled structures in the crystal. Accepted: 25 January 2021 Published: 27 January 2021 Presently, KTP is considered a promising material for converting infrared (IR) laser ra- diation to terahertz waves [3–8]. It is important for the development of small-size terahertz

Publisher’s Note: MDPI stays neutral (THz) radiation sources tunable in a wide frequency range and possessing high spectral with regard to jurisdictional claims in brightness. Among the tasks that could be solved with the help of such sources one should published maps and institutional affil- notice gas analysis, in particular, the construction of a terahertz Light Detection and Rang- iations. ing (LiDAR) for monitoring minor gas components in the ground layer of the atmosphere on kilometer-length paths for the environment and climate control [9,10]; investigation of nuclear spin isomers of molecules and their conversion [11,12]; development of compact accelerators for charged particles [13]; the advancement of to new spectral ranges [14] and study of selective effects of THz radiation on living organisms [15]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. In several works, the terahertz optical properties of KTP crystals were studied in This article is an open access article detail, and the collinear phase-matching conditions for the terahertz difference frequency distributed under the terms and generation (DFG) under IR laser pumping were calculated [3–6]. Numerical simulations conditions of the Creative Commons presented in [4] also confirm the possibility of frequency conversion within the terahertz Attribution (CC BY) license (https:// range. The possibility of generating THz waves by stimulated polariton scattering (SPS) creativecommons.org/licenses/by/ was experimentally demonstrated by the group of Prof. Yen-Chieh Huang [7,8]. In this 4.0/).

Crystals 2021, 11, 125. https://doi.org/10.3390/cryst11020125 https://www.mdpi.com/journal/crystals Crystals 2021, 11, 125 2 of 7

case, due to the higher laser-induced damage threshold, KTP was shown to be a more efficient converter than niobate [8]. It was demonstrated recently that cooling the KTP crystals to liquid nitrogen tem- perature significantly decreases their terahertz absorption coefficient [3,4]. This should increase the efficiency of the KTP crystal in THz photonics applications. Along with this, thermoelectric measurements demonstrated that the electrical conductivity mechanism of KTP crystals along the c-axis also changes its character with cooling. It was shown in [16,17] that at temperatures below −73 ◦C, electrons are the predominant charge carriers. In the temperature range of (−73)–(−23) ◦C, the conductivity is bipolar, and at T > −23 ◦C, cationic conductivity is dominant. The exact temperature values can vary due to crystal quality and doping. The observed change in the dielectric response of the crystal with temperature is determined by the gradual freezing of the motion of the K+ . This can potentially lead to an abrupt change in the terahertz dielectric response since it is mainly determined by phonon modes below 6 THz which are associated with vibrations of the potassium sublattice of the crystal [18]. An abrupt change in the dielectric response can potentially influence the efficiency of nonlinear THz-wave generation. Previously, KTP crystals were investigated only at room and liquid nitrogen tempera- tures. For a full characterization of the KTP crystal, it is also important to study in detail its terahertz optical properties at intermediate temperatures. This is the goal of the present work in which we comprehensively investigate KTP terahertz optical properties in the temperature range from −192 ◦C to +150 ◦C using terahertz time-domain spectroscopy. To increase measurement accuracy in the subterahertz range, samples with a thickness of about 3 mm were studied. The results are presented in the frequency range of 0.2−1 THz limited by the absorption of the thick samples and the dynamic range of the spectrometer.

2. Materials and Methods Two a- and b-cut samples with the dimensions of 9 × 9 × 3 mm3 were fabricated from KTP crystals. The crystals were grown by the and supplied by Castech Inc. (Fujian, China). Large facets were polished to high optical quality. The measured direct current conductivity of the samples along the c-axis was ~10-10 Ohm−1·cm−1 at room temperature. As in previous works [2–4], the following correspondence between the optical and crystallographic axes was accepted: x, y, z → a, b, c. The measurements were carried out using a conventional terahertz time-domain spec- trometer developed at the Institute of Automation and Electrometry SB RAS (Figure1 ). The spectrometer is based on the radiation of the second harmonic of a femtosecond Er-fiber laser (Toptica Photonics AG, Munich, Germany, λ = 775 nm, τ = 130 fs, P = 100 mW). An in- terdigitated photoconductive antenna (Batop GmbH, Jena, Germany) is used to generate THz radiation. Detection is carried out by the free-space electro-optic sampling technique in a 2-mm-thick ZnTe crystal [19,20]. The spectral range of the system is 0.2−2.5 THz; the dynamic range is more than 70 dB at the frequency of 0.3 THz. THz signals are acquired with a time step of 125 fs in the 60 ps range which corresponds to the spectral resolution of ~20 GHz. Statistical errors of the measured parameters are determined from four ex- perimental sets. Metal grid polarizers are used to increase the polarization contrast of the system. Before the measurement, samples are positioned so that the linear polarization of the THz wave is parallel to the principal optical axis under study. Processing of terahertz signals and calculation of the optical properties of samples were carried using the algorithm from [21]. Crystals 2021, 11, x FOR PEER REVIEW 3 of 7 Crystals 2021, 11, 125 3 of 7

Figure 1. Block diagram of the terahertz optical path of the spectrometer. Generator—interdigitated photoconductiveFigure 1. Block diagram antenna; of Detector—2the terahertz mm-thickoptical path ZnTe; of the OAPM—off-axis spectrometer. Generator—interdigitated parabolic mirror with photoconductive antenna; Detector—2 mm-thick ZnTe; OAPM—off-axis parabolic mirror with 50.6 50.6 mm diameter and focal length; P—metal grid polarizer; L—plano-concave TPX lens with mm diameter and focal length; P—metal grid polarizer; L—plano-concave TPX lens with 100 mm 100 mm effective focal length. effective focal length. A nitrogen bath cryostat with fused silica windows was used in the study. The cryo- A nitrogen bath cryostat with fused silica windows was used in the study. The cry- stat was equipped with a resistive heater attached to a neck of a sample holder. ostat was equipped with a resistive heater attached to a neck of a copper sample holder. This custom-made sample holder had two identical holes with a diameter of 7 mm in its This custom-made sample holder had two identical holes with a diameter of 7 mm in its lower flat part (Figure1). One hole accommodated samples under investigation; the other waslower used flat to part record (Figure reference 1). THzOne signalshole accommodated by moving the entiresamples cryostat under by investigation; a motorized lin-the earother translation was used stage to record (Newport, reference RI, USA) THz perpendicular signals by moving to the THzthe entire beam. cryostat The temperature by a mo- wastorized measured linear usingtranslation a chromel-alumel stage (Newport, thermocouple RI, USA) perpendicular fixed near the to sample. the THz The beam. temper- The aturetemperature was stabilized was measured with an using accuracy a chromel-al of ±0.5 umel◦C using thermocouple a computer-controlled fixed near the thermal sample. regulatorThe temperature TRM251 was (Oven, stabilized Moscow, with Russia). an accuracy of ±0.5 °C using a computer-controlled thermal regulator TRM251 (Oven, Moscow, Russia). 3. Results 3. ResultsSpectral dependences of KTP absorption coefficients at the temperatures of −192 ◦C; −150 Spectral◦C; −100 dependences◦C; −50 ◦C; of 25KTP◦C; absorption 50 ◦C; 100 coe◦Cffi andcients 150 at the◦C temperatures are shown in of Figure −192 2°C;. Absorption−150 °C; −100 coefficients °C; −50 °C; increase 25 °C; as50 the °C; radiation100 °C and frequency 150 °C are increases, shown whichin Figure is associated 2. Absorp- withtion thecoefficients presence increase of phonon as modesthe radiation above 1frequency THz. As shownincreases, earlier which in [ 3is], theassociated absorption with maximathe presence of phonon of phonon modes modes for αz aboveat room 1 THz. temperature As shown are nearearlier 1.75 in THz[3], the and absorption 2.2 THz; ◦ formaximaαy, near of phonon 2.15 THz; modes and forfor ααxz, at near room 2.44 temperature THz. At 25 areC near the values1.75 THz of and the absorption2.2 THz; for −1 −1 coefficientαy, near 2.15αz inTHz; the and range for of α 0.5x, near−1 THz2.44 increaseTHz. At from25 °С 5 the cm valuesto 15 of cm the absorption, while the othercoeffi- Crystals 2021, 11, x FOR PEER REVIEWtwo absorption z coefficients are close to each other, α ≈ α −,1 and they are−1 approximately2 of 3 five cient α in the range of 0.5−1 THz increase from x5 cmy to 15 cm , while the other two timesabsorption smaller coefficients than αz, which are close is in goodto each agreement other, αx with ≈ αy the, and previous they are measurements approximately [3 ,five4]. times smaller than αz, which is in good agreement with the previous measurements [3,4].

FigureFigure 2. Absorption 2. Absorption coefficients coefficients of of the thex x( a(a),), yy (b),), and and z z(c()c axes) axes at various at various temperatures. temperatures. Figure 2. Absorption coefficients of the x (a), y (b), and z (c) axes at various temperatures. Note that in [3,4], a weak broad absorption peak with a maximum of ~30 cm−1 in the −1 vicinityNote of 0.9that THz in [3,4], (λ = a 333 weakµm) broad was detectedabsorption in thepeakα zwithspectrum. a maximum This peak of ~30 is cm absent in inthe ourvicinity case atof all0.9 temperatures.THz (λ = 333 μ Perhapsm) was detected this indicates in the a α betterz spectrum. quality This of the peak KTP is absent crystals in studiedour case in at the all presenttemperatures. work. Perhaps Previously this observed indicates absorptiona better quality peak of can the be KTP associated crystals withstudied the in wavevector the present selection work. Previously rule breakdown observed due absorption to defect-induced peak can be disorder. associated As with a result,the wavevector the manifestation selection of rule acoustic breakdown phonon due oscillation to defect-induced can be seen disorder. in the low-frequency As a result, the absorptionmanifestation spectrum. of acoustic A similarphonon effectoscillation was observedcan be seen in in LiNbO the low-frequency3 crystal using absorption Raman spectroscopyspectrum. A [similar22]. In theeffect work was of observed Mounaix etin al.LiNbO [23],3 the crystal spectral using resolution Raman andspectroscopy spectral range[22]. In did the not work allow of unambiguous Mounaix et al. detection [23], the of spectral the peak; resolution however, and the absorptionspectral range coefficient did not

Figure 3. Dispersion of the refractive indices of the x (a), y (b), and z (c) axes at different temperatures. Symbols are measured data; continuous lines are approximations by Sellmeier equations.

Figure 4. Temperature dependence of the Sellmeier coefficients for the x (a), y (b), and z (c) axes. Symbols represent measured values; solid lines are the linear approximations.

Crystals 2021, 11, x FOR PEER REVIEW 4 of 7

Crystals 2021, 11, 125 4 of 7 allow unambiguous detection of the peak; however, the absorption coefficient of the slow −1 axisof the at 1 slow THz axis had at a 1value THz close had ato value 20 cm close that to corresponds 20 cm−1 that to corresponds our present toresult. our presentFigure 2a,b demonstrate that the values of αx and αy are below 5 cm−1 at frequencies−1 ≤ 0.75 THz at result. Figure2a,b demonstrate that the values of αx and αy are below 5 cm at frequencies all≤ 0.75temperatures. THz at all Upon temperatures. cooling to Upon −192 cooling°C, the absorption to −192 ◦C, along the absorptionall axes decreases along all to axesthe −1 valuesdecreases < 1 cm to the. values <1 cm−1. The temperature dependencies of the KTP refractive index spectra are presented in Crystals 2021, 11, x FOR PEER REVIEW The temperature dependencies of the KTP refractive index spectra are presented2 of 3 in FigureFigure 33.. TheThe datadata demonstrate demonstrate an an increase increase in thein the refractive refractive index index with with increasing increasing frequency. fre- quency.This is alsoThis dueis also to due the crystalto the crystal phonon phonon modes modes above above 1 THz. 1 THz. These These results results are in are good in goodagreement agreement with with the data the obtaineddata obtained at room at temperatureroom temperature and liquid and nitrogenliquid nitrogen temperature tem- z y peraturein the previous in the previous works. works. The refractive The refractive indices indices at 1 THz at 1 THz are n arez = 4.00,n = 4.00,ny = n 3.35, = 3.35,and and the ◦ thebirefringence at room at room temperature temperature is ∆ nis= Δnnz =− nnz y−= n 0.65.y = 0.65. At −At192 −192C, °C, the the refractive refractive indices in- dicesare ~10% are ~10% lower lower than than those those at room at room temperature. temperature. It can It becan seen be seen from from Figure Figure3 that 3 withthat withthe temperaturethe temperature change, change, the refractive the refractive index curvesindex curves shift in shift parallel, in parallel, practically practically without withoutchanging changing their shape. their shape.

Figure 2. Absorption coefficients of the x (a), y (b), and z (c) axes at various temperatures.

Figure 3. Dispersion of the refractive indices of the x (a), y (b), and z (c) axes at different temperatures. Symbols are Figuremeasured 3. Dispersion data; continuous of the linesrefractive are approximations indices of the byx (a Sellmeier), y (b), and equations. z (c) axes at different temperatures. Symbols are measured data; continuous lines are approximations by Sellmeier equations. The dispersion of the refractive indices in the range 0.2−1 THz for all temperatures is approximatedThe dispersion in the of form the refractive of Sellmeier indices equations: in the range 0.2−1 THz for all temperatures is approximated in the form of Sellmeier equations: B λ2 2 𝐵λ i 𝑛 n=i 𝐴= +Ai + (1)(1) λ 2 λ −𝐶− Ci Figure 3. Dispersion ofwhere the refractive Ai, Bi, indicesCi are ofthe the Sellmeier x (a), y (b ),coefficients and z (c) axes determ at differentined temperatures.from the experimental Symbols are data us- where A , B , C are the Sellmeier coefficients determined from the experimental data using measured data; continuousing thelines least-squares arei approximationsi i fit; iby stands Sellmeier for equations x, y, z; λ. is the wavelength in μm. The behavior of the the least-squares fit; i stands for x, y, z; λ is the wavelength in µm. The behavior of the Sellmeier coefficients with respect to the temperature change is shown in Figure 4. Sellmeier coefficients with respect to the temperature change is shown in Figure4.

Figure 4. Temperature dependence of the Sellmeier coefficients for the x (a), y (b), and z (c) axes. Symbols represent Figure 4. Temperature dependence of the Sellmeier coefficients for the x (a), y (b), and z (c) axes. Symbols represent measured values; solid lines are the linear approximations. Figure measured4. Temperature values; solid dependence lines are the of linearthe Sellmeier approximations. coefficients for the x (a), y (b), and z (c) axes. Symbols represent measured values; solid lines are theThe linear data approximations. in Figure4 show that the temperature dependences of Sellmeier coefficients

are well approximated by linear functions:

Ai(T) = A0i + δAiT Bi(T) = B0i + δBiT (2) Ci(T) = C0i + δCiT

Crystals 2021, 11, x FOR PEER REVIEW 5 of 7

The data in Figure 4 show that the temperature dependences of Sellmeier coeffi- cients are well approximated by linear functions:

Ai(T) = A0i + δAiT

Crystals 2021, 11, 125 Bi(T) = B0i + δBiT 5 of(2) 7

Ci(T) = C0i + δCiT The values of these coefficients obtained from experimental data are summarized in Table1The. values of these coefficients obtained from experimental data are summarized in Table 1. Table 1. The values of the Sellmeier coefficients. Table 1. The values of the Sellmeier coefficients. −4 −1 −4 −1 2 2 −1 Axis A0 δA × 10 ,K B0 δB × 10 ,K C0, µm δC, µm ·K Axis A0 δA × 10−4, K−1 B0 δB × 10−4, K−1 C0, μm2 δC, μm2·K−1 x 8.89 7.32 1.35 2.65 12,848.15 1.28 хy 8.89 9.14 7.32 9.36 1.35 1.31 2.65 4.54 12,848.15 12,857.17 1.28 1.68 yz 9.14 11.08 9.36 22 1.31 3.67 4.54 21.3 12,857.17 10,566.31 1.68 0.83 z 11.08 22 3.67 21.3 10,566.31 0.83 TheThe absenceabsence ofof aa strongstrong deviationdeviation fromfrom thethe linear linear dependence dependence maymay indicateindicate thatthat the the changechange inin thethe conductionconduction mechanismmechanism inin thethe crystalcrystal makesmakes anan insignificantinsignificant contributioncontribution to to thethe terahertzterahertz opticaloptical properties.properties. WeWe alsoalso estimatedestimated thethe deviationdeviation ofof thethe opticaloptical axisaxis directiondirection inin the the studied studied spectral spectral range. The value of the angle VZ is determined using the expression [24]: range. The value of the angle VZ is determined using the expression [24]: s n2 − n2 𝑛nz 𝑛 −𝑛y x sinsin 𝑉Vz== (3)(3) 2 − 2 𝑛ny 𝑛 n−𝑛z nx

wherewhere the the refractive refractive indices indices are are taken taken from from the the approximation. approximation. Figure Figure5 shows 5 shows the temper-the tem- atureperature dependence dependence of V ofZ for VZ thefor the frequencies frequencies of 0.2 of and0.2 and 1 THz. 1 THz.

FigureFigure 5.5.Temperature Temperature dependence dependence of of the theV VZZ angleangle forfor 0.20.2 andand 11 THz.THz.

◦ TheThe angleangle VVZZ forfor 0.20.2 THzTHz changeschanges fromfrom 12.612.6° atat thethe liquidliquid nitrogennitrogentemperature temperature toto ◦ 16.116.1°at at roomroom temperature.temperature. InIn ourour previous previous measurements measurements [ 4[4],], the the change change in in the the angle angleV VZZ ◦ ◦ waswas 3.5 3.5 times times smaller, smaller, from fromV VZ Z= =17 17°at atthe theliquid liquid nitrogen nitrogen temperature temperature to toV VZ Z= =18 18°at atroom room temperature.temperature.

4.4. ConclusionsConclusions ThisThis paperpaper presented the results results of of the the study study of of the the optical optical properti propertieses of ofKTP KTP crystals crys- − ◦ talsin the in frequency the frequency range range of 0.2–1 of 0.2–1 THz THzand the and temperature the temperature range rangeof (−192)–150 of ( 192)–150 °C. The re-C. The refractive indices of the optical indicatrix of the crystal were measured with linearly fractive indices of the optical indicatrix of the crystal were measured with linearly po- polarized terahertz radiation. The dispersion of the refractive indices was approximated in the form of Sellmeier equations. Experiments showed that the temperature dependence of the Sellmeier coefficients for all three crystal axes is close to linear. This indicated no extremum in the vicinity of the activation temperature of the ionic conductivity of the KTP crystal. The absorption coefficient αz exhibited smooth dependence with respect to the crystal temperature. The absence of a broad weak absorption peak in the vicinity of 0.9 THz, which was observed in other studies, was most likely due to the better quality of the KTP crystal used in present study. Crystals 2021, 11, 125 6 of 7

Based on the conducted studies, we conclude that the change in the mechanism of KTP electrical conductivity along the c-axis with its cooling influences the terahertz optical properties insignificantly. Therefore, this should not affect the nonlinear optical processes occurring within the terahertz range or based on interactions with phonons that are associated with vibrations of potassium sublattice. Since the millimeter-wave range (<300 GHz) is free from strong absorption lines of atmospheric water, cooled high-quality KTP crystals can be of interest for developing a terahertz LiDAR. Indeed, KTP crystals have lower absorption and birefringence compared to the family of doped and undoped LiNbO3 crystals [25] and can be used for efficient generation of THz radiation by collinear phase matching. The change in nZ with cooling down to liquid nitrogen temperature is 0.12 in our case, which is comparable to its variation from crystal to crystal. We estimate this can lead to a shift in the phase-matching curves within 1◦ which can be considered insignificant for the case of free space optical alignment where it is possible to compensate for the difference by rotating the crystal. Otherwise, according to the phase-matching curves from [5], 1-degree detuning can lead to a significant shift in the generated difference frequency. Consequently, it should be taken into account for the correct design of nonlinear integrated terahertz photonic devices.

Author Contributions: Conceptualization, V.A.; spectra measurement and original draft preparation A.R. and V.A.; experimental data processing and analysis, A.R. and A.M.; writing—review and editing, N.N. and A.M.; supervision, N.N.; All authors have read and agreed to the published version of the manuscript. Funding: The work is supported by the Russian Science Foundation, project No 17-12-01418. ¯ Informed Consent Statement: Not applicable. Acknowledgments: The authors thank P. L. Chapovsky and N. V. Surovtsev for useful suggestions and help in the manuscript preparation. The authors acknowledge the Shared Equipment Center CKP “Spectroscopy and Optics” of the Institute of Automation and Electrometry SB RAS for the provided terahertz spectrometer and the Shared Equipment Center CKP “VTAN” (ATRC) of the NSU Physics Department for the high-performance terahertz polarizers provided for measurements. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Bolt, R.J.; van der Mooren, M. Single Shot Bulk Damage Threshold and Conversion Efficiency Measurements on Flux Grown KTiOPO4 (KTP). Opt. Commun. 1993, 100, 399–410. [CrossRef] 2. Nikogosyan, D.N. Nonlinear Optical Crystals: A Complete Survey; Springer: New York, NY, USA, 2005; pp. 54–60. ISBN 0387220224. 3. Antsygin, V.D.; Kaplun, A.B.; Mamrashev, A.A.; Nikolaev, N.A.; Potaturkin, O.I. Terahertz Optical Properties of Potassium Titanyl Phosphate Crystals. Opt. Express 2014, 22, 25436. [CrossRef] 4. Mamrashev, A.; Nikolaev, N.; Antsygin, V.; Andreev, Y.; Lanskii, G.; Meshalkin, A. Optical Properties of KTP Crystals and Their Potential for Terahertz Generation. Crystals 2018, 8, 310. [CrossRef] 5. Huang, J.-G.; Huang, Z.-M.; Nikolaev, N.A.; Mamrashev, A.A.; Antsygin, V.D.; Potaturkin, O.I.; Meshalkin, A.B.; Kaplun, A.B.; Lanskii, G.V.; Andreev, Y.M.; et al. Phase Matching in RT KTP Crystal for Down-Conversion into the THz Range. Laser Phys. Lett. 2018, 15, 075401. [CrossRef] 6. Nikolaev, N.A.; Lansky, G.V.; Andreev, Y.M.; Ezhov, D.M.; Krekov, M.G.; Lisenko, A.A. β-BBO, LBO, AND KTP Nonlinear Crystals as Sources of Millimeter-Wave Radiation. Russ. Phys. J. 2020, 63, 1025–1029. [CrossRef] 7. Wu, M.-H.; Tsai, W.-C.; Chiu, Y.-C.; Huang, Y.-C. Generation of ~100 KW Narrow-Line Far-Infrared Radiation from a KTP off-Axis THz Parametric Oscillator. Optica 2019, 6, 723. [CrossRef] 8. Wu, M.-H.; Chiu, Y.-C.; Wang, T.-D.; Zhao, G.; Zukauskas, A.; Laurell, F.; Huang, Y.-C. Terahertz Parametric Generation and Amplification from Potassium Titanyl Phosphate in Comparison with and . Opt. Express 2016, 24, 25964. [CrossRef] 9. Bigourd, D.; Cuisset, A.; Hindle, F.; Matton, S.; Fertein, E.; Bocquet, R.; Mouret, G. Detection and Quantification of Multiple Molecular Species in Mainstream Cigarette Smoke by Continuous-Wave Terahertz Spectroscopy. Optics Lett. 2006, 31, 2356. [CrossRef] Crystals 2021, 11, 125 7 of 7

10. Hsieh, Y.D.; Nakamura, S.; Abdelsalam, D.G.; Minamikawa, T.; Mizutani, Y.; Yamamoto, H.; Iwata, T.; Hindle, F.; Yasui, T. Dynamic Terahertz Spectroscopy of Gas Molecules Mixed with Unwanted Aerosol under Atmospheric Pressure Using Fibre-Based Asynchronous-Optical-Sampling Terahertz Time-Domain Spectroscopy. Sci. Rep. 2016, 6, 28114. [CrossRef] 11. Mamrashev, A.A.; Maximov, L.V.; Nikolaev, N.A.; Chapovsky, P.L. Detection of Nuclear Spin Isomers of Water Molecules by Terahertz Time-Domain Spectroscopy. IEEE Trans. Terahertz Sci. Technol. 2018, 8, 13–18. [CrossRef] 12. Zhukov, S.S.; Balos, V.; Hoffman, G.; Alom, S.; Belyanchikov, M.; Nebioglu, M.; Roh, S.; Pronin, A.; Bacanu, G.R.; Abramov, P.; et al. Rotational Coherence of Encapsulated Ortho and Para Water in Fullerene-C60 Revealed by Time-Domain Terahertz Spectroscopy. Sci. Rep. 2020, 10, 18329. [CrossRef][PubMed] 13. Nanni, E.A.; Huang, W.R.; Hong, K.H.; Ravi, K.; Fallahi, A.; Moriena, G.; Dwayne Miller, R.J.; Kärtner, F.X. Terahertz-Driven Linear Electron Acceleration. Nat. Commun. 2015, 6, 9486. [CrossRef] 14. Hafez, H.A.; Kovalev, S.; Tielrooij, K.; Bonn, M.; Gensch, M.; Turchinovich, D. Terahertz Nonlinear Optics of Graphene: From Saturable Absorption to High-Harmonics Generation. Adv. Opt. Mater. 2020, 8, 1900771. [CrossRef] 15. Cherkasova, O.P.; Serdyukov, D.S.; Ratushnyak, A.S.; Nemova, E.F.; Kozlov, E.N.; Shidlovskii, Y.V.; Zaytsev, K.I.; Tuchin, V.V. Effects of Terahertz Radiation on Living Cells: A Review. Opt. Spectrosc. 2020, 128, 855–866. [CrossRef] 16. Antsigin, V.D.; Gusev, V.A.; Semenenko, V.N.; Yurkin, A.M. Ferroelectric and Nonlinear Optical Properties of Ferroelectric- Superionic Ktp. Ferroelectrics 1993, 143, 223–227. [CrossRef] 17. Urenski, P.; Gorbatov, N.; Rosenman, G. Dielectric Relaxation in Flux Grown KTiOPO4 and Isomorphic Crystals. J. Appl. Phys. 2001, 89, 1850–1855. [CrossRef] 18. Kugel, G.E.; Brehat, F.; Wyncke, B.; Fontana, M.D.; Marnier, G.; Carabatos Nedelec, C.; Mangin, J. The Vibrational Spectrum of a Ktiopo4 Studied by Raman and Infrared Reflectivity Spectroscopy. J. Phys. C Solid State Phys. 1988, 21, 5565–5583. [CrossRef] 19. Wu, Q.; Zhang, X.C. Free-Space Electro-Optic Sampling of Terahertz Beams. Appl. Phys. Lett. 1995, 67, 3523. [CrossRef] 20. Mamrashev, A.A.; Potaturkin, O.I. Characteristics of the System of Polarization-Optical Detection of a Pulsed Terahertz Spectrom- eter. Optoelectron. Instrum. Data Process. 2011, 47, 332–337. [CrossRef] 21. Duvillaret, L.; Garet, F.; Coutaz, J.L. A Reliable Method for Extraction of Material Parameters in Terahertz Time-Domain Spectroscopy. IEEE J. Sel. Topics Quantum Electron. 1996, 2, 739–745. [CrossRef] 22. Surovtsev, N.V.; Malinovskii, V.K.; Pugachev, A.M.; Shebanin, A.P. The Nature of Low-Frequency Raman Scattering in Congruent Melting Crystals of Lithium Niobate. Phys. Solid State 2003, 45, 534–541. [CrossRef] 23. Mounaix, P.; Sarger, L.; Caumes, J.P.; Freysz, E. Characterization of Non-Linear Potassium Crystals in the Terahertz Frequency Domain. Opt. Commun. 2004, 242, 631–639. [CrossRef] 24. Dmitriev, V.G.; Gurzadyan, G.G.; Nikogosyan, D.N. Handbook of Nonlinear Optical Crystals. In Optical Sciences; Springe: Berlin/Heidelberg, Germany, 1991; Volume 64, p. 16; ISBN 978-3-662-13832-8. 25. Pálfalvi, L.; Hebling, J.; Kuhl, J.; Péter, A.; Polgár, K. Temperature and Composition Dependence of the Absorption and Refraction of Mg-Doped Congruent and Stoichiometric LiNbO3 in the THz Range. In Proceedings of the Optics InfoBase Conference Papers, Optical Society of America (OSA), Vienna, Austria, 6 February 2005; p. WB4.