All three components of the refractive index of a GUHP crustal in the visible, near-infrared, and terahertz spectra were thoroughly studied. The results obtained allow for the calculation with high accuracy of phase matching conditions for nonlinear frequency conversion of different laser sources.

Gallium selenide nonlinear optical crystals are effective up- and down-converters of terahertz and infrared frequencies. Their nonlinear characteristics at the telecommunication wavelength have not received enough attention, particularly for sulfur-doped crystals. In order to close this gap, we put forward data on the optical and electro-optical characteristics of GaSe(1-x)Sx crystals (where x = 0, 0.03, 0.12, 0.16, and 0.22) at the wavelength of 1.55 micrometers and in the terahertz range. This study provides an estimate of the GaSe:S crystal’s nonlinear coefficient d22(ω1, ω2, ω3) responsible for the terahertz wave generation by means of optical rectification and difference frequency generation, where ω1 ≈ ω2 – corresponds to the near infrared range, and ω3 belongs to the subterahertz frequencies. We assume that the value of d22 lies within the range from 13.9 to 20.5 pm/V for the wavelengths of 0.63 to 1.55μm. The potential of S-doped GaSe crystals to be used as nonlinear optical converters for photonic devices operating at telecom wavelengths is discussed.

Terahertz (THz) frequency generation via nonlinear optical techniques is of particular interest due to the immense potential of this type of radiation in various scientific fields, ranging from medicine to telecommunications. Selecting suitable nonlinear media for laser frequency down-conversion presents a challenging task. Considering an approach that uses nonlinear crystals with high radiation resistance, pumped by intense laser pulses near their damage threshold, we suggest the crystal of bismuth triborate (BiB3O6, BIBO). Compared to other borate-class crystals, BIBO exhibits relatively high coefficients of quadratic susceptibility. In this paper, we have studied the optical properties of BIBO samples in a wide spectral range from 0.1 to 2.1 THz at temperatures of 473, 383, 295, and 77 K using Terahertz Time-Domain Spectroscopy (THz-TDS). Furthermore, we simulated collinear three-wave interactions with nonzero efficiency for difference frequency generation (DFG) in the THz range. For the pump wavelengths of about 800 nm, we determined phase-matching (PM) conditions and compared the generation efficiency for different crystal cuts. The potential of utilizing BIBO crystal for terahertz frequency generation is discussed.

Recently, the semiorganic crystal of guanylurea hydrogen phosphite was revealed as an extremely efficient narrow-band terahertz radiation source. It is proposed that the emission is closely connected with the excitement of high-quality phonon oscillations, which in turn necessitates a detailed characterization of the optical properties of the crystal in this range. The extreme dispersion of the angle between dielectric and crystallographic coordinates, permitted by the monoclinic syngony of the crystal, was estimated to be 2.3° per 100 GHz. The dielectric susceptibility of the crystal measured by means of time-domain spectroscopy is determined by the high-Q phonon at 1.45 THz for ETHz||x, which is responsible for the efficient terahertz generation. Also, two modes were found at 1.02 and 0.92 THz for ETHz||z and ETHz||y, respectively. All three components of the refractive index were measured in the range of 0.4–2 THz. We believe that modification of the crystal structure will allow tuning the characteristics of the narrowband terahertz source based on it.

Based on electro-optical measurements in the THz region and comparison with data from other authors, estimates of the nonlinear optical coefficient of GaSe crystals with varied degrees of sulfur doping are provided. The d22 value for GaSe was 13.9 pm/V at a laser wavelength of 1.55 microns. The acquired data are compared with the values of nonlinear coefficients for undoped GaSe samples provided in the works of other authors. The qualitative correspondence of experimental measurements of the nonlinearity coefficient and the empirical model based on Miller's rule in the range of 0.63 – 1.55 microns is shown. The inaccuracy of theoretical curve in comparison to the experimental one is around 10%. The techniques described in the paper for calculating the nonlinear optical coefficient's magnitude and the results obtained will make it possible to establish the foundation for the creation of radiophotonics devices based on unalloyed and sulfur-doped GaSe crystals.

We studied terahertz (THz) optical and dielectric properties of stoichiometric lithium tantalate (sLT) from room down to liquid nitrogen temperature in the range of 0.15–1.8 THz using terahertz time-domain spectroscopy. Two-oscillator Lorentz models were fitted well to the crystal properties for both ordinary and extraordinary waves. We also studied changes in sLT ultraviolet (UV) absorption from room to liquid nitrogen temperature. The measurements showed a significant drop in absorption in both the THz and UV ranges with cooling. According to these results cooling should increase lithium tantalate potential in optical-to-terahertz conversion of high-power 800-nm radiation. The measured properties can be used in designing nonlinear optical conversion schemes and devices based on sLT.

We present the thorough studies of optical properties of BiB 3 O 6 (BIBO) crystal in the millimeter-wave (subterahertz) range. We observe a large birefringence Δn = n Z -n X = 1.5 and the values of absorption coefficients of all three axes to be less than 0.5 cm −1 at the frequency of 0.3 THz. The difference from visible range in angle ϕ between the dielectric axis z and crystallophysical axis X is found to be more than 6°. The simulated phase-matching curves in the xz plane of the crystal show the optimal value of the angle θ to be around 25.5°±1° for an efficient millimeter-wave generation under the pump of 1064 nm laser radiation.

Due to their high optical damage threshold, borate crystals can be used for the efficient nonlinear down-conversion of terawatt laser radiation into the terahertz (THz) frequency range of the electromagnetic spectrum. In this work, we carried out a thorough study of the terahertz optical properties of the lithium tetraborate crystal (Li2B4O7; LB4) at 295 and 77 K. Approximating the terahertz refractive index in the form of Sellmeier’s equations, we assessed the possibility of converting the radiation of widespread high-power laser sources with wavelengths of 1064 and 800 nm, as well as their second and third harmonics, into the THz range. It was found that four out of eight types of three-wave mixing processes are possible. The conditions for collinear phase matching were fulfilled only for the o − e → o type of interaction, while cooling the crystal to 77 K did not practically affect the phase-matching curves. However, a noticeable increase of birefringence in the THz range with cooling (from 0.12 to 0.16) led to an increase in the coherence length for o − o → e and e − e → e types of interaction, which are potentially attractive for the down-conversion of ultrashort laser pulses.

We present the thorough studies of dielectric properties of BiB3O6 (BIBO) crystal in the subterahertz range. We observe a large birefringence Δn = nZ −nX = 1.5 and the values of absorption coefficients of all three axes to be less than 0.5 cm−1 at the frequency of 0.3 THz. The difference from visible range in angle Φ between the dielectric axis z and crystallophysical axis χ is found to be more than 6°. The simulated phase-matching curves in the xz plane of the crystal show the optimal value of the angle θ to be around 25.5°±1° for an efficient millimeter-wave generation under the pump of 1064 nm laser radiation.

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 dispersion 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.

We suggest a method to simultaneously measure terahertz optical properties of crystals along two axes in terahertz time-domain spectroscopy using two broadband polarizers. We demonstrate the method by measuring temperature dependence of refractive index and absorption coefficient of lithium triborate crystal along different optical axes.

We propose a method for measuring the terahertz properties for two principal optical axes of anisotropic crystals without optical activity using terahertz time-domain spectroscopy (THz-TDS). The method put forward in this paper utilizes the inherent polarization sensitivity of the THz-TDS electro-optic detection system. We demonstrate the practical application of the method by measuring the temperature dependence of the refractive index and the absorption coefficient of a lithium triborate crystal for three optical axes.

В работе предложен метод исследования ТГц свойств анизотропных материалов без оптической активности, учитывающий чувствительность систем электрооптической регистрации импульсных ТГц спектрометров к поляризации ТГц излучения. Для этого проанализирована зависимость чувствительности системы регистрации на основе кристаллов типа цинковой обманки со срезами (110) и (111) от поляризации терагерцового и зондирующего лазерного излучения. Обнаружено, что одинаковая чувствительность может быть достигнута для двух ортогональных поляризаций терагерцового излучения. Так, для ТГц излучения линейно поляризованного под углами ±45° относительно осей z и (−211) кристаллов-детекторов со срезами (110) и (111), соответственно, достигается одинаковая чувствительность со значением √2 2 от максимальной. Это справедливо для излучения зондирующего лазера, поляризованного под углами 0° или 90°.

We study optical properties of 1% Nd-doped KGW crystal by terahertz time-domain spectroscopy in the range of 0.2–2.1 THz. We find dispersion of the measured refractive indices in the studied frequency range. Their values for the three optical axes are ng = 3.75, nm = 3.37, np = 3.40 at 1 THz indicating the birefringence of Δn = ng – nm ≈ 0.38. We observe an increasing absorption for all three axes with dichroism up to 20 cm−1 at 2 THz. We attribute the main source of the dispersion and increasing absorption to phonon oscillations at 2.58 and 2.7 THz outside of the studied frequency range.

Кристаллы семейства боратов нашли широкое применение в различных областях нелинейной оптики благодаря их преимуществам: относительно большому эффективному нелинейному коэффициенту, широкому спектральному диапазону прозрачности, высокому порогу разрушения и т. д. [1].К типичным представителям кристаллов этого семейства относятся бета-борат бария (β-BaB 2 O 4 , BBO), триборат лития (LiB 3 O 5 , LBO) и триборат висмута (BiB 3 O 6 , BIBO).В последнее время исследователи начали проявлять интерес к применению этих материалов в терагерцовой (ТГц) области спектра.Измерены их терагерцовые оптические свойства -показатель преломления и коэффициент поглощения при комнатной температуре и температуре жидкого азота [2-4].В работе рассмотрены перспективы нелинейного преобразования излучения волоконных лазеров (длины волн 1 мкм и 1,5 мкм) в терагерцовое за счет генерации разностных частот (ГРЧ).Наиболее перспективным представляется преобразование в излучение суб-ТГц (миллиметрового) диапазона благодаря большей длине когерентности и низкому поглощению на фононных модах.Этот диапазон актуален для телекоммуникационных систем нового поколения, в т. ч. 6G.В качестве примера на рис. 1 приведены расчётные кривые коллинеарного фазового согласования для процесса ГРЧ в кристалле BBO и сравнение его коэффициента поглощения с другими боратами в ТГц области.Рис. 1. Кривые фазового синхронизма ГРЧ в кристалле BBO при накачке на длине волны 1,064 мкм при температуре 300 К (сплошная линия) и 77 К (пунктир), слева.Спектры поглощения кристаллов семейства боратов в ТГц спектральном диапазоне, справа. Литература

We report terahertz optical properties of KGW single crystal. The dispersion of the refractive index is approximated in the form of Sellmeier equations. Possibilities of the crystal applications for optical-to-terahertz and terahertz-to-terahertz radiation conversion are discussed.

We report terahertz optical properties of 1% Nd-doped potassium gadolinium tungstate (KGW) monocrystals studied by terahertz time-domain spectroscopy in the range of 0.25–2.1 THz. KGW refractive index and absorption coefficient are measured for waves polarized parallel to all three axes of optical indicatrix Ng, Nm, Np. Typical values for the refractive indices are ng = 3.75, nm = 3.37, np = 3.40 at 1 THz corresponding to rather large birefringence of ng – nm ≈ 0.38. We find that KGW exhibits dispersion in the studied spectral range with the refractive index ng, for example, significantly increasing by the value of 0.5. We approximate the dispersion of the refractive index in the form of Sellmeier equations. Absorption coefficient behaves similarly for all three optical axes and it is less than 5 cm-1 for the frequencies below 1 THz. It rises significantly up to 50 cm-1 at higher frequencies where we also observe dichroism of more than 10 cm-1. Observed properties can be attributed to vibrational modes at 2.58 and 3.39 THz found by Raman and IR spectroscopy in previous studies. However, we find no particular features in the spectral range under investigation in contrast to Raman and IR spectroscopy results. Relatively high cubic nonlinearity of KGW crystal, its small low-frequency terahertz absorption and high birefringence indicate an opportunity for its usage for optical-to-optical and optical-to-terahertz conversion.