Spectral-temperature dependences of the refractive indices of nonlinear LBO crystals 30 × 30 × 10 mm3 in size are derived by terahertz time-domain spectroscopy in the 0.05–0.5-THz range upon heating from 20 to 200°C. Intersection of the spectral dependences of the refractive index components nx and ny is found at ∼84°C. The results can be used for selection of types and phase-matching conditions for frequency conversion into the long-wavelength part of the THz region, which is promising for monitoring the gas composition of the atmosphere due to the low sensitivity to aerosol attenuation.

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.

Water molecules exist in nature in the form of nuclear spin isomers ortho-H2O and para-H2O with the total spin of hydrogen nuclei I = 1 and 0, respectively. The effect of the vibrational excitation of the molecule on the conversion rate of nuclear spin isomers is studied in this work. It is found that the conversion of isomers is anomalously accelerated (by a factor of about 50) if the normal vibration v2 is excited. The spin conversion in the state 2v2 is accelerated by a factor of 260 compared to the ground state of the molecule. The effect is due to the rotational distortion of the water molecule and to the appearance of levels of ortho- and para-isomers of H2O with close energies in vibrational states v2 and 2v2. The predicted acceleration of conversion in the vibrationally excited states of water molecules can be used to enrich their spin isomers by laser radiation.

We have studied the structure, lattice dynamics, and dielectric properties (terahertz spectroscopy) of heteroepitaxial Sr0.5Ba0.5Nb2O6 films grown on MgO (001) substrates by rf cathode sputtering in an oxygen atmosphere. The results demonstrate that the films are heteroepitaxial and homogeneous and have high structural perfection. According to analysis of Raman spectra of the films measured at various temperatures in the range 299–433 K, the behavior of individual vibrational modes shows anomalies at T ≃ 390 ± 10 K, related to the phase transition. The unit cell of barium strontium niobate films 2.4–2.5 μm in thickness is shown to undergo strain both along and across the plane of the interface. Presumably, this is responsible for the increase in the width of the ferroelectric phase transition according to Raman spectroscopy data and also for the observed variations in the dielectric properties of the films in the terahertz frequency range.

Spectral dependences of components of the refractive index and absorption coefficient in a Li 2 B 4 O 7 (LB4) nonlinear crystal are determined in the THz range for the first time. Measurements have been carried out in the 0.15–1.6 THz (180–2000 μm) spectral range. The measured refractive index components are approximated by the Sellmeier dispersion equations subsequently used to determine the possible interaction types and to calculate the phase-matching angles for THz wave generation at the difference frequency between a Nd:YAG laser and a tunable laser source with a close wavelength. The efficiency of the process is estimated.

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.

Существующая тенденция роста несущих частот оптических линий связи (длина волны 1,5 мкм) до сотен ГГц [1] требует развития миллиметровых (субтерагерцовых) технологий. В частности, идет поиск материалов, обеспечивающих эффективное взаимодействие оптического и ИК излучения с миллиметровыми волнами (100–300 ГГц). Поскольку существует проблема ограниченного быстродействия устройств «классической» электроники на обозначенных частотах, пропагандируется оптико-оптическое преобразование лазерного излучения в миллиметровый диапазон и обратно, т.е. минуя устройства классической электроники. Такое преобразование может быть основано на принципах нелинейной кристаллооптики.

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.

The monoclinic crystal of guanylurea(1+) hydrogen phosphite (GUHP) is a novel efficient nonlinear-optical medium having high χ (3) and χ (2) values. We used THz-TDS with high polarization contrast to study the absorption coefficient and the refractive index of the crystal in the range of 0.2—1.5 THz. We found that the dielectric frame xyz at 1 THz is rotated 16 degrees from the visible range. Absorption spectra show peaks at 0.93, 1.02, and 1.45 THz representing high-Q resonances. The last two modes show very high Raman activity that can be associated with the interlayer collective movement of the crystal. We assume that GUHP has the potential to be a basis for active and passive terahertz devices manufacturing.

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.

Conversion of water nuclear-spin isomers by the quantum relaxation phenomenon is revisited and the high stability of water isomers is predicted. As an example, the isomer lifetime is found equal to 18 hours at room temperature and the water vapor pressure equal to similar or equal to 5 torr. Our calculations correct the prediction of rather short lifetime of water spin isomers, approximate to 1 s, made by Cacciani et al. [Phys. Rev. A 85, 012521 (2012)].

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°.

Optical properties of single axes Li2B4O7 (LB4) crystal are defined at room temperature in the spectral range 0.03˗0.5 THz. Dispersion of the refractive index components are approximated in the form of Sellmeier equations. Dispersion properties were used to determine phase-matching conditions for THz wave generation by collinear difference frequency generation processes. To the damage threshold under the pump by train of hundreds of 60 fs pulses of Ti:sapphire laser operating at 950 nm is found to be 250 TW/cm2, as well as the coherence length and efficiency of all possible types of three wave interactions are defined.

For the first time, we present the spectra of all three components of the refractive index and absorption coefficient of the KTiOAsO4 (KTA) crystal measured by the means of terahertz time-domain spectroscopy in the range of 0.3–2.1 THz. The dispersion of the refractive index is approximated and presented in the form of the Sellmeier equation. We observe a large birefringence ΔnZ-X ≈ 0.76 and dichroism which is attributed to a strong absorption peak in the vicinity of ~1.23–1.25 THz for the Z-axis. However, the crystal can be considered as almost uniaxial due to a close value of nX and nY as well as αX ≈ αY in the region below 0.5 THz. Moreover, KTA crystals can satisfy the phase-matching condition in principal XZ-plane for THz emission on difference frequency generation mechanism. Therefore, the crystal could be considered as an efficient candidate for terahertz wave generator under intense laser pump.

We present the spectra of the refractive index and the absorption coefficient of BiB3O6 (BIBO) crystal measured by the means of terahertz time-domain spectroscopy in the frequency range of 0.18–2.75 THz. We observe a large birefringence Δn = nZ − nX = 1.5 and the values of absorption coefficients of all three axes to be less than 5 cm−1 in the range below 1 THz. We report weak absorption lines in the vicinity of 2.28 and 2.6 THz for the Z-axis. Our results show that at room temperature BIBO has a lower absorption than other popular oxygen-containing nonlinear crystals: β–BBO, LBO, KTP, and LN. Thus, we assume the crystal to be an efficient THz radiation generator under intense laser pumping.

The high-performance broadband terahertz polarizers based on one-dimensional metal grids on carrying thin-film polymeric substrates were developed and manufactured. THz time-domain spectroscopy was used to study their power transmission (PT) and extinction ratio (ER) in the range of 0.2–2 THz. A tandem structure of two polarizers patterned on top of 4-μm-thick polypropylene film showed the best overall characteristics: PT > 95%, while ER exceeded the system dynamic range which spanned from 10 7 :1 to 10 4 :1. The developed polarizers can be used for polarization-sensitive measurements of anisotropic and optically active materials.