A generator of far-infrared and terahertz radiation in nonlinear metamaterials exhibiting negative index of refraction

Laser oscillation conditions are analyzed in nonlinear composite metamaterials exhibiting a negative index of refraction. It is demonstrated that strong quadratic and cubic nonlinearities can be simultaneously realized in such structures. Based on the described composite material, a scheme of an optically pumped parametric generator of terahertz radiation is proposed.     

Coherent diffraction radiation as a source of radiation in far-infrared and terahertz range

The results of measurements of the coherent diffraction radiation yield generated by femtosecond electron beam are presented. It is shown that the simulated and experimental data agreed quite well. The coherent diffraction radiation can be considered as a real candidate to develop a source of radiation in far-infrared and terahertz range.     

Highly selective terahertz optical frequency comb generator

Using a 10.5-GHz resonant electro-optic modulator placed inside a resonant optical cavity, we generated an optical frequency comb with a span wider than 3 THz. The optical resonator consists of three mirrors, with the output coupler being a thin Fabry-Perot cavity with a free spectral range of 2 THz and a finesse of 400. Tuning this filter cavity onto resonance with a particular high-order sideband permits efficient output coupling of the desired sideband power from the comb generator, while keeping all other sidebands inside for continued comb generation. This spectrally pure output light was then heterodyne detected by another laser with a frequency offset of the order of 1 THz.     

Generating terahertz radiation by means of optical rectification

Various means of raising the efficiency of generating wideband terahertz radiation on the basis of optical rectification are analyzed.     

Excitonic optical absorption in semiconductors under intense terahertz radiation

The excitonic optical absorption of GaAs bulk semiconductors under intense terahertz （THz） radiation is investigated numerically. The method of solving initial-value problems, combined with the perfect matched layer technique, is used to calculate the optical susceptibility. In the presence of a driving THz field, in addition to the usual exciton peaks, 2p replica of the dark 2p exciton and even-THz-photon-sidebands of the main exciton resonance emerge in the continuum above the band edge and below the main exciton resonance. Moreover, to understand the shift of the position of the main exciton peak under intense THz radiation, it is necessary to take into consideration both the dynamical Franz-Keldysh effect and ac Stark effect simultaneously. For moderate frequency fields, the main exciton peak decreases and broadens due to the field-induced ionization of the excitons with THz field increasing. However, for high frequency THz fields, the characteristics of the exciton recur even under very strong THz fields, which accords with the recent experimental results qualitatively.     

Transmission of terahertz radiation using a microstructured polymer optical fiber

A hollow-core microstructured polymer optical fiber was analyzed in the terahertz (THz) region. Spectral analysis of time domain data shows propagation of THz waves in both the hollow-core and the microstructured cladding with a time delay of approximately 20 ps. The frequency range and shift of the transmission bands between different sized waveguides suggested photonic bandgap or resonant guidance. Finite-difference time domain calculations agree relatively well to the experimental transmission results. Propagation losses were estimated to be as low as 0.9 dB/cm.     

Pump-enhanced optical parametric oscillator generating continuous wave tunable terahertz radiation

We demonstrate a tunable cw terahertz (THz) parametric oscillator based on periodically poled MgO-doped lithium niobate, directly converting the 1030 nm pump wave into the THz regime. The tunability ranges from 1.2 to 2.9 THz at output power levels between 0.3 and 3.9 μW. To overcome the high pump threshold caused by THz absorption in the nonlinear crystal, we employ an enhancement cavity with a finesse of 500 at the pump wavelength. The intracavity pump threshold at 1.4 THz is measured to be 350 W for a crystal length of 2.5 cm.     

Transient optical modulation properties in the terahertz metamaterial of split ring resonators

The ultrafast optical modulation properties of split ring resonators are characterized by utilizing optical pump-terahertz probe spectroscopy.The experimental results show that when the terahertz electric vector is perpendicular to the gap of the split ring resonator,resonant absorption can be quenched significantly under high pump excitation.However,when the terahertz electric vector is parallel to the gap,the resonant absorption is less sensitive to pump excitation due to the structural properties of the metamaterial.Our numerical simulations also demonstrate that the pump pulse significantly influences the split ring resonator current by generating carriers in the substrate.     

Generation of terahertz radiation upon the optical breakdown of a gas

Terahertz radiation of plasma oscillations excited upon the optical (axicon) breakdown of a gas in the presence of external fields of other frequency ranges has been analyzed. It has been shown that the spectra and intensity of oscillations and radiation generated by them depend strongly on the character of the spatiotemporal evolution of the formed plasma. The intensity is maximal upon the rapid formation of the plasma with a sharp boundary and decreases to very low values for objects with a smooth density profile. New schemes and regimes of break-down have been proposed, which make possible, according to preliminary estimates, an emitted energy of about 10 mJ with moderate intensities of ionizing laser radiation.     

Generation of terahertz radiation in the regime of resonant-nonresonant optical rectification

The generation of broadband terahertz radiation in quadratically nonlinear crystals containing additional resonant impurities having a permanent dipole moment has been theoretically analyzed. It has been shown that the contributions to the intensity of the generated radiation from resonance and nonresonance mechanisms of optical rectification are nonadditive. In semiconductor nanostructures and systems of organic molecules, the energy efficiency of generation due to the resonance mechanism can reach about 0.001, which is much higher than the corresponding efficiencies of the currently widespread tilted front technique. The spectra of the input optical pulses exhibit a noticeable red shift due to the resonance-nonresonance rectification.     

On the generation of terahertz radiation by a phase-modulated optical pulse

The problem of terahertz radiation generation in a quadratic nonlinear medium by a nanosecond phase-modulated optical pulse possessing the properties of supercontinuum has been investigated. The possibility of generating both broadband and quasi-monochromatic terahertz signals has been demonstrated. The conditions have been revealed under which the generation efficiency of a phase-modulated nanosecond optical pulse is greater than that of a spectrally confined femtosecond signal.     

Observation of broadband self-amplified spontaneous coherent terahertz synchrotron radiation in a storage ring

Bursts of coherent synchrotron radiation at far-infrared and millimeter wavelengths have been observed at several storage rings. A microbunching instability has been proposed as the source for the bursts. However, the microbunching mechanism has yet to be elucidated. We provide the first evidence that the bursts are due to a microbunching instability driven by the emission of synchrotron radiation in the bunch. Observations made at the Advanced Light Source are consistent with the values predicted by the proposed microbunching model. These results demonstrate a new instability regime for high energy synchrotron radiation sources and could impact the design of future sources.     

ZnTe晶体中光学整流产生的THz辐射及其电光探测研究

借助抽运-探测技术研究了ZnTe晶体中光学整流产生的太赫兹(THz)辐射，利用ZnTe晶体的线性电光效应探测THz辐射场分布，观察到了较窄(约为0.2 ps)的THz场分布及相应较宽(响应超过4 THz，半峰宽约为2.4 THz)的THz频谱，并运用琼斯矩阵对实验结果进行了理论拟合. 研究了飞秒激光脉冲波长(750—850 nm)、脉冲宽度(56—225 fs)和晶体旋转与THz辐射产生的关系. 同时改变探测光偏振方向进行偏振调制，并从理论上分析了偏振调制对THz辐射探测的影响. 关键词： THz辐射 光学整流 电光探测 ZnTe     

Generation of terahertz radiation by optical rectification in presence of resonance impurities

We demonstrate the possibility of an efficient generation of terahertz radiation under optical rectification in a uniaxial crystal containing impurities with resonance two-level quantum transitions. The group velocity of an optical pulse entering the regime of self-induced transparency decreases to the value of phase velocity of the terahertz radiation, which results in phase matching of long- and short-wavelength waves. The terahertz generation is accompanied by strong self-modulation of the optical pulse. Two-component soliton-like structures can be formed at long distances accompanied by spectral red shift of the optical component.     

Detection of freely propagating terahertz radiation by use of optical second-harmonic generation

We report the application of electric-field-induced optical second-harmonic generation as a new technique for measuring the field of freely propagating terahertz radiation. Using silicon as the nonlinear medium, we demonstrate subpicosecond time resolution and a sampling signal that varies linearly with the terahertz electric field. This approach, which is attractive for centrosymmetric media, permits a significantly broadened class of materials to be exploited for free-space sampling measurements.     

Effects of gain saturation on terahertz radiation via optical difference frequency generation

Numerical studies on stability of terahertz (THz) radiation generation based on difference frequency generation in nonlinear crystals are reported. When the gain saturation is achieved at a wavelength that corresponds to an optimal crystal length, the maximal output of THz radiation could be obtained. As a result of crystal absorption, the gain saturation region of THz radiation is unstable region for output. The stability of THz radiation is determined by the stability of pump in the stable region where behind of the gain saturation.     

Study of terahertz radiation generated by optical rectification on thin gold films

Emission of terahertz (THz) radiation as a result of optical rectification of intense femtosecond laser pulses on thin gold films has been studied by time-domain THz spectroscopy. The THz amplitude was measured as a function of film thickness and incidence angle. The experiments reveal that the emitted THz field is suppressed for a thickness below 100 nm, which gives evidence of the nonlocal character of the response. The variation of incidence angle allows us to estimate the components of susceptibility tensor chi2ijk. For thicker films and near grazing incidence, the emitted THz field attains a peak value of 4 kV/cm.     

可调太赫兹与光学超材料

本文对可调太赫兹与光学超材料的研究进展进行了综述,并对其发展趋势和应用前景进行了展望。可以预见,可调超材料将继续成为超材料研究中的热点课题,并将成为引领光学器件和光学系统变革的潜在技术途径,对光学和太赫兹技术的发展将产生深远的影响。     

The generation of terahertz radiation via optical rectification in the self-induced transparency regime

The possibility of efficient terahertz generation via optical rectification in electro-optic crystal doped with the two-level resonant impurities is demonstrated. Under conditions of the self-induced transparency regime the laser pulses slows down to achieve the phase matching condition. The terahertz generation is accompanied by periodic modulation and spectral deformation of the laser pulse. The most efficient generation occurs under condition of small negative detuning from the resonant frequency of the two-level system.     

Generating terahertz radiation via optical rectification in nonlinear crystals: Theory and experimental results

Key principles of terahertz radiation generation via laser pulse optical rectification are reviewed. The development of theoretical concepts is considered in connection with recent experimental results. The usability of resonant effects and metamaterials for the further development of this technique of generation is analyzed.