Terahertz time-domain spectrometer with module heads coupled to photonic crystal fiber

We constructed optical-fiber-based THz time-domain spectrometers (THz TDSs) with standard single-mode fibers (SSMFs) and large-mode-area photonic crystal fibers (LMA PCFs) and compared those to THz waves and spectra. The optical fibers are used for guiding optical pulses from ultra-fast lasers to a THz emitter and detector. The LMA-PCF-based THz TDS exhibits increased bandwidth from 1 to 2 THz and increased field amplitude by a factor of four compared with the SSMF-based THz TDS under the relatively higher excitation power of ultra-fast lasers. This improvement results from LMA PCFs that are suitable for high-power transmission without introducing nonlinear effects. We also fabricated compact THz emitter and detector module heads, which are connected with LMA PCFs. The LMA-PCF-based THz TDS had THz radiation power and bandwidth comparable with those obtained by a conventional THz TDS with a free-space optical arrangement. PACS 42.72.Ai; 42.65.-k; 42.81.DP     

An Ultra-Broadband Chirality Selective Metastructure Absorber using High Impedance Surface

In this paper, an ultra-wideband chirality selective metastructure absorber is proposed that enables differential absorption and reflection of circularly polarized waves in the terahertz (THz) range. The structure achieves circular dichroism (CD) by using asymmetrically split metal rings as fundamental meta-atoms. Most critically, the high impedance surface and air-resonant cavities are inserted separately in the meta-atoms and dielectric substrate to enhance CD and broaden the bandwidth of absorption. The metastructure absorber can achieve more than 90% absorption of right circularly polarized waves at 0.675–1.244 THz, and it can maintain more than 90% reflection of left circularly polarized waves at 0.607–1.229 THz without changing the direction of rotation. Besides, its CD can reach more than 80% at 0.687–1.213 THz with a relative bandwidth of 55.3%. Spin-selective absorption, which is closely related to breaking chiral symmetry, is investigated through power loss distribution, wide-angle incidence, and scan parameter optimization. The proposed strategy is further validated in the THz band, and the polarization selection and manipulation techniques can be applied to chiral sensing/radio-thermometry, circular polarization detectors/lasers, and molecular spectroscopy.     

利用空气来探测脉冲太赫兹波

曾有报道利用飞秒激光脉冲通过空气中的三阶光学非线性过程产生高强度的太赫兹波．理论上，作为太赫兹波产生的逆过程，有可能实现用空气作为介质探测脉冲太赫兹波．作者以空气或激光诱导的空气等离子体作为介质，通过测量太赫兹波场诱导产生的二次谐波信号，首次实现了宽带太赫兹波的时间分辨探测，本文介绍了空气中太赫兹波的非相干和相干探测的实验结果和理论分析．迄今为止，这种具有突破意义的太赫兹波空气电离相干探测法（THz-ABCD）的频谱宽度可以超过8THz，实现动态范围可达30dB。     

Temperature dependent narrow-band terahertz pulse generation in periodically poled crystals via difference frequency generation

Femtosecond optical pulse is used to generate narrow-band terahertz pulses depending on a quasi-phase-matched condition in periodically poled lithium niobate (PPLN) and stoichiometric lithium tantalate (PPSLT) crystals by difference frequency generation. The origin of narrow-band THz generation proved that the two frequency components of the fs pulse contribute to the frequency mixing. By cryogenic cooling, the absorption of THz waves in the crystal is significantly reduced which results in efficient THz generation. Simultaneously generated forward and backward THz pulses were 1.38 and 0.65 THz with as narrow as the bandwidth of 32 GHz in the PPSLT sample. Temperature dependence of the generated THz waveforms had good agreement with the simulation result using one dimensional plane-wave propagation model.     

Laser-induced photoacoustics influenced by single-cycle terahertz radiation

Laser-induced plasma acoustic waves are enhanced under the illumination of single-cycle terahertz (THz) radiation, making THz-enhanced acoustics (TEA) a useful method for THz wave detection. During a single-cycle THz pulse with its peak field of 100 kV/cm, a pressure enhancement of 10% is observed throughout the acoustic spectrum up to 140 kHz, and the TEA signal is found to increase linearly with THz wave intensity. By using dual-color laser excitation to manipulate free electron drift, it is possible to modulate the enhanced acoustic signal and recover a coherent THz time-domain waveform by simply "listening" to the plasma.     

High-power terahertz-wave generation using DAST crystal and detection using mid-infrared powermeter

The exact power output of a table-top-sized terahertz (THz)-wave source using a nonlinear optical process has not been clarified because detectors for these experiments [Si bolometer, deuterated triglycine sulfate (DTGS), etc.] are not calibrated well. On the other hand, powermeters for the mid-infrared (mid-IR) region are well established and calibrated. We constructed a high-power dual-wavelength optical parametric oscillator with two KTP crystals as a light source for difference frequency generation. The obtained powers of dual waves were 21 mJ at ~1300 nm, ten times higher than that of the previous measurement. The device provides high-power THz-wave generation with ~100 times greater output power than that reported in previous works. A well-calibrated mid-IR powermeter at ~27 THz detected the generated THz wave; its measured energy was 2.4 microJ. Although the powermeter had no sensitivity in the lower-frequency range (below 20 THz), the pulse energy at such a low-frequency region was estimated in reference to the output spectrum obtained using a DTGS detector: the energy would be from about the submicrojoule level to a few microjoules in the THz-wave region.     

Efficient generation of high-power quasi-single-cycle terahertz pulses from a single infrared beam in a second-order nonlinear medium

It is shown that the coherence lengths for terahertz (THz) generation based on difference-frequency generation within an ultrafast infrared pulse can be sufficiently long for a wide bandwidth about each phase-matching wavelength owing to a slight dispersion in the THz region. As a result, quasi-single-cycle THz pulses can be efficiently generated. An efficient conversion for the parametric process is made possible not only by use of the wide phase-matching bandwidth but also by optimization of the pulse width for each peak THz frequency. I have investigated the strong-pump regime and found the limits to the conversion efficiencies to generate high peak intensities efficiently for THz waves with which to explore nonlinear regimes of the THz interactions.     

激光等离子体太赫兹辐射源的频率控制

实验研究了双色超快强激光场作用于氮气分子束所产生的宽带太赫兹(THz)辐射光谱随等离子体介质的密度和长度的依赖关系, 发现THz辐射的中心频率随等离子体密度提高和长度减小而增大(0.8-1.4 THz), 且谱宽也随之增加(0.78-1.53 THz). 分析和计算表明, 太赫兹光谱的变化由等离子体振荡频率和谱宽决定. 该发现为等离子体宽带太赫兹辐射源的光谱操控提供了新思路.     

激光等离子体光丝中太赫兹频谱的调控

研究了双色激光场激发空气成丝产生太赫兹辐射频谱的变化规律.实验观察到随驱动光功率和光丝长度增加,太赫兹光谱主要发生红移的现象.分析表明,由于等离子体密度的增加,太赫兹辐射的趋肤深度减小,等离子体吸收主导了红移的发生.在光丝足够短的条件下,趋肤深度远大于光丝长度,从而产生等离子体振荡主导的太赫兹辐射光谱蓝移.本研究为超快宽带太赫兹辐射的频谱调控提供了新思路.     

Multifunctional Device for Circular to Linear Polarization Conversion and Absorption

In this paper, a metastructure multifunctional device for circular-to-linear polarization conversion (PC) and perfect absorption is proposed in which the electrical conductivity of the silicon material is controlled by light, thus changing the function of the device. The paper also explores three methods of optimizing bandwidth and their mechanisms, which are analyzed by means of current and energy density diagrams. The unit structure of this device adopts a 2 × 2 array, which is used for differentiated reflection of circular polarization waves, and forms linear polarization waves after reflection. In the other state, ultrawideband absorption can be achieved by changing the conductivity of silicon by external optical pumping, and the bandwidth is widened by inserting air resonators. In general, the device can form a PC at 0.89–1.31 THz with a relative bandwidth of 38% when there is no illumination. The resulting linear polarization wave has an axial ratio greater than 19 dB. When the silicon is excited by light resulting in a stable conductivity of around 9000 S m⁻¹, the absorption band is 0.89–2.01 THz, the relative bandwidth is 77%, and the absorption rate is above 90%. This device can be used for communication, electromagnetic cloaking, and modulation.     

Achromatic terahertz quarter-wave retarder in reflection mode

A compact achromatic quarter-wave retarder (QWR) operating in reflection mode is designed for using in terahertz region. It is a composite device utilizing form birefringence and Fabry–Pérot (FP) interference. Under illumination of plane waves with incidence angle of 45°, from 1.8 THz to 2.8 THz, the QWR achieved only ±2° variation around 90° phase retardation, enlarging the working bandwidth of ordinary QWR greatly. An analytical model combining transmission-line (TL) theory with effective medium theory (EMT) is presented and results agree well with the time-consuming numerical calculation. The 38 μm thick construction is simple and easy for fabrication by the existing lithographic technique and a promising application in terahertz or other frequency region is believed.     

Thermally stimulated terahertz radiation of plasmon–phonon polaritons in GaAs

The thermally stimulated excitation of radiative modes of surface plasmon–phonon polaritons in GaAs followed by the high-power terahertz (THz) radiation selective emission is studied and experimentally observed. The selective high-power THz radiation emitters in the 7–8 and 10–15 THz frequency ranges based on the heated highly doped (n>5?10¹⁷ cm^?3) GaAs plates are proposed.     

Spectra of Stimulated Electromagnetic Emission of the Ionosphere Sweeping of the Pump Wave Frequency Near Gyroharmonics. II. Discussion of the Results

Alternative mechanisms of generation of the stimulated electromagnetic emission (SEE) excited in the ionosphere by high-power radio waves are analyzed on the basis of measurements of the SEE spectra obtained during the pump-wave frequency sweeping near the forth (n = 4) and fifth (n = 5) harmonics of the electron gyrofrequency nf_ce [1] and their comparison with the existing physical models. A method for determination of the magnetic field strength and plasma density near the double-resonance region in the ionosphere is developed. It is shown that the generation of the broad upshifted maximum (BUM) feature in the SEE spectrum should occur several kilometers below the double-resonance altitude. A role of high-frequency plasma modes and small-scale magnetic field-aligned irregularities, excited under ionosphere pumping by a high-power radio wave, in the formation of SEE spectra is demonstrated. It is shown that the difference in the emission intensities for f₀ ≲ nf_ce and f₀ > nf_ce is related to different regions (altitudes) at which the plasma waves exist in these cases. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 7, pp. 553–570, July 2008.     

Coherent control of THz wave generation in ambient air

Our study of THz wave generation in the pulsed laser induced air plasma with individually controlled phase, polarization, and amplitude of the optical fundamental wave (omega) and its second harmonic (2omega) indicates that the third-order nonlinear optical process mixing the omega and 2omega beams in the ionized plasma is the main mechanism of the efficient THz wave generation. The polarity and the strength of the emitted THz field are completely controlled by the relative phase between the omega and 2omega waves. The measured THz field amplitude is proportional to the pulse energy of the fundamental beam and to the square root of the pulse energy of the second-harmonic beam once the total optical pulse energy exceeds the plasma formation threshold. The optimal THz field is achieved when all waves (omega, 2omega, and THz waves) are at the same polarization in the four-wave-mixing process.     

Wave Generation in a Warm Magnetized Multi‐Component Plasma

The resonant interaction between three waves propagating perpendicularly to an external magnetic field in a plasma is considered. We present the explicit expressions for the three wave coupling coefficients of a warm multi‐component plasma. The results of previous work on the generation of THz radiation by laser plasma interaction are significantly improved. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tunable few-cycle and multicycle coherent terahertz radiation from relativistic electrons

We report the generation of tunable, narrow-band, few-cycle and multicycle coherent terahertz (THz) pulses from a temporally modulated relativistic electron beam. We demonstrate that the frequency of the THz radiation and the number of the oscillation cycles of the THz electric field can be tuned by changing the modulation period of the electron beam through a temporally shaped photocathode drive laser. The central frequency of the THz spectrum is tunable from ～0.26 to 2.6 THz with a bandwidth of ～0.16 THz.     

Generation of widely tunable Fourier-transform-limited terahertz pulses using narrowband near-infrared laser radiation

Widely tunable, Fourier-transform-limited pulses of terahertz (THz) radiation have been generated using (i) crystals of the highly nonlinear organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST), (ii) zinc telluride (ZnTe) crystals, (iii) gallium phosphide (GaP) crystals, and (iv) low-temperature-grown gallium arsenide (LTG-GaAs) photomixers with THz spiral antennas. Outputs from two narrowband (Δν < 1 MHz, λ ∼ 800 nm) cw titanium-doped sapphire (Ti:Sa) ring lasers with a well-controlled frequency difference were shaped into pulses using acousto-optic modulators (AOM), coupled into an optical fiber, pulse amplified in Nd:YAG-pumped Ti:Sa crystals and used as optical sources to pump the THz emitters. The THz radiation was detected over a broad frequency range and its bandwidth was determined to be ∼10 MHz. The spectroscopic potential of the THz source is illustrated by the absorption spectrum of a pure rotational transition of OCS.     

Plasma wave field effect transistor as a resonant detector for 1 terahertz imaging applications

The possibility of using plasma wave field effect transistor in a time domain terahertz (THz) spectroscopy setup is presented. We demonstrate that High Electron Mobility Transistors (HEMTs) is an efficient device for detection of pulsed terahertz electric fields generated with a femtosecond laser oscillator. The response was observed in the frequency range of about 1 THz, far above the cutoff frequency of the transistors at room temperature. We show that the physical mechanism of the detection is related to the plasma waves excited in the transistor channel and that significant improvement of the active device can be achieved by increasing the drain current. The two-dimensional terahertz imaging applications clearly demonstrate that plasma wave nanometer HEMT should be employed as efficient future detectors in a matrix configuration.     

An Ultra-Wideband Janus Metastructure with Graphene for Detector Based on Anapole Mode in the Terahertz Region

In this paper, an ultra-wideband Janus metastructure (MS) utilizing anapole mode for detector in the terahertz (THz) range by graphene is proposed. Specifically, when Fermi level (E_f) is set to 0.9 eV, the MS demonstrates ultra-broadband absorption exceeding 0.9 from 0.754 to 5 THz in the −z-direction with a relative bandwidth of 147.6 %, in which perfect absorption of over 98% develops from 3.24 to 5 THz. In the case of the +z-direction, the absorptivity maintains around 0.6 within the 0.745 ∼ 5 THz range. As E_f equals 0 eV, the difference in absorption between the −z-direction and +z-direction exceeds 0.9 from 4.49 to 4.76 THz. The study also explores the MS for refractive index sensing near 3.71 THz by a unique difference detection, measuring two refractive index ranges: 1.2 ∼ 2.6 and 4.5 ∼ 4.7, with corresponding sensitivities of 0.0450 and 0.0304, respectively. Owing to its highly symmetrical structure, the MS is insensitive to the polarization state of the electromagnetic (EM) waves, performing remarkable angular stability as the incident angle varies from 0 to 60 degrees in the −z-direction. These splendid properties make the design a good candidate for biomedical sensing, EM cloaking, and full-space EM wave control.