Terahertz Wave Detection Research Articles

Laser Air Photonics: Covering the ＂Terahertz Gap＂ and Beyond

Laser air photonics involves the interaction of high-peak-power femtosecond laser pulses with air. Through the ionization process, the very air that we breath is capable of generating terahertz (THz) electromagnetic field strengths greater than 1 MV/cm, useful bandwidths of over 100 THz, and highly directional emission patterns. Following the ionization of air, the emitted air-plasma uorescence or acoustics can serve as an omnidirectional, broadband, THz wave sensor. Frequencies lying within the THz band were historically termed the ”THz gap,” due to the relative difficulty for generating and detecting the radiation at these frequencies. Here we review significant advances in laser air photonics that help to close this ”gap,” enabling ultra-broadband THz wave generation and detection through nonlinear optical processes. Research opportunities and applications including non-destructive evaluation, material characterization, and feasibility for remote sensing, are discussed.

Terahertz wave detection using a low-temperature-grown GaAs photoconductive antenna excited by 1.5µm pulses

Terahertz wave detection using a low-temperature-grown GaAs photoconductive antenna excited by 1.5µm pulses

Enhanced Photoresponse of Plasmonic Terahertz Wave Detector Based on Silicon Field Effect Transistors with Asymmetric Source and Drain Structures

We investigate the enhanced effects of asymmetry ratio variations of the source and drain area in silicon (Si) field-effect transistor (FET). Photoresponse according to the variation of asymmetry difference between the width of source and drain are obtained by using the plasmonic terahertz (THz) wave detector simulation based on technology computer-aided design (TCAD) with the quasi-plasma 2DEG model. The simulation results demonstrate the potential of Si FETs with asymmetric source and drain structures as the promising plasmonic THz detectors.

Investigation of ultra-broadband terahertz time-domain spectroscopy with terahertz wave gas photonics

Recently, air plasma, produced by focusing an intense laser beam to ionize atoms or molecules, has been demonstrated to be a promising source of broadband terahertz waves. However, simultaneous broadband and coherent detection of such broadband terahertz waves is still challenging. Electro-optical sampling and photoconductive antennas are the typical approaches for terahertz wave detection. The bandwidth of these detection methods is limited by the phonon resonance or carrier’s lifetime. Unlike solid-state detectors, gaseous sensors have several unique features, such as no phonon resonance, less dispersion, no Fabry-Perot effect, and a continuous renewable nature. The aim of this article is to review the development of a broadband terahertz time-domain spectrometer, which has both a gaseous emitter and sensor mainly based on author’s recent investigation. This spectrometer features high efficiency, perceptive sensitivity, broad bandwidth, adequate signal-to-noise ratio, sufficient dynamic range, and controllable polarization. The detection of terahertz waves with ambient air has been realized through a third order nonlinear optical process: detecting the second harmonic photon that is produced by mixing one terahertz photon with two fundamental photons. In this review, a systematic investigation of the mechanism of broadband terahertz wave detection was presented first. The dependence of the detection efficiency on probe pulse energy, bias field strength, gas pressure and third order nonlinear susceptibility of gases were experimentally demonstrated with selected gases. Detailed discussions of phase matching and Gouy phase shift were presented by considering the focused condition of Gaussian beams. Furthermore, the bandwidth dependence on probe pulse duration was also demonstrated. Over 240 times enhancement of dynamic range had been accomplished with n-hexane vapor compared to conventional air sensor. Moreover, with sub-20 fs laser pulses delivered from a hollow fiber pulse compressor, an ultra-broad spectrum covering from 0.3 to 70 THz was also showed. In addition, a balanced detection scheme using a polarization dependent geometry was developed by author to improve signal-to-noise ratio and dynamic range of conventional terahertz air-biased-coherent-detection (ABCD) systems. Utilizing the tensor property of third order nonlinear susceptibility, second harmonic pulses with two orthogonal polarizations was detected by two separated photomultiplier tubes (PMTs). The differential signal from these two PMTs offers a realistic method to reduce correlated laser fluctuation, which circumvents signal-to-noise ratio and dynamic range of conventional terahertz ABCD systems. A factor of two improvement of signal-to-noise ratio was experimentally demonstrated. This paper also introduces a unique approach to directly produce a broadband elliptically polarized terahertz wave from laser-induced plasma with a pair of double helix electrodes. The theoretical and experimental results demonstrated that velocity mismatch between excitation laser pulses and generated terahertz waves plays a key role in the properties of the elliptically polarized terahertz waves and confirmed that the far-field terahertz emission pattern is associated with a coherent process. The results give insight into the important influence of propagation effects on terahertz wave polarization control and complete the mechanism of terahertz wave generation from laserinduced plasma. This review provides a critical understanding of broadband terahertz time-domain spectroscopy (THz-TDS) and introduces further guidance for scientific applications of terahertz wave gas photonics.

(211) oriented ZnTe growth on m‐plane sapphire by MBE

AbstractSingle‐crystalline and single domain ZnTe thin films are sought for high‐performance terahertz wave detectors, and ZnTe/sapphire heterostructures were considered since the Electro‐Optical (EO) effect could be obtained only from epilayers. ZnTe epilayers were grown on m‐plane sapphire substrates by molecular beam epitaxy, and the potential of single domain epilayers was explored. Through the X‐ray diffraction pole figure measurement it was confirmed that one (100) oriented ZnTe domain along with two kinds of (211) oriented domains were formed on the m‐plane sapphire when the layer was grown at 340 °C. When the layer was grown at 350 °C, the (211) oriented domain dominated the film. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Terahertz electro-optical detection: optical phase or energy measurements

Models of different available single-photodetector electro-optic sampling schemes are considered from the unified theoretical position. It is taken into account that under electro-optic terahertz wave detection, not only do the polarization states of the femtosecond optical pulse components change but the modules of their amplitudes are also varied. As a result, information concerning the terahertz wave can be obtained not only using the conventional ellipsometry readout scheme (probe-phase sampling) but also by simple measuring of the induced power of the optical pulses (probe-energy sampling). Spectral sensitivities of both of these electro-optic sampling methods are calculated for the cases of nonlinear-optical crystals with zinc-blende symmetry (like ZnTe) and the crystals with one active component of the second-order optical susceptibility tensor (like PPLN). It is found that the ratio between spectral sensitivities of the pure probe-phase and pure probe-energy schemes is proportional to the ratio between the optical and terahertz wave frequencies in all types of the crystals. It is shown that the signal in the near-zero optical transmission point scheme, which is the best for terahertz imaging, has a mixed character. While the contribution of the probe-phase sensitivity appears to be due to spatially nonuniform residual birefringence of the nonlinear zinc-blende crystal, the probe-energy part of the sensitivity has a uniform distribution and can be increased by the angle misalignment of the optical polarization element.

Non-ellipsometric detection of terahertz radiation using heterodyne EO sampling in the Cherenkov velocity matching scheme

A new electro-optic (EO) sampling scheme, which we refer to as "heterodyne EO sampling", for detection of pulsed terahertz (THz) waves is proposed and experimentally demonstrated. In this heterodyne EO sampling scheme, the intensity change in the optical probe pulse induced by a THz field in a nonlinear crystal is measured without any polarization optics. Applied in combination with the non-collinear Cherenkov velocity matching technique, this method allows one to detect pulsed THz waves efficiently and easily using a simpler optical setup as compared to the conventional ellipsometric EO sampling method.

Theoretical Modeling of a Photoconductive Antenna in a Terahertz Pulsed System

The most common device for terahertz (THz) wave generation and detection is a THz photoconductive antenna. This paper examines the factors which affect the radiated power and optical-to-THz power conversion efficiency of the antenna. A novel equivalent circuit model using lumped elements is developed for analyzing the performance of these antennas. In this model, whilst keeping the simplicity of the lumped element approach, the underlying physical behavior of the device is taken into account when calculating the circuit elements. Based on the model, the influence of various parameters on the optical-to-THz power conversion efficiency and radiated power is then investigated. The simulated results agree well with published measured results. The model predicts that an increase in the laser power and/or bias voltage, and a reduction in reflections from the air-substrate can improve the optical-to-THz power conversion efficiency of the device. This novel model is very useful for both designing a THz antenna and tuning a THz system to achieve maximized optical-to-THz power conversion efficiency and THz radiated power.

A polarization-independent and ultra-broadband terahertz metamaterial absorber studied based on circular-truncated cone structure

In this paper, we present an ultra-broadband polarization-independent terahertz (THz) metamaterial absorber (MA) made of circular truncated cone metamaterial. Absorptivity higher than 92.3% at normal incidence is obtained in a wide range of frequencies from 2 to 10 THz. We employ an isotropic metamaterial cell which consists of alternating layers of Au metal and SiO2 dielectric spacer. The absorption spectra of the THz MA are calculated using the finite-difference time domain (FDTD) method within the CST Microwave Studio 2009 in the frequency range of 0–10 THz. Our broadband absorber can be regarded as a group of micro-absorbers perpendicularly stacked and their absorption peaks coupling to each other to form an ultra broadband absorption. This THz MA has the advantages of broadband, polarization-independent and fabrication facility, and thus can be widely applied in THz wave harvesting, detection, spectrum imaging and stealthy technology.

Single-slot-dipole Coupled YBCO Josephson Junctions for Terahertz Wave Detectors

Abstract In this paper, we propose a terahertz wave detector using a grain boundary Josephson junctions (GBJJs) with a single slot-dipole antenna (SDA). A full-wavelength single SDA structure designed for 220 GHz was patterned with an Au on an YBa2Cu3O7-δ (YBCO) GBJJ covered with an insulating benzocyclobutene (BCB). The current-voltage characteristic of the detector showed clear Shapiro steps for radio frequency wave irradiation at frequencies around the designed one.

Modulation of electron trajectories inside a filament for single-scan coherent terahertz wave detection

A synthesized fundamental optical beam and its second harmonic laser field is phase modulated to dynamically reshape the electron momentum distribution inside an air-plasma filament. The net electron motion becomes a combination of the initial laser “kick,” from the remaining optical field, and the terahertz field present at the time of its ionization. The time dependent Schrödinger equation is solved to map the net electron velocity distribution as the phase between the two color beams is changed, and single-scan coherent terahertz wave detection through air-plasma fluorescence is experimentally demonstrated.

Enhancement of Terahertz Pulse Emission by Optical Nanoantenna

Bridging the gap between ultrashort pulsed optical waves and terahertz (THz) waves, the THz photoconductive antenna (PCA) is a major constituent for the emission or detection of THz waves by diverse optical and electrical methods. However, THz PCA still lacks employment of advanced breakthrough technologies for high-power THz emission. Here, we report the enhancement of THz emission power by incorporating optical nanoantennas with a THz photoconductive antenna. The confinement and concentration of an optical pump beam on a photoconductive substrate can be efficiently achieved with optical nanoantennas over a high-index photoconductive substrate. Both numerical and experimental results clearly demonstrate the enhancement of THz wave emission due to high photocarrier generation at the plasmon resonance of nanoantennas. This work opens up many opportunities for diverse integrated photonic elements on a single PCA at THz and optical frequencies.

Terahertz wave detection method based on multi-reflection optical lever with nano-scale precision

Terahertz wave detecting method based on multi-reflection optical lever with nano-scale displacement measuring precision is presented. Multi-reflection optical lever is composed of a pair of plane micro-mirrors, and detecting material is coated on one mirror to absorb terahertz radiation. Affected by radiant thermal effects, tiny mirror deformation is produced while displacement between mirrors is changed. These variations proportional to radiation are amplified and measured by multi-reflection optical lever, and then terahertz wave power can be obtained. Theoretical displacement measuring precision of multi-reflection optical lever method is better than 1 nm. Experimental results show that measuring stability of this method is better than Knife-edge filter method. This method achieves resolution of 4 nm, sensitivity of 5.95 nm/mV, and measurement range of 30 \mu m.

Recent Progresses in Terahertz Wave Air Photonics

Terahertz (THz) wave generation and detection using gaseous medium as both the THz wave emitter and sensor, also termed “THz wave air photonics”, has attracted much scientific attention. We report the most recent progresses in THz generation and detection in laser-induced gas plasma. The overall emphasis of this paper is on THz wave detection with gaseous medium, while in the generation part, emphasis will be on the case with two-color laser excitation.

An ion-charged carbon nanotube oscillator beyond the terahertz regime

A frequency-tunable nano-oscillator consisting of a K+ ion and a negatively-charged carbon nanotube (CNT) was proposed. The ion induces a strong dielectric response on the wall of the CNT, which can be described through a self-consistent classical molecular dynamics based on the atomistic moment method. The calculation results indicate that the stability and the oscillating frequency of the oscillator strongly depend on the quantity of charge on the CNT. As the extra charge on the CNT increases, the oscillator becomes more stable, and the oscillating frequency increases successively, from 0.23 to 1.31 THz. The high frequency and stability of the system are due to the strong axial Coulomb restoring force and its small fluctuations in the cross section of the CNT. This kind of nano-oscillator can be used as a frequency-tunable terahertz wave detector.

Tunable continuous-wave terahertz generation/detection with compact 155 μm detuned dual-mode laser diode and InGaAs based photomixer

We demonstrate a tunable continuous-wave (CW) terahertz (THz) homodyne system with a novel detuned dual-mode laser diode (DML) and low-temperature-grown (LTG) InGaAs photomixers. The optical beat source with the detuned DML showed a beat frequency tuning range of 0.26 to over 1.07 THz. Log-spiral antenna integrated LTG InGaAs photomixers are used as THz wave generators and detectors. The CW THz radiation frequency was continuously tuned to over 1 THz. Our results clearly show the feasibility of a compact and fast scanning CW THz spectrometer consisting of a fiber-coupled detuned DML and photomixers operating in the 1.55-μm range.

“All air–plasma” terahertz spectroscopy

We demonstrate terahertz wave generation and detection capabilities up to 6 THz without the need for solid state materials, biased electrodes, or forward propagating signal collection. An "all air-plasma" terahertz system is used to encode explosive material resonant signatures into the 357 nm nitrogen fluorescence line of a bichromatic field-induced laser plasma filament. These results show the practicability to extend these measurements to remote locations where terahertz pulse information is no longer limited by water vapor absorption, phonon resonance, or signal collection directionality.

Focus on lasers and imaging

Focus on lasers and imaging

A Method of Calibration of Terahertz Wave Brightness Under Nonlinear-Optical Detection

A new method is proposed for calibration of the terahertz wave spectral brightness. The method is based on the use of quantum and thermal field fluctuations as intrinsic references under nonlinear-optical detection. Both effective brightness of the thermal fluctuations and temperature of the laser-pumped nonlinear-optical crystal can be measured absolutely within the calibration procedure also. It is shown that the accuracy of the terahertz wave brightness calibration is strongly dependent on the value of the terahertz wave losses in the nonlinear-optical crystal. The scheme of quasi-phase matched terahertz wave detection in periodically poled lithium niobate is considered. Dispersion of the absorption coefficient in the terahertz frequency range is measured by a three-frequency interference method under spontaneous parametric down-conversion for nominally pure and Mg-doped lithium niobate crystals. The influence of the terahertz radiation losses in Mg-doped lithium niobate crystals is estimated.

Influence of magnetic field on terahertz wave generation in photorefractive periodically poled lithium niobate crystal

By employing femtosecond pump-probe configuration, we successfully realized narrowband terahertz wave generation and detection in both photorefractive periodically poled lithium niobate (PPLN) and periodically poled Mg:LiNb(3) (PP-Mg:LN) crystal. Using an applied magnetic field, we achieved modulation of the terahertz wave in a photorefractive PPLN crystal. The terahertz wave depends strongly on the magnitude of the applied magnetic field in the photorefractive PPLN crystal. Terahertz wave independence of the magnetic field in PP-Mg:LN crystal was also identified. The interaction of the magnetic field and photorefractive PPLN crystal is believed to occur due to the Lorentz force, which results in the buildup of a space-charge field in a photorefractive PPLN crystal.