Room-temperature terahertz detection based on CVD graphene transistor

We report the fabrication and characterization of a single-layer graphene field-effect terahertz detector, which is cou- pled with dipole-like antennas based on the self-mixing detector model. The graphene is grown by chemical vapor deposi- tion and then transferred onto an SiO2/Si substrate. We demonstrate room-temperature detection at 237 GHz. The detector could offer a voltage responsivity of 0.1 V/W and a noise equivalent power of 207 nW/Hz 1/2. Our modeling indicates that the observed photovoltage in the p-type gated channel can be well fit by the self-mixing theory. A different photoresponse other than self-mixing may apply for the n-type gated channel.

Terahertz Orbital Angular Momentum: Generation, Detection and Communication

To accommodate the ever-increasing wireless capacity,the terahertz(THz)orbital angular momentum(OAM)beam that combines THz radiation and OAM technologies has attracted much attention recently,with contributing efforts to explore new dimensions in the THz region.In this paper,we provide an overview of the generation and detection techniques of THz-OAM beams,as well as their applications in communications.The principle and research status of typical generation and detection methods are surveyed,and the advantages and disadvantages of each method are summarized from a viewpoint of wireless communication.It is shown that developing novel THz components in generating,detecting and(de)multiplexing THz-OAM beams has become the key engine to drive this direction forward.Anyway,beneficial from the combination of infinite orthogonal modes and large bandwidth,THz-OAM beams will have great potential in delivering very large capacity in next generation wireless communications.

太赫兹谐波混频检测器变频效率测量

太赫兹波是电磁波谱中最后一个未被全面研究开发的频段,它的开发和利用具有重大的科学价值。利用高温超导双晶结制备的太赫兹谐波混频探测器,具有灵敏度高、变频效率高、谐波次数高、工作频带宽、噪声系数低等优点。基于GM制冷机搭建了高温超导双晶结太赫兹混频系统,并采用了准光学系统增强结与太赫兹波的耦合。在2.7 K的温度下对结进行了测量,获得了结对623 GHz太赫兹波的响应,并且成功进行了254次谐波混频实验,中频输出信噪比达10 dB。对高温超导约瑟夫森双晶结的变频效率进行了计算,并且测量和分析了变频效率与本振功率、谐波混频次数的关系。

Plasma-based terahertz wave photonics in gas and liquid phases

Ultra-broadband,intense,coherent terahertz(THz)radiation can be generated,detected,and manipulated using laser-induced gas or liquid plasma as both the THz wave transmitter and detector,with a frequency coverage spanning across and beyond the whole THz gap."Such a research topic is termed plasma-based THz wave photonics in gas and liquid phases."In this paper,we review the most important experimental and theoretical works of the topic in the non-relativistic region with pump laser intensity below 1018 W/cm^(2).

A proposed approach for detecting terahertz pulses by using double few-cycle laser pulses with opposite carrier envelope phases

Previous research shows that few-cycle laser（FCL） pulses with low energy and without a bias field can be used to coherently detect terahertz（THz） pulses. As we know, it is very difficult to stabilize the carrier envelope phase（CEP） of FCL pulses, i.e., there are some random fluctuations for the CEP. Here we theoretically investigate the influence of such instability on the accuracy of THz detection. Our results show that although there is an optimum CEP for THz detection, the fluctuations of the CEP will lead to terrible thorns on the detected THz waveform. In order to solve this problem, we propose an approach using two few-cycle laser pulses with opposite CEPs, i.e., their CEPs are differed by π.

Improved common-path spectral interferometer for single-shot terahertz detection

To seek high signal-to-noise ratio(SNR) is critical but challenging for single-shot intense terahertz(THz)coherent detection. This paper presents an improved common-path spectral interferometer for single-shot THz detection with a single chirped pulse as the probe for THz electro-optic(EO) sampling. Here, the spectral interference occurs between the two orthogonal polarization components with a required relative time delay generated with only a birefringent plate after the EO sensor. Our experiments show that this interferometer can effectively suppress the noise usually suffered in a non-common-path interferometer. The measured single-shot SNR is up to 88.85, and the measured THz waveforms are independent of the orientation of the used Zn Te EO sensor, so it is easy to operate and the results are more reliable. These features mean that the interferometer is quite qualified for applications where strong THz pulses, usually with single-shot or low repetition rate, are indispensable.

An InP-based heterodimensional Schottky diode for terahertz detection

We present an InP-based heterodimensional Schottky diode（HDSD）,which has so far never been reported in the literature.Compared to a GaAs-based HDSD,the InP-based HDSD is expected to have better high frequency performance and operational conditions resulting from its higher mobility and concentration of 2D electron gas（2DEG） as well as its smaller Schottky barrier height.The cutoff frequency of the InP-based HDSD obtained by the AC measurement is more than 500 GHz,which shows its potential application in terahertz detection.

Efficiency of terahertz detection in electro-optic polymer sensors with interdigitated coplanar electrodes

We discuss the efficiency of an electro-optic （EO） polymer sensor with interdigitated coplanar electrodes. The developed EO sensor is used to detect terahertz radiation via EO sampling. Results show that the sensor improves more significantly detection sensitivity than does a sensor with sandwich configurations.

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. Electroptical 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.