Terahertz Quantum-well Photodetector Research Articles

A terahertz near-field nanoscopy revealing edge fringes with a fast and highly sensitive quantum-well photodetector

SummaryWe demonstrate the successful implementation of a terahertz (THz) quantum-well photodetector (QWP) for effective signal collection in a scattering-type scanning near-field optical microscope (s-SNOM) system. The light source is an electrically pumped THz quantum cascade laser (QCL) at 4.2 THz, which spectrally matches with the peak photoresponse of THz QWP. The sensitive THz QWP has a low noise equivalent power (NEP) of about 1.1 pW/Hz0.5 and a spectral response range from 2 to 7 THz. The fast-responding capability of the THz QWP is vital for detecting the rapidly tip-modulated THz light which can effectively suppress the background noise. The THz images of the nanostructure demonstrate a spatial resolution of about 95 nm, corresponding to ∼λ/752 at 4.2 THz. We experimentally investigate and theoretically interpret the formation of the fringes which appear at the edge position of a gold stripe in the THz near-field image.

Wide spectrum terahertz quantum well photodetector

A 45°-incident facet wide-spectrum terahertz (THz) quantum well photodetector (QWP) is presented. It peaks at ∼2.0 THz with a full width at half maximum of 3.0 THz. The response frequency covers 1.5–7.6 and 8.8–20.3 THz. It has the highest responsivity of 0.18 A W−1 and a best specific detectivity of 3.5 × 1010 cm Hz0.5 W−1. It is an initial attempt to achieve a wide spectrum THz QWP below 3.0 THz, which has promising applications in THz radar or THz imaging.

Fast terahertz reflective confocal scanning imaging with a quantum cascade laser and a photodetector

For the terahertz imaging systems, the fast detection of terahertz light is deemed as a key technology. We present a fast terahertz reflective confocal scanning imaging system employing a fast terahertz quantum-well photodetector (QWP) and a fast rotating translational platform. The excellent performance of the terahertz QWP indicates that the device can rapidly detect the 4.3 THz radiation generated from a pulsed electrically pumped terahertz quantum cascade laser, which is used as the terahertz source of the imaging system. Two 200 μm pinholes are placed in the two focuses to filter out the scattered light and improve the resolution of the imaging system, we achieve a lateral resolution better than 110 μm and an axial resolution of about 320 μm. The terahertz two-dimension images of some objects are obtained within 5 s with a high contrast. The three-dimensional sections of the object demonstrate the great axial selectivity of the terahertz imaging system.

Reflective scanning imaging based on a fast terahertz photodetector

The fast detection of terahertz radiation is recognized as a key technology of terahertz imaging systems. We realize a terahertz imaging system employing a terahertz quantum-well photodetector (QWP) and a terahertz quantum cascade laser (QCL). The detector can rapidly detect the 4.3THz light generated from a pulsed electrically-pump terahertz QCL, which is used as the terahertz source of the imaging system. The object is placed on a rotary scanning platform to realize fast scanning. A practical detection method of the terahertz pulse signal is employed to extract the amplitude information from the terahertz signal and improve the signal-to-noise ratio (SNR) of the system. The electrical and optical performances of the fast terahertz QWP are characterized, and the results show that the terahertz QWP can completely meet the fast detection requirement of the terahertz imaging system. The terahertz images of some invisible objects are obtained with a high contrast within 5 s. The imaging system owns a resolution of 0.3 mm and a circular imaging region with a diameter of 100 mm. An image processing algorithm is applied in this system to solve the noise problem and improve the quality of the images. The imaging results indicate that terahertz QWP has a good application prospect in nondestructive inspection applications and the research of the fast physical or chemistry process.

Terahertz imaging using photomixers based on quantum well photodetectors

Due to the fast intersubband transitions, the terahertz (THz) quantum well photodetector (QWP) is supposed to work fast. Recently it has been demonstrated that the THz QWP can detect the THz light modulated at 6.2 GHz and therefore it can be used as a photomixer [H. Li et al., Sci. Rep. 7, 3452 (2017)]. In this work, the authors report a novel active THz imaging using THz QWP photomixers. The THz radiation source used for this imaging application is a multi-mode THz quantum cascade laser (QCL) operating in continuous wave mode. When the fast THz QWP is illuminated by the multi-mode THz radiation, the intermediate frequency signal that is resulted from the frequency beating between the neighbouring THz modes of the QCL can be extracted from the QWP mesa for imaging applications. Employing the technique, the frequency can be down-converted from the THz range to the microwave regime. And therefore, the signal can then be amplified, filtered, and detected using the mature microwave technology.

Direct detection of a fast modulated terahertz light with a spectrally matched quantum‐well photodetector

A fast detection system based on a terahertz quantum-well photodetector (QWP) and a fast modulated terahertz quantum-cascade laser is presented. Response waveforms and eye diagrams under different modulation frequencies are acquired. Employing the current measurement system without any high speed optimisation obtains the highest response rate of 0.5 GHz. The experiment demonstrates that the QWP is potentially useful for fast detection and high speed communication applications.

High-Temperature Photon-Noise-Limited Performance Terahertz Quantum-Well Photodetectors

In this paper, we propose using a terahertz quantum-well photodetector (THz QWP) in combination with a terahertz source to realize a detection system with photon-noise limited performance (PLIP) at high temperatures. Systematical investigations on the high-temperature performances of THz QWPs, including required signal power density for PLIP, detectivity, and the signal-to-noise ratio, have been carried out by elaborating their dark current mechanism and photocurrent response both experimentally and theoretically. We also present the optimal doping concentration of THz QWPs designed for different peak wavelengths and the resulting optimum performance regarding the above three key parameters. Numerical results show that optimal designed QWP with peak response frequency of 5.5 THz is expected to achieve PLIP at 77 K at signal power density at 819 W/cm $^{{{2}}}$ and above. This work gives a precise description of PLIP performance of THz QWPs and will open ways for new applications for high-temperature detection in the THz regime.

20 Mbps wireless communication demonstration using terahertz quantum devices

A wireless terahertz (THz) communication link is demonstrated, in which a THz quantum cascade laser and a THz quantum-well photo-detector (QWP) serve as the emitter and receiver, respectively. With the help of the well-matched THz QWP, the optical collection efficiency has greatly been improved. A data signal transmitted over 2.2 m with a low bit error rate (≤1×10−8) and data rate as high as 20 Mbps is achieved, which are almost 1 order of magnitude higher than that previously reported.

Reflection imaging with terahertz quantum‐cascade laser and quantum‐well photodetector

A reflection imaging system based on the terahertz quantum-well photodetector is presented. Reflected terahertz imaging of a logo on a universal serial bus (USB) flash disk is demonstrated. A spatial resolution better than 0.4 mm is achieved. This experiment demonstrates that the quantum-well photodetector is potentially useful for reflection imaging applications.

Terahertz quantum-well photodetectors: Design, performance, and improvements

Theoretical studies and numerical simulations on design, performance, and improvements of terahertz quantum-well photodetector (THz QWP) are presented. In the first part of this paper, we discuss the device band structure resulting from a self-consistent solution and simulation results. First, the temperature dependence of device characteristics is analyzed. Next, we deduce the condition of optimal doping concentration for maximizing dark current limited detectivity Ddet* when QWP is lightly doped. Accordingly, unlike in previously published reports, doping concentration is not fixed and is selected by the above condition. In the second part of this paper, we propose two schemes for improving operation temperature. The first is to incorporate an optical antenna which focuses incident THz wave. Numerical results show that the QWP with peak frequency higher than 5.5 THz is expected to achieve background-noise-limited performance at 77 K or above when employing a 106 times enhancement antenna. The second scheme is to use a laser as the signal source to achieve photon-noise-limited performance (PLIP) at high temperatures. Simulations show that when operating below critical temperature QWPs in the range of 1 ∼ 7 THz can reach PLIP under practical illumination intensities.

Asymmetric Fabry–Perot Oscillations in Metal Grating-Coupled Terahertz Quantum Well Photodetectors

Asymmetric Fabry-Perot oscillations are observed in a high-resolution photocurrent spectrum of a 1-D metal grating-coupled terahertz quantum well photodetector (THzQWP). This behavior is carefully studied through analysis of the field in the device obtained by the finite element method. It is found that such asymmetric oscillation is a pure near-field effect caused by the phase shift of the reflected wave at the grating surface. Our findings also indicate that, because of the long wavelength in the THz range, the near field properties of a microstructured surface could be extracted through the photocurrent measurement on a THzQWP.

Terahertz Imaging With Quantum-Well Photodetectors

We present an imaging system based on a terahertz (THz) quantum-well photodetector (QWP). The THz QWP possessing high sensitivity at THz frequency band (0.3-10 THz) is utilized as a detector for imaging. We obtained images of a concealed sample in envelope by detecting the transmitted THz radiation from a black-body source as background radiation. This letter shows the potential of THz QWPs for imaging.

Experimental realization of wireless transmission based on terahertz quantumcascade laser

A terahertz wireless transmission system is constructed by using a continuous wave terahertz quantum-cascade laser (THz QCL) and a spectrally-matched terahertz quantum-well photodetector (THz QWP). The transmission bandwidth of the system is measured. The transmission processes at 4.13 THz of a picture file are demonstrated by employing the above system, and the both message totally consistent with the original one are received correctly. Hence, the feasibility of wireless transmission based on THz QCL and THz QWP devices is confirmed. Finally, we analyze the transmission rate of the demonstrating system and give the improving methods.

Strong enhancement of terahertz response in GaAs/AlGaAs quantum well photodetector by magnetic field

A strong enhancement of terahertz (THz) response in a GaAs/AlGaAs quantum well photodetector (QWP) is observed under perpendicular magnetic field. Photocurrent spectra show that besides partial contribution from an increase in the detector differential resistance, improvement of photoconductive gain induced by electron localization and Landau quantization of the in-plane electron motion in quantum wells is the predominant underlying mechanism. This study sheds light on the transport mechanism in THz QWPs and provides a possible means for enhancing THz QWP’s response.

Time-Resolved Thermal Quenching of THz Quantum Cascade Lasers

The thermal dynamic behaviors of terahertz quantum cascade lasers are investigated using a spectrally matching terahertz quantum-well photodetector. The THz output power of a lasing device under a pulse excitation is found to decrease with time, which is attributed to the increase of active-region temperature. The measurements show that the duration of output lasing pulse ranges from a few microseconds up to 77 ¿s, depending on device bias and heat-sink temperature. A theoretical analysis based on a two-dimensional heat diffusion model is presented. The model calculates lasing quenching time based on an ¿average active-region temperature¿ criterion as well as a ¿reserve of gain¿ criterion. The best fit for quenching time is found when the vertical thermal conductivity in the quantum well region is two orders of magnitude lower than that of substrate. The measured lasing quenching time is in very good agreement with theoretical modeling calculated using both criteria.

Emission spectra of terahertz quantum-cascade lasers based on the terahertz quantum-well photodetectors

The emission of a 41 THz quantum cascade laser (THz QCL) has been measured by a spectrally-matching terahertz quantum-well photodetector (THz QWP). The measured emission spectra have been analyzed with respect to the spectral shape and peak position. The relative light power of the laser under different drive currents have been estimated from the emission spectra and then the lasing domain and threshold current density of the laser has been determined. The temperature-dependent response of THz QWP to the laser has also been investigated. The analyzed results indicate that the THz QWPs could be a good detector for characterizing the emission properties of the THz QCLs and may be the receivers in the terahertz communications.

Temperature dependence of current–voltage characteristics of terahertz quantum-well photodetectors

We have performed current–voltage (I–V) measurements on a terahertz quantum-well photodetector (QWP) at different temperatures and employed an emission-capture model to simulate the I–V curves. A temperature-dependent vertical electron drift mobility has been used to fit the curves from 7 K to 20 K. Photocurrents caused by 300 K background radiation have also been measured at different temperatures and a background-limited infrared performance (blip) temperature of 12 K for this terahertz detector has been determined. The current–temperature (I–T) curves derived from the measured dark I–V curves indicate that the thermionic emission process is the major mechanism for dark current in this terahertz detector.

Effect of optical phonons on the spectral shape of terahertz quantum-well photodetectors

Abstract We calculate the spectral response of a terahertz (THz) quantum-well photodetector designed for 7 THz detection peak frequency. We incorporate the effect of GaAs optical phonons which give rise to a strong absorption in the region of 34–36 meV and result in an increase in reflection due to the large refractive index ( n ) change around this region. Comparing the calculated spectral shape with experiments, we show the improvement over the standard expressions.