Ziran Zhao

Ultra‐Broadband Photodetector for the Visible to Terahertz Range by Self‐Assembling Reduced Graphene Oxide‐Silicon Nanowire Array Heterojunctions

This natural energy gap can be tuned by controlling the degree of RGO reduc-tion. The apparent energy gap ranged from 10 to 50 meV (corresponding to the energy of 124–24.8 µm electromag-netic waves) and approached zero with extensive reduc-tion. The naturally narrow energy gap in RGO is suitable for detection of mid-infrared (MIR) (2.5–30 µm) or even THz (30–3000 µm) waves. Recently, Chitara et al. reported NIR (1.55 µm) photodetectors based on extensively reduced RGO.

Compressive sensing for direct millimeter-wave holographic imaging.

Direct millimeter-wave (MMW) holographic imaging, which provides both the amplitude and phase information by using the heterodyne mixing technique, is considered a powerful tool for personnel security surveillance. However, MWW imaging systems usually suffer from the problem of high cost or relatively long data acquisition periods for array or single-pixel systems. In this paper, compressive sensing (CS), which aims at sparse sampling, is extended to direct MMW holographic imaging for reducing the number of antenna units or the data acquisition time. First, following the scalar diffraction theory, an exact derivation of the direct MMW holographic reconstruction is presented. Then, CS reconstruction strategies for complex-valued MMW images are introduced based on the derived reconstruction formula. To pursue the applicability for near-field MMW imaging and more complicated imaging targets, three sparsity bases, including total variance, wavelet, and curvelet, are evaluated for the CS reconstruction of MMW images. We also discuss different sampling patterns for single-pixel, linear array and two-dimensional array MMW imaging systems. Both simulations and experiments demonstrate the feasibility of recovering MMW images from measurements at 1/2 or even 1/4 of the Nyquist rate.

Terahertz absorbance spectrum fitting method for quantitative detection of concealed contraband

We present a quantitative method for the nondestructive detection of concealed contraband based on terahertz transmission spectroscopy. Without knowing the prior information of barrier materials, the amount of concealed contraband can be extracted by approximating the terahertz absorbance spectrum of the barrier material with a low-order polynomial and then fitting the measured absorbance spectrum of the inspected object with the polynomial and the known standard spectrum of this kind of contraband. We verify the validity of this method using a sample of explosive 1,3,5-trinitro-s-triazine (RDX) covered with several different barrier materials which are commonly encountered in actual inspection, and good agreement between the calculated and actual value of the amount of RDX is obtained for the experiments performed under both nitrogen and air atmospheres. This indicates that the presented method can achieve quantitative detection of hidden contraband, which is important for security inspection applications.

Continuous-wave terahertz phase imaging using a far-infrared laser interferometer

Terahertz phase imaging can reveal the depth information of an optically opaque object and provide much better contrast for weak-absorption materials. We demonstrate a continuous-wave terahertz interferometric imaging method in which a far-infrared laser interferometer is used to measure the phase distribution with diffraction-limited lateral resolution and subwavelength axial resolution. An improved four-step phase-shifting algorithm is introduced to retrieve the phase map with very high accuracy and low distortion. The relative depth profiles of two transparent samples are successfully extracted by using this method. Experimental results verify that terahertz interferometric imaging in combination with the phase-shifting technique enables effective reconstruction of the phase image of the object under test.

Design and construction of muon tomography facility based on MRPC detector for high-Z materials detection

Muon tomography with cosmic ray muons is a novel technology for high-Z material detection. The tomographic imaging is based on multiple coulomb scattering of cosmic ray muons in matter, which requires large-scale, high-efficiency, high spatial resolution detectors for tracking of incoming and outgoing muons. From previous studies, the multi-gap resistive plate chamber (MRPC) would be an excellent and inexpensive choice for this application, which also offers the possibility of introducing energy information of muons by TOF measurement for image reconstruction of muon tomography. A prototype of muon tomography facility with 6 layers large-scale MRPC detectors has been designed and is under construction, the preliminary results of muon tomography and design details of the MRPC detectors and electronics were presented.

Photocurrent response of carbon nanotube–metal heterojunctions in the terahertz range

We investigate the optoelectronic properties of a carbon nanotube (CNT)-metal heterostructure in the terahertz range. On the basis of terahertz time-domain spectroscopy characterization of a double-walled CNT (DWNT) film, we present and analyze the photocurrent measurement for a DWNT-nickel heterojunction illuminated by continuous-wave terahertz radiation. A significant current across the junction directly induced by terahertz excitation is observed and a negative photoconductivity behavior is found to occur in the device. The photocurrent shows a linear response to the bias voltage and the illumination power within the examined range. These phenomena support the feasibility of using CNT-metal heterojunctions as novel terahertz detectors.

Restoration of terahertz signals distorted by atmospheric water vapor absorption

Terahertz spectroscopic measurements under ordinary atmospheric conditions may suffer interferences from water vapor absorption in the ambient air. A manifold of narrow absorption lines appears in the terahertz spectrum at particular frequencies corresponding to the pure rotational transitions of water molecules. For real-world data, such effect results in unwanted spectral artifacts in the deconvolved spectrum of the examined sample and thus complicates its frequency-dependent characterization. In this paper we use a signal postprocessing algorithm consisting of line shape fitting and spectral subtraction procedures to eliminate the water lines. Restoration of terahertz signals from simulated data and low-humidity measurements is first demonstrated to validate the algorithm. Furthermore, to overcome the difficulty of eliminating strong lines which lead to possible excessive absorption under high-humidity environment, we propose to modify the objective function in spectral subtraction by smoothing the res...

Suppression of spectral interferences due to water-vapor rotational transitions in terahertz time-domain spectroscopy

Absorption lines of atmospheric water vapor are commonly present in terahertz spectra measured in ambient air. These spectral interferences are caused by the rotational transitions of water molecules. In this study, we develop an effective method for suppression of the water-vapor effects by modeling the absorption lines using Lorentzian line shape and spectroscopic parameters extracted from the HITRAN database and then subtracting them iteratively in the frequency domain. The free-space terahertz pulse and the absorbance spectrum of an explosive material are restored successfully in the experimental verification.

Terahertz photodetector based on double-walled carbon nanotube macrobundle-metal contacts

We report on the characterization of a terahertz (THz) photodetector with an extremely simple structure consisting of only a macroscopic bundle of double-walled carbon nanotubes (DWCNTs) suspended between two metal electrodes. Polarization-sensitive, broadband, and significant photoresponse occurring at the DWCNT-metal contacts under THz illumination are observed with room-temperature photocurrent and photovoltage responsivities up to ∼16 mA/W and ∼0.2 V/W at 2.52 THz, respectively. Scanning photocurrent measurements provide evidence that the photothermoelectric mechanism dominates the detector response. The simple geometry and compact nature of our device make it suitable for integration and show promising applications for THz detection.

Calibration of a thermal detector for pulse energy measurement of terahertz radiation.

We present a calibration method for measuring the terahertz pulse energy through a conventional thermal power detector. Short terahertz pulses were generated by mechanically modulating a continuous wave source with a chopper containing a narrow slot and detected by a Golay cell. We use a calibrated calorimeter to monitor the total source power so we can know the terahertz pulse energy in advance. The Golay detector response to rectangular pulses is theoretically analyzed and the peak amplitude of its output signal is found to be the relevant parameter to determine the pulse energy. We accomplish absolute calibration for the pulse responsivity of the Golay cell by examining the linear correlation between the output signal and the incident energy.