Low-Temperature Thermal Transport Characteristics in Epitaxial Bilayer Graphene Microbridges.

In this paper, we present a study of the thermal transport of epitaxial bilayer graphene microbridges. The thermal conductance of three graphene microbridges with different lengths was measured at different temperatures using Johnson noise thermometry. We find that with the decrease of the temperature, the thermal transport in the graphene microbridges switches from electron-phonon coupling to electron diffusion, and the switching temperature is dependent on the length of the microbridge, which is in good agreement with the simulation based on a distributed hot-spot model. Moreover, the electron-phonon thermal conductance has a temperature power law of T3 as predicted for pristine graphene and the electron-phonon coupling coefficient σep is found to be approximately 0.18 W/(m2 K4), corresponding to a deformation potential D of 55 eV. In addition, the electron diffusion in the graphene microbridges adheres to the Wiedemann-Franz law, requiring no corrections to the Lorentz number.

A novel method for the measurement of superconducting transmission lines at terahertz frequencies.

Characterizing the properties (e.g., effective dielectric constant εeff, attenuation constant α, and characteristic impedance Z0) of terahertz (THz) superconducting transmission lines is of particular interest in designing on-chip integrated THz bandpass filters, which are a critical component for THz astronomical instruments, such as multi-color camera and broadband imaging spectrometers. Here, we propose a novel method for the characterization of three parameters (εeff, α, and Z0) of THz superconducting transmission lines. This method measures the ratio of the THz signal powers through two different-length branches of the superconducting transmission line to be measured. In addition, only one measurement is required for an all-in-one device chip, including an antenna, a half-power divider, the superconducting transmission line to be measured, and two detectors. The key point is that the superconducting transmission line to be measured is impedance-mismatched with the two integrated detectors. The method is validated through simulation and measurement for superconducting coplanar waveguide transmission lines around 400 GHz.

A triple band local oscillator coupling system based on a phase grating for five-meter Dome-A terahertz explorer telescope.

For heterodyne receivers, broadband and integrated local oscillator (LO) coupling schemes are crucial. In this paper, we present a novel terahertz broadband receiver system based on a one-pixel phase grating. We designed a phase grating that can simultaneously couple the LO signals at 690, 850, and 1300 GHz to a superconducting hot-electron bolometer mixer at the incident angles of 25.7°, 20.6°, and 13.3°, respectively. At these three frequencies, the measured diffraction efficiencies of the phase grating are as high as 70%, 73%, and 65%, and the total power coupling efficiencies of the LO coupling system are 22%, 23%, and 23%, in good agreement with the simulated results. This efficient and highly integrated multiband receiver system has potential applications in the proposed Five-meter Dome-A Terahertz Explorer telescope.

Faking photon number on a transition-edge sensor.

We study potential security vulnerabilities of a single-photon detector based on superconducting transition-edge sensor. In one experiment, we show that an adversary could fake a photon number result at a certain wavelength by sending a larger number of photons at a longer wavelength, which is an expected and known behaviour. In another experiment, we unexpectedly find that the detector can be blinded by bright continuous-wave light and then, a controlled response simulating single-photon detection can be produced by applying a bright light pulse. We model an intercept-and-resend attack on a quantum key distribution system that exploits the latter vulnerability and, under certain assumptions, able to steal the key.

Development of terahertz two-dimensional phase gratings for multiple beam generation based on a high-accuracy phase retrieval algorithm.

High efficiency and accuracy phase gratings are of crucial importance for large format heterodyne array receivers at terahertz frequencies. Here, by developing a design approach that can create gratings with arbitrary two-dimensional diffraction distributions, we have realized a reflective metallic phase grating that generates 2×2 diffraction beams at 0.85 THz. The measured total power efficiency of the diffraction beam pattern is 81.9%, which demonstrates at least 17% improvement in efficiency compared with the standard pseudo-2D Fourier phase grating. In addition, we report the realization of up to 10×10 diffraction beam two-dimensional phase grating designs at terahertz wavelengths, using an adaptation of the Gerchberg-Saxton (GS) scheme known as the Mixed-Region-Amplitude-Freedom algorithm. Rigorous full wave simulation proves the efficiency and accuracy of the design, which overcomes the inaccurate intensity of the beam distribution drawbacks originated from the standard GS algorithm. The results pave the way for the development of large-pixel terahertz multi-beam heterodyne receivers.

Optical Sensors and Sensing, 2019: introduction to the joint feature issue.

This joint feature issue of Optics Express and Applied Optics highlights contributions from authors who presented their latest research at the OSA Optical Sensors and Sensing Congress, held in San Jose, California, USA from 25-27 June 2019. The joint feature issue comprises 6 contributed papers, which expand upon their respective conference proceedings. The published papers introduced here cover a range of timely research topics in optics and photonics for active open-path sensing, radiometry, and adaptive optics and fiber devices.

Wide-field optical design for a 60 m submillimeter telescope.

We present a practical wide-field optical design for a 60 m aperture submillimeter telescope, which is currently under conceptual design study in China. The telescope is specified to operate over a wavelength range of 0.65-3 mm and provide a wide field of view (FOV) of 1° in diameter. We designed an F/6 Ritchey Chrétien (RC) system with a quasi-planar tertiary corrector, which cancels all spherical, coma, and astigmatism aberrations. It also achieves a good balance among the mirror sizes, central obscuration, and focal-plane curvature. The problems of focal surface curvature and nontelecentricity are treated in the subfield instrumental design, which employs a simple silicon wedge prism to obtain flat and telecentric focal planes for each subfield instrument module. Our studies show that by such a design, more than ${{10}^5}$105 detector pixels can be efficiently and uniformly fed at the shortest wavelength band with Strehl ratios above 0.85 across the entire 1° FOV. Several practical issues related to the telescope optics are also discussed.

Optical Sensors and Sensing 2019: introduction to the joint feature issue.

This joint feature issue of Optics Express and Applied Optics highlights contributions from authors who presented their latest research at the OSA Optical Sensors and Sensing Congress, held in San Jose, California, USA, from 25-27 June 2019. The joint feature issue comprises six contributed papers, which expand upon their respective conference proceedings. The published papers introduced here cover a range of timely research topics in optics and photonics for active open-path sensing, radiometry, and adaptive optics and fiber devices.

High-Speed Device-Independent Quantum Random Number Generation without a Detection Loophole.

Quantum mechanics provides the means of generating genuine randomness that is impossible with deterministic classical processes. Remarkably, the unpredictability of randomness can be certified in a manner that is independent of implementation devices. Here, we present an experimental study of device-independent quantum random number generation based on a detection-loophole-free Bell test with entangled photons. In the randomness analysis, without the independent identical distribution assumption, we consider the worst case scenario that the adversary launches the most powerful attacks against the quantum adversary. After considering statistical fluctuations and applying an 80 Gb×45.6 Mb Toeplitz matrix hashing, we achieve a final random bit rate of 114 bits/s, with a failure probability less than 10^{-5}. This marks a critical step towards realistic applications in cryptography and fundamental physics tests.

Demonstration of a fully integrated superconducting receiver with a 2.7 THz quantum cascade laser.

We demonstrate for the first time the integration of a superconducting hot electron bolometer (HEB) mixer and a quantum cascade laser (QCL) on the same 4-K stage of a single cryostat, which is of particular interest for terahertz (THz) HEB/QCL integrated heterodyne receivers for practical applications. Two key issues are addressed. Firstly, a low power consumption QCL is adopted for preventing its heat dissipation from destroying the HEB's superconductivity. Secondly, a simple spherical lens located on the same 4-K stage is introduced to optimize the coupling between the HEB and the QCL, which has relatively limited output power owing to low input direct current (DC) power. Note that simulation techniques are used to design the HEB/QCL integrated heterodyne receiver to avoid the need for mechanical tuning. The integrated HEB/QCL receiver shows an uncorrected noise temperature of 1500 K at 2.7 THz, which is better than the performance of the same receiver with all the components not integrated.

A quasi-optical vector near-field measurement system at terahertz band.

This paper describes a vector near-field measurement system at terahertz band based on a high sensitivity superconducting receiver equipped with a quasi-optical probe for high resolution near-field sensing. A novel single-receiver rather than commonly used dual-receiver configuration is adopted for vector measurement. Performances of the measurement system including stability and dynamic range are studied. Vector near-field measurement of a diagonal feedhorn at 850 GHz is presented and shows good agreement with simulation and direct far-field measurement.

Effects of laser-plasma interactions on terahertz radiation from solid targets irradiated by ultrashort intense laser pulses.

Interactions of 100-fs laser pulses with solid targets at intensities of 10(18) W/cm(2) and resultant terahertz (THz) radiation are studied under different laser contrast ratio conditions. THz emission is measured in the specular reflection direction, which appears to decrease as the laser contrast ratio varies from 10(-8) to 10(-6). Correspondingly, the frequency spectra of the reflected light are observed changing from second harmonic dominant, three-halves harmonic dominant, to vanishing of both harmonics. Two-dimensional particle-in-cell simulation also suggests that this observation is correlated with the plasma density scale length change. The results demonstrate that the THz emission is closely related to the laser-plasma interaction processes. The emission is strong when resonance absorption is a key feature of the interaction, and becomes much weaker when parametric instabilities dominate.