RESUMO
We demonstrate terahertz (THz) wave generation by wavelength conversion in a ridge-type/bulk periodically poled lithium niobate (RT-/bulk-PPLN) under almost the same experimental conditions. When using the RT-PPLN, the ridge structure works as a slab waveguide for the incident pump beam (wavelength: â¼1â µm), and the generated THz wave (â¼200â µm) was emitted uniformly from the entire side surface of the crystal. The RT-PPLN has a much higher conversion efficiency from the pumping beam to the THz wave than the bulk-PPLN, and the ratio improved several ten times compared with those of previous studies.
RESUMO
The realization of high-quality quantum emitters that can operate at room temperature is important for accelerating the application of quantum technologies, such as quantum communication, quantum information processing, and quantum metrology. In this work, we study the photon-antibunching properties on room-temperature emission from individual colloidal quantum dots (CQDs) using superconducting-nanowire single-photon detectors and temporal filtering of the photoluminescence decay curve. We find that high single-photon purities and high photon-generation rates can be simultaneously achieved by removing the signals originating from the sequential two-photon emission of biexcitons created by multiple excitation pulses. We successfully demonstrate that the ultrahigh performance of the room-temperature single-photon sources showing g(2)(0) ⪠10-2 can be confirmed by the ultralow-dark-count detection of the temporally purified single photons. These findings provide strong evidence for the attractiveness of CQDs as candidates for high-quality room-temperature quantum light sources.
RESUMO
Using a frequency-tunable optical comb generated from a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) and a nonlinear optical fiber, we demonstrated a frequency measurement of continuous terahertz wave sources with the frequency of 0.1 and 0.6 THz by an electro-optic sampling method. We clearly observed beat signals between the terahertz source and the optical two-tone extracted from the optical comb, allowing us to determine the absolute frequency. Owing to the wide comb spacing of the MZ-FCG, this method has a high potential for the high-speed measurement of the frequency of terahertz wave sources.
RESUMO
Multi-input multi-output (MIMO) transmission of two millimeter-wave radio signals seamlessly converted from polarization-division-multiplexed quadrature-phase-shift-keying optical signals is successfully demonstrated, where a radio access unit basically consisting of only optical-to-electrical converters and a radio receiver performs total signal equalization of both the optical and the radio paths and demodulation with digital signal processing (DSP). Orthogonally polarized optical components that are directly converted to two-channel radio components can be demultiplexed and demodulated with high-speed DSP as in optical digital coherent detection. 20-Gbaud optical and radio seamless MIMO transmission provides a total capacity of 74.4 Gb/s with a forward error correction overhead of 7%.
RESUMO
The generation of a 40-Gb/s 16-QAM radio-over-fiber (RoF) signal and its demodulation of the wireless signal transmitted over free space of 30 mm in W-band (75-110 GHz) is demonstrated. The 16-QAM signal is generated by a coherent polarization synthesis method using a dual-polarization QPSK modulator. A combination of the simple RoF generation and the versatile digital receiver technique is suitable for the proposed coherent optical/wireless seamless network.
RESUMO
The broadband wavelength tunability of femtosecond pulse generation using a Mach-Zehnder-modulator-based flat-comb generator (MZ-FCG) and a dispersion-flattened dispersion-decreasing fiber (DF-DDF) was demonstrated. Near-Fourier-transform-limit picosecond pulses generated from the MZ-FCG were compressed into femtosecond pulses by adiabatic soliton compression. By tuning the wavelength of the input cw light, 200 fs, 10 GHz pulses were generated in the wavelength range of 1,535 to 1,570 nm. Such wide-range wavelength tunability was realized by both the independence of a comb-flattening condition from the inputted wavelength and the dispersion flatness of the DF-DDF.
RESUMO
By combining a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) with a dispersion-flattened dispersion-decreasing fiber, femtosecond pulses have been generated from a cw light. Near-Fourier-transform-limit picosecond pulses generated from the MZ-FCG were compressed into femtosecond order by pulse compression. Our system enables flexible tuning of the repetition rate and pulse width, because those depend on the driving signal of the MZ-FCG. Pulse trains of 200 fs width were continuously and stably generated without mode hopping, with a repetition rate range from 5 to 17 GHz. Our system consists of a modulator and compression fiber; thus, the configuration is simpler and more stable.
RESUMO
We have demonstrated the operation of a frontside-illuminated GaAs/AlGaAs quantum-well photodetector based on intersubband absorption in a quantum well (QW) with a targeted peak frequency of 3 THz. A multiple-quantum-well structure consists of 20 periods of 18 nm QWs interleaved by 80 nm barriers with an Al alloy content of 2%. We measured the following performance characteristics: dark current, responsivity, and spectral response. A responsivity of 13 mA/W at an electric bias of 40 mV and an operating temperature of 3 K was obtained with a peak response close to the designed detection frequency. The dark current density was a few microA/cm2 and was limited by thermally assisted tunneling through the barriers. We looked also at possible designs to optimize the device's performance.
RESUMO
A new method of fabricating multilayer optical coatings used at terahertz frequencies has been developed. Using plasma-enhanced chemical-vapor deposition, a multilayer antireflection coating for germanium optics at terahertz frequencies was fabricated. The coating consists of amorphous silicon and silicon-oxide layers. The transmittance and structure of the coating were experimentally investigated. The transmittance spectrum of the coating on the Ge substrate shows a wideband antireflection behavior in the 40-120 cm(-1) region.
RESUMO
A method of manufacturing optical coatings for germanium optics used at terahertz frequencies has been developed. The various optical coatings used at terahertz frequencies are difficult to manufacture conventionally because these coatings must be as thick as several tens of micrometers, which is far thicker than those used in the optical region. One way to overcome this problem is to form a silicon oxide layer through plasma-enhanced chemical-vapor deposition, with silane (SiH4) as a source gas. Using this method, I formed 21-microm-thick silicon oxide films as antireflection (AR) layers for germanium optics and obtained low reflection at 1.7 THz (wavelength, lambda = 175 microm). This method is easily applied to large-aperture optics and micro-optics as well as to optics with a complex surface form. The AR coatings can also be formed for photoconductive detectors made from germanium doped with gallium at a low temperature (160 degrees C); this low temperature ensures that the doped impurities in the germanium do not diffuse. Fabrication of optical coatings upon substrates that have refractive indices of 3.84-11.7 may also be possible by control of the refractive indices of the deposited layers.
