Coherent acoustic phonons are generated at terahertz frequencies when semiconductor quantum-well nanostructures are illuminated by femtosecond laser pulses. These phonons-also known as nanoacoustic waves-typically have wavelengths of tens of nanometres, which could prove useful in applications such as non-invasive ultrasonic imaging and sound amplification by the stimulated emission of radiation. However, optical diffraction effects mean that the nanoacoustic waves are produced with spot sizes on the micrometre scale. Near-field optical techniques can produce waves with smaller spot sizes, but they only work near surfaces. Here, we show that a far-field optical technique--which suffers no such restrictions--can be used to spatially manipulate the phonon generation process so that nanoacoustic waves are emitted with lateral dimensions that are much smaller than the laser wavelength. We demonstrate that nanoacoustic waves with wavelengths and spot sizes of the order of 10 nm and 100 nm, respectively, can be generated and detected.
We report InAs/InGaAs quantum dot (QD) waveguide photodetectors (PD) monolithically grown on silicon substrates. A high-crystalline quality GaAs-on-Si template was achieved by aspect ratio trapping together with the combined effects of cyclic thermal annealing and strain-balancing layer stacks. An ultra-low dark current of 0.8 nA and an internal responsivity of 0.9 A/W were measured in the O band. We also report, to the best of our knowledge, the first characterization of high-speed performance and the first demonstration of the on-chip photodetection for this QD-on-silicon system. The monolithically integrated waveguide PD shares the same platform as the previously demonstrated micro-ring lasers and can thus be integrated with laser sources for power monitors or amplifiers for pre-amplified receivers.
Ultrabroadband millimeter-wave and subterahertz waveforms offer significant potential, from ultrahighspeed communications to high-resolution radar. Electronic generation of broadband arbitrary waveforms at these frequencies suffers from limited digital-to-analog converter speed and high timing jitter. Photonicassisted techniques, such as those based on optical shaping and frequency-to-time mapping, can overcome these difficulties. Nevertheless, previous photonic arbitrary waveform generation demonstrations are confined to microwave and low millimeter-wave frequencies due to limited optical-to-electrical conversion bandwidth. Here, by utilizing cutting-edge photodetector technology, we report the first generation, wireless transmission, and measurement of substantially complex and highly stable arbitrary waveforms in the W-band (75-110 GHz). These waveforms exhibit high time-bandwidth products up to 600, arbitrarily extendable repetition periods, and phase-noise performance substantially better than state-of-the-art electronic arbitrary waveform generators. The utility of the generated waveforms is demonstrated in multi-target ranging, where a depth resolution of 3.9 mm, unprecedented in the W-band, is achieved over more than 5 m.
Ultra-high-speed photodetectors and printed-circuit antennas construct photonic transmitters. In this letter, we demonstrate a terahertz ͑THz͒ photonic transmitter: edge-coupled membrane photonic transmitters based on metal-semiconductor-metal traveling-wave photodetectors, which are fabricated with low-temperature-grown GaAs photoabsorption layers. With a membrane-based and edge-coupled structure, the demonstrated photonic transmitters can eliminate the requirement of Si lenses and attain an over 20 times higher optical-to-THz power conversion efficiency 2ϫ10 Ϫ4 than vertical illuminated photonic transmitters with Si lenses at the same operation frequency.
We demonstrate a photonic microwave generator on the heterogeneous silicon-InP platform. Waveguide photodiodes with a 3 dB bandwidth of 65 GHz and 0.4 A/W responsivity are integrated with lasers that tune over 42 nm with less than 150 kHz linewidth. Microwave signal generation from 1 to 112 GHz is achieved.
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