Feasibility analysis of phonon lasers

We identify the main reason impeding coherent generation of phonons in solid state - the inherently high density of phonon states. Based on the results of our analysis we formulate a set of conditions that may make phonon lasing practical and point out the most promising mechanism of phonon lasing: LO/spl rarr/LA+TO in InP. We then develop a complete set of phonon laser equations and evaluate the threshold, output power and efficiency of phonon lasers based on InP MESFET. We show that one can obtain high-power 1.26-THz coherent phonon emission with pump power as small as a few milliwatts and up to 5% slope efficiency. Potential applications are also discussed.     

Transversely-pumped counter-propagating optical parametricoscillators and amplifiers: conversion efficiencies and tuning ranges

We show that the second-order optical nonlinearities in the waveguides can be used to implement transversely-pumped counter-propagating optical parametric oscillators and amplifiers. By changing the incident angle of the pump beam, one can tune the output frequency in a large range. For the optimal semiconductor structure and with the presence of the horizontal and vertical cavities, the threshold pump powers for the oscillation can be as low as ~100 μW. There is no threshold for amplifying the incident beam. Combining the oscillators or amplifiers with vertical-cavity surface-emitting lasers, it is feasible for us to implement compact and efficient electrically-pumped mid-infrared light sources or amplifiers. The quasiphase matching is achieved by spatially modulating second-order susceptibility along the growth direction based on semiconductor alternating thin layers or asymmetric quantum well domain structures     

Practical aspects of lasing without inversion in various media

A rigorous density-matrix-based theory of lasing based on optically induced or autoionization-induced coherence between the levels is developed. The balance equations are obtained, and the conditions for lasing threshold are derived as a function of pumping strength and relaxation rates. Connection between the lasing without inversion and Raman and parametric processes is established. The main conclusion is that true CW lasing without inversion can indeed be obtained but only in a system where pumping and relaxation rates are favorable for attaining conventional lasing with population inversion. Practical implications of it, especially for the intersubband lasers, are discussed     

High‐Order Shift Current Induced Terahertz Emission from Inorganic Cesium Bromine Lead Perovskite Engendered by Two‐Photon Absorption

High‐order nonlinear optical phenomena are interesting from a fundamental point of view as they reveal intrinsic symmetries of the materials. Potentially they can also be used for practical optoelectronic applications. High‐order shift current is one of these phenomena, and yet it has never been detected in experiments, primarily due to the difficulty in conventional contact detection. In this work, the shift current due to the two‐photon absorption (TPA) from all‐inorganic perovskite CsPbBr₃ is first observed by a contactless and nondestructive terahertz (THz) emission method. The results reveal that the THz emission is dominated by a high‐order shift current (fourth‐order nonlinear optical effect) with the below‐bandgap photon energy of femtosecond laser excitation. The high‐order shift current origins from the broken inversion symmetry induced by the dynamic stereochemical activity of the Pb²⁺ lone pair. A microscopic TPA‐assisted nonlinear optical model is presented to describe the photophysical process of the THz emission. The model matches well with the quadratic pump fluence and angular dependence of the THz emission. This work can not only open a new venue for the all‐inorganic perovskite‐based nonlinear optoelectronics and THz devices, but also afford a THz technology for the high‐order nonlinear effect analysis.     

Closed-Loop Bias Control of Optical Quadrature Modulator

We describe a relatively simple but effective closed-loop bias control of LiNbO₃ quadrature modulator (QM) for optical quaternary phase-shift-keyed (QPSK) transmission. Simulation and experimental results of the feedback control loop for maintaining near optimal operating points of the QM are described for 12.5-GSym/s optical QPSK. The control loop uses nonhigh-speed off-the-shelf components independent of the symbol rate. Similar performance of the control loop at higher symbol rates using the same components is expected     

A low-crosstalk semiconductor optical amplifier

A new type of semiconductor optical amplifiers (SOAs) that can have a very low-channel crosstalk is proposed and demonstrated. By arranging the order of the gain materials with different saturation power, the SOA crosstalk is greatly reduced. This new type of SOA can be an ideal gain material for wavelength division multiplexed (WDM) amplification and integration in the access and metro environment.     

Single longitudinal mode operation of the electron-beam-pumped semiconductor laser

The influence of the external and internal resonator parameters on the laser emission spectral and spatial characteristics has been investigated. Single longitudinal mode operation of the electron-beam-pumped visible semiconductor laser with an external resonator has been observed. A single-mode peak power of 0.8 W and beam divergence of less than 3° were achieved.     

Theory of backward second-harmonic and third-harmonic generationusing laser pulses in quasi-phase-matched second-order nonlinear medium

Recently, we have observed backward second-harmonic generation (SHG) using subpicosecond and nanosecond laser pulses in periodically poled lithium niobate crystals. The conversion efficiency in terms of energy density per pulse measured in the experiments agrees well with that based on our theory. In this paper, we present our theoretical investigation of this process in detail. Although the temporal behaviors of the second-harmonic radiation are determined by the same functions as those for the forward SHG, the amounts of pulse broadening and conversion efficiencies are quite different for the two configurations. In addition, we have investigated saturation of the conversion efficiencies. We have also considered effective third-harmonic generation based on cascaded second-order nonlinear processes. For a few optimized spatial periods of the domains, we have estimated the conversion efficiencies     

Backward optical parametric oscillators and amplifiers

Degenerate backward optical parametric oscillators and amplifiers have been considered, for the first time, to the best of our knowledge. When the pump intensity is four times the threshold pump intensity, the conversion efficiency reaches a maximum value. On the other hand, for nondegenerate optical parametric oscillators, the conversion efficiency always increases as the pump intensity increases. This behavior is different from those for forward optical parametric oscillators. In either configuration, the oscillation can occur without an external feedback. There is, however, a distributed feedback provided through the opposite propagation directions of the signal and idler. The threshold pump intensities for the oscillators can be achieved by the lasers currently available based on quasi-phase matching in several structures. As the input intensity for the backward parametric wave increases, the gain for this wave decreases dramatically if the pump intensity is on the order of the threshold or higher. When the input intensity is much larger than the threshold pump intensity, there is almost no gain regardless of the level of the pump intensity