Generally, waste heat is redundantly released into the surrounding by anthropogenic activities without strategized planning. Consequently, urban heat islands and global warming chronically increases over time. Thermophotovoltaic (TPV) systems can be potentially deployed to harvest waste heat and recuperate energy to tackle this global issue with supplementary generation of electrical energy. This paper presents a critical review on two dominant types of semiconductor materials, namely gallium antimonide (GaSb) and indium gallium arsenide (InGaAs), as the potential candidates for TPV cells. The advantages and drawbacks of non-epitaxy and epitaxy growth methods are well-discussed based on different semiconductor materials. In addition, this paper critically examines and summarizes the electrical cell performance of TPV cells made of GaSb, InGaAs and other narrow bandgap semiconductor materials. The cell conversion efficiency improvement in terms of structural design and architectural optimization are also comprehensively analyzed and discussed. Lastly, the practical applications, current issues and challenges of TPV cells are critically reviewed and concluded with recommendations for future research. The highlighted insights of this review will contribute to the increase in effort towards development of future TPV systems with improved cell conversion efficiency.

Three-dimensional (3D) glass chips are promising waveguide platforms for building hybrid 3D photonic circuits due to their 3D topological capabilities, large transparent windows, and low coupling dispersion. At present, the key challenge in scaling down a benchtop optical system to a glass chip is the lack of precise methods for controlling the mode field and optical coupling of 3D waveguide circuits. Here, we propose an overlap-controlled multi-scan (OCMS) method based on laser-direct lithography that allows customizing the refractive index profile of 3D waveguides with high spatial precision in a variety of glasses. On the basis of this method, we achieve variable mode-field distribution, robust and broadband coupling, and thereby demonstrate dispersionless LP₂₁-mode conversion of supercontinuum pulses with the largest deviation of <0.1 dB in coupling ratios on 210 nm broadband. This approach provides a route to achieve ultra-broadband and low-dispersion coupling in 3D photonic circuits, with overwhelming advantages over conventional planar waveguide-optic platforms for on-chip transmission and manipulation of ultrashort laser pulses and broadband supercontinuum.

Saturable absorbers (SAs) have been regarded as a low‐cost yet robust and compact solution to pulse generation in different fiber and solid‐state ultrafast lasers. Following the development of different artificial SAs and semiconductor SA mirrors, carbon nanotube (CNT) and graphene are discovered and employed as SAs for solid‐state as well as fiber lasers. In the past decade, a growing number of SAs based on new materials like MXenes and metal halide perovskites have emerged, while CNT and graphene are still irreplaceable by new materials due to their fascinating saturable absorption properties. Instead of searching for new SAs, the investigation of transient dynamics for mode‐locked pulse generation still relies on the use of CNT and graphene SAs in the fiber laser system due to their reproducible performance and facile fabrication. In this review, a comparison between CNT and graphene will be thoroughly studied in terms of optical nonlinearity, relaxation and recovery time, saturation intensity or fluence, modulation depth, non‐saturable loss, and damage threshold dynamics. In the end, this review will address the challenges in the selection of SAs and provides recommendations for CNT and graphene SAs used in different mode‐locked fiber lasers.

We demonstrated an all-fiber Er-doped Mamyshev oscillator with short pulse duration and high peak power. Through the dispersion management technique and accumulation of sufficient nonlinear phase shift, stable pulses with repetition rate of 6.55 MHz, 20-dB spectral bandwidth of 78.2 nm, pulse duration of 72 fs and peak power of 86 kW were achieved directly without external compression. Simulations have been performed to show the direct generation of ultrafast laser pulse from the Mamyshev oscillator based on the extended nonlinear Schrdinger equation. To the best of our knowledge, this is by far the shortest pulse duration, the highest experimental peak power Er-doped Mamyshev oscillator.