We investigate thermoelectric pumping in wide-bandgap GaN based light-emitting diodes (LEDs) to take advantage of high junction temperature rather than avoiding the problem of temperature-induced efficiency droop through external cooling. We experimentally demonstrate a thermally enhanced 450 nm GaN LED, in which nearly fourfold light output power is achieved at 615 K (compared to 295 K room temperature operation), with nearly no reduction in the wall-plug efficiency (i.e., electrical-optical energy conversion efficiency) at bias V<ℏω/q. The LED is shown to work in a mode similar to a thermodynamic heat engine operating with charged carriers pumped into the active region by a combination of electrical work and Peltier heat (phonons) drawn from the lattice. In this optimal operating regime at 615 K, the LED injection current (3.26 A/cm2) is of similar magnitude to the operating point of common high power GaN based LEDs (5–35 A/cm2). This result suggests the possibility of removing bulky heat sinks in current high power LED products thus realizing a significant cost reduction for solid-state lighting.

Light detection and ranging (LiDAR) sensors enable precision sensing of an object in 3D. LiDAR technology is widely used in metrology, environment monitoring, archaeology, and robotics. It also shows high potential to be applied in autonomous driving. In traditional LiDAR sensors, mechanical rotator is used for optical beam scanning, which brings about limitations on their reliability, size, and cost. These limitations can be overcome by a more compact solid‐state solution. Solid‐state LiDAR sensors are commonly categorized into the following three types: flash‐based LiDAR, microelectromechanical system (MEMS)‐based LiDAR, and optical phased array (OPA)‐based LiDAR. Furthermore, advanced optics technology enables novel nanophotonics‐based devices with high potential and superior advantages to be utilized in a LiDAR sensor. In this review, LiDAR sensor principles are introduced, including three commonly used sensing schemes: pulsed time of flight (TOF), amplitude‐modulated continuous wave TOF, and frequency‐modulated continuous wave. Recent advances in conventional solid‐state LiDAR sensors are summarized and presented, including flash‐based LiDAR, MEMS‐based LiDAR, and OPA‐based LiDAR. The recent progress on emerging nanophotonics‐based LiDAR sensors is also covered. A summary is made and the future outlook on advanced LiDAR sensors is provided.

including high refractive index contrast with its oxide material, low loss at communication wavelength regime, and significant thermo-optic coefficient for tuning. Contributed by these advantages, various photonic components have been demonstrated on integrated Si photonics platform, including mode couplers, [3][4][5][6] tunable filters, [7][8][9][10] and optical modulators. [11][12][13][14] However, Si itself is an indirect bandgap material, which limits its light emission efficiency. Laser sources on Si remain to be the challenging component for photonics integration. The requirements of an integrated laser source not only cover laser performance parameters such as optical power, threshold, pumping scheme, and stability, but also low-cost and high-volume production. Researchers are trying to come up with different ways to integrate lasers on Si. The most common way is to integrate III-V lasers on Si photonics platform, through bonding integration or direct growth approach. Alternatively, group IV materials such as germanium (Ge) and germanium tin (GeSn) alloy-based lasers show promise, with significant research progress in the past decade. Raman effect has also been explored to demonstrate lasers on Si. Such kind of laser enables lasing from the Si material itself and has drawn a lot of interest in the research community. Also, rare earth (RE) elements can be doped within photonics layer to make lasers on Si, which is similar to the idea of RE-doped optical fiber laser. RE-doped laser has the advantage of low noise and high thermal stability. Comprehensive reviews on Si-integrated lasers were published more than 6 years ago. [15,16] In the meanwhile, to the best of our knowledge, the review for the most recent progress on Siintegrated lasers covering the above-mentioned approaches is still lacking.In this review, we have summarized the recent development progress of lasers integrated on Si in the past two decades, with the focus on the wavelength regimes for communication. These lasers are categorized based on their gain media, including III-V semiconductor laser, Ge/GeSn laser, Si-based Raman laser, and RE-doped laser on Si. For III-V laser, different integration approaches are discussed, covering flip-chip integration, transfer printing integration, hybrid bonding, and direct growth method. The review is organized in the following way: it starts from background information as presented in Section 1. III-V laser, Ge/GeSn laser, Raman laser, and RE-doped laser