The fast development of mid-wave infrared photonics has increased the demand for high-performance photodetectors that operate in this spectral range. However, the signal-to-noise ratio, regarded as a primary figure of merit for mid-wave infrared detection, is strongly limited by the high dark current in narrow-bandgap materials. Therefore, conventional mid-wave infrared photodetectors such as HgCdTe require cryogenic temperatures to avoid excessively high dark current. To address this challenge, we report an avalanche photodiode design using photon-trapping structures to enhance the quantum efficiency and minimize the absorber thickness to suppress the dark current. The device exhibits high quantum efficiency and dark current density that is nearly three orders of magnitude lower than that of the state-of-the-art HgCdTe avalanche photodiodes and nearly two orders lower than that of previously reported AlInAsSb avalanche photodiodes that operate at 2 µm. Additionally, the bandwidth of these avalanche photodiodes reaches ~7 GHz, and the gain–bandwidth product is over 200 GHz; both are more than four times those of previously reported 2 µm avalanche photodiodes.

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Recently, advances in imaging and LIDAR applications have stimulated the development of high-sensitivity receivers that operate at wavelengths of ≥ 2 µm, which has driven research on avalanche photodiodes (APDs) that operate in that spectral region. High quantum efficiency is a key performance parameter for these photodetectors. Increasing the thickness of the absorption region is a straightforward approach to increase the quantum efficiency. However, the primary source of dark current is the narrow-bandgap material used for 2-µm detection. Increasing its thickness results in higher noise. In this paper, we describe two approaches to enhance the quantum efficiency, both of which are superior to a conventional anti-reflection (AR) coating. For normal incidence at 2 µm, finite-difference time-domain (FDTD) simulations show the absorption can be enhanced by more than 100% with a triangular-lattice photonic crystal, and nearly 400% by applying a metal grating. This is achieved by coupling normal incidence light into the laterally propagating modes in the device. Moreover, the significantly higher absorption of the metal grating compared to the photonic crystal is due to the high coupling efficiency provided by the metal grating. This work provides promising methods and physical understanding for enhancing the quantum efficiency for 2-µm detection without increasing absorber thickness, which also enables low dark current and high bandwidth.

Lithium niobate on insulator (LNOI) has become an intriguing platform for integrated photonics for applications in communications, microwave photonics, and computing. Whereas, integrated devices including modulators, resonators, and lasers with high performance have been recently realized on the LNOI platform, high-speed photodetectors, an essential building block in photonic integrated circuits, have not been demonstrated on LNOI yet. Here, we demonstrate for the first time, heterogeneously integrated modified uni-traveling carrier photodiodes on LNOI with a record-high bandwidth of 80 GHz and a responsivity of 0.6 A/W at a 1550-nm wavelength. The photodiodes are based on an n-down InGaAs/InP epitaxial layer structure that was optimized for high carrier transit time-limited bandwidth. Photodiode integration was achieved using a scalable wafer die bonding approach that is fully compatible with the LNOI platform.

The morphology evolution of self-organized nanopatterns induced during Ar+ ion bombardment (IB) with Mo co-deposition on fused silica (SiO2) surfaces at different incidence angles and fluences was investigated by using atomic force microscopy and transmission electron microscopy. For pure IB at incidence angles from 30° to 70°, SiO2 surfaces evolve from being flat, via ripples, to direction-transversed ripples. In contrast, at the same ion fluence and incidence angles, the simultaneous Mo co-deposition leads to significant terraced structures with significantly enhanced roughness and wavelength. Our observations show that the concurrent Mo co-deposition during IB can reduce the critical incidence angle and the fluence level of terrace formation. Owing to the guidance of the IB-induced morphology, at incidence angles where a well-ordered ripple-mode can be generated, well-ordered terrace morphology is more likely to be formed. Terraced structures are initiated and further grow until the appearance of the nonlinear phase, i.e., where the ripple amplitude is sufficiently high. The enhanced terrace morphology on smooth SiO2 results from the interplay between pure IB and Mo co-deposition. The phase separation is attributed to the formation of crystalline MoOx on the side facing the impurity.