We present a detailed investigation on the effect of functional group modulation at the edges of carbon quantum dots (CQDs) on the fluorescence from the CQDs. The CQDs attached by N, S, and P elements are synthesized via pyrolysis of a mixture of citric acid and NH3H2O, H2SO4, and H3PO4, respectively. Thus, part of –COOH at the edges of CQDs can be converted into –C=O and functional groups such as –NH2, –SO2, –HSO3, and –H2PO4 can connect to the carbon bonds. We find that the formation of the N/S/P-CQDs can reduce the amount of –COOH that attaches to the edges of sp2-conjugated π-domains located at centers of these CQDs. This effect can result in the reduction of the non-radiative recombination for electronic transition in these CQDs. As a result, the quantum yield (QY) for fluorescence from the CQDs can be efficiently enhanced. We demonstrate experimentally that the QYs for N/S/P-CQDs can reach up to 18.7%, 29.7%, and 10.3%, respectively, in comparison to 9% for these without functional group modulation. This work can provide a practical experimental approach in improving the optical properties of fluorescent CQDs.

A 1280 × 1,024 In_0.53Ga_0.47As short wave infrared (SWIR) focal plane array (FPA) detector with a planar-type back-illuminated process has been fabricated. With indium bump flip-chip bonding techniques, the InGaAs photodiode arrays were hybrid-integrated to the CMOS readout integrated circuit (ROIC) with correlated double sampling (CDS). The response spectrum is 0.9–1.7 μm. The test results show that the dark current density is 2.25 nA/cm² at 25 °C, the detectivity D^* is up to 1.1 × 10¹³ cm · Hz^1/2/W, the noise electron is as low as 48 e⁻ under correlated double sampling mode, the quantum efficiency is 88% at 1550 nm, and the operability is more than 99.9%. Moreover, the dark current and noise electron have been studied theoretically in depth. The results indicate that the diffusion current is the main contribution of the dark current, and the readout integrated circuit noise electron is the main source of FPA noise.

PIN InGaAs short wavelength infrared (SWIR) focal plane array (FPA) detectors have attracted extensive attention due to their high detectivity, high quantum efficiency, room temperature operation, low dark current, and good radiation resistance. Furthermore, InGaAs FPA detectors have wide applications in many fields, such as aviation safety, biomedicine, camouflage recognition, and infrared night vision. Recently, extensive research has been conducted on the extension of the response spectrum from short wavelength infrared (SWIR) to visible light (VIS) through InP substrate removal and reserving the n-InP contact layer. However, there is little research on the absorption of InGaAs detectors in the ultraviolet (UV) band. In this paper, we present an ultra-broadband UV–VIS–SWIR 640 × 512 15 μm InGaAs FPA detector by removing the n-InP contact layer in the active area and reserving the InP contact layer around the pixels for n contact, creating incident light to be directly absorbed by the In_0.53Ga_0.47As absorption layer. In addition, the optical absorption characteristics of InGaAs infrared detectors with and without an n-InP contact layer are studied theoretically. The test results show that the spectral response is extended to the range of 200–1700 nm. The quantum efficiency is higher than 45% over a broad wavelength range of 300–1650 nm. The operability is up to 99.98%, and the responsivity non-uniformity is 3.28%. The imaging capability of InGaAs FPAs without the n-InP contact layer has also been demonstrated, which proves the feasibility of simultaneous detection for these three bands.

A newly designed room-temperature, sine-wave gated, InGaAs/InP SPAD based photon counting system with dead-time mitigation is described in this work. In this system, an InGaAs/InP SPAD was developed and used as the detector. To mitigate SPAD's dead time and enable room temperature operation, a sine-wave gated quenching circuit is employed for rapid quenching and post-pulse suppression. Furthermore, phase modulation schemes are utilized to mitigate dead-time that enables SPAD to detect incident photons with unknown arrival times when applied in Optical Time Domain Reflectometry (OTDR). Results show that the system developed can operate under room temperature with a dark count rate of less than 8 kcounts/s at bias voltage of 67 V. At this bias voltage, the photon efficiency reaches 9.5% at 1400 nm. A narrow time jitter of 146 ps was achieved and the afterpulsing possibility measured was around 10% at bias voltage of 65 V. To evaluate the system's performance, a photon counting optical time domain reflectometer system was built and tested using FC/PC connectors. It has been proved that the system is able to detect reflected signals from all connectors and accurately measure their position, eliminating the problem of dead-time in the sine wave gated quenching circuit.

Traditional InP/InGaAs single-photon avalanche detectors require cooling to approximately 240 K to reduce dark counts, necessitating the use of semiconductor thermoelectric cooling, which increases both volume and power consumption. Through optimization of the material structure and fabrication processes via a single diffusion process, we have developed a high-performance 1550 nm single-photon detector that operates at room temperature (293 K). At a detection efficiency of 20.1%, the dark count rate achieved was 6.82 kHz, with an afterpulse probability of 0.15% and a timing jitter of 120 ps under gated operation. This is the lowest dark count rate reported to date for a 1550 nm single-photon detector operating at room temperature. Furthermore, in a purely passive quenching mode, we obtained a dark count rate of 32.6 kHz with a detection efficiency of 10.3% at room temperature by integrating a SiO2/Cr film resistor through a sputtering process into the detector. Our scheme provides an alternative choice for room-temperature single-photon detectors and should significantly advance their application in compact, low-power devices.