A high-brightness, droop-free, and speckle-free InGaN/GaN quantum well blue superluminescent diode (SLD) was demonstrated on a semipolar (2021) GaN substrate. The 447-nm emitting SLD has a broad spectral linewidth of 6.3 nm at an optical power of 123 mW. A peak optical power of 256 mW was achieved at 700 mA CW injection current. By combining YAG:Ce phosphor, SLD-generated white light shows a color-rendering index (CRI) of 68.9 and a correlated color temperature (CCT) of 4340 K. The measured frequency response of the SLD revealed a -3 dB bandwidth of 560 MHz, thus demonstrating the feasibility of the device for both solid-state lighting (SSL) and visible-light communication (VLC) applications. © 2016 Optical Society of America.

We implemented a tunable dual-longitudinal-mode spacing InGaN/GaN green (521–528 nm) laser diode by employing a self-injection locking scheme that is based on an external cavity configuration and utilizing either a high-or partial-reflecting mirror. A tunable longitudinal-mode spacing of 0.20 – 5.96 nm was accomplished, corresponding to a calculated frequency difference of 0.22–6.51 THz, as a result. The influence of operating current and temperature on the system performance was also investigated with a measured maximum side-mode-suppression ratio of 30.4 dB and minimum dual-mode peak optical power ratio of 0.03 dB. To shed light on the operation of the dual-wavelength device arising from the tunable longitudinal-mode spacing mechanism, the underlying physics is qualitatively described. To the best of our knowledge, this tunable longitudinal-mode-spacing dual-wavelength device is novel, and has potential applications as an alternative means in millimeter wave and THz generation, thus possibly addressing the terahertz technology gap. The dual-wavelength operation is also attractive for high-resolution imaging and broadband wireless communication.

We demonstrate narrow-line green laser emission at 513.85 nm with a linewidth of 31 pm and side-mode suppression ratio of 36.9 dB, operating under continuous-wave injection at room temperature. A high-order (40th) distributed-feedback surface grating fabricated on multimode InGaN-based green laser diodes via a focused ion beam produces resolution-limited, single-mode lasing with an optical power of 14 mW, lasing threshold of 7.27 kA cm−2, and maximum slope efficiency of 0.32 W A−1. Our realization of narrow-line green laser diodes opens a pathway toward efficient optical communications, sensing, and atomic clocks.

Self-assembled nanowires are posed to be viable alternatives to conventional planar structures, including the nitride epitaxy for optoelectronic, electronic and nano-energy applications. In many cases, current injection and extraction at the nanoscopic scale are essential for marked improvement at the macroscopic scale. In this investigation, we study the mechanism of nanoscale current injection and the origin of improvement of the flow of charged carriers at the group-III nitride semiconductor surface and metal-semiconductor interfaces. Conductive atomic force microscopy (c-AFM) and Kelvin probe force microscopy (KPFM) enable a rapid analysis of the electrical and morphological properties of single and ensemble nanostructures. The surface potential and current injection of AlGaN nanowire-based LEDs are spatially mapped before and after surface treatment with KOH solution. Treated-nanowires showed an improved current spreading and increased current injection by nearly 10×, reduced sub-turn-on voltage (as low as 5 V), and smaller series resistance. The reduced contact potential confirms the lower semiconductor/metal barrier, thus enabling larger carriers flow, and correlates with the 15% increase in injection efficiency in macroscopic LEDs. The improvement leads to the normalization of nanoscale electrical conducting properties of UV AlGaN-based nanowire-LEDs and lays the foundation for the realization of practical nanowire-based device applications.

To allow for reliable wireless optical links in realistic underwater environments, we study the dependence of turbulence-induced fading on the wavelength using a laser-based white-light interrogator in emulated realistic conditions. We experimentally show that the scintillation index decreases significantly with the increase of wavelength. The results are verified for longer distances using a Monte Carlo simulation. We numerically and experimentally demonstrate that the use of longer wavelengths lowers the bit error ratio by as much as three orders of magnitude. We conclude that using green light is more reliable in turbulent channels than blue. The correlation between different wavelengths under turbulence is studied, which was made possible by the use of the laser-based white-light interrogator.

Underwater wireless optical communication (UWOC) can offer reliable and secure connectivity for enabling future internet-of-underwater-things (IoUT), owing to its unlicensed spectrum and high transmission speed. However, a critical bottleneck lies in the strict requirement of pointing, acquisition, and tracking (PAT), for effective recovery of modulated optical signals at the receiver end. A large-area, high bandwidth, and wide-angle-of-view photoreceiver is therefore crucial for establishing a high-speed yet reliable communication link under non-directional pointing in a turbulent underwater environment. In this work, we demonstrated a large-area, of up to a few tens of cm2, photoreceiver design based on ultraviolet(UV)-to-blue color-converting plastic scintillating fibers, and yet offering high 3-dB bandwidth of up to 86.13 MHz. Tapping on the large modulation bandwidth, we demonstrated a high data rate of 250 Mbps at bit-error ratio (BER) of 2.2 × 10−3 using non-return-to-zero on-off keying (NRZ-OOK) pseudorandom binary sequence (PRBS) 210-1 data stream, a 375-nm laser-based communication link over the 1.15-m water channel. This proof-of-concept demonstration opens the pathway for revolutionizing the photodetection scheme in UWOC, and for non-line-of-sight (NLOS) free-space optical communication.

We report the use of 3D-printed microscale spiral phase plates to generate orbital angular momentum (OAM) carrying beams. We confirm that the generated beams have high purity, and we have successfully tested them to convey data signals with low bit error rates at the wavelength of 980 nm. This method will open new opportunities for generating OAM beams for many applications in optical communications, including free-space optics, as well as underwater, chip-to-chip, and quantum communications.

Optical wireless communication (OWC) using the ultra-broad spectrum of the visible-to-ultraviolet (UV) wavelength region remains a vital field of research for mitigating the saturated bandwidth of radio-frequency (RF) communication. However, the lack of an efficient UV photodetection methodology hinders the development of UV-based communication. The key technological impediment is related to the low UV-photon absorption in existing silicon photodetectors, which offer low-cost and mature platforms. To address this technology gap, we report a hybrid Si-based photodetection scheme by incorporating CsPbBr3 perovskite nanocrystals (NCs) with a high photoluminescence quantum yield (PLQY) and a fast photoluminescence (PL) decay time as a UV-to-visible colour-converting layer for high-speed solar-blind UV communication. The facile formation of drop-cast CsPbBr3 perovskite NCs leads to a high PLQY of up to ~73% and strong absorption in the UV region. With the addition of the NC layer, a nearly threefold improvement in the responsivity and an increase of ~25% in the external quantum efficiency (EQE) of the solar-blind region compared to a commercial silicon-based photodetector were observed. Moreover, time-resolved photoluminescence measurements demonstrated a decay time of 4.5 ns under a 372-nm UV excitation source, thus elucidating the potential of this layer as a fast colour-converting layer. A high data rate of up to 34 Mbps in solar-blind communication was achieved using the hybrid CsPbBr3–silicon photodetection scheme in conjunction with a 278-nm UVC light-emitting diode (LED). These findings demonstrate the feasibility of an integrated high-speed photoreceiver design of a composition-tuneable perovskite-based phosphor and a low-cost silicon-based photodetector for UV communication.

In recent years, β-Ga2O3/NiO heterojunction diodes have been studied, but reports in the literature lack an investigation of an epitaxial growth process of high-quality single-crystalline β-Ga2O3/NiO thin films via electron microscopy analysis and the fabrication and characterization of an optoelectronic device based on the resulting heterojunction stack. This work investigates the thin-film growth of a heterostructure stack comprising n-type β-Ga2O3 and p-type cubic NiO layers grown consecutively on c-plane sapphire using pulsed laser deposition, as well as the fabrication of solar-blind ultraviolet-C photodetectors based on the resulting p-n junction heterodiodes. Several characterization techniques were employed to investigate the heterostructure, including X-ray crystallography, ion beam analysis, and high-resolution electron microscopy imaging. X-ray diffraction analysis confirmed the single-crystalline nature of the grown monoclinic and cubic (2̅01) β-Ga2O3 and (111) NiO films, respectively, whereas electron microscopy analysis confirmed the sharp layer transitions and high interface qualities in the NiO/β-Ga2O3/sapphire double-heterostructure stack. The photodetectors exhibited a peak spectral responsivity of 415 mA/W at 7 V reverse-bias voltage for a 260 nm incident-light wavelength and 46.5 pW/μm2 illuminating power density. Furthermore, we also determined the band offset parameters at the thermodynamically stable heterointerface between NiO and β-Ga2O3 using high-resolution X-ray photoelectron spectroscopy. The valence and conduction band offsets values were found to be 1.15 ± 0.10 and 0.19 ± 0.10 eV, respectively, with a type-I energy band alignment.

We experimentally introduce a normalized differential method to enhance the time domain signal-to-noise ratio (SNR) of an optical fiber distributed acoustic sensor (DAS). The reported method is calibrated against the typical differential method in noisy DAS systems, including those utilizing a relatively wide linewidth laser or few-mode fiber. In these two systems, the normalized differential method respectively identifies the position information of various vibration events with 1.7 dB and 0.53 dB SNR improvement. We further demonstrate the ability to locate positions along a fiber that are subjected to vibrations of frequencies higher than the theoretical maximum, but without determining these frequencies.