### Recent Submissions

• #### A highly sensitive, large area, and self-powered UV photodetector based on coalesced gallium nitride nanorods/graphene/silicon (111) heterostructure

(Applied Physics Letters, AIP Publishing, 2020-11-13) [Article]
In this paper, we demonstrate an ultraviolet photodetector (UV-PD) that uses coalesced gallium nitride (GaN) nanorods (NRs) on a graphene/Si (111) substrate grown by plasma-assisted molecular beam epitaxy. We report a highly sensitive, self-powered, and hybrid GaN NR/graphene/Si (111) PD with a relatively large 100 mm2 active area, a high responsivity of 17.4 A/W, a high specific detectivity of 1.23 1013 Jones, and fast response speeds of 13.2/13.7 ls (20 kHz) under a UV light of 355 nm at zero bias voltage. The results show that the thin graphene acts as a perfect interface for GaN NRs, encouraging growth with minimum defects on the Si substrate. Our results suggest that the GaN NR/graphene/Si (111) heterojunction has a range of interesting properties that make it well-suited for a variety of photodetection applications.
• #### Performance Characterization of High and Low Power Prism based Tunable Blue Laser Diodes Systems

(IEEE, 2020-11-13) [Conference Paper]
Comparison of high- and low-power tunable external-cavity blue laser-diode system demonstrates a tunability of 10.6 and 4nm, respectively, with a corresponding SMSR as high as 35 and 32dB and linewidth as low as 97 and 59pm, while showcasing high stability at extreme operating conditions.
• #### Large-Scale Sub-1-nm Random Gaps Approaching the Quantum Upper Limit for Quantitative Chemical Sensing

(Advanced Optical Materials, Wiley, 2020-10-27) [Article]
Metallic nanostructures with nanogap features can confine electromagnetic fields into extremely small volumes. In particular, as the gap size is scaled down to sub-nanometer regime, the quantum effects for localized field enhancement reveal the ultimate capability for light–matter interaction. Although the enhancement factor approaching the quantum upper limit has been reported, the grand challenge for surface-enhanced vibrational spectroscopic sensing remains in the inherent randomness, preventing uniformly distributed localized fields over large areas. Herein, a strategy to fabricate high-density random metallic nanopatterns with accurately controlled nanogaps, defined by atomic-layer-deposition and self-assembled-monolayer processes, is reported. As the gap size approaches the quantum regime of ≈0.78 nm, its potential for quantitative sensing, based on a record-high uniformity with the relative standard deviation of 4.3% over a large area of 22 mm × 60 mm, is demonstrated. This superior feature paves the way towards more affordable and quantitative sensing using quantum-limit-approaching nanogap structures.
• #### Survey of energy-autonomous solar cell receivers for satellite–air–ground–ocean optical wireless communication

(Progress in Quantum Electronics, Elsevier BV, 2020-10-14) [Article]
With the advent of the Internet of Things, energy- and bandwidth-related issues are becoming increasingly prominent in the context of supporting the massive connectivity of various smart devices. To this end, we propose that solar cells with the dual functions of energy harvesting and signal acquisition are critical for alleviating energy-related issues and enabling optical wireless communication (OWC) across the satellite–air–ground–ocean (SAGO) boundaries. Moreover, we present the first comprehensive survey on solar cell-based OWC technology. First, the historical evolution of this technology is summarized, from its beginnings to recent advances, to provide the relative merits of a variety of solar cells for simultaneous energy harvesting and OWC in different application scenarios. Second, the performance metrics, circuit design, and architectural design for energy-autonomous solar cell receivers are provided to help understand the basic principles of this technology. Finally, with a view to its future application to SAGO communication networks, we note the challenges and future trends of research related to this technology in terms of channel characterization, light source development, photodetector development, modulation and multiplexing techniques, and network implementations.
• #### Diffused-Line-of-Sight Communication for Mobile and Fixed Underwater Nodes

(IEEE Photonics Journal, IEEE, 2020-10-13) [Article]
The misalignment of mobile underwater wireless optical communication (UWOC) systems, compounded by misalignment in underwater mobility scenarios, is a practical problem that can be resolved through various means. This work describes a pulse-position modulation (PPM) based diffused-line-of-sight UWOC system that offers a solution to this issue. PPM is found to be power-efficient and, in terms of bit error ratio performances, outperforms on-off keying modulation and orthogonal frequency-division multiplexing modulation in complex dynamic underwater channels. Through indoor experiments and an outdoor deployment, herein, we validated the robustness of our PPM-based mobile UWOC system. This work is expected to shed light on practical implementation of UWOC network for relieving the strict positioning-acquisition-and-tracking requirements when underwater apparatus is transmitting or receiving signals on-the-fly.
• #### Nanoporous GaN/n-type GaN: a cathode structure for ITO-free perovskite solar cells

(ACS Energy Letters, American Chemical Society (ACS), 2020-09-17) [Article]
Introducing suitable electron/hole transport layers and transparent conductive layers (TCLs) into perovskite solar cells (PSCs) is key to enhancing the selective extraction of charge carriers and reducing surface recombination losses. Here, we introduce nanoporous gallium nitride (NP GaN)/n-type GaN (n-GaN) as a dual-function cathode structure for PSCs, acting as both the TCL and the electron transport layer (ETL). We demonstrate that the hierarchical NP GaN structure provides an expanded interfacial contact area with the perovskite absorber, while the n-GaN under the NP GaN displays high transmittance in the visible spectrum as well as higher lateral electric conductivity than that of a conventional ITO film. Prototype MAPbI3 PSCs based on this NP GaN/n-GaN cathode structure (without an extra ETL) show a power conversion efficiency of up to 18.79%. The NP GaN/n-GaN platform demonstrated herein paves the way for PSCs to take advantage of the widely available heterostructures of mature III-nitride-based technologies.
• #### 1.5-Gbit/s Filter-free Optical Communication Link based on Wavelength-selective Semipolar ($20\overline{21}$) InGaN/GaN Micro-photodetector

(OSA Technical Digest, 2020-09-14) [Conference Paper]
We report on wavelength-selective semipolar (2021) InGaN/GaN micro-photodetector with broad modulation bandwidth of 293.52 MHz, outperforming polar-based devices. A 1.5-Gbit/s data rate was achieved without the need of spectral-efficient modulation format.
• #### Single/Multi-wavelength green laser diode system

(OSAIEEE, 2020-09-11) [Conference Paper]
Single and two-stage self-injection locking in InGaN/GaN laser diode is presented. Near single-mode emission with 34pm linewidth, and simultaneous locking of four longitudinal modes with appreciable 18dB SMSR and <2.5dB peak power ratio is achieved.
• #### Quantifying the Transverse-Electric-Dominant 260 nm Emission from Molecular Beam Epitaxy-Grown GaN-Quantum-Disks Embedded in AlN Nanowires: A Comprehensive Optical and Morphological Characterization

(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2020-09-01) [Article]
There has been a relentless pursuit of transverse electric (TE)-dominant deep ultraviolet (UV) optoelectronic devices for efficient surface emitters to replace the environmentally unfriendly mercury lamps. To date, the use of the ternary AlGaN alloy inevitably has led to transverse magnetic (TM)-dominant emission, an approach that is facing a roadblock. Here, we take an entirely different approach of utilizing a binary GaN compound semiconductor in conjunction with ultrathin quantum disks (QDisks) embedded in AlN nanowires (NWs). The growth of GaN QDisks is realized on a scalable and low-cost Si substrate using plasma-assisted molecular beam epitaxy as a highly controllable monolayer growth platform. We estimated an internal quantum efficiency of ∼81% in a wavelength regime of ∼260 nm for these nanostructures. Additionally, strain mapping obtained by high-angle annular dark-field scanning transmission electron microscopy is studied in conjunction with the TE and TM modes of the carrier recombination. Moreover, for the first time, we quantify the TE and TM modes of the PL emitted by GaN QDisks for deep-UV emitters. We observed nearly pure TE-polarized photoluminescence emission at a polarization angle of ∼5°. This work proposes highly quantum-confined ultrathin GaN QDisks as a promising candidate for deep-UV vertical emitters.
• #### Demonstration of a low-complexity memory-polynomial-aided neural network equalizer for CAP visible-light communication with superluminescent diode

Visible-light communication (VLC) stands as a promising component of the future communication network by providing high-capacity, low-latency, and high-security wireless communication. Superluminescent diode (SLD) is proposed as a new light emitter in the VLC system due to its properties of droop-free emission, high optical power density, and low speckle-noise. In this paper, we analyze a VLC system based on SLD, demonstrating effective implementation of carrierless amplitude and phase modulation (CAP). We create a low-complexity memory-polynomial-aided neural network (MPANN) to replace the traditional finite impulse response (FIR) post-equalization filters of CAP, leading to significant mitigation of the linear and nonlinear distortion of the VLC channel. The MPANN shows a gain in Q factor of up to 2.7 dB higher than other equalizers, and more than four times lower complexity than a standard deep neural network (DNN), hence, the proposed MPANN opens a pathway for the next generation of robust and efficient neural network equalizers in VLC. We experimentally demonstrate a proof-of-concept 2.95-Gbit/s transmission using MPANN-aided CAP with 16-quadrature amplitude modulation (16-QAM) through a 30-cm channel based on the 442-nm blue SLD emitter.
• #### Optical properties and first principles study of CH3NH3PbBr3 perovskite structures for solar cell application

(Springer Singapore, 2020-08-15) [Conference Paper]
Solution-processed organic–inorganic hybrid perovskites have attracted attention as light-harvesting materials for solar cells and photonic applications. The present study focusses on cubic single crystal; microstructures of CH3NH3PbBr3 perovskite fabricated by a one-step solution based self-assembly method. It is seen that, in addition to the nucleation from the precursor solution, the crystallization occurs when the solution was supersaturated, followed by formation of small nucleus of CH3NH3PbBr3 that will self-assembled into bigger hollow cubes. A 3D fluorescence microscope investigation of hollow cubes confirmed the formation of hollow plates on the bottom, then the growth starts from the perimeter and propagate to the center of the cube. Furthermore, the growth in the (001) direction follows a layer-by-layer growth model to form a complete cube, confirmed by SEM observations. To get more insights into the structural and optical properties, density functional theory (DFT) simulations were conducted. The density of state (DOS) calculations revealed that the valence band maximum (VBM) consists of states contributed by Br and Pb, which agrees with the X-ray photoelectron spectroscopy valence band (XPSVB) measurements.
• #### Roadmap to Free Space Optics

(Journal of the Optical Society of America B, The Optical Society, 2020-08-14) [Article]
With the ever-increasing demand for data and radio frequency spectrum becoming congested, Free Space Optical (FSO) communication may find a niche for situations where fiber is too expensive or too difficult to install. FSO is a cross-disciplinary field that draws from radio and fiber communication, astronomy, and even quantum optics, and it has seen major advances over the last three decades. In this tutorial-style review, we provide a broad overview of many of the important topics required to design, develop, and research the next generation of FSO technology.
• #### AquaE-lite Hybrid-Solar-Cell Receiver-Modality for Energy-Autonomous Terrestrial and Underwater Internet-of-Things

(IEEE Photonics Journal, IEEE, 2020-08-04) [Article]
Our goal is to develop an energy-autonomous solar cell receiver that can be integrated with a variety of smart devices to implement the Internet of Things in next-generation applications. This paper details efforts to develop such a prototype, called AquaE-lite. Owing to the capability of detecting low-intensity optical signals, 20-m and 30-m long-distance lighting and optical wireless communication with data rates of 1.6 Mbit/s and 1.2 Mbit/s have been achieved on a laboratory testbed, respectively. Moreover, field trials on an outdoor solar cell testbed and a port (turbid water) of the Red Sea have been conducted. Under bright sunlight, energy autonomy and 1.2-Mbit/s optical wireless communication over a transmission distance of 15 m have been implemented, which demonstrated that AquaE-lite with an elaborate receiver circuit has excellent performance in energy harvesting and resistance to background noise. In a more challenging underwater environment, 1.2-Mbit/s signals were successfully received over a transmission distance of 2 m. It indicates that energy-autonomous AquaE-lite with large detection area has promising prospects in future underwater mobile sensor networks to significantly relieve the requirement of pointing, acquisition and tracking while resolving the energy issues.
• #### Functionalization of Magnetic Nanowires for Active Targeting and Enhanced Cell Killing Efficacy

(ACS Applied Bio Materials, American Chemical Society (ACS), 2020-07-08) [Article]
Conventional chemotherapy and radiation therapy are often insufficient in eliminating cancer and are accompanied by severe side effects, due to a lack in the specificity of their targeting. Magnetic iron nanowires have made a great contribution to the nanomedicine field because of their low toxicity and ease of manipulation with the magnetic field. Recently, they have been used in magnetic resonance imaging, wireless magneto-mechanical, and photothermal treatments. The addition of active targeting moieties to these nanowires thus creates a multifunctional tool that can boost therapeutic efficacies through the combination of different treatments towards a specific target. Colon cancer is the third most commonly occurring cancer, and 90±2.5% of colon cancer cells express the glycoprotein CD44. Iron nanowires with an iron oxide surface are biocompatible, multifunctional materials that can be controlled by magnetic fields and heated by laser irradiation. Here, they were functionalized with anti-CD44 antibodies and used for in a combination therapy that included magneto-mechanical and photothermal treatments on colon cancer cells. The functionalization resulted in a threefold increase of nanowire internalization in colon cancer cells compared to control cells and did not affect the antigenicity and magnetic properties. It also increased the efficacy of killing from 35±1% to more than 71±2%, whereby the combination therapy was more effective than individual therapies alone.
• #### Prism-based tunable InGaN/GaN self-injection locked blue laser diode system: study of temperature, injection ratio, and stability

(Journal of Nanophotonics, SPIE-Intl Soc Optical Eng, 2020-07-02) [Article]
A quasicontinuously wavelength tuned self-injection locked blue laser diode system employing a prism is presented. A rigorous analysis of the injection ratio (IR) in the form of three systems, namely high (HRS, ∼ − 0.7 dB IR), medium (MRS, ∼ − 1.5 dB IR), and low (LRS, ∼ − 3.0 dB IR) reflection systems, showed a direct relationship with the wavelength tunability whereas the usable system power exhibited an inverse correlation. In particular, MRS configuration demonstrated a concurrent optimization of tuning window and system power, thus emerging as a highly attractive candidate for practical realization. Moreover, a comprehensive investigation on two distinct MRS configurations employing different commercially available InGaN/GaN blue lasers, i.e., MRS-1 and MRS-2, displayed a wavelength tunability (system power) of ∼8.2 nm (∼7.6 mW) and ∼6.3 nm (∼11.6 mW), respectively, at a low injection current of 130 mA. In addition, both MRS configurations maintained high-performance characteristic with corresponding average optical linewidths of ∼80 and ∼58 pm and a side-mode-suppression-ratio of ≥12 dB. Lastly, a thorough stability analysis of HRS and MRS configurations, which are more prone to system instabilities due to elevated IRs, is performed at critical operation conditions of a high injection current of ≥260 mA and a temperature of 40°C, showing an extended stable performance of over 120 min, thus further substantiating the promising features of the prism-based systems for practical applications.
• #### Iridocytes Mediate Photonic Cooperation Between Giant Clams (Tridacninae) and Their Photosynthetic Symbionts

(Frontiers in Marine Science, Frontiers Media SA, 2020-06-19) [Article]
Iridocytes, containing multiple stacks of proteinaceous platelets and crystalized guanine, alternating with thin cytoplasm sheets, are specialized cells that act as multilayer nanoreflectors. Convergence evolution led to their arising across a broad range of organisms, including giant clams of the Tridacninae subfamily – the only sessile and photosymbiotic organism, among animals known to possess iridocytes. Through the interference of light with their nanoscale architecture, iridocytes generate “structural colors,” which are reported to serve different purposes, from intra-species communication to camouflage. In giant clams, iridocytes were previously reported to promote a lateral- and forward scattering of photosynthetically productive radiation (PAR) into the clam tissue, as well as the back reflection of non-productive wavelengths. Hence, they are assumed to promote an increased efficiency in the use of available solar energy, while simultaneously preventing photodamage of the algal symbionts. We report the use of guanine crystals within Tridacna maxima giant clam iridocytes as a basis for photonic cooperation between the bivalve host and their photosynthetic symbionts. Our results suggest that, in addition to the previously described scattering processes, iridocytes absorb potentially damaging UV radiation (UVR) and, through successive emission, emit light at longer wavelengths, which is then absorbed by the photosynthetic pigments of the algal symbionts. Consequently, both, host and algal symbionts are sheltered from (potentially) damaging UVR, while the available solar energy within the PAR spectrum increases, thereby potentially enhancing photosynthetic and calcification rates in this large bivalve. Further, our results suggest that this photonic cooperation could be responsible for the broad repertoire of colors that characterizes the highly diverse mantle patterns found in T. maxima.
• #### Characterization of epitaxial titanium nitride mediated single-crystal nickel oxide grown on MgO-(100) and Si-(100)

(AIP Advances, AIP Publishing, 2020-06-12) [Article]
Single-crystal nickel oxide (NiO) was grown, using epitaxial titanium nitride (TiN) as a preorienting interlayer, on both the lattice-matching substrate of magnesium oxide in the (100) surface orientation, MgO-(100), and a lattice-mismatched silicon (100) substrate, Si-(100), by high-temperature pulsed-laser deposition. To the best of the authors’ knowledge, this is the first report of its kind in the literature. The high-temperature sputter-deposited TiN interlayer is crucial for forming a bottom contact for the implementation of a device, and as a lattice-matching layer for NiO and MgO. The structural, surface-related, and elemental properties of the as-grown NiO/TiN/MgO(100) and NiO/TiN/Si(100) samples were determined by high-resolution transmission electron microscopy (HRTEM), x-ray diffraction (XRD), thinfilm x-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy in conjunction with energy-dispersed x-ray spectroscopy. XRD rocking curve data confirmed that the NiO layers were single crystalline on both template substrates, and the structural quality of NiO/TiN on the lattice-matching MgO substrate surpassed that on the Si substrate. XRD φ-scan data confirmed the cube-on-cube stacking of NiO and TiN. An analysis of HRTEM fast Fourier transform (FFT) images further confirmed the single crystallinity of the NiO and TiN layers, while lattice mismatches at the NiO/TiN, TiN/MgO, and TiN/Si interfaces were examined using the FFT line profile measurements of HRTEM. The resulting thin film of single-crystalline NiO can be used as a transparent conducting electrode in group-III oxide and group-III nitride semiconductor devices, and in such electrochemical processes as solar hydrogen generation and nitrogen reduction reactions.
• #### Aqua-Fi: Delivering internet underwater using wireless optical networks

(IEEE Communications Magazine, Institute of Electrical and Electronics Engineers (IEEE), 2020-06-09) [Article]
In this article, we demonstrate bringing the Internet to underwater environments by deploying a low power and compact underwater optical wireless system, called Aqua-Fi, to support today's Internet applications. Aqua-Fi uses an LED or laser to support bidirectional wide-range communication services with different requirements, low cost, and simple implementation. LEDs introduce robust short distance solutions with low power requirements. However, laser extends the communication distance and improves the transmission rate at the cost of higher power requirements. Throughout this work, we discuss the proposed Aqua-Fi system architecture, limitations, and solutions to improve data rates and deliver reliable communication links.
• #### Sensing within the OTDR dead-zone using a two-mode fiber

(Optics Letters, The Optical Society, 2020-05-20) [Article]
An optical time-domain reflectometer (OTDR) is incapable of providing sensing or diagnostic information within dead-zones. We use a two-mode fiber (TMF) and a photonic lantern to completely overcome the main OTDR's dead-zone originating from the front facet of optical fiber. This is achieved by injecting the optical pulses of the OTDR in the form of the fundamental LP01 mode and meanwhile collecting the Rayleigh signals associated with the higher-order modes. Using the reported TMF-based OTDR, we accurately sense the position and frequency of a vibration event located within the dead-zone as a proof-of-concept demonstration.