Wafer-Scale Synthesis of 2D Dirac Heterostructures for Self-Driven, Fast, Broadband Photodetectors.

Type-II Dirac semimetal platinum ditelluride (PtTe2) is a promising functional material for photodetectors because of its specially tilted Dirac cones, strong light absorption, and high carrier mobilities. The stack of two-dimensional (2D) Dirac heterostructures consisting of PtTe2 and graphene could overcome the limit of detection range and response time occurring in the heterostructures of graphene and other low-mobility and large-gap transition metal dichalcogenides (TMDs). Here, we report an approach for achieving highly controllable, wafer-scale production of 2D Dirac heterostructures of PtTe2/graphene with tunable thickness, variable size, and CMOS compatibility. More importantly, the optimized recipes achieve the exact stoichiometric ratio of 1:2 for Pt and Te elements without contaminating the underlayer graphene film. Because of the built-in electric field at the junction area, the photodetectors based on the PtTe2/graphene heterostructure are self-driven with a broadband photodetection from 405 to 1850 nm. In particular, the photodetectors have a high responsivity of up to ∼0.52 AW-1 (without bias) and a fast response time of ∼8.4 µs. Our work demonstrated an approach to synthesizing hybrid 2D Dirac heterostructures, which can be applied in the integration of on-chip, CMOS-compatible photodetectors with near-infrared detection, high sensitivity, and low energy consumption.

Controlling the lasing modes in random lasers operating in the Anderson localization regime.

Random lasers, which rely on random scattering events unlike traditional Fabry-Pérot cavities, are much simpler and cost-effective to fabricate. However, because of the chaotic fluctuations and instability of the lasing modes, controlling the lasing properties is challenging. In this study, we use random InP nanowire (NW) arrays that operate in the Anderson localization regime with stable modes as the random lasers. We show that by changing the design parameters of the NW arrays, such as filling factor, dimensions of the NWs, degree of randomness, and the size of the array, the properties of the lasing modes including the number of modes, lasing wavelengths, and lasing threshold can be controlled.

Efficient and Tunable Reflection of Phonon Polaritons at Built-In Intercalation Interfaces.

Phonon polaritons-light coupled to lattice vibrations-in polar van der Waals crystals offer unprecedented opportunities for controlling light at the nanoscale due to their anisotropic and ultralow-loss propagation. While their analog plasmon polaritons-light coupled to electron oscillations-have long been studied and exhibit interesting reflections at geometrical edges and electronic boundaries, whether phonon polaritons can be reflected by such barriers has been elusive. Here, the effective and tunable reflection of phonon polaritons at embedded interfaces formed in hydrogen-intercalated α-MoO3 flakes is elaborated upon. Without breaking geometrical continuity, such intercalation interfaces can reflect phonon polaritons with low losses, yielding the distinct phase changes of -0.8π and -0.3π associated with polariton propagation, high efficiency of 50%, and potential electrical tunability. The results point to a new approach to construct on-demand polariton reflectors, phase modulators, and retarders, which may be transplanted into building future polaritonic circuits using van der Waals crystals.

Understanding the role of facets and twin defects in the optical performance of GaAs nanowires for laser applications.

GaAs nanowires are regarded as promising building blocks of future optoelectronic devices. Despite progress, the growth of high optical quality GaAs nanowires is a standing challenge. Understanding the role of twin defects and nanowire facets on the optical emission and minority carrier lifetime of GaAs nanowires is key for the engineering of their optoelectronic properties. Here, we present new insights into the microstructural parameters controlling the optical properties of GaAs nanowires, grown via selective-area metal-organic vapor-phase epitaxy. We observe that these GaAs nanowires have a twinned zinc blende crystal structure with taper-free {110} side facets that result in an ultra-low surface recombination velocity of 3.5 × 104 cm s-1. This is an order of magnitude lower than that reported for defect-free GaAs nanowires grown by the vapor-liquid-solid technique. Using time-resolved photoluminescence and cathodoluminescence measurements, we untangle the local correlation between structural and optical properties demonstrating the superior role of the side facets in determining recombination rates over that played by twin defects. The low surface recombination velocity of these taper-free {110} side facets enable us to demonstrate, for the first time, low-temperature lasing from bare (unpassivated) GaAs nanowires, and also efficient room-temperature lasing after passivation with an AlGaAs shell.

Managing Resonant and Nonresonant Lasing Modes in GaAs Nanowire Random Lasers.

Random lasers are promising, easy-to-fabricate light sources that rely on scattering instead of well-defined optical cavities. We demonstrate random lasing in GaAs nanowires using both randomly oriented and vertically aligned arrays. These configurations are shown to lase in both resonant and nonresonant modes, where aligned nanowires support predominantly resonant lasing and randomly oriented favors nonresonant lasing. On the basis of numerical simulations, aligning the nanowires increases the system's scattering efficiency leading to higher quality factor modes and thus favoring the resonant modes. We further demonstrate two methods to optically suppress resonant mode lasing by increasing the number of excited modes. The light output-light input curves show a pronounced kink for the resonant lasing mode while the nonresonant mode is kink-free. The resonant lasing modes may be used as tunable lasers, and the nonresonant modes exhibit near-thresholdless amplification. Switching between lasing modes opens up new opportunities to use lasers in broader applications.

Ultrathin Ta2O5 electron-selective contacts for high efficiency InP solar cells.

Heterojunction solar cells with transition-metal-oxide-based carrier-selective contacts have been gaining considerable research interest owing to their amenability to low-cost fabrication methods and elimination of parasitic absorption and complex semiconductor doping process. In this work, we propose tantalum oxide (Ta2O5) as a novel electron-selective contact layer for photo-generated carrier separation in InP solar cells. We confirm the electron-selective properties of Ta2O5 by investigating band energetics at the InP-Ta2O5 interface using X-ray photoelectron spectroscopy. Time-resolved photoluminescence and power dependent photoluminescence reveal that the Ta2O5 inter-layer also mitigates parasitic recombination at the InP/transparent conducting oxide interface. With an 8 nm Ta2O5 layer deposited using an atomic layer deposition (ALD) system, we demonstrate a planar InP solar cell with an open circuit voltage, Voc, of 822 mV, a short circuit current density, Jsc, of 30.1 mA cm-2, and a fill factor of 0.77, resulting in an overall device efficiency of 19.1%. The Voc is the highest reported value to date for an InP heterojunction solar cells with carrier-selective contacts. The proposed Ta2O5 material may be of interest not only for other solar cell architectures including perovskite cells and organic solar cells, but also across a wide range of optoelectronics applications including solid state emitting devices, photonic crystals, planar light wave circuits etc.

Optical Study of p-Doping in GaAs Nanowires for Low-Threshold and High-Yield Lasing.

Semiconductor nanowires suffer from significant non-radiative surface recombination; however, heavy p-type doping has proven to be a viable option to increase the radiative recombination rate and, hence, quantum efficiency of emission, allowing the demonstration of room-temperature lasing. Using a large-scale optical technique, we have studied Zn-doped GaAs nanowires to understand and quantify the effect of doping on growth and lasing properties. We measure the non-radiative recombination rate ( knr) to be (0.14 ± 0.04) ps-1 by modeling the internal quantum efficiency (IQE) as a function of doping level. By applying a correlative method, we identify doping and nanowire length as key controllable parameters determining lasing behavior, with reliable room-temperature lasing occurring for p ≳ 3 × 1018 cm-3 and lengths of ≳4 µm. We report a best-in-class core-only near-infrared nanowire lasing threshold of ∼10 µJ cm-2, and using a data-led filtering step, we present a method to simply identify subsets of nanowires with over 90% lasing yield.

Large-Scale Statistics for Threshold Optimization of Optically Pumped Nanowire Lasers.

Single nanowire lasers based on bottom-up III-V materials have been shown to exhibit room-temperature near-infrared lasing, making them highly promising for use as nanoscale, silicon-integrable, and coherent light sources. While lasing behavior is reproducible, small variations in growth conditions across a substrate arising from the use of bottom-up growth techniques can introduce interwire disorder, either through geometric or material inhomogeneity. Nanolasers critically depend on both high material quality and tight dimensional tolerances, and as such, lasing threshold is both sensitive to and a sensitive probe of such inhomogeneity. We present an all-optical characterization technique coupled to statistical analysis to correlate geometrical and material parameters with lasing threshold. For these multiple-quantum-well nanolasers, it is found that low threshold is closely linked to longer lasing wavelength caused by losses in the core, providing a route to optimized future low-threshold devices. A best-in-group room temperature lasing threshold of ∼43 µJ cm-2 under pulsed excitation was found, and overall device yields in excess of 50% are measured, demonstrating a promising future for the nanolaser architecture.

Excited State Biexcitons in Atomically Thin MoSe2.

The tightly bound biexcitons found in atomically thin semiconductors have very promising applications for optoelectronic and quantum devices. However, there is a discrepancy between theory and experiment regarding the fundamental structure of these biexcitons. Therefore, the exploration of a biexciton formation mechanism by further experiments is of great importance. Here, we successfully triggered the emission of biexcitons in atomically thin MoSe2, via the engineering of three critical parameters: dielectric screening, density of trions, and excitation power. The observed binding energy and formation dynamics of these biexcitons strongly support the model that the biexciton consists of a charge attached to a trion (excited state biexciton) instead of four spatially symmetric particles (ground state biexciton). More importantly, we found that the excited state biexcitons not only can exist at cryogenic temperatures but also can be triggered at room temperature in a freestanding bilayer MoSe2. The demonstrated capability of biexciton engineering in atomically thin MoSe2 provides a route for exploring fundamental many-body interactions and enabling device applications, such as bright entangled photon sources operating at room temperature.

Single n+-i-n+ InP nanowires for highly sensitive terahertz detection.

Developing single-nanowire terahertz (THz) electronics and employing them as sub-wavelength components for highly-integrated THz time-domain spectroscopy (THz-TDS) applications is a promising approach to achieve future low-cost, highly integrable and high-resolution THz tools, which are desirable in many areas spanning from security, industry, environmental monitoring and medical diagnostics to fundamental science. In this work, we present the design and growth of n+-i-n+ InP nanowires. The axial doping profile of the n+-i-n+ InP nanowires has been calibrated and characterized using combined optical and electrical approaches to achieve nanowire devices with low contact resistances, on which the highly-sensitive InP single-nanowire photoconductive THz detectors have been demonstrated. While the n+-i-n+ InP nanowire detector has a only pA-level response current, it has a 2.5 times improved signal-to-noise ratio compared with the undoped InP nanowire detector and is comparable to traditional bulk THz detectors. This performance indicates a promising path to nanowire-based THz electronics for future commercial applications.

Design and Room-Temperature Operation of GaAs/AlGaAs Multiple Quantum Well Nanowire Lasers.

We present the design and room-temperature lasing characteristics of single nanowires containing coaxial GaAs/AlGaAs multiple quantum well (MQW) active regions. The TE01 mode, which has a doughnut-shaped intensity profile and is polarized predominantly in-plane to the MQWs, is predicted to lase in these nanowire heterostructures and is thus chosen for the cavity design. Through gain and loss calculations, we determine the nanowire dimensions required to minimize loss for the TE01 mode and determine the optimal thickness and number of QWs for minimizing the threshold sheet carrier density. In particular, we show that there is a limit to the minimum and maximum number of QWs that are required for room-temperature lasing. Based on our design, we grew nanowires of a suitable diameter containing eight uniform coaxial GaAs/AlGaAs MQWs. Lasing was observed at room temperature from optically pumped single nanowires and was verified to be from TE01 mode by polarization measurements. The GaAs MQW nanowire lasers have a threshold fluence that is a factor of 2 lower than that previously demonstrated for room-temperature GaAs nanowire lasers.

Doping-enhanced radiative efficiency enables lasing in unpassivated GaAs nanowires.

Nanolasers hold promise for applications including integrated photonics, on-chip optical interconnects and optical sensing. Key to the realization of current cavity designs is the use of nanomaterials combining high gain with high radiative efficiency. Until now, efforts to enhance the performance of semiconductor nanomaterials have focused on reducing the rate of non-radiative recombination through improvements to material quality and complex passivation schemes. Here we employ controlled impurity doping to increase the rate of radiative recombination. This unique approach enables us to improve the radiative efficiency of unpassivated GaAs nanowires by a factor of several hundred times while also increasing differential gain and reducing the transparency carrier density. In this way, we demonstrate lasing from a nanomaterial that combines high radiative efficiency with a picosecond carrier lifetime ready for high speed applications.

Enhanced luminescence from GaN nanopillar arrays fabricated using a top-down process.

We report the fabrication of GaN nanopillar arrays with good structural uniformity using the top-down approach. The photoluminescence intensity from the nanopillar arrays is enhanced compared to the epilayer. We use finite difference time domain simulations to show that the enhancement in photoluminescence intensity from the nanopillar arrays is a result of anti-reflection properties of the arrays that result in enhanced light absorption and increase light extraction efficiency compared to the epilayer. The measured quantum efficiency of the nanopillars is comparable to that of an epitaxially grown GaN epilayer.

Mode Profiling of Semiconductor Nanowire Lasers.

We experimentally determine the lasing mode(s) in optically pumped semiconductor nanowire lasers. The spatially resolved and angle-resolved far-field emission profiles of single InP nanowire lasers lying horizontally on a SiO2 substrate are characterized in a microphotoluminescence (µ-PL) setup. The experimentally obtained polarization dependent far-field profiles match very well with numerical simulations and enable unambiguous identification of the lasing mode(s). This technique can be applied to characterize lasing modes in other type of nanolasers that are integrated on a substrate in either vertical or horizontal configurations.

Polarized light absorption in wurtzite InP nanowire ensembles.

We investigate the absorption properties of ensembles of wurtzite (WZ) InP nanowires (NWs) by high-resolution polarization-resolved photoluminescence excitation (PLE) spectroscopy at T = 10 K. The degree of linear polarization of absorbed light, ρ(abs), resulting from the PLE spectra is governed by a competition between the dielectric mismatch effect and the WZ selection rules acting differently on different optical transitions. These two contributions are deconvoluted with the help of finite-difference time-domain simulations, thus providing information about the symmetry of the three highest valence bands (A, B, and C) of WZ InP and the extent of the spin-orbit interaction on these states. Moreover, ρ(abs) shows two characteristic dips corresponding to the two sharp A and B exciton resonances in the PLE spectra. A model developed for the dip in A provides the first experimental evidence of an enhancement in the dielectric mismatch effect originating from the Coulomb interaction between electron and hole.

An order of magnitude increase in the quantum efficiency of (Al)GaAs nanowires using hybrid photonic-plasmonic modes.

We demonstrate 900% relative enhancement in the quantum efficiency (QE) of surface passivated GaAs nanowires by coupling them to resonant nanocavities that support hybrid photonic-plasmonic modes. This nonconventional approach to increase the QE of GaAs nanowires results in QE enhancement over the entire nanowire volume and is not limited to the near-field of the plasmonic structure. Our cavity design enables spatially and spectrally tunable resonant modes and efficient in- and out-coupling of light from the nanowires. Furthermore, this approach is not fabrication intensive; it is scalable and can be adapted to enhance the QE of a wide range of low QE semiconductor nanostructures.

Selective-area epitaxy of pure wurtzite InP nanowires: high quantum efficiency and room-temperature lasing.

We report the growth of stacking-fault-free and taper-free wurtzite InP nanowires with diameters ranging from 80 to 600 nm using selective-area metal-organic vapor-phase epitaxy and experimentally determine a quantum efficiency of ∼50%, which is on par with InP epilayers. We also demonstrate room-temperature, photonic mode lasing from these nanowires. Their excellent structural and optical quality opens up new possibilities for both fundamental quantum optics and optoelectronic devices.

Periodic dielectric structures for light-trapping in InGaAs/GaAs quantum well solar cells.

We study dielectric diffraction gratings for light-trapping in quantum well solar cells and compare their performance with plasmonic and Lambertian light-trapping structures. The optimum structural parameters are identified for symmetric uni-periodic, symmetric bi-periodic and asymmetric bi-periodic gratings. The enhancement in short-circuit current density from the quantum well region with respect to a reference cell with no diffraction grating is calculated. The ratio of this enhancement to the maximum achievable enhancement (i.e. no transmission losses) is 33%, 75% and 74%, respectively for these structures. The optimum asymmetric and symmetric bi-periodic structures perform closest to Lambertian light-trapping, while all three optimum grating structures outperform optimum plasmonic light-trapping. We show that the short-circuit current density from the quantum well region is further enhanced by incorporating a rear reflector.

Design considerations for semiconductor nanowire-plasmonic nanoparticle coupled systems for high quantum efficiency nanowires.

The optimal geometries for reducing the radiative recombination lifetime and thus enhancing the quantum efficiency of III-V semiconductor nanowires by coupling them to plasmonic nanoparticles are established. The quantum efficiency enhancement factor due to coupling to plasmonic nanoparticles reduces as the initial quality of the nanowire increases. Significant quantum efficiency enhancement is observed for semiconductors only within about 15 nm from the nanoparticle. It is also identified that the modes responsible for resonant enhancement in the quantum efficiency of an emitter in the nanowire are geometric resonances of surface plasmon polariton modes supported at the nanowire/nanoparticle interface.

Polarization tunable, multicolor emission from core-shell photonic III-V semiconductor nanowires.

We demonstrate luminescence from both the core and the shell of III-V semiconductor photonic nanowires by coupling them to plasmonic silver nanoparticles. This demonstration paves the way for increasing the quantum efficiency of large surface area nanowire light emitters. The relative emission intensity from the core and the shell is tuned by varying the polarization of the excitation source since their polarization response can be independently controlled. Independent control on emission wavelength and polarization dependence of emission from core-shell nanowire heterostructures opens up opportunities that have not yet been imagined for nanoscale polarization sensitive, wavelength-selective, or multicolor photonic devices based on single nanowires or nanowire arrays.