An electrically pumped surface-emitting semiconductor green laser

Advancing on-chip Kerr optical parametric oscillation towards coherent applications covering the green gap

Optical parametric oscillation (OPO) in Kerr microresonators can efficiently transfer near-infrared laser light into the visible spectrum. To date, however, chromatic dispersion has mostly limited output wavelengths to >560 nm, and robust access to the whole green light spectrum has not been demonstrated. In fact, wavelengths between 532 nm and 633 nm, commonly referred to as the "green gap", are especially challenging to produce with conventional laser gain. Hence, there is motivation to extend the Kerr OPO wavelength range and develop reliable device designs. Here, we experimentally show how to robustly access the entire green gap with Kerr OPO in silicon nitride microrings pumped near 780 nm. Our microring geometries are optimized for green-gap emission; in particular, we introduce a dispersion engineering technique, based on partially undercutting the microring, which not only expands wavelength access but also proves robust to variations in resonator dimensions. Using just four devices, we generate >150 wavelengths evenly distributed throughout the green gap, as predicted by our dispersion simulations. Moreover, we establish the usefulness of Kerr OPO to coherent applications by demonstrating continuous frequency tuning (>50 GHz) and narrow optical linewidths (<1 MHz). Our work represents an important step in the quest to bring nonlinear nanophotonics and its advantages to the visible spectrum.

Architecture for Surface-Emitting Lasers with On-Demand Lasing Wavelength by Nanowire Optical Cavities

Despite the importance and exciting progress of surface-emitting (SE) semiconductor lasers, we have limited choices of lasing wavelength even today. From an application viewpoint, it is desirable to have an architecture that can allow SE lasing in a wide spectral range, based on the need of applications. Herein, we demonstrate a path for SE lasers with lasing wavelength on demand by exploiting III-nitride nanowire optical cavities formed by low-temperature selective area epitaxy (SAE), combined with fine-tuning of substrate patterns and photonic bands. Moreover, in this study, we focus on the device demonstration in the ultraviolet (UV) spectral range, considering the severe lag in developing SE lasers in the UV wavelength range compared to longer wavelengths, e.g., near-infrared (NIR), as well as the potential applications enabled by UV lasers such as solar blind optical wireless communications. Ultralow threshold wavelength-tunable SE UV lasing is achieved by optical pumping. Moreover, SE UV lasing under direct electric current injection is also achieved. This study not only represents an important step in the journey of SE UV laser development but, more importantly, it lays the ground for SE lasers with lasing wavelength on demand, broadly from NIR to UV.

Topological photonic crystal nanowire array laser with bulk states

A topological photonic crystal InGaAsP/InP core-shell nanowire array laser with bulk states operating in the 1550 nm band is proposed and simulated. By optimizing the structure parameters, high Q factor of 1.2 × 105 and side-mode suppression ratio of 13.2 dB are obtained, which are 28.6 and 4.6 times that of a uniform nanowire array, respectively. The threshold and maximum output are 17% lower and 613% higher than that of the uniform nanowire array laser, respectively, due to the narrower nanowire slits and stronger optical confinement. In addition, a low beam divergence angle of 2° is obtained due to the topological protection. This work may pave the way for the development of high-output, low-threshold, low-beam-divergence nanolasers.

Design of double-lattice GaN-PCSEL based on triangular and circular holes

We have theoretically designed a double-lattice photonic crystal surface-emitting laser (PCSEL) based on triangular and circular holes. In the design, porous-GaN which has the properties of lower refractive index and high quality stress-free homo-epitaxy with GaN, was first proposed to be the cladding layer for GaN-PCSEL. The finite difference-time domain (FDTD), the plane wave expansion (PWE), and the rigorous coupled-wave analysis (RCWA) method were employed in the investigation. Our simulations achieved a radiation constant of up to 50 cm-1 and a slope efficiency of more than 1 W/A while maintaining a low threshold gain. We conducted a systematic study on the effects of the filling factor, etching depth, and holes shift, on the performance of the PCSEL. The findings indicate that increasing the filling factor improves the radiation constant and slope efficiency. Asymmetric hole patterns and varying etching depths have a similar effect. The introduction of asymmetric patterns and a double lattice in the photonic crystal breaks the symmetry of electric fields in the plane, while different etching depths of the two holes break the symmetry in the vertical direction. Additionally, altering the shift of the double lattice modifies the optical feedback in the resonators, resulting in variations of cavity loss and confinement factor.

High-sensitivity hemoglobin detection based on polarization-differential spectrophotometry

Hemoglobin content is recognized as a momentous and fundamental physiological indicator, especially the precise detection of trace hemoglobin is of great significance for early diagnosis and prevention of tumors, cancer, organic injury, etc. Therefore, high-sensitivity hemoglobin detection is imperative. However, effective detection methods and reliable detection systems are still lacking and remain enormous challenges. Herein, we present a synthetical strategy to break through the existing bottleneck based on polarization-differential spectrophotometry and high-performance single-frequency green fiber laser. Importantly, this framework not only has precisely extracted the two-dimensional information of intensity and polarization during the interaction between laser and hemoglobin, but also has taken advantage of the high monochromaticity and fine directivity in the optimized laser source to reduce the undesirable scattered disturbance. Thus, the hemoglobin detection sensitivity of 7.2 × 10-5 g/L has advanced a hundredfold compared with conventional spectrophotometry, and the responsive dynamic range is close to six orders of magnitude. Results indicate that our technology can realize high-sensitivity detection of trace hemoglobin content, holding promising applications for precision medicine and early diagnosis as an optical direct and fast detection method.

Functionalizing nanophotonic structures with 2D van der Waals materials

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Topological photonic crystal nanowire array laser with edge states

A topological photonic crystal InGaAsP/InP core-shell nanowire array laser operating in the 1550 nm wavelength band is proposed and simulated. The structure is composed of an inner topological nontrivial photonic crystal and outer topological trivial photonic crystal. For a nanowire with height of 8 µm, high quality factor of 4.7 × 104 and side-mode suppression ratio of 11 dB are obtained, approximately 32.9 and 5.5 times that of the uniform photonic crystal nanowire array, respectively. Under optical pumping, the topological nanowire array laser exhibits a threshold 27.3% lower than that of the uniform nanowire array laser, due to the smaller nanowire slit width and stronger optical confinement. Moreover, the topological NW laser exhibits high tolerence to manufacturing errors. This work may pave the way for the development of low-threshold single-mode high-robustness nanolasers.

Ultralow threshold surface emitting ultraviolet lasers with semiconductor nanowires

Surface-emitting (SE) semiconductor lasers have changed our everyday life in various ways such as communication and sensing. Expanding the operation wavelength of SE semiconductor lasers to shorter ultraviolet (UV) wavelength range further broadens the applications to disinfection, medical diagnostics, phototherapy, and so on. Nonetheless, realizing SE lasers in the UV range has remained to be a challenge. Despite of the recent breakthrough in UV SE lasers with aluminum gallium nitride (AlGaN), the electrically injected AlGaN nanowire UV lasers are based on random optical cavities, whereas AlGaN UV vertical-cavity SE lasers (VCSELs) are all through optical pumping and are all with large lasing threshold power densities in the range of several hundred kW/cm² to MW/cm². Herein, we report ultralow threshold, SE lasing in the UV spectral range with GaN-based epitaxial nanowire photonic crystals. Lasing at 367 nm is measured, with a threshold of only around 7 kW/cm² (~ 49 μJ/cm²), a factor of 100× reduction compared to the previously reported conventional AlGaN UV VCSELs at similar lasing wavelengths. This is also the first achievement of nanowire photonic crystal SE lasers in the UV range. Further given the excellent electrical doping that has already been established in III-nitride nanowires, this work offers a viable path for the development of the long-sought-after semiconductor UV SE lasers.

Fabry-Perot Oscillation and Resonance Energy Transfer: Mechanism for Ultralow-Threshold Optically and Electrically Driven Random Laser in Quasi-2D Ruddlesden-Popper Perovskites

The recently emerged metal-halide hybrid perovskite (MHP) possesses superb optoelectronic features, which have obtained great attention in solid-state lighting, photodetection, and photovoltaic applications. Because of its excellent external quantum efficiency, MHP has promising potential for the manifestation of ultralow threshold optically pumped laser. However, the demonstration of an electrically driven laser remains a challenge because of the vulnerable degradation of perovskite, limited exciton binding energy (E_b), intensity quenching, and efficiency drop by nonradiative recombinations. In this work, based on the paradigm of integration of Fabry-Perot (F-P) oscillation and resonance energy transfer, we observed an ultralow-threshold (∼250 μWcm^-2) optically pumped random laser from moisture-insensitive mixed dimensional quasi-2D Ruddlesden-Popper phase perovskite microplates. Particularly, we demonstrated an electrically driven multimode laser with a threshold of ∼60 mAcm^-2 from quasi-2D RPP by judicious combination of a perovskite/hole transport layer (HTL) and an electron transport layer (ETL) having suitable band alignment and thickness. Additionally, we showed the tunability of lasing modes and color by driving an external electric potential. Performing finite difference time domain (FDTD) simulations, we confirmed the presence of F-P feedback resonance, the light trapping effect at perovskite/ETL, and resonance energy transfer contributing to laser action. Our discovery of an electrically driven laser from MHP opens a useful avenue for developing future optoelectronics.

InGaN micro-light-emitting diodes monolithically grown on Si: achieving ultra-stable operation through polarization and strain engineering

Micro or submicron scale light-emitting diodes (µLEDs) have been extensively studied recently as the next-generation display technology. It is desired that µLEDs exhibit high stability and efficiency, submicron pixel size, and potential monolithic integration with Si-based complementary metal-oxide-semiconductor (CMOS) electronics. Achieving such µLEDs, however, has remained a daunting challenge. The polar nature of III-nitrides causes severe wavelength/color instability with varying carrier concentrations in the active region. The etching-induced surface damages and poor material quality of high indium composition InGaN quantum wells (QWs) severely deteriorate the performance of µLEDs, particularly those emitting in the green/red wavelength. Here we report, for the first time, µLEDs grown directly on Si with submicron lateral dimensions. The µLEDs feature ultra-stable, bright green emission with negligible quantum-confined Stark effect (QCSE). Detailed elemental mapping and numerical calculations show that the QCSE is screened by introducing polarization doping in the active region, which consists of InGaN/AlGaN QWs surrounded by an AlGaN/GaN shell with a negative Al composition gradient along the c-axis. In comparison with conventional GaN barriers, AlGaN barriers are shown to effectively compensate for the tensile strain within the active region, which significantly reduces the strain distribution and results in enhanced indium incorporation without compromising the material quality. This study provides new insights and a viable path for the design, fabrication, and integration of high-performance µLEDs on Si for a broad range of applications in on-chip optical communication and emerging augmented reality/mixed reality devices, and so on.

Electrically driven single microwire-based single-mode microlaser

Engineering the lasing-mode oscillations effectively within a laser cavity is a relatively updated attentive study and perplexing issue in the field of laser physics and applications. Herein, we report a realization of electrically driven single-mode microlaser, which is composed of gallium incorporated zinc oxide microwire (ZnO:Ga MW) with platinum nanoparticles (PtNPs, d ~ 130 nm) covering, a magnesium oxide (MgO) nanofilm, a Pt nanofilm, and a p-type GaN substrate. The laser cavity modes could resonate following the whispering-gallery mode (WGM) among the six side surfaces by total internal reflection, and the single-mode lasing wavelength is centered at 390.5 nm with a linewidth of about 0.18 nm. The cavity quality factor Q is evaluated to about 2169. In the laser structure, the usage of Pt and MgO buffer layers can be utilized to engineer the band alignment of ZnO:Ga/GaN heterojunction, optimize the p-n junction quality and increase the current injection. Thus, the well-designed device structure can seamlessly unite the electron-hole recombination region, the gain medium, and optical microresonator into the PtNPs@ZnO:Ga wire perfectly. Such a single MW microlaser is essentially single-mode regardless of the gain spectral bandwidth. To study the single-mode operation, PtNPs working as superabsorber can engineering the multimode lasing actions of ZnO:Ga MWs even if their dimensions are typically much larger than that of lasing wavelength. Our findings can provide a straightforward and effective scheme to develop single-mode microlaser devices based on one-dimensional wire semiconductors.

Progress of GaN-Based Optoelectronic Devices Integrated with Optical Resonances

Being direct wide bandgap, III-nitride (III-N) semiconductors have many applications in optoelectronics, including light-emitting diodes, lasers, detectors, photocatalysis, etc. Incorporation of III-N semiconductors with high-efficiency optical resonances including surface plasmons, distributed Bragg reflectors and micro cavities, has attracted considerable interests for upgrading their performance, which can not only reveal the new coupling mechanisms between optical resonances and quasiparticles, but also unveil the shield of novel optoelectronic devices with superior performances. In this review, the content covers the recent progress of GaN-based optoelectronic devices integrated with plasmonics and/or micro resonators, including the LEDs, photodetectors, solar cells, and light photocatalysis. The authors aim to provide an inspiring insight of recent remarkable progress and breakthroughs, as well as a promising prospect for the future highly-integrated, high speed, and efficient GaN-based optoelectronic devices.

Net Optical Gain Coefficients of Cu+ and Tm3+ Single-Doped and Co-Doped Germanate Glasses

Broadband tunable solid-state lasers continue to present challenges to scientists today. The gain medium is significant for realizing broadband tunable solid-state lasers. In this investigation, the optical gain performance for Tm³⁺ and Cu⁺ single-doped and co-doped germanate glasses with broadband emissions was studied via an amplified spontaneous emission (ASE) technique. It was found that the net optical gain coefficients (NOGCs) of Tm³⁺ single-doped glass were larger than those for Cu⁺ single-doped glass. When Tm³⁺ was introduced, the emission broadband width of Cu⁺-doped glass was effectively extended. Moreover, it was found that for the co-doped glass the NOGCs at the wavelengths for Tm³⁺ and Cu⁺ emissions were larger than those of Tm³⁺ and Cu⁺ single-doped glasses at the same wavelengths. In addition, the NOGC values of Tm³⁺ and Cu⁺ co-doped germanate glasses were of the same order of magnitude, and were maintained in a stable range at different wavelengths. These results indicate that the Tm³⁺ and Cu⁺ co-doped glasses studied may be a good candidate medium for broadband tunable solid-state lasers.

Gridization-Driven Mesoscale Self-Assembly of Conjugated Nanopolymers into Luminescence-Anisotropic Photonic Crystals

Organic semiconducting emitters integrated with butterfly-mimetic photonic crystals (PhCs) are fascinating for dramatic advantages over light outcoupling efficiency and multifunctional strain sensors, as well as the key step toward electrically pumped lasers. Herein, an unprecedentedly direct mesoscale self-assembly into 1D PhCs is reported through a covalently gridization-driven approach of wide-bandgap conjugated polymers. A simple solvent-casting procedure allows for in situ self-assembly of the state-of-the-art conjugated nanopolymer, poly{[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]grid}-co-{[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]grid} (PODPFG), into well-defined multilayer architectures with an excellent toughness (30-40 J m^-3 ). This ordered meso-architecture shows a typical Bragg-Snell diffraction behavior to testify the PhC nature, along with a high effective refractive index (1.80-1.88) and optical transmittance (85-87%). The PhC films also exhibit an angle-dependent blue/green photoluminescence switching, an electroluminescence efficiency enhancement by 150-250%, and an amplified spontaneous emission enhancement with ultralow waveguide loss coefficient (2.60 cm^-1 ). Gridization of organic semiconductors offers promising opportunities for cross-scale morphology-directed molecular design in multifunctional organic mechatronics and intelligences.

Interfacial modulation and plasmonic effect mediated high-brightness green light sources in a single Ga-doped ZnO microwire based heterojunction

Heterostructure manufacturing has been extensively studied as indispensable footstones in the progressive semiconductor optoelectronic devices due to their constituent materials, interfacial states and electronic transport capabilities, thus enabling competitive candidates...

Facet-selective morphology-controlled remote epitaxy of ZnO microcrystals via wet chemical synthesis

We report on morphology-controlled remote epitaxy via hydrothermal growth of ZnO micro- and nanostructure crystals on graphene-coated GaN substrate. The morphology control is achieved to grow diverse morphologies of ZnO from nanowire to microdisk by changing additives of wet chemical solution at a fixed nutrient concentration. Although the growth of ZnO is carried out on poly-domain graphene-coated GaN substrate, the direction of hexagonal sidewall facet of ZnO is homogeneous over the whole ZnO-grown area on graphene/GaN because of strong remote epitaxial relation between ZnO and GaN across graphene. Atomic-resolution transmission electron microscopy corroborates the remote epitaxial relation. The non-covalent interface is applied to mechanically lift off the overlayer of ZnO crystals via a thermal release tape. The mechanism of facet-selective morphology control of ZnO is discussed in terms of electrostatic interaction between nutrient solution and facet surface passivated with functional groups derived from the chemical additives.

Monolithic integration of multicolor InGaN LEDs with uniform luminescence emission

We report the demonstration of monolithic integration of multicolor LEDs with highly spatially uniform emission wavelength. LEDs with colors ranging from green to orange are realized in a single selective area epitaxy process, and pronounced emission peak with very narrow spectral linewidth from photonic crystal effect is also achieved simultaneously. The In contents and emission colors are tuned by precisely controlling the nanowire emitter diameter and spacing. The emission wavelengths exhibit small variations of only a few nanometers among countless individual nanowire emitters over a sub-mm² area region.

Chip-Scale Droop-Free Fin Light-Emitting Diodes Using Facet-Selective Contacts

Sub-micron-size light sources are currently extremely dim, achieving nanowatt output powers due to the current density and temperature droop. Recently, we reported a droop-free fin light-emitting diode (LED) pixel that at high current densities becomes a laser with record output power in the microwatt range. Here, we show a scalable method for selectively metallizing fins via their nonpolar side facet that allows electrical injection to sub-200 nm wide n-ZnO fins on p-GaN with at least 0.8 μm² active area. Electrically addressable fin LEDs are fabricated in a linear array format using standard 2 μm resolution photolithography. Electroluminescence analysis across different pixels shows that the fin acts as the active region of the LED and generates a narrow-band ultraviolet emission between ≈368 and ≈390 nm. Investigating fins at high current densities, ranging from 100 to 2000 kA/cm², shows that their emission increases without any decline even as the junction temperature reaches a range of 200-340 °C. The absence of electron leakage to p-GaN at high injection levels and an undetectable electron-hole escape from the fin at high temperatures indicate that the fin shape is highly efficient in controlling the nonradiative recombination pathways such as Auger recombination. The fin LED geometry is expected to enable the realization of high-brightness arrays of light sources at sub-micron-size regimes suitable for operation at high temperatures and high current densities.

High-efficiency broadband tunable green laser operation of direct diode-pumped holmium-doped fiber

Green laser sources have become increasingly important for the application in scientific research and industry. Although several laser approaches have been investigated, the development of green lasers with the necessary efficiency and spectral characteristics required for practical deployment continues to attract immense interest. In this study, the efficient green laser operation of a Ho³⁺-doped fluoride fiber directly pumped by a commercial blue laser diode (LD) is experimentally investigated at various active fiber lengths. In the free-running laser, the slope efficiency was optimized up to 59.3% with 543.9 nm lasing, with respect to the launched pump power, using a 20-cm long active fiber. This is the maximum slope efficiency reported to date for a green fiber laser. A maximum output power of 376 mW at 543.5 nm was achieved by using a 17-cm long active fiber pumped at a maximum available launched pump power of 996 mW. Moreover, broadband tuning operation was demonstrated by employing a range of active fiber lengths, together with an intracavity bandpass filter. The operating wavelength was tunable from 536.3 nm to 549.3 nm. A maximum tuning power achieved was 118 mW at 543.4 nm for a 17-cm long active fiber. Moderate Ho³⁺-doped fiber length is shown to be effective in producing a high performance of a green fiber laser. The short-length of the active fiber considerably extends the green short wavelength operation due to limited reabsorption of the signal below 540 nm.

An electrically driven whispering gallery polariton microlaser

Near-infrared micro/nanolaser devices utilizing low-dimensional semiconductors can provide essential building blocks to achieve integrated optoelectronic devices and circuitry for advanced functionalities and are compatible with on-chip technologies. Although significant progress has been made through using narrow-band semiconductor micro/nanostructures to realize near-infrared stimulated radiation at room temperature, severe challenges still remain involving much lower quantum efficiencies and higher auger recombination. Herein, we report an experimental realization of a current-injection semiconductor polariton device made of a ZnO microwire via Ga-doping (ZnO:Ga MW) and p-type GaAs template. The device can emit polaritonic illumination directly from sharp edges of the hexagonal MW. The experimental results of angle-resolved electroluminescence measurements reveal a typical anticrossing feature between excitons and cavity modes, unambiguous evidence of the strong exciton-polariton coupling, with corresponding Rabi splitting energy extracted to be about 195 meV. As the applied bias goes above a certain value, electrically driven whispering gallery lasing action was achieved in the near-infrared spectrum, and the lasing features can be assigned to the exciton-polariton effect. The results not only can afford insights into the development of low-threshold coherent light sources via the exciton-polariton effect, but also can expand the fabrication of low-dimensional, near-infrared microlaser devices.

Implementing an Insect Brain Computational Circuit Using III-V Nanowire Components in a Single Shared Waveguide Optical Network

Recent developments in photonics include efficient nanoscale optoelectronic components and novel methods for subwavelength light manipulation. Here, we explore the potential offered by such devices as a substrate for neuromorphic computing. We propose an artificial neural network in which the weighted connectivity between nodes is achieved by emitting and receiving overlapping light signals inside a shared quasi 2D waveguide. This decreases the circuit footprint by at least an order of magnitude compared to existing optical solutions. The reception, evaluation, and emission of the optical signals are performed by neuron-like nodes constructed from known, highly efficient III-V nanowire optoelectronics. This minimizes power consumption of the network. To demonstrate the concept, we build a computational model based on an anatomically correct, functioning model of the central-complex navigation circuit of the insect brain. We simulate in detail the optical and electronic parts required to reproduce the connectivity of the central part of this network using previously experimentally derived parameters. The results are used as input in the full model, and we demonstrate that the functionality is preserved. Our approach points to a general method for drastically reducing the footprint and improving power efficiency of optoelectronic neural networks, leveraging the superior speed and energy efficiency of light as a carrier of information.