Search

Abstract We propose a novel design for multi-junction photonic crystal surface emitting lasers (PCSELs) operating in higher-order waveguide modes to minimize internal losses. This paper details the design and simulation of a 2-junction PCSEL, including calculations of confinement factors, coupling coefficients, radiation loss, and threshold currents. We compare the performance of 2 J-PCSELs and 1 J-PCSELs, demonstrating the potential for highly efficient multi-junction PCSELs with improved power conversion efficiency and output power.

We analyzed the optimal waveguide structure of two types of InGaN-based photonic crystal surface-emitting lasers (PCSELs) to suppress the coupling with leaky modes via mode simulations. To minimize the threshold material gain (gth), we calculated the confinement factor and quality factor of PCSELs with varying waveguide layer thicknesses in the separate confinement heterostructure (SCH) layer. The optical mode intensity profile revealed the coupling between the fundamental mode of SCH and parasitic leaky modes in the cladding layer or substrate as the primary root cause of the low-quality factor and high threshold gain of PCSELs. The asymmetric nature of the SCH structure yielded the optimal waveguide structure to be dependent on the position of the air holes. With a proper waveguide thickness and air hole depth, the optimized threshold modal gain of PCSELs with the n-side air holes can be less than 30 cm−1.

Abstract Photonic-crystal surface-emitting lasers have many promising properties over traditional semiconductor lasers and are regarded as the next-generation laser sources. However, the minimum achievable lasing threshold of PCSELs is still several times larger than that of VCSELs, and limiting its applications especially if the required power is small. Here, we propose a new design that reduces the gain region in the lateral plane by using selective quantum-well intermixing to reduce the threshold current of PCSELs. By performing theoretical calculations, we confirmed that the threshold current can be lowered by a factor of two to three while keeping the PCSEL’s advantage of small divergence angle.

Abstract 2D materials have manifested themselves as key components toward compact integrated circuits. Because of their capability to circumvent the diffraction limit, light manipulation using surface plasmon polaritons (SPPs) is highly‐valued. In this study, plasmonic photodetection using graphene as a 2D material is investigated. Non‐scattering near‐field detection of SPPs is implemented via monolayer graphene stacked under an SPP waveguide with a symmetric antenna. Energy conversion between radiation power and electrical signals is utilized for the photovoltaic and photoconductive processes of the gold‐graphene interface and biased electrodes, measuring a maximum photoresponsivity of 29.2 mA W−1. The generated photocurrent is altered under the polarization state of the input light, producing a 400% contrast between the maximum and minimum signals. This result is universally applicable to all on‐chip optoelectronic circuits.

Herein, it is shown how the novel layout and arrangement of electrodes of a vertical‐cavity surface‐emitting laser (VCSEL) array can simultaneously improve its high‐speed data transmission performance and the brightness of the output beam. In contrast to the layout of the traditional VCSEL array with its isolated mesas and a single electrode to electrical parallel arrangement of all active elements, the new inverse design can effectively reduce the pitch size between neighboring light emission apertures thereby allowing significant downscaling of the whole active area of the array and high brightness output. Moreover, there are two separate electrodes in demonstrated compact 7 × 7 VCSEL array, one for pure dc current injection and the other for large ac signal modulation. Compared with the single electrode reference device, the demonstrated array shows heavier dampening of the electrical–optical (E–O) frequency response, a wider maximum 3‐dB E–O bandwidth (17 vs 13 GHz), and a Gaussian‐like optical far‐field pattern with a higher brightness output (65.95 vs 40.8 kW cm−2 sr−1), under the same high output power (≈145 mW). The advantages of this novel VCSEL array lead to a much better quality of 32 Gbps eye‐opening with a higher brightness output.

Abstract This study conducts comprehensive performance analyses of a commercial photonic-crystal surface-emitting laser (PCSEL) via small-signal measurement and the bit-error-rate test. Meanwhile, the radio frequency characteristics of the PCSEL are unveiled for the first time. Compared to the vertical-cavity surface-emitting lasers, the PCSEL shows great potential for a broader optical bandwidth that is benefited from the high optical-confinement factor. A maximum bandwidth of around 2.32 GHz is experimentally observed when the PCSEL was biased at 340 mA. Moreover, a theoretical calculation was applied to shed light on the characteristics of the small-signal measurement, providing a deep insight into the corresponding intrinsic response model. The signal transmission capability of the PCSEL was investigated as well. The maximum bit rate and corresponding rise time transmitted at 500 Mbps are 1.2 Gbps and 186.16 ps, respectively. Thus, a high-speed PCSEL can be realised with a shrunk form factor, serving as a promising candidate for the next-generation light sources in high-speed optical communication.

We would like to celebrate Prof [...]

Abstract We designed and fabricated a photonic crystal surface emitting laser (PCSEL) with vertically integrated diffractive optical elements on their top to study the mechanism of static beam steering on a single chip. The deflected output beam by the self-formed periodic ITO cladding layer of the PCSEL can be further steered by changing the grating period and azimuthal angle of the diffractive gratings relative to the photonic crystal. Through the analysis of photonic band structure and lasing characteristics, the periodic ITO structure is coupled to the photonic crystal band, whereas the integrated grating serves the diffractive function only. The findings pave the way for the design of PCSELs enabling single or multiple output beam with varying direction capability. This type of laser is regarded as an ideal light source for various applications, such as light detection and ranging and three-dimensional sensing systems.

Abstract We systematically studied the characteristics of hybrid perovskite-based surface plasmon nanolasers. If one changes the anion composition of perovskites, the emission wavelength can be easily tuned. We conducted in full-spectrum modeling that featured hybrid perovskite nanowires placed on different SiO2-coated metallic (Au, Ag, and Al) plates. The proposed nanocavities that supported plasmonic gap modes exhibited distinguished properties of nanolasers, such as low-transparency threshold-gain and low lasing threshold. The corresponding experimental results for the MAPbBr3 nanolaser on Ag revealed the low-threshold operation. These superior features were attributed to enhanced light-matter interaction with strong coupling. Therefore, the proposed scheme, integrated with hybrid perovskite as gain material, provides an excellent platform for nanoscale plasmon lasing in the visible to near-infrared spectra.

The ZnO‐based Tamm plasmon‐polariton (TPP) ultraviolet laser is realized with the strong electric field confinement in the active layer. A coupling between TPPs and photons with a Rabi splitting of ≈30 meV is observed at room temperature. Furthermore, the TPP lasing at 373 nm is clearly observed by optical pumping. The corresponding lasing characteristics, such as threshold energy, linewidth, and angular dispersion curve are verified. These results form a basis for better understanding of the TPPs lasing mechanisms.

Photonic crystal surface-emitting lasers (PCSELs) hold promising properties of both edge emitting lasers (EELs) and vertical-cavity surface-emitting lasers (VCSELs). They possess high output power while radiating light vertically, being thought of as the next generation laser source. One of the main advantages of PCSELs is their scalability of size and power, which makes them applicable to high power applications or long-distance detection. However, due to problems such as current injection and mode competition, there are limits on their dimensions. To further increase the power, the capability of two-dimensional array integration paves the way. In this work, we demonstrate a new method to fabricate PCSEL arrays by defining an isolation pattern. We also investigate the influence of aperture size and array arrangement on lasing performance.

In this study, a paradigm for modulating the light emission performance of photonic-crystal surface-emitting laser (PCSEL) via GaN high electron mobility transistor (HEMT) driving circuit is proposed for the first time. For light detection and ranging (LiDAR) system, a faster pulse repetition frequency with shorter pulse width can provide not only high resolution but also sufficiently precise range resolution. Hereupon, comprehensive analyses for such an integrated system are conducted with not only electro-optical responses but also the corresponding optical behaviors. The relevant electrical characteristics of the employed GaN HEMT are examined at first. Next, the integrated system on a matrix board with its corresponding circuit topology is discussed, illustrating the relevant operating principles. Thereby, sufficient systematical scrutinization for relevant light emissions is performed for both photodiode responses and the optical behaviors under different conditions, paving a holistic panorama for the LiDAR system. Thus, prospects for the next generation LiDAR system in high-power and high-speed operation can be expected.

We study the lateral optical confinement effect on the performance and lasing characteristics of GaN-based vertical-cavity surface-emitting lasers (VCSELs). The anti-guiding and guiding structures are designed by calculating the effective refractive index difference of active and cladding regions. By varying the compensation layer configuration and thickness, the designed effective refractive index of the active region is 0.44% higher than peripheral for the guiding structure. The threshold current density of VCSEL with the guiding structure is 25.4 kA/cm2, which is almost half of that with the anti-guiding structure showing 53.5 kA/cm2. For the anti-guiding structure, the signals from both active and cladding regions are observed by the far-field angle-resolved spectrum at 1.4 × Ith driving current, which agrees well with our simulation. Throughout the study, we show the advantage of the guiding structure both theoretically and experimentally. Moreover, an effective guiding structure is obtained by a proper design and simple device process, which will be very promising for mass production.

Abstract Polyimides (PIs) possess excellent mechanical properties, thermal stability, and chemical resistance and can be converted to carbon materials by thermal carbonization. The preparation of carbon nanomaterials by carbonizing PI‐based nanomaterials, however, has been less studied. In this work, the fabrication of PI nanofibers is investigated using electrospinning and their transformation to carbon nanofibers. Poly(amic acid) carboxylate salts (PAASs) solutions are first electrospun to form PAAS nanofibers. After the imidization and carbonization processes, PI and carbon nanofibers can then be obtained, respectively. The Raman spectra reveal that the carbon nanofibers are partially graphitized by the carbonization process. The diameters of the PI nanofibers are observed to be smaller than those of the PAAS nanofibers because of the formation of the more densely packed structures after the imidization processes; the diameters of the carbon nanofibers remain similar to those of the PI nanofibers after the carbonization process. The thermal dissipation behaviors of the PI and carbon nanofibers are also examined. The infrared images indicate that the transfer rates of thermal energy for the carbon nanofibers are higher than those for the PI nanofibers, due to the better thermal conductivity of carbon caused by the covalent sp2 bonding between carbon atoms.

Since the first demonstration of (Al, In, Ga)N-based blue vertical-cavity surface-emitting lasers (VCSELs) in 2008, the maximum output power (Pmax) and threshold current density (Jth) has been improved significantly after a decade of technology advancements. This article reviewed the key challenges for the realization of VCSELs with III-nitride materials, such as inherent polarization effects, difficulties in distributed Bragg’s reflectors (DBR) fabrication for a resonant cavity, and the anti-guiding effect due to the deposited dielectrics current aperture. The significant tensile strain between AlN and GaN hampered the intuitive cavity design with two epitaxial DBRs from arsenide-based VCSELs. Therefore, many alternative cavity structures and processing technologies were developed; for example, lattice-matched AlInN/GaN DBR, nano-porous DBR, or double dielectric DBRs via various overgrowth or film transfer processing strategies. The anti-guiding effect was overcome by integrating a fully planar or slightly convex DBR as one of the reflectors. Special designs to limit the emission polarization in a circular aperture were also summarized. Growing VCSELs on low-symmetry non-polar and semipolar planes discriminates the optical gain along different crystal orientations. A deliberately designed high-contrast grating could differentiate the reflectivity between the transverse-electric field and transverse-magnetic field, which restricts the lasing mode to be the one with the higher reflectivity. In the future, the III-nitride based VCSEL shall keep advancing in total power, applicable spectral region, and ultra-low threshold pumping density with the novel device structure design and processing technologies.

One of the main reasons that the emission efficiency of GaN-based light-emitting diodes (LEDs) decreases significantly as the emission wavelength shorter than 300 nm is the low light extraction efficiency (LEE). Especially in deep ultra-violet (DUV) LEDs, light propagating outside the escape cone and being reflected back to the semiconductor or substrate layer is absorbed not only by active layers but also by p-type layers with narrower bandgaps and electrodes that are neither transparent nor reflective of the DUV wavelength. In this report, we propose a DUV LED structure with mesh p-GaN/indium-tin-oxide (ITO) contacts and a Ti/Al/Ni/Au layer as a reflective layer to improve LEE. The mesh p-GaN/ITO DUV LED showed an output power of 12% higher than that from the conventional DUV LED due to the lower light absorption at 280 nm.

Strains and V-shaped pits are essential factors for determining the efficiency of GaN-based light-emitting diodes (LEDs). In this study, we systematically analyzed GaN LED structures on patterned sapphire substrates (PSSs) with two types of growth temperature employed for prestrained layers and three different thickness of n-type GaN layers by using cathodoluminescence (CL), microphotoluminescence (PL), and depth-resolved confocal Raman spectroscopy. The results indicated that V-pits formation situation can be analyzed using CL. From the emission peak intensity ratio of prestrained layers and multiple quantum wells (MQWs) in the CL spectrum, information regarding strain relaxation between prestrained layers and MQWs was determined. Furthermore, micro-PL and depth-resolved confocal Raman spectroscopy were employed to validate the results obtained from CL measurements. The growth conditions of prestrained layers played a dominant role in the determination of LED performance. The benefit of the thick layer of n-GaN was the strain reduction, which was counteracted by an increase in light absorption in thick n-type doped layers. Consequently, the most satisfactory LED performance was observed in a structure with relatively lower growth temperature of prestrained layers that exhibited larger V-pits, leading to higher strain relaxation and thinner n-type GaN layers, which prevent light absorption caused by n-type GaN layers.

In this study, we theoretically designed and experimentally fabricated an InGaN vertical-cavity surface-emitting laser (VCSEL) with a tunnel junction (TJ) structure. From numerical simulation results, the optical loss of the device can be reduced by a TJ structure. Additionally, the leakage current of the VCSEL with TJ structure was much smaller than that of the VCSEL with an Indium-Tin-Oxide (ITO) layer. We have been demonstrated that laser output performance is improved by using the TJ structure when compared to the typical VCSEL structure of the ITO layer. The output power obtained at 2.1 mW was enhanced by a factor of 3.5 by the successful reduction of threshold current density (Jth) from 12 to 8.5 kA/cm2, and the enlarged slope efficiency was due to less absorption in VCSEL with a TJ structure. Finally, the samples passed the high temperature (70 °C) and high operation current (1.5 × Jth) test for over 500 h.

Whispering gallery mode (WGM) lasers and resonators are key building blocks for photonic integrated circuits. The quality factor and resonant wavelength are strong functions of the cavity size. Nanoscale WGM cavities suffer from a low-quality factor due to prominent scattering loss. However, the quality factor could be enhanced by forming an optically-coupled rod array or photonic molecules. Through simulations, we revealed how rod-to-rod optical coupling influenced the threshold pumping level and dominant mode selection, where the trend showed good agreement with the experimental observation. According to the simulation, the quality factor could be enhanced by up to eight times by forming a six-rod photonic molecule. The quality factor and effective mode were both superior to the single rods with the same wafer device footprint.

In this paper, influence of a V-pit embedded inside the multiple quantum wells (MQWs) LED was studied. A fully three-dimensional stress-strain solver and Poisson-drift-diffusion solver are employed to study the current path, where the quantum efficiency and turn-on voltage will be discussed. Our results show that the hole current is not only from top into lateral quantum wells (QWs) but flowing through shallow sidewall QWs and then injecting into the deeper lateral QWs in V-pit structures, where the V-pit geometry provides more percolation length for holes to make the distribution uniform along lateral MQWs. The IQE behavior with different V-pit sizes, threading dislocation densities, and current densities were analyzed. Substantially, the variation of the quantum efficiency for different V-pit sizes is due to the trap-assisted nonradiative recombination, effective QW ratio, and ability of hole injections.

The crystal quality, electrical and optical properties of GaN based light emitting diodes (LEDs) with ex-situ sputtered physical vapor deposition (PVD) aluminum nitride (AlN) nucleation layers were investigated. It was found that the crystal quality in terms of defect density and x-ray diffraction linewidth was greatly improved in comparison to LEDs with in-situ low temperature GaN nucleation layer. The light output power was 3.7% increased and the reverse bias voltage of leakage current was twice on LEDs with ex-situ PVD AlN nucleation layers. However, larger compressive strain was discovered in LEDs with ex-situ PVD AlN nucleation layers. The study shows the potential and constrain in applying ex-situ PVD AlN nucleation layers to fabricate high quality GaN crystals in various optoelectronics.