The advancements in integrated thin-film lithium niobate on insulator (LNOI) platform have significantly enhanced the performance of various integrated electro-optic devices, including modulators, external cavity diode lasers, and optical frequency comb generators. Additionally, the development of LNOI has facilitated applications at shorter wavelengths, due to its wide transparency window. The coupling efficiency between laser diodes (LDs) and LN chips is commonly enhanced by designing spot size converters (SSCs). However, achieving high-efficiency SSCs is more challenging at shorter wavelength due to the smaller mode area and the increased confinement of the optical field in LN waveguides. In this study, we present a spot size converter based on hybrid SiN-LN structure, for low-loss light coupling between a III– V gain chip and a LNOI waveguide at 780nm. The parameters of SiN waveguide, LN taper and SiO2 spacing layer has been optimized in order to enhance the matching of effective refractive indices. The entire SSC structure can be fabricated with two steps of photolithography and etching, demonstrating high fabrication tolerance. Simulations indicate that the coupling losses between the output mode of the LD and the fundamental mode of the LN waveguide are 0.41dB/facet for TE mode and 0.55dB/facet for TM mode at a wavelength of 780nm. Our design is intended to offer efficient light coupling from LD to LNOI chips at short wavelength range, characterized by its simple process and high fabrication tolerance.
KEYWORDS: Quantum chromodynamics, Transition metals, Quantum efficiency, Absorption, Sensors, Electric field sensors, Doping, Modulation, Quantum wells, Electron transport
This work proposes a novel way to regulate the electron quantum states of quantum cascade detectors (QCDs) by utilizing localized built-in electric field introduced by modulation doping. The mechanism that how the localized built-in electric field influences extraction efficiency is studied by analyzing the quantum transitions in a simplified three-quantum-well model. The calculation results show that, by introducing the localized built-in electric field, a transition energy close to the LO phonon energy can be more easily realized with almost unchanged transition matrix element. The transition matrix element can be enlarged by the localized built-in electric field with almost unchanged transition energy. The calculated extraction efficiency is below 65% for the standard QCD structures without localized built-in electric field, whereas for the structures with localized built-in electric field, the extraction efficiency can reach above 80%. From experimental results, a higher extraction efficiency of photo-generated electrons of 89% is obtained for the proposed QCD structure, comparing with 63% for the standard QCD structure. The peak response wavelengths of two structures are both around 4.5 μm. At temperatures ranging from 40K to 210K, the photocurrents of the structure with localized built-in electric field are over 55% larger than those of the standard structure. To sum up, the localized built-in electric field can be utilized to regulate the electron states besides the layer thickness and material composition of QCDs.
In this paper, we investigate the fabrication of high aspect ratio photonic crystal air holes in AlGaAs materials using general inductively coupled plasma (ICP) dry etching system. We propose dividing the long etching process into multiple short-time etching segments during the ICP etching process, so that there is enough time to exhaust the etch products out from the bottom of the holes before the next etching segment, which is beneficial for deep air hole etching. Simultaneously, a novel method to suppress lateral penetration of holes by in-situ sidewall passivation is proposed, which can be realized by inserting one oxygen plasma treatment between two etching segments. This method also allows the optimization of etch rate to be achieved independent of sidewall passivation. Our experiment results show that the sidewall passivation has a crucial influence on the etched morphology of air holes. Without sidewall passivation, the air holes are lateral penetrated in the middle. While with appropriate oxidation for sidewall passivation, deep air holes with high verticality are obtained. Finally, high-aspect-ratio air holes with a diameter of 130 nm and a depth of over 1.5 μm are successfully manufactured.
In this paper, we have fabricated and packaged a blue micro-LED with a diameter of 50-μm based on a single layer of InGaN QD micro-LED and present a new method to calculate the junction capacitance of micro-LEDs under forward voltage using the forward AC small-signal method. The results confirm that QD micro-LEDs, like commercial LEDs, show obvious negative capacitances at low frequencies and large voltages. The values of negative capacitance at high frequency and low voltage are so small and can be ignored, or there is no negative capacitance. We have also concluded the empirical expressions for negative capacitance, voltage, and frequency.
A GaN surface emitting laser (SEL) based on angular-symmetry-breaking concentric-ring surface grating (ASB-CRSG) is proposed in this paper. The second-order CRSG located in the p-contact and p-cladding of an EPI wafer of GaN FP laser is adopted to select the radial mode and couple the optical power vertically out of the laser cavity. As the zero-order azimuthal CRSG with a two-lobe far field has the lowest mode loss in the angular-symmetric CRSG, the first-order ASB is adopted by the removal of two circular sections of GaN epitaxial layers to break the angular symmetry of the lasing modes. The simulation results show that degenerate modes in angular-symmetric CRSG have different mode losses with the help of the first-order ASB and the bigger breaking angles of CRSG results to higher loss difference between the first-order and other azimuthal modes. The loss and divergence angle decrease with the increasing area of CRSG, and the deeper CRSG results to the higher out-plane coupling. The first-order azimuthal mode has the lowest mode loss whose value is ~ 84% of that of the second-lowest-loss mode. A single-lobe far-field with a divergence angle of 1.33° in the wavelength of 450nm will be realized by an ASB-CRSG with the diameter of 10.6μm, the breaking angles of 12° and the depth of 325nm. Therefore, the single-mode operation of the first-order azimuthal mode which has a single-lobe far field is expected with the combination of the second-order CRSG and the first-order ASB.
A hybrid integration method of back-illuminated modified uni-traveling carrier photodiode (MUTC-PD) on silicon-oninsulator (SOI) is demonstrated. Compared with the die-to-die bonding of unprocessed III-V die, this hybrid bonding method, implemented by a flip-chip bonding machine, is more convenient and flexible, thus providing a more direct path to utilizing high-speed PDs in integrated microwave photonics on SOI. As a result, the integrated photodetector exhibits a 3-dB bandwidth of 30 GHz, showing no degradation compared with the bandwidth before bonding.
The distributed feedback (DFB) laser is a key component for fiber communication due to its single-mode performance, but it usually requires complex and expensive regrowth after grating definition. The laterally-coupled distributed feedback (LC-DFB) laser has the advantage of a simple fabrication process without epitaxial regrowth, but the LC-DFB laser usually has a low coupling coefficient as the optical feedback is provided by the evanescent field and Fabry-Parot (FP) longitudinal modes arise from the pair of parallel cleaved facets. In this work, a triangular prism etched facet is proposed to suppress the FP longitudinal modes from cleaved facets of a 1.3 μm LC-DFB laser. The length-width ratio of a triangular prism facet is optimized on the compromise between the reflection and length by finite difference time domain (FDTD) method. The vertical etched facet with depth of 4 μm and tip curvature of 100 nm and the lateral gratings with depth of 1.8 μm and gap of 200 nm are fabricated by inductively coupled plasma (ICP) etching with the gas mixtures of Cl2/CH4/Ar and CH4/H2/Ar, respectively. The FP longitudinal modes of the etched-facet laterally-coupled distributed feedback (EF-LC-DFB) laser are effectively suppressed compared to the counterpart of cleaved facets, and the stable single-mode operation of EF-LC-DFB is demonstrated with the side mode suppression ratio (SMSR) of 54.35 dB.
Optical phased arrays (OPAs) are widely used in many applications to realize high-speed optical beam scanning. At present OPAs often suffer from limited scanning range. Here we propose a circular optical phased array (COPA) based on silicon photonics platform. According to our simulations, by positioning the OPA units in a circle and adopting a specific phase distribution, the COPA can realize 360° constant amplitude scanning. In addition, the design of the disk grating coupler, which is the key device of the COPA, is presented. The COPA is believed to have great potential for applications where a wide scanning range is mandatory.
Recently the GaN/AlN multi-quantum-well structure avalanche photodiode (MAPD) has been demonstrated with PMT-like multiplication gain larger than 1E4. In this work, the photocurrent of GaN/AlN MAPD has been investigated and negative differential conductance (NDC) is found in the photocurrent characteristic of MAPD. Through self-consistent calculation, conduction band structure and discrete energy states in each quantum well layer have been obtained for MAPD. The discrete states drop down and align with the conduction band edge of absorption layer around the NDC peak voltage, so the NDC feature is proposed as resonant tunneling of photoelectrons into MQW structure. The proposed resonant tunneling process is confirmed by the observation of resonant tunneling peaks in a specially designed resonant tunneling diode simulating the band profile of MAPD. The finding of NDC feature is beneficial for understanding and increasing the quantum efficiency of MAPD, since the photoelectron blocking at AlN barrier is greatly reduced by the resonant tunneling process.
Measurement of carrier lifetime is very important to understand the physics in light-emitting diodes (LEDs), as it builds a link between carrier concentration and excitation power or current density. In this paper, we present our study on optical and electrical characterizations on carrier lifetimes in polar InGaN-based LEDs. First, a carrier rate equation model is proposed to explain the non-exponential nature of time-resolved photoluminescence (TRPL) decay curves, wherein exciton recombination is replaced by bimolecular recombination, considering the influence of polarization field on electron-hole pairs. Then, nonradiative recombination and radiative recombination coefficients can be deduced from fitting and used to calculate the radiative recombination efficiency. By comparing with the temperature-dependent photoluminescence (TDPL) and power-dependent photoluminescence (PDPL), it is found these three methods provide the consistent results. Second, differential carrier lifetimes depending on injection current are measured in commercial near-ultraviolet (NUV), blue and green LEDs. It is found that carrier lifetime is longer in green one and shorter in NUV one, which is attributed to the influence of polarization-induced quantum confined Stark effect (QCSE). This result implies the carrier density is higher in green LED while lower NUV LED, even the injection current is the same. By ignoring Auger recombination and fitting the efficiency–current and carrier lifetime–current curves simultaneously, the dependence of injection efficiency on carrier concentration in different LED samples are plotted. The NUV LED, which has the shallowest InGaN quantum well, actually exhibits the most serious efficiency droop versus carrier concentration. Then, the approaches to overcome the efficiency droop are discussed.
The high-gain photomultiplier tube (PMT) is the most popular method to detect weak ultra-violet signals which attenuate quickly in atmosphere, although the vacuum tube makes it fragile and difficult to integrate. To overcome the disadvantage of PMT, an AlN/GaN periodically–stacked-structure (PSS) avalanche photodiode (APD) has been proposed, finally achieving good quality of high gain and low excessive noise. As there is a deep г valley only in the conduction band of both GaN and AlN, the electron transfers suffering less scattering and thus becomes easier to obtain the threshold of ionization impact. Because of unipolar ionization in the PSS APD, it works in linear mode. Four prototype devices of 5-period, 10-period, 15-period, and 20-period were fabricated to verify that the gain of APD increases exponentially with period number. And in 20-period device, a recorded high and stable gain of 104 was achieved under constant bias. In addition, it is proved both experimentally and theoretically, that temperature stability on gain is significantly improved in PSS APD. And it is found that the resonant enhancement in interfacial ionization may bring significant enhancement of electron ionization performance. To make further progress in PSS APD, the device structure is investigated by simulation. Both the gain and temperature stability are optimized alternatively by a proper design of periodical thickness and AlN layer occupancy.
The next generation infrared (IR) detection technology demands for very-large-format focal plane arrays (FPAs) with
high performance. Semiconductor up-converters can convert IR photons to near-infrared (NIR) photons, and can be
potential candidates for large-format IR imaging since the mechanical bonding with the read-out circuits can be avoided.
However, previously reported up-converters and corresponding up-conversion systems suffer from low detectivity
because of the trade-off between responsivity and dark current. To solve this issue, a cascade infrared up-converter
(CIUP) is demonstrated in this work. Based on a quantum cascade transport mechanism, high IR responsivity is achieved
while the dark current is maintained fairly low. A 4-μm InGaAs/AlGaAs CIUP has been fabricated, and both the CIUP
and up-conversion system are under background-limited infrared performance (BLIP) regime below 120 K. The upconversion
efficiency is 2.1 mW/W at 3.3 V and 78 K. Taking shot noise as the main noise in the up-conversion system,
the BLIP detectivity of the system is 2.4×109 Jones at 3.3 V and 78 K, higher than the semiconductor up-converters at
similar wavelengths reported so far. To further improve the CIUP performance, an AlInP hole-blocking layer is
introduced taking place of the AlAs layer. AlInP/GaAs has larger valence band discontinuity than AlAs/GaAs, showing
the advantage of tightly confining injected holes into the emission quantum well. By adopting the AlInP hole-blocking
layer, the quantum efficiency and detectivity of the up-conversion system can be enhanced.
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