Optically pumped flexible GaN-based ultraviolet VCSELs.

Flexible optoelectronic platforms, which integrate optoelectronic devices on a flexible substrate, are promising in more complex working environments benefiting from the mechanical flexibility. Herein, for the first time to the best of our knowledge, a flexible GaN-based vertical cavity surface-emitting laser (VCSEL) in the ultraviolet A (UVA) range was demonstrated by using a thin-film transfer process based on laser lift-off (LLO) and spin-coating of a flexible substrate. The lasing wavelength is 376.5 nm with a linewidth of 0.6 nm and threshold energy of 98.4 nJ/pulse, corresponding to a threshold energy density of 13.9 mJ/cm2. The flexible substrate in this study is directly formed by spin-coating of photosensitive epoxy resin, which is much simplified and cost-effective, and a 2-in. wafer scale GaN-based membrane can be successfully transferred to a flexible substrate through this method. Such flexible UVA VCSELs are promising for the development of next-generation flexible and wearable technologies.

Optical gain at 1.55 µm of Er(TMHD)3 complex doped polymer waveguides based on the intramolecular energy transfer effect.

Based on the intramolecular energy transfer mechanism between organic ligand TMHD (2, 2, 6, 6-tetramethyl-3, 5-heptanedione) and central Er3+ ions, optical gains at 1.55 µm were demonstrated in three structures of polymer waveguides using complex Er(TMHD)3-doped polymethylmethacrylate (PMMA) as the active material. With the excitation of two low-power UV light-emitting diodes (LEDs) instead of 980 or 1480 nm lasers, relative gains of 3.5 and 4.1 dB cm-1 were achieved in a 1-cm-long rectangular waveguide with an active core of Er(TMHD)3-doped PMMA polymer. Meanwhile, relative gain of 3.0 dB cm-1 was obtained in an evanescent-field waveguide with cross-section of 4 × 4 µm2 using passive SU-8 polymer as core and a ∼1-µm-thick Er(TMHD)3-doped PMMA as upper cladding. By growing a 100 nm thick aluminum mirror and active lower cladding, the optical gain was doubled to 6.7 dB cm-1 in evanescent-field waveguides because of the stimulated excitation of Er3+ ions in the upper and lower cladding and the improved absorption efficiency.

Two-step spin coating CsPbBr3 thin films and photodetectors in the atmosphere.

Recently, inorganic halide perovskites, especially CsPbBr3, have been attracting attention because of their high efficiency, wide color gamut, and narrow luminescent spectrum. To elevate the perovskite devices' performance, optimizations of crystalline quality, device structures, and fabrication process are essential. Currently, the state-of-the-art fabrication approach of CsPbBr3 is spin-coating in an inert environment (nitrogen, argon, etc.), which requires temperature and humidity control. In this work, a CsPbBr3-based visible photodetector (PD) is realized in a humid atmosphere, whose performances were comparable to those reported in an inert glovebox. The dependencies of responsivity and transient time on CsBr coating layer numbers and electrode period were also investigated. The best device performance was obtained with 4 layers of CsBr coating with a responsivity of 107.2 mA/W, detectivity of 4.29 × 1010 Jones, and quantum efficiency of 25.4%. The rise time of the 3-4-layer CsBr-coated PD was reduced by the higher crystalline quality and carrier mobility, while the decay time of the 1-layer CsBr-coated PD was faster since the dense defect induced non-radiative recombination centers. With the period T increasing, the responsivity decreased, while the transient times increased. We believe that our results could benefit the future optimization of perovskite materials and PDs.

Flexible GaN-based ultraviolet microdisk lasers on PET substrate.

Flexible optoelectronics is a technique for fabricating optoelectronic devices on a flexible substrate. Compared with conventional devices, flexible optoelectronic devices can be used in more complex working environments benefiting from the mechanical flexibility. Herein, for the first time to the best of our knowledge, a flexible GaN-based microdisk laser on a polyethylene terephthalate (PET) substrate in the ultraviolet A (UVA) range was demonstrated by using thin film transfer process based on laser lift-off (LLO). The lasing wavelength is 370.5 nm with a linewidth of 0.15 nm and a threshold power density of 200 kW/cm2. Additionally, a distributed Bragg reflector (DBR) was deposited on the backside of the microdisk as the bottom mirror between GaN microdisk and PET substrate, which can provide better mode confinement inside the microdisk and increases the oscillation intensity. The lasing wavelength of the flexible laser shows a 2-nm redshift under different bending curvature of the substrate, which is promising for applications such as mechanical sensing.

Current spreading structure of GaN-based vertical-cavity surface-emitting lasers.

Indium tin oxide (ITO) is often used as a current spreading layer in the GaN-based vertical-cavity surface-emitting lasers (VCSELs). However, the absorption coefficient of ITO is significant, which reduces the laser output power, raises the threshold, and makes VCSELs hardly lase in the ultraviolet range. To find a transparent conductive structure that can replace ITO, we propose a periodic p-AlGaN/u-GaN/p-GaN structure. In the simulation of light-emitting diodes, the optimized parameter is obtained with multi-period 10 nm p-Al0.1Ga0.9N/2 nm u-GaN/8 nm p-GaN combined with n-GaN/n-Al0.2Ga0.8N in the n region. Applying the structure to 435 nm VCSELs and comparing it to a common VCSEL with the ITO current spreading layer, it can be found that the new structure reduces the threshold from 9.17 to 3.06 kA/cm2. The laser power increases from 1.33 to 15.4 mW. The optimized structure has a high laser power and a lower threshold, which can be used in future investigations.

Dual-wavelength switching in InGaN quantum dot micro-cavity light-emitting diodes.

Dual-wavelength switchable emission has been demonstrated in InGaN quantum dot (QD) micro-cavity light-emitting diodes (MCLEDs). By simply modulating the injected current levels, the output of the device can be dynamically tuned between the two distinct cavity modes at 498.5 and 541.7 nm, exhibiting deterministic mode switching in the green spectral range. Owing to the microcavity effect, high spectral purity with a narrow linewidth of 0.21 nm was obtained. According to the experimental and theoretical results, it can be concluded that the dual-wavelength switching for the investigated MCLEDs is ascribed to the broad and tunable gain of a thin InGaN QD active region, together with the mode selection and enhancement effect of the cavity. To provide additional guidelines for controllable dual-wavelength switchable operation in nitride-based light-emitting devices, detailed design and fabrication strategies are discussed. This work presents an effective method to achieve mode switching for practical applications such as multi-wavelength optical recording, frequency mixing, flip-flop and optical switches.

GaN-based green resonant-cavity light-emitting diodes with Al mirror and copper plate.

In this Letter, GaN-based green resonant-cavity light-emitting diodes (RCLEDs) with a low-cost aluminum (Al) metal bottom mirror, a dielectric top mirror, and a copper (Cu) supporting plate were fabricated. The green-emitting epitaxial wafer was grown on a patterned sapphire substrate (PSS) to ensure high crystal quality (CQ). Laser lift-off (LLO) of the PSS and electrical plating of a Cu supporting plate were then carried out to realize the vertical device structure. The emission wavelength and full width at half maximum (FWHM) of the main emission peak of the device are ∼518 nm and 14 nm, respectively. Under the current density of 50 A/cm2, a relatively high light output power (LOP) of 11.1 mW can be obtained from the green RCLED. Moreover, when the current injection is 20 mA (8 A/cm2), the corresponding forward bias voltage is as low as ∼2.46 V. The reasons for the low operating voltage and high LOP can be attributed to the improvement of CQ, the release of residual compressive stress of the GaN-based epilayer due to the removal of PSS, and better heat dissipation properties of the Cu supporting plate.

Electrically injected GaN-based microdisk towards an efficient whispering gallery mode laser.

III-nitrides based microdisks with the mushroom-type shape are key components for integrated nanophotonic circuits. The air gap undercut in the mushroom-type microdisk is essential for maintaining vertical optical confinement, but this structure is still facing the difficulty of electrical injection. In this work, we demonstrate an electrically injected GaN-based microdisk of such structure. The device is featured with a copper substrate and copper supporting pedestal, through which current can be efficiently injected into the microdisk with low leakage current (less than 10 nA). Bright emission at ∼420 nm was demonstrated from the microdisk under current injection. The copper substrate and supporting pedestal can also extract thermal energy out of the microdisk effectively, and the structure in this work shows a low thermal resistance of ∼788.86 K/W. Low threshold lasing action at ∼405 nm was realized under the optically pumped condition and the threshold energy is ∼35 nJ/pulse. Clear whispering gallery modes were observed and the Q factor is as high as 4504, indicating the high quality of the microdisk cavity. This work is the first step towards low threshold efficient electrically injected microdisk laser with a mushroom-type shape.

Demonstration of optical gain at 1550 nm in an Er3+-Yb3+ co-doped phosphate planar waveguide under commercial and convenient LED pumping.

A 980 nm semiconductor laser is always selected as the pump source for erbium-ytterbium co-doped optical waveguide amplifiers. In this work, two low-cost blue-violet LEDs, rather than an expensive 980 nm laser, were used to pump an Er3+-Yb3+ co-doped phosphate planar waveguide. When the signal power was 0.4 mW at a 1550 nm wavelength, internal optical gains of about 4.1 and 4.5 dB/cm were respectively obtained under the excitations of a 32 mW/cm2, 275 nm LED and a 914 mW/cm2, 405 nm LED. It was found that 51.17% of the total Er3+ ions in the 2H9/2 state contributed to the luminescence at 1550 nm, and a theoretical model of gain simulation was established under the excitation of a 405 nm LED. The calculated gain of about 4.1 dB/cm was found to be in accordance with the experimental optical gain results.

Investigation of CsPbBr3 CVD dynamics at various temperatures.

Since the emerging development of CsPbBr3 perovskite, chemical vapor deposition (CVD) has become one of the most promising fabrication techniques by which to precisely deposit uniform perovskite thin films. However, there have been few reports on the growth dynamics and chemical reaction parameters (e.g., activation energy) for perovskite CVD. In this work, different deposition rates of CVD-grown CsPbBr3 thin films were obtained at different substrate temperatures. Dynamics equations were developed to relate the inflow rates, desorption coefficients and concentrations of reactants on the substrates. Only a small amount of reactant became activated at low temperature and a small amount of PbBr2 resided on the substrate at high temperature, and accordingly the maximal deposition rate was achieved at 250 °C. The Arrhenius activation energy of CVD-grown CsPbBr3 was also calculated, and found to be 31.64 kJ mol-1. We believe that our work provides a detailed picture of perovskite CVD growth.

Low threshold continuous-wave lasing of yellow-green InGaN-QD vertical-cavity surface-emitting lasers.

Low threshold continuous-wave (CW) lasing of current injected InGaN quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs) was achieved at room temperature. The VCSEL was fabricated by metal bonding technique on a copper substrate to improve the heat dissipation ability of the device. For the first time, lasing was obtained at yellow-green wavelength of 560.4 nm with a low threshold of 0.61 mA, corresponding to a current density of 0.78 kA/cm2. A high degree of polarization of 94% were measured. Despite the operation in the range of "green gap" of GaN-based devices, single longitudinal mode laser emission was clearly achieved due to the high quality of active region based on InGaN QDs and the excellent thermal design of the VCSELs.

Enhancing Performance of GaN/Ga2 O3 P-N Junction Uvc Photodetectors via Interdigitated Structure.

Ga2 O3 -based Ultraviolet-C photodetector (UVCPD) is considered the most promising UVCPD at present and is divided into Metal-Semiconductor-Metal (MSM) and PN junction types. Compared with MSM-PDs, PN-PDs exhibit superior transient performance due to the built-in electric field. However, current Ga2 O3 -based PN-PDs lack consideration for carrier collection and electric field distribution. In this study, PN-PDs with an interdigital n-Ga2 O3 layer and finger electrodes are fabricated on p-GaN/n-Ga2 O3 epilayers. Ultrafast response times of 31 µs (1/e decay) and 2.76 µs (fast component) are realized, which outperforms all Ga2 O3 UVC-PDs up to now. Under 0 V self-powered, the responsivity (0.25 A W-1 ) of interdigital PD is enhanced by the interdigital electrode structure due to increasing carriers' collection length. Under bias, the performances of interdigital PD with 41.7 A W-1 responsivity and 8243 selection ratios are significantly elevated by enhancing the built-in electric field in the Ga2 O3 region, which is 34.76 and 39.4 times those of traditional PDs, respectively. The intrinsic enhancing mechanism of interdigital structure is also investigated by interdigital PDs with various electrode spacings and perimeters. In summary, this paper not only reports a highly performed interdigitated structure p-GaN/n-Ga2 O3 UVCPDs, but also provides guidelines for structure design in Ga2 O3 -based PN-PDs.

High-Gain of NdIII Complex Doped Optical Waveguide Amplifiers at 1.06 and 1.31 µm Wavelengths Based on Intramolecular Energy Transfer Mechanism.

Chelate phosphine oxide ligand (9,9-dimethyl-9H-xanthene-4,5-diyl) bis (diphenylphosphineoxide) (XPO) is prepared as a neutral ligand to synthesize complex Nd (TTA)3 (XPO) (TTA = 2-thenoyltrifluoroacetone). An appropriate energy gap between the XPO and TTA ligands, which can support two additional energy transfer routines from the first excited triplet state (T1 ) energy level of the XPO to that of the TTA, improves energy transfer in the Nd complex. Based on intramolecular energy transfer mechanism, optical gains at 1.06 and 1.31 µm are demonstrated in Nd (TTA)3 (XPO)-doped polymer waveguides with the excitation of low-power light-emitting diodes (LEDs) instead of semiconductor lasers as pump sources. Using the vertical top-pumping mode of a 365 nm LED, relative gains of 22.5 and 8.4 dB cm-1 are obtained at 1.06 and 1.31 µm, respectively, in a 0.2 cm long embedded waveguide with a cross-section of 8 × 5 µm2 . The active core layer is Nd (TTA)3 (XPO)-doped SU-8 polymer. Moreover, relative gains are achieved in evanescent-field waveguide with a cross-section of 6 × 4 µm2 . The 21.0 and 5.6 dB cm-1 relative gains are achieved at 1.06 and 1.31 µm, respectively, with a net gain of 13.8 ± 0.3 dB cm-1 obtained at 1.06 µm in a 0.9 cm long SU-8 waveguide with Nd (TTA)3 (XPO)-doped polymethylmethacrylate as upper cladding.

Optical Amplification at 637 and 1067 nm Based on Organic Molecule AQ(PhDPA)2 and NdIII Complex Codoped Polymer Waveguides.

Based on the molecular energy transfer mechanism, relative gains at 1067 and 637 nm wavelengths are achieved in thermally activated delayed fluorescence molecule AQ(PhDPA)2 and Nd complex with chelating phosphine oxide as ligands codoped polymer waveguides, with the excitation of low-power UV light-emitting diodes (LEDs) instead of traditional semiconductor lasers as pump sources. For AQ(PhDPA)2 -Nd(DBTTA)3 (DBFDPO) (DBTTA = dibenzotetrathienoacene, DBFDPO = 4,6-bis (diphenylphosphoryl) dibenzofuran) -codoped polymethylmethacrylate (PMMA), and AQ(PhDPA)2 -Nd(DBTTA)3 (FDPO) (FDPO = 9,9-bis (diphenylphosphorylphenyl) fluorene)-codoped PMMA polymers with a mass ratio of 1:4 respectively, when they are spin-coated as upper claddings, the relative gains of 2.2 and 1.8 dB cm-1 at 1067 nm are obtained in evanescent-field waveguides with cross-section of 4 × 8 µm2 under excitation of 300 mW 405 nm LED, and the gains of 3.9 and 4.9 dB cm-1 at 637 nm are achieved with pumping of 530 mW 450 nm LED respectively. By growing a 100 nm-thick aluminum reflector with the waveguides, the optical gain at 1067 and 637 nm can be enhanced to 3.5 and 6.1 dB cm-1 , corresponding to AQ(PhDPA)2 -Nd(DBTTA)3 (DBFDPO) and AQ(PhDPA)2 -Nd(DBTTA)3 (FDPO)-codoped PMMA polymers, respectively.

Green Vertical-Cavity Surface-Emitting Lasers Based on InGaN Quantum Dots and Short Cavity.

Room temperature low threshold lasing of green GaN-based vertical cavity surface emitting laser (VCSEL) was demonstrated under continuous wave (CW) operation. By using self-formed InGaN quantum dots (QDs) as the active region, the VCSEL emitting at 524.0 nm has a threshold current density of 51.97 A cm-2, the lowest ever reported. The QD epitaxial wafer featured with a high IQE of 69.94% and the δ-function-like density of states plays an important role in achieving low threshold current. Besides, a short cavity of the device (~ 4.0 λ) is vital to enhance the spontaneous emission coupling factor to 0.094, increase the gain coefficient factor, and decrease the optical loss. To improve heat dissipation, AlN layer was used as the current confinement layer and electroplated copper plate was used to replace metal bonding. The results provide important guidance to achieving high performance GaN-based VCSELs.

Exciton-phonon interaction in quasi-two dimensional layered (PEA)2(CsPbBr3)n-1PbBr4 perovskite.

Two-dimensional (2D) Ruddlesden-Popper perovskites with bulky organic cations have attracted extensive attention in light-emitting devices and photovoltaics due to their robust environment stability, tunable luminescent color, strong exciton binding and promising efficiency. A quantum well (QW) structure is spontaneously formed by sandwiching PbBr4 layers into bulky organic cations. However, some intrinsic excitonic mechanisms in these materials still need to be elucidated. In this study, the exciton-phonon interaction of quasi-2D (PEA)2(CsPbBr3)n-1PbBr4 with different PbBr4 layer numbers (n) was analyzed by temperature-varied photoluminescence (PL), scanning electron microscopy (SEM) and powder X-ray diffraction (PXRD). The mechanism of bandgap shifting with temperature was found to be dominated by the thermal expansion effect in the large-n 2D and bulk perovskite, and gradually switched to exciton-phonon interaction in the n = 1 (PEA)2PbBr4 phase, indicating enhanced exciton-phonon interaction in the thinner quantum well structure. Further analysis showed that the enhanced exciton-phonon interaction originated from the longitudinal optical phonon-exciton Fröhlich interaction rather than acoustic phonon-exciton coupling. We believe that our results will benefit the further optimization of light-emitting devices based on 2D perovskites.

Quantum dot vertical-cavity surface-emitting lasers covering the 'green gap'.

Semiconductor vertical-cavity surface-emitting lasers (VCSELs) with wavelengths from 491.8 to 565.7 nm, covering most of the 'green gap', are demonstrated. For these lasers, the same quantum dot (QD) active region was used, whereas the wavelength was controlled by adjusting the cavity length, which is difficult for edge-emitting lasers. Compared with reports in the literature for green VCSELs, our lasers have set a few world records for the lowest threshold, longest wavelength and continuous-wave (CW) lasing at room temperature. The nanoscale QDs contribute dominantly to the low threshold. The emitting wavelength depends on the electron-photon interaction or the coupling between the active layer and the optical field, which is modulated by the cavity length. The green VCSELs exhibit a low-thermal resistance of 915 kW-1, which benefits the CW lasing. Such VCSELs are important for small-size, low power consumption full-color displays and projectors.

Performance enhancement of GaN-based light emitting diodes by transfer from sapphire to silicon substrate using double-transfer technique.

GaN-based light emitting diodes (LEDs) fabricated on sapphire substrates were successfully transferred onto silicon substrates using a double-transfer technique. Compared with the conventional LEDs on sapphire, the transferred LEDs showed a significant improvement in the light extraction and thermal dissipation, which should be mainly attributed to the removal of sapphire and the good thermal conductivity of silicon substrate. Benefited from the optimized wafer bonding process, the transfer processes had a negligible influence on electrical characteristics of the transferred LEDs. Thus, the transferred LEDs showed a similar current-voltage characteristic with the conventional LEDs, which is of crucial importance for practical applications. It is believed that the double-transfer technique offers an alternative way to fabricate high performance GaN-based thin-film LEDs.

Well-width dependence of the emission linewidth in ZnO/MgZnO quantum wells.

Photoluminescence (PL) spectra were measured as a function of well width (LW) and temperature in ZnO/Mg0.1Zn0.9O single quantum wells (QWs) with graded thickness. The emission linewidth (full width at half maximum) was extracted from the emission spectra, and its variation as a function of LW was studied. The inhomogeneous linewidth obtained at 5 K was found to decrease with increasing LW from 1.8 to 3.3 nm due to the reduced potential variation caused by the LW fluctuation. Above 3.3 nm, however, the linewidth became larger with increasing LW, which was explained by the effect related with defect generation due to strain relaxation and exciton expansion in the QW. For the homogenous linewidth broadening, longitudinal optical (LO) phonon scattering and impurity scattering were taken into account. The LO phonon scattering coefficient ΓLO and impurity scattering coefficient Γimp were deduced from the temperature dependence of the linewidth of the PL spectra. Evident reduction of ΓLO with decreasing LW was observed, which was ascribed to the confinement-induced enhancement of the exciton binding energy. Different from ΓLO, a monotonic increase in Γimp was observed with decreasing LW, which was attributed to the enhanced penetration of the exciton wave function into the barrier layers.