Monolithic multi-wavelength lasing from multi-sized microdisk lasers.

This paper demonstrates monolithic multi-wavelength lasing through fabrication of multi-sized microdisks on a green-emitting thin film sample. The different dimensions of the microdisks incur different extent of strain relaxation, thus changing the emission/gain spectra due to the reduction of the quantum confined Stark effect. Under room-temperature optical pumping, lasing thresholds of 15.1 mJ/cm2, 2.9 mJ/cm2, and 5.3 mJ/cm2 with Q factors of 2370, 2060, and 4308 are realized, respectively, for fabricated microdisks with diameters of 950 nm, 6 µm, and 10 µm. By exciting the microdisks with a pump laser spot diameter of 2 mm, simultaneous multi-wavelength lasing action is thus observed. The strain relaxation effect is confirmed by the shift of the E2 (high) Raman peak from 563.2 cm-1 to 561.5 cm-1 as the diameter of the fabricated microdisk reduces.

Development of chipscale InGaN RGB displays using strain-relaxed nanosphere-defined nanopillars.

Chip-scale red, green and blue (RGB) light emission on an InGaN/GaN multi-quantum well wafer adopting a top-down fabrication approach is demonstrated in this study, facilitated by shadow-masked nanosphere lithography for precise site-controlled nano-patterning. Exploiting the strain relaxation mechanism by fabricating arrays of nanosphere-defined nanopillars of two different dimensions utilizing a sequential shadow-masked nanosphere coating approach into the blue and green light-emitting pixel regions on a red-light emitting InGaN/GaN wafer, RGB light emission from a monolithic chip is demonstrated. The micro-sized RGB light-emitting pixels emit at 645 nm-680 nm, 510 nm-521 nm and 475 nm-498 nm respectively, achieving a maximum color gamut of 60% NTSC and 72% sRGB. Dimensional fluctuations of the nanopillars of 73% and 71% for the green and blue light-emitting pixels, respectively, are estimated from scanning electron microscope images of the fabricated device, corresponding to fluctuations in spectral blue-shifts of 5.4 nm and 21.2 nm as estimated by strain-coupledk·pSchrödinger calculations, consistent with observations from micro-photoluminescence (µ-PL) mapping which shows deviations of emission wavelengths for the RGB light-emitting pixels to be 8.9 nm, 14.9 nm and 23.7 nm, respectively. The RGB pixels are also configured in a matrix-addressable configuration to form an RGB microdisplay, demonstrating the feasibility of the approach towards chip-scale color displays.

GaN microdisk with direct coupled waveguide for unidirectional whispering-gallery mode emission.

Microdisks are excellent whispering-gallery mode (WGM) optical resonators, but their emissions are invariably in-plane isotropic due to their circularities and thus difficult to be extracted efficiently. In this work, a waveguide with a width of 0.16 µm directly coupled to a microdisk with a diameter of 10 µm is fabricated on a 0.77 µm thick GaN thin film containing InGaN/GaN multi-quantum wells. This eliminates the need for precision patterning required by evanescent coupling schemes in which coupling gaps of the order of tens of nanometers must be maintained. The fabrication was carried out using nanosphere and nanowire lithography. Non-evanescent coupling of WGMs to the waveguide from the microdisk is successfully demonstrated.

Wafer-scale Fabrication of Non-Polar Mesoporous GaN Distributed Bragg Reflectors via Electrochemical Porosification.

Distributed Bragg reflectors (DBRs) are essential components for the development of optoelectronic devices. For many device applications, it is highly desirable to achieve not only high reflectivity and low absorption, but also good conductivity to allow effective electrical injection of charges. Here, we demonstrate the wafer-scale fabrication of highly reflective and conductive non-polar gallium nitride (GaN) DBRs, consisting of perfectly lattice-matched non-polar (11-20) GaN and mesoporous GaN layers that are obtained by a facile one-step electrochemical etching method without any extra processing steps. The GaN/mesoporous GaN DBRs exhibit high peak reflectivities (>96%) across the entire visible spectrum and wide spectral stop-band widths (full-width at half-maximum >80 nm), while preserving the material quality and showing good electrical conductivity. Such mesoporous GaN DBRs thus provide a promising and scalable platform for high performance GaN-based optoelectronic, photonic, and quantum photonic devices.

Nanocathodoluminescence Reveals Mitigation of the Stark Shift in InGaN Quantum Wells by Si Doping.

Nanocathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high efficiency light emitting diodes. We observe a variation in the emission wavelength of each quantum well, in correlation with the Si dopant concentration in the quantum barriers. This is reproduced by band profile simulations, which reveal the reduction of the Stark shift in the quantum wells by Si doping. We demonstrate nanocathodoluminescence is a powerful technique to optimize doping in optoelectronic devices.