Flexible Glass Hybridized Colloidal Quantum Dots for Gb/s Visible Light Communications

Color converting films of colloidal quantum dots (CQDs) encapsulated with flexible glass are integrated with microsize GaN LEDs (μLEDs) in order to form optical sources for high-speed visible light communications (VLC). VLC is an emerging technology that uses white and/or colored light from LEDs to combine illumination and display functions with the transmission of data. The flexible glass/CQD format addresses the issue of limited modulation speed of typical phosphor-converted LEDs while enhancing the photostability of the color converters and facilitating their integration with the μLEDs. These structures are less than 70 μm in total thickness and are directly placed in contact with the polished sapphire substrate of 450-nm-emitting μLEDs. Blue-to-green, blue-to-orange and blue-to-red conversion with respective forward optical power conversion efficiencies of 13%, 12% and 5.5% are reported. In turn, free-space optical communications up to 1.4 Gb/s VLC is demonstrated. Results show that CQD-converted LEDs pave the way for practical digital lighting/displays with multi-Gb/s capability

Stripe excitation of high gain media with disorder

We discuss the spatial and spectral intensity distribution of emission in optical gain media under stripe excitation in the case where the intensity reaches saturation level. It is found that modes propagating along the stripe in different directions are spatially separated if they affect each other due to saturation. The investigation includes the effects of wavelength-dependent inhomogenities, such as localized losses and reflective perturbations. Even relatively small distortions of this kind are found to cause considerable spatial and spectral redistribution of the intensity compared to an ideal disorder-free medium. Our results indicate that a simple ansatz may describe some mechanisms that lead to the formation of random laser action that is commonly observed in high gain media, such as organic semiconductors. Furthermore, consequential difficulties of gain measurements in such media using stripe excitation experiments are highlighted

Micro-structured light emission from planar InGaN light-emitting diodes

Investigation of the surface modification of the p-type layer in GaN light-emitting diodes (LEDs) by exposure to a trifluoromethane plasma is reported. It is found that the plasma treatment reduces the conductivity of the p-GaN by several orders of magnitude, and when applied at room-temperature through a patterned mask, localized current channels into the active region of a p–i–n device are created. This provides a novel approach to laterally modulate the light emission from an LED over essentially planar areas. This technique allows the projection of high-resolution images from non-pixelated devices, and an example application of maskless pattern transfer with sub-micron features into photoresist is demonstrated

Micro-structured light emission from planar InGaN light-emitting diodes

Investigation of the surface modification of the p-type layer in GaN light-emitting diodes (LEDs) by exposure to a trifluoromethane plasma is reported. It is found that the plasma treatment reduces the conductivity of the p-GaN by several orders of magnitude, and when applied at room-temperature through a patterned mask, localized current channels into the active region of a p–i–n device are created. This provides a novel approach to laterally modulate the light emission from an LED over essentially planar areas. This technique allows the projection of high-resolution images from non-pixelated devices, and an example application of maskless pattern transfer with sub-micron features into photoresist is demonstrated

Micro-structured light emission from planar InGaN light-emitting diodes

Investigation of the surface modification of the p-type layer in GaN light-emitting diodes (LEDs) by exposure to a trifluoromethane plasma is reported. It is found that the plasma treatment reduces the conductivity of the p-GaN by several orders of magnitude, and when applied at room-temperature through a patterned mask, localized current channels into the active region of a p–i–n device are created. This provides a novel approach to laterally modulate the light emission from an LED over essentially planar areas. This technique allows the projection of high-resolution images from non-pixelated devices, and an example application of maskless pattern transfer with sub-micron features into photoresist is demonstrated

Size-dependent efficiency and efficiency droop of blue InGaN micro-light emitting diodes

The mechanisms of size-dependent efficiency and efficiency droop of blue InGaN micro-pixel light emitting diodes (LEDs) have been investigated experimentally and by simulation. Electrical characterisation confirms the improvement of current spreading for smaller LEDs, which enables the achievement of the higher efficiency at high injection current densities. Owing to the higher ratio of sidewall perimeter to mesa area of smaller LEDs, a lower efficiency was observed at a low injection current density, resulting from defect-related Shockley-Read-Hall non-radiative recombination. We demonstrate that such sidewall etch defects can be partially recovered by increased thermal annealing time, consequently improving the efficiency at low current densitie

Sub-micron lithography using InGaN micro-LEDs : mask-free fabrication of LED arrays

The fabrication of gallium-nitride (GaN)-based light-emitting diode (LED) arrays by a direct writing technique, itself using micron-sized LEDs (micro-LEDs), is reported. CMOSdriven ultraviolet GaN-based micro-LED arrays are used to pattern photoresist layers with feature sizes as small as 500 nm. Checkerboard-type square LED array devices are then fabricated using such photoresist patterns based on either single pixel or multipixel direct writing, and implemented as part of a completely mask-less process flow. These exemplar arrays are composed of either 450-nm-emitting 199 × 199 μm2 pixels on a 200-μm pitch or 520-nm-emitting 21 × 18 μm2 pixels on a 23-μm pitch. Fill factors of 99% and 71.5% are achieved with optical output power densities per pixel of 5 and 20 W/cm2 at 90- and 6-mA dc-injected currents, respectively

Colloidal quantum dot random laser

We report random laser action in a system where optical amplification is provided by colloidal quantum dots (CQDs). This system is obtained by depositing from solution CdSe/ZnS core-shell CQDs into rough micron-scale grooves fabricated on the surface of a glass substrate. The combination of CQD random packing and of disordered structures in the glass groove enables gain and multiple scattering. Upon optical excitation, random laser action is triggered in the system above a 25-mJ/cm2 threshold. Single-shot spectra were recorded to study the emission spectral characteristics and the results show the stability of the laser mode positions and the dominance of the modes close to the material gain maximum

Flexible distributed-feedback colloidal quantum dot laser

Flexible distributed feedback colloidal quantum dot laser patterned by a submicron grating structure is reported. An exposed standard commercial blank digital versatile disk (DVD) was used as the mask to fabricate a submicron scale grating structure, providing a simple and low-cost approach. A UV transparent polymerisable host matrix, 1,4-cyclohexanedimethanol divinyl ether (CHDV), was then photo-cured to replicate the DVD grating structure and peeled off from the mask, resulting in a flexible grating. The thickness of the grating substrate can be controlled in the range of 50 to 300 μm. A frequency-tripled Q-switched Nd:YAG laser (5-ns pulse, 10-Hz repetition rate and 355-nm excitation wavelength) was then used to photo-pump the CQD sample. The emission evolution was investigated and a 4 mJ/cm2 lasing threshold was found, comparing favourably to other reports of CQD lasers. The polarisation of the laser emission revealed that the DFB CQD laser favours a linearly polarized (TE) emission. During the experiment, the laser performance, operating above lasing threshold at room temperature and ambient atmosphere, was stable over more than 10,000 pump pulses

Modification of emission wavelength in organic random lasers based on photonic glass

Control of the emission wavelength of a random laser (RL) system over a 7-nm waveband is demonstrated using a green-emitting π-conjugated polymer infiltrated into a photonic glass formed by nano/micro-size monodisperse silica spheres. The use of a solution-based conjugated polymer enables the complete filling of the voids within the photonic glass without suffering from quenching and the gain can therefore be maximized. The emission wavelength of these structures is set by a combination of the material system spectral gain and of the transport mean free path, the latter being controlled by the mean diameter of the spheres in the nano-scale range. Transport mean free paths of photons in the RL’s active region are calculated using Mie scattering theory and corroborated with coherent backscattering measurements. Further wavelength modification is also possible by changing the pump spot size and the pump fluence