David Massoubre

Visible-Light Communications Using a CMOS-Controlled Micro-Light- Emitting-Diode Array

We report the high-frequency modulation of individual pixels in 8 × 8 arrays of III-nitride-based micro-pixellated light-emitting diodes, where the pixels within the array range from 14 to 84 μ m in diameter. The peak emission wavelengths of the devices are 370, 405, 450 and 520 nm, respectively. Smaller area micro-LED pixels generally exhibit higher modulation bandwidths than their larger area counterparts, which is attributed to their ability to be driven at higher current densities. The highest optical -3 dB modulation bandwidths from these devices are shown to be in excess of 400 MHz, which, to our knowledge, are the highest bandwidths yet reported for GaN LEDs. These devices are also integrated with a complementary metal-oxide-semiconductor (CMOS) driver array chip, allowing for simple computer control of individual micro-LED pixels. The bandwidth of the integrated micro-LED/CMOS pixels is shown to be up to 185 MHz; data transmission at bit rates up to 512 Mbit/s is demonstrated using on-off keying non return-to-zero modulation with a bit-error ratio of less than 1 × 10-10, using a 450 nm-emitting 24 μm diameter CMOS-controlled micro-LED. As the CMOS chip allows for up to 16 independent data inputs, this device demonstrates the potential for multi-Gigabit/s parallel data transmission using CMOS-controlled micro-LEDs.

High-Speed Visible Light Communications Using Individual Pixels in a Micro Light-Emitting Diode Array

The high-frequency modulation of individual pixels in III-nitride-based micro-pixel light-emitting diode arrays, where each array consists of 16 × 16 individually addressable 72-μm-diameter pixels, are reported. The devices investigated have peak emission wavelengths at 370, 405, and 450 nm, respectively. The optical -3-dB modulation bandwidth of a typical pixel from the 450-nm-emitting device was found to be approximately 245 MHz. Data transmission at rates of up to 1 Gb/s is demonstrated from a single pixel emitting at 450 nm, using on-off keying nonreturn-to-zero modulation, with a bit-error ratio of less than 1 × 10-10. Such devices have potential for free-space or fiber-coupled visible light communications.

Size-dependent light output, spectral shift, and self-heating of 400 nm InGaN light-emitting diodes

We have systematically investigated the impact of device size scaling on the light output, spectral shift, and self-heating of 400 nm InGaN light-emitting diodes (LEDs). Devices with diameters in the range 20–300 μm have been studied. It is shown that smaller LED pixels can deliver higher power densities (despite the lower absolute output powers) and sustain higher current densities. Investigations of the electroluminescence characteristics of differently sized pixels against current density reveal that the spectral shift is dominated by blueshift at the low current density level and then by redshift at the high current density level, owing to the competition between the bandgap shrinkage caused by self-heating and band-filling effects. The redshift of the emission wavelength with increasing current density is much faster and larger for the bigger pixels, suggesting that the self-heating effect is also size dependent. This is further confirmed by the junction-temperature rise measured by the established s...

1.5 Gbit/s Multi-Channel Visible Light Communications Using CMOS-Controlled GaN-Based LEDs

An on-chip multi-channel visible light communication (VLC) system is realized through a blue (450 nm) GaN-based micron-size light-emitting diode (μLED) array integrated with complementary metal-oxide-semiconductor (CMOS) electronics. When driven by a custom-made CMOS driving board with 16 independent parallel data input ports, this μLED array device is computer controllable via a standard USB interface and is capable of delivering high speed parallel data streams for VLC. A total maximum error-free data transmission rate of 1.5 Gbit/s is achieved over free space by modulating four μLED pixels simultaneously using an on-off key non-return to zero modulation scheme. Electrical and optical crosstalk of the system has also been investigated in detail and the further optimization of CMOS design to minimize the crosstalk is proposed.

Thermal and optical characterization of micro-LED probes for in vivo optogenetic neural stimulation

Within optogenetics there is a need for compact light sources that are capable of delivering light with excellent spatial, temporal, and spectral resolution to deep brain structures. Here, we demonstrate a custom GaN-based LED probe for such applications and the electrical, optical, and thermal properties are analyzed. The output power density and emission spectrum were found to be suitable for stimulating channelrhodopsin-2, one of the most common light-sensitive proteins currently used in optogenetics. The LED device produced high light intensities, far in excess of those required to stimulate the light-sensitive proteins within the neurons. Thermal performance was also investigated, illustrating that a broad range of operating regimes in pulsed mode are accessible while keeping a minimum increase in temperature for the brain (0.5°C). This type of custom device represents a significant step forward for the optogenetics community, allowing multiple bright excitation sites along the length of a minimally invasive neural probe.

Individually Addressable AlInGaN Micro-LED Arrays With CMOS Control and Subnanosecond Output Pulses

We report the fabrication and characterization of an ultraviolet (370 nm) emitting AlInGaN-based micro-light- emitting diode (micro-LED) array integrated with complementary metal-oxide-semiconductor control electronics. This configuration allows an 8 × 8 array of micro-LED pixels, each of 72-mum diameter, to be individually addressed. The micro-LED pixels can be driven in direct current (dc), square wave, or pulsed operation, with linear feedback shift registers (LFSRs) allowing the output of the micro-LED pixels to mimic that of an optical data transmitter. We present the optical output power versus drive current characteristics of an individual pixel, which show a micro-LED output power of up to 570 muW in dc operation. Representative optical pulse trains demonstrating the micro-LEDs driven in square wave and LFSR modes, and controlled optical pulsewidths from 300 ps to 40 ns are also presented.

Active-Matrix GaN Micro Light-Emitting Diode Display With Unprecedented Brightness

Displays based on microsized gallium nitride light-emitting diodes possess extraordinary brightness. It is demonstrated here both theoretically and experimentally that the layout of the n-contact in these devices is important for the best device performance. We highlight, in particular, the significance of a nonthermal increase of differential resistance upon multipixel operation. These findings underpin the realization of a blue microdisplay with a luminance of 106 cd/m2.

Nanoscale-accuracy transfer printing of ultra-thin AlInGaN light-emitting diodes onto mechanically flexible substrates

The transfer printing of 2 μm-thick aluminum indium gallium nitride (AlInGaN) micron-size light-emitting diodes with 150 nm (±14 nm) minimum spacing is reported. The thin AlInGaN structures were assembled onto mechanically flexible polyethyleneterephthalate/polydimethylsiloxane substrates in a representative 16 × 16 array format using a modified dip-pen nano-patterning system. Devices in the array were positioned using a pre-calculated set of coordinates to demonstrate an automated transfer printing process. Individual printed array elements showed blue emission centered at 486 nm with a forward-directed optical output power up to 80 μW (355 mW/cm2) when operated at a current density of 20 A/cm2.

Modulation bandwidth studies of recombination processes in blue and green InGaN quantum well micro-light-emitting diodes

We report modulation bandwidth measurements on a number of InGaN-based quantum well LEDs emitting at 450 and 520 nm wavelengths. It is shown that for these devices the data can be interpreted in terms of Auger recombination, by taking account of the carrier density dependence of the radiative coefficient. We find values for the Auger coefficient of (1±0.3)×10−29cm6s−1 at 450 nm and (3±1)×10−30cm6s−1 at 520 nm.

All-optical extinction-ratio enhancement of a 160 GHz pulse train by a saturable-absorber vertical microcavity

A vertical-access passive all-optical gate has been used to improve the extinction ratio of a 160 GHz picosecond pulse train at 1555 nm. An extinction ratio enhancement of 6 dB is observed within an 8 nm bandwidth. Such a device is a promising candidate for low-cost all optical reamplication and reshaping (2R) regeneration at 160 Gbits/s.