Gigabit per second visible light communication based on AlGaInP red micro-LED micro-transfer printed onto diamond and glass.

Full-color smart displays, which act both as a display and as a high-speed visible light communication (VLC) transmitter, can be realized by the integration of red-green-blue micron-sized light emitting diodes (micro-LEDs) onto a common platform. In this work, we report on the integration of aluminum gallium indium phosphide red micro-LEDs onto diamond and glass substrates by micro-transfer printing and their application in VLC. The device on-diamond exhibits high current density and bandwidth operation, enabled by diamond's superior thermal properties. Employing an orthogonal frequency division multiplexing modulation scheme, error-free data rates of 2.6 Gbps and 5 Gbps are demonstrated for a single micro-LED printed on-glass and on-diamond, respectively. In a parallel configuration, a 2x1 micro-LED array achieves error-free data rates of 3 Gbps and 6.6 Gbps, on-glass and on-diamond, respectively.

Direct integration of micro-LEDs and a SPAD detector on a silicon CMOS chip for data communications and time-of-flight ranging.

We present integration of singulated micron-sized light emitting diodes (micro-LEDs) directly onto a silicon CMOS drive chip using a transfer printing method. An 8x8 micro-LED device array with individual control over each pixel is demonstrated with modulation bandwidths up to 50 MHz, limited by the large modulation depth of the driver chip. The 2 kHz frame rate CMOS driver also incorporates a Single Photon Avalanche Diode device thus allowing detection and transmission functionality on a single integrated chip. Visible light communications at data rates up to 1 Mbps, and time-of-flight ranging with cm-scale resolution are demonstrated using this hybrid integrated system.

Gallium nitride micro-light-emitting diode structured light sources for multi-modal optical wireless communications systems.

Gallium nitride-based light-emitting diodes (LEDs) have revolutionized the lighting industry with their efficient generation of blue and green light. While broad-area (square millimetre) devices have become the dominant LED lighting technology, fabricating LEDs into micro-scale pixels (micro-LEDs) yields further advantages for optical wireless communications (OWC), and for the development of smart-lighting applications such as tracking and imaging. The smaller active areas of micro-LEDs result in high current density operation, providing high modulation bandwidths and increased optical power density. Fabricating micro-LEDs in array formats allows device layouts to be tailored for target applications and provides additional degrees of freedom for OWC systems. Temporal and spatial control is crucial to use the full potential of these micro-scale sources, and is achieved by bonding arrays to pitch-matched complementary metal-oxide-semiconductor control electronics. These compact, integrated chips operate as digital-to-light converters, providing optical signals from digital inputs. Applying the devices as projection systems allows structured light patterns to be used for tracking and self-location, while simultaneously providing space-division multiple access communication links. The high-speed nature of micro-LED array devices, combined with spatial and temporal control, allows many modes of operation for OWC providing complex functionality with chip-scale devices. This article is part of the theme issue 'Optical wireless communication'.

On-chip GaN-based dual-color micro-LED arrays and their application in visible light communication.

Integrated multi-color micron-sized light emitting diode (micro-LED) arrays have been demonstrated in recent years for display applications; however, their potential as visible light communication (VLC) transmitters is yet to be fully explored. In this work, we report on the fabrication and characterization of on-chip dual-color micro-LED arrays and their application in VLC. For this purpose, blue-green and blue-violet micro-LED arrays were fabricated by transfer printing blue-emitting micro-LEDs onto the substrate of green and violet micro-LEDs, respectively. The potential of these dual-color micro-LED arrays as VLC transmitters is demonstrated with respective error-free data rates of 1.79 and 3.35 Gbps, achieved by the blue-green and blue-violet devices in a dual wavelength multiplexing scheme.

Broadly tunable femtosecond pulses around 2.06 µm from a diode-pumped Tm3+-doped solid-state laser source.

We report on a broadly tunable diode-pumped femtosecond Tm:LuScO3 laser source around 2.06 µm. Tuning was obtained through the use of a steeply diving birefringent filter, maintaining sub-600 fs pulses over a tuning range of 2019-2110 nm. The minimum pulse duration of 240 fs was recorded at a central wavelength of 2080 nm with an average output power of 93 mW. Higher output coupling of 2% resulted in a narrower tuning range of 2070-2102 nm with generated pulses as short as 435 fs and an average output power of 119 mW at 2090 nm.

Hybrid integration of an evanescently coupled AlGaAs microdisk resonator with a silicon waveguide by nanoscale-accuracy transfer printing.

Hybrid integration of a III-V microdisk resonator on a silicon-on-insulator waveguide platform is demonstrated. Transfer printing with nanoscale accuracy is used to micro-assemble an evanescently coupled all-pass microdisk resonator with a targeted coupler gap of 100 nm using pre-fabricated AlGaAs and silicon components. Transmission measurements show hybrid resonances with a loaded Q-factor of 7×103 and a cavity finesse of over 100.

Diode-pumped femtosecond Tm3+-doped LuScO3 laser near 2.1 µm.

We report on the first demonstration, to the best of our knowledge, of a diode-pumped Tm:LuScO3 laser. Efficient and broadly tunable continuous wave operation in the 1973-2141 nm region and femtosecond mode-locking through the use of an ion-implanted InGaAsSb quantum-well-based semiconductor saturable absorber mirror are realized. When mode-locked, near-transform-limited pulses as short as 170 fs were generated at 2093 nm with an average output power of 113 mW and a pulse repetition frequency of 115.2 MHz. Tunable picosecond pulse generation was demonstrated in the 2074-2104 nm spectral range.

Transfer-printed micro-LED and polymer-based transceiver for visible light communications.

Visible light communications (VLC) is an emerging technology that uses LEDs, such as found in lighting fixtures and displays, to transmit data wirelessly. Research has so far focused on LED transmitters and on photoreceivers as separate, discrete components. Combining both types of devices into a single transceiver format will enable bi-directional VLC and offer flexibility for the development of future advanced VLC systems. Here, a proof of concept for an integrated optical transceiver is demonstrated by transfer printing a microsize LED, the transmitter, directly onto a fluorescent optical concentrator edge-coupled to a photodiode, the receiver. This integrated device can simultaneously receive (downlink) and transmit (uplink) data at rates of 416 Mbps and 165 Mbps, respectively. Its capability to operate in optical relay mode at 337 Mbps is experimentally demonstrated.

InGaN µLEDs integrated onto colloidal quantum dot functionalized ultra-thin glass.

Red-, orange-, and green-emitting integrated optoelectronic sources are demonstrated by transfer printing blue InGaN µLEDs onto ultra-thin glass platforms functionally enhanced with II-VI colloidal quantum dots (CQDs). The forward optical power conversion efficiency of these heterogeneously integrated devices is, respectively, 9%, 15%, and 14% for a blue light absorption over 95%. The sources are demonstrated in an orthogonal frequency division multiplexed (OFDM) visible light communication link reaching respective data transmission rates of 46 Mbps, 44 Mbps and 61 Mbps.

Fluorene-containing tetraphenylethylene molecules as lasing materials.

A series of star-shaped oligofluorene molecules, each containing a TPE core, have been specifically designed and produced to show effective aggregation-induced emission (AIE). Each molecule differs either in the number of fluorene units within the arms (e.g., 1 or 4, compounds 4 and 5), or the terminal group positioned at the end of each arm (e.g., H, TMS, or TPA, compounds 4, 6, and 7). Although they are all poor emitters in solution phase they become efficient yellow-green luminogens in the condensed state. Their AIE properties were investigated in THF/H2O mixtures, with each molecule exhibiting a clear emission enhancement at specific water contents. An all-organic distributed feedback (DFB) laser was fabricated using compound 4 as the gain material and exhibited an average threshold energy fluence of 60 ± 6 µJ/cm2 and emission in the green region. Furthermore, piezofluorochromism studies on a thin film of this material displayed a linear dependence of the amplified spontaneous emission (ASE) peak position on applied pressure, indicating potential applications as lasing-based pressure sensors. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 734-746.

1.9 µm waveguide laser fabricated by ultrafast laser inscription in Tm:Lu2O3 ceramic.

The ultrafast laser inscription technique has been used to fabricate channel waveguides in Tm3+-doped Lu2O3 ceramic gain medium for the first time to our knowledge. Laser operation has been demonstrated using a monolithic microchip cavity with a continuous-wave Ti:sapphire pump source at 796 nm. The maximum output power achieved from the Tm:Lu2O3 waveguide laser was 81 mW at 1942 nm. A maximum slope efficiency of 9.5% was measured with the laser thresholds observed to be in the range of 50-200 mW of absorbed pump power. Propagation losses for this waveguide structure are calculated to be 0.7 dB⋅cm-1 ± 0.3 dB⋅cm-1 at the lasing wavelength.

Visible light communication using InGaN optical sources with AlInGaP nanomembrane down-converters.

We report free space visible light communication using InGaN sources, namely micro-LEDs and a laser diode, down-converted by a red-emitting AlInGaP multi-quantum-well nanomembrane. In the case of micro-LEDs, the AlInGaP nanomembrane is capillary-bonded between the sapphire window of a micro-LED array and a hemispherical sapphire lens to provide an integrated optical source. The sapphire lens improves the extraction efficiency of the color-converted light. For the case of the down-converted laser diode, one side of the nanomembrane is bonded to a sapphire lens and the other side optionally onto a dielectric mirror; this nanomembrane-lens structure is remotely excited by the laser diode. Data transmission up to 870 Mb/s using pulse amplitude modulation (PAM) with fractionally spaced decision feedback equalizer is demonstrated for the micro-LED-integrated nanomembrane. A data rate of 1.2 Gb/s is achieved using orthogonal frequency division multiplexing (ODFM) with the laser diode pumped sample.

RGB and white-emitting organic lasers on flexible glass.

Two formats of multiwavelength red, green and blue (RGB) laser on mechanically-flexible glass are demonstrated. In both cases, three all-organic, vertically-emitting distributed feedback (DFB) lasers are assembled onto a common ultra-thin glass membrane substrate and fully encapsulated by a thin polymer overlayer and an additional 50 µm-thick glass membrane in order to improve the performance. The first device format has the three DFB lasers sitting next to each other on the glass substrate. The DFB lasers are simultaneously excited by a single overlapping optical pump, emitting spatially separated red, green and blue laser output with individual thresholds of, respectively, 28 µJ/cm(2), 11 µJ/cm(2) and 32 µJ/cm(2) (for 5 ns pump pulses). The second device format has the three DFB lasers, respectively the red, green and blue laser, vertically stacked onto the flexible glass. This device format emits a white laser output for an optical pump fluence above 42 µJ/cm(2).

CdS(x)Se(1-x)/ZnS semiconductor nanocrystal laser with sub 10kW/cm(2) threshold and 40nJ emission output at 600 nm.

A colloidal quantum dot laser emitting at 600 nm with a sub 10kW/cm(2) threshold at 5ns pulse pumping is reported. The device has a second order distributed feedback cavity for vertical emission and incorporates a bilayer planar waveguide structure based on a film of yellow-orange alloyed-core/shell CdS(x)Se(1-x)/ZnS quantum dots over-coated with polyvinyl alcohol. A study of the amplified spontaneous regime indicates that the quantum dot gain region behaves like a quasi-three level system and that the bilayer structure design increases the modal gain compared to a single layer of quantum dots. An output of 40nJ per pulse is measured for a total pump-to-signal efficiency above threshold of 3%.

Capability of GaN based micro-light emitting diodes operated at an injection level of kA/cm².

Different size InGaN/GaN based micro-LEDs (µLEDs) are fabricated. An extremely high injection level above 16 kA/cm2 is achieved for 10 µm-diameter LED. The lateral current density and carrier distributions of the µLEDs are simulated by APSYS software. Streak camera time resolved photoluminescence (TRPL) results show clear evidence that the band-gap renormalization (BGR) effect is weakened by strain relaxation in smaller size µLEDs. BGR affects the relaxation of free carriers on the conduction band bottom in multiple quantum wells (MQWs), and then indirectly affects the recombination rate of carriers. An energy band model based on BGR effect is made to explain the high-injection-level phenomenon for µLEDs.

Heterogeneous integration of gallium nitride light-emitting diodes on diamond and silica by transfer printing.

We report the transfer printing of blue-emitting micron-scale light-emitting diodes (micro-LEDs) onto fused silica and diamond substrates without the use of intermediary adhesion layers. A consistent Van der Waals bond was achieved via liquid capillary action, despite curvature of the LED membranes following release from their native silicon growth substrates. The excellence of diamond as a heat-spreader allowed the printed membrane LEDs to achieve optical power output density of 10 W/cm(2) when operated at a current density of 254 A/cm(2). This high-current-density operation enabled optical data transmission from the LEDs at 400 Mbit/s.

Diode-pumped, mechanically-flexible polymer DFB laser encapsulated by glass membranes.

A diode-pumped, mechanically-flexible organic distributed-feedback laser that is fully encapsulated with ultra-thin glass is reported. The organic laser is excited by 450 nm laser diode and emits at 537 nm with an oscillation threshold of 290 W/cm². The encapsulation format of the device results in a photostability that is improved by two orders of magnitude compared to a non-encapsulated reference device while maintaining mechanical flexibility thanks to an overall device thickness below 105 µm. The laser is also wavelength-tunable between 535 nm and 545 nm by bending the ultra-thin glass structure.

Nanosecond colloidal quantum dot lasers for sensing.

Low-threshold, gain switched colloidal quantum dot (CQD) distributed-feedback lasers operating in the nanosecond regime are reported and proposed for sensing applications for the first time to the authors' knowledge. The lasers are based on a mechanically-flexible polymeric, second order grating structure overcoated with a thin-film of CQD/PMMA composite. The threshold fluence of the resulting lasers is as low as 0.5 mJ/cm² for a 610 nm emission and the typical linewidth is below 0.3 nm. The emission wavelength of the lasers can be set at the design stage and laser operation between 605 nm and 616 nm, while using the exact same CQD gain material, is shown. In addition, the potential of such CQD lasers for refractive index sensing in solution is demonstrated by immersion in water.

Distributed feedback laser for biosensing applications.

We present an organic semiconductor distributed feedback laser biosensor and demonstrate its sensing capabilities. Optimization of the gain layer thickness to maximize the response to refractive index changes and bulk sensing results are shown. Desthiobiotin-avidin sensing assay results are also presented and the potential to perform multiple, repeated sensing measurements is demonstrated.

Broadly tunable femtosecond mode-locking in a Tm:KYW laser near 2 µm.

Efficient mode-locking in a Tm:KY(WO(4))(2) laser is demonstrated by using InGaAsSb quantum-well SESAMs. Self-starting ultrashort pulse generation was realized in the 1979-2074 nm spectral region. Maximum average output power up to 411 mW was produced around 1986 nm with the corresponding pulse duration and repetition rate of 549 fs and 105 MHz respectively. Optimised pulse durations of 386 fs were produced with an average power of 235 mW at 2029 nm.