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. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.


Introduction
InGaN-based visible light emitting diodes (LEDs) are widely used in solid-state lighting and display technologies [1,2]. Because of their high modulation bandwidth, ranging from 20 MHz for large-area LEDs [3,4] up to several hundreds of megaHertz for microsize LEDs (µLEDs) [5,6], they can also double as wireless transmitters in visible light communications (VLC) applications [7]. In particular, µLEDs have been demonstrated in VLC links with data transmission rates approaching 8 Gbps [6]. In a VLC system, LED transmitters are intensity modulated at high speeds and photoreceivers convert the optical signals into the electric domain. Research has so far mainly considered LED transmitters and photoreceivers as discrete devices. VLC demonstrations have also been principally unidirectional, the downlink using VLC and the uplink another technology like Wi Fi (wireless fidelity) or infrared communications. Here, in order to enable bi-directional VLC, we devise an integrated transceiver with a µLED as the transmitter. The signal-to-noise ratio (SNR) at the receiver and the overall bandwidth are key factors that affect the performance of VLC. Advanced modulation schemes, e.g. orthogonal frequency division multiplexing (OFDM), are often implemented in order to maximize utilization of the available bandwidth [6]. The size of the photoreceiver can be increased to enhance the SNR but this is at the detriment of the bandwidth as photoreceivers with large active areas tend to be slow [8]. One solution is to keep the active area of the receiver small and increase the amount of light arriving onto it with an optical element known as a fluorescent concentrator [9,10], the downshifted light is waveguided to the edge of the concentrator where a small area receiver is positioned. Unlike lenses, these fluorescent concentrators make use of fluorescence (and an accompanying Stokes shift) and not exclusively reflection and refraction and can therefore be used to circumvent the étendue limit. This enables the collection of an incident signal over a wide area and its concentration onto the detector whilst maintaining high bandwidth and excellent field of view of the receiver. In the following we report an integrated optical transceiver that combines a 450 nm transfer printed (TP) µLED (the transmitter) with such a fluorescent optical concentrator (the receiver). Transfer printing is a heterogeneous integration technique where micro-and nano-sized optoelectronic devices are "printed" onto non-native, capability enhancing, substrates [11][12][13][14][15][16][17][18][19]. The advantage of a TP µLED is for integration of the transmitter with the concentrator receiver in a small form factor. Using a full µLED-on-sapphire device, rather than a TP µLED, would lead to a device with at least twice the thickness and furthermore raise the issue of cross-talk at the receiver side. The performance of GaN-on-sapphire µLEDs for VLC is typically better than GaN-onSi devices [20] in terms of bandwidth and optical power. An alternative would be to transfer print µLEDs fabricated on sapphire and then released from their substrate by laser lift off [11]. On the other hand, the utilization of Si offers cost effectiveness, potential for scalability and a wide choice of substrate removal approaches. We note that TP µLEDs used this demonstration have been shown to have modulation bandwidths of 100s of MHz, and data transmission rates of 120 Mbps on flexible and silica substrates [21,22].
We describe the design and fabrication of an optical transceiver device and assess its operation using ODFM as the VLC modulation scheme. The device can simultaneously receive and transmit VLC signals, i.e. it can implement both the VLC downlink and uplink. It benefits from the wide-angle detection capability of the optical concentrator and can also operate as an optical relay, thereby providing a solution to the line-of-sight limitations of indoor communications [9]. The schematic in Fig. 1 shows the concept of the integrated optical transceiver device with the µLED on the top surface of the optical concentrator. Fabrication first entails TP the blue-emitting µLED onto a 1 mm-thick 25 mm × 75 mm glass substrate (microscope slide). TP makes use of the reversible adhesion of an elastomeric stamp to pick-up, transfer and print objects (here the µLED) onto a non-native substrate. Previously used in the fabrication of mechanically-flexible inorganic semiconductor devices [17,18], we have demonstrated the technique for printing InGaN µLEDs with < 200 nm resolution onto optically inactive substrates [21,23] and onto multi-color devices for VLC applications [22,24]. Prior to TP, the 100 µm × 100 µm µLED is fabricated in a suspended format from a GaN-on-Si LED epistructure. The Si substrate is underetched with potassium hydroxide (KOH) to leave ultra-thin µLED platelets, suspended by 10 µm-wide anchors that can be easily removed from their growth wafer with an elastomeric stamp. Figure 2(a) shows a schematic of the TP process. After TP, the µLED contacts are defined by photolithography [21]. Figures 3(a)-3(c) show microscope images of the µLED TP on the glass slide (seen on the bottom right of the three images) with metal tracks added. The metal track design gives scope to print a 6 × 6 array of µLEDs, but this is beyond this first device demonstration. Figure 3(d) gives the current-voltage characteristic for such a TP µLED, these start to emit light around 3.5 V. It is important to note that these µLEDs can have some leakage current, and so we only see light-emission after around 1 mA of drive current. This is possibly due to damage to the device sidewalls occurring when the Si substrate is removed, leaving a rough surface and increasing opportunities for tunneling. The current-optical power characteristic for these TP µLEDs is given in Fig. 3(e) and the emission spectrum when printed on the glass slide can be seen in Fig. 3(f). These TP µLEDs emit at 460 nm, and achieve over 50 µW of optical power at 3.5 mA.

Design and fabrication
Before removal from their Si substrate, these µLEDs have a lower turn on voltage and begin to emit light at around 2.8 V as reported in [21]. The optical power of 100 µm x 100 µm InGaN-on-Si µLEDs, fabricated in the same way but without any underetching of the Si substrate, is difficult to measure though, as the flip-chip configuration of these µLEDs means light is not significantly transmitted until the Si substrate is removed. Top-emitting, broad-area, µLEDs processed on the same wafer as the TP µLEDs used in this work show a spectral emission around 450 nm. We attribute the 10 nm redshift between the spectral emission on Si and the TP µLEDs (see Fig. 3(f)) to strain effects induced when the µLED is removed from its growth substrate.
The glass substrate integrated with the TP µLED forms the top cladding of the transceiver. The other part of the transceiver is fabricated by blade coating a 70 nm-thick fluorescent film on a second microscope slide -this glass slide is the bottom cladding of the fluorescent concentrator, see Fig. 2(b). The fluorescent material is a conjugated poly (p-phenylene vinylene) copolymer, commonly known as "super yellow" (SY), acquired from Merck KGaA (PDY 132). The absorption and typical emission of SY respectively peak at 450 nm and 560 nm [25]. While we focus here on demonstrating a device concept and a stability study was not conducted, we did not observe any degradation of the fluorescence during experiments. We note that SY is a relatively stable copolymer, in widespread use in OLEDs, and given the device geometry, it is encapsulated by glass, which is beneficial for stability. Initially a 4 mg/ml solution of SY in chlorobenzene was prepared. An automatic film applicator (ZAA 2300, Zehntner GmbH) was used to blade coat the solution onto the glass. The temperature was set to 40 • C, the speed to 30 mm/s and the height of the blade (ZUA 2000.80) was 100 µm above the microscope slide. A circular area (diameter 3 mm) of the SY film directly under the targeted position of the TP LED was mechanically removed. This is to prevent absorption of µLED light emitted from the TP µLED, which falls within the absorption band of SY, by the optical concentrator as this would otherwise cause interference, at the receiver end, when the device simultaneously transmits and receives. Furthermore, the µLED transmits through the glass cladding as shown in Fig. 1 and any absorption would diminish the optical power of the transmitter. To finalize the transceiver, the microscope slide with the SY film was coated with the optical epoxy NOA 13685 (Norland Inc.), placed in contact with the backside of the glass cladding on which the µLED was printed, and UV cured (365 nm, 7 J/cm2). For the experiments described in section 3, the edge of the optical concentrator was coupled to a Hamamatsu (S8664-50K) avalanche photodiode (APD). This APD has a peak sensitivity at 600 nm (spectral response range 320-1000 nm), a quantum efficiency of 70 % (at 420 nm) [8], and a 3-dB bandwidth of 60 MHz but the overall bandwidth of the system is set by the pulse-shaping filter (see section 3 for details).

VLC demonstration and discussion
Two separate experiments were performed to demonstrate the capability of the transceiver device for VLC. The first consisted of operating the device to simultaneously transmit and receive independent data (transceiver mode). In the second experiment the device was operated to receive and re-transmit data (optical relay mode). In both these experiments, an external µLED source (respectively blue, 450 nm, and 3 violet, 405 nm, µLEDs in series) and a second APD are used to respectively send a signal to and receive a signal from the transceiver. The typical electrical-to-optical modulation bandwidth of these external LEDs is above 100 MHz. The maximum optical power is approximately 9 mW at 175 mA and 9.5 mW at 45 mA for the blue and 3 series violet emitting LEDs, respectively. Information on the design, fabrication, and characteristics of the blue µLED used as an external source in this demonstration can be found in [26]. The power versus injection current for the 3 violet µLEDs is plotted in Fig. 4. The setup used for the experiments is shown in Fig. 2(c). The TP LED and the external LED source are driven with a direct current (DC) bias from a power supply combined by a bias-tee with a modulated alternating current (AC) signal from a signal generator. The modulated signal is based on an OFDM data encoding scheme. To generate this OFDM signal, an incoming stream of bits was modulated into M-ary quadrature amplitude modulated (M-QAM) symbols where M is the constellation size. Hermitian symmetry and fast Fourier transformation (FFT) operation are then applied to NFFT M-QAM symbols. 1024 subcarriers are equally spaced over the system bandwidth. The resulting OFDM stream drives the µLEDs through an arbitrary waveform  generator (AWG) Agilent®81180A with a sampling frequency of 720 MS/sec (transceiver mode experiment) or 1200 MS/sec (optical relay mode experiment). A root raised cosine (RRC) pulse shaping filter is used with an oversampling factor of 8, which results in a single side modulation bandwidth of 45 MHz for the transceiver mode experiment and 75 MHz for the optical relay mode experiment. This is because of the higher SNR in the optical relay experiment, which we believe is related to the addition of a small lens directly under the TP µLED (see below). Light from the external µLED source is collected by the optical concentrator and waveguided to the edge facet of the structure to the APD at the position labeled A in Fig. 2(c). This optical link is referred to hereafter as link A, a spectrum of the light collected here is shown in Fig. 5. This shows a major peak at 563 nm from the color-converted SY light and a smaller peak at 543 nm from unconverted blue light from the incident µLED . Light from the TP µLED (460 nm) is emitted down and straight through the glass and epoxy layers to the external APD at the position labeled B in Fig. 2(c). This optical link is referred to in the following as link B. The data rates given in the following are for a bit error rate (BER) target of 3.8 × 10 -3 [27]. The drive conditions in this first experiment are 6.1 V and 3 mA for the TP µLED and the 4.1 V and 24 mA for the external µLED. The optical power of these µLEDs is approximately 44.5 µW for the TP µLED and 2 mW for the external µLED at these drive currents. The typical electrical-to-optical modulation bandwidth for the TP µLEDs on glass is around 50 MHz [23]. The SNR for both links is shown in Fig. 6(a). The higher SNR for link A is due to the higher optical power of the external µLED. Figure 6(b) shows the data rates for both link A and link B. The link B with the TP µLED transmitter reaches a data rate of 165 Mbps. The link between the external µLED and the transceiver (link A) has a data transmission rate of 416 Mbps. These performance values are consistent with the SNR of both links. The sharp drop in the SNR as the modulation bandwidth is increased past 40 MHz is attributed to the pulse shaping filter.
The second experiment demonstrates the integrated transceiver operating as an optical relay in decode and forward regime. The experimental setup is similar, but the external source is an array of 3 violet LEDs (405 nm) in place of the blue LED. The violet LEDs are used in this case to boost the received optical power at APD A. It also shows the wavelength flexibility at detection of the device while still demonstrating a viable VLC system, with violet LEDs an emerging enabling technology for white light solid state lighting as well as for communications [6,28,29]. The three violet µLEDs were driven in series, with a combined current of 30 mA (at 12V), the optical power of the three violet µLEDs was 7 mW. A lens (focal length 2.5 mm, diameter 2.5 mm) is also added onto the underside of the device, directly under the TP µLED, to increase the light collected by APD B. The emitted light of the external LEDs is collected by the fluorescent concentrator and detected at APD A. This detected signal is sent to an oscilloscope (MSO7104B) and demodulated offline in MATLAB, where all the signal processing operations are performed as described in [30]. The demodulated -QAM symbols are then remodulated and fed to the TP µLED. The TP µLED was driven at 1 mA (at 5.7V). The three violet µLEDs have a modulation bandwidth of 350 MHz, while the TP µLEDs typically have electrical-to-optical modulation bandwidths of just over 20 MHz at 1 mA [23]. The light from the TP µLED is emitted through the glass structure and is in turn detected at APD B. Below forward error correction (FEC) level of 3.8 × 10 -3 , the data rate is 375 Mbps for link A and 337 Mbps for link B, as shown in 6(c). The optical relay functions at up to 337 Mbps, as it is limited by the speed of the slowest component -in this case the TP µLED. The improvement in data transmission rate from the TP µLED (link B) between the first experiment (165 Mbps) and the second experiment (337 Mbps) is attributed to the integration of a lens to more effectively transmit light from the TP µLED to the APD. This is a first concept device demonstration, and tests are done at short-range in order to define the ultimate performance. For longer range VLC demonstrations, it can be noted that µLEDs have been demonstrated in free-space VLC at data transmission rates in excess of 1 Gb/s over distances of up to several meters. Hence we believe this concept device could potentially be extended to communicate over comparable distances in the future [31,32]. Transfer printing arrays of µLEDs, rather than a single µLED as in this demonstration, will increase the light output power and therefore the range of the device. Additionally, increasing the light output power will increase the SNR, and consequently the data transmission rate. As seen in Fig. 3(e), these µLEDs typically rollover at 4-6 mA, and so can't be driven to high currents -limiting their power. Printing an array of µLEDs would also go some way to negating this issue.
One of the main visions of VLC is that conventional room lighting can be used to transmit data through free space. We therefore chose here GaN-on-sapphire µLEDs as the external sources (rather than the same µLEDs used for TP but before removal from their Si substrate) , as they are the conventional materials for solid-state lighting fixtures.
In any real-life VLC system, it would be important to minimize the required power of this device to integrate with other electronic components; one way to reduce this would be as in [33].
While this demonstration has focused on the VLC applications of this device, it could just as easily be applied to biomedical applications, such as optogenetics and biosensing, as demonstrated in [34][35][36][37], to wirelessly transmit signals to analysis equipment.

Conclusion
In conclusion, we have demonstrated, for the first time to our knowledge an integrated optical transceiver and repeater for visible light communications. The device is based on a transfer printed µLED as the transmitter, and a fluorescent concentrator as the receiver. It was experimentally demonstrated that the optical transceiver can achieve beyond 100 Mbps data rates for reception and transmission. We believe that this new VLC device will be very suitable for implementing smart devices and enable IoT applications.