## 5 III-V-on-Silicon Devices for Optical Sensing Applications
### III-V-on-Silicon Broadband Light Sources for Optical Sensing Applications
#### 5.1.2 Power-Efficient Single Spatial Mode LEDs  
Similar to superluminescent LEDs, power efficient LEDs would be of great use in sensing applications. Especially if this integrated LED can combine efficiency and fabrication simplicity, one can think of high-volume low-cost spectroscopic sensors based on these devices. Using heterogeneous integration, very high refractive index contrast III-V membranes can be realized. In a high refractive index contrast waveguide, the efficiency of capturing the spontaneous emission into a single spatial mode is boosted compared to classical low index contrast structures. While electrical injection of such thin membranes is difficult, an optically pumped membrane LED can be envisioned, pumped by an efficient waveguide coupled laser sources, such as a VCSEL. Avoiding the electrical pumping also makes the device fabrication more straightforward. Figure 21: **(a)** On-chip spectra when pumping the different sections of the superluminescent LED. The dashed, dash-dotted, solid and dotted lines indicate the section at 1300 nm, 1380 nm, 1460 nm and 1540 nm respectively. **(b)** On-chip spectrum when pumping the different sections of the superluminescent LED differently to maximize optical bandwidth. The pumping currents were 70 mA, 50 mA, 300 mA and 140 mA for the sections at 1300 nm, 1380 nm, 1460 nm and 1540 nm respectively. Reproduced from [60]. Figure 20: Illustration of superluminescent LED comprised of four band gaps connected to a silicon waveguide circuit. Reproduced from [60]. We have been investigating the integration of thin InP membranes on top of an SOI waveguide to create an efficient optically pumped waveguide coupled LED [61]. The thin membranes are obtained by bonding an ultra-thin III-V stack onto the SOI chips via DVS-BCB bonding, as shown in Figure 22. In combination with an appropriate waveguide design, the membrane (LED) can be optically pumped via the single mode SOI waveguide underneath. A significant power fraction of the entire spontaneously emitted light couples back into the single mode of the SOI waveguide. The device in Figure 22a consists of an SOI photonic wire waveguide that tapers down, squeezing the pumping mode into the III-V membrane. The III-V is shaped as a ridge waveguide and has a taper connection to the silicon as well. The membrane material is chosen to strongly absorb the optical pump. The absorbed light will generate electron-hole pairs that lead to spontaneous emission, which is collected efficiently into a single spatial mode in the III-V membrane waveguide because of the high refractive index contrast. By optimizing the confinement of the pump light in the III-V material and the spontaneous emission collection efficiency in the SOI waveguide, an optimum membrane thickness of 115 nm is obtained. For the layer stack reported in [61], the confinement factor in the four quantum well structure is 18.4%. The spontaneous emission collection efficiency into the fundamental guided mode in the SOI, is around 10%. The spontaneous emission couples to both forward and backward propagating modes in the SOI, meaning it is possible to collect light both in transmission and in reflection. Given the strong pump absorption, the carrier concentration will be highest at the first taper and lower towards the end of the III-V waveguide due to pump depletion. For the signal in transmission, there will be an optimum device length: a very long device may not be pumped completely to transparency and hence reabsorb the spontaneous emission. The spontaneous emission in reflection does not suffer from this problem, but then a photonic integrated circuit to (de)multiplex signal and pump has to be designed. Figure 22b shows the LED spectra collected both in transmission and reflection for a 50 \(\upmu\)m long LED. Optical pumping was done at a wavelength of 1310 nm using 0.5 mW waveguide coupled power. The collected spectrum  
Figure 22: **(a)** Illustration of an optically pumped membrane LED. (inset) SEM image of the fabricated device.