Hybrid integration of III-V semiconductor lasers on silicon waveguides using optofluidic microbubble manipulation

Optofluidic manipulation mechanisms have been successfully applied to micro/nano-scale assembly and handling applications in biophysics, electronics, and photonics. Here, we extend the laser-based optofluidic microbubble manipulation technique to achieve hybrid integration of compound semiconductor microdisk lasers on the silicon photonic circuit platform. The microscale compound semiconductor block trapped on the microbubble surface can be precisely assembled on a desired position using photothermocapillary convective flows induced by focused laser beam illumination. Strong light absorption within the micro-scale compound semiconductor object allows real-time and on-demand microbubble generation. After the assembly process, we verify that electromagnetic radiation from the optically-pumped InGaAsP microdisk laser can be efficiently coupled to the single-mode silicon waveguide through vertical evanescent coupling. Our simple and accurate microbubble-based manipulation technique may provide a new pathway for realizing high precision fluidic assembly schemes for heterogeneously integrated photonic/electronic platforms as well as microelectromechanical systems.


Supplementary Figure S1
Laser beam propagation and absorption after microbubble generation   Assuming negligible internal flows inside the microbubble at the steady state, the interfacial force by the shear stress at the latitude angle θ can be written as 1

Supplementary Figure S2
where R and σ(θ) stand for the radius of the microbubble and surface tension, respectively, as indicated in Fig.

S2a. Its vertical component (F σ,z ) is given by
where T i indicates the interfacial temperature between the microbubble and the water. To estimate the amount of force under the experiment condition, the radius of the microbubble, R, was set to be 5 μm, and its shape was considered as being almost hemispheric (θ max =101º). The experimental results of surface tension at water-air interface 2 as a function of temperature was used to approximate the surface tension gradient term (∂σ/∂T=0.17 mN/m) as a constant value (Fig. S2b). The interfacial temperature distribution in Fig. S2c was obtained from numerical simulations assuming that the input laser power was 8 mW and its beam spot diameter was 1.8 µm.
We also assumed that the laser beam was applied at 3 µm away from the microdisk center to emulate actual experiment conditions. The air-water interfacial temperature was assumed to vary linearly with the vertical direction (z) whose fitting result is given by T i = 43.7-0.57×10 6 z. Applying the linear fitting results, the vertical thermocapillary force of ~12 nN is obtained.

Preparation of semiconductor microdisk lasers
For semiconductor microdisk fabrication, compound semiconductor epitaxial layers were grown by metalorganic chemical vapor deposition on an InP substrate. The InGaAsP bulk layer has a photoluminescence peak at around 1550 nm at room temperature. The details of the layer structure are shown in Table S1. Photolithography was used to define circular microdisk patterns with diameters ranging from 5 to 20 µm. The InP/InGaAsP layer was then chemically etched using bromic acid (HBr), phosphoric acid (H 3 PO 4 ), and potassium dichromate (K 2 Cr 2 O 7 ) solutions 3 , which preserve a photoresist layer during the etching process for reliable dimension control. After removing the photoresist mask, the InP substrate was further selectively etched with diluted hydrochloric acid (HCl) to create pedestals supporting the microdisk structures (Fig. S3a). Optical characterization results of a typical microdisk laser with a diameter of ~8 µm placed on the pedestal can be found in Fig. 4c. When further etched with HCl, the semiconductor microdisks can be released from the substrate. The released microdisks were then immersed in deionized water and transferred to the silicon substrate (PDMS well shown in Fig. S4) Table S1. Epitaxial layer structure for the semiconductor microdisk fabrication. Figure S3. SEMs of (a) a microdisk on the InP pedestal layer and (b) a microdisk after transferring onto a SiO 2 /Si wafer. The diameter and thickness of the microdisk are ~5 µm and ~350 nm, respectively. Figure S4. Experimental setup for optofluidic thermocapillary manipulation using photothermal microbubble generation.

Optofluidic manipulation setup
The overall experimental setup schematically described in Fig. S4 is similar to a typical upright optical microscope with an additional laser excitation port. A visible light source was used to illuminate and visualize the microdisks and the substrate, and their optical image was projected to a charge-coupled device (CCD) camera for real-time observation. The microfluidic chamber for aqueous solution containment consists of a simple polydimethylsiloxane (PDMS) well with a ~2 mm height, as shown in the inset of Fig. S4. The semiconductor microdisks released from the InP substrate were dispersed in deionized water and transferred into this well for further optofluidic manipulation and assembly on the silicon waveguides. Figure S5 illustrates examples of translation and orientation control for the rod-like objects using the suggested microbubble manipulation technique. The 350 nm thick semiconductor objects were obtained by the same fabrication procedure explained above. The width and length of the semiconductor block are ~4 µm and ~23 µm, respectively. After a microbubble is generated on one side of the rod-like object, it can be rotated by moving the laser beam in a tangential direction to the microbubble.   Average 269 ± 53 Table S2. Assembly accuracy of the microdisk placed near the silicon waveguides.

Manipulation and orientation control of non-circular objects
In our experiments, the manipulation accuracy can be judged by the lateral gap between the edges of the microdisk and the silicon waveguide. The lateral misalignments are measured for five different example cases with various microdisk diameters ranging from 5 to 16 µm. Average misalignment was measured to be 269 nm ± 53 nm as summarized in Table S2.