DOI: 10.25549/usctheses-c40-369134
title: Flexible micro-scale vertical-cavity surface-emitting lasers via printing-enabled materials assembly

description: Vertical cavity surface emitting lasers (VCSELs) represent ubiquitous light sources that have made significant contributions to diverse modern technological applications where VCSELs provide many inimitable advantages over light emitting diodes or edge-emitting lasers. Although VCSELs in their conventional formats operating on their rigid growth wafer have been successfully exploited over the past decades, capabilities to incorporate VCSELs on non-native substrates and to heterogeneously assemble them with dissimilar materials and devices into integrated systems hold great potential for a variety of unconventional utilities that are not available in traditional VCSEL technologies as well as for improvement of thermal and optical properties. Such novel device platforms promise to accelerate accomplishing a variety of unprecedented applications with unique advantages including programmable spatial layouts, efficient utilization of expensive epitaxial materials, as well as thin, lightweight, and flexible constructions. ❧ This dissertation highlights the development of new strategies and designs that allows for nonintrusive, intimate integration of VCSELs and dissimilar opto-electronic devices (e.g., photodetectors) with various substrates without compromising original device performances. With the unique concepts and the design guidelines established through this thesis, unprecedented device styles for many applications ranging from the flexible infrared laser display to the wearable laser Doppler Velocimeter are presented. The dissertation consists of three main categories: i) transfer-printing of micro-sized VCSELs (micro-VCSELs) without materials degradation during fabrication, and their operation on various substrate, ii) thermal engineering for boosting laser power of micro-VCSELs printed on a flexible substrate, and iii) applications of micro-VCSELs heterogeneously integrated with Si or GaAs photodetectors in order to achieve the flexible fluorescence chemical sensors as well as the wearable laser Doppler sensors for blood flow rate monitoring. ❧ The first part of this dissertation details the materials designs and fabrication strategies for heterogeneous integration of micro-VCSELs onto various materials. Beyond the conventional VCSEL structures, specialized epitaxial design such as base and sacrificial layers as well as distinctive fabrication concepts like triple mesa structure enabled successful demonstration of defect-free release and transfer-printing of fully-functional ultrathin, micro-VCSELs onto unlimited choices of non-native substrates without compromising intrinsic materials properties. Although the printed micro-VCSELs showed decrease in optical power whose extent depends on the substrate thermal conductivity, adjusting device design parameters such as adhesive thickness and operating the micro-VCSELs in pulse mode suggest ways to overcome the device heating issues. Thermal transfer simulation based on finite element analysis support the experimental results. Lastly, with presented materials and fabrication approaches, a couple of unconventional classes of optoelectronic devices are demonstrated.  One is a 10 × 10 addressable arrays of micro-VCSELs integrated on a thin PET substrate to form flexible near-infrared (NIR) laser displays, and the other is heterogeneously integrated micro-VCSELs with an array of silicon photodiodes on plastics to construct large area assemblies of light source and photodetector, capable of spatiotemporal sensing and imaging. ❧ The second part introduces relatively practical, yet highly effective routes to dramatically enhance the optical power of micro-VCSELs printed on a flexible substrate that has two to three order lower thermal conductivity than source wafer. Despite attractive advantages of flexible format, micro-VCSELs printed on a polymeric substrate with organic adhesive exhibited a significant reduction of the optical output power mainly because of comparatively lower heat removal rate and resultant temperature increase in the laser cavity, associated with a spectral mismatch between emission of gain medium and cavity resonance at elevated temperatures. Therefore, in this chapter, we present the strategies that can overcome these performance limitations of printed micro-VCSELs through engineering substrates with metal composites. Although incorporated metal assemblies are much thinner (~a few μm) than the typical substrates (a few ten μm ~ 1 mm), they facilitate sufficient heat removal from the micro-VCSELs. Furthermore, bi-layer composite metal structure serves even higher laser power than one on source wafer. Systematic studies of electrical and optical characteristics of devices together with three-dimensional thermal modeling based on finite element analysis (FEA), provide quantitative description of underlying materials science and physics, as well as design factors for optimized performance. ❧ In the other parts of this dissertation, a couple of advanced platforms that cannot be achievable through the conventional VCSEL technologies are demonstrated via heterogeneous integration of micro-VCSELs with other dissimilar device components. In one chapter, fluorescence chemical sensors constituted of micro-VCSELs and 3-μm-thick single crystalline Si photodetectors, integrated on foreign substrates for portable analysis system are exhibited. Such a new strategy provides useful chemical sensing platforms, allowing for unlimited device configuration and unrestricted materials choice with minimal production cost. To maximize the performance of sensing capability, several optical components such as back surface reflector, anti-reflection layer, and long-pass optical filter are incorporated as the device elements. Transfer matrix method is used to conduct optical simulation for prediction and optimization of the system. PDMS fluidic channel device enables chemical dye solution to flow-in an individual device assembly (i.e. a couple of micro-VCSEL and Si PD) such that the consumption of sample solution containing target chemical substance can be minimized. Moreover, real-time change in chemical concentration in solution can be captured with the fluidic channel device. Mechanically compliant format of the constituting device elements including flexible substrates and PDMS fluidic channel suggests the potential use of the resultant devices as portable electronics. ❧ In the following two chapters, integrated, miniaturized VCSEL-based Doppler flowmeters are demonstrated for real-time measurement of blood flow in human body. Laser Doppler flowmetry (LDF) is a non-invasive, highly-sensitive, and reproducible technique that is one of the most suitable ways for time-resolved assessment of blood flow in tissue. However, its applications for continuous monitoring of blood flow to prevent diseases related to disturbed blood stream such as pressure sore and frostbite have been practically limited because of relative motion of the probe to the body (i.e., motion artifacts) and the non-portable size of analytical equipment. To overcome those issues, in the earlier chapter, we demonstrated the integrated optoelectronic probes, consisting of a micro-VCSEL and a GaAs-based photodetector as a coherent light source and a signal collector, respectively. The printed micro-VCSELs with a circular oxide aperture diameter of ~5 μm exhibited a few-mode lasing spectrum at the optical output power of 1 mW. The blood perfusion and the velocity measurement in a human fingertip with the presented device validated its capabilities as a laser Doppler blood flow monitor. In the latter chapter, on top of the developed probe head, we proposed unique approaches for achieving epidermal wireless laser Doppler sensing systems. Substrates with low elastic modulus and serpentine-shaped metal interconnects provide high stretchability to the wearable devices. The proposed devices will be used for monitoring the responses of the blood flow under various external provocations, especially, for detection of pressure ulcers that often happen to long-stay patients.