Air-suspended single-mode polymer waveguides towards highly sensitive optofluidic sensors
Creators: name: Prokop, Christoph
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400908
subject: Microelectronics
classificationCode: 400909
subject: Photonic and electro-optical devices, sensors and systems (excl. communications)

description: In this thesis, a polymer lamination method is presented that enables efficient light coupling into air-suspended polymer waveguides by grating couplers which can then be exploited for research towards a large range of applications in the field of microfluidics, optics and optofluidics. This approach is especially useful for highly sensitive integrated optofluidic sensors as has been demonstrated in this work.<br><br>Due to the low refractive index contrast available between various polymer materials, the efficiency of miniaturized polymer photonic and optofluidic devices is often limited. In this work, a new lamination technique is presented that allows the bonding of unstructured and structured layers of SU-8 photoresist down to sub-micron thicknesses onto microchannels fabricated in KMPR photoresist using a flexible PDMS carrier stamp. This approach enables the creation of air-suspended SU-8 structures with the highest possible refractive index contrast. Such layers can be bonded down to a thickness of 0.5 µm which in principal allows guidance of light of infrared wavelengths within the single-mode regime.<br><br><br>In order to exploit this approach for highly sensitive optofluidic sensing applications, light needs to be coupled into such layers. In this work light coupling is achieved using air-suspended SU-8 rib waveguides and butt coupling, as well as using air-suspended SU-8 surface grating couplers.<br><br>Particularly, air-suspended SU-8 grating couplers are very interesting. After numerical simulations, air-suspended SU-8 grating couplers have been fabricated based on the developed lamination method. Transmission measurements have shown that a single grating coupler exhibits approximately 8 dB coupling loss at a centre wavelength of 1557 nm indicating a coupling efficiency of about 16%. In addition, the thermal sensitivity of the air-suspended SU-8 grating couplers has been studied by numerical simulations followed by experimental evaluation.<br><br>Even though the achieved coupling efficiencies are lower than for SOI couplers, mainly due to current fabrication limits of the master structures, the fabrication method employing widely used polymer materials has the advantage that multiple air-suspended structures can directly be created in only one simple lamination process without the need of additional etching steps.<br>Finally, different optofluidic sensor concepts are proposed based on the demonstrated SU-8 bonding method and the air-suspended SU-8 waveguide grating couplers. Experimental transmission measurements have shown that a first sensor concept exhibits a refractive index sensitivity of approximately 400 nm RIU-1 according to the wavelength shift and 17 dB RIU-1 due to the intensity loss which is similar to the results of the numerical simulations. In comparison to previously shown sensing applications using grating coupler structures, the analytes can directly be probed in-line due to the combination of a microfluidic channel and air-suspended grating couplers making the proposed sensor concept highly applicable for in-line polymer optofluidics and suitable for low-cost optofluidics and photonics sensor concepts.<br><br>However, in order to detect even the smallest differences in analytes, higher sensitivities would be required. Therefore, a second sensor concept aims to increase the sensitivity by introducing a long-period grating into an air-suspended SU-8 waveguide in order to enhance the light-matter interaction strength. Numerical simulations have been carried out to obtain a set of parameters for a multi-mode long-period grating sensor for TE polarized light at a resonance wavelength of 1550 nm. Transmission calculations as a function of wavelength have shown that such sensor can exhibit a very high refractive index sensitivity of about 6000 nm RIU-1.<br><br>In summary, this thesis introduces a new polymer lamination method for thin, structured air-suspended SU-8 films down to sub-micron thicknesses. In fact, such films are thin enough to provide single-mode guidance of light in planar waveguides in the infrared wavelength regime. By exploiting air-suspended SU-8 grating couplers, light can be efficiently coupled into such waveguides. Different applications in the fields of microfluidics, optics and optofluidics are introduced in this thesis and it appears that the thin air-suspended SU-8 films can be robust enough even for the realisation of real world applications.