Fluid intracavity laser diode (FILD) sensor

Abstract

Laser based microfluidic devices, especially ones in which a microfluidic channel is an integral part of the laser cavity, are very attractive for biomedical diagnostics applications. The fluid intracavity laser diode (FILD) sensor constructed in this research work is an electrically pumped vertical cavity surface emitting laser (VCSEL) based microfluidic device. It has the potential to detect different biological cells in a fluid. The FILD sensor was constructed by attaching a bottom emitting VCSEL above an external dielectric mirror with an intervening ~10 ?m thick photoresist spacer which forms the sidewall of the fluidic channel. The VCSEL contained a complete top DBR mirror and gain region, but only a partially reflecting bottom mirror so that the external dielectric mirror completed the resonator cavity. The external dielectric mirror was made on a BK7 polished substrate with a high reflective coating (~99%) at the laser wavelength (980nm). The sensor was assembled by heating the dielectric mirror with patterned photoresist at 125 °C and attaching the VCSEL die to the softened photoresist. The result was a closed fluidic cavity formed between the dielectric mirror and the VCSEL diode. This closed fluidic channel allowed fluids and other biological samples through the reservoirs to the cavity of the laser. Lasing operation of FILD sensors were observed under CW and pulsed input current conditions. Output power vs. input current characteristics of various FILD sensors were measured under pulsed, room temperature conditions, for different fluids inside the fluidic cavity. The majority of the sensors exhibited a trend of increase in slope efficiency and decrease in threshold current with increase in fluid index of the fluidic cavity. Spectral measurements of a FILD sensor were also performed, which showed ~1nm wavelength shift with change in homogeneous fluid index from 1 to 1.33. Modulation of transverse confinement of laser beam was also observed when 10 ?m diameter polystyrene spheres, used as prototype biological cells, were flown inside the FILD's fluidic cavity. Several theoretical phenomena were investigated to explain the modulation trends of FILD's output characteristics due to change in fluid index. Complete qualitative and quantitative analysis of these phenomena are presented in the form of a thesis chapter and appendices.