Fabrication and characterization of electrically pumped VCSEL based intracavity fluidic biosensor

Abstract

There is an increasing need to quickly and cheaply detect the presence of biological agents introduced into the atmosphere by terrorists or on the battlefield. In order to accomplish this requires a biosensor and a method of integrating it with electronics and an alarm or display. The overall device must be small, easily manufactured and simple to use in order that it be sufficiently portable to allow being carried in many different types of situations. This thesis reports the fabrication and characterization of such a sensor based on an electrically pumped vertical-cavity surface-emitting laser (VCSEL). The prototype biosensor structure reported here is composed of a "half VCSEL" separated from a glass top plate coated with a dielectric distributed bragg reflector (DBR) mirror by a ~ 5 μm thick photoresist spacer. Simulations indicate that the emission wavelength for the sensor is shifted quasi-continuously with respect to the change of the cell refractive index and/or the fluidic cavity thickness. Fabrication of the VCSEL biosensor required the development of suitable techniques for forming the fluidic cavity, a fluid delivery system, proper sealing and forming the VCSEL. All of these were accomplished. The fluidic cavity, which provides suitable spacing and fluid seal, was formed by heating patterned photoresist at 90 °C with external force applied. The fluid was introduced into the VCSEL cavity from a capillary tube butted up against the side of the cavity. The tube/cavity connection was successfully sealed with a deposited epoxy gasket. Forming a suitable n-ohmic contact was the most difficult step in VCSEL fabrication because the n mirror is very thin. The assembled structure was shown to emit light but apparently does not lase. The functionality of the biosensor was demonstrated by the presence of a 3.2 nm wavelength shift of the ~840 nm peak when DI water was injected into the empty fluidic cavity. The emission spectra are complex and the further study and analysis are required to interpret the measurements.