  Silicon p-n junction diodes emit low-intensity, broad-spectrum light near
1120 nm in forward bias and between 400-900 nm in reverse bias (avalanche). For
the first time, we experimentally achieve optical absorption sensing of pigment
in solution with silicon micro LEDs designed in a standard silicon-on-insulator
CMOS technology. By driving a single LED in both forward and avalanche modes of
operation, we steer its electroluminescent spectrum between visible and
near-infrared (NIR). We then characterize the vertical optical transmission of
both visible and NIR light from the LED through the same micro-droplet specimen
to a vertically mounted discrete silicon photodiode. The effective absorption
coefficient of carmine solution in glycerol at varying concentrations were
extracted from the color ratio in optical coupling. By computing the
LED-specific molar absorption coefficient of carmine, we estimate the
concentration (0.040 mo/L) and validate the same with a commercial
spectrophotometer (0.030 mol/L ). With a maximum observed sensitivity of 1260
/cm /mol L, the sensor is a significant step forward towards low-cost
CMOS-integrated optical sensors with silicon LED as the light source intended
for biochemical analyses in food sector and plant/human health.
