Most engineers are familiar with the basic laser pointer physical parameters that need to be measured, such as temperature, pressure, position, speed, and distance. But there are other key parameters that are also very important. An example of this is the measurement of light refractive index (RI), which is also part of many liquid processing processes, such as beer brewing, wine fermentation, evaluation of water content in honey, and hydrocarbon distillation. It is even useful in medicine because cancer cells or abnormal cells have a slightly higher RI value than normal cells.
The problem is that even a miniaturized RI sensor the size of a coin is larger than many applications. This is exactly the problem that the University of Michigan team hopes to change by using laser emission of nanowires excited by a light source. The team described in detail the research conducted by the team in a paper entitled “Refractive Index Sensing Based on Semiconductor Nanowire Lasers”.
In this study, a cadmium sulfide (CdS) wire with a diameter of 204 nm and a length of 15 μm was used as the core design of the optical cavity. The high refractive index of the nanowire makes the incident light have a high reflectivity factor at the end face and is guided by the nanowire. This arrangement forms a Fabry-Perot cavity, which when activated by an external light source also forms a green laser pointer. The laser wavelength formed in this way depends on the refractive index around it. By measuring the wavelength of the laser output, the refractive index of the liquid immersed in the nanowire can be determined.