Refractive Index Sensing Based on Semiconductor Na
Most engineers are familiar with the basic physical parameters that need to be measured, such as temperature, pressure, position, speed and distance. But there are other key parameters is also very important. One example is the measurement of the refractive index (RI), which is part of many liquid processing processes such as brewing, wine fermentation, assessment of moisture content in honey, and hydrocarbon distillation. Even in medicine is also useful, because the cancer cells or abnormal cells RI value is slightly higher than normal cells.
The problem is that even miniaturized RI sensors, such as coin-sized sizes, are larger than many applications. It is also the problem that the University of Michigan team hopes to change by using most powerful laser emission from nanowires excited by light sources. The team described the team's research in detail in a paper entitled "Refractive Index Sensing Based on Semiconductor Nanowire Lasers".
In this study, a cadmium sulfide (CdS) line with a diameter of 204 nm and a length of 15 μm was used as the core design of the optical cavity (Fig. 1). The high refractive index of the nanowires causes the incident light to have a high reflectivity factor at the end face and be guided by the nanowires. This arrangement forms the Fabry-Burrow cavity, which, when activated by an external light source, forms a laser. The green laser pointer wavelength thus formed depends on the refractive index around it. By measuring the wavelength of the laser output, the refractive index of the liquid immersed in the nanowires can be determined.
The manufacturing process uses a semiconductor-like process in which cadmium sulfide (CdS) nanowires (refractive index n = 2.67) are grown on silicon wafer substrates using chemical vapor transmission. The nanowires are then deposited directly on the silicon glass cover and then exposed to low pressure plasma. This plasma treatment is used to ensure that the nanowires are fixed on the glass surface for use in experiments in liquid environments.
In experiments, the nanowires were immersed in a mixture of ethanol and toluene at a volume ratio of 10:0, 9:1, 8:2 And 7:3, equivalent to refractive indices of 1.365, 1.39, 1.339, and 1.407, respectively. The nanowire is excited by a pulsed optical parametric oscillator (5 ns pulse width, 20 Hz repetition frequency, and 479 nm wavelength). The pump beam is then focused using a 50 x objective lens with a focal size of 60 M. Figure 2 shows the resulting green astronomy laser spectra of each refractive index, using the same excitation energy density of 4 J/ mm 2.
The test results also show that the wavelength effect is also very sensitive. For example, when the refractive index increases from 1.365 to 1.407, there is a significant measurable redshift from 504.35 nm to 505.23 nm.
Further analysis of the data based on the 0.22nm line width of the purple laser pointer peak and other factors shows that the overall quality factor (FOM) of the design is 96.
The semiconductor nanowire sensor is not only the recently announced 24-fold increase in the results achieved using the resonant scattering method, but also its manufacturing complexity is low. It can be internalized into living cells and senses the refractive index of each fraction.
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