In optics the refractive index (or index of refraction) n of a substance (optical medium) is a number that describes how light, or any other radiation, propagates through that medium.
Refractive index is the basic property of optical crystals, and it is an important parameter to study the properties of crystals, such as spectrum and laser. Refractive index of materials varies with the wavelength, and the measured value is very important for device design. Take our 4H silicon carbide (SiC) wafer for example, its refractive refractive index decreases with the increasing of wavelength, please refer to 2.2 Optical Property of 4H N Type SiC Wafer (link: https://www.powerwaywafer.com/4h-n-type-sic.html). SiC refraction index 2.55 (infrared; all polytypes).
The concept of refractive index is widely used within the full electromagnetic spectrum, from x-rays to radio waves. It can also be used with wave phenomena other than light (e.g. sound) In this case the speed of sound is used instead of that of light and a reference medium other than vacuum must be chosen.
For infrared light refractive indices can be considerably higher. Germanium is transparent in a wavelength of 589 nanometers and has a refractive index of about 4, making it an important material for infrared optics. More details please refer to our product of Germanium (Ge) Window ( https://www.powerwaywafer.com/germanium-window.html).
How to Measure Refractive Index of Crystal ?
There are many methods for measuring the refractive index of crystals, which are listed as follows:
| Methods | Advantages | Disadvantages |
| Ellipsometry | High measurement accuracy, non-destructive to the sample, especially suitable for the measurement of optical parameters of thin film layers | This method is only suitable for the measurement of isotropic media. In addition, using this method to process experimental data is tedious and requires a high level of surface flatness. |
| Prism coupling method (quasi-waveguide method) | Only angle measurement is required, which is convenient and accurate. | This method requires that the refractive index of the sample must be larger than that of the substrate, which makes some samples with low refractive index cannot be measured by this method. |
| Interference method | Not only can measure transparent films, weakly absorbing films and non-transparent films, but also suitable for birefringent films. | The thickness and refractive index of the film cannot be obtained simultaneously. In addition, it is difficult to determine the number q of the dislocation fringes of the interference fringes. The contrast of the interference fringes formed by the low reflectivity film is low, which will bring measurement errors, and the film must have steps, the measurement process is complicated to adjust, and the film surface is easy to wear, etc. These all bring inconvenience to the measurement. |
| V-prism method | This method does not measure the thin film directly, but measures the composite material solution supplemented by the quasi-waveguide method. The principle of the method is simple, the operation is easy, and the data processing is not complicated. | The measurement accuracy is not very high, and the thickness of the film cannot be obtained at the same time. |
| Transmission line method | Only using the transmittance curve spectrum of the sample, the experimental process is relatively simple, and the calculation process can be realized by computer programming, which is also relatively simple, fast and accurate. | The quality of the sample is relatively high, and the film must be uniform and the surface is parallel. |
Among all the methods, transmission spectrum can obtain refractive index values over a wide spectral range. However, the V-prism method, interferometric method, self-collimation method and minimum deflection angle method are limited by the measurement light source, usually only the refractive index data of several wavelengths can be obtained. Obviously, the former can better give the refractive index distribution in a certain spectral range than the latter.
If necessary, we can offer optical semiconductor wafers with refraction index testing.
For more information, please contact us email at victorchan@powerwaywafer.com and powerwaymaterial@gmail.com.