The phenomenon of polarization of light is based on the waveform character of the light. Light is a transverse electromagnetic wave, with electric and magnetic fields oscillating normal to the direction of propagation. Typically, as shown in the figure on the right, oscillations can be vertical.
A polarizer then filters out those fractions of light which oscillate in parallel with the filter centerline of the polarizer. If, for example, the orientation of an electromagnetic wave is 45° relative to an ideal polarizer's direction of filtering, it will transmit an electromagnetic wave which, while the direction of oscillation is in parallel with the filtering direction (polarization axis), has 50% of the intensity of incident light. If, however, the angle formed between the wave and the polarization axis is 90°, light is not transmitted any more. In contrast, with the angle being 0°, the wave would travel through the polarizer without any restriction.
This phenomenon is not limited to the visible-wavelength range (light visible by the human eye, also referred to as the rainbow spectrum); it does occur throughout the electromagnetic spectrum, hence also in the infrared (IR) and the ultraviolet (UV) ranges. Each of the varied technical uses to which this phenomenon may be put calls for a polarizer to be employed for the particular wavelength range. In addition to the optical parameters, there are requirements to be met in terms of geometric dimensions, resistance and useful life.
Nanoparticles: What is their mode of action?
Spherical nanoparticles (colloids) of silver, embedded in sodium-silicate glass, absorb light at a wavelength of approx. 410 nm. The band form is roughly equivalent to a Gaussian distribution. The physical location is a function of various factors:
- Colloid size;
- Ambient material(s);
- Colloid material.
Polarization does not take place.
Things turn different when the colloids take the shape of spherical ellipsoids. Then, the ratio of semi-axes becomes an additional factor governing the physical location of the absorption bands as a function of the direction of oscillations relative to the semi-axes, i.e. the light is filtered in an approach depending on the polarizing direction. This causes one absorption band (- vector in parallel with the shorter semi-axis b) to travel into the UV range, whereas for the orthogonally polarized light the band travels through the visible-wavelength range as far as the IR. If colloids having different ratios of semi-axes are embedded in the glass, an interference of individual bands occurs, virtually resulting in a broadening of the absorption bands. The geometry of this absorption band is governed by the distribution curve through the ratios of semi-axes of the spheroids.
