Optics in Quantum structures

OPTICS IN QUANTUM STRUCTURES

Optical properties

Nanomaterials have attracted much attention due to their novel optical properties that are markedly different from bulk materials.

The reason for this change in optical properties is due to the quantum confinement of electrons in nanomaterials and surface plasma resonance.

Surface Plasmon

Surface plasmon is the natural oscillation of electron gas inside the nanosphere. It can appear on the surface of bulk solids, thin films, and nanoparticles. On the surface of bulk solids, surface plasmons appear as a propagating wave parallel to the surface.

When the nanosphere size is smaller than the wavelength of incident light, the frequency of surface plasmon becomes comparable to the frequency of radiation due to constructive interference. Then a resonance occurs and surface plasmon resonance (SPR) is generated (Fig. 5.27).

Other Factors that Contribute to Optical Properties

Efficient energy and charge transfer in nanoscale dimension further contribute to the novel properties. Linear and nonlinear optical properties of the material can be finely tuned by controlling their dimension and surface chemistry.

The optical properties of nanomaterials depend on their size, shape, surface characteristics, doping, and interaction with the surrounding environment or other nanostructures.

A change in size of the CdSe semiconductor nanoparticles alters their optical properties. (A 2.3 nm CdSe emits blue light, whereas a 5.5 nm CdSe emits red light) A change in the size of metal nanoparticles causes some change in their optical properties.

Quantum Size Effect

Quantum size effect is most significant in semiconductor nanoparticles. In semiconductors, the bandgap energy is of the order of a few electron volts.

dia As the size of the particle increases, absorption shifts towards the shorter wavelength (blue shifts) indicates increase in the bandgap energy.

Luminescence

The Semiconductor nanoparticles show luminescence when excited by electrons, photons, or electric field. Photoluminescence (fluorescence or phosphorescence) occurs when the external stimulus is due to photons.

When luminescence is observed by the application of an electric field, it is termed electroluminescence.

It can be tuned to the desired wavelength for nanomaterials as their bandgap can be tuned with particle size.

Nanomaterials show high quantum efficiency for cathodoluminescence also, in which luminescence is produced by electrons of very high energy incident on materials.

Finally, thermoluminescence is very strong for nanomaterials which have defect levels and larger number of surface atoms that can act as efficient electron/hole traps.

Optical Reflectance

Optical reflectance is defined by the fraction of incident light reflected from the surface of a material. Metals show high reflectance. It is due to the presence of a partially filled conduction band. Thus, absorption and reflection of photons take a continuum of energies from the infrared to visible region.

Semiconductors, on the other hand, show low absorption and reflectance in infrared range and increases drastically at the bandgap and increases further in the ultraviolet region.

Direct optical transitions for bulk and nanomaterials are shown in fig. 5.28. Absorption edge of nanomaterials denotes trailing effect as compared to bulk due to variation in density of states, surface restructuring and a random distribution of impurities.

Applications Based on Optical Properties

There are some fascinating applications of the optical properties of nanomaterials in the areas of optical detectors, lasers, sensors, imaging, phosphors, displays, solar cells, photocatalysis, photoelectrochemistry, and biomedicine. Some of these applications are listed as follows.

1. Suitable for optoelectronic materials such as switches, amplifiers, gratings, splitters, isolators, lasers, and detectors

2. Widely used in polymers to increase their refractive index, which makes them suitable as optical components

3. Useful in preparing abrasion-resistant coatings whose optical clarity can be enhanced

4. Nanoparticles in castings improve shielding against electromagnetic fields in computers

5. LCDs and organic LEDs developed using nanomaterials show better resolution of images.