University of Bath

Dept. of Physics

Introduction to Optical Spectroscopy

What is optical spectroscopy?

Most physicists take the word spectrum to mean a graph of intensity versus energy for some type of particle or wave. In optical spectroscopy, the wave concerned is an electromagnetic wave (that is, a light wave, hence the term optical) and optical spectroscopy thus consists of the measurement of the intensity of the light emitted by the light source of interest as a function of the energy or, equivalently, the wavelength of the light. It is the wavelength (or mixture of wavelengths) of a light beam that determines its colour.

The materials that interest us are semiconductors and those that we study have the ability to emit light when they are held at low temperatures and are illuminated by a laser (a powerful light source). There is a decrease in the energy of the light waves emitted by the semiconductor materials; this corresponds to an increase in the wavelength of the light emitted, or a shift in its colour from the blue part of the spectrum towards the red. This is shown in the picture below, in which a green laser beam shines on a ruby crystal which then emits red light; this phenomenon is known as photoluminescence.

Note that the spectrum of the light emitted by the ruby is not as intense as that of the incident laser beam (the difference is actually much larger than shown here) and note also that the ruby spectrum is not simply a shifted replica of the laser spectrum, but consists of more than one line. The explanation of this effect and of the processes that generate these different lines gives a very detailed understanding of the electronic processes occurring within the crystal and, in a semiconductor material, this understanding can be extremely valuable in optimising its properties for device applications.

What equipment is required?

Since the light beam emitted by the source must be broken up into its different components with different wavelenths (and, thus, colours), the most familiar form of spectroscopic tool is the prism, which is famous for its ability to reveal the spectrum of visible light. However, diffraction gratings have several advantages and are used in most modern instruments, together with detectors sensitive enough to detect single light wave-packets (photons). As suggested above, for the spectroscopy of semiconductors, a laser is necessary to cause the emission of light from the sample. Finally, since the samples must usually be held at low temperatures, a cryostat is required in which the sample is cooled to as low as 1.5 degrees above absolute zero (the cryostat has windows in order that optical experiments are still possible while the sample is cold).

What is Raman spectroscopy?

Experimentally, Raman spectroscopy is similar to photoluminescence spectroscopy. However, in the case of Raman spectroscopy, the light emitted after one special type of process is investigated. In photoluminescence (above), the wavelength of the laser could be changed to, for instance, the blue region, but the ruby spectrum would be unchanged and would remain red. However, in the process of Raman scattering, exact amounts of energy (or quanta) are transferred to or from the material by the incident light. The sizes of these quanta are determined by basic properties of the material (for instance, the frequency of vibration of its atoms) and so, unlike the case of photoluminescence, the useful information is now the energy shift between the incident and emitted light waves, rather than their separate energies. Raman spectroscopy is the measurement of these energy shifts, often called Raman shifts.

What is spin-flip Raman spectroscopy?

One of the most specialised and unusual forms of Raman spectroscopy involves the application of a strong magnetic field to a semiconductor (also still at low temperatures). The electrons (whose behaviour makes semiconductors what they are and determines all their useful properties) can be thought of as being similar to small bar magnets and must, therefore, line up in the applied magnetic field. A fixed amount of energy is required to flip an electron into the "upside-down" position, the amount being determined by the strength of its magnetic moment which is itself determined by the details of the material of interest. Since light is able to flip the orientation of the electrons, one can determine the strength of the magnetic moment of the electrons from the Raman shift of the emitted light waves. From this information, one can obtain a better understanding of the factors governing the more familiar and useful properties of the semiconductor, for example, the ability of its electrons to carry an electrical current.