Optical characterization is a key method for testing the optical quality and performance of the components of an optical system, as well as giving information about the structure of a surface. Optical quality is related to the optical properties of such elements and has also been widely used to study the interactions between particles inside materials, for which supercontinuum lasers open a wide range of possibilities.
Different methods for optical characterization can be described. In these techniques, the photons coming from a light source are used to illuminate a sample of interest. Due to the interactions between light and matter, optical transitions (absorption, excitation and emission) inside the material take place. After these interactions, light with a specific intensity and spectrum is emitted. This radiation is specific for each particle inside of a material and it shows their specific absorption and emission bands (Figure 1), which give information about the part of the spectrum that was absorbed or emitted by the material.
Such specificity resides in the fact that each atom has a different electronic structure, meaning that for them to undergo a transition, the incoming photons need to have a specific energy and wavelength, that must match the energy between the lower and the excited states of that atom.
Figure 1. Absorption and emission bands of hydrogen atom in the Visible range of the spectrum.
Spectroscopy for optical characterization of devices
Spectroscopy is one method that can be used for the optical characterization of devices . It consists of a technique that studies the electromagnetic field of the light that is absorbed or emitted by a surface. For such purpose, several elements are needed: a light source and an optical device called spectrometer (Figure 2). A spectrometer is an optical system that comprises a component that is able to disperse or split the incoming light like a prism or a diffraction grating, and a detection device with which information about the physical properties of the material are provided by the emitted or absorbed wavelengths. Traditional light sources that have been used in combination with spectrometers include LEDs, arc lamps, tungsten lamps and different laser diodes.
Figure 2. Schematic representation of the working principle of a spectrometer.
Several types of spectroscopy techniques can be described. They can be classified according to the light source that is used to perform the measurements (i.e infrared spectroscopy, UV-Vis spectroscopy, nuclear magnetic resonance spectroscopy or x-ray spectroscopy), or by the nature of the interactions between light and matter that is studied (absorption, transmission or reflectance spectroscopy). In this second type of classification, the difference among the techniques resides in the part of the electromagnetic radiation that is being studied: absorbed radiation by the sample (Absorption Spectroscopy), transmitted radiation by the sample (Transmission Spectroscopy), or reflected radiation by the sample (Reflection Spectroscopy) .
One technique that is particularly useful for Chemistry and Life Sciences is absorption spectroscopy, also referred to as spectrophotometry. It measures the absorbed radiation from a material, providing information about its composition .
With the optical configuration explained before (Figure 2), the illuminating radiation will pass through the sample. Some of this radiation passes without loss, although another part of the light is attenuated or absorbed. The obtained spectrum will show the absorption bands of the sample (Figure 1), which can be compared with the existing literature about the absorption spectrum of previously known molecules (such as C-C or H2O transition wavelength ranges) leading to the knowledge of the molecular structure of the sample.
For absorption spectroscopy, several light sources have been traditionally implemented and sometimes more than one illumination system is needed in order to cover the full spectrum of a certain material. In this scope, Ultraviolet-Visible Infra-Red wavelengths are particularly interesting for the fields of chemistry and life sciences. The reason for that is related to the wavelengths of the incoming photons that cause the transitions of the molecules falling in that part of the spectrum .
Alternative instrumentation for absorption spectroscopy:
Some examples of implemented light sources include LEDs, that emits narrow bandwidth light; or halogen lamps, that emits a wide range of wavelengths including near-UV light to near-IR light, although their efficiency and their life span are short due to the emission of heat, that can lead to the damage of the sample. For these reasons, our supercontinuum laser called Iceblink supposes a versatile tool for spectroscopic measurements, where the stability (< 0.5 % std. dev), flat spectrum and broad emission are essential requirements.
Regarding this last important requirement, with the Iceblink, the surface of interest can be illuminated with a spectrum that covers the whole visible and NIR light (from 450 to 2300 nm). Consequently, as each molecule has its own absorption spectra, this means that with our laser, it is possible to identify and study the structure of a material which is inhomogeneous (i.e. whose composition comprises different molecules).