Due to the inherent transparency of diamond across a wide wavelength range, optical methods of characterization have long been the go-to methods for characterizing inclusions, impurities, and color-causing defects. Except for diamond window applications, it is usually these features that make the diamond most interesting or valuable.
Diamond has the widest optical transparency band of all known solids, which ranges from 0.22 um (fundamental absorption edge) to the far-infrared. Only the intrinsic vibrational absorption band of moderate intensity between 2.5 and 7 um disturbs the perfection of diamond’s transparency in the infrared region. Being transparent in the ultraviolet, visible and infrared spectral regions, diamond provides many opportunities for lattice defects to reveal the optical activity of their electronic and vibrational transitions. The large bandgap energy (5.49 eV) is a particularly favorable condition in the case of luminescence, because the radiative electronic transitions require that both the ground and excited electronic states lie within the bandgap. The high mechanical hardness and thermal conductivity of diamond greatly support its optical applications making diamond optics very stable and resistant in many respects. When discussing the optical properties of a material its optical centers should be considered carefully, because their properties and abundance determine almost all optical performance of the material. Besides, the content of optical centers is the main parameter of the optical characterization of the material. So far more than 150 vibrational and more than 500 electronic optical centers have been detected in diamond within the spectral range of 20 to 0.17 um;
Optical Properties of Diamond: A Data Handbook (c) 2001
by Alexandre Zaitsev
Available Measurements (Metrology):
Optical Microscopy
Incident visible light (400 nm-700 nm wavelength) is observed in reflection or transmission as visible light. Observation under magnification with visible light is all that is required to evaluate the color and clarity of a diamond gemstone. As such, it is the oldest and best understood of the methods listed here.
Birefringence
Two linear polarizing filters, which each permit visible light to pass of a certain polarization are placed 90 degrees rotated from each other, nominally blocking all light. A diamond is placed between the two polarizing filters. Strain features within the diamond shift the polarization of the light which has already passed through the first filter, which shows up as bright regions when viewed through the second filter. A diamond with no strain, as is common only in HPHT-grown material, does not influence the polarization of the light passing through it, thus appearing entirely dark, featureless, near-invisible. Low levels of strain produce white light when viewed in this way. Higher levels of strain induce a yellow color in the transmitted light, while still higher levels of strain produce all the colors of the rainbow.
It can be helpful to visualize strain as twisting the light, which then makes it intuitive that birefringence is indicative not only of the absolute amount of twist, but also can show gradients. Sharp, well-defined strain features, particularly with color such as in the right-most image, are of much more concern as a fracture risk than the diffuse features visible in the left and center image.
A birefringence image is strongly dependent on the thickness of the material it is examining. A diamond that is twice as thick will induce twice as much polarization twist even if the strain concentration is unchanged.
Infrared Spectroscopy (FTIR)
Infrared spectroscopy of diamond is useful to evaluate the overall transmissivity of the diamond as well as for the identification of certain impurity color centers.
Despite the “perfect” transmissivity in the UV and visible (excepting the characteristic features above) the theoretical limit for diamond transmissivity is a mere 72% owing to its high index of refraction as compared to air. For optical diamond applications in the IR, it would not be uncommon to specify a transmissivity of >70% at the wavelengths of interest. For this reason, even a “perfect” diamond can appear to be light gray due to the light lost due to reflection. When in a media other than air, or with the application of an anti-reflection (AR) coating, transmission of diamond can approach 100%. This same hard-to-imitate high index of refraction has long been useful to gemologists seeking to quickly and unequivocally identify a gemstone as diamond.
UV-VIS
Like the infrared spectrum above, absorption and transmission measurements can be collected in the ultra-violet and visible spectra (200 nm -700 nm). Diamonds whose color is due to absorption (primarily gray, yellow, and brown) can have that color absorption feature measured quantitatively via UV-VIS.
UV Fluorescence
UV Fluorescence is a technique of primary importance to those in the gem trade seeking to understand the source of a diamond. Not knowing any applications for this technique in technical diamond markets, suffice it to say that natural diamonds and lab-grown diamonds frequently respond differently from each other in the presence of short-wave and/or long-wave UV.
Raman
Named after Dr. C.V. Raman, who discovered/invented the technique, Raman spectroscopy illuminates the diamond with a bright monochromatic source, such as a laser. Features within the diamond, and notably the diamond lattice itself, will absorb that light, frequency-shift it, and re-emit in such a way that it can be measured via a spectrometer.
Certain features, such as the characteristic 1332 cm-1 emission peak of diamond, have been measured precisely in both the peak width and peak center wavelength in order to derive information about the strain and crystalline perfection of the diamond. This level of close analysis requires a sophisticated and expensive (typically $25k-$300k) instrument.
Photoluminesence (PL and Cryo-PL)
Photoluminesence uses the same setup as a Raman instrument in that a laser illuminates the sample and a spectrometer measures the frequency-shifted emission. In practice, the techniques are done simultaneously and in the same way. Here, the change in name reflects a change in the physics phenomena involved.
Photoluminesence is an excellent and robust technique for the detection and quantification of graphitic carbon inclusions and nitrogen (via emission from nitrogen-vacancy complexes).
Because the PL peak from nitrogen is smeared out by room temperature phonon activity, a far more sensitive measurement can be obtained if the diamond is cooled to liquid nitrogen temperatures (approx -200 degrees C, or -321 F). When cooled in this way, the technique becomes known as cryogenic photoluminesence (Cryo-PL).