The Verdet constant is an optical "constant" that describes the strength of the Faraday effect for a particular material.
The Verdet constant for most materials is extremely small and is wavelength dependent. It is strongest in substances containing paramagnetic ions such as terbium. The highest Verdet constants are found in terbium doped dense flint glasses or in crystals of terbium gallium garnet (TGG). This material has excellent transparency properties and is very resistant to laser damage.
Although the Faraday effect is not chromatic (i.e. it doesn't depend on wavelength), the Verdet constant is quite strongly a function of wavelength. At 632.8 nm, the Verdet constant for TGG is reported to be -134 rad/T·m whereas at 1064 nm, it has fallen to -40 rad/T·m. This behavior means that the devices manufactured with a certain degree of rotation at one wavelength, will produce much less rotation at longer wavelengths. Many Faraday rotators and isolators are adjustable by varying the degree to which the active TGG rod is inserted into the magnetic field of the device. In this way, the device can be tuned for use with a range of lasers within the design range of the device. Truly broadband sources (such as ultra-short pulse lasers and the tunable vibronic lasers ) will not see the same rotation across the whole wavelength band.
The Verdet constant is named after the French physicist Émile Verdet .