These parameters are needed for the further development of diamond lasers.
A method of measuring two key parameters of H3 color centers in diamond has been created as part of a continuing effort to develop tunable, continuouswave, visible lasers that would utilize diamond as the lasing medium. (An H3 color center in a diamond crystal lattice comprises two nitrogen atoms substituted for two carbon atoms bonded to a third carbon atom. H3 color centers can be induced artificially; they also occur naturally. If present in sufficient density, they impart a yellow hue.) The method may also be applicable to the corresponding parameters of other candidate lasing media. One of the parameters is the number density of color centers, which is needed for designing an efficient laser. The other parameter is an optical-absorption cross section, which, as explained below, is needed for determining the number density.
The present method represents an improvement over prior methods in which optical-absorption measurements have been used to determine absorption cross sections or number densities. Heretofore, in order to determine a number density from such measurements, it has been necessary to know the applicable absorption cross section; alternatively, to determine the absorption cross section from such measurements, it has been necessary to know the number density. If, as in this case, both the number density and the absorption cross section are initially unknown, then it is impossible to determine either parameter in the absence of additional information.
In the present method, the needed additional information is extracted from the saturation characteristics of the bulk material: As a laser gain medium (in this case, diamond) absorbs more power from a pump light beam, it begins to absorb a smaller fraction of the incident power. The intensity level at which this saturation effect is observed depends on the atomic-absorption cross section of the material (in this case, the absorption cross section per H3 color center), but not on the number density of color centers. Thus, by measuring the saturation characteristics of the material, the absorption cross section per color center can be determined unambiguously, without knowledge of the number density. Once the absorption cross section is known, the number density of color centers can be determined, in the conventional manner, from measurements of absorption in the bulk material at intensity below the saturation level.
More specifically, the method is based largely on the following principles:
- The absorption cross section (σ) of a
single color center can be obtained
from a set of measurements of the absorption
saturation irradiance (Isat)
and the decay time (τ) of the fluorescence
excited by the absorption. For
this purpose, Isat can be identified as
the level of the probe-beam irradiance
above which fluorescence decreases
and the fraction of incident probe
light transmitted through the specimen
increases. The absorption cross
section can then be calculated by use
of the equation
σ = hc/λpIsatτ,
- where h is Planck’s constant, c is the speed of light, and λp is the wavelength of the monochromatic light used to probe the specimen.