Absorption In Optical Materials Research Articles

Investigation of allowed and forbidden electronic transitions in rare earth doped materials for laser cooling application by thermal lens spectroscopy

In this work Thermal Lens Spectroscopy (TLS) is used for sensitive characterization of absorption in optical materials with particular application for solid-state laser cooling (optical refrigeration). TLS is used to characterize key parameters involved in 4f-4f transition in Yb3+:YLF and 4f-5d transition in Ce3+:YAG systems. The results confirm the excellent laser cooling results found in previous works on Yb3+:YLF crystal. In addition, although no cooling effect was observed in Ce3+:YAG sample, the TLS showed that this material exhibits promising characteristics towards laser cooling applications, such as high external quantum efficiency (>92%), and low parasitic background absorption of (1.2 ± 0.2) × 10−4 cm−1 at 885 nm. The key mechanisms that appear to inhibit net cooling in Ce3+:YAG are attributed to the intrinsic excited state absorption and/or 4f-4f transitions between the two lowest levels of Ce3+. This investigation was turned possible due to the high sensitivity of TLS for very low absorptions.

Optimally designed narrowband guided-mode resonance reflectance filters for mid-infrared spectroscopy

An alternative to the well-established Fourier transform infrared (FT-IR) spectrometry, termed discrete frequency infrared (DFIR) spectrometry, has recently been proposed. This approach uses narrowband mid-infrared reflectance filters based on guided-mode resonance (GMR) in waveguide gratings, but filters designed and fabricated have not attained the spectral selectivity (≤ 32 cm−1) commonly employed for measurements of condensed matter using FT-IR spectroscopy. With the incorporation of dispersion and optical absorption of materials, we present here optimal design of double-layer surface-relief silicon nitride-based GMR filters in the mid-IR for various narrow bandwidths below 32 cm−1. Both shift of the filter resonance wavelengths arising from the dispersion effect and reduction of peak reflection efficiency and electric field enhancement due to the absorption effect show that the optical characteristics of materials must be taken into consideration rigorously for accurate design of narrowband GMR filters. By incorporating considerations for background reflections, the optimally designed GMR filters can have bandwidth narrower than the designed filter by the antireflection equivalence method based on the same index modulation magnitude, without sacrificing low sideband reflections near resonance. The reported work will enable use of GMR filters-based instrumentation for common measurements of condensed matter, including tissues and polymer samples.

Nonlinear absorption in optical materials at a wavelength of 193 nm

The coefficients of two-photon absorption of 30- and 70-ns ArF laser pulses in high-purity CaF2, BaF2, and Al2O3 crystals are measured by the method of nonlinear transmission to be 2.2 ± 0.8, 2.3 ± 0.8, and 5.6 ± 2.2 cm GW-1, respectively. The thresholds of the laser breakdown of the surface of crystals are determined.

Grain size effect on the spectral characteristics of CV using photoacoustic spectroscopy

Photoacoustic spectroscopy (PAS) offers a unique method to measure optical absorption of materials in the powder form. In this work we employ PAS to study the change in the peak absorption of the dye crystal violet (CV) in the powder form, as a function of the grain size which was carefully controlled. It is observed that as the grain size is comparable to the incident radiation wavelength, there is an increase in the blue shift with decreasing the grain size.

Measurement of surface absorption in optical materials by the method of laser photothermal radiometry

The present work is devoted to the development of the method of laser photothermal radiometry with a view to increasing its spatial resolution over the depth of the specimen when surface layers of the substance of thickness about 1 μm or less are to be investigated. As an example, results of an investigation of radiation absorption in the surface layer of a lithium niobate crystal are presented. The absorption index measured at a wavelength of 1.08 μm was ∼0.6 cm−1.

Optical Depth Profiling of Thin Films by Impulse Mirage Effect Spectroscopy. Part I: Theory

Impulse mirage effect/photothermal deflection spectrometry may be used to detect depth-dependent optical absorption in materials, through the time dependence of the probe beam deflection signal occurring in response to sample irradiation with a short excitation pulse. In this work a theoretical expression was derived for the normal and transverse photothermal deflection signals which occur in a sample with homogeneous thermal properties but where optical absorptivity varies with depth from the surface. An analytical solution of moderate simplicity is obtained for several cases of experimental interest, with three-dimensional heat conduction effects included. The depth profile resolution obtained with the mirage effect method is critically dependent on the distance between the sample layer probed and the offset position of the probe beam in the fluid layer above the sample. Saturation conditions and conditions for obtaining optimal depth resolution of continuous and discrete optical profiles are examined in detail.

A technique for measuring radiation-induced infrared absorption in optical materials located in or near a nuclear reactor

A technique for measuring radiation-induced absorption in optical materials located in a nuclear reactor is given. This technique uses optical fibers to couple a probe beam into the reactor volume and requires a minimum of effort to facilitate. Reactor vibrations have a minimal effect on the optical alignment and, consequently, accurate absorption measurements can be made. The technique is illustrated by measurements of radiation-induced absorption in infrasil silica at 1.06, 1.73, and 2.03 μm.

UV photon-induced absorption in multilayer dielectric mirrors

We have begun measurements of UV-induced absorption in optical materials using an in situ testing system we have constructed. Samples are exposed in a vacuum system to the intense, tunable photon beam produced by the TOK undulator mounted on the NSLS VUV ring. The absorption of the samples is measured in the visible spectral region during the exposure. We present our results concerning the dependence of the induced absorption on the photon energy and flux. Using a differentially pumped isolation line, we can also modify the residual-gas environment around the samples during exposure to study the deposition of carbonaceous films. We describe our measurement of the carbon film growth caused by UV interaction with CO gas.

Theory and optimization of the parameters of a polarization method for the measurement of weak absorption in optical materials

A theoretical analysis is made of a polarization calorimetric method for the determination of weak absorption in optical materials. Several variants of the method are considered and the optimal conditions for its application are identified. The sensitivity of the method is ~1 dB/km for a sample 1 cm thick. It is shown that the method can be used to determine absorption in optically anisotropic materials.

Transmittance and reflectance of crystalline quartz and high- and low-water content fused silica from 2 μm to 1 mm

The transmittances and reflectances of cultured crystalline quartz, Suprasil, Suprasil W, and Infrasil were compared over the wavelength region from 2 to 1000 μm. The high-water content of Suprasil and the low-water content of cultured crystalline quartz, Suprasil W, and Infrasil were determined by their transmittances measured at 2.73 μm where water content causes high absorption in optical materials. The fact that the fused silicas, both with high- and low-water content, had identical far-IR transmittances and that their transmittances were greatly inferior to that of crystalline quartz led to the conclusion that their inferior transmittance is due to their amorphous structure and not to their water content.