Bulk Optical Materials Research Articles

Double-pulse-laser volumetric modification of fused silica: the effect of pulse delay on light propagation and energy deposition.

Volumetric modification of dielectrics by ultrashort laser pulses is a complex dynamic phenomenon involving material photoexcitation and associated nonlinear processes. To achieve control over modification, it is necessary to gain a deep insight into the dynamics of laser-excited processes that can be realized using double-laser-pulse experiments with different time separations supported by numerical simulations. In this paper, we apply this approach to investigate fused silica modification with femtosecond laser pulses that provides time-resolved information about the dynamic behavior of the laser-excited bandgap material. It is shown that the laser-generated free-electron plasma causes a shielding effect for the following pulse with a characteristic duration of ∼600 fs after the pulse action. Within this time interval, the second pulse produces a reduced modification as compared to a longer time separation between pulses. For double pulses with different energies, it was found that the volumetric modification is stronger when a lower-energy pulse couples with material first. This is explained by the combination of the effects of the re-excitation of self-trapped excitons, which are generated as a result of free electron recombination and associated light shielding. Experimental results are supported by numerical simulations of double laser pulse propagation in nonlinear media based on Maxwell's equations. Our findings offer a route for better controlling the inscription of 3D photonic structures in bulk optical materials.

Room temperature synthesis of CdSe/CdS triangular nanoemitters and their stabilization in colloidal state and sol-gel glass.

Heterostructured cadmium-based core-shell nanoparticles (NPs) are the subject of research because of not only fundamental scientific advances but also a range of technological applications. To increase the range of applications of nanoparticles, it is possible to immobilise them in sol-gel glass that can be easily manufactured and shaped, keeping the properties of the dispersed particles. This allows the creation of new bulk optical materials with tailored properties, opening up opportunities for various technological applications such as lighting or sensing. Herein we report the synthesis of core-shell CdSe/CdS triangular-shaped nanoparticles under an atmosphere of oxygen and at room temperature. A detailed characterisation of the obtained NPs was carried out. The interesting effect of the gelling agent (tetra-n-butylammonium fluoride) on the triangular nanoparticles in solution and the stability of the emission properties over time was investigated. Sol-gel glasses with entrapped triangular NPs were prepared, and their photoluminescence properties were compared with those obtained in colloidal solutions.

Index Modulation Embedded in Type I Waveguide Written by Femtosecond Laser in Fused Silica.

Slit-shaped laser beams focused in bulk optical materials can realize embedded waveguides with circular cross sections consisting of positive index change type I traces. In these kinds of waveguide traces, a peculiar periodical refractive index modulation was observed in type I waveguides with two different femtosecond lasers. The direction of refractive index modulation can be controlled with the slit configuration, and its period can be controlled by mechanical perturbation of the stages and the scanning speed. We argue that platform perturbation and dynamical thermal transport processes during the scan are generating factors in the appearance of this modulation. The embedded microstructures in waveguides can provide spectrum modulation, which may have potential applications in optical sensing, filtering, and phase control.

Resonant dielectric metasurfaces in strong optical fields

Optical materials are undergoing revolutionary transformations driven by nanotechnology. Our ability to engineer structures at a scale smaller than the wavelength of light enables new properties and functionalities otherwise not available in natural bulk optical materials. A class of such components—dielectric metasurfaces—employs two-dimensional arrays of designer resonant nanoscale elements whose optical response is defined by their geometry. While linear regimes of interactions between dielectric metasurfaces and moderately intense light have already formed a mature field of applied research and engineering, new frontiers are being actively explored in the nonlinear optical regime describing interactions of metasurfaces with strong optical fields. In this Research Update, we cover the most recent progress along with several directions of research within the field of nonlinear optics of dielectric metasurfaces. Specifically, we review approaches to design and fabricate metasurfaces with high local field enhancements that facilitate nonlinear light–matter interactions, outline nonlinearity-enabled functionalities of dielectric metasurfaces, explore resonant metasurfaces in the strong-field non-perturbative regime, and discuss the implications of the time-variant refractive index in metasurfaces that interact with strong optical fields produced by laser pulses.

The effect of gamma-ray irradiation on bulk optical plastic materials

We present our study of the effect of gamma-ray irradiation on bulk optical plastic materials as follows: three types of sheets commonly used for eyeglass lenses (CR-39, MR-8 and Trivex), cyclo olefin polymer ZEONEX E48R, styrene methyl methacrylate NAS21, polymethyl methacrylate PLEXIGLAS®7N, polyphenylsulfone EUROPLEX® PPSU, and poly(n-methyl methacrylimide PLEXIMID®8813. We used the panoramic gamma-radiation facility with 60Co source with applied doses 14.6 kGy and 54.2 kGy. For the thought properties, we measured the transmission spectra from 200 to 1800 nm and refractive indices using the prism coupling technique at six wavelengths 532.0, 654.2, 846.4, 1308.2, 1549.1, and 1652.1 nm. We also studied the possibility of using several approximation methods for evaluating the refractive indices including Cauchy, Cauchy–Schott, Herzberger, Sellmeier, Sellmeier-2, and Sellmeier with an infrared correction. The measurements showed that the most precise determination model of the refractive indices was obtained by the Sellmeier approximation with an infrared correction. The transmission spectra measurement showed that the CR-39 and EUROPLEX® PPSU polymers did not change significantly after the irradiation and the refractive indices measurement showed that ZEONEX E48R and EUROPLEX® PPSU did not change the refractive indices values.

Growth and Decay Kinetics of Radiation-Induced Attenuation in Bulk Optical Materials

Using a dedicated setup, the growth and decay kinetics of the radiation-induced attenuation (RIA) have been characterized during and after steady-state 40-keV X-ray exposures of bulk (1-cm thickness) optical materials. Such materials served to design the lenses of radiation-hardened cameras of the FURHIS Project intended to withstand MGy (SiO2) dose levels associated with ITER remote handling operations. RIA is monitored in situ in the 400- to 800-nm spectral domain for glass types ranging from radiation sensitive (BK7) to radiation hardened ones: pure silica, BK7G18, K5G20, and SF6G05 from SCHOTT. In addition to the detailed characterization of these various glasses, our study highlights the systematic underestimation of the RIA measured by the usual qualification approach consisting in postirradiation RIA measurements after dose steps. The error due to this underestimation is shown to strongly depend on the glass type, very large for the BK7G18 glass. This material is associated with high transient RIA, its loss levels exceed the ones of its nonrad-hard counterpart (BK7) at low dose levels. For the development of MGy radiation-hardened systems using these glasses, a vulnerability study of the in situ response of glass lenses appears mandatory. By improving our knowledge on the origin of these transient losses, appropriate hardening techniques to maintain the camera photometry budget will be tested in the future.

「バルク光学材料開発の最前線」特集号によせて

「バルク光学材料開発の最前線」特集号によせて

Laser interactions with embedded Ca metal nanoparticles in single crystal CaF2

Single crystal calcium fluoride (CaF2) is an important material for vacuum-ultraviolet optics. Nevertheless, prolonged exposure to energetic radiation can color the material by producing calcium metal nanoparticles. We compare the effectiveness of laser conditioning treatments at wavelengths ranging from the near infrared to the deep ultraviolet in removing this coloration. Treatments at 157, 532, and 1064 nm can significantly reduce the visible coloration due to nanoparticles. In contrast, irradiation at 248 nm has little effect at fluences below the damage threshold for the material employed in this work. We present evidence that the effect of laser irradiation on coloration is principally thermal and is largely confined to the first 50 ns after each laser pulse. We attribute the wavelength dependence of the bleaching process to the wavelength dependence associated with Mie absorption by metal nanoparticles. The consequences of these observations with regard to laser conditioning processes in bulk optical materials are discussed.

Refractive-index dispersion measurement of bulk optical materials using a fiber raman laser widely tunable in the visible and near-infrared

We propose a simple, highly sensitive fiber-optic autocollimation method for refractive-index dispersion measurement of solid-state and liquid bulk optical materials using a double-pass fiber Raman laser with Littrow-prism-tuned emission. The optical fiber is a key element of the scheme and serves simultaneously as a point laser source for the test, as a highly sensitive point receiver (or spatial filter) of the autocollimation backreflectance signal and as a medium for nonlinear frequency conversion and generation of a broadband continuum spectrum. When the Raman medium is a graded-index multimode fiber with powerful pumping (over 100 kW) using the second harmonic of a Q-switched Nd:YAG laser (λp=532nm), we obtain widely tunable (0.54-1.01 μm) generation in both the visible and near-IR ranges. The results obtained in the refractive-index dispersion measurements are fitted to the Sellmeier dispersion equation and the standard deviation of the experimental data from the analytical curve does not exceed 5x10-5.

A fiber-optic autocollimation refractometric method for dispersion measurement of bulk optical materials using a widely tunable fiber Raman laser

A simple and highly sensitive fiber-optic autocollimation method for refractive-index dispersion measurement of solid-state and liquid bulk optical materials is presented. The method is based on the use of a double-pass multimode fiber Raman laser which generates widely tunable emission in a broadband (0.54-1.01 /spl mu/m) continuous spectral range at pumping with the second harmonic of a Q-switched Nd:YAG laser. The optical fiber is used in a specific multifunctional regime where, because of its micrometric core dimensions, it serves simultaneously as a point laser source for the formation of a collimated input emission, as a highly sensitive receiver of the autocollimation backreflectance, and as a medium for nonlinear frequency conversion and generation of a broad-band continuum spectrum. The experimental results obtained in the refractive-index dispersion measurements are fitted to the Sellmeier dispersion equation and used to determine the material dispersion and additional dispersive characteristics of the test optical material.

Breakdown threshold and plasma formation in femtosecond laser–solid interaction

Combining femtosecond pump–probe techniques with optical microscopy, we have studied laser-induced optical breakdown in optically transparent solids with high temporal and spatial resolution. The threshold of plasma formation has been determined from measurements of the changes of the optical reflectivity associated with the developing plasma. It is shown that plasma generation occurs at the surface. We have observed a remarkable resistance to optical breakdown and material damage in the interaction of femtosecond laser pulses with bulk optical materials.

Wet chemical synthesis of bulk optical materials

Wet chemical processing has a large number of both proven and potential applications in different areas of glasses and ceramics. In optics where homogeneity and high purity are of utmost importance, the inherent advantages of sol-gel methods can be fully exploited. The optical applications of wet chemical methods are widespread and range from direct casting of SiO2 lenses to semiconductor-doped glasses. The most notable benefits of wet chemical approaches and progress achieved to date in a number of the most attractive representative areas are surveyed.

Optical properties of chalcogenide glasses

Over the past two decades chalcogenide glasses have been researched in order to assess their suitability as passive bulk optical component materials for 3–5μm and 8–12μm infrared applications. This research has led to a greater understanding of the physical properties of these materials, and the present paper concentrates on the optical properties and applications of these bulk chalcogenide glasses. The various factors influencing the intrinsic and extrinsic optical loss mechanisms in materials are discussed, numerical data on the basic optical properties of chalcogenide glasses is presented and applications are discussed.

Variable refractive index optical coatings

The discrete and restricted values of refractive index of the bulk optical materials at present available are a serious limitation on the usefulness of these materials for optical coatings. This limitation can be overcome by utilizing the atom-by-atom condensation feature of the growth of vapour-deposited thin films, which allows the homogeneous mixing of different materials irrespective of their solubility restrictions. We have used this feature of co-deposition of different materials to form mixed optical materials of variable refractive index, the variation being determined by the composition of the source material. Measurements of the optical constants of these films, prepared by co-evaporation of mixtures of ZnS and MgF 2 of various compositions, have been made. The refractive index of the mixed films is found to be in good agreement with the values predicted on the basis of the Lorentz-Lorentz theory. In addition, the optical equivalence of alternate layers of ZnS and MgF 2 with step thicknesses ranging from 5 to 250 Å has been studied. For step thicknesses less than 100 Å, the optical properties of the composite films are equivalent to those for the homogeneously mixed films. For larger step thicknesses, considerable and complicated interference effects are observed. Thus, variable refractive index composite films can be prepared by (a) co-deposition and (b) deposition of alternate discrete layers of different materials as long as the step thickness does not exceed about 100 Å. Furthermore, these techniques of obtaining optical films of mixed materials can be extended to any combination of materials and therefore they open up a new field in materials technology.