Annelise During

Observation by photothermal microscopy of increased silica absorption in laser damage induced by gold nanoparticles

In order to understand laser-induced damage in glass, we subjected engineered SiO2 thin films containing sub-micron gold inclusions to high fluences, and observed the results using several means of analysis. We found decoupling in time between the emission of gold and that of silicon with samples containing gold spheres of diameter 3 nm. We have analyzed the changes in the silica optical absorption at 1064 nm, using photothermal deflection microscopy. We find, upon exceeding a sharp fluence threshold, a thousand-fold increase in absorption of the silica matrix around the inclusion. We conclude that ions from the inclusion permeate the surrounding silica, and form a highly absorbent mixture.

Integrated photothermal microscope and laser damage test facility for in-situ investigation of nanodefect induced damage.

An integrated setup allowing high resolution photothermal microscopy and laser damage measurements at the same wavelength has been implemented. The microscope is based on photothermal deflection of a transmitted probe beam : the probe beam (633 nm wavelength) and the CW pump beam (1.06 microm wavelength) are collinear and focused through the same objective. In-situ laser irradiation tests are performed thanks to a pulsed beam (1.06 microm wavelength and 6 nanosecond pulse). We describe this new facility and show that it is well adapted to the detection of sub-micronic absorbing defects, that, once located, can be precisely aimed and irradiated. Photothermal mappings are performed before and after shot, on metallic inclusions in dielectric. Results obtained on gold inclusions of about 600 nm in diameter embedded in silica are presented.

Multiwavelength imaging of defects in ultraviolet optical materials

Laser-induced damage in bare glass substrates and thin films has long been widely acknowledged as a localized phenomenon associated with the presence of micrometer and submicrometer scale defects. The scanning of both optical absorption and scattering allows us to discriminate between absorbing and nonabsorbing defects and can give specific information about the origin of the defects. We investigate the spectral properties of defects in thin films and fused-silica surfaces. Absorbing and scattering defects are studied at different wavelengths in the ultraviolet, visible, and infrared ranges. Absorbing defects are shown to be highly wavelength dependent, whereas we have observed significant correlation between scattering defects.

Photothermal microscopy and laser damage in optical components

The development of applications of high power lasers require new characterization techniques for studying behavior of optical materials under intensive illumination, laser damage phenomena. Destructive investigations in silica have led to the conclusion that absorbing defects, typically a few nanometers in size, were responsible for laser damage initiation. The measured precursor densities are very low. The understanding of the true nature of these defects and damage phenomena requires the development of non destructive evaluation techniques with both high spatial resolution and high detectivity. The capability of collinear photothermal deflection to reach sub-micrometric resolution by reduction of the pump beam diameter has been theoretically and experimentally explored. Its ability to detect single absorbing particles has been studied. Currently 100-nm-diameter gold inclusions can be imaged with a signal-to-noise ratio of 8 at the wavelength 1064nm. Such a photothermal microscope has been coupled with an experimental set-up allowing damage threshold measurement at the same wavelength. Thus behavior of 100-nm-gold inclusions in silica can be studied under irradiation. Further improvements by decreasing wavelength, increasing modulation frequency and by using piezoelectric translation stages, will allow to study 10-nm-inclusions. We present an overview of last developments in the field of photothermal microscopy in connection with laser damage.

Photothermal microscopy for the study of laser damage initiation by measurement of absorption modification of silica seeded with gold nanoparticles

In order to understand the mechanisms of laser damage initiation, we study “model” samples constituted of pure silica seeded with 3 nm gold particles. Numerical simulations are performed with a 1-D hydrodynamic code to determine the laser light absorption by a spherical nanoparticle. This code also simulates the thermal conduction, radiative transfer and ionization by UV light emitted by the heated metallic particles. The setup used for experimental studies is a high resolution, high sensitivity photothermal microscope. This setup allows correlation between optical absorption and laser irradiation. We observe the silica transformation in terms of absorption modification as a function of the irradiation fluence. The morphology of irradiated samples surface is observed thanks to “Nomarski” and “atomic force” microscopy and compared to photothermal microscopy results. A correlation is observed between flaked silica and strongly absorbing areas.

Photothermal microscopy for in-situ study of laser damage induced by gold inclusions

A photothermal microscope has been combined with an experimental set-up allowing damage threshold measurements at the same wavelength. The microscope is based on photothermal deflection of the transmitted probe beam: the CW pump beam (1.06 μm wavelength) and the probe beam are collinear and focused through the same objective. The diameter of the pump beam on the sample surface is 1 μm. Laser damage thresholds are measured thanks to a pulsed beam (1.06 μm wavelength and 6 nanosecond pulse) and the spatial position of the pulsed beam is controlled by a CCD camera. This experimental setup has been used to study the behavior of metallic inclusions in dielectric materials in laser damage processes. Results are presented with gold inclusions of about 600 nm in diameter in silica.