ABSTRACT
An optical picowatt calorimeter at 4 K is demonstrated to measure absorption spectra from 330 nm to 1700 nm of optical samples. The minimum detectable absorbed power is 10 pW, resulting in absorption sensitivities of 0.3 ppm for 30 µW of incident power from a tunable source and 0.6 ppb for 15 mW laser excitation. Active temperature stabilization of main components of the cryogen-free cryostat and the use of paramagnetic temperature sensors with superconducting quantum interference device readout provided a low noise environment (700 nK temperature rms) and nK temperature resolution.
ABSTRACT
With ever-increasing laser power, the requirements for ultraviolet (UV) coatings increase continuously. The fundamental challenge for UV laser-resistant mirror coatings is to simultaneously exhibit a high reflectivity with a large bandwidth and high laser resistance. These characteristics are traditionally achieved by the deposition of laser-resistant layers on highly reflective layers. We propose a "reflectivity and laser resistance in one" design by using tunable nanolaminate layers that serve as an effective layer with a high refractive index and a large optical bandgap. An Al2O3-HfO2 nanolaminate-based mirror coating for UV laser applications is experimentally demonstrated using e-beam deposition. The bandwidth, over which the reflectance is >99.5%, is more than twice that of a traditional mirror with a comparable overall thickness. The laser-induced damage threshold is increased by a factor of ~1.3 for 7.6 ns pulses at a wavelength of 355 nm. This tunable, nanolaminate-based new design strategy paves the way toward a new generation of UV coatings for high-power laser applications.
ABSTRACT
HfO2/SiO2 bilayer coatings and multilayer high-reflection coatings without and with a modified co-evaporated interface (MCEI) have been prepared. An MCEI is designed to be evaporated at an oxygen-deficient environment to achieve higher absorption than the conventional discrete interface. Capacitance-voltage measurements and absorption measurements demonstrate that an MCEI increases the trap density and leads to higher absorption. The laser-induced damage threshold and nano-indenter test results indicate that the MCEI multilayer coating exhibits better laser resistance and mechanical property, despite the larger absorption. The experimental results suggest that adhesive force between layers plays a more important role in nanosecond laser damage resistance than interface absorption.
ABSTRACT
Hafnium oxide thin films with varying oxygen content were investigated with the goal of finding the optical signature of oxygen vacancies in the film structure. It was found that a reduction of oxygen content in the film leads to changes in both, structural and optical characteristics. Optical absorption spectroscopy, using nanoKelvin calorimetry, revealed an enhanced absorption in the near-ultraviolet (near-UV) and visible wavelength ranges for films with reduced oxygen content, which was attributed to mid-gap electronic states of oxygen vacancies. Absorption in the near-infrared was found to originate from structural defects other than oxygen vacancy. Luminescence generated by continuous-wave 355-nm laser excitation in e-beam films showed significant changes in the spectral profile with oxygen reduction and new band formation linked to oxygen vacancies. The luminescence from oxygen-vacancy states was found to have microsecond-scale lifetimes when compared with nanosecond-scale lifetimes of luminescence attributed to other structural film defects. Laser-damage testing using ultraviolet nanosecond and infrared femtosecond pulses showed a reduction of the damage threshold with increasing number of oxygen vacancies in hafnium oxide films.
