This work explores reactive ion etching parameters in order to identify and optimize key characteristics in gratings that govern their overall performance, including minimization of sidewall and trench structural defects and modification of fused silica via intrinsic molecular-level defects. This study is performed using grating-like samples with 5-µm-wide lines and trenches generated in fused silica by the photolithographic process and inductively coupled plasma-reactive ion etching. The analysis compiles metrology, simulation, and damage-testing results to obtain a better understanding of how to modify the fabrication process of gratings toward achieving better laser-induced–damage performance.
Transverse stimulated raman scattering (TSRS) in potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (DKDP) plates for polarization control is a well-recognized limitation, giving rise to parasitic energy conversion and laser-induced damage. TSRS amplification is a coherent process that grows exponentially and distributed nonuniformly in the crystal and at the crystal surfaces. To understand the growth and spatial distribution of TSRS energy, a modeling approach was developed to simulate operational conditions in inertial confinement fusion-class laser systems. The result helps assess upper limits for operational conditions and guide possible ways to suppress the TSRS gain.
The transverse spontaneous Raman scattering in a configuration analogous to the geometry of a large-aperture potassium dihydrogen phosphate polarization rotator plate for inertial confinement fusion-class lasers is investigated. The experimental setup enabled effectively measuring the transverse Raman scattering in 360° around the beam propagation direction for any crystal optic-axis (OA) alignment. Results reveal the angular dependence of the transverse Raman scattering signal as a function of (1) the angle between the OA and the pump-beam propagation and (2) the angle of the X-Y axis around the OA. These results enable the consideration of optimal crystal cut designs for specific applications.
The interactions of microparticles of different materials located on the surface of a multilayer dielectric mirror with intense 1053-nm laser pulses of varying fluence and duration (10 and 0.6 ps) are investigated. The particles caused localized intensification of the electric field, which becomes the dominant mechanism for the onset of damage and secondary contamination of the mirror at fluences far below the pristine (without particles) laser-induced-damage threshold of the mirror. Several interaction mechanisms leading to material modification and damage are identified, including the localized field intensification by multibeam interference and particle-induced microlensing, plasma-induced scalding, and secondary contamination via nanoparticle generation and particle melting. The resulting morphologies were observed to be vulnerable to damage growth and additional damage initiation when irradiated by subsequent pulses.
Multilayer dielectric (MLD) diffraction gratings are a key component for the construction of high-peak-power, pulse-compressed laser systems. While a great deal of effort has been devoted to the design of optimal grating structures and the etching of these structures into the MLD coating, there has not been the same effort put into the optimization of the MLD coating itself. The primary characteristics of the multilayer that must be considered during design include minimization of the standing wave created in the photoresist because of the reflectivity of the coated optical surface, creation of a sufficiently high reflectivity at the use wavelength and incidence angle in a dry environment, proper balance of the individual layer materials to yield a coating with an overall neutral or slightly compressive stress, and a high laser-damage threshold for the wavelength and pulse duration of use. This work focuses on the modification of a standard MLD mirror, while considering these characteristics, to allow the fabrication of a diffraction grating with higher efficiency and laser-damage threshold than is typically achieved. Scanning electron microscopy (SEM) images of the grating structures demonstrate smoother shapes with lower roughness due to the holographic exposure. Damage testing performed at 1053 nm with a pulse width of 10 ps demonstrates the MLD coating has a sufficiently high laser-damage threshold to form the basis of reflection gratings that survive in high-fluence applications.
The OMEGA EP Facility includes two high-energy, short-pulse laser beams that will be focused to high intensity in the OMEGA target chamber, providing backlighting of compressed fusion targets and investigating the fast-ignition concept. To produce 2.6-kJ output energy per beam, developments in grating compressor technology are required. Gold-coated diffraction gratings limit on-target energy because of their low damage fluence. Multilayer dielectric (MLD) gratings have shown promise as high-damage-threshold, high-efficiency diffraction gratings suitable for use in high-energy chirped-pulse amplification [ B. W. Shore et al., J. Opt. Soc. Am. A14, 1124 (1997).] Binary 100-mm-diam MLD gratings have been produced at the Laboratory for Laser Energetics (LLE) using large-aperture, holographic exposure and reactive ion-beam etching systems. A diffraction efficiency of greater than 99.5% at 1053 nm has been achieved for gratings with 1740 grooves/mm, with a 1:1 damage threshold of 5.49 J/cm2 diffracted beam fluence at 10 ps. To demonstrate the ability to scale up to larger substrates, several 100-mm substrates have been distributed over an aperture of 47 × 43 cm and successfully etched, resulting in high efficiency over the full aperture. This paper details the manufacture and development of these gratings, including the specifics of the MLD coating, holographic lithography, reactive ion etching, reactive ion-beam cleaning, and wet chemical cleaning.
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