TelAztec has developed a high-transmission, Random Anti-Reflective (RAR) nano-texturing plasma process for the highly useful nonlinear optical crystal zinc-germanium phosphide (ZnGeP₂ , ZGP) [1,2]. ZGP is a low absorption mid-IR transmitting material with a high nonlinear coefficient, significant birefringence, high thermal conductivity, and a moderately high pulsed laser damage threshold [3]. This combination of properties makes ZGP the best choice for generating agile wavelength agile Mid-IR laser light sources critical for defense and medical applications.
An issue with ZGP is it’s high refractive index that for polished crystal facets, will reflect over 26% of Mid-IR light, dramatically reducing the conversion efficiency without an effective AR treatment. Conventional multi-layer thin-film interference AR coating stacks are often deposited to combat these losses over the broad operating bandwidth. Yet, this approach has its limitations. It requires numerous layers of alternating dissimilar materials, which can be costly and challenging to implement without the risk of film delamination, thermal accumulation, and laser damage as the stack approaches several microns in thickness. The innovative RAR nano-texturing solution replaces the complex stack of thin-films with a surface relief texture etched directly into the ZGP crystal material. The fine and densely packed features built into the ZGP surface impart a graded index AR function that is inherently robust and operates over the same wide bandwidth as the material itself. The RAR process is based on a patented gas-plasma chemistry that requires a single fabrication step that takes a fraction of the machine time typically required to deposit conventional thin-film AR coatings. This simplicity and efficiency significantly reduces manufacturing costs and provides a robust and long lasting optic.
The image above shows TelAztec CTO Anthony Manni operating a mid-IR FTIR spectrometer to make transmission measurements of a 1x1cm ZGP crystal after RAR nano-texturing. The nano-textured ZGP crystal surface appears quite black in visible light as shown by the expanded inset image. Scanning electron microscope (SEM) images reveal a carpet-like texture with randomly distributed features that are about 100-200nm wide and 400nm high. A plot of the measured spectral transmission through the 1.25mm thick ZGP crystal before (black curve) and after one-facet RAR nano-texturing (green curves), is shown below. The dashed gray curve shows the maximum theoretical external transmission efficiency, calculated using Beer’s Law and measured complex refractive index data for ZGP [4].
In addition to the RAR nano-texture, TelAztec can fabricate Motheye AR nano-structures that consist of a periodic array of cones etched into the ZGP crystal surface [1-3]. Though more costly due to the need for lithography to originate the Motheye pattern, a Motheye AR texture has previously been more readily fabricated. A plot of the transmission of ZGP crystals with a Motheye AR array structure etched in one surface, is shown below (green curves). The black curve in the figure is the measured transmission of the ZGP crystal before the Motheye AR nano-texturing. Note that the maximum attainable transmission of a ZGP crystal with one surface AR is about 72.5%. SEM images of the Motheye AR structure are also shown along with an image of a typical ZGP crystal.
The applications of ZGP include mid-IR optical parametric oscillators (OPOs), harmonic generation of CO₂ and CO lasers, continuously tunable MWIR-LWIR lasers via combined parametric nonlinear processes, and generation of terahertz (THz) range frequencies [5-7]. With the enhanced laser damage resistance and transmission enabled by RAR Nano-Texturing, ZGP has the potential to revolutionize the field, unlocking a whole new class of high-energy IR lasers for materials processing and defense applications.
References:
[1] Douglas S. Hobbs, “Laser damage threshold measurements of anti-reflection microstructures operating in the near UV and mid-infrared,” Proc. SPIE 7842, Laser-Induced Damage in Optical Materials, 78421Z (2010)
[2] Bruce D. MacLeod, Douglas S. Hobbs, Ernest Sabatino III, “Moldable AR microstructures for improved laser transmission and damage resistance in CIRCM fiber optic beam delivery systems,” Proc. SPIE 8016, Window and Dome Technologies and Materials XII, 80160Q (2011)
[3] Kevin T. Zawilski, Scott D. Setzler, Peter G. Schunemann, and Thomas M. Pollak, “Increasing the laser-induced damage threshold of single-crystal ZnGeP2,” J. Opt. Soc. Am. B 23, 2310-2316 (2006)
[4] Anne Hildenbrand, Christelle Kieleck, Aleksey Tyazhev, Georgi Marchev, Georg Stöppler, Marc Eichhorn, Peter G. Schunemann, Vladimir L. Panyutin, Valentin P. Petrov, “Laser damage of the nonlinear crystals CdSiP2 and ZnGeP2 studied with nanosecond pulses at 1064 and 2090 nm,” Opt. Eng. 53(12) 122511 (2014)
[5] Peter G. Schunemann, Kevin T. Zawilski, Leonard A. Pomeranz, Daniel J. Creeden, and Peter A. Budni, “Advances in nonlinear optical crystals for mid-infrared coherent sources,” J. Opt. Soc. Am. B 33, D36-D43 (2016)
[6] Magnus W. Haakestad, Helge Fonnum, and Espen Lippert, “Mid-infrared source with 0.2 J pulse energy based on nonlinear conversion of Q-switched pulses in ZnGeP2,” Opt. Express 22, 8556-8564 (2014)
[7] Yudin, Nikolay, Mikhail Zinoviev, Vladimir Kuznetsov et al. “Effect of Dopants on Laser-Induced Damage Threshold of ZnGeP2” Crystals 13, no. 3: 440. (2023)