3D Meta-Optics for High-Energy Lasers

Engineering electromagnetic fields in 3D dielectric structures to change the way components are designed and fabricated for high-energy laser (HEL) applications.

September 1, 2019

Air Force Research Laboratory, Arlington, Virginia

The scope of this research radically changed the way novel components are designed and fabricated for high-energy applications. This 3D Meta-Optics platform was, and is, essentially engineering the electromagnetic fields in 3D dielectric structures. The results have provided a class of transformational optical components that can be integrated at all levels throughout a high-energy laser system.

A Guided-Mode Resonance Filter (GMRF) is an optically resonant structure consisting of a subwavelength grating (SWG) and a high-index waveguide layer fabricated on a supporting substrate. The devices work based on the electromagnetic coupling of a narrow spectral band from the incident light, due to diffraction by the SWG into leaky waveguide modes, which in turn re-couples to the SWG, and exit the device from the incidence side. This effect results in high reflectance at the desired wavelength band of interest.

The advantage of such devices over conventional optical filters such as the Distributed Bragg Reflectors (DBR) is in their simple construction. The GMRF devices require few layers of dielectrics, compared to their conventional counterpart, while providing equivalent or better spectral characteristics in terms of achieving high reflectance, suppressed sideband reflections, polarization sensitivity and, arbitrarily narrow resonance line widths.

The cost and complexity associated with the GMRF device fabrication is greatly reduced when wafer scale fabrication is used. This project presents the design and fabrication of mid-infrared optical reflection filters, based on GMRF principles, for applications at 2.94m and based on hafnium dioxide (HfO₂)/quartz material system. The wavelength band has potential applications in high power lasers based on Er:YAG which are used in sensors, dentistry, laser surgery, spectroscopy, IR countermeasures, etc. Narrowband devices based on a silicon nitride/soda lime glass system and broadband GMRF devices based on a germanium/fused silica system have been reported for use in absorption spectroscopy, VCSELs, and similar mid-IR applications.

The mid-IR GMRF devices described in this paper use a 2D periodic square lattice of holes to form the SWG. The period of the SWG is 1.90m and the hole radius is 0.76m. The major challenge in the mid-IR arises due to the limited choices of material substrates that have little or no absorption and the fabrication challenges of such material systems using conventional lithographic techniques. A material system that has low absorption and an adequate refractive index contrast would be the ideal choice for high reflectance mirrors.

Infrared grade quartz substrates (Heraeus Infrasil 301) were used for this design. The SWG was formed into the quartz substrate (nsub = 1.4206), with a depth of 270nm. HfO₂ (nwg = 1.9695) conformally overcoats the patterned SWG and functions as the waveguide layer. It was chosen for its high power damage threshold properties.

The device spectral performance was simulated using Rigorous Coupled Wave Analysis (RCWA), assuming plane wave illumination. The symmetry of the square lattice SWG results in polarization insensitive device design at normal incidence. The simulated resonance has a peak reflectance of 100% at 2962.5nm with a FWHM of 35nm.

This work was done by Glenn Boreman and Eric Johnson of the University of North Carolina at Charlotte for the Air Force Research Laboratory.For more information, download the Technical Support Package (free white paper) here under the Photonics category. AFRL-0276