Spectral modification of array truncation effects in infrared frequency selective surfaces

AbstractInfrared frequency selective surfaces (FSS) are widely used in quasi-infinite, planar configurations; however, applications for finite arrays exist as well. Here, a square loop infrared FSS was designed with an infinite array resonance near 10μm when illuminated at 60° off-normal. Along with the quasi-infinite array, a patterned area containing finite arrays of 7×7 square loops of this design was fabricated and characterized to have a resonance which was blue-shifted due to the effects of truncation. To counteract the effects of truncation, two geometrically modified arrays of 7×7 square-loop elements were designed and fabricated to shift the resonant wavelength approximately back to that of the infinite array

Large area infrared frequency selective surface with dimensions reproducible by optical lithography

An infrared frequency selective surface (FSS) with absorptive resonance near 6.5 mu m was fabricated by electron-beam lithography using a patch design with dimensions reproducible by optical-projection lithography. By selective wet etching along with reactive-ion etching, the sample was divided into miniature FSS particles, which were released from the substrate. A large number of such particles could be implemented as a large area, conformal coating. Spectral reflectivity of the full FSS array as well as the FSS particles was measured and compared to electromagnetic simulations. To show the feasibility of this approach, the full array FSS design was fabricated using a g-line (lambda = 436 nm) 5 x projection lithography stepper and compared to the array fabricated by electron-beam lithography using scanning electron microscopy and Fourier transform infrared spectroscopy. Even though the resolution of the g-line stepper led to a poor fabrication output, the optical resonance was found to be robust, with only slight detuning attributed to the Changes in unit cell geometry. This work highlights the utility of optical-projection lithography, coupled with the releasable particle fabrication procedure, to create a large area, conformal coating with specific infrared spectral properties

Near- and far-field spectroscopic imaging investigation of resonant square-loop infrared metasurfaces

Optical metamaterials have unique properties which result from geometric confinement of the optical conductivity. We developed a series of infrared metasurfaces based on an array of metallic square loop antennas. The far-field absorption spectrum can be designed with resonances across the infrared by scaling the geometric dimensions. We measure the amplitude and phase of the resonant mode as standing wave patterns within the square loops using scattering-scanning near-field optical microscopy (s-SNOM). Further, using a broad-band synchrotron-based FTIR microscope and s-SNOM at the Advanced Light Source, we are able to correlate far-field spectra to near-field modes of the metasurface as the resonance is tuned between samples. The results highlight the importance of multimodal imaging for the design and characterization of optical metamaterials