Far-infrared absorber based on standing-wave resonances in metal-dielectric-metal cavity

Abstract

Thin-film resonant absorbers for the far-IR spectral range were fabricated, characterized, and modeled. The 3-μm-thick structure comprises a periodic surface array of metal squares, a dielectric spacer and a metallic ground plane. Up to 95% absorption for the fundamental band at ~53.5μm wavelength (5.6 THz) is achieved experimentally. Absorption bands are independent of the structure period and only weakly dependent on polarization and incident angle. The results are well explained in terms of standing-wave resonances within individual metal-dielectric-metal cavities. The structure has application as a wavelength selective coating for far-IR bolometers.

Infra-red spectral microscopy of standing-wave resonances in single metal-dielectric-metal thin-film cavity

“…This confirms our observation from simulated spectra as function of period, that when the squares are very close, a slight redshift appears for the fundamental due to couplings with next neighbors that increase the inertia of the oscillators. 15 The resonance positions reported here for mid-IR devices agree with the theory less well than they did for our far-IR devices. 15 We note that the two classes of device are not simply scaled replicas of each other.…”

“…1b) explain the positions of a series of resonances in our films. 15 Incident light polarizes the squares, resulting in edge dipoles that are secondary sources of radiation, which can propagate evanescently under the squares. The resonance wavelength is given by,…”

“…These structured films are usually much thinner than their resonance wavelengths, 10 and the periodicity of the structures that comprise them is similarly sub-wavelength. Independence on polarization and incidence angle have been demonstrated [11][12][13][14][15] . *janardan..nath@knights.ucf.edu Structured thin-film absorbers typically comprise 3 layers, namely a top surface metal with sub-wavelength patterning, a dielectric spacer, and a metal ground plane on suitable substrate [3][4][5][6][7][8][9][10][11][12][13][14][15] .…”

“…1. 15 Successes of the model included prediction of the fundamental and higher order resonance wavelengths, the dependence of these resonances on square size and dielectric thickness, the independence of the resonances on the period of the structure, and the insensitivity to polarization or incidence angle. 15 Our calculations supposed that each unit cell acts independently, with no dependence of resonance wavelength on the structure's period.…”

Unidirectional reflectionless light propagation at exceptional points

“…Tuning the geometry rather than the refractive index can reduce the difficulty in the experimental realization of such structures. Among different plasmonic waveguide structures, MDM plasmonic waveguides are of particular interest [108][109][110][111][112][113][114][115][116], because they support modes with deep subwavelength scale over a very wide range of frequencies extending from DC to visible [117] and are relatively easy to fabricate [118,119]. The waveguide widths w, w 1 , and w 2 are set to be 50, 20, and 100 nm, respectively ( Figure 9A).…”