All-Semiconductor Negative-Index Plasmonic Absorbers

We demonstrate epitaxially grown all-semiconductor thin-film midinfrared plasmonic absorbers and show that absorption in these structures is linked to the excitation of highly confined negative-index surface plasmon polaritons. Strong ($>98\%$) absorption is experimentally observed, and the spectral position and intensity of the absorption resonances are studied by reflection and transmission spectroscopy. Numerical models as well as an analytical description of the excited guided modes in our structures are presented, showing agreement with experiment. The structures investigated demonstrate a wavelength-flexible, all-semiconductor, plasmonic architecture with potential for both sensing applications and enhanced interaction of midinfrared radiation with integrated semiconductor optoelectronic elements.

Figure 1

Experimental (red lines), numerically modeled (blue lines), and $T$-matrix calculated (green lines) (a) reflection and (b) transmission spectra for the unpatterned, as-grown perfect absorber sample. Experimental transmission has been scaled by $\times 35$. Inset shows a schematic of the sample epitaxial layer structure.

Figure 2

(a) TM (solid lines) and TE (dotted lines) experimental (red) and FDTD calculated (blue) reflection for a PA sample with $\mathrm{\Lambda }=4\mu \mathrm{m}$, $w=2.1\mu \mathrm{m}$, and $d=210\mathrm{nm}$. Experimental transmission is shown in black. (b) Schematic of PA absorber structure and (c) atomic force micrograph (AFM) of the PA characterized in (a), with inset showing an AFM line scan (dashed line) across the sample surface.

Figure 3

(a) Scatter plot showing experimental absorption intensity ($1-R_{\mathrm{TM}}/R_{\mathrm{TE}}$) at resonance for a range of samples as a function of fill factor ($x$ axis) and etch depth in nm (color scale). (b) TE (dashed lines) and TM (solid lines) experimental reflection spectra for features [marked with horizontal lines in (a)] with a range of lateral geometries and etch depths between 180 and 220 nm. (c) Experimental (red squares) and modeled (blue stars) absorption for all features characterized as a function solely of etch depth. Modeled data use samples with $\mathrm{\Lambda }=4\mu \mathrm{m}$ and $w=2.4\mu \mathrm{m}$ and varying etch depth $d$. (d) TE (dashed lines) and TM (solid lines) experimental reflection spectra for samples with similar fill factor and varying etch depth $d$ [marked with vertical lines in (a)].

Figure 4

(a) Dispersion relations of the guided modes supported by the noncorrugated stacks. Black, blue, and red curves correspond to MDM, ADM, and undercut AMDM structures; thin gray line represents dispersion of the plane wave in vacuum. Note that the high-index modes have negative refractive index for shorter wavelength and positive index for longer wavelength. (b)– (d) Reflection spectrum for different etch depths and periodicity as a function of fill factor and wavelength. The parameters for the plots are (b) $\mathrm{\Lambda }=2\mu \mathrm{m}$, $d=110\mathrm{nm}$, (c) $\mathrm{\Lambda }=4\mu \mathrm{m}$, $d=220\mathrm{nm}$, (d) $\mathrm{\Lambda }=2\mu \mathrm{m}$, $d=220\mathrm{nm}$; white squares correspond to spectral positions of intersections of collective resonances formed by guided modes in corrugated systems and fundamental half-wavelength resonances of AMDM (b) and ADM [(c) and (d)] modes.