Abstract
A two-dimensional nanotrench cavity grating on a thick gold film was fabricated by using e-beam lithography. Optical reflection spectra from the fabricated device were measured at oblique angles of incidence for TE and TM polarizations. Near perfect light absorption was observed at different wavelengths for TE and TM polarizations at oblique angles of incidence. The peak absorption wavelength of TM polarization red-shifts significantly as angle of incidence increases. The peak absorption wavelength of TE polarization blue-shifts slightly as incident angle increases. Using finite-difference time-domain (FDTD) simulations, two orders of magnitude magnetic field enhancement was revealed inside nanotrenches, indicating strong light trapping inside the nanostructure. The fabricated device was investigated as a refractive index chemical sensor. It was found that sensitivity increases for TM polarization and decreases for TE polarization when angle of incidence increases from zero.
Highlights
Light trapping and enhanced light absorption in metal nanostructures [1,2,3,4,5] potentially are significant for many applications such as biosensing, enhanced photodetection, and solar energy harvesting
A two-dimensional nanotrench cavity grating on a thick gold film was fabricated by using e-beam lithography
Optical reflection spectra from the fabricated device were measured at oblique angles of incidence for TE and TM polarizations
Summary
Light trapping and enhanced light absorption in metal nanostructures [1,2,3,4,5] potentially are significant for many applications such as biosensing, enhanced photodetection, and solar energy harvesting. Polarization independent light trapping in 2D nanotrench metal surface gratings at normal incidence was recently investigated with numerical simulations [16]. For 2D symmetric nanostructure metal surface gratings at oblique angles of incidence, plasmon resonance intuitively depends on the polarization and varies with the angle of incidence. We fabricated a 2D nanotrench surface cavity grating on a thick gold film surface and investigated plasmon resonance and light trapping in the 2D nanotrench grating device at oblique angles of incidence for TE (s) and TM (p) polarizations, respectively. It was found that plasmon resonance enhanced light absorption behaves very differently for TE and TM polarization excitations at oblique angles of incidence. For TM polarization excitation, plasmon resonance wavelength is strongly sensitive to the change of angle of incidence and red-shifts significantly as the angle of incidence increases. The sensitivity of the refractive index sensor exhibits opposite trends with increase of angle of incidence for TE and TM polarizations
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