Near perfect light trapping in a 2 D gold nanotrench grating at oblique angles of incidence and its application for sensing

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 timedomain (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. ©2016 Optical Society of America OCIS codes: (250.5403) Plasmonics; (310.6628) Subwavelength structures, nanostructures; (130.6010) Sensors; (350.2450) Filters, absorption. References and links 1. F. Frederix, J. M. Friedt, K. H. Choi, W. Laureyn, A. Campitelli, D. Mondelaers, G. Maes, and G. Borghs, “Biosensing based on light absorption of nanoscaled gold and silver particles,” Anal. Chem. 75(24), 6894–6900 (2003). 2. J. Cesario, R. Quidant, G. Badenes, and S. Enoch, “Electromagnetic coupling between a metal nanoparticle grating and a metallic surface,” Opt. Lett. 30(24), 3404–3406 (2005). 3. S. Mokkapati, F. J. Beck, A. Polman, and K. R. Catchpole, “Designing periodic arrays of metal nanoparticles for light-trapping applications in solar cells,” Appl. Phys. Lett. 95(5), 053115 (2009). 4. L. B. Luo, L. H. Zeng, C. Xie, Y. Q. Yu, F. X. Liang, C. Y. Wu, L. Wang, and J. G. Hu, “Light trapping and surface plasmon enhanced high-performance NIR photodetector,” Sci. Rep. 4, 3914 (2014). 5. C. F. Guo, T. Sun, F. Cao, Q. Liu, and Z. Ren, “Metallic nanostructures for light trapping in energy-harvesting devices,” Light Sci. Appl. 3(4), e161 (2014). 6. G. H. Derrick, R. C. McPhedran, D. Maystre, and M. Neviere, “Crossed grating: A theory and its applications,” Appl. Phys. 18(1), 39–52 (1979). 7. T. Lopez-Rios, D. Mendoza, F. J. Garcia-Vidal, J. Sanchez-Dehesa, and B. Pannetier, “Surface shape resonances in lamellar metallic gratings,” Phys. Rev. Lett. 81(3), 665–668 (1998). 8. E. Popov, S. Enoch, and N. Bonod, “Absorption of light by extremely shallow metallic gratings: metamaterial behavior,” Opt. Express 17(8), 6770–6781 (2009). 9. N. Bonod, G. Tayeb, D. Maystre, S. Enoch, and E. Popov, “Total absorption of light by lamellar metallic gratings,” Opt. Express 16(20), 15431–15438 (2008). 10. E. Popov, D. Maystre, R. C. McPhedran, M. Nevière, M. C. Hutley, and G. H. Derrick, “Total absorption of unpolarized light by crossed gratings,” Opt. Express 16(9), 6146–6155 (2008). 11. J. Le Perchec, P. Quémerais, A. Barbara, and T. López-Ríos, “Why metallic surfaces with grooves a few nanometers deep and wide may strongly absorb visible light,” Phys. Rev. Lett. 100(6), 066408 (2008). 12. A. Dhawan, M. Canva, and T. Vo-Dinh, “Narrow groove plasmonic nano-gratings for surface plasmon resonance sensing,” Opt. Express 19(2), 787–813 (2011). 13. A. Polyakov, S. Cabrini, S. Dhuey, B. Harteneck, P. J. Schuck, and H. A. Padmore, “Plasmonic light trapping in nanostructured metal surfaces,” Appl. Phys. Lett. 98(20), 203104 (2011). 14. A. Polyakov, H. A. Padmore, X. Liang, S. Dhuey, B. Harteneck, J. P. Schuck, and S. Cabrini, “Light trapping in plasmonic nanocavities on metal surfaces,” J. Vac. Sci. Technol. B. 296(06), 06FF01 (2011). Vol. 24, No. 15 | 25 Jul 2016 | OPTICS EXPRESS 17259


Introduction
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.Surface plasmon resonance enhanced light absorption in subwavelength period metal surface gratings has been extensively investigated for more than a decade [6][7][8][9][10][11][12].Polarization dependent light trapping in one-dimensional nanotrench gratings was recently investigated [13][14][15].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.However, the polarization dependence and variation with angle of incidence have not been investigated previously.In this work, 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.For TE polarization excitation, plasmon resonance wavelength is not sensitive to change of the angle of incidence.Strong magnetic field enhancement inside nanotrench cavities was found at the plasmon resonance wavelength.Also, the fabricated 2D nanotrench surface cavity device was used as refractive index chemical sensor.The sensitivity of the refractive index sensor exhibits opposite trends with increase of angle of incidence for TE and TM polarizations.

Device fabrication and measurement
Figure 1(a) shows the schematic of a 2D nanotrench surface cavity grating on a gold surface.The cross section of nanotrenches is rectangular.The nanotrench grating is etched into a thick metal film surface.The width of the nanotrenches is w and the depth is h.The period is p in both x and y dimensions.We fabricated a nanotrench grating on a 500 nm thick gold film sputtered on a glass wafer.In the fabrication process, we first deposited a 10 nm titanium layer on the glass substrate as an adhesion layer.Then a 500 nm gold film was sputtered on the titanium adhesion layer.After the thick gold film was deposited, an electron-beam resist (Zep 520A) layer was spin-coated on the gold film surface.The thickness of the e-beam resist layer was approximately 80 nm.After spin-coating of the e-beam resist, the sample was baked for 30 minutes at 120 °C temperature in an oven.Using e-beam lithography, a 2D nanotrench grating pattern was written in the e-beam resist layer.The acceleration voltage of e-beam was 30 kV and the electron dosage was 180 µC/cm 2 .After e-beam writing, the sample was developed in an e-beam resist developer (ZED N50) for 40 seconds, and then rinsed in isopropyl alcohol (IPA) solution for 1 minute.Next, the sample was baked in a 120 °C oven for 30 minutes for hardening the e-beam resist as a mask for follow-up plasma etching.Dry plasma etching was carried out by using argon (Ar) ion plasma in a reactive ion etching (RIE) equipment for 20 minutes.After the plasma etching, the e-beam resist residues were stripped off by dipping the device in a heated e-beam resist remover solution (remover PG, MicroChem) at 130 °C for 20 hours.In the final processing step, the sample was thermally annealed on a hotplate at the temperature of 250 °C for 5 minutes.It was discovered earlier in our group that annealing gold nanostructures at 250 °C slightly increases plasmon resonance frequency of gold nanostructures.
Figure 1(b) shows a scanning electron microscope (SEM) picture of the fabricated device.The period of nanotrench grating is 400 nm in x and y dimensions.We first measured optical reflectivity at normal incidence.The solid line curve in Fig. 1(c) shows the measured reflectivity spectrum.The reflectivity minimum is 3.5% at the wavelength of 575.5 nm.Then using finite-different time-domain (FDTD) simulations (Lumerical Solutions, Inc.), the measured reflectivity spectrum was fitted by adjusting the width (w) and depth (h) of the nanotrenches with the fixed period of 400 nm.The dashed blue line curve in Fig. 1(c) shows the FDTD simulation fitted reflectivity spectrum.The best-fit nanotrench geometric parameters are: 44 nm width (w) of 44 nm and 34 nm depth (h).Optical reflectivity spectra from the fabricated device were also measured at different oblique angles of incidence by using a super-continuum broadband light source with a spectral range from 400 nm to 2400 nm.The broadband light was first collimated by a microscope objective.The collimated beam of 3 mm diameter size passed through a linear polarizer.The linearly polarized light was focused onto the device by using an optical lens of 40 cm focal length.The focused beam size on the device is approximate 150 microns in diameter.An optical mirror was inserted between the focal lens and the device for varying the angle of incidence.Reflected light from the device was collected by an optical fiber collimator and sent to an optical spectrometer (StellarNet SR 50).In our measurements, the angle of incidence was set at 10°, 20°, 30°, and 40° for TE and TM polarizations, respectively.Figures 2(a) and 2(b) show the measured reflectivity spectra at different angles of incidence for TE and TM polarizations.The black, red, green and blue line curves represent the reflectivity spectra at the incident angle of 10°, 20°, 30° and 40°, respectively.For TE polarization excitation, no significant resonance wavelength shift is seen as the incident angle increases from Fig. 2(a).A slight blue shift of the resonance wavelength is seen as a result of increasing the incident angle to 40°.The resonance wavelength shifts slightly from 585.5 nm at 10° to 575.5 nm at 40°.This is because the electric field of TE polarization angular incidence is in the y direction since the plane of incidence is in the x-z plane.TE polarization light excites plasmon resonance in the x direction.Therefore, resonance wavelength remains the same approximately while the angle of incidence changes.However, for excitation with TM polarization, resonance wavelength shifts to longer wavelength as an increasing the angle of incidence.It can be seen from Fig. 2(b) that resonance wavelength shifts from 587.5 nm at 10° to 695 nm at 40°.The resonance wavelengths at 20° and 30° are 613 nm and 653 nm, respectively.The total shift of resonance wavelength is 107.5 nm from 10° to 40° incident angle.For TM polarization excitation, the direction of electric field changes as the incident angle increases.The direction of electric field has an angle with respect to the gold surface, the tangential component of electric field changes with the incident angle.The resonance wavelength shifts to red in response to the change of the incident angle.It is also seen in Fig. 2(b) that increasing the angle of incidence narrows the linewidth of the resonance spectrum, which is due to less coupling of incident light to the nanotrench plasmonic resonators.Each nanotrench is a plasmonic resonator behaving as a vertical split-ring resonator.These plasmon resonators are coupled together through surface plasmon waves on the metal surface.

Simulations and discussion
To understand angular and polarization dependence of light trapping in the 2D nanotrench grating, optical reflectivity and near electric and magnetic field distributions were calculated by using a finite-different time-domain (FDTD) simulation software code from Lumerical Solutions, Inc.In FDTD simulations, the plane of incidence is the x-z plane.The boundary condition is set as Bloch boundary in the x direction, and periodic in the y direction.Perfectly matched layer (PML) boundaries were set in the z direction.Optical constants of gold were taken from reference [17].A 2D plane monitor was placed on an x-y plane 10 nm above gold nanotrench surface to capture the electric field distributions.The electric field distributions at the resonance wavelength of 575.5 nm under TE and TM polarization excitations are shown in Figs.3(a) and 3(b) respectively.Another 2D plane monitor was placed on the cross-section of a nanotrench in the x-z plane.Figures 3(c) and 3(d) show electric and magnetic field enhancement distributions at the cross-section of a nanotrench at normal incidence.The electric and magnetic field enhancement profiles are plotted in logarithmic scale.The electric/magnetic field enhancement is defined as the ratio of the calculated local electric/magnetic field amplitude over the field amplitude of incident plane wave.It is seen in Fig. 3(c) that electric field enhancement maximizes at the edges of the nanotrenches.The magnetic field enhancement of two orders of magnitude is observed inside the nanotrench as shown in Fig. 3(d).The magnetic field enhancement is caused by the plasmon surface electric current along the side wall of the nanotrenches.Reflectivity spectra were calculated at different angles of incidence for TE and TM polarizations and are plotted in Fig. 4. The black, red, green and blue line curves represent the reflectivity spectra at incident angle of 10°, 20°, 30° and 40°, respectively.In Fig. 4(a), a minimum of 3.7% is observed at wavelength of 597.6 nm for 10° incident angle As the incident angle increases to 20°, 30°, and 40°, the resonance wavelength shifts to 596.6 nm, 595.6 nm and 591.1 nm, correspondingly.The slightly blue shift matches well with the experimental result.As seen in Fig. 4(b), the resonance wavelength shifts to longer wavelength as the incident angle increases for TM polarization.The resonance wavelength is 601.6 nm, 615.2 nm, 645.7 nm and 686.8 nm corresponding to incident angle of 10°, 20°, 30°, and 40°, respectively.The linewidth of resonance spectrum decreases as the angle of incidence increases, which is due to the reduced energy coupling and dissipation to the nanotrenches.Simulation results agree well with the experimental results shown in Fig. 2(b) for the trends of peak absorption wavelength shift.For oblique angle incidence of TM polarization, the electric field of excitation has an angle with respect to the metal surface.It excites surface plasmons in the direction with an oblique angle with respect to metal surface.Therefore, the plasmon resonance is sensitive to the change of angle of incidence.For oblique angle incidence of TE polarization, electric field is always parallel to the gold metal surface regardless the change of incident angle.Therefore, increasing angle of incidence of TE polarization does not change much the plasmon resonance wavelength To investigate electromagnetic field enhancement inside nanotrenches, two point monitors were placed in a nanotrench at two locations.The first point monitor is placed at the center of the nanotrench opening (x = 0 nm, z = 0 nm) aligned with the top gold film surface as indicated by the dot in the inset of the Fig. 5.We calculated the electric and magnetic field enhancements at this point monitor.Figure 5 shows calculated electric and magnetic field enhancement versus wavelength for different incident angles.Figure 5(a) shows electric field enhancement of TE polarization excitation at different angles of incidence.The electric field enhancement is defined as the calculated electric field amplitude normalized to the electric field amplitude of the incident plane wave.It is seen from Fig. 5(a) that the electric field enhancement decreases as the angle of incidence increases.Figure 5(b) shows the magnetic field enhancement at the point monitor for TE polarization excitation.It is seen that the magnetic field enhancement decreases and resonance wavelength remains about same as increasing the incident angle.Figure 5(c) shows electric field enhancement of TM polarization at the monitor location.It can be seen that the electric field enhancement increases as angle of incidence increases from normal incidence to 30°, and then start to decrease at 40°.The resonance wavelength shifts significantly as increasing the angle of incidence.Figure 5(d) shows the magnetic field enhancement for TM polarization excitation.The magnetic field enhancement increases and resonance wavelength shifts as increasing the incident angle.A strong asymmetric Fano resonance lineshape is observed in the near electric and magnetic field resonance spectra for TM polarization excitation.The strong Fano resonance observed for TM polarization excitation is due to the strong coupling of localized plasmon resonance in each nanotrench cavity and the propagating surface plasmon resonance excited by the grating in the x-direction.It is interesting to see that the resonance lineshape in the reflectivity spectrum is different from the resonance line shapes of the near electromagnetic field.The second point monitor is indicated as the dot in the inset of Fig. 6. Figure 6(a) shows calculated electric field enhancement for TE polarization excitation at different angles of incidence at this point monitor location.The electric field enhancement is defined as the calculated electric field amplitude normalized to the electric field amplitude of the incident optical wave.From Fig. 6(a), it can be seen that the electric field decreases as the angle of incidence increases and the resonance wavelength remains about same for different angles of incidence.Figure 6(b) shows the magnetic field enhancement of TE polarization at different angles of incidence.The magnetic field enhancement is defined the same way as the electric field enhancement.From Fig. 6(b), it can be seen that the magnetic field decreases as the angle of incidence increases and the resonance wavelength remains insensitive to the change of angle of incidence.Figure 6(c) shows the electric field enhancement for TM polarization at different angles of incidence.The electric field first increases as the incident angle increases from normal incidence to 30°, then decreases as the incident angle increases to 40°.The resonance wavelength shifts significantly when the incident angle increases.Figure 6(d) shows magnetic field enhancement for TM polarization.The magnetic field increases and resonance wavelength shifts significantly when the angle of incidence increases.

A refractive index chemical sensor with the metal nanotrench grating
The fabricated nanotrench device was used as a refractive index chemical sensor.The sensitivity of refractive index chemical sensor was obtained by measuring the shift of the peak absorption wavelength caused by different chemicals applied to the surface.Distilled (DI) water with refractive index of 1.33 and isopropyl alcohol (IPA) with refractive index of 1.37 were used as two chemicals for characterizing the refractive index chemical sensor.The measured reflectivity spectra at the normal incidence are plotted and shown in Fig. 7.The black, red and blue lines represent the measurement results when the device was exposed to air, DI water, and IPA, respectively.The resonance wavelength exhibits a red shift as the refractive index of the surrounding medium increases.When the device was exposed to DI water and IPA, two dips in the reflectivity spectra were observed.The reflectivity dip happened at the wavelength of 731.5 nm when DI water was applied onto the device surface and 755 nm when IPA was applied.Another absorption dip at shorter wavelength was observed at 532 nm for DI water and 545.5 nm for IPA.The absorption dip at the shorter wavelength corresponds to the second order plasmon resonance mode.The resonance wavelength shift of the second order mode is less than the shift of the fundamental resonance mode.The refractive index sensitivity was calculated by the resonance wavelength shift over the change of the refractive index.In this work, we focus primarily on the sensitivity of the fundamental resonance mode.The fundamental mode sensitivity is 478 nm per refractive index unit (RIU), calculated based on the resonance wavelength shift from water to IPA at normal resonance.The second order mode sensitivity is 270 nm/RIU, which is less than the sensitivity of the fundamental resonance mode.This result is consistent with previously reported plasmon sensor sensitivity of higher order plasmon resonance mode [18].Next, we investigated sensitivity of the refractive index sensor at different angles of incidence for TE and TM polarization excitations.Figure 8 shows the measured reflectivity spectra for different chemicals applied to the device surface at 10°, 20°, 30° and 40° incident angles, respectively.The incident light is TE polarized.The black, red and blue lines represent measurement results when the device was exposed to air, DI water, and IPA, respectively.It can be seen that as the refractive index of surrounding medium increases, the resonance wavelength shifts to longer wavelength.As the angle of incident increases, the second order mode is gradually weakened and becomes less sensitive to the change of the surrounding refractive index.For fundamental mode resonance shown in Fig. 8(a), the resonance wavelength shifts from 715 nm for DI water to 737.5 nm for IPA.As the angle of incidence increases, resonance wavelength changes from 710 nm to 730 nm in Fig. 8(b) and increases from 698.5 nm to 716.5 nm in Fig. 8(c).Observations show when the incident angle increases to 40°, the resonance wavelength shifts from 693 nm to 707.5 nm as shown in Fig. 8(d).The sensitivities obtained at different angles of incidence are summarized in Table 1.It is concluded that for TE polarization excitation, the fundamental mode sensitivity decreases as the angle of incidence increases.
Reflectivity spectra of TM polarization excitation at different angles of incidence are measured and plotted in Fig. 9.In Fig. 9(a), it can be seen that the fundamental mode resonance wavelength is 742 nm in DI water and 766.5 nm in IPA, respectively, and the second order mode resonance wavelengths is 606 nm in DI water and 618 nm in IPA respectively.In Fig. 9(b), the fundamental mode resonance wavelength is 764 nm in DI water and 794 nm in IPA, and the second order mode resonance wavelength is 614 nm in DI water and 625 nm in IPA, respectively.In Fig. 9(c), the second order mode resonance wavelength is 620.5 nm in DI water and 630 nm in IPA, respectively.The fundamental mode resonance wavelength is 808 nm and 833.5 nm.As shown in Fig. 9(d), the second order mode resonance wavelength is 634 nm in DI water and 640 nm in IPA, respectively and the fundamental mode resonance wavelength is 844.5 nm and 871 nm.The fundamental mode resonance linewidth decreases as the angle of incidence increases, indicating less coupling of incident light to the plasmon resonance mode and less energy dissipation in nanotrench cavities.Measured sensitivities corresponding to different incident angles and polarizations are summarized in Table 1.It is seen that the fundamental mode sensitivity increases and the second order mode sensitivity decreases as the angle of incidence increases for TM polarization excitation.Table 1 summarizes measured sensitivities at different angles of incidence of TE and TM polarizations.For TE polarization, only the fundamental mode resonance occurs and the sensitivity decreases as angle of incidence increases.Sensitivity is 478 nm/RIU at normal incidence and decreases to 296 nm/RIU at incident angle of 40°.For TM polarization excitation, fundamental mode sensitivity increases from 478 nm/RIU at normal incidence to 544 nm/RIU at 40° angle of incidence.However, second order mode is less sensitive than fundamental mode and decreases from 270 nm/RIU to 122 nm/RIU as the incident angle increases from zero to 40 degrees.The large sensitivity is found at large angle of incidence of TM polarization excitation.For TE polarization incidence, we calculated reflectivity spectra for three different surrounding media at angle of incidence of 10, 20, 30, 40 degrees, respectively.The results are plotted and shown in Fig. 10.In Fig. 10(a), it can be seen that at 10 degrees incident angle, resonance wavelength shifts from 718.2 nm to 735.7 nm when the surrounding index of refraction changes from 1.33 to 1.37.At 20 degrees incident angle, resonance wavelength shifts from 712.6 nm to 729.6 nm as seen in Fig. 10(b).At 30 degrees incident angle, resonance wavelength shifts from 706.7 nm to 723.2 nm as seen in Fig. 10(c).At 40 degrees incident angle, resonance wavelength shifts from 705.2 nm to 721.2 nm due to the change of index of refraction of the surrounding medium as shown in Fig. 10(d).The red-shift of resonance wavelengths is caused by the coupling of plasmon resonance with the polarization dipoles in the surrounding dielectric medium, which slows down the plasmon resonance.For TM polarization excitation, we also calculated reflectivity spectra corresponding to three different surrounding media at angle of incidence of 10, 20, 30, 40 degrees, respectively.The results are plotted in Fig. 11.For TM polarization at 10 degrees of angle of incidence, the fundamental mode resonance wavelength shifts from 734.7 nm to 753.7 nm as the refractive index of surrounding medium changes from 1.33 to 1.37, as seen in Fig. 11(a).The second order mode wavelength shifts from 634.5 nm to 653.5 nm.At the incident angle of 20 degree, the fundamental mode resonance wavelength shifts from 780.4 nm to 801.9 nm, while the second order mode absorption wavelength shifts from 634.2 nm to 653 nm due to the change of surrounding medium as shown in Fig. 11(b).As shown in Fig. 11(c) at the angle of incidence of 30 degrees, the fundamental mode resonance wavelength shifts from 839.5 nm to 864 nm due to the surrounding medium index of refraction change from 1.33 to 1.37, while the second order mode absorption wavelength shifts from 640.2 nm to 658 nm.As shown in Fig. 11(d) at the angle of incidence of 40 degrees, the fundamental mode resonance shifts from 911.2 nm to 937.7 nm wavelength, while the second order mode absorption wavelength shifts from 687 nm to 704 nm, caused by the change of the surrounding medium index of refraction from 1.33 to 1.37.The red-shift of resonance wavelengths is caused by the coupling of plasmon resonance to the polarization dipoles in the nanocavities.Higher refractive index dielectric materials have higher electric dipole polarization density, therefore resulting in stronger coupling to surface plasmons and more red-shift of plasmon resonance wavelength.We also calculated refractive index sensitivities based on resonance wavelength shifts caused by chemicals with different indices of refraction.Calculated refractive index sensitivities at different angles of incidence for TE and TM polarizations are summarized in Table 2.In Table 2, it can be seen that calculated sensitivities follow the same trends as the measured sensitivities shown in Table 1.For TE polarization, the fundamental mode sensitivity decreases as angle of incidence increases.For TM polarization, the fundamental mode sensitivity increases as angle of incidence increases.Large sensitivity is found at large angle of incidence for the fundamental mode resonance of TM polarization.For the second order resonance mode of TM polarization, the sensitivity decreases as angle of incidence increases.The increased sensitivity at large oblique angle of incidence of TM polarization is due to strong coupling of localized plasmon resonance mode inside nanotrench cavities with propagating surface plasmon resonance mode supported by the periodic grating structure.For oblique angle incidence of TM polarization, the surface plasmon polarization has an oblique angle with respect to the metal surface.Therefore, it enhances the interaction of surface plasmons with electric dipoles inside nanotrench cavities.For oblique angle incidence of TE polarization, surface plasmon polarization is always parallel to the gold metal surface regardless the change of incident angle.Increasing angle of incidence of TE polarization decreases the coupling from incident light to surface plasmons.Therefore, it reduces the sensitivity for oblique angle incidence of TE polarization.

Summary
In this work, a 2D nanotrench surface cavity subwavelength grating was fabricated on a gold film surface by using e-beam lithography.Reflectivity spectra from the fabricated device were measured for TE and TM polarization excitations respectively, at various angles of incidence to investigate angular and polarization dependence of light absorption in the 2D nanotrench structure.Near perfect light absorption was found to occur at different wavelengths between TE and TM polarizations at same oblique angles of incidence.The resonance wavelength changes differently with angle of incidence for TE and TM polarizations.As the angle of incidence increases, the resonance wavelength has significant red-shift for TM polarization and has a small blue-shift for TE polarization excitation.A refractive index chemical sensor was demonstrated by applying liquid chemicals onto the device surface and measuring the shift of the peak absorption wavelength.It was found that fundamental mode sensitivity increases and the second order mode sensitivity decreases when the angle of incidence increases for TM polarization excitation.For TE polarization excitation, sensitivity decreases as the angle of incidence increases.FDTD simulations were carried out to fit measured reflectivity spectra and to obtain electromagnetic field enhancement inside nanotrench cavities.Strong magnetic field enhancement inside the nanotrench cavites was found due to the coupled local plasmon resonance in the nanotrench cavity grating.Also strong Fano resonance was revealed in the near field of the device with TM polarization excitation at large oblique angles of incidence.The 2D nanotrench cavity grating investigated in this work can be potentially used for biochemical sensors and also can be used for optical color filters.

Fig. 1 .
Fig. 1.(a) Schematic of the 2D nanotrench grating on a gold surface.(b) A SEM picture of a fabricated device.(c) Measured (solid line) and calculated (dashed line) reflectivity spectra at normal incidence.A strong absorption is observed at 575.5 nm wavelength.

Fig. 2 .
Fig. 2. Measured reflectivity spectra at different angles of incidence for (a) TE polarization excitation and (b) TM polarization excitation.

Fig. 3 .
Fig. 3. Electric field and magnetic field enhancement at the near perfect absorption wavelength of 575.5 nm.(a) Top view of electric field enhancement distribution for TE polarization excitation, the electric field in the y direction.(b) Top view of the electric field enhancement distribution for TM polarization excitation, the electric field is in the x direction.(c) Electric field enhancement of TM polarization excitation is in the x direction.(d) Magnetic field enhancement of TM polarization excitation.

Fig. 5 .
Fig. 5. Electromagnetic field enhancement versus wavelength at the middle of a nanotrench opening (z = 0) for different incident angles.(a) Electric field enhancement of TE polarization.(b) Magnetic field enhancement of TE polarization.(c) Electric field enhancement of TM polarization.(d) Magnetic field enhancement of TM polarization.

Fig. 6 .
Fig. 6.Electromagnetic field enhancement versus wavelength at the center of a nanotrench for different angles of incidence.(a) Electric field enhancement for TE polarization.(b) Magnetic field enhancement for TE polarization.(c) Electric field enhancement for TM polarization.(d) Magnetic field enhancement for TM polarization.

Fig. 7 .
Fig. 7. Measured reflectivity spectra at normal incidence when the device was exposed to air, water (H 2 0), and IPA liquid.