Source: http://aoot.osa.org/ome/abstract.cfm?uri=ome-9-4-1826
Timestamp: 2019-04-21 02:54:09+00:00

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Surface nano-gratings of different periods are fabricated on a Ga-doped ZnO (GaZnO) thin film with a high electron concentration for the study of their surface plasmon (SP) resonance behaviors in the near-infrared range. The dispersion curve of the surface plasmon polariton (SPP) based on the ellipsometry measurement of the GaZnO dielectric constant helps in designing the grating period for effective SPP excitation. Spectral depressions of grating reflection under certain incident polarization conditions, corresponding to SP resonance features, are observed in the wavelength range between 1400 and 2200 nm. From the numerical simulation of light scattering from a GaZnO grating structure based on the measured dielectric constant and the fitted Drude model, we can identify either SPP or localized SP modes among the observed SP resonance features. Essentially, it is difficult to excite below 1600 nm SPP at an air/GaZnO interface due to its lossy nature. The potential application of SP resonance on GaZnO is evaluated.
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Fig. 1. (a): Wavelength-dependent refractive index, n, (blue curve with the left ordinate) and extinction coefficient, k, (black curve with the right ordinate) of the GaZnO thin film based on ellipsometry measurement. (b): Real part, ɛ’, (blue continuous curve with the left ordinate) and imaginary part, ɛ”, (black continuous curve with the right ordinate) of the dielectric constant of the grown GaZnO thin film. Here, the red and green dashed curves are plotted for fitting the measured results of ɛ’ and ɛ” based on the Drude model.
Fig. 2. Continuous (dashed) blue curve on the right: Dispersion curve of SPP at the smooth air/GaZnO interface based on the measurement data (the fitted Drude model). “T” indicates the turning point of the dispersion curve. Continuous and dashed green curves on the left (Continuous and dashed red curves in the middle): Dispersion curves of SPP on an air/GaZnO interface grating structure with Λ1 = 750 nm (Λ2 = 1100 nm) based on the measurement data and the fitted Drude model, respectively. The slant dotted lines show the light lines in air with different incident angles, as labeled. The horizontal dashed lines and the intersection points labeled by “A” and “B” indicate the possible conditions for exciting SPP.
Fig. 3. Schematic illustration of the GaZnO nano-grating structure and the definitions of structure parameters and coordinate system. The incident angle with respect to the z-axis is defined as θ (φ) in the x-z (y-z) incidence plane.
Fig. 4. (a) and (b): Plane-view SEM images of the GaZnO grating of Λ1 = 750 nm with two different magnifications. (c): AFM image (3 µm x 3 µm in size) of the GaZnO grating of Λ1 = 750 nm. (d): Line-scan profile of the AFM image in part (c).
Fig. 5. (a)-(d): Results similar to Figs. 4(a)–4(d), respectively, for the GaZnO grating of Λ2 = 1100 nm.
Fig. 6. (a) and (b): Reflectance spectra of a GaZnO (GZO) thin-film structure (d = 300 nm) at various incident angles under the conditions of TE and TM polarizations, respectively. (c) [(d)]: Reflectance spectra of the GaZnO grating of Λ1 = 750 nm at different incident angles, θ (φ), under the TE- (TM-) polarized condition when light is incident in the x-z (y-z) plane.
Fig. 7. (a) [(b)]: Reflectance spectra of the GaZnO grating of Λ1 = 750 nm at different incident angles, θ (φ), under the TM- (TE-) polarized condition when light is incident in the x-z (y-z) plane. (c) and (d): Results similar to parts (a) and (b), respectively, for the GaZnO grating of Λ2 = 1100 nm. The vertical dashed lines indicate the wavelength of systematic perturbation caused by the change of grating set in the measurement system.
Fig. 8. (a) and (b): Simulation results of reflectance spectra at different incident angles under the conditions of TE- and TM-polarized incidences, respectively, from the air side of the GaZnO thin-film structure, corresponding to the experimental results shown in Figs. 6(a) and 6(b), respectively. (c) [(d)]: Simulated reflectance spectra of the GaZnO grating of Λ1 = 750 nm at different incident angles, θ (φ), under the TM- (TE-) polarized incidence condition when light is incident in the x-z (y-z) plane, corresponding to the experimental results shown in Fig. 7(a) [7(b)].
Fig. 9. (a1)-(a12): Charge distributions on the surface of the GaZnO grating with Λ1 = 750 nm at the wavelength of 1500 nm under the TM-polarized excitation when light is incident in the x-z plane at the incident angle of θ = 5 degrees. Parts (a1) through (a12) show the instantaneous charge distributions at the times t = jT/12 (j = 0-11), respectively, where T is the period of electromagnetic oscillation. The blue and red colors represent the opposite charges. (b1)-(b12): Results of charge distributions on the surface of the GaZnO grating with Λ1 = 750 nm similar to parts (a1)-(a12) except that the resonance wavelength is 1620 nm and the incident angle is θ = 60 degrees.
Fig. 10. (a) and (b): Distributions of electric field magnitude in the grating structure of Λ1 = 750 nm, corresponding to the cases in Figs. 9(a1)–9(a12) and 9(b1)–9(b12) at the wavelengths of 1500 and 1620 nm, respectively. (c) and (d): Distributions of electric field magnitude in the grating structure of Λ2 = 1100 nm, corresponding to the cases in Figs. 12(a1)–12(a12) and 12(b1)–12(b12) at the wavelengths of 1610 and 2030nm, respectively.
Fig. 11. (a) and (b): Simulated results similar to those in Figs. 8(c) and 8(d), respectively, for the GaZnO grating of Λ2 = 1100 nm.
Fig. 12. (a1)-(a12): Charge distributions on the surface of the GaZnO grating, similar to Figs. 9(a1)–9(a12), respectively, for the grating of Λ2 = 1100 nm at the wavelength of 1610 nm under the TM-polarized excitation when light is incident in the x-z plane at the incident angle of θ = 5 degrees. (b1)-(b12): Results of charge distributions on the surface of the GaZnO grating with Λ2 = 1100 nm, similar to parts (a1)-(a12) except that the resonance wavelength is 2030nm and the incident angle is θ = 45 degrees.
Fig. 13. (a): Real (with the left ordinate) and imaginary (with the right ordinate) parts of dielectric constants, i.e., ɛ’ and ɛ’’, respectively, of GaZnO, Au, and Ag. (b): SPP propagation lengths at the air/GaZnO, air/Au, and air/Ag interfaces. (c): Lateral penetration depths of SPP at the air/GaZnO, air/Au, and air/Ag interfaces on the air (with the left ordinate) and material (with the right ordinate) sides.
Fig. 14. (a): Experimental results of GaZnO absorption in the grating structure of Λ1 = 750 nm under the labelled four excitation conditions with the incident angle at 30 degrees. (b): Simulation results corresponding to the experimental data in part (a).

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