Reflectance spectra of two porous Si layers formed at two different current densities. The inset shows the fast Fourier transform of each spectrum.
(a) Experimental and (b) theoretical reflectance spectra of a single PSRF in air (nitrogen in the case of the experiment) and (c) in the presence of saturated ethanol vapor. The filter was prepared by applying a sinusoidal current density oscillating between 88 and . The frequency and number of cycles were and 60, respectively.
Experimental resonant shifts of a PSRF at room temperature and the critical radius of the capillary condensation calculated from Eq. (1) as a function of ethanol vapor partial pressures exposed to the filter. The -coordinate term represents the ratio of the analyte partial pressure to its saturation vapor pressure .
Resonant wavelength shift of a PSRF upon exposure to a saturated ethanol vapor (in , ) as a function of time. The introduction and purging of the ethanol pulse are indicated on the trace.
Experimental and simulated reflectance spectra of a double rugate filter, consisting of two filters with equal thickness, prepared by using the same current densities and different frequencies (0.52 and ). The photonic resonances for the top and bottom filters are centered at 480 and , respectively.
Shift of the resonant bands of the double filter in saturated ethanol vapor as a function of top filter thickness: (a) 2, (b) 7, and (c) . The maximum steady-state peak shifts were obtained after a exposure time. The photonic resonances (in nitrogen) for the top and bottom filters are centered at 480 and , respectively.
Transient response of a double filter in saturated ethanol vapor. The shift in position of the photonic resonance for each filter is monitored as a function of time after the introduction of vapor. The photonic resonance for the “upper” and “lower” filters are originally centered at 480 and , respectively. The relative wavelength shift for a given analyte concentration is greater for the longer wavelength resonance. The inset shows the observed spectral response at the earlier exposure time.
Calculated change in the peak positions of a double PSRF whose resonant wavelengths are originally centered at 800 and for the lower and upper filters, respectively, as a function of the volume percentage of the liquid condensate in the porous nanostructure. The inset shows the estimated spectral response at the earlier exposure time.
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