We study the effect of charge state conversion of nitrogen vacancy (NV) centers on the local density of states (LDOS) induced modifications in emission lifetime. The pump-dependent decay rates reveal two power regimes according to the dominance of photoionization of NV centers or the effect of LDOS. The extent of lifetime modification is achieved at low pump power, whereby the emission rate of zero phonon line is enhanced and the rate of phonon sideband emission is suppressed. At high pump power, the effect of modified emission rate is demeaned due to increased photoionization of NV centers.
We discuss a promising method to assess the refractive index profile of vertically aligned disordered Silicon nanowire arrays. The aberration-free micro-reflectivity set-up equipped with an in-situ optical microscope is designed to measure the reflectivity from 4μm2 area of the nanowires. The spatial- and polarization-dependent reflectivity values along the nanowire length is used to estimate the refractive index profile. The transfer matrix method involving the estimated refractive index profiles is employed to corroborate the measured reflectivity values. The disordered Silicon nanowires with gradient refractive index profile can suppress 96 % reflectivity irrespective of direction, wavelength, and polarization which make them a potential candidate for photon management applications.
We report the experimental observation of multiple Bragg diffraction that occurs when the tip of the incident wave vector lies on a line joining the L and W points in the Brillouin zone of a face-centered cubic (FCC) photonic crystal. The multiple Bragg diffraction is analyzed for photonic crystals with different lattice constants and refractive index contrasts. Angle-dependent reflectance spectroscopy indicates strong hybridization of diffraction resonances when the tip of the incident wave vector crosses the W point and the multiple Bragg diffraction is seen to be extended over an angular range of 8 deg around the W point. We also observe a new diffraction resonance in the short-wavelength region for wave vectors shifting toward the W point in the hexagonal facet of the FCC Brillouin zone. Each diffraction resonance is fitted using the Bragg’s law for different planes in the FCC photonic crystal taking into account the internal angle between the planes. The diffraction resonances in the multiple Bragg diffraction regime are assigned to FCC crystal planes with Miller indices (111), (200), (1¯11), and (220). Our results have implications for diverse kinds of wave propagations in periodic structures and applications in light emission, sensing, and structural color pigments.
We present a systematic analysis of three dimensionally ordered photonic crystals made of dyed-polystyrene spheres fabricated using the newly reported inward growing self assembling method. The SEM and AFM images show good ordering of the spheres with (111) plane parallel to the substrate. The photonic stop band is tuned by varying the angle of incidence of light and made to coincide with the emission wavelength of the dyes. The multiple Bragg diffraction effects observed at high angles of incidence are interpreted. The high reflectance values obtained along with the observation of the higher order bands, and the photonic stop band splitting at high angles of incidence show the superior quality of the photonic crystals fabricated using these dyed polystyrene spheres in less than three hours.