Strongly Modified Spontaneous Emission Rates in Diamond-Structured Photonic Crystals

The spontaneous emission decay dynamics of nanocrystal quantum dots embedded into biotemplated titania photonic crystals with a diamond-based lattice are investigated. Modification of the decay rate of quantum dot emission over wide frequency bandwidths in the visible by the photonic crystals is observed. Frequency-dependent analysis reveals both inhibition and enhancement of emission with a radiative lifetime variation by more than a factor of 10.

Figure 1

(a) Calculated photonic band structure for a diamond-based lattice of air cylinders surrounded by dielectric with refractive index of 2.2; shown is the low-frequency region around the overlapping PSGs. Calculations are based on scanning electron microscopy images of the titania PC lattice ($a=354\pm 9\mathrm{nm}$) used in this study (b). Inset in (b) shows the dielectric model for the band structure calculations. (c) Corresponding calculated DOS of PC lattice described in (a). See supplemental material [33] for details of photonic band structure and DOS calculation. Scanning electron microscopy image was adapted from Ref. 29.

Figure 2

(a) Selected PL emission decay curves plotted on a normalized log scale of QDs embedded into various titania PCs. QD emission in the region of strong PSG overlap (solid line; calculated lifetime of $99\pm 2\mathrm{ns}$) and at the low-frequency band edge (dotted; calculated lifetime of $8\pm 1\mathrm{ns}$) of the titania PC with a diamond-based lattice (for a full range of decay curves, see supplemental material [33]). QD emission inside the titania inverse-opal $\Gamma \mathrm{\text{−}}L$ PSG (dashed; calculated lifetime of $20\pm 1\mathrm{ns}$) and in the titania reference sample outside of any PSGs (dashed-dotted; calculated lifetime of $14\pm 1\mathrm{ns}$). Reported lifetimes reflect the peak of the log-normal distribution of the decay curve fitting. (b) Decay rates of QD spontaneous emission over a broad frequency range of the band structure of the titania PC with a diamond-based lattice, including regions of normal, enhanced, and inhibited emission. All decay rates are given relative to the decay rates of the same QDs in a titania reference sample outside any PSGs. Vertical error bars indicate the variation of the measured lifetime over several spatial positions in the sample. Horizontal bars represent the spectral width over which the measurements were made.

Figure 3

(a) Calculated photonic band structure for the biopolymeric (chitin) PC used in this study; shown is the low-frequency region with three overlapping PSGs. (b) PL emission decay curves plotted on a normalized log scale for QDs in the region of overlapping PSGs (top) and far away from any PSGs (bottom). The inset in (b) shows the calculated DOS for the biopolymeric (chitin) PC. Both decay curves were measured at $18,350{\mathrm{cm}}^{-1}$ in two different isomorphic structures with different lattice constants. The reduced frequency positions are indicated by the two arrows and the width of the arrows resembles the lattice constant uncertainty of these biological photonic structures.