Multiple Higher-Order Stop Gaps in Infrared Polymer Photonic Crystals

Engineering of stop gaps between higher photonic bands provides an alternative to miniaturization of photonic crystals. Femtosecond laser microfabrication of highly correlated void channel polymer microstructures results in photonic crystals with large stop gaps and a multitude of higher-order gaps in the mid- and near-infrared spectral regions. The gap wavelengths obey Bragg’s law. Consistent with theory, varying the woodpile structure unit cell allows for tuning the number of higher-order gaps, and transitions from mere resonant Bragg scattering to stop band total reflection are observed.

Figure 1

(a) Photonic band structures of woodpile-type polymer void channel photonic crystals for ratios $\delta z/\delta x$ of 1.0 (right) and 2.0 (left, only $\Gamma X^{\prime }$), as calculated for $\delta z=2.5\mathrm{\mu }\mathrm{m}$ and elliptical channel cross section of diameters 750 nm and $1.125\mathrm{\mu }\mathrm{m}$. The number of higher-order gaps strongly depends on $\delta z/\delta x$. Inset: Geometrical arrangement of void channels and corresponding first Brillouin zone. (b) Dependence of the gap/midgap ratio on the filling ratio for various values of $\delta z/\delta x$.

Figure 2

Infrared spectra of woodpile structures at constant $\delta z=1.6\mathrm{\mu }\mathrm{m}$ with $\delta x$ varying from 1.14 to $1.6\mathrm{\mu }\mathrm{m}$. (a) As measured; dashed line: polymer absorption. (b) Fundamental photonic band gap and (c) first higher-order gap after baseline correction. (d) Linear dependence of the gap wavelength on the inverse in-plane channel spacing.

Figure 3

Higher-order bandgaps in transmission (a) and reflection (b) for constant $\delta z/\delta x=2.0$ with layer spacing $\delta z$ varying from 2.4 to $2.7\mathrm{\mu }\mathrm{m}$. Dips in transmission correspond to spikes in reflection. (c) First and (d) second higher-order gap wavelengths as a function of $\delta z$ with linear curve fit and photonic band theory results.

Figure 4

(a) Transmission and (b) reflection infrared spectra at constant $\delta z=2.55\mathrm{\mu }\mathrm{m}$ and varying unit cell geometry ($\delta z/\delta x=2.13–1.0$). Note the emergence of higher-order band gaps ($m=2,3$) with increasing $\delta z/\delta x$. (c) Calibrated peak reflectances reveal the transformation of Bragg resonances into photonic band gap reflections.