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Impact of GaP layer deposition upon photonic bandgap behaviour of opal

Thin (less than 4 nm thick) film layers of InP and GaP were deposited onto the inner surface of synthetic opal to enhance the refractive index contrast of this photonic crystal. The MOCVD process was deliberately modified to produce in-void chemical synthesis of compound semiconductors. The overall effect upon the photonic bandgap (PBG) behaviour appears to be large in coated opals as compared with bare opal. Apart from the `red' shift of the stop-band in accordance with Bragg's law, the angular dispersion was found to be squeezed by nearly a factor of two, within the same range of angles of oblique light incidence, compared with bare opal. A trend has been demonstrated towards overlapping of the stop-bands over a wider range of directions in the photonic crystal as coating thickness increases. Improved overlap of the stop-bands results in the stronger suppression/enhancement of the spontaneous emission from GaP-coated opal as compared with bare opal.