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The temporal evolution of the photonic stop band shift governed by the photoinduced phase transition in a three-dimensional photonic crystal based on opal-vanadium dioxide has been studied. After the ultrafast shift of the Bragg reflectivity peak by 25 meV, occurring during the femtosecond laser pulse excitation, a further increase of the shift up to ϳ60 meV is observed for ϳ500 ps. The theoretical analysis shows that the phase transition during the laser pulse occurs in a limited fraction of opal pores filled with vanadium dioxide, while the later temporal component reflects the spatial evolution of the phase boundary inside each pore.Three-dimensional ͑3D͒ photonic crystals ͑PhCs͒ are prospective materials for nanophotonics. 1 For example, ultrafast switching of the photonic stop band ͑PSB͒ can be a basis for the manipulation of light beams in PhCs on a subpicosecond time scale. 2-6 One way to switch the spectral position of a PSB is to use a photoinduced phase transition, accompanied by permittivity changes of the constituents forming the 3D PhC. The PSB switching time in this case may be less than 10 −12 s. 6 Common to all of them is the excitation of the PhC material by intense laser pulses with a photon energy higher than its fundamental band gap to generate hot carriers which, in turn, induce the phase transition directly or indirectly via generation of phonons. In both cases the temporal and spatial evolutions of the photoinduced phase transition during and after the laser pulse are governed by the kinetic properties of the photoexcited quasiparticles ͑electrons, phonons, plasmons, etc.͒.A material which shows such an ultrafast photoinduced phase transition and can be used as basis for a 3D PhC is vanadium dioxide ͑VO 2 ͒, 7 The first-order semiconductormetal phase transition of VO 2 occurs at T c = 68°C, and is accompanied by strong changes in the dielectric constant for visible light. 8 Note that in near-infrared region the absorbance of VO 2 metallic nanoclusters are strongly changed due to the effect of surface-plasmon resonance. 9,10 The kinetics of the phase transition in VO 2 films has been studied in detail for different time scales. 10-14 The absorption length in VO 2 for visible light excitation is ϳ100 nm ͑Ref. 13͒ and is comparable with the size of the opaline pores occupied by VO 2 ͑hereafter VO 2 elements͒ forming PhCs with a PSB in the visible region. 15 Therefore, it may happen that the photoexcitation does not penetrate far enough to induce the phase transition instantaneously by a laser pulse within the whole volume of the VO 2 element. Instead, after photoexcitation the evolution of the temporal and spatial distributions of the two phases in the 3D PhC governs the PSB position and width.In the present work, we study the change of the PSB due to the photoinduced phase transition in a 3D opal-VO 2 PhC with time on scales from 10 −13 to 5 ϫ 10 −10 s. The results show that an ultrafast shift of the PSB by 25 meV during the laser pulse is followed by a further, subsequent shift up to values ...

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