0021-3640/04/8010- $26.00 © 2004 MAIK “Nauka/Interperiodica” 0633 JETP Letters, Vol. 80, No. 10, 2004, pp. 633–637. Translated from Pis’ma v Zhurnal Éksperimental’noœ i Teoreticheskoœ Fiziki, Vol. 80, No. 10, 2004, pp. 737–742. Original Russian Text Copyright © 2004 by Gusev, Martemyanov, Soboleva, Dolgova, Fedyanin, Aktsipetrov. The nonlinear optics of microstructures with a pho- tonic band gap is one of the fields of modern optics [1]. The simplest object of this class is a Bragg reflector consisting of layers with a thickness of about optical wavelength and periodically alternating refractive indi- ces. The reflection spectrum of the Bragg reflector con- tains a frequency band that is characterized by a high reflectance and corresponds to the photonic band gap. A microcavity is obtained by doubling the optical thickness of one of the central layers of the Bragg reflector (1D photonic crystal) and has the resonance state of the electromagnetic field, i.e., a mode whose frequency coincides with the center of the photonic band gap. If the optical thicknesses of several layers in the Bragg reflector are changed, the coupled microcav- ities with several eigenmodes are formed [2]. The sim- plest coupled microcavity has two identical microcav- ity layers separated by the intermediate Bragg reflector. Interchange of electromagnetic-field energy between the microcavities, which is determined by the reflec- tance of the intermediate reflector; removes the degen- eration of modes and leads to their frequency–angular splitting, which is a measure of the coupling between the microcavities [3]. The reflection spectrum of such coupled microcavities has two dips in the photonic band gap, which correspond to resonances of incident light with the modes of coupled microcavities [4]. In coupled microcavities, the enhancement of cubic nonlinear optical processes, such as third-harmonic generation, can be observed. The mechanism of such an enhancement, as in the case of the recently observed effect of the enhancement of second-harmonic genera- tion [5], is an increase in the energy density of the fun- damental field within the microcavities due to the spa- tial localization of the fundamental radiation in reso- nance with one of the modes of the coupled microcavities. This mechanism is predominant in the case of single microcavities [6], where the contribution to the third-harmonic field from the microcavity layer is much larger than the contributions from the Bragg- reflector layers [7]. In coupled microcavities, an addi- tional mechanism of the enhancement of the third-har- monic generation is possible. It is the constructive interference of the third-harmonic waves from micro- cavity-layer areas. In this case, the intensity of the third harmonic is maximal in the frequency–angular position shifted from the maximum localization of the funda- mental field in a sample. In this work, we present the observation of the reso- nance enhancement of the optical third-harmonic gen- eration in 1D coupled microcavities based on mesopo- rous silicon upon the angular tuning of fundamental radiation near their split modes. It is shown that the angular spectrum of the third-harmonic intensity depends on the coupling of microcavities (the reflec- tance of the intermediate Bragg reflector). The square susceptibility of mesoporous silicon is low, because the initial silicon crystal is centrosymmetric. This excludes the cascade third-harmonic generation in coupled microcavities and enables one to attribute the detected radiation to the direct generation via the cubic suscep- tibility of mesoporous silicon. A mechanism of the χ ˆ 3 (29 Optical Third-Harmonic Generation in Coupled Microcavities Based on Porous Silicon D. G. Gusev*, M. G. Martemyanov, I. V. Soboleva, T. V. Dolgova, A. A. Fedyanin**, and O. A. Aktsipetrov Department of Physics, Moscow State University, Vorob’evy gory, Moscow, 119992 Russia * e-mail: denis@shg.ru ** e-mail: fedyanin@shg.ru Received July 12, 2004; in final form, October 11, 2004 The resonance features of the third-harmonic generation have been observed in 1D coupled microcavities con- sisting of three Bragg reflectors and two identical half-wave layers of mesoporous silicon. The third-harmonic intensity increases by a factor of about 10 3 in the resonance of fundamental radiation with each of the modes of coupled microcavities. It has been shown that the resonance positions in the angular spectra of the third-har- monic intensity depend on the coupling between microcavities that is determined by the transmission of the intermediate Bragg reflector. In the framework of the transfer-matrix method with nonlinear sources, it has been shown that the basic mechanism of the enhancement of the third-harmonic generation in coupled microcavities based on porous silicon is the constructive interference of the partial third-harmonic waves that are generated by near-surface layers. © 2004 MAIK “Nauka/Interperiodica”. PACS numbers: 42.65.Ky; 42.70.Qs; 78.67.–n; 78.67.Pt