JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 12, No. 1, January 2010, p. 57 Phase locked second and third harmonic localization in semiconductor cavities V. ROPPO a,b , C. COJOCARU a , G. D’AGUANNO b , F. RAINERI c,d , J. TRULL a , Y. HALIOUA c,e , R. VILASECA a , R. RAJ c , M. SCALORA b a Universitat Politècnica de Catalunya, Dept. de Física i Eng. Nuclear, Colom 11, 08222 Terrassa, SPAIN b C. M. Bowden Research Facility, US Army, RDECOM, Redstone Arsenal, AL 35803, USA c Laboratoire de Photonique et de Nanostructures (CNRS), Marcoussis, FRANCE d Université Paris-Diderot, 75205 Paris Cedex 13, FRANCE e Ghent University-IMEC, Depart. of Information Technology- Sint-Pietersnieuwstraat 41, 9000 Gent, BELGIUM We study the enhancement of the second and third harmonic generation using ultra short pulses in a cavity environment, focusing on the role of the phase locking phenomena. Despite the fact that the cavity is only resonant at the fundamental frequency and the harmonics are tuned in a spectral range of huge nominal absorption, we predict and experimentally observe the harmonics become localized inside the cavity leading to relatively large conversion efficiencies. This unique behavior reveals new optical phenomena and new applications for opaque nonlinear materials (i.e. semiconductors) in the visible and UV ranges. (Received Keywords: Nonlinear optics, Harmonics generation 1. Introduction The study of harmonics generation begins in the early 1960s with the theoretical and experimental works on second harmonic (SH) generation [1, 2]. By now second harmonic generation (SHG) is a well-understood phenomenon that finds practical applications in many areas. Second and higher harmonics arise thanks to the nonlinear relation that links the polarization to the electromagnetic fields. In most cases this relation can be assumed to behave linearly. However, when the intensity of the incident pump field or fields is high enough and/or if the nonlinear coefficients of the material are relatively high, nonlinear effects may become important. To date, most researchers’ efforts have been directed at improving the efficiency of the harmonic generation process by engineering new materials with higher effective nonlinear coefficients, accompanied by phase and group velocity matching [3-6]. This is why most studies have been concerned with maximizing conversion efficiencies through the achievement of phase matching (PM) conditions, i.e. an attempt at equalizing the relative phases of the beams to ensure maximum energy transfer from the fundamental (FF) beam to the second or third harmonic (TH) signals. Outside of PM condition the efficiency decreases rapidly [7]. The resulting low conversion efficiency is the reason why propagation phenomena in the mismatched regime remain largely unexplored. 2. Phase locked harmonic localization Recently, an effort was initiated to more systematically study the dynamics of second and third harmonic generation in transparent and opaque materials under conditions of phase mismatch [8-11]. The dynamic of the phase locking (PL) mechanism is deeply related with the harmonic generation theory. As outlined in the seminal work [2], the energy transfer between the fundamental field and its generated SH always happens at the interface and it is limited near it. Then, how much near depends on the working conditions and the dimensions of the pulses and samples. Generally speaking, the SH pulse will always experience an index of refraction bigger than the fundamental and thus will travel slower. The exchange of energy will take place from the interface up to when the walk-off of the two pulses is complete and the harmonic pulse is not longer under the influence of the fundamental pulse due the velocity difference. With these considerations in mind, it not surprising to bury the fact that when a fundamental (pump) pulse crosses an interface between a linear and a nonlinear medium there are always three generated SH and/or TH components. One component is generated backward into the incident medium, due the index mismatch between the materials that form the interface and the other two are generated forward. The basis for understanding the generation of two distinct forward-moving signals is based on the mathematical solution of the homogeneous and inhomogeneous wave equations at the SH frequency [2]. Continuity of the tangential components of all the fields at the boundary leads to generation of the two forward-