Enhanced nonlinear optics in resonant GaAs gratings: harmonic generation and optical bistability D. de Ceglia* a , G. D’Aguanno a , N. Mattiucci a , M. A. Vincenti a , M. J. Bloemer b , M. Scalora b a AEgis Technologies Inc., 401 Jan Davis Dr, Huntsville – AL, 35806, USA b Charles M. Bowden Research Center, Redstone Arsenal, Huntsville, AL 35898, USA ABSTRACT We present a study on nonlinear optical processes in GaAs gratings, made by perforating a single layer of GaAs with very narrow slits. Large enhancement of conversion efficiency, for both second and third harmonic generation, is predicted when a TE-polarized pump field excites the guided mode resonances of the grating. At the onset of these modes the spectrum near the pump wavelength shows abrupt changes of linear transmission and reflection that follow a typical Fano-like shape. Under these circumstances, the grating provides dramatic enhancement of local fields and fosters favorable conditions for harmonic generation processes, even in regimes of strong linear absorption at the harmonic wavelengths. In a GaAs grating pumped at 1064nm, we predict second (532nm) and third (354nm) harmonic conversion efficiencies several orders of magnitude larger than conversion rates achievable in either bulk or etalon structures made of the same material. These efficiencies are not influenced by linear absorption, and they are unrelated to grating thickness. We discuss the influence of self-phase modulation on the harmonic generation conversion efficiencies. Finally, we also analyze self phase modulation effects on resonant gratings tuning the input signal at guided mode resonances, demonstrating the possibility of triggering optical bistability at relatively low switching intensities. Keywords: Nonlinear optics, grating, guided mode resonances 1. INTRODUCTION Gratings are characterized by two types of Wood’s anomalies: (i) the Wood-Rayleigh anomalies, associated with sharp variations of diffraction efficiencies at the onset of diffracted orders [1]; and (ii) resonance type anomalies, generated by diffracted orders phase-matched to resonant modes of the grating [2]. Guided mode resonances (GMRs) [3] belong to the latter type of anomalies. The diffraction grating obtained by modulating the index of a waveguide along its propagation direction displays abrupt changes of diffraction efficiency whenever the propagation modes of the waveguide are excited. These anomalies appear regardless of the nature of the excited modes, which may be TE or TM- polarized, and can involve surface waves, like surface plasmon polaritons in metallic gratings, or core-guided modes. When the perturbation of the waveguide is weak, spectral and angular bandwidths of the resulting resonances are usually very narrow, a few nanometers or less, making these gratings highly selective filters. We study harmonic generation and self-phase modulation effects in a 1D, free-standing, semiconductor grating with subwavelength slits using a 1064nm pump wavelength (see Figure 1(a) for a description of the structure). The grating momentum is exploited to excite TE- polarized waveguide modes that form Fano-like resonances [4] in the diffraction efficiency spectrum near the pump field wavelength. We stress that the second harmonic (SH) falls in the visible range, where linear absorption of GaAs yields an attenuation length of only 125nm, while the third harmonic (TH) falls at 354nm, where absorption lengths are even smaller (in Figure 1(b) the pump, SH and TH wavelengths are plotted along with the chromatic dispersion of GaAs). Therefore, any attempt to increase the conversion efficiency based on phase-matching techniques would fail, since the necessary homogeneous harmonic component is absorbed, leaving only the phase-locked (PL) component intact. In fact, the PL component displays the same propagation properties (absorption and phase) as the fundamental wavelength [5]. Thus, the PL component survives even if the material at the SH wavelength is characterized by huge absorption, in either bulk or cavity environments [6], provided the pump is tuned to a region of relative transparency. *deceglia@nanogenesisgroup.com : phone 1 256 955-6278 Metamaterials: Fundamentals and Applications IV, edited by Allan D. Boardman, Nader Engheta, Mikhail A. Noginov, Nikolay I. Zheludev, Proc. of SPIE Vol. 8093, 80932D · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.893696 Proc. of SPIE Vol. 8093 80932D-1 Downloaded from SPIE Digital Library on 15 Sep 2011 to 136.205.47.226. Terms of Use: http://spiedl.org/terms