Nonlinear probing of the fundamental lattice vibration of polar crystals is shown to reveal insight into higher-order cohesive lattice forces. With a free-electron laser tunable in the far infrared we experimentally investigate the dispersion of the second-order susceptibility due to the phonon resonance in GaAs. We observe a strong resonance enhancement of second harmonic light generation at half the optical phonon frequency, and in addition a minimum at a higher frequency below the phonon frequency. Measuring this frequency and comparison to a theoretical model allows the determination of competing higher-order lattice forces. DOI: 10.1103/PhysRevLett.90.055508 PACS numbers: 63.20.Ry, 42.65.An, 42.65.Ky, 78.20.-e Since the invention of lasers the field of nonlinear optics evolved rapidly starting with the observation of frequency doubling of a pulsed ruby laser in crystalline quartz [1]. Since these times a multitude of materials has been investigated and developed in order to yield high second-order nonlinear susceptibilities for frequency conversion, enabling the generation of light from the mid IR to the UV covering many wavelengths where no laser source directly emits radiation. However, in the THz frequency range (1-10 THz) a remarkable gap exists in the availability of tunable and intense light sources with the consequence that nonlinear optics in this frequency range remains to great extents unexplored [2]. Thussince the pioneering work of Faust and Henry on frequency mixing with a HeNe laser and a far-infrared (FIR) H 2 O laser in GaP [3] and microwave-mixing experiments in a variety of crystals by Boyd et al. [4] only a few experiments on the dispersion of the nonlinear susceptibility in the THz frequency range have been performed [5][6][7]. The lack of firm experimental data on a large number of technologically important crystals has been an obstacle for the development of quantitative theories. In the last decade intense free-electron lasers (FEL) tunable in the FIR have become available for the study of nonlinear optical phenomena. Recently, the first quantum cascade laser has been realized in a GaAs=AlGaAs heterostructure emitting at 4.4 THz, which may open the door for integrated nonlinear optics in this frequency range [8]. Here we report on a study of second harmonic generation (SHG) in the range of 4.0 to 6.0 THz in thin GaAs films performed with a FEL. This experiment provides insight into the influence of higherorder terms of the lattice potential on the nonlinear susceptibility in the THz frequency range.The nonlinear optical susceptibility in semiconductors in the THz frequency range is governed by the superposition of electronic and ionic contributions. Faust and Henry were the first to determine the dispersion of the THz nonlinear susceptibility in GaP [3]. They observed a resonance enhancement at the TO phonon frequency and showed that the ionic and electronic contributions are of opposite sign, leading to a cancellation of both contributions below the phonon resonance. They showed...

Dykes composed of basic rocks and granite are formed due to interactions between melts in a wide range of conditions, from contrasting compositions and fluid saturation rates to various tectonic settings and processes at different depths. Textures and petrochemical characteristics of the dykes are thus widely variable. This paper is focused on composite dykes observed in the West Sangilen region in SouthEast Tuva, Russia. The Sangilen wedge is a fragment of the Early Caledonian orogenic structure of the Tuva-Mongolia Massif which evolved in a succession of geodynamic settings, from collision (transpression, 570-480 Ma) to transform faulting (transtension, 480-430 Ma). Intensive tectonic deformation facilitated massive basic-rock and granite magmatism at various layers of the crust and associated heating and metamorphism of the rocks (510-460 Ma). Basic-rock-granite composite dykes were formed in the above-mentioned period in various tectonic settings that controlled conditions of dyke intrusions and their compositions. We distinguish two groups of composite dykes observed on two sites, in the area between the Erzin and Naryn rivers and on the right bank of the Erzin river (Strelka and Erzin Sites, respectively) (Fig. 1). The dykes in both groups originated from one and the same basic-rock melt source. However, mingling of the contrasting melts was carried out by different mechanisms as suggested by the proposed intrusion models. In the area between the Erzin and Naryn rivers (Strelka Site), the host rock of the composite dykes is granite of the Nizhneerzin massif. The mingling dykes are composed of amphibole gabbro and monzogabbro, granosyenite and twofeldspar granite. Contacts between basic and felsic rocks vary from smooth contrasting to complex 'lacerated' flameshaped, and gradual transition zones are present (Fig. 6). The dykes were formed at mesoabyssal or abyssal depths, and the subliquidus heat regime was thus maintained for a long time, and even the smallest portions of the basic-rock melt were consolidated through quite a long period of time. As a consequence, indicators of deformation are lacking in the composite dykes, while transition zones and hybridization are present. On the right bank of the Erzin river (Ersin Site), the dykes cut through migmatite-granite of the Erzin formation in the same-name tectonic zone. Contacts with host rocks are transverse. Melanocratic rocks are represented by smallgrained diorite and quartz diorite, and the felsic composite dykes are composed of medium-and small-grained twofeldspar granite and leukogranite. Transition zones, hornfelsing and contact alterations are absent at contacts of all the types (Fig. 8). The composite dykes of this type intruded and emplaced when the shear zone was subject to extension and fragmentation, which predetermined active intrusion of basic and, possibly, felsic melts through conjugated faults. Crystallization of the melts was rapid, and their potential heat impact on the adjoining rocks was thus excluded, as evidenced by the presence of ...

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We have measured an enhancement factor of Raman signal up to 30 times using a Fabry-Pérot structure made of porous silicon (PS) layers of different porosity. The obtained enhancement was due to the coupling of the laser radiation and Stokes photons of porous silicon with the microcavity mode at the optimal laser beam incidence and scattering angles. Our results provide a way to increase the sensitivity of Raman spectroscopy for studying the species inside porous silicon which can considerably influence the properties of this material and hence of PS based devices.