Optical harmonic generation occurs when high intensity light (>10 W m) interacts with a nonlinear material. Electrical control of the nonlinear optical response enables applications such as gate-tunable switches and frequency converters. Graphene displays exceptionally strong light-matter interaction and electrically and broadband tunable third-order nonlinear susceptibility. Here, we show that the third-harmonic generation efficiency in graphene can be increased by almost two orders of magnitude by controlling the Fermi energy and the incident photon energy. This enhancement is due to logarithmic resonances in the imaginary part of the nonlinear conductivity arising from resonant multiphoton transitions. Thanks to the linear dispersion of the massless Dirac fermions, gate controllable third-harmonic enhancement can be achieved over an ultrabroad bandwidth, paving the way for electrically tunable broadband frequency converters for applications in optical communications and signal processing.

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Layered hybrid metal-halide perovskites with non-centrosymmetric crystal structure are predicted to show spin-selective band splitting from Rashba effects. Thus, fabrication of metal-halide perovskites with defined crystal symmetry is desired to control the spin-splitting in their electronic states. Here, we report the influence of halogen parasubstituents on the crystal structure of benzylammonium lead iodide perovskites (4-XC 6 H 4 CH 2 NH 3 ) 2 PbI 4 (X = H, F, Cl, Br). Using X-ray diffraction and second-harmonic generation, we study structure and symmetry of single crystal and thin film samples. We report that introduction of a halogen atom lowers the crystal symmetry such that the chlorine-and bromine-substituted structures are non-centrosymmetric. The differences can be attributed to the nature of the intermolecular interactions between the organic molecules. We calculate electronic band structures and find good control of Rashba splittings. Our results present a facile approach to tailor hybrid layered metal halide perovskites with potential for spintronic and non-linear optical applications.

Optical harmonic generation occurs when high intensity light (> 10 10 W/m 2 ) interacts with a nonlinear material. Electrical control of the nonlinear optical response enables applications such as gate-tunable switches and frequency converters. Graphene displays exceptionally strong-light matter interaction and electrically and broadband tunable third order nonlinear susceptibility. Here we show that the third harmonic generation efficiency in graphene can be tuned by over two orders of magnitude by controlling the Fermi energy and the incident photon energy. This is due to logarithmic resonances in the imaginary part of the nonlinear conductivity arising from multi-photon transitions. Thanks to the linear dispersion of the massless Dirac fermions, ultrabroadband electrical tunability can be achieved, paving the way to electrically-tuneable broadband frequency converters for applications in optical communications and signal processing.The response of a material to interaction with an optical field can be described by its polarization[1]:where E is the incident electric field and ǫ 0 is the permittivity of free space. χ (1) (dimensionless) is the linear susceptibility, while the tensorsare the second-and third-order nonlinear susceptibilities [2]. Thanks to the nonlinear terms of P , new frequencies can be generated inside a material due to harmonic generation[3] and frequency mixing[4]. E.g., in Second Harmonic Generation (SHG) an incident electromagnetic wave with angular frequency ω 0 = 2πν, with ν the photon frequency, generates via χ (2) a new electromagnetic wave with frequency 2ω 0 [3]. The SHG efficiency (SHGE) is defined as the ratio between the SH intensity and the intensity of the incoming light. Analogously, Third Harmonic Generation (THG) is the emission of a photon with energy triple that of the incident one. The THG efficiency (THGE) is defined as the ratio between the TH intensity and the intensity of the incoming light. Second-order nonlinear processes are also known as three-wave-mixing, as they mix two optical fields to produce a third one [5]. Third-order nonlinear processes are known as four-wave-mixing (FWM)[5], as they mix three fields to produce a fourth one. Nonlinear optical effects are exploited in a variety of applications, including laser technology[6], material processing[7] and telecommunications [8]. E.g., to generate new photon frequencies (532nm from SHG in a Nd:YAG laser at 1.06µm) [9] or broadly tuneable ultrashort pulses (fs-ps) by optical parametric amplifiers (OPAs)[10] and optical parametric oscillators (OPOs) [11]. High harmonic generation is also used for extreme UV light [12] and attosecond pulse generation [13], while difference frequency generation is used to create photons in the THz range [14].Second order nonlinear effects can only occur in materials without inversion symmetry, while third order ones occur in any system independent of symmetry[15], and they thus represent the main intrinsic nonlinear response for most materials. THG intensity enhancement was achieved...

Non-radiative recombination in all-inorganic CsPbBr3 perovskite films was dramatically reduced upon partial replacement of lead ions with magnesium, leading to efficient and stable perovskite light-emitting diodes.