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Motivated by the nonlinear Hall effect observed in topological semimetals, we studied the photocurrent by the quantum kinetic equation. We recovered the shift current and injection current discovered by Sipe et al., and the nonlinear Hall current induced by Berry curvature dipole (BCD) proposed by Inti Sodemann and Liang Fu. Especially, we further proposed that 3-form tensor can also induce photocurrent, in addition to the Berry curvature and BCD. This work will supplement the existing mechanisms for photocurrent. In contrast to the shift current induced by shift vector, all photocurrents induced by gradient/curl of Berry curvature, and high rank tensor require circularly polarized light and topologically non-trivial band structure, viz. non-vanishing Berry curvature.
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Although Weyl fermions have proven elusive in high-energy physics, their existence as emergent quasiparticles has been predicted in certain crystalline solids in which either inversion or time-reversal symmetry is brokencite{WanPRB2011,BurkovPRL2011, WengPRX2015,HuangNatComm2015}. Recently they have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention cite{XuScience2015, LvPRX2015, YangNatPhys2015}. The question that arises now is whether these materials will exhibit novel, enhanced, or technologically applicable electronic properties. The TMMPs are polar metals, a rare subset of inversion-breaking crystals that would allow spontaneous polarization, were it not screened by conduction electrons cite{anderson1965symmetry,shi2013ferroelectric,kim2016polar}. Despite the absence of spontaneous polarization, polar metals can exhibit other signatures of inversion-symmetry breaking, most notably second-order nonlinear optical polarizability, $chi^{(2)}$, leading to phenomena such as optical rectification and second-harmonic generation (SHG). Here we report measurements of SHG that reveal a giant, anisotropic $chi^{(2)}$ in the TMMPs TaAs, TaP, and NbAs. With the fundamental and second harmonic fields oriented parallel to the polar axis, the value of $chi^{(2)}$ is larger by almost one order of magnitude than its value in the archetypal electro-optic materials GaAs cite{bergfeld2003second} and ZnTe cite{wagner1998dispersion}, and in fact larger than reported in any crystal to date.
Following our ab initio nonlinear optical (NLO) materials design guidelines, in this Letter, we discovered a novel type of structure to realize potential deep-ultraviolet (DUV) NLO performance in the classical beryllium borate system. By densely stacking the NLO-active layered frameworks, the key design scheme for the structural evolution from the (Be2BO3F2) layers in KBe2BO3F2 (KBBF) to the novel (Be2BO5H3) layers in berborite is illustrated. Based on available experimental results and systematical theoretical evaluation from first principles, the NLO properties of berborite are further obtained as comparable as the only pratical DUV NLO crystal KBBF. It is demonstrated that berborite can achieve available DUV phase-matched output with strong NLO effect for the practically important 177.3 nm and 193.7 nm lasers. Once obtained with sizable single crystal, it can be applied as a promising DUV NLO crystal.
We present an analytical model describing complex dynamics of a hybrid nonlinear system consisting of interacting carbon nanotubes (CNT) and a plasmonic metamaterial. Our model is based on the set of coupled equations, which incorporates well-established density matrix formalism appropriate for quantum systems (CNT are described as a two level system) and harmonic-oscillator approach ideal for modelling sub-wavelength plasmonic and optical resonators. We show that the saturation nonlinearity of CNT increases multifold in the resonantly enhanced near field of a metamaterial. In the framework of our model, we discuss the effect of inhomogeneity of the CNT layer (band gap value distribution) on the nonlinearity enhancement. It is shown, that the Purcell effect is indistinguishable from the field enhancement and is described by the same phenomenological constant.
We grow accustomed to the notion that optical susceptibilities can be treated as a local property of a medium. In the context of nonlinear optics, both Kerr and Raman processes are considered local, meaning that optical fields at one location do not produce a nonlinear response at distinct locations in space. This is because the electronic and phononic disturbances produced within the material are confined to a region that is smaller than an optical wavelength. By comparison, Brillouin interactions can result in a highly nonlocal nonlinear response, as the elastic waves generated through the Brillouin process can occupy a region in space much larger than an optical wavelength. The nonlocality of these interactions can be exploited to engineer new types of processes, where highly delocalized phonon modes serve as an engineerable channel that mediates scattering processes between light waves propagating in distinct optical waveguides. These types of nonlocal optomechanical responses have been recently demonstrated as the basis for information transduction, however the nontrivial dynamics of such systems has yet to be explored. In this work, we show that the third-order nonlinear process resulting from spatially extended Brillouin-active phonon modes involves mixing products from spatially separated, optically decoupled waveguides, yielding a nonlocal joint-susceptibility. We further explore the coupling of multiple acoustic modes and show that multi-mode acoustic interference enables a tailorable nonlocal-nonlinear susceptibility, exhibiting a multi-pole frequency response.
We conduct a theoretical study of the bistable optical response of a nanoparticle heterodimer comprised of a closely spaced semiconductor quantum dot and metal nanoparticle. The bistable nature of the response results from the interplay between the quantum dots optical nonlinearity and its self-action (feedback) originating from the presence of the metal nanoparticle. We show that the feedback is governed by a complex valued coupling parameter $G$. Both the real and imaginary parts of $G$ ($G_mathrm{R}$ and $G_mathrm{I}$) play an important role in the occurrence of bistability, which is manifested in an S-shaped dependence of the quantum dot excited state population on the intensity of the external field, and hysteresis of the population. From our calculations, we find that at $G_mathrm{R} = 0$, the critical value for bistability to occur is $G_mathrm{I} = 8Gamma$, whereas at $G_mathrm{I} = 0$, the critical value of $G_mathrm{R} = 4Gamma$, where $Gamma$ is the polarization dephasing rate. Thus, there exist two different (limiting) mechanisms of bistability, depending on whether $G_mathrm{R}$ is much larger or much smaller than $G_mathrm{I}$. We also calculate the bistability phase diagram within the systems parameter space: spanned by $G_mathrm{R}$, $G_mathrm{I}$ and $Delta$, the latter being the detuning between the driving frequency and the transition frequency of the quantum dot. Additionally, switching times from the lower stable branch to the upper one (and {it vise versa}) are calculated as a function of the intensity of the driving field. We show that the conditions for bistability to occur can be realized, for example, for a heterodimer comprised of a closely spaced CdSe (or CdSe/ZnSe) quantum dot and a gold nanosphere.
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