Enhanced second-harmonic generation for multiple wavelengths by defect modes in one-dimensional photonic crystals

We present an effective design of aperiodically stacked layers of nonlinear material and air, sandwiched by two truncated photonic crystals, in terms of the simulation annealing method. The constructed structure can achieve multiple-wavelength second-harmonic generation (SHG) at the preassigned wavelengths. We derive a general solution of SHG in 1D inhomogeneous systems and apply it to evaluate the SHG conversion efficiency. Numerical simulations show that the conversion efficiency of SHG can be significantly enhanced when the fundamental wave frequencies are assigned to the designed defect states.     

A way for enhancing second harmonic generation in one-dimensional nonlinear photonic crystals

We investigate one-dimensional nonlinear photonic crystals consisting of ferroelectric domains with the modulated polarization directions. Significant enhancement of second harmonic generation (SHG) is observed from numerical simulations when the frequency of fundamental wave is aimed at the photonic band edge. We devise the model structure with optimal configuration of the polarization directions of the ferroelectric domains in terms of simulated annealing algorithm. The conversion efficiencies of the ‘forward’ and ‘backward’ SHGs can be engineered.     

Collinear second harmonic generation of 20 wavelengths in a single two-dimensional decagonal nonlinear photonic quasi-crystal

We report on the observation of quasi-phase matched collinear second harmonic generation (SHG) at 20 wavelengths in a two-dimensional nonlinear photonic quasi-crystal with decagonal lattice. We show that at some wavelengths the second harmonics are generated via standard quasi-phase matching, namely a reciprocal vector exists that equals the phase-mismatch vector, while at others the SHG is as a result of the projection-based quasi-phase matching in which the momentum conservation is satisfied up to a projection of a reciprocal vector onto the direction of propagation. In spite of different generation mechanisms, the reciprocal vectors (or their projections) involved in the collinear QPM SHG can be described by a generalized equation.     

Resonant behavior and selective switching of stop bands in three-dimensional photonic crystals with inhomogeneous components

We demonstrate that, in contrast with the well-studied photonic crystals consisting of two homogeneous components, photonic crystals comprised of inhomogeneous or multiple (three or more) components may bring new opportunities to photonics due to the discovered quasiperiodic resonant behavior of their (hkl) stop bands as a function of the reciprocal lattice vector. A resonant stop band cannot be switched off for any permittivity of structural components. Tuning the permittivity or structural parameters allows the selective on-off switching of nonresonant (hkl) stop bands. This independent manipulation of light at different Bragg wavelengths provides a new degree of freedom to design selective optical switches and waveguides. Transmission experiments performed on synthetic opals confirmed the theoretical predictions.     

Second order optical susceptibilities in alkali halide crystals due to FA color centers

This paper presents the first calculation of the second order optical nonlinearity of the F_A color center for second harmonic generation (SHG) in KCl:Li crystals. The real and imaginary components of the nonlinear susceptibility evaluated over a wide spectral range enclosing the resonance region of the center show interesting feature. Second harmonics generation and three wave mixing processes are nonlinear optical effects based on the second order susceptibility of materials and are very effective in providing laser radiation over a wide range of wavelengths. The density matrix formalism is employed and the equation of motion is solved by second order perturbation to evaluate the nonlinear susceptibility for SHG. It is found that the system shows large resonance-enhanced second order susceptibilities for color center concentration of 10²³ m⁻³. A scheme of phase matching in terms of anomalous dispersion of the centers is discussed.     

Broadly wavelength- and pulse width-tunable high-repetition rate light pulses from soliton self-frequency shifting photonic crystal fiber integrated with a frequency doubling crystal

Soliton self-frequency shift (SSFS) in a photonic crystal fiber (PCF) pumped by a long-cavity mode-locked Cr:forsterite laser is integrated with second harmonic generation (SHG) in a nonlinear crystal to generate ultrashort light pulses tunable within the range of wavelengths from 680 to 1800?nm at a repetition rate of 20?MHz. The pulse width of the second harmonic output is tuned from 70 to 600?fs by varying the thickness of the nonlinear crystal, beam-focusing geometry, and the wavelength of the soliton PCF output. Wavelength-tunable pulses generated through a combination of SSFS and SHG are ideally suited for coherent Raman microspectroscopy at high repetition rates, as verified by experiments on synthetic diamond and polystyrene films.     

Two-color nonlinear localized photonic modes

We analyze second-harmonic generation (SHG) at a thin effectively quadratic nonlinear interface between two linear optical media. We predict multistability of SHG for both plane and localized waves, and also describe two-color localized photonic modes composed of a fundamental wave and its second harmonic coupled together by parametric interaction at the interface.     

Second- and third-harmonic generations for a nondilute suspension of coated particles with radial dielectric anisotropy

We derive expressions for the effective nonlinear susceptibility tensors for both the second harmonic generation (SHG) and induced third harmonic generation (THG) of nonlinear composite materials, in which nondilute coated particles with radial dielectric anisotropy are randomly embedded in the linear host. Two types of coated particles are considered. The first is that the core possesses a second order nonlinear susceptibility and the shell is linear and radially anisotropic, while the second is that the core is linear with radial anisotropy and the shell has a second order nonlinear susceptibility. We observe greatly enhanced SHG and THG susceptibilities at several surface plasmon resonant frequencies. For the second model, due to the coating material being metallic, there exists two fundamental resonant frequencies ω_c1 and ω_c2, whose difference ω_c2-ω_c1 is strongly dependent on the interfacial parameter and the radial dielectric anisotropy. Furthermore, in both systems, the adjustment of the dielectric anisotropy results in larger enhancement of both SHG and induced THG susceptibilities at surface plasmon resonant frequencies than the corresponding isotropic systems. Therefore, both the core-shell structure and the dielectric anisotropy play important roles in determining the nonlinear enhancement and the surface resonant frequencies.     

Modulation of the second-harmonic generation in MoS_2 by graphene covering

Nonlinear optical frequency mixing,which describes new frequencies generation by exciting nonlinear materials with intense light field,has drawn vast interests in the field of photonic devices,material characterization,and optical imaging.Investigating and manipulating the nonlinear optical response of target materials lead us to reveal hidden physics and develop applications in optical devices.Here,we report the realization of facile manipulation of nonlinear optical responses in the example system of MoS₂ monolayer by van der Waals interfacial engineering.We found that,the interfacing of monolayer graphene will weaken the exciton oscillator strength in MoS₂ monolayer and correspondingly suppress the second harmonic generation(SHG)intensity to 30%under band-gap resonance excitation.While with off-resonance excitation,the SHG intensity would enhance up to 130%,which is conjectured to be induced by the interlayer excitation between MoS₂ and graphene.Our investigation provides an effective method for controlling nonlinear optical properties of two-dimensional materials and therefore facilitates their future applications in optoelectronic and photonic devices.     

Simultaneous frequency doubling and polarization coupling in aperiodically poled lithium niobate

We proposed a method to simultaneously implement quasi-phase-matching (QPM) second harmonic generation (SHG) and polarization coupling for arbitrary wavelengths in an aperiodically poled lithium niobate (APPLN). It allows for modulation of both polarization and magnitude of the SHG light by an external electric field. A method called the self-adjusting algorithm (SA) is suggested for designing the aperiodic microstructure. The numerical simulation shows that it is the competition between the SHG and polarization coupling processes, which is dominated by the external electric field, that makes the output tunable.     

Second harmonic generation in composites of ellipsoidal particles with core–shell structure

We study the enhancement of the second-harmonic generation (SHG) coefficient in a random composite consisting of ellipsoidal particles with a core–shell structure in a linear dielectric host. The material making up the ellipsoidal core is assumed to be dielectric, but with a nonlinear susceptibility for SHG. The coating material is assumed to be metallic with a linear susceptibility. The effective SHG coefficient is derived and its expression is related to various local field factors. The numerical calculations of the effective SHG response per unit volume of nonlinear material can be greatly enhanced at certain frequencies. For coated ellipsoidal particles, the core–shell structure and the particle shape allow for tuning of the resonance through the choice of material parameters and/or the ratio of the core to shell volume fraction and the depolarization factor of the particles.     

Periodic,quasi‐periodic,and random quadratic nonlinear photonic crystals

Quadratic nonlinear photonic crystals are materials in which the second order susceptibility χ⁽²⁾ is spatially modulated while the linear susceptibility remains constant. These structures are significantly different than the more common photonic crystals, in which the linear susceptibility is modulated. Nonlinear processes in nonlinear photonic crystals are governed by the phase matching requirements, which are determined by the reciprocal lattice of these crystals. Therefore, the modulation of the nonlinear susceptibility enables to engineer the spatial and spectral response in various three‐wave mixing processes. It enables to support the efficient generation of new optical frequencies at multiple directions. We analyze three wave mixing processes in nonlinear photonic crystals in which the modulation is either periodic, quasi‐periodic, radially symmetric or even random. We discuss both one‐dimensional and two‐dimensional modulations. In addition to harmonic generations, we outline several new possibilities for all‐optical control of the spatial and polarization properties of optical beams in specially designed nonlinear photonic crystals.     

Second harmonic generation in planar optical waveguides

We study optical second harmonic generation (SHG) in planar waveguide structures composed of several layers with different dielectric constants. We develop a general formalism for the calculation of mode generation by a planar antenna embedded in the waveguide. As an application we consider a monolayer of high second-order susceptibility adsorbed at the interface between two layers of the waveguide structure. Periodic modulation of the nonlinear susceptibility allows phase matching leading to dramatically enhanced second harmonic intensities. We investigate the SHG-efficiency of various experimentally realizable geometries.     

Surface second-harmonic generation based on periodically poled LiNbO3 nonlinear optical crystal

We have generated a second-harmonic generation(SHG) of a Q-switched microchip Nd:YAG laser on the surface of a periodically poled LiNbO3(PPLN) nonlinear crystal near the grazing incidence angle.Three individual SHG waves as transmitted homogeneous,inhomogeneous and reflected radiations have been generated and their intensities are measured and characterized within a desirable range of about 10 different incidence angles of the Nd:YAG laser as pump source on the PPLN surface.The basic of surface nonlinear radiation is also investigated and similar results are calculated and extracted from the theory.Comparison between calculated and measured data shows that they are in good agreement with each other.     

Band structure and reflectance for a nonlinear one-dimensional photonic crystal

We consider a model for a one-dimensional photonic crystal formed by a succession of nonlinear Kerr-type equidistant spaceless interfaces immersed in a linear medium. We calculate analytically the band structure of this system as a function of the incident wave intensity, and find two main tendencies: the appearance of prohibited bands, and the separation and narrowing of these bands. We consider as well a finite version of this photonic crystal for a limited number of alternating linear and non linear set of stacks for which we calculate reflectance as a function of the electromagnetic wave intensity, band index and number of periods. A system with these features can be constructed by alternating very thin slabs of a nonlinear soft matter material with thicker solid films, which can be used to design a device to control light propagation for specific wavelength intervals and light intensities of the same propagating signal.     

Computing for second harmonic generation in one-dimensional nonlinear photonic crystals

A numerical study of second harmonic generation (SHG) in one-dimensional nonlinear photonic crystals based on full nonlinear system of equations, implemented by a combination of the method of finite elements and fixed-point iterations, is reported. This model is derived from a nonlinear system of Maxwell’s equations, which partly overcomes the known shortcoming of some existing models relied on the undepleted-pump approximation. We derive a general solution of SHG in one-dimensional nonlinear photonic crystals structures. The convergence of our method is fast. Numerical simulations also show the conversion efficiency of SHG can be significantly enhanced when the frequencies of the fundamental wave are located at the photonic band edges or are assigned to the designed defect states.     

A diffractive study of optical parametric interactions in nonlinear photonic crystals

With the nonlinear diffraction concept, we present a diffractive study of optical parametric interactions in nonlinear photonic crystals. The nonlinear diffraction concept enables the design of complicated nonlinear photonic crystal structures in an intuitive way. We show that there are two basic linear sequences, the anti-stacking and the para-stacking sequences, existing in a one-dimensional structure; and we present the realization of multiple phase-matching resonances in the combination of the two basic sequences. The parameters affecting the structure factor of a two-dimensional nonlinear photonic crystal are investigated, which indicate that not only the Ewald construction but also the relative domain size determines two-dimensional nonlinear diffractions.     

Enhanced nonlinear optical response of one-dimensional metal-dielectric photonic crystals

We describe a new type of artificial nonlinear optical material composed of a one-dimensional metal-dielectric photonic crystal. Because of the resonant nature of multiple Bragg reflections, the transmission within the transmission band can be quite large, even though the transmission through the same total thickness of bulk metal would be very small. This procedure allows light to penetrate into the highly nonlinear metallic layers, leading to a large nonlinear optical response. We present experimental results for a Cu/SiO(2) crystal which displays a strongly enhanced nonlinear optical response (up to 12X) in transmission.     

Epitaxial growth of 2D gallium selenide flakes for strong nonlinear optical response and visible-light photodetection

As an emerging group III−VI semiconductor two-dimensional (2D) material, gallium selenide (GaSe) has attracted much attention due to its excellent optical and electrical properties. In this work, high-quality epitaxial growth of few-layer GaSe nanoflakes with different thickness is achieved via chemical vapor deposition (CVD) method. Due to the non-centrosymmetric structure, the grown GaSe nanoflakes exhibits excellent second harmonic generation (SHG). In addition, the constructed GaSe nanoflake-based photodetector exhibits stable and fast response under visible light excitation, with a rise time of 6 ms and decay time of 10 ms. These achievements clearly demonstrate the possibility of using GaSe nanoflake in the applications of nonlinear optics and (opto)-electronics.     

Frequency doubling of ultrashort pulses in a nonlinear photonic strontium tetraborate crystal

The second harmonic generation (SHG) of the femtosecond titanium-sapphire laser radiation was experimentally studied in the mode of nonlinear diffraction in the nonlinear photonic strontium tetraborate crystal. The spectrum of reciprocal lattice vectors of domain structures includes components responsible for the efficient frequency-tunable SHG in the spectral range of 355–510 nm. The maximum SHG efficiency was 1.9%. The SHG spectrum narrowing does not exceed 10–20%. The measured spectral dependence of the nonlinear diffraction angle is in good agreement with calculation.