Strong localization of near-field second-harmonic generation for nonlinear mesoscopic surface structures

In this paper, noise characteristics of second-harmonic generation (SHG) in periodically poled lithium niobate (PPLN) using the quasi-phase-matching (QPM) technique are studied experimentally. In the experiment, a 0.78-mm second-harmonic (SH) wave was generated when a 1.55-mm fundamental wave passed through a PPLN crystal (bulk or annealed proton-exchanged waveguide). The fundamental and SH waves were then separated through a beam splitter and sent to two photodetectors, respectively. The time-domain and frequency-domain characteristics of the fundamental and SH waves were analyzed. By using the pump-probe method, the noise characteristics were studied further when 532-nm irradiation light co-propagated with the 1.55-mm fundamental light in the PPLN crystals. It is found that for the bulk and waveguides of PPLN crystals, the SH wave has a higher relative noise level than the corresponding fundamental wave. For the same fundamental wave, the SH wave has lower noise in a bulk crystal than in a waveguide, and in MgO-doped PPLN than in undoped PPLN. In addition, the photorefractive effect incurred by the irradiation light can influence the SHG noise.

The grating-assisted nonlinear directional couplers are investigated by using the singular perturbation technique with multiple space scales. One of the waveguides of the nonlinear directional couplers has second order nonlinear cladding whose surface has a grating profile. The period of the grating is determined to satisfy the quasi-phase matching condition between the fundamental wave and second harmonic (SH) wave. From the solvability condition to have nontrivial solutions for each order, the coupled-mode equations between fundamental and SH waves are derived. In the numerical examples, lithium niobate (LiNbO/sub 3/) is chosen as second-order nonlinear medium. After the coupled-mode equations are examined numerically, it is found that the SH wave separates from the fundamental wave due to the difference of the coupling length between the fundamental and SH waves.

In this paper, a 1D nonlinear plasma photonic crystal (NPPC) structure composed of polarized ferroelectric crystals and nonlinear plasma periodic alternation is proposed. The transfer matrix method is employed to analyze the second harmonic generation (SHG) problem of this structure. In the designed NPPCs, the fundamental wave (FW) operates in the gigahertz (GHz) band and the nonlinear plasma is controlled by an external high‐intensity control wave (CW). Numerical simulations are performed to investigate the effects of different incident angles and external CW intensities on the total conversion efficiency (T‐con) of the second harmonic wave (SHW). Additionally, the internal electric field distribution and incident light intensity within the nonlinear structure are analyzed. The importance of the relationship between the FW frequency and photonic band gap (PBG) in enhancing SHG is summarized. The results demonstrate that the optimal structure can be obtained by changing the structural parameters, such that the FW and SHW are tuned to the edge of the PBG. At this point, the electromagnetic field density is large, the group velocity is small, the local field is enhanced, and the nonlinear optical interaction is increased, resulting in a significant increase in the T‐con of the SHW.

The visibility of inverted domain structures using the second harmonic generation (SHG) microscope is discussed based upon the three-wave coupling equations. Without reference second harmonic (SH) waves (non-interference case), the SHG intensity at a domain boundary decreases steeply along the perpendicular direction to the domain wall, which is observed as a dark line. Thus the non-interference SHG microscope reveals the location of domain boundaries but not the domain polarity which the SHG interference microscope can do. In the case of periodically poled domain structures, the clear observation criterion is expressed with the wave-number offset between the fundamental and SH waves.

The nonlinear optical properties of one-dimensional all-solid-state photonic-crystal microcavities (MCs) are experimentally studied by second-harmonic generation (SHG) spectroscopy in both the frequency and the wave-vector domains. The studied single and coupled MCs are formed by the alternating of mesoporous silicon layers of different porosities. When the fundamental radiation is in resonance with the MC mode the second-harmonic intensity is enhanced by a factor of approximately 102. The resonant SHG response is compared with the off-resonance response, as the fundamental wavelength is outside the photonic bandgap. The splitting of the modes of two identical coupled MCs is observed in the wave-vector domain spectrum of enhanced SHG. The SHG enhancement is attributed to the combined effects of the spatial localization of the fundamental field in the MC spacer and the fulfillment of the phase-matching conditions. The confinement of the resonant fundamental field is probed directly at the MC cleavage by a scanning near-field optical microscope. The role of the phase matching that is associated with the giant effective dispersion in the spectral vicinity of the MC mode is deduced from a comparison with the SHG peaks at both edges of the photonic bandgap.

We predict that a different type of second-harmonic generation (SHG) in nonlinear optical fibers is possible for the waves excited from a continuous-wave background. We show that in a normal dispersion regime and near the zero-dispersion point of a single-mode optical fiber the phase-matching condition of the SHG can be satisfied by suitably choosing the wave vectors and frequencies of fundamental and second-harmonic waves. Using a multiscale method the nonlinearly coupled envelope equations for the SHG are derived and their explicit solutions are provided and checked by numerical simulation.

We propose a monolithic topological photonic circuit that integrates second-harmonic generation (SHG) and wavelength demultiplexing in a single platform. Leveraging the finite element method (FEM) for electromagnetic simulations, a sandwich waveguide heterostructure with air-gap-tuned edge states simultaneously generates SHG at 280.6 THz and routes fundamental (FW) and second-harmonic waves (SHW) through topologically distinct paths: FW propagates via a nontrivial edge state (near-zero backscattering), while SHW transmits through a trivial boundary mode (optimized dispersion). Simulated results demonstrate greater than 99.88% separation efficiency—matching the state-of-the-art in topological isolation systems. The dual-functionality design eliminates hybrid-component losses, yielding a normalized SHG efficiency of η =139.4 % W −1 cm −2 . Field profiles confirm that the spatial isolation of FW/SHW outputs a minimum crosstalk suppression of −29.2 dB . This work establishes a scalable paradigm for topology-enabled nonlinear light management in quantum frequency conversion and photonic signal processing.

In this study we have investigated the electrical poling effects in a p(DR1M) azo-polymer thin film and in related diffraction gratings inscribed on other films. Their optical properties were determined using polarization measurements in UV-visible spectroscopy, in confocal Raman micro-spectrometry and in second harmonic generation (SHG) in the transmission mode. Near-field scanning optical microscopy (NSOM) experiments together with SHG responses in reflection were also performed in order to retrieve simultaneously the topography and the SHG contrasts of the surface gratings. This multitechnique approach led to the determination of the first even and odd parity Legendre polynomials, from ⟨P 1⟩ up to ⟨P 4⟩, allowing the establishment of precise chromophore orientational distribution functions. In particular, from SHG near-field scanning optical microscopy (NSOM) experiments we have evidenced that poled gratings with large diffraction efficiencies and χ(2) nonlinear susceptibilities exhibit new sub-structures with puzzling differences from their topographical AFM images. A combination of all the linear and nonlinear results allows us, for the first time, to explain the peculiar orientational and mechanical properties in the peak areas of the surface relief modulations, at near each half-period grating (Λ/2 ≈ 0.7μm). An efficient poling of the azobenzene dye groups in such gratings thus leads to new interesting properties.

In a quasi-phase-matched (QPM) nonlinear optical waveguide, the efficiency of second harmonic generation (SHG) is proportional to the square of effective nonlinear coefficient determined by the overlap between the waveguide modes involved in SHG and the domain-inversion grating. Thus the domain-inversion grating should be fabricated to have the maximum overlap between the guided modes and the grating, after selecting a proper waveguide geometry for the modes of both the fundamental wave and second-harmonic wave to be tightly overlapped. For that purpose, it is necessary to control the domain-inversion depth in a QPM LiNbO/sub 3/ waveguide. In the conventional Ti-indiffused domain inversion of LiNbO/sub 3/, the depth of the domain-inversion grating may be controlled by the Ti-film thickness and the Ti-indiffusion conditions (i.e. temperature and time). For the control of the domain-inversion depth it prefers the Ti diffusion only in the depth direction. However, the lateral diffusion of the Ti film and the outdiffusion of Li/sub 2/O occur at the Ti-free region simultaneously with the diffusion of Ti film in the depth direction. In this paper, we report a new periodic domain-inversion method for a QPM LiNbO/sub 3/ waveguide, which is amenable to the control of depth to maximize the efficiency of SHG. Because the method does not form the planar domain-inverted layer, it is effective to the case that the guided modes are well-confined near the surface.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Near-field scanning optical microscope (NSOM) lithography is one of optical technologies for planar structure fabrication, where exposure process is performed by optical near field produced at tip of fiber probe. Maskless exposure of defined regions is performed so that different periodic and predefined arrangement can be achieved. In this contribution, NSOM lithography is presented as effective tool for semiconductor device surface patterning. Non-contact mode of NSOM lithography was used to pattern planar predefined structures in GaAs, AlGaAs and GaP surfaces. In this way, GaAs/AlGaAs-based LED with patterned structure in the emitting surface was prepared, where patterned air holes show enhancement of radiation in comparison with the surrounding surface. Furthermore, NSOM in combination with lift-off technique was used to prepare metal-catalyst particles on GaP substrate for subsequent growth of GaP nanowires which can be used in photovoltaic applications.

Since the invention of the quantum cascade (QC) laser, which was demonstrated by Faist et al. in 1994, there has been much progress and advancement to include a wide range of wavelengths on GaAs and InP based material. Specifically, the operating range includes mid- to far-infrared with peak power levels in the watt range and above room temperature pulsed operation for wavelengths from 4.5 to 16 mum. There have been three distinctive designs of the QCL active region- the 'vertical' and 'diagonal' transitions and the 'superlatttice'- all of which are used with either surface-plasmon or conventional dielectric waveguides . Then, there is also the multiple wavelength superlattice, bi-directional, dual wavelength and the injector less quantum cascade lasers. With all the progress made thus far, there is still much work to be done in producing QC lasers that emit efficiently in the atmospheric window (3-5 mum wavelength range), which is important for free-space communications and gas sensing applications. Two known approaches in achieving QC lasers with shorter wavelength emission are through material selection, i.e. wider bandgap materials and type-II structures. There is an alternative approach that also holds much promise: exploitation of the nonlinear optical processes. Within the QC laser intracavity, higher order harmonic generation can be employed because both the fundamental and second-harmonic waves can propagate freely within the cavity without being reabsorbed by the semiconductor. Conveniently, the energy generated in this approach by the fundamental mode and second-harmonic generation is well below the selected material bandgap energy, which is still in the transparency region of the material. Appropriately, our work involves the study of strained active regions in AlGaAs/GaAs QC lasers with the goal of enhancing the nonlinear susceptibility and producing lasers that emit in the 3-5 mum wavelength range.

Optical second-harmonic generation (SHG) spectroscopy has been used to determine the field enhancements from Au nanoparticles on a silicon substrate. Au particles with diameters from 30 to 250 nm have been deposited on a Si substrate passivated by a 1 nm thick surface oxide. The linear optical spectra are dominated by a horizontal plasmon resonance near 1.0 eV, and the experimental spectra are modelled by the island film model in order to extract the linear properties of the metal particles. SHG spectroscopy from this system shows resonances from the metal particles and from the silicon/oxide substrate. By following the evolution of these Si resonances with the size of the Au particles, the field enhancement in the Si surface has been modelled. The effect of the Au particles on SHG at the Si E1 resonance is a combination of charge transfer through the thin oxide that changes the space charge region and an enhancement of the optical field in a thin surface layer of the Si substrate.

In this Letter, a new method is presented to calculate the interactive length between the fundamental wave and second harmonic generation (SHG) for the configuration of total internal reflection on the inner surface of a nonlinear crystal. Three independent experiments are designed to measure the bandwidths of this second harmonic wave. The theoretical expression of the intensity of SHG is obtained through a nonlinear coupled wave equation. The interactive length of this phase-matched SHG can be calculated mathematically by utilizing the measured bandwidths and the intensity equation. There is no existing method to obtain the interactive length either from theoretical calculations or by experimental measurement. This method can be applied to estimate the extremely short interactive volume in nonlinear processes.

ABSTRACTIn order to obtain efficient light trapping within a thin-film silicon solar cell, randomly textured interfaces are used. The texture can be introduced by wet-chemical etching in diluted hyrdofluoric acid (HF). By varying of the HF concentration, a continuous transition to smaller surface structures can be achieved. Near-field scanning optical microscopy is applied to measure scattered light with sub-wavelength resolution. On those different surfaces, using Fourier high-pass filters on the measured near-field images, surface features with a high light trapping potential are identified. Finally, criteria for optimized scattering surfaces are obtained.

We analyze two-color spatially localized nonlinear modes formed by parametrically coupled fundamental and second-harmonic fields excited at quadratic (or chi(2)) nonlinear interfaces embedded in a linear layered structure--a quadratic nonlinear photonic crystal. For a periodic lattice of nonlinear interfaces, we derive an effective discrete model for the amplitudes of the fundamental and second-harmonic waves at the interfaces (the so-called discrete chi(2) equations) and find, numerically and analytically, the spatially localized solutions--discrete gap solitons. For a single nonlinear interface in a linear superlattice, we study the properties of two-color localized modes, and describe both similarities to and differences from quadratic solitons in homogeneous media.