Resonant Second Harmonic Generation Research Articles

Observation of quantum-correlated twin beams in cascaded nonlinear interactions.

We report on the generation of twin beams through a cascaded process of optical parametric oscillation in a doubly resonant second-harmonic generation system. These bright beams exhibit strong quantum correlations, enabling the observation of up to 5 dB of noise reduction in their intensity difference below the standard quantum limit.

Resonant second-harmonic generation of self-focused oblique incident laser beam in plasma

The present manuscript aims to realize the effect of self-focusing on second-harmonic generation (SHG) by oblique incident laser beams in plasma. When an intense laser beam is obliquely incident on the plasma, it is resolved into reflected and refracted components, and resonant SHG is produced in the reflected component on account of the self-focusing of the incident laser. With the help of paraxial approximation, we have obtained a mathematical expression for the beam width parameter and for the normalized amplitude of the second harmonic. It has been observed that the amplitude of the second-harmonic pulse is significantly influenced by the intensity of the incident laser beam, the wiggler magnetic field, and the normalized plasma frequency. The analysis was performed for different values of the angle of incidence and laser parameters. The results show stronger self-focusing and, hence, efficient SHG when the angle of incidence is the critical angle.

Resonant second-harmonic generation in an array of magnetized anharmonic carbon nanotubes

Resonant second-harmonic generation in an array of magnetized anharmonic carbon nanotubes

Doubly resonant second-harmonic generation in a fiber-based tunable open microcavity

Doubly resonant second-harmonic generation in a fiber-based tunable open microcavity

Optical frequency combs in dispersion-controlled doubly resonant second-harmonic generation.

We report on the experimental realization and a systematic study of optical frequency comb generation in doubly resonant intracavity second harmonic generation (SHG). The efficiency of intracavity nonlinear processes usually benefits from the increasing number of resonating fields. Yet, achieving the simultaneous resonance of different fields may be technically complicated, all the more when a phase matching condition must be fulfilled as well. In our cavity we can separately control the resonance condition for the fundamental and its second harmonic, by simultaneously acting on an intracavity dispersive element and on a piezo-mounted cavity mirror, without affecting the quasi-phase matching condition. In addition, by finely adjusting the laser-to-cavity detuning, we are able to observe steady comb emission across the whole resonance profile, revealing the multiplicity of comb structures, and the substantial role of thermal effects on their dynamics. Lastly, we report the results of numerical simulations of comb dynamics, which include photothermal effects, finding a good agreement with the experimental observations. Our system provides a framework for exploring the richness of comb dynamics in doubly resonant SHG systems, assisting the design of chip-scale quadratic comb generators.

Resonant Second-Harmonic Generation as a Probe of Quantum Geometry.

Nonlinear responses are actively studied as probes of topology and band geometric properties of solids. Here, we show that second harmonic generation serves as a probe of the Berry curvature, quantum metric, and quantum geometric connection. We generalize the theory of second harmonic generation to include Fermi surface effects in metallic systems, and finite scattering timescale. In doped materials the Fermi surface and Fermi sea cause all second harmonic terms to exhibit resonances, and we identify two novel contributions to the second harmonic signal: a double resonance due to the Fermi surface and a higher-order pole due to the Fermi sea. We discuss experimental observation in the monolayer of time reversal symmetric Weyl semimetal WTe_{2} and the parity-time reversal symmetric topological antiferromagnet CuMnAs.

Dynamically tunable second-harmonic generation using hybrid nanostructures incorporating phase-change chalcogenides

Abstract Nonlinear metasurfaces with high conversion efficiencies have been vastly investigated. However, strong dynamic tunability of such devices is limited in conventional passive plasmonic and dielectric material platforms. Germanium antimony telluride (GST) is a promising phase-change chalcogenide for the reconfiguration of metamaterials due to strong nonvolatile changes of the real and imaginary parts of the refraction index through amorphous-crystalline phase change. The orderly structured GST has an even higher potential in tunable second-harmonic generation (SHG) with a non-centrosymmetric crystal structure at the crystalline phase, while the amorphous phase of GST does not exhibit bulk second-order nonlinearity. Here, we experimentally demonstrate SHG switches by actively controlling the crystalline phase of GST for a GST-based hybrid metasurface featuring a gap-surface plasmon resonance, and a quarter-wave asymmetric Fabry–Perot (F–P) cavity incorporating GST. We obtain SHG switches with modulation depths as high as ∼ 20 dB for the wavelengths at the on-state resonance. We also demonstrate the feasibility of multi-level SHG modulation by leveraging three controlled GST phases, i.e., amorphous, semi-crystalline, and crystalline, for the gap-surface plasmon hybrid device, which features stronger light–matter interaction and has higher resonant SHG efficiencies than the asymmetric F–P cavity device at respective GST phases. This research reveals that GST-based dynamic SHG switches can be potentially employed in practical applications, such as microscopy, optical communication, and photonic computing in the nonlinear regime.

Nonlinear Exciton‐Mie Coupling in Transition Metal Dichalcogenide Nanoresonators

AbstractThanks to a high refractive index, giant optical anisotropy, and pronounced excitonic response, bulk transition metal dichalcogenides (TMDCs) have recently been discovered to be an ideal foundation for post‐silicon photonics. The inversion symmetry of bulk TMDCs, on the other hand, prevents their use in nonlinear‐optical processes such as second‐harmonic generation (SHG). To overcome this obstacle and broaden the application scope of TMDCs, MoS2 nanodisks are engineered to couple Mie resonances with C‐excitons. As a result, their alliance produces 23‐fold enhancement of SHG intensity with respect to the resonant SHG from a high‐quality exfoliated MoS2 monolayer under C‐exciton excitation. Furthermore, SHG demonstrates a strongly anisotropic response typical of a MoS2 monolayer due to the single‐crystal structure of the fabricated nanodisks, providing a polarization degree of freedom to manipulate SHG. Hence, these results significantly improve the potential of bulk TMDCs enabling an avenue for next‐generation nonlinear photonics.

Effects of hydrostatic pressure, temperature and Al-concentration on the second-harmonic generation of tuned quantum dot/ring under a perpendicular magnetic field

The simultaneous effects of hydrostatic pressure, temperature and Al-concentration on the second-harmonic generation (SHG) coefficient for GaAs/Ga1−xAlxAs tuned quantum dot/quantum ring (QD/QR) under parabolic-inverse squared potential plus the modified Gaussian potential in a perpendicular magnetic field are studied in detail. The analytical expression of the SHG coefficient can be obtained using the density matrix theory and iterative method. The numerical results of GaAs/Ga1−xAlxAs tuned QD/QR are given. The result of calculation illuminates that the applied magnetic field, Al-concentration, hydrostatic pressure, temperature and the harmonic confinement frequency can make an impact on energy level interval, the product of matrix elements, transition frequency and other factors so that the peak value of SHG coefficient and resonance peak changing under the action of these factors. Furthermore, the depth and the radius of the QD have a bearing on the SHG coefficient.

Optimization of second harmonic generation in Au/Si core-shell nanoparticles

This study numerically investigated second harmonic generation (SHG) and the linear optical responses in core–shell nanoparticles comprising a gold core and silicon shell. The effects of the core radius and shell thickness were studied based on the resonance wavelength shifts, linear responses, and SHG efficiency in numerical simulations. By accurately setting the shell thickness, a model was designed where the resonance wavelength was almost constant relative to variations in the core radius for a specific shell thickness (i.e., the resonance wavelength did not shift). Thus, it was possible to optimize the efficiency of the linear optical responses and SHG in the core–shell nanoparticles by only changing the core radius. The desired efficiency can be achieved for this nanoparticle in specific applications by first tuning the resonance wavelength by changing the shell thickness and then by optimizing the efficiency by changing the core radius. The results obtained in this study may facilitate the development and optimization of optical nanodevices for different applications in nanophotonics and nano-optics, such as nanolasers and nanosensors.

Organic Nanocrystal Fabrication Using the Process of Resonant Second-Harmonic Generation of Light.

Laser ablation with the use of ultra-short laser pulses is a widely used technique for the fabrication of nanoparticles of metals, inorganic and hybrid materials. However, fabrication of fragile organic nanocrystals via laser ablation is rarely used due to easy photodegradation of molecules. The method employing laser irradiation of the target material is beneficial as no other chemicals are used in the production of nanoparticles, except for a given material and a solvent. In this work, we test the concept of formation of nonlinear optical (NLO) organic nanocrystals dispersion in water by irradiation of the microcrystals of the NLO material with nonabsorbed infrared nanosecond light pulses. These pulses, due to a nonlinear optical process active in a noncentrosymmetric organic crystal, such as those studied in this work, DCNP dye (3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole), produce nanosecond pulses of second-harmonic (SH) light. Due to doubling of photon energy, they are reabsorbed in the volume of DCNP microcrystals and thermal shocks fracture them into nanometer size crystals. To the best of our knowledge, such process and its interpretation have not been described yet in the literature.

Frequency-domain model of optical frequency-comb generation in optical resonators with second- and third-order nonlinearities

We developed a frequency-domain model describing optical frequency-comb generation in optical resonators with second- and third-order nonlinearities. Compared with time-domain models, our model in principle allows one to express the cavity dispersion accurately, avoiding the dispersion being truncated beyond a certain order. Moreover, the frequency-domain model can readily include frequency dependence of system parameters, such as the linear absorption and the cavity coupling ratio. To demonstrate the validity of our model, we numerically simulated quadratic combs in a singly resonant second-harmonic generation cavity and Kerr combs in a micro-resonator as two examples. The simulated results obtained from the frequency-domain model agree well with those given by previous time-domain models. A system containing both second- and third-order nonlinearities can give rise to many novel physical dynamics. The developed frequency-domain model will contribute to understanding optical frequency-comb generation assisted by multi-order nonlinear processes in various optical resonators.

Second harmonic generation of cosh-Gaussian laser beam in magnetized plasma

In the present paper, second harmonic generation (SHG) of cosh-Gaussian laser in a magnetized plasma is analyzed. During laser propagation through plasma, electrons acquire the oscillatory velocity and result density perturbation. The density oscillations beat with the oscillatory velocity to produce second harmonic current which drives SHG. Wiggler magnetic field adds the additional momentum to the photons of second harmonic and fulfills the phase matching condition which results in resonant SHG. Wiggler magnetic field also helps to maintain the cyclotron frequency due to which plasma electrons remain confined within the plasma region and results SHG of higher efficiency. Using paraxial approximation, we have derived the equation for the amplitude of SHG and studied its variation for different values of intensity parameter of incident laser, wiggler magnetic field, decentered parameter and plasma density. The efficiency of SHG is significant at higher values of intensity of incident laser, wiggler field, and plasma density as observed in our analysis.

Doubly resonant second-harmonic generation of a vortex beam from a bound state in the continuum

Second-harmonic generation in nonlinear materials can be greatly enhanced by realizing doubly resonant cavities with high quality factors. However, fulfilling such doubly resonant condition in photonic crystal (PhC) slab cavities is a long-standing challenge, because of the difficulty in engineering photonic bandgaps around both frequencies. Here, by implementing a second-harmonic bound state in the continuum (BIC) and confining it with a heterostructure design, we show the first doubly resonant PhC slab cavity with 2.4 × 10 − 2 W − 1 intrinsic conversion efficiency under continuous-wave excitation. We also report the confirmation of highly normal-direction concentrated far-field emission pattern with radial polarization at the second harmonic frequency. These results represent a solid verification of previous theoretical predictions and a cornerstone achievement, not only for nonlinear frequency conversion but also for vortex beam generation and prospective nonclassical sources of radiation.

Remarkably enhanced first hyperpolarizability and nonlinear refractive index of novel graphdiyne-based materials for promising optoelectronic applications: A first-principles study

A new functionalized graphdiyne-based structure was designed by adsorbing single and double AM3O (AM = Li, Na, K) on graphdiyne (GDY) surface, namely (AM3O)n@GDY (AM = Li, Na, K; n = 1, 2). Density-functional theory calculations have been used to unravel the relations between the geometric structure and nonlinear optical (NLO) properties, e.g., Hyper-Rayleigh Scattering coefficient (βHRS) and nonlinear refractive index (n2). The frequency-dependent electro-optic Pockel's effect (EOPE) and second-harmonic generation (SHG) were explored to design the efficient NLO materials. Our calculations reveal that the AM3O molecule has the strong interactions with the GDY unit, and the intramolecular charge transfer is significant for determining the NLO properties. Large first hyperpolarizabilities (βtot) for single-adsorbed GDY have been found, especially for K3O@GDY (∼3.06 × 105 a.u.), rather than double-adsorbed GDY. Moreover, the small transition energies with large oscillator strength are a decisive factor for enhancing the βtot values. In the dynamic NLO process, the EOPE and SHG resonances can be enhanced at ω = 0.10 a.u., while single-adsorbed GDY clusters have large nonlinear refractive index and improved Hyper-Rayleigh Scattering response. All of these results will inevitably stimulate further synthesis of GDY-based optoelectronic materials with high NLO properties for future applications.

Quadratic soliton combs in doubly resonant second-harmonic generation.

We report a theoretical investigation of quadratic frequency combs in a dispersive second-harmonic generation cavity system. We identify different dynamical regimes and demonstrate that the same system can exhibit both bright and dark localized cavity solitons in the absence of a temporal walk-off.

Second Harmonic Spectroscopy of Surface Lattice Resonances.

Because of their large figures of merit, surface lattice resonances (SLRs) in metal nanoparticle arrays are very promising for chemical and biomolecular sensing in both liquid and gas media. SLRs are sensitive to refractive index changes both near the surface of the nanoparticles (surface sensitivity) and in the volume between them (bulk sensitivity). Because of its intrinsic surface-sensitivity and a power law dependence on electric fields, second harmonic generation (SHG) spectroscopy can improve upon both the surface and volume sensitivities of SLRs. In this report on SHG spectroscopy of plasmonic nanoparticles, we show that the SHG signal is greatly increased (up to 450 times) by the SLRs. We also demonstrate very narrow resonances in SHG intensity (∼5 nm fwhm). We illustrate how the SHG resonances are highly sensitive to SLRs by varying the fundamental wavelength, angle of incidence, nanoparticle material, and lattice constant of the arrays. Finally, we identify an SHG resonance (10 nm fwhm) that is electric dipole forbidden and can be attributed to higher-order multipoles, enhanced by the strong near-fields of SLRs. Our results open up new and very promising avenues for chemical and biomolecular sensing based on SHG spectroscopy of SLRs.

Electrical Double Layer Probed by Surface-Specific Vibrational Technique

In this issue of Chem, Paesani and co-workers present a computational analysis of sum-frequency generation (SFG) spectra of water to disentangle the interfacial χ(2) and bulk χ(3) contributions. SFG line shapes are in good agreement with experimental results. The analysis reveals the molecular response of interfacial water, which is usually masked by the bulk water spectra at the water-charged surfactant interface.

Element Selectivity in Second-Harmonic Generation of GaFeO3 by a Soft-X-Ray Free-Electron Laser

Nonlinear optical frequency conversion has been challenged to move down to the extreme ultraviolet and x-ray region. However, the extremely low signals have allowed researchers to only perform transmission experiments of the gas phase or ultrathin films. Here, we report second harmonic generation (SHG) of the reflected beam of a soft x-ray free-electron laser from a solid, which is enhanced by the resonant effect. The observation revealed that the double resonance condition can be met by absorption edges for transition metal oxides in the soft x-ray range, and this suggests that the resonant SHG technique can be applicable to a wide range of materials. We discuss the possibility of element-selective SHG spectroscopy measurements in the soft x-ray range.

Cascading second-order nonlinear processes in a lithium niobate-on-insulator microdisk

Whispering-gallery-mode (WGM) microcavities are very important in both fundamental science and practical applications, among which on-chip second-order nonlinear microresonators play an important role in integrated photonic functionalities. Here we demonstrate resonant second-harmonic generation (SHG) and cascaded third-harmonic generation (THG) in a lithium niobate-on-insulator (LNOI) microdisk resonator. Efficient SHG in the visible range was obtained with only several mW input powers at telecom wavelengths. THG was also observed through a cascading process, which reveals simultaneous phase matching and strong mode coupling in the resonator. Cascading of second-order nonlinear processes gives rise to an effectively large third-order nonlinearity, which makes on-chip second-order nonlinear microresonators a promising frequency converter for integrated nonlinear photonics.