Second-harmonic Radiation Research Articles

Second-harmonic radiation by on-chip integrable mirror-symmetric nanodimers with sub-nanometric plasmonic gap

Abstract Second-harmonic generation (SHG) facilitated by plasmonic nanostructures has drawn considerable attention, owing to its efficient frequency up-conversion at the nanoscale and potential applications in on-chip integration and nanophotonic devices. Herein, we present a nanodimer array fabricated by nanoimprinting, composed of nanofinger-pair symmetrically leaning at an off-angle with a well-defined sub-nanometric gap. Commonly, geometric symmetry would suppress the far-field SHG due to the near-field cancelling of symmetric surface SH polarization. However, we find that the light-induced surface SH polarization distribution along the wave-vector of incidence could be influenced by the off-angle, which is consistent to the requirement of SH polarization symmetry-breaking in symmetric metallic nanocavity. A dramatic enhancement of far-field SHG is achieved by tuning the off-angle of nanofinger-pair, even approaching up to over 4 orders of magnitude for an optimal value. The demonstration of SHG enhancement on our well-defined plasmonic nanodimer provides a new way of on-chip integration to activate high-efficient SH radiation, which might be potential for applications in novel nonlinear optical nanodevices with remarkable efficiency and sensitivity.

Evolution of level population in Ar interacting with XFEL pulses: impact of resonant absorptions

Theoretical exploration of the population dynamics at fine-structure levels of Ar atoms interacting with ultrafast ultraintense x-ray free electron laser (XFEL) pulses is conducted. A time-dependent rate equation based on a detailed-level accounting approach is applied for tracking population levels, encompassing microscopic atomic processes such as photoexcitation, radiative decay, photoionization and Auger decay. A Monte Carlo algorithm is implemented to solve large-scale rate equations efficiently. The primary investigation centers on generating Ar17+ through resonant absorption by the second-harmonic radiation of the x-ray pulse. The calculated population ratios of Ar17+ to Ar16+ align well with the experimental measurements (LaForge et al 2021 Phys. Rev. Lett. 127 213202). In comparison to the transition energy of the strongest line, (1s2)0→(1s1/22p3/2)1 of Ar16+, there is a distinct ∼25 eV red shift in the peak ratio, which is attributed to the presence of intricate resonant channels in the lower ionization stages. The results demonstrate the sensitivity of the population ratio Ar17+/Ar16+ to the laser pulse parameters such as x-ray pulse duration, bandwidth and the contribution of second-harmonic radiation, indicating their potential as diagnostic tools in future experiments.

Cherenkov second harmonic generation of femtosecond laser pulses in a homogeneous nonlinear crystal

In experiments on second harmonic (SH) generation (SHG), a conical structure of radiation has been observed. In the present study, a non-stationary theory of SH excitation of ultrashort laser pulses with phase modulation has been developed, which explains the properties of such a structure as Cherenkov radiation. Under phase-mismatched interactions, a maximum of the SH spectrum is observed at the Cherenkov angle, which is determined by the ratio of the SH and laser radiation phase velocities. It is shown that tightly focused laser beams are preferred to observe Cherenkov SHG. The SH spectral width depends on the group velocity mismatch and is more complicated on the excited radiation spectrum. The SH energy can be proportional to the crystal length or group delay length depending on their ratio. We also demonstrate that a complex angular distribution of spectral components (an angular chirp) appears within the SH cross-section.

Spatial shaping of low- and high-order harmonics generated using vortex beams

We demonstrate the generation of the low- and high-order harmonic vortex beams from a single spiral phase plate illuminated by different laser wavelengths. The second harmonic (532 nm) originates from the application of the wavefront-structured 1064 nm femtosecond pulses with fractional orbital angular momentum (OAM) during propagation through a lithium triborate crystal, while the third harmonic (500 nm) originates from the application of the wavefront-structured near-IR (1500 nm) femtosecond pulses with integer OAM during propagation through a 150 μm thick fused silica plate. The topological charges (TCs) of the second and third harmonics are measured and compared. The increase in TC and the peculiarities in OAM variations during modification of the polarisation of the incident radiation are analysed and discussed. The two-colour-pump-driven second-harmonic vortex radiation interacted with an Ar gas jet to generate vortex harmonics up to the 14th order with double-lobe complex spatial profiles in the extreme ultraviolet region.

Laser ablation microsampling of copper phthalocyanine blue by the second-harmonic radiation of Nd:YAG laser

An original nonlinear optical method employing laser ablation with single nanosecond laser pulses at a wavelength of λ = 532 nm is used to study organic paint layer (copper phthalocyanine blue (PB15)). Two processes (photochemical reaction and formation of craters), which appear to be nonlinear result from the pulsed radiation. It is shown that the nonlinear optical procedure (laser ablation microsampling (LAMS)) can be used to obtain microparticles of PB15 sufficient for the further analysis. The Raman spectra of the surface of the paint layer prior to and after LAMS and microparticles are obtained. Spectral data, including those in the low-frequency region, are discussed.

Rainbow Cherenkov Second-Harmonic Radiation

Nonlinear Cherenkov radiation, an intriguing noncollinear optical parametric process with versatile longitudinal phase matching, is commonly realized in engineered nonlinear photonic structures. Here, we report on the observation of ultrabroadband rainbow Cherenkov second-harmonic generation (UBR CSHG) in a single periodically poled lithium niobate (PPLN) nonlinear crystal driven by a near-infrared high-peak-power ultrashort femtosecond pump laser. Multiple colored nonlinear radiation bands along the direction of Cherenkov conical angles are simultaneously exhibited, leading to a beautifully colorful rainbow picture never reported before. We systematically analyze the multicolor spectral characteristics of the UBR CSHG in the whole conical direction and explicitly clarify their phase-matching conditions. We show that this UBR CSHG interaction is subject to remarkable spectral broadening owing to both the third-order nonlinearity effect induced by the high-peak-power ultrashort pump laser and the reciprocal lattice vector of the PPLN crystal, which eventually enables the generation of three bright dominating colors---red, orange-yellow, and green bands---and their automatic dispersion in space. The experimental results suggest rich physics of nonlinear optical interactions and indicate the possibility to engineer broadband nonlinear frequency conversion for applications in compact colored laser sources and others.

Radially Polarized Second-Harmonic Generation from a Single-Element Nanoantenna via Dark Plasmon Coupling of Nonlinear Polarization

Controlling the process of second-harmonic (SH) generation at the nanoscale is important in photonic applications but remains challenging in nanophotonics. Herein, based on theoretical and experimental studies, we found that a cross-shaped single-element plasmonic nanoantenna resonating with bright and dark modes at the fundamental (excitation) and SH wavelengths, respectively, generates radially polarized ring-shaped SH radiations with axial symmetry by exciting the bright dipole mode with linearly polarized light. Mode expansion analysis of the generated SH radiation revealed that the radial polarization arises from a dark breathing mode in which coherent dipoles are radially distributed along the arms of the cross-shaped antenna. This study reveals that surface SH polarization generated by the electric near-field of the bright mode at the fundamental wavelength efficiently is coupled to the dark modes rather than the bright mode because its spatial distribution has the same symmetry as the dark modes. By engineering bright and dark modes at the fundamental and SH wavelengths, respectively, novel plasmonic nanodevices that simultaneously perform polarization control and efficient wavelength conversion are expected.

Control of second-harmonic generation in all-dielectric intersubband metasurfaces by controlling the polarity of χ(2).

All-dielectric metasurfaces have recently led to a paradigm shift in nonlinear optics as they allow for circumventing the phase matching constraints of bulk crystals and offer high nonlinear conversion efficiencies when normalized by the light-matter interaction volume. Unlike bulk crystals, in all-dielectric metasurfaces nonlinear conversion efficiencies primarily rely on the material nonlinearity, field enhancements, and the modal overlaps, therefore most efforts to date have only focused on utilizing these degrees of freedom. In this work, we demonstrate that for second-harmonic generation in all-dielectric metasurfaces, an additional degree of freedom is the control of the polarity of the nonlinear susceptibility. We demonstrate that semiconductor heterostructures that support resonant nonlinearities based on quantum-engineered intersubband transitions provide this new degree of freedom. We can flip and control the polarity of the nonlinear susceptibility of the dielectric medium along the growth direction and couple it to the Mie-type photonic modes. Here we demonstrate that engineering the χ(2) polarity in the meta-atom enables the control of the second-harmonic radiation pattern and conversion efficiency. Our results therefore open a new direction for engineering and optimizing second-harmonic generation using all-dielectric intersubband nonlinear metasurfaces.

Optical second-harmonic response of axially symmetric media under tightly focused excitation: linear versus radial polarization

We compared the optical second-harmonic (SH) response of axially symmetric media excited by tightly focused radially and linearly polarized beams. The simulation performed showed a significant redistribution of energy between the lobes/cones of the SH radiation pattern, depending on the focusing accuracy in the case of a purely surface/interface nonlinearity. This phenomenon is due to the change in the contributions of second-order susceptibility tensor components to SH generation as we demonstrated via a comparison of SH patterns related to different components of the tensor. Replacement of the interface with a nonlinear layer leads to noticeable broadening of SH radiation patterns, which can allow one to distinguish surface nonlinearity from bulk. The model developed is applicable for SH generation by interfaces, films, and poled glasses.

Separating Non-linear Optical Signals of a Sample from High Harmonic Radiation in a Soft X-ray Free Electron Laser

Second harmonic generation (SHG) is a unique non-linear optical effect that can be used to investigate chemical states of molecules at surfaces/interfaces. By the use of soft X-rays, SHG gains element selectivity through the inner-core excitation resonance. However, it is challenging to observe SHG signals separately from the second-order light generated from the undulator. Here, we report a new ellipsometry method for soft X-ray SHG to suppress the contribution of second-harmonic radiation from the light source. Through measurements of a GaAs(100) crystal, we demonstrate that pure SHG signals can be obtained for the horizontally polarized component. The present method is generally applicable regardless of the incident photon energy and hence the absorption edge of the targeted materials. If combined with optical filters blocking the second-harmonic radiation and equipped with soft X-ray phase shifters, the method allows one to obtain further information from SHG signals such as tensor components of second-order non-linear susceptibility.

Tunable unidirectional nonlinear emission from transition-metal-dichalcogenide metasurfaces

Nonlinear light sources are central to a myriad of applications, driving a quest for their miniaturisation down to the nanoscale. In this quest, nonlinear metasurfaces hold a great promise, as they enhance nonlinear effects through their resonant photonic environment and high refractive index, such as in high-index dielectric metasurfaces. However, despite the sub-diffractive operation of dielectric metasurfaces at the fundamental wave, this condition is not fulfilled for the nonlinearly generated harmonic waves, thereby all nonlinear metasurfaces to date emit multiple diffractive beams. Here, we demonstrate the enhanced single-beam second- and third-harmonic generation in a metasurface of crystalline transition-metal-dichalcogenide material, offering the highest refractive index. We show that the interplay between the resonances of the metasurface allows for tuning of the unidirectional second-harmonic radiation in forward or backward direction, not possible in any bulk nonlinear crystal. Our results open new opportunities for metasurface-based nonlinear light-sources, including nonlinear mirrors and entangled-photon generation.

Study of optical guiding of the Hermite–Gaussian laser beam in preformed collisional parabolic plasma channel and second harmonic generation

Abstract In the present work, the scheme of optical guiding of the Hermite–Gaussian laser beam and the generation of second-harmonic 2ω radiation (ω being the frequency of incident beam) is presented in plasma having the preformed collisional plasma channel in which density variation is parabolic. The nonlinear coupling of excited electron plasma wave with the carrier or incident beam results in the production of second harmonics of the latter. The method of moments is used for finding the coupled differential equations for the beam diameter to study the dynamics of the Hermite–Gaussian laser beam in plasma under the effect of the collisional parabolic channel. For numerical simulations, the Runge–Kutta fourth-order numerical method is used. Standard perturbation theory gives the equation for excitation of electron plasma wave which further acts as the source term for the second harmonic generation. The numerical results show that the preformed plasma channel has a significant effect on the guiding as well as on the 2ω generation of the Hermite–Gaussian laser beam in plasma.

Modeling of surface-induced second-harmonic generation from multilayer structures by the transfer matrix method.

We analytically and numerically investigate surface second-harmonic generation (SHG) from a stack of dielectric layers. We develop a theoretical formalism based on the transfer matrix method for the calculation of the surface-driven second-harmonic radiation from multilayer structures and elaborate it for the case of ultrathin dielectric layers using a power series expansion to derive the effective surface nonlinear tensor for the whole stack. We show that for deeply subwavelength thicknesses of the layers the surface responses from all interfaces can efficiently sum up, leading to largely enhanced efficiency of SHG. As a result, such surface-driven nonlinearity can become comparable to the bulk nonlinearity in noncentrosymmetric semiconductors and can yield high performance for nonlinear nanophotonic applications.

Second harmonic generation from metal nanoparticle dimer: an analytical approach in dipole approximation

In this theoretical study, the problem of Second Harmonic Generation (SHG) in the interaction of laser beam with a Metallic Nanoparticle (MNP) dimer is considered. Using a classical electrodynamics approach, the nonlinear interaction of laser beam fields with Nanoparticles (NPs) is considered taking into account the dipole-dipole interparticle interactions. Analytical formulae are derived for the effect of dipole-dipole interaction on the Second Harmonic (SH) radiation power for two different polarizations of laser beam. It is found that the interaction causes the substantial enhancement of the SH radiation power while for the case when the laser beam field is parallel to the dimer axis, this enhancement is larger. Additionally, the dipole-dipole interaction of NPs leads to the redshift of the peak value with respect to the individual NP radiation. The resonance frequency displacement of the parallel case is more than that of the perpendicular one. The effect of particles size and interparticle separation on the SHG is studied.

Optical Second-Harmonic Generation of Terahertz Field from n-type InSb Semiconductors

In this paper, optical second-harmonic generation of terahertz field from n-type InSb semiconductors is investigated. The inter-band transitions generate electron-hole charge carriers in narrow bandgap semiconductors. The nonlinear interaction of terahertz fields with charge carriers within the absorption depth produces a nonlinear current at twice of the fundamental THz frequency, which generates second-harmonic terahertz photons. The conversion efficiency of second-harmonic generation is enhanced by the resonant density perturbations of the charge carriers. The results based on computational fluid dynamics show the generation of second-harmonic radiation spectrally centered at 2 THz frequency with 8 MeV energy. This mechanism for optical second-harmonic generation of terahertz may open new realms of semiconductor characterization, for which terahertz techniques are ideally suited.

Compact Filtering Patch Antenna Arrays for Marine Communications

A planar 2 × 2 filtering patch antenna array with high selectivity and wide stopband is investigated for marine communications. The 2 × 2 patch array is composed of a driven patch and four identical parasitic patches placed nearby. A forked-microstrip-line-coupled rectangular slot is used to excite the driven patch, while the neighboring parasitic patches are capacitively coupled by the driven patch. Owing to the hybrid feeding effect of the forked feed line, a radiation null can be generated at the lower band-edge. Simultaneously, another radiation null can be generated at the upper band-edge owing to the radiation cancellation effect between the driven patch and the parasitic patches. Consequently, no power dividing feeding network is required for the 2 × 2 array, and the quasi-elliptic bandpass filtering response is achieved without utilizing any complex filtering/non-filtering circuits. In addition, the proposed filtering array exhibits a wide stopband capable of suppressing the second-harmonic radiation effectively. Based on this 2 × 2 array, a larger 4 × 4 filtering array is further designed by just using a traditional four-way power dividing network. For validation, both of the 2 × 2 and 4 × 4 filtering patch arrays operating at 5.25 GHz are fabricated and tested. The measured results show that the two patch arrays have comparable impedance bandwidths of about 4.4%. The in-band peak gains are given by 11.1 and 15.5 dBi respectively, which are suitable for the long-distance shore-to-ship communications.

Multipolar second-harmonic generation from high-Q quasi-BIC states in subwavelength resonators

Abstract We put forward the multipolar model which captures the physics behind linear and nonlinear response driven by high-quality (high-Q) supercavity modes in subwavelength particles. We show that the formation of such trapped states associated with bound states in the continuum (quasi-BIC) can be understood through multipolar transformations of coupled leaky modes. The quasi-BIC state appears with increasing the order of the dominating multipole, where dipolar losses are completely suppressed. The efficient optical coupling to this state in the AlGaAs nanodisk is implemented via azimuthally polarized beam illumination matching its multipolar origin. We establish a one-to-one correspondence between the standard phenomenological non-Hermitian coupled-mode theory and multipolar models. The derived multipolar composition of the generated second-harmonic radiation from the AlGaAs nanodisk is then validated with full-wave numerical simulations. Back-action of the second-harmonic radiation onto the fundamental frequency is taken into account in the coupled nonlinear model with pump depletion. A hybrid metal-dielectric nanoantenna is proposed to augment the conversion efficiency up to tens of per cent due to increasing quality factors of the involved resonant states. Our findings delineate novel promising strategies in the design of functional elements for nonlinear nanophotonics applications.

Anomalous absorption due to development of return current instability

We examine the anomalous absorption effect due to ion–acoustic turbulence (IAT) driven by the return current instability in a plasma corona in relation to relevant conditions of the direct-drive inertial confinement fusion scheme of the Russian megajoule-energy level laser facility. We find that for the plasma corona created by a carbon ablator, the IAT contributes not more than 10% to absorption for intensities ≳ 7 × 1014 W/cm2 at second-harmonic radiation.

Influence of a Fundamental Frequency Pulse Chirp on the Spectral and Temporal Second-Harmonic Characteristics

The results of investigation of the possibility of reducing the duration of the second-harmonic radiation pulse generated by the starting complex of an THL-100 laser system by conversion of a pre-chirped fundamental frequency pulse are presented. A front-end compressor of the complex is used to obtain a fundamental harmonic pulse chirp at a central wavelength of 950 nm with duration of 70 fs. It is shown that a small excess of the group velocity dispersion (±3200–4000 fs2) of fundamental frequency radiation increases the width of the second-harmonic radiation spectrum by a factor of 1.5–2. A compensation for the excess dispersion of radiation pulses with broadened spectrum makes it possible to reduce the second-harmonic radiation pulse duration approximately proportionally to its spectral broadening and to obtain a minimal pulse duration of 35 fs.

Near-Surface Plasma Formed by Dual-Pulse Laser Action at Two Wavelengths on Titanium in Air

Spectra and structures of near-surface plasma torches were studied experimentally by measuring the specific mass loss and plume luminosity caused by dual-pulse laser radiation at wavelengths 1064 and 532 nm on titanium in air as functions of the time between laser pulses and their sequence. Dependences of the laser-plasma temperature and concentration of charged particles on the paired laser pulse parameters were established at laser radiation power densities q1064 ≤ 3.1·109 and q532 ≤ 2.7·109 W/cm2 for λ = 1064 and 532 nm. The optimal conditions for recording erosion plasma spectra had the initial effect from second-harmonic radiation with a time interval between pulses of 4–5 μs; for specific mass loss, 3–6 μs. The results were important for enhancing the efficiency of laser emission spectral analysis and laser-plasma processing of materials.