Unconventional high-harmonic generation in resonant membrane metasurfaces

The local optical field enhancement which can occur at the end of a nanometer-size metallic tip has given rise to both increasing interest and numerous theoretical works on near-field optical microscopy. In this article we report direct experimental observation of this effect and present an extensive study of the parameters involved. Our approach consists in making a “snapshot” of the spatial distribution of the optical intensity in the vicinity of the probe end using photosensitive azobenzene-containing films. This distribution is coded by optically induced surface topography which is characterized in situ by atomic force microscopy using the same probe. We perform an extensive analysis of the influence of several experimental parameters. The results are analyzed as a function of the illumination parameters (features of the incident laser beam, exposure time, illumination geometry) as well as the average tip-to-sample distance and tip geometry. The results obtained provide substantial information about the tip’s field. In particular, they unambiguously demonstrate both the nanometric spatial confinement of the tip field and the evanescent nature of the nanosource excited at the tip’s end. Most of the experimental results are illustrated by numerical calculations based on the finite element method and commented using the literature on the subject. Additionally, we discuss the origin of the optically induced topography on a nanometer scale and present some preliminary results of the apertureless near-field optical lithography based on local field enhancement. Our approach constitutes a useful tool to investigate the near-field of apertureless probes and should enable the optimization of the nanosource for any experiment requiring local optical excitation of the matter.

The electromagnetic ‘ponderomotive force’ was discovered in 1957 in a radio-frequency experiment by Boot and Harvie 1. The force arises due to gradients in an inhomogeneous oscillating electric field, and can be used to control and accelerate electrons, with applications in ion traps, plasma accelerators and high-harmonic generation. However, being a second-order nonlinear effect in the applied field, the ponderomotive force is relatively weak. The availability of high-power, short laser pulses has enabled new practical realizations of this effect 2. In a Letter in this issue, Batelaan and co-workers predict that metal nanoantennas can produce sufficient ponderomotive force to deflect electrons 4. The phenomenon relies on the surface plasmon resonance properties of the nanoantenna, which produces a high local field enhancement combined with large field gradients in a small volume around the nanoantenna. To achieve the required field strengths, they propose to use high energy laser pulses of femtosecond time duration, tuned to the surface plasmon resonance wavelength. These short laser pulses offer yet another advantage, as they allow producing an ultrafast modulation of an incident electron beam. Such an electron switch would be of interest for a range of applications, for example ultrafast electron microscopy 4. Ultrafast electron microscopy is a powerful tool for characterizing chemical and physical processes at nanometer length scales and ultrafast time scales. Currently, the generation of ultrafast electron pulses is done by directly exciting the electron emission source with a high-power UV laser pulse. Plasmonic electron switches may be a feasibly alternative route toward the development of ultrafast electron beams at higher repetition frequencies and using lower-power femtosecond oscillators. As a first step, the authors measured the coherence time of surface plasmon modes in an array of nanoparticles, so-called optical antennas 3. A short, resonant laser pulse can drive a coherent plasmon oscillation, however this oscillation rapidly dephases via collisions of the electrons (intraband) or through excitation of electron-hole pairs (interband). Surface plasmons in isolated metal nanospheres have a dephasing time of several femtoseconds, depending on the type of metal antenna under study and details of the damping mechanism. Detailed measurements of plasmon dephasing were pioneered in the 1990s by Franz Aussenegg and co-workers, who used high harmonic generation as a nonlinear process to make an autocorrelation of the plasmon pulse 5. High-harmonic generation in nanoparticles is complicated, since second-harmonic requires noncentrosymmetric structures, while third harmonic signals are comparably weak. Becker et al. use a different, more direct method of measuring both the amplitude and phase of the plasmon field using cross-correlation of the plasmon field with a reference pulse with known width taken directly from the laser. The prospect of electron beam modulation using ultrafast plasmon fields is promising as a fundamental phenomenon and may further open up the field of ultrafast electron microscopy. Both the coherence time and local field enhancement can be tailored by the geometry of the nanoantenna, allowing for a rational design of nanostructures optimized for specific applications 6. Of particular interest will also be the application of techniques from coherent control to achieve local plasmonic hot-spots in both space and time 7, potentially resulting in even stronger field gradients at shorter time scales. Several hurdles will have to be overcome, related to the high field strengths required and the breakdown of metal nanoantenna arrays under such high power excitation. Similar problems are encountered in higher-harmonic generation experiments, and require careful optimization of material properties of the antennas and substrate. However, if successful, the plasmonic antenna approach may result in a new generation of electron modulation devices.

Resonant metasurfaces can greatly trap the light fields, so that they are widely used to enhance light–matter interactions at the nanoscale, such as promoting nonlinear effects of materials. Lithium niobate (LN) is an excellent nonlinear optical material and is often employed to generate harmonic signals. In this Letter, we numerically study the second harmonic generation (SHG) characteristics of the LN metasurface based on the quasi-bound states in the continuum (QBIC). The designed BIC and excited QBIC metasurfaces always hold C4v symmetry, and the BIC is demonstrated to degenerate into two BICs due to the anisotropic characteristics of LN. Moreover, the excited two high Q-factor QBICs can effectively enhance the SHG in LN, although the device maintains C4v symmetry, the SHG signal still shows polarization dependence. In addition, with the increase of Q-factor of quasi-BIC, the power and conversion efficiency (η) of SHG increase significantly. The calculated η can reach 6.04 × 10−3 and can be further improved when the resonance mode is closer to BIC. These results have important implications for high-quality nonlinear light sources based on LN materials.

We present high-quality (high-Q) Fano resonances in all-dielectric metasurfaces consisting of a periodic array of air holes on silicon (Si) film, deposited on the top of quartz substrate. With the control of the radius difference Δr and center distance Δd between the air holes, two asymmetric all-dielectric metasurfaces are proposed to achieve extremely high-Q Fano resonances. Numerical method with finite difference time domain and equivalent circuit model is employed to analyse the excitation mechanism of the sharp Fano resonances. It is shown that the high-Q Fano resonances come from the interference of two Fabry-Perot resonances, resulting in an extremely narrow window. Moreover, we also demonstrate that the high-Q Fano resonances can also be realized as electromagnetic wave is obliquely incident on the symmetric all-dielectric metasurface. Finally, we show the high-Q Fano resonances caused by asymmetric configurations can coexist with the Fano resonances in the symmetric configuration induced by oblique incidence. As a result, a tri-band Fano resonance is obtained. It is expected that our results will provide important mechanisms for tuning and switching a wide variety of optical devices such as angular sensors, filters, switches, and modulators.

We designed a multifunctional all-dielectric metasurface employing cuboid structures patterned with bow-tie-shaped nanoholes; it exhibits multiple Fano resonances induced by quasi-bound states in the continuum (quasi-BICs) through structural asymmetry. Among them, several resonant modes demonstrated high quality factors in the range of 103–104, along with near-unity modulation depth and strong spectral contrast. The optical responses were analyzed utilizing the finite-difference time-domain (FDTD) method, with Fano profiles fitted to theoretical models and the BIC-governed modes validated via the inverse-square ratio law. Furthermore, multipolar decomposition and electromagnetic spatial field profiles revealed the origins of the resonance, while LC circuit modeling provided additional physical insight into the Fano profiles. The proposed metasurface also exhibited strong polarization dependence, indicating its potential for active optical switching. Finally, the refractive-index sensing performance, including the potential for detecting Vibrio cholerae in an appropriate environment, reached a sensitivity of 342 nm per RIU and a figure of merit of 217.14 RIU−1. Advancing the control of high-Q quasi-BIC Fano resonances, this study highlights Fano resonators’ potential for refractive-index sensing and active switching.

We study the resonant enhancement of linear and nonlinear chiroptical effects in planar silicon metasurfaces with an in-plane asymmetry supporting multipolar Mie resonances and quasi-bound states in the continuum (quasi-BICs). We demonstrate theoretically and observe in experiment the pronounced linear circular dichroism at the quasi-BIC resonances originating from the interaction of modes with the substrate. We further find that both local field enhancement and third-harmonic signal are large for Mie resonances and some quasi-BIC modes due to the critical coupling. We demonstrate experimentally a strong nonlinear chiroptical response associated with high efficiency of the third-harmonic generation and large nonlinear circular dichroism varying from +0.918 ± 0.049 to −0.771 ± 0.004 for the samples with different asymmetries. We reveal the nonreciprocal nature of nonlinear chirality governed by the microscopic symmetry of nonlinearities and macroscopic symmetries of the meta-atom and metasurface lattice. We believe our results suggest a general strategy for engineering nonlinear chiroptical response in dielectric resonant metasurfaces.

We explore the outcomes of detailed microscopic models by calculating second- and third-harmonic generation from thin film surfaces with discontinuous free-electron densities. These circumstances can occur in structures consisting of a simple metal mirror, or arrangements composed of either different metals or a metal and a free electron system like a conducting oxide. Using a hydrodynamic approach we highlight the case of a gold mirror, and that of a two-layer system containing indium tin oxide (ITO) and gold. We assume the gold mirror surface is characterized by a free-electron cloud of varying density that spills into the vacuum, which as a result of material dispersion exhibits epsilon-near-zero conditions and local field enhancement at the surface. For a bylayer consisting of a thin ITO and gold films, if the wave is incident from the ITO side the electromagnetic field is presented with a free-electron discontinuity at the ITO/gold interface, and wavelength-dependent, epsilon-near-zero conditions that enhance local fields and conversion efficiencies, and determine the surface's emission properties. We evaluate the relative significance of additional nonlinear sources that arise when a free-electron discontinuity is present, and show that harmonic generation can be sensitive to the density of the screening free-electron cloud, and not its thickness. Our findings also suggest the possibility to control surface harmonic generation through surface charge engineering.

Resonant metasurfaces are an attractive platform for enhancing the nonlinear optical processes, such as second harmonic generation (SHG), since they can generate large local electromagnetic fields while relaxing the phase-matching requirements. Here, we demonstrate visible range, continuous wave (CW) SHG by combining the attractive material properties of gallium phosphide with high quality-factor photonic modes enabled by bound states in the continuum. We obtain efficiencies around 5e-5% W-1 when the system is pumped at 1200 nm wavelength with CW intensities of 1 kW/cm2. Moreover, we measure external efficiencies of 0.1% W-1 with pump intensities of only 10 MW/cm2 for pulsed irradiation. This efficiency is higher than the values previously reported for dielectric metasurfaces, but achieved here with pump intensities that are two orders of magnitude lower. These results take metasurface-based SHG a step closer to practical applications.

We have theoretically studied high harmonic generation (HHG) in solids driven simultaneously by a mid-infrared (MIR) laser and a high-order harmonic pulse with energy around the band gap between the valence band and conduction band. By adding this resonant harmonic light pulse with the relative intensity ratio of 4%, the high-order harmonic emission from the crystal is enhanced by 1–2 orders of magnitude. The yield of HHG in solid increases monotonically with the relative strength of the resonant harmonic pulses. In addition, we also found that HHG dynamics from the k channel around the boundary of the Brillouin zone can be selectively enhanced by adjusting the frequency of the resonant high-order harmonic pulse. The resonance-enhanced HHG and k channel selection effect in solids is also investigated by using the three-band semi-conductor Bloch equation for HHG in ZnO. We also find that the harmonic in the plateau region driven by adding a resonant light field to the strong MIR driving field has less red-shifted compared with the case driven by the MIR driving field alone.

We review recent progress towards imaging the electronic wavefunctions and nuclear dynamics of small molecules using the high order harmonics emitted when a molecule experiences an intense laser field. We illustrate that the essence of high harmonic emission is contained in the recombination amplitude between the continuum portion of the electronic wavefunction, that is formed through field ionization and which is accelerated and driven back to recollide in the laser field, and the bound electronic state. We review for the non-specialist some recent experimental and theoretical work dealing with high harmonic generation (HHG) in molecules. Particular attention is paid to two types of experiment recently performed in our group. The first of these types of experiment is the measurement of signatures of molecular electronic structure using HHG from molecules with a fixed orientation in space. The second is the use of HHG to track extremely fast proton rearrangement following ionization in light molecules, using the intrinsic temporal variation of the recolliding electron energy to extract these dynamics from measurements of the high harmonics.

The extremely local electric field enhancement and light confinement is demonstrated in dielectric waveguide with corner and gap geometry. The numerical results reveal the local electric field enhancement in the vicinity of the apex of fan-shaped waveguide. Classical electromagnetic theory predicts that the field enhancement and confinement abilities increase with decreasing radius of rounded corner ($r$) and gap ($g$), and show singularity for infinitesimal $r$ and $g$. For practical parameters with $r=g=10\,\mathrm{nm}$, the mode area of opposing apex-to-apex fan-shaped waveguides can be as small as $4\times10^{-3}A_{0}$ ($A_{0}=\lambda^{2}/4$), far beyond the diffraction limit. This way of breaking diffraction limit with no loss outperforms plasmonic waveguides, where light confinement is realized at the cost of huge intrinsic loss in the metal. Furthermore, we propose a structure with dielectric bow-tie antenna on a silicon-on-insulator waveguide, whose field enhancement increases by one order. The lossless dielectric corner and gap structures offer an alternative method to enhance the light-matter interaction without metal nano-structure, and will find applications in quantum electrodynamics, sensors and nano-particle trapping.

The local electric field enhancements around various nanoshell structures are investigated using the Finite Difference Time Domain (FDTD) method. The method provides a convenient and systematic approach for calculating several physical properties of nanostructures, including the optical absorption and scattering cross sections as well as the local electromagnetic fields. The method is applied to single uniform nanoshells as well as nanoshells with surface defects and structural distortions. The results show that, while defects can significantly affect local electric field enhancements, far field results such as extinction spectra can be remarkably insensitive to defects and distortions.

Non-volatile liquid organic semiconducting materials have received much attention as emerging functional materials for organic electronic and optoelectronic devices due to their remarkable advantages. However, charge injection and transport processes are significantly impeded at interfaces between electrodes and liquid organic semiconductors, resulting in overall lower performance compared to conventional solid-state electronic devices. Here we successfully demonstrate efficient charge injection into solvent-free liquid organic semiconductors via cracked metal structures with a large number of edges leading to local electric field enhancement. For this work, thin metal films on deformable polymer substrates were mechanically stretched to generate cracks on the metal surfaces in a controlled manner, and charge injection properties into a typical non-volatile liquid organic semiconducting material, (9-2-ethylhexyl)carbazole (EHCz), were investigated in low bias region (i.e., ohmic current region). It was found that the cracked structures significantly increased the current density at a fixed external bias voltage via the local electric field enhancement, which was strongly supported by field intensity calculation using COMSOL Multiphysics software. We anticipate that these results will significantly contribute to the development and further refinement of various organic electronic and optoelectronic devices based on non-volatile liquid organic semiconducting materials.

Summary form only given. High harmonic generation is unique as a source of coherent soft-X-ray (XUV) light because of its extremely short pulse duration, high spatial coherence, and compact size. Several theories have predicted that attosecond pulses might be generated using high order harmonic generation excited by ultrashort pulses. However, to date the fastest harmonic pulse duration measurements published were performed using laser pulses longer than 25 fs. In this work, we experimentally demonstrate a cross-correlation measurement which sets an upper limit of /spl sim/10 fs on the pulse duration of XUV harmonics.

Upconversion nanoparticles (UCNPs) convert near-infrared excitation into visible emission with efficiencies far greater than those of two-photon absorption or second harmonic generation, enabling upconversion with low intensity, incoherent light. For widespread applications, however, further enhancement of upconversion efficiency is desired. Photonic crystal (PhC) structure embedded with UCNPs provides a new way to engineer the photonic environment and enhance upconversion luminescence. We incorporate silica-coated UCNPs into a two-dimensional (2D) thin film PhC structure, which exhibits enhanced local electric field at the near-infrared (NIR) excitation wavelength of UCNPs. Thanks to the nonlinearity of the upconversion process, the local field enhancement is amplified and results in a significantly enhanced luminescence intensity. We observed approximately 130- and 350-fold enhancements for green and red luminescence, respectively, and present a detailed analysis of the enhancement mechanism. Unlike the plasmonic nanostructure, which tends to cause severe luminescence quenching, the purely dielectric photonic crystal structure generally shows little quenching and provides a good alternative for many applications.