Plasmon‐Enhanced Second‐ and Third‐Order Harmonic Generation in Spherical Free‐Electron Nanoclusters with Diffuse Surface

The nonlinear scattering of a laser pulse off spherical nanoclusters with free electrons and with a diffuse surface is examined in the collisionless hydrodynamics approximation in the framework of perturbation theory with respect to the laser pulse intensity, as well as of the steady‐state approximation. In a previous publication [S.V. Fomichev and W. Becker, Phys. Rev. A 81 , 063201 (2010)] we reported the full nonlinear hydrodynamic model of forced collective electron motion confined to a cluster with diffuse surface and introduced two different perturbation theories corresponding to different laser intensity regimes. In the current paper, in the framework of this hydrodynamic model we focus on the properties of plasmon resonance‐enhanced third‐harmonic generation in a spherical cluster and its dependence on the shape of its diffuse surface whose role increases for nonlinear processes. At the same time, the quadrupole second‐harmonic generation in a spherical cluster is also inspected as a necessary intermediate step. Both cold metal clusters in vacuum or in a dielectric surrounding and hot laser‐heated and laser‐ionized clusters are considered within the same approach for a wide range of the fundamental laser frequency. Nonlinear laser excitation of the dipole plasmon Mie resonance in spherical clusters, as well as of other respective multipole plasmon resonances is investigated analytically and numerically in detail (position, width, and strength) versus the cluster‐surface diffuseness, the outer ionization degree in charged clusters, the electron‐density diffuseness, and their interplay. Under certain conditions, depending on the various cluster parameters, different secondary nonlinear resonances are found. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

自生磁场对相对论谐波辐射的影响

分析了自生磁场对相对论谐波辐射的影响，得出结论，自生磁场对强激光在欠稠密等离子 产生的相对论相干波辐射有重要作用，自生磁场激发偶次谐波辐射，并对奇次谐波辐射产生影响，对二次，三次谐波作了详细分析，发现，自生磁场激发二次谐波辐射，而对三次谐波辐射有削弱作用，并且它还使谐波的失相时间延长。     

Enhanced Photoelectric and Photothermal Responses on Silicon Platform by Plasmonic Absorber and Omni‐Schottky Junction

Recent progresses in plasmon‐induced hot electrons open up the possibility to achieve photon harvesting beyond the fundamental limit imposed by band‐to‐band transitions in semiconductors. To obtain high efficiency, both the optical absorption and electron emission/collection are crucial factors that need to be addressed in the design of hot electron devices. Here, we demonstrate a photoresponse as high as 3.3mA/W at 1500nm on a silicon platform by plasmonic absorber (PA) and omni‐Schottky junction integrated photodetector, reverse biased at 5V and illuminated with 10mW. The PA fabricated on silicon consists of a monolayer of random Au nanoparticles (NPs), a wide‐band gap semiconductor (TiO₂) and an optically thick Au electrode, resulting in broadband near‐infrared (NIR) absorption and efficient hot‐electron transfer via an all‐around Schottky emission path. Meanwhile, time and spectral‐resolved photoresponse measurements reveal that embedded NPs with superior absorption resembling plasmonic local heating sources can transfer their energy to electricity via the photothermal mechanism, which until now has not been adequately assessed or rigorously differentiated from the photoelectric process in plasmon‐mediated photon harvesting nano‐systems.     

Enhanced Raman scattering based on fabry‐perot like resonance in a metal‐cladding waveguide

A liquid‐core metal‐cladding waveguide structure of millimeter scale is designed to enhance Raman signal via the excitation of Fabry–Perot‐like resonance. Theoretically, an oscillating field distribution covering the whole guiding layer is generated by the multireflection at the two metal interfaces. The large detection area covers the whole sample chamber because of the oscillating nature of excited high order modes with concentrated intensity. By adding metal nanoparticles, the Fabry–Perot‐like resonance can be combined with local surface plasmons resonance to further enhance the light‐matter interaction with the target molecules, which is also confirmed by the experimental results. Copyright © 2015 John Wiley & Sons, Ltd.     

Red/Near‐Infrared Emissive Metalloporphyrin‐Based Nanodots for Magnetic Resonance Imaging‐Guided Photodynamic Therapy In Vivo

Near‐infrared emissive (NIR) porphyrin‐implanted carbon nanodots (PCNDs or MPCNDs) are prepared by selectively carbonization of free base or metal complexes [M = Zn(II) or Mn(III)] of tetra‐(meso‐aminophenyl)porphyrin in the presence of citric acid. The as‐prepared nanodots exhibit spontaneously NIR emission, small size, good aqueous dispersibility, and favorable biocompatibility characteristic of both porphyrins and pristine carbon nanodots. The subcellular localization experiment of nanodots indicates a lysosome‐targeting feature. And the in vitro photodynamic therapy (PDT) results on HeLa cells indicate the nanodots alone have no adverse effect on tumor cells, but display remarkable photodynamic efficacy upon irradiation. Moreover, MnPCNDs containing paramagnetic Mn(III) ions, which possesses good biocompatibility, NIR luminescence, and magnetic resonance imaging and efficient singlet oxygen production, are further studied in magnetic resonance imaging‐guided photodynamic therapy in vivo.     

An Infrared Metamaterial Broadband Absorber Based on a Simple Titanium Disk with High Absorption and a Tunable Spectral Absorption Band

A metamaterial absorber is proposed that functions in the medium- (3–5 µm) and long-wavelength (8–12 µm) infrared (medium-wavelength infrared, MWIR, and long-wavelength infrared, LWIR, respectively) regions. The proposed design, which consists of periodic cells, can be tuned to achieve single-band or dual-band light absorption by changing the periodicity of the structure. Each cell forming the metamaterial absorber consists of a bottom metal plate (Al), a top metal disk (Ti), and an intermediate dielectric medium (Si or ZnS) in which a metal disk (Ti) is embedded. For a period of 0.85 µm, the absorber achieves broadband absorption in the LWIR region, with an average absorption of 92.1%. Further, the absorber shows acceptable tolerance to irradiation at oblique incidence. For a period of 2 µm, a peak absorption of 99.05% is achieved in the MWIR region, thereby providing dual-band absorption. Tuning the periodicity of the structure enhances the localized surface plasmon resonance, with the absorption mechanism explained by establishing an equivalent parallel LC circuit. The absorption properties demonstrated by the proposed metamaterial absorber are promising for thermal imaging and infrared spectroscopy.     

三阶非线性效应对三次谐波振荡转换的影响

使用快速傅里叶变换和四阶龙格-库塔法,对KDP晶体内以Ⅰ／Ⅱ类角度失谐设置方式的高强度激光三次谐波振荡转换进行了研究,考虑了谐波转换过程中的三阶非线性x^[3]、衍射、离散等效应,并着重研究了KDP晶体的三阶非线性效应对高强度激光三次谐波转换的影响。研究表明,三阶非线性效应降低了三次谐波振荡转换效率,增大了相位扰动对3ω光束的强度调制,然而通过增加二倍频的失谐角△θ8,可以避免三阶非线性效应对三次谐波振荡转换带来的不利影响。     

Imaging the Magnetic Reversal of Isolated and Organized Molecular‐Based Nanoparticles using Magnetic Force Microscopy

In the race towards miniaturization in nanoelectronics, magnetic nanoparticles (MNPs) have emerged as potential candidates for their integration in ultrahigh‐density recording media. Molecular‐based materials open the possibility to design new tailor‐made MNPs with variable composition and sizes, which benefit from the intrinsic properties of these materials. Before their implementation in real devices is reached, a precise organization on surfaces and a reliable characterization and manipulation of their individual magnetic behavior are required. In this paper, it is demonstrated how molecular‐based MNPs are accurately organized on surfaces and how the magnetic properties of the individual MNPs are detected and tuned by means of low‐temperature magnetic force microscopy (LT‐MFM) with variable magnetic field. The magnetization reversal on isolated and organized MNPs is investigated; in addition, the temperature dependence of their magnetic response is evaluated.     

Controlling fluorescence emission with split‐ring‐resonator‐based plasmonic metasurfaces

Metasurfaces, which consist of resonant metamaterial elements in the form of two‐dimensional thin planar structures, retain great capabilities in manipulating electromagnetic wave and potential applications in modifying interaction with fluorescent molecules. The metasurfaces with magnetic responses are favorable to weakening fluorescence quenching while less investigated in controlling fluorescence. In this paper, we demonstrate control over fluorescence emission by engineering the magnetic and electric modes in plasmonic metasurfaces consisting of 45‐nm‐thick gold split‐ring‐resonators (SRRs). The fluorescence emission exhibits an enhancement factor of ∼18 and is predominantly x‐polarized with assistance of the magnetic mode excited by oblique incidence with an x‐polarized electric field. The magnetic and electric modes excited by oblique incidence with a y‐polarized electric field contribute to the rotation of emission polarization with respect to the incident polarization. The results demonstrate manipulating the interaction of fluorescent emitters with different resonant modes of the SRR‐based metasurface at the nanoscale by the polarization of incident light, providing potential applications of metasurfaces in a wide variety of areas, including optical nanosources, fluorescence spectroscopy and compact biosensors.

     

Intensity‐Enhanced Apodization Effect on an Axially Illuminated Circular‐Column Particle‐Lens

A particle can function as a refractive lens to focus a plane wave, generating a narrow, high intensive, weak‐diverging beam within a sub‐wavelength volume, known as the ‘photonic nanojet’. It is known that apodization method, in the form of an amplitude pupil‐mask centrally situated on a particle‐lens, can further reduce the waist of a photonic nanojet, however, it usually lowers the intensity at the focus due to blocking the incident light. In this paper, the anomalously intensity‐enhanced apodization effect was discovered for the first time via numerical simulation of focusing of the axially illuminated circular‐column particle‐lenses, and a greater than 100% peak intensity increase was realised for the produced photonic nanojets.     

Theoretical Exploration of the Isotopic Effect on Molecular High-Order Harmonic Generation

The isotopic effect on the generation of the molecular high-order harmonics is studied by numerically solving the one-dimensional time-dependent Schrödinger equation when the model hydrogen molecule ions/hydrogen deuterium molecule ions are exposed to an intense laser pulse. To explain the effect more clearly, not only the ionization probabilities but also the electron–nuclear probability density distributions and time-frequency profiles are calculated. The results show that more intense harmonics are generated in the asymmetric diatomic molecule ions/hydrogen deuterium molecule ions than those of hydrogen molecule ions. Moreover, the interference minimum in the harmonic spectra is investigated by adjusting the laser intensity and the initial vibrational state. It is shown that the interference minimum is sensitive to the laser intensity and the initial vibrational level for hydrogen molecule ions; in contrast, it is only dependent on the initial vibrational level for hydrogen deuterium molecule ions.     

二次谐波产生中入射啁啾脉冲对相位的影响

讨论了基波为叔率啁啾脉冲的二次谐波产生过程，结果表明选取合适的啁啾参量可以提高二次谐波的光强，但同时会使其波形和相位发生变化，三次相位的畸变主要取决于入射的基波啁啾脉冲，而与相位匹配的失谐量关系不大。     

Light Trapping and Down‐Shifting Effect of Periodically Nanopatterned Si‐Quantum‐Dot‐Based Structures for Enhanced Photovoltaic Properties

Periodically nanopatterned Si structures have been prepared by using a nanosphere lithography technique. The formed nanopatterned structures exhibit good anti‐reflection and enhanced optical absorption characteristics. The mean surface reflectance weighted by AM1.5 solar spectrum (300–1200 nm) is as low as 5%. By depositing Si quantum dot/SiO₂ multilayers (MLs) on the nanopatterned Si substrate, the optical absorption is higher than 90%, which is significantly improved compared with the same multilayers deposited on flat Si substrate. Furthermore, the prototype n‐Si/Si quantum dot/SiO₂ MLs/p‐Si heterojunction solar cells has been fabricated, and it is found that the external quantum efficiency is obviously enhanced for nanopatterned cell in a wide spectral range compared with the flat cell. The corresponding short‐circuit current density is increased from 25.5 mA cm^?2 for flat cell to 29.0 mA cm^?2 for nano‐patterned one. The improvement of cell performance can be attributed both to the reduced light loss and the down‐shifting effect of Si quantum dots/SiO₂ MLs by forming periodically nanopatterned structures.     

Tuning of near‐ and far‐field properties of all‐dielectric dimer nanoantennas via ultrafast electron‐hole plasma photoexcitation

All‐optical ultrafast signal modulation and routing by low‐loss nanodevices is a crucial step towards an ultracompact optical chip with high performance. Here, we propose a specifically designed silicon dimer nanoantenna, which is tunable via photoexcitation of dense electron‐hole plasma with ultrafast relaxation rate. On the basis of this concept, we demonstrate the effect of beam steering by up to 20 degrees through simple variation of the intensity of incident light. The effect, which is suitable for ultrafast light routing in an optical chip, is demonstrated both in the visible and near‐IR spectral regions for silicon‐ and germanium‐based nanoantennas. We also reveal the effect of electron‐hole plasma photoexcitation on the local density of states (LDOS) in the dimer gap and find that the orientation averaged LDOS can be altered by 50%, whereas modification of the projected LDOS can be even more dramatic, almost five‐fold for transverse dipole orientation. Moreover, our analytical model sheds light on the transient dynamics of the studied nonlinear nanoantennas, yielding all temporal characteristics of the suggested ultrafast nanodevice. The proposed concept paves the way to the creation of low‐loss, ultrafast, and compact devices for optical signal modulation and routing.

     

五配位铝强化硅铝固体酸的固体核磁共振研究

Brønsted酸（B酸）是无定型硅铝（ASAs）表面最重要的催化活性位点.通常认为B酸位的形成只依赖于不饱和四配位铝（Al^IV）,且仅具有弱B酸性.通过合成五配位铝（Al^V）富集的ASAs能够大幅提升高铝硅比（Al/Si）时的B酸含量及强度,克服传统Al^IV富集的ASAs的酸性强化瓶颈.本文介绍了Al^V在ASAs酸性强化及合成单原子催化剂中的重要作用.通过采用多种二维固体核磁共振（SSNMR）及原位质子NMR技术,证明了Al^V能够大量富集在ASAs表面,着重介绍了两种基于Al^V的新B酸位的形成机制,并阐明了Al^V诱导单原子催化剂在ASAs表面形成的机理.     

Second Harmonic Generation of Self Focused Cosh‐Gaussian Laser Beam in Collisionless Plasma

This paper presents an investigation of self‐focusing of a Cosh‐Gaussian (ChG) laser beam and its effect on second harmonic generation in collisionless plasma. In the presence of ChG laser beam the carriers get redistributed from high field region to low field region on account of ponderomotive force as a result of which a transverse density gradient is produced in the plasma which in turn generates an electron‐plasma wave at pump frequency. Generated plasma wave interacts with the incident laser beam and hence generates its second harmonics. Moment theory has been used to derive differential equation governing the evolution of spot size of ChG laser beam propagating through collisionless plasma. The differential equation so obtained has been solved numerically. The effect of decentered parameter, intensity of ChG laser beam and density of plasma on self‐focusing of the laser beam and second harmonic yield has been investigated. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron‐Nuclear Dynamics in Molecular Harmonic Generation Driven by a Plasmonic Nonhomogeneous Field

Electron (z)‐nuclear (R) dynamics in the molecular high‐order harmonic generation (MHHG) from H₂⁺ driven by the plasmonic nonhomogeneous field, generated by the surface plasmon polaritons in the bowtie‐shaped nanostructure, have been theoretically investigated through solving the two dimensional time‐dependent Schrödinger equation with the Non‐Bohn‐Oppenheimer approximation. It is found that (i) due to the plasmonic enhancement of the laser intensity, the harmonic cutoff can be extended when the spatial position of H₂⁺ is away from the gap center of the nanostructure. However, due to the limit of the gap size, the threshold value of the harmonic cutoff can be obtained at a given position of H₂⁺. (ii) Due to the asymmetric enhancement of the laser intensity in space, the extended higher harmonics are respectively from E(t) > 0 a.u. or E(t) < 0 a.u. for the cases of the positive and the negative spatial position of H₂⁺. As a result, the intensities of the extended higher harmonics are different and can be controlled by changing the carrier‐envelope phase and the pulse duration of the laser field. (iii) In the few‐cycle pulse duration, the MHHG mainly comes from the multi‐photon resonance ionization (MPRI), while as the pulse duration increases, the MPRI, the charge‐resonance enhanced ionization (CREI) and even the dissociative ionization (DI) are contributed to the MHHG. Moreover, as the spatial position of H₂⁺ moves, the contributions of the MHHG from the MPRI, the CERI and the DI can be controlled. (iv) The contributions of the MHHG from the two‐H nuclei have been investigated and found that when E(t) > 0 a.u., the intensities of the harmonics from the negative‐H is higher than those from the positive‐H; while when E(t) < 0 a.u., the intensities of the harmonics from the positive‐H plays the main role in the MHHG. Moreover, the multi‐minima, caused by the two‐center interference can also be found. (v) Finally, by superposing a properly selected harmonics, a single isolated attosecond pulse (SIAP) with the full width at half maximum (FWHM) of 34 as can be obtained.     

Nonreciprocal transmission in a nonlinear photonic‐crystal Fano structure with broken symmetry

Nanostructures that feature nonreciprocal light transmission are highly desirable building blocks for realizing photonic integrated circuits. Here, a simple and ultracompact photonic‐crystal structure, where a waveguide is coupled to a single nanocavity, is proposed and experimentally demonstrated, showing very efficient optical diode functionality. The key novelty of the structure is the use of cavity‐enhanced material nonlinearities in combination with spatial symmetry breaking and a Fano resonance to realize nonreciprocal propagation effects at ultralow power and with good wavelength tunability. The nonlinearity of the device relies on ultrafast carrier dynamics, rather than the thermal effects usually considered, allowing the demonstration of nonreciprocal operation at a bit‐rate of 10 Gbit s⁻¹ with a low energy consumption of 4.5 fJ bit⁻¹.