Photothermal metasurface with polarization and wavelength multiplexing.

Controlling temperature distribution at the micro/nano-scale brings new applications in many fields such as physics, chemistry and biology. This paper proposes a photothermal metasurface that employs polarization and wavelength multiplexing to regulate various temperature distributions at the micro/nano-scale. Such a photothermal metasurface is numerically validated by the finite element method. Firstly, the inversion algorithm is used to calculate the thermal power density distribution, which is decided by a given temperature distribution. Then, based on the bottom-up design method, (a) the library of absorption cross sections of gold nanoparticles is established by resizing nanoparticles; (b) the single pixel is constructed for wavelength and polarization multiplexing; (c) the overall structure of a photothermal metasurface is optimized and established. Finally, four given temperature distributions, combining the multiplexing of two orthogonal polarizations and two wavelengths, are achieved in the same area. The simulation results well confirm the feasibility of photothermal multiplexing. Such photothermal metasurface provides solutions for flexible control of temperature distribution at the micro/nano-scale.

Electromagnetically induced transparency enabled by quasi-bound states in the continuum modulated by epsilon-near-zero materials.

Highly tunable electromagnetically induced transparency (EIT) with high-quality-factor (Q-factor) excited by combining with the quasi-bound states in the continuum (quasi-BIC) resonances is crucial for many applications. This paper describes all-dielectric metasurface composed of silicon cuboid etched with two rectangular holes into a unit cell and periodically arranged on a SiO2 substrate. By breaking the C2 rotational symmetry of the unit cell, a high-Q factor EIT and double quasi-BIC resonant modes are excited at 1224.3, 1251.9 and 1299.6 nm with quality factors of 7604, 10064 and 15503, respectively. We show that the EIT resonance is caused by destructive interference between magnetic dipole resonances and quasi-BIC dominated by electric quadrupole. Toroidal dipole (TD) and electric quadrupole (EQ) dominate the other two quasi-BICs. The EIT window can be successfully modulated with transmission intensity from 90% to 5% and modulation depths ranging from -17 to 24 dB at 1200-1250 nm by integrating the metasurface with an epsilon-near-zero (ENZ) material indium tin oxide (ITO) film. Our findings pave the way for the development of applications such as optical switches and modulators with many potential applications in nonlinear optics, filters, and multichannel biosensors.

Broadband zero backward scattering by all-dielectric core-shell nanoparticles.

Efficiently controlling the direction of optical radiation at nanoscale dimensions is essential for various nanophotonics applications. All-dielectric nanoparticles can be used to engineer the direction of scattered light via overlapping of electric and magnetic resonance modes. Herein, we propose all-dielectric core-shell SiO2-Ge-SiO2 nanoparticles that can simultaneously achieve broadband zero backward scattering and enhanced forward scattering. Introducing higher-order electric and magnetic resonance modes satisfies the generalized first Kerker condition for breaking through the dipole approximation. Zero backward scattering occurs near the electric and magnetic resonant regions, this directional scattering is therefore efficient. Adjusting the nanoparticles' geometric parameters can shift the spectral position of the broadband zero backward scattering to the visible and near-infrared regions. The wavelength width of the zero backward scattering could be enlarged as high as 142 and 63 nm in the visible and near-infrared region. Due to these unique optical features the proposed core-shell nanoparticles are promising candidates for the design of high-performance nanoantennas, low-loss metamaterials, and photovoltaic devices.

All-silicon-based nano-antennas for wavelength and polarization demultiplexing.

We propose an all-silicon-based nano-antenna that functions as not only a wavelength demultiplexer but also a polarization one. The nano-antenna is composed of two silicon cuboids with the same length and height but with different widths. The asymmetric structure of the nano-antenna with respect to the electric field of the incident light induced an electric dipole component in the propagation direction of the incident light. The interference between this electric dipole and the magnetic dipole induced by the magnetic field parallel to the long side of the cuboids is exploited to manipulate the radiation direction of the nano-antenna. The radiation direction of the nano-antenna at a certain wavelength depends strongly on the phase difference between the electric and magnetic dipoles interacting coherently, offering us the opportunity to realize wavelength demultiplexing. By varying the polarization of the incident light, the interference of the magnetic dipole induced by the asymmetry of the nano-antenna and the electric dipole induced by the electric field parallel to the long side of the cuboids can also be used to realize polarization demultiplexing in a certain wavelength range. More interestingly, the interference between the dipole and quadrupole modes of the nano-antenna can be utilized to shape the radiation directivity of the nano-antenna. We demonstrate numerically that radiation with adjustable direction and high directivity can be realized in such a nano-antenna which is compatible with the current fabrication technology of silicon chips.

Hot-Electron Intraband Luminescence from GaAs Nanospheres Mediated by Magnetic Dipole Resonances.

Significantly enhanced electric field in plasmonic hot spots can dramatically increase the linear and nonlinear absorption of light, leading to a high-temperature electron gas which radiates, through mainly intraband transition, a broadband luminescence quite similar to blackbody radiation. Here, we demonstrate that such hot-electron intraband luminescence (HEIL) can also be achieved by exploiting the significantly enhanced electric field at the magnetic dipole resonances of gallium arsenide (GaAs) nanospheres (NSs). We show that monocrystalline GaAs NSs with distinct electric and magnetic dipole (ED and MD) resonances can be obtained by using femtosecond laser ablation and annealing. Significantly enhanced second harmonic generation and broadband HEIL are observed when the MD resonances of such GaAs NSs are resonantly excited. The lifetime of the HEIL is found to be as short as ∼82 ps, indicating a significant enhancement in radiative intraband transition rate. We reveal that the slope extracted from the dependence of the HEIL intensity on the irradiance is linearly proportional to the energy of the emitted photon. The existence of distinct ED and MD resonances in combination with a direct bandgap makes GaAs NSs an attractive candidate for constructing novel all-dielectric metamaterials and active photonic devices.

Hot luminescence from gold nanoflowers and its application in high-density optical data storage.

Gold nanoflowers with feature sizes ranging from several tenths to several hundred nanometers were synthesized by using the one-pot method. They were formed by the self-organization of gold nanoparticles of several nanometers and exhibited broad extinction spectra in the near infrared spectral range. Randomly distributed hot spots originating from the strongly localized modes were generated in gold nanoflowers and their appearances exhibited strong dependences on both the polarization and wavelength of the excitation light. Under the excitation of femtosecond laser pulses, such hot spots emitted efficient hot luminescence spanning the visible to near infrared spectral range. Distinct from the hot luminescence of single hot spots formed on rough gold and silver surfaces, the hot luminescence from gold nanoflowers composed of a large number of hot spots exhibited excitation-intensity dependence quite similar to the emission spectrum. It was demonstrated that the polarization- and wavelength-dependent hot luminescence of gold nanoflowers can be utilized to realize optical data storage with high density and low energy.

Magnetic Fano resonance of heterodimer nanostructure by azimuthally polarized excitation.

The optical properties of a Si-Au heterodimer nanostructure, which is composed of an Au split nanoring surrounded by a Si nanoring with a larger diameter, are investigated both theoretically and numerically. It is found that a pure magnetic plasmon Fano resonance can be achieved in the Si-Au heterodimer nanostructure when it is excited by an azimuthally polarized beam. It is revealed that the pure magnetic Fano resonance is generated by the destructive interference between the magnetic dipole resonance of the Si nanoring and the magnetic dipole resonance of the Au split nanoring. A coupled oscillator model is employed to analyze the Fano resonance of the Si-Au heterodimer nanostructure. The pure magnetic response of the Si-Au heterodimer nanostructure is verified by the current density distributions and the scattering powers of the electric and magnetic multipoles. The Fano resonance in the Si-Au heterodimer nanostructure exhibits potential applications of low-loss magnetic plasmon resonance in the construction of artificial magnetic metamaterials.

Polarization beam splitters, converters and analyzers based on a metasurface composed of regularly arranged silicon nanospheres with controllable coupling strength.

A metasurface composed of regularly arranged silicon (Si) nanospheres (NSs) with coupling was investigated both theoretically and numerically based on the Mie theory, the simple Lorentz line shape model and the finite-difference time-domain technique. By deliberately controlling the coupling strength between Si NSs through the design of the lattice constants of a rectangular lattice, polarization beam splitters, converters and analyzers with good performance can be successfully constructed. A square lattice as well as a large incidence angle was employed to build the polarization beam splitters and converters. At an incidence angle of 80°, the polarization beam splitters can completely reflect the s-polarized light and transmit the p-polarized light in a wavelength region of 510-620 nm. For a circularly polarized light incident on the polarization converters, one can get s-polarized light in the reflection direction and p-polarized light in the transmission direction. For the polarization beam analyzers, a rectangular lattice with deliberately chosen lattice constants was employed and the transmissivity of a linearly polarized light can be continuously adjusted from 0 to ~0.90 by simply rotating the metasurface. We revealed that the broadening of either the electric dipole resonance or the magnetic dipole resonance or both of them, which is induced by the asymmetric coupling of Si NSs, is responsible for the modification in the transmissivity spectrum of the metasurface. Our findings provide a guideline for designing photonic devices based on the metasurfaces composed of Si NSs with controllable coupling strength.

Effects of substrates on the nonlinear optical responses of two-dimensional materials.

We investigated numerically and experimentally the achievement of strongly localized electric field and significantly enhanced second harmonic generation (SHG) in two-dimensional (2D) materials by using dielectric-metal hybrid substrates. Based on the theory of thin film interference, it was revealed that the strongest localization of electric field in a 2D material, which corresponds to the largest absorption in the metal film, could be achieved by minimizing the reflection of the combined structure (i.e., 2D material + hybrid substrate) because the transmission through the combined structure was negligible. By using MoS2 as an example, it was demonstrated that a SHG enhancement factor of ~6 could be achieved in the 17-nm-thick MoS2 layer on an Au/SiO2 substrate as compared with the single-layer MoS2 on the commonly used SiO2/Si substrates with highly efficient SHG. By employing a SiO2-SnO2/Ag/SiO2 substrate in which a 20-nm-thick dielectric film of SiO2-SnO2 was inserted in between the MoS2 layer and the Ag film, a SHG enhancement factor as large as ~18 could be realized in the 9-nm-thick MoS2 layer. Numerical simulations based on the finite-difference time-domain technique were employed to derive the enhancement factors for SHG and it was revealed that for thick MoS2 layers the SHG intensity is dominated mainly by the localization of electric field induced by the dielectric-metal hybrid substrates. The dependence of the SHG enhancement factor on the thickness of the MoS2 layer was found to be modified when the dielectric-metal hybrid substrates were adopted.

Three-photon-induced blue emission with narrow bandwidth from hot flower-like ZnO nanorods.

ZnO nanorods (NRs) self-organized into flowers were synthesized at different temperatures ranging from 100°C to 180°C by using the hydrothermal method. The existence of Zn interstitials (Zn(i)) was confirmed by X-ray photoelectron spectroscopy and a larger amount of Zn(i) was found in the ZnO NRs prepared at higher temperatures. A redshift of the emission peak of more than 15 nm was observed for the ZnO NRs under single photon excitation. The nonlinear optical properties of the flower-like ZnO NRs were characterized by using focused femtosecond laser light and strong three-photon-induced luminescence was observed at an excitation wavelength of ~750 nm. More interestingly, a large redshift of the emission peak was observed with increasing excitation intensity, resulting in efficient blue emission with a narrow bandwidth of ~30 nm. It was confirmed that the large redshift originates from the heating of the ZnO NRs to a temperature of more than 800°C and the closely packed ZnO NRs in the flowers play a crucial role in heat accumulation. The stable and efficient three-photon-induced blue emission from such ZnO NRs may find potential applications in the fields of optical display, high-temperature sensors and light therapy of tumors.

Formation of 100-nm periodic structures on a titanium surface by exploiting the oxidation and third harmonic generation induced by femtosecond laser pulses.

Periodic surface structures with periods as small as about one-tenth of the irradiating femtosecond (fs) laser light wavelength were created on the surface of a titanium (Ti) foil by exploiting laser-induced oxidation and third harmonic generation (THG). They were achieved by using 100-fs laser pulses with a repetition rate of 1 kHz and a wavelength ranging from 1.4 to 2.2 µm. It was revealed that an extremely thin TixOy layer was formed on the surface of the Ti foil after irradiating fs laser light with a fluence smaller than the ablation threshold of Ti, leading to a significant enhancement in THG which may exceed the ablation threshold of TixOy. As compared with Ti, the maximum efficacy factor for TixOy appears at a larger normalized wavevector in the direction perpendicular to the polarization of the fs laser light. As a result, the THG-dominated laser ablation of TixOy induces 100-nm periodic structures parallel to the polarization of the fs laser light. The depth of the periodic structures was found to be ~10 nm by atomic force microscopy and the formation of the thin TixOy layer was verified by energy dispersive X-ray spectroscopy.

Efficient blue light emission from In0.16Ga0.84N/GaN multiple quantum wells excited by 2.48-µm femtosecond laser pulses.

We report on the efficient blue light emission from In0.16Ga0.84N/GaN multiple quantum wells excited by femtosecond laser pulses with long wavelengths ranging from 1.24 to 2.48 µm. It is found that the trap states in GaN barrier layers lead to an efficient cascade multiphoton absorption in which the carriers are generated through simultaneous absorption of n (n=1 and 2) photons to the trap states, followed by simultaneous absorption of m (m=3, 4, and 5) photons to the conduction band. The dependence of the upconversion luminescence on excitation intensity exhibits a slope between n and n+m, which is in good agreement with the prediction based on the rate equation model.

Controllable color display induced by excitation-intensity-dependent competition between second and third harmonic generation in ZnO nanorods.

We investigated the second and third harmonic generation (SHG and THG) in ZnO nanorods (NRs) by using a femtosecond laser (optical parametric amplifier with tunable wavelengths) with a long excitation wavelength of 1350 nm and a low repetition rate of 1 kHz. The damage threshold for ZnO NRs in this case was sufficiently large, enabling us to observe the competition between SHG and THG. The transition from red to blue emission and the mixing of red and blue light with different ratios were successfully demonstrated by simply varying excitation intensity, implying the potential applications of ZnO NRs in all-optical display.

Gradient-induced long-range optical pulling force based on photonic band gap.

Optical pulling provides a new degree of freedom in optical manipulation. It is generally believed that long-range optical pulling forces cannot be generated by the gradient of the incident field. Here, we theoretically propose and numerically demonstrate the realization of a long-range optical pulling force stemming from a self-induced gradient field in the manipulated object. In analogy to potential barriers in quantum tunnelling, we use a photonic band gap design in order to obtain the intensity gradients inside a manipulated object placed in a photonic crystal waveguide, thereby achieving a pulling force. Unlike the usual scattering-type optical pulling forces, the proposed gradient-field approach does not require precise elimination of the reflection from the manipulated objects. In particular, the Einstein-Laub formalism is applied to design this unconventional gradient force. The magnitude of the force can be enhanced by a factor of up to 50 at the optical resonance of the manipulated object in the waveguide, making it insensitive to absorption. The developed approach helps to break the limitation of scattering forces to obtain long-range optical pulling for manipulation and sorting of nanoparticles and other nano-objects. The developed principle of using the band gap to obtain a pulling force may also be applied to other types of waves, such as acoustic or water waves, which are important for numerous applications.

Asymmetric femtosecond laser ablation of silicon surface governed by the evolution of surface nanostructures.

The femtosecond laser ablation of silicon surface near the ablation threshold was investigated and the preferential ablation along different directions was observed in different stages. It was found that the ripples formed in the initial stage facilitate the ablation along the direction perpendicular to the ripples, leading to the formation of an elliptical ablation area. With increasing length and depth of the ripples, however, nanohole arrays formed in the ripples will modify the distribution of electric field which benefits the ablation along the direction parallel to the ripples. Consequently, the ablation area is gradually changed to a circular one after irradiating sufficient number of pulses.

Size dependent competition between second harmonic generation and two-photon luminescence observed in gold nanoparticles.

We investigate systematically the competition between the second harmonic generation (SHG) and two-photon-induced luminescence (TPL) that are simultaneously present in Au nanoparticles excited by using a femtosecond (fs) laser. For a large-sized (length ~ 800 nm, diameter ~ 200 nm) Au nanorod, the SHG appears to be much stronger than the TPL. However, the situation is completely reversed when the Au nanorod is fragmented into many Au nanoparticles by the fs laser. In sharp contrast, only the TPL is observed in small-sized (length ~ 40 nm, diameter ~ 10 nm) Au nanorods. When a number of the small-sized Au nanorods are optically trapped and fused into a large-sized Au cluster by focused fs laser light, the strong TPL is reduced while the weak SHG increases significantly. In both cases, the morphology change is characterized by scanning electron microscope. In addition, the modification of the scattering and absorption cross sections due to the morphology change is calculated by using the discrete dipole approximation method. It is revealed that SHG is dominant in the case when the scattering is much larger than the absorption. When the absorption becomes comparable to or larger than the scattering, the TPL increases dramatically and will eventually become dominant. Since the relative strengths of scattering and absorption depend strongly on the size of the Au nanoparticles, the competition between SHG and TPL is found to be size dependent.

Highly localized, efficient, and rapid photothermal therapy using gold nanobipyramids for liver cancer cells triggered by femtosecond laser.

In this study, the photothermal effect and up-conversion florescence imaging effect of gold nanobipyramids in liver cancer cells are investigated theoretically and experimentally to explore the photothermal ablation tumor therapy with higher photothermal conversion efficiency, shorter laser action time, smaller action range and lower laser power. The small-size gold nanobipyramids with good biocompatibility and infrared absorption peak located in the first biological window are synthesized. Femtosecond laser is focused on the nanobipyramids clusters in cells and the cells die after being irradiated for 20 s at a power as low as 3 mW. In contrast, the control cells die after irradiation with 30 mW laser for 3 min. The theoretical simulation results show that: under femtosecond laser irradiation, the local thermal effect of gold nanoclusters is produced in the range of hundreds of square nanometers and the temperature rises by 516 °C in 106 picoseconds. This therapy reduces the treatment time to seconds level, and the treatment range to square micrometer level, the power to milliwatt level. In this treatment, cells die by apoptosis rather than necrosis, which reduces inflammation. This result opens up a new way to develop photothermal ablation therapy with less side effects and more minimally invasive.

Role of interfering optical fields in the trapping and melting of gold nanorods and related clusters.

We investigate the simultaneous trapping and melting of a large number of gold (Au) nanorods by using a single focused laser beam at 800 nm which is in resonance with the longitudinal surface plasmon resonance of Au nanorods. The trapping and melting processes were monitored by the two-photon luminescence of Au nanorods. A multi-ring-shaped pattern was observed in the steady state of the trapping process. In addition, optical trapping of clusters of Au nanorods in the orbits circling the focus was observed. The morphology of the structure after trapping and melting of Au nanorods was characterized by scanning electron microscope. It was revealed that Au nanorods were selectively melted in the trapping region. While Au nanorods distributed in the dark rings were completely melted, those located in the bright rings remain unmelted. The multi-ring-shaped pattern formed by the interference between the incident light and the scattered light plays an important role in the trapping and melting of Au nanorods.

Assembling of three-dimensional crystals by optical depletion force induced by a single focused laser beam.

We proposed a method to assemble microspheres into a three-dimensional crystal by utilizing the giant nonequilibrium depletion force produced by nanoparticles. Such assembling was demonstrated in a colloid formed by suitably mixing silica microspheres and magnetic nanoparticles. The giant nonequilibrium depletion force was generated by quickly driving magnetic nanoparticles out of the focusing region of a laser light through both optical force and thermophoresis. The thermophoretic binding of silica beads is so tight that a colloidal photonic crystal can be achieved after complete evaporation of solvent. This technique could be employed for fabrication of colloidal photonic crystals and molecular sieves.

High spatial frequency periodic structures induced on metal surface by femtosecond laser pulses.

The high spatial frequency periodic structures induced on metal surface by femtosecond laser pulses was investigated experimentally and numerically. It is suggested that the redistribution of the electric field on metal surface caused by the initially formed low spatial frequency periodic structures plays a crucial role in the creation of high spatial frequency periodic structures. The field intensity which is initially localized in the grooves becomes concentrated on the ridges in between the grooves when the depth of the grooves exceeds a critical value, leading to the ablation of the ridges in between the grooves and the formation of high spatial frequency periodic structures. The proposed formation process is supported by both the numerical simulations based on the finite-difference time-domain technique and the experimental results obtained on some metals such as stainless steel and nickel.