In order to obtain optical nonlinear materials with high transparency and low propagation loss in the visible and infrared range, noble metal materials in the off-resonant band have become a hot spot in the optical field in recent years. Therefore, the nonlinear absorption characteristics of platinum nanoparticles (PtNPs) with the surface plasmon resonance (SPR) in the ultraviolet band were investigated with multi-wavelength (500–700 nm) nanosecond Z-scan technology. The measurement results showed that the SPR wavelength of PtNPs was far away from the excitation wavelength, but there were still the saturated absorption (SA) and the reverse saturated absorption (RSA) phenomena, and the size of nonlinear absorption was related to the excitation wavelength and the excitation energy. When the excitation wavelength was constant, with the increase in excitation energy, PtNPs converted from SA to RSA. When the excitation energy was constant, with the excitation wavelength approaching SPR, PtNPs also converted from SA to RSA. The SA and RSA phenomena in the off-resonant region were complementary to the systematic study of the nonlinearity of PtNPs.

The multi-channel high-efficiency absorber in the mid-infrared band has broad application prospects. Here, we propose an SiC-photonic crystal (PhC) heterostructure-SiC structure to realize the absorber. The absorption characteristics of the structure are studied theoretically. The results show that the structure can achieve high-efficiency multi-channel absorption in the mid-infrared range. The absorption peaks come from the coupling of the dual Tamm phonon polariton (TPhP) mode formed at the interface between the two SiC layers and the photonic crystal, and the optical Tamm state (OTS) mode formed in the PhC heterostructure. By adjusting the thickness of the air dielectric layer and the period of the PhC in the heterostructure, the mode coupling intensity can be regulated; thereby, the position and intensity of the absorption peak can be adjusted. In addition, the absorption peaks of TE and TM polarized light can be controlled by changing the incident angle. Adjusting the incident angle can also control the excitation and intensity of the epsilon-near-zero (ENZ) phonon polariton mode produced by TM polarized light. This kind of light absorber may have potential applications in sensors, filters, modulators, switches, thermal radiators, and so on.

Two-dimensional materials, such as transition metal dichalcogenides (TMDs), exhibit intriguing physical properties that lead to both fundamental research and technology development. The recently emerged platinum diselenide (PtSe2), as a new member of the TMDs, has attracted increasing attention because of its good air stability, large refractive index and high electron mobility. However, being atomically thin significantly hinders its interaction with light, severely limiting the spontaneous or stimulated linear and nonlinear emission. Particularly, its nonlinear up-converted emission has not been fully exploited yet. Here, we experimentally observed the distinct enhancement of nonlinear up-converted luminescence of CVD-grown PtSe2 atomic layers on a SiO2/Si substrate with the assistance of the Fabry-Perot cavity resonance. The laser irradiance dependent luminescence study reveals the three-photon process of this nonlinear emission for the first time. Compared with non-resonant excitation, the luminescence enhancement can be up to six times because of the optical interference induced local field enhancement at the excitation wavelength. Leveraging this three-photon luminescence, nonlinear optical imaging and encryption were demonstrated for exploring information security applications. These results will pave the way for integrating nonlinear optical devices with the PtSe2 2D material.

A bifunctional tunable metamaterial composed of pattern metal structure, graphene, and strontium titanate (STO) film is proposed and studied numerically and theoretically. The dual plasmon-induced transparency (PIT) window is obtained by coupling the bright state cut wire (CW) and two pairs of dark state dual symmetric semiring resonators (DSSRs) with different parameters. Correspondingly, slow light effect can also be realized. When shifting independently, the Fermi level of the graphene strips, the amplitudes of the two PIT transparency windows and slow light effect can be tuned, respectively. In addition, when independently tuning the temperature of the metamaterial, the frequency of the dual PIT windows and slow light effect can be tuned. The physical mechanism of the dual-PIT was analyzed theoretically by using a three-harmonic oscillator model. The results show that the regulation function of the PIT peak results from the change of the oscillation damping at the dark state DSSRs by tuning conductivity of graphene. Our design presents a new structure to realize the bifunctional optical switch and slow light.

All-dielectric nanoparticles, as the counterpart of metallic nanostructures have recently attracted significant interest in manipulating light-matter interaction at a nanoscale. Directional scattering, as an important property of nanoparticles, has been investigated in traditional high refractive index materials, such as silicon, germanium and gallium arsenide in a narrow band range. Here in this paper, we demonstrate that a broadband forward scattering across the entire visible range can be achieved by the low loss TiO2 nanoparticles with moderate refractive index. This mainly stems from the optical interferences between the broadband electric dipole and the magnetic dipole modes. The forward/backward scattering ratio reaches maximum value at the wavelengths satisfying the first Kerker's condition. Experimentally, the femtosecond pulsed laser was employed to splash different-sized nanoparticles from a thin TiO2 film deposited on the glass substrate. Single particle scattering measurement in both the forward and backward direction was performed by a homemade confocal microscopic system, demonstrating the broadband forward scattering feature. Our research holds great promise for many applications such as light harvesting, photodetection and on-chip photonic devices and so on.

Hot electron intraband luminescence from plasmonic nanostructures is of critical importance for integrated photonic devices and applications in ultracompact nanospectrometer, bioimaging, information encryption et al. Although, the past few decades have witnessed tremendous progress in enhancing the luminescence efficiency of plasmonic nanostructures, the luminescence is usually unpolarized or partially polarized and difficult to be tailored because of its incoherent and broadband feature, significantly limiting its applications. Here the current limitation, demonstrating extremely polarized hot electron intraband luminescence with record-high degree of linear polarization (approximate to 1) and approximate to 40 000-fold enhancement from judiciously designed aluminium (Al) plasmonic nanostructures, is broken through. The designed nanostructures exhibit strong polarization-dependent anisotropic scattering across the visible band which efficiently modulates the luminescence in orthogonal directions. Leveraging this appealing feature, high-contrast analyzer controlled optical image encryption and camouflage for information security applications are demonstrated. This research lays the groundwork for integrated nonlinear photonic devices based on complementary metal-oxide-semiconductor (CMOS)-compatible plasmonic materials and paves the way for ultracompact on-chip photonic devices demanding polarized white light sources.

We propose and demonstrate a tunable dual-band mid-infrared absorber structure based on the coupling effect of a surface plasmon polariton (SPP) and Tamm phonon-polariton (TPhP). The structure is composed of the distributed Bragg reflector (DBR), air layer, SiC and graphene ribbons. In the air layer, the graphene ribbons are embedded to realize the localized SPP (LSPP), which makes the structure support both the graphene LSPP (GLSPP) and TPhP. The absorption properties of the structure are investigated theoretically and numerically. It is found that strong coupling of the GLSPP and TPhP can be realized by choosing reasonable parameters, which causes a dual-frequency perfect absorption and makes the maximum Rabi splitting of the coupled mode reach 5.76 meV. Furthermore, the mode coupling and absorption intensity can be tuned by adjusting the thickness of the air layer and the Fermi level of the graphene ribbons. This work might provide new possibilities for the development of mid-infrared band sensors, filters and emitters based on the coupling of multiple modes.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with tantalizing layer-dependent electronic and optical properties have emerged as a paradigm for integrated flat opto-electronic devices, but their widespread applications are hampered by challenges in deterministic fabrication with demanded shapes and thicknesses, as well as light field manipulation in such atomic-thick layers with negligible thicknesses compared to the wavelength. Here we demonstrate ultra-sensitive light field manipulation in full visible ranges based on MoS 2 laser prints exfoliated with nanometric precisions. The nontrivial interfacial phase shifts stemming from the unique dispersion of MoS 2 layers integrated on the metallic substrate empower an ultra-sensitive resonance manipulation up to 13.95 nm per MoS 2 layer across the entire visible bands, which is up to one-order-of-magnitude larger than their counterparts. The interlayer van der Waals interactions and the anisotropic thermal conductivity of layered MoS 2 films endow a laser exfoliation method for on-demand patterning MoS 2 with atomic thickness precision and subwavelength feature sizes. With this, nanometric flat color prints and further amplitude-modulated diffractive components for binocular stereoscopic images can be realized in a facile and lithography-free fashion. Our results with demonstrated practicality unlock the potentials of, and pave the way for, widespread applications of emerging 2D flat optics.

The comparative study on nonlinear absorption of WS 2 nanosheets was performed by using nanosecond Z-scan measurements at 532 nm and 500 nm. It was found that, WS 2 nanosheets exhibit saturable absorption (SA) at low excitation intensities, and the saturable intensity at 500 nm is much stronger than that at 532 nm, which implies that resonance plays main role. With the increase of incident intensity, a switch from SA to reverse saturable absorption (RSA) occurs. Besides, ultrafast dynamics of WS 2 nanosheets were investigated by using femtosecond pump-probe technique. Photo-dynamics process was found to contain a double exponential energy relaxation with a fast decay component (14 ps) and a slow one (145 ps).

Nonlinear absorption of multi-layer black phosphorus nanosheets (BPs) were investigated using open aperture Z-scan method with 6 ns nanosecond laser pulses at 532 nm. It was found that, at lower intensities, BPs behave as saturable absorbers, but at higher intensities a shift from saturable absorption to reverse saturable absorption occurs. The transition process was analyzed in terms of the bleaching of ground-state and the excited-state absorption. We investigate the ultrafast dynamics by using pump - probe technique. The investigation implies that BPs has great potential applications in both mode-locking and optical liming in the visible range for ultrafast laser pulses.

In this paper, using the new WSe2 nanosheets as the saturable absorber (SA), a passively Q-switched (PQS) Tm:YAP laser was faultlessly realized. A 1.29-W average output power at 1988.3 nm and a 392.7-ns pulse duration were obtained with a 12.3% optical conversion efficiency, and a 11.3-mu J pulse energy and a 28.9-W peak power were gained with a 12.5-W pump power. In addition, the beam quality factors of Tm:YAP laser in X and Y directions were less than 1.2 under continuous wave (CW) and PQS mode.

In order to study the extinction characteristics of gold-silver nanoshuttles, finite difference time domain (FDTD) method was used to simulate the models of gold-silver nanoshuttles with different diameter-length ratios, diameters and cone angles. The simulation results show that with the increase of aspect ratio, the longitudinal absorption peak of the gold-silver nanoshuttles shifts red, while its transverse absorption peak does not change significantly; the longitudinal absorption peak also shifts red with the increase of the length of the central nanorod; and the transverse absorption peak of the gold-silver nanoshuttles shifts blue with the increase of the angle of cone.

The nonlinear absorption and ultrafast dynamics process of WS2 and WS2-Ag nanocomposite were investigated by using nanosecond Z-scan measurements and femtosecond time-resolved transient absorption spectrum, respectively. Z-scan measurements showed that WS2 exhibit saturation absorption (SA) at low excitation intensities, with the increase of incident intensity, a switch from SA to reverse saturation absorption (RSA) occurs, but WS2-Ag nanocomposite exhibit SA at the same intensity. Transient absorption spectrum provided information show that the response time did not change much. The investigation shows that WS2 and WS2-Ag nanocomposite can be used for mode-locker and optical switcher.

Nonlinear optical absorption of Au-Ag nanoshuttles (NSs) was studied using an open-aperture Z-scan experiment with a 532 nm nanosecond laser at different energies. It was found that, when the laser energy is relatively low, the Au-Ag NSs exhibit saturated absorption (SA). When the laser energy is high, a conversion from SA to reverse saturated absorption (RSA) occurs. The ultrafast dynamic process of Au-Ag NSs was also investigated by using a femtosecond pump-probe technique. It is found that the process contains a fast and slow decay component that depends strongly on the laser intensity. Furthermore, when the probe wavelength is far away from the plasma resonance peak, the decay shows modulation due to the vibration mode of the coherent excitation.

Featured by prominent flexibility and fidelity in producing sophisticated stereoscopic structures transdimensionally, three-dimensional (3D) laser printing technique has vastly extended the toolkit for delivering diverse functional devices. Yet chiral nanoemitters heavily resorting to artificial structures that manifest efficient emission and tightly confined light-mater interactions simultaneously remains alluring but dauntingly challenging for this technique at this moment. In this work, we assert the chiral photoluminescence is implemented from silver nanostructures of optical duality in one go via a twofold three-dimensional laser printing scheme. Such laser printing protocol allows the highly desired duality by simultaneously producing uniformly distributed fluorescent silver nanoclusters and aggregated plasmonic silver nanoparticles to tightly confine chiral interactions at the nanoscale. A helical emitter of 550 nm-helix-diameter as fabricated has seen a record-high luminescence anisotropic factor with the absolute value up to 0.58, which is two orders of magnitude greater than fluorescent chiral silver clusters. This method holds great promise for future versatile applications in chiroptical nanodevices.

Surface plasmon can be produced in graphene in the mid-infrared and terahertz waveband regimes, and the regulation for surface plasmon can be achieved by a reasonable design. On the basis of above, a resonant tunable structure was designed. By depositing single layers of graphene ribbons with different widths on a dielectric substrate, discontinuities in nanoscale were introduced, thereby effectively controlling the interaction of graphene with light. The spectral and electromagnetic field distributions of the structure were theoretically studied using the finite difference time domain method. The results showed that when the designed structure was coupled with the incident light, there would be multiple resonance enhanced absorption peaks. By changing the number, width and distance of the graphene ribbons in each period, the number, position, intensity of the resonance peak can be controlled. In addition, the Fermi energy level of graphene can be changed by applying different bias voltages, so the position and intensity of resonance peak can be adjusted dynamically. Therefore, with this structure graphene plasmon resonance can be regulated over a wide spectral range. This study provides a theoretical basis for the design of the graphene-based sensors, filters and absorbers in infrared regime.

The nonlinear absorption and ultrafast dynamic process of WS2 nanosheets were investigated by using a broadband (ranging from 450 to 700 nm) nanosecond Z-scan technique. Z-scan measurements showed that WS2 nanosheets exhibit saturable absorption (SA), and the magnitude of SA is wavelength-dependent. Besides, the ultrafast dynamics process of WS2 was also investigated with femtosecond transient absorption spectrum. It was found that there is a double-exponential energy relaxation in the process. The investigation shows that WS2 nanosheets can be used for ultrashort pulse generation and a wide spectral range optical absorber. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

In order to study the extinction characteristics of the periodic structures of Au and Ag nanospheres, an Au–Ag nanosphere periodic array structure has been established. Firstly, the geometry structure and research methods of Au–Ag structure are introduced. Secondly, the LSPR absorption spectra of Au–Ag nanospheres periodic array and Au and Ag nanospheres periodic array were compared by the finite difference time domain method. Then, the simulation and analysis of periodic array of Au–Ag nanospheres with different sizes and spacings are carried out. Finally, the Au–Ag nanosphere periodic array structures in different media environments are simulated and analyzed, and the refractive index sensitivity is obtained. The simulation results show that Au–Ag nanoparticles periodic array structure has two LSPR absorption peaks, and the two peaks position relative to the separate Au and Ag nanoparticles periodic arrays of LSPR absorption peak showed blueshift; in the same spacing condition, with the increase of radius r, the absorption peak wavelength of the double peak showed redshift firstly and then blueshift; when the radius r is constant, the LSPR peak wavelength showed redshift with the increase of the spacing L; owing to Au nanospheres contribution in Au–Ag nanoparticles, the refraction absorption peak with the refractive index of the fitting rate sensitivity is 101.11 nm/RIU, refractive index sensitivity of 82.985 nm/RIU is higher than that of Au nanoparticles periodic array alone; and the contribution by Ag nanospheres in Au–Ag nanoparticles resulted in refraction absorption peak with the refractive index of the fitting rate sensitivity of 130.3 nm/RIU is lower than that of Ag nanoparticles periodic array of individual Refractive index sensitivity 154.23 nm/RIU. The structure basically meets the requirements of optical sensing accuracy and can be applied to filters.

Extraordinary optical transmission (EOT) in subwavelength metal structures has been studied widely. Herein, we propose a strategy for tuning the EOT of the bullseye structure. Specifically, the bullseye structure was immersed in a nonlinear medium, and a controlling light was employed to change the refractive index of the medium. At different intensities and distributions of controlling light, the transmission property of signal light in the bullseye structure was simulated. The results show that a variable transmission spectrum in the bullseye structure can be realized. Moreover, the position of the central transmission peak shifts linearly with the increasing intensity of controlling light.

Aluminum (Al) plasmonic nanostructures have recently demonstrated remarkable optical nonlinear phenomena, such as enhanced second harmonic (SH) generation. However, the relatively weak field enhancement resulted from large optical losses associated with aluminum nanostructures in combination with the difficulties in controlling the emission polarization pose as a challenge for SH enhancement and tuning. In this paper, we show that the SH emission of aluminum nanostructures can be efficiently enhanced with the polarization properties simultaneously tunable by using metal-insulator-metal (MIM) nanostructures, constituting of Al cross nanoantenna arrays on top of Al mirrors with a SiO2 spacing layer. Specifically, femtosecond laser beam with a linear polarization parallel to one arm illuminates on the structure while the orthogonal arms were physically modified by the laser-induced photothermal reshaping technique to control the SH radiation by the plasmonic resonances. Under the resonance at the SH wavelength, we observed one order of magnitude larger emission enhancement compared to that at the off-resonant condition. Interestingly, the polarization states can be well manipulated simultaneously by controlling the resonances of the orthogonal arms. The enhanced SH conversion and tunable polarization states pave the way for the development of nonlinear optical sources and advanced functional metasurfaces.