Arbitrarily control of electromagnetic field in femto-nano spatiotemporal scale can be realized by illuminating metallic nanoparticle with well-defined femtosecond laser pulse, which is at heart of current ambitious research endeavors in nanophotonics. However, the quantitative relation of the behavior of localized field with the incident laser pulse hasn’t yet been revealed. Here, active switching of localized near field is achieved by single chirped laser pulse in asymmetric Au nanocross system within the pulse duration using Finite Differential Time Domain algorithm. Temporal interval of energy switching between the two poles of nanocross is determined by the chirp of the laser pulse According to the temporal evolution, we found that field enhancement is asymmetric for positively and negatively chirped pulses as the consequence of the imbalance response of the plasmonic field around resonant frequency. It is demonstrated that under the excitation pulse of specific spectrum, field enhancement can also be effectively modulated by chirped laser pulse, providing a new degree of freedom for manipulating the dynamics of localized surface plasmons in nanoparticle.
We present a simple quad-band terahertz (THz) metamaterial absorber for perfect absorption. Unlike most reported absorbers, the proposed structure achieves perfect absorption based on high-order plasmon resonance, with the absorptivity of the first three peaks all >99.4 % . First, the resonance absorption mechanism of the absorber involving fundamental resonance mode and high-order resonance mode is analyzed in detail. Next, the sensing performance is also studied by changing the ambient refractive index, the numerical results show that the designed multiband metamaterial absorber is very sensitive to small changes in refractive index, and appears highly sensitive in its sensing performance. In particular, figure of merit of the last absorption peak reaches 26.6, which is far superior to most multiband metamaterial absorbers operating in the THz regime. The proposed simple structure not only greatly simplifies the fabrication, but also is insensitive to the polarization of incident waves. The results show that the quad-band metamaterial absorber has potential applications in sensing, imaging, detection, and other fields.
The remote excitation based on the propagating surface plasmons (PSP) is becoming increasingly extensive. Here, we study the dynamics of the localized surface plasmons (LSP) remotely excited by PSP on the gold film using the finitedifference time-domain numerical simulations method. The results show that the spectra of LSP excited by PSP change along with its excitation location, showing a unique phenomenon that the spectra exhibit a redshift tendency compared to the LSP excited by the traditional laser source. By comparing with the dynamics of LSP excited by a laser source, the unique near-field characteristics of that excited by PSP can be obtained. Furthermore, we find that the dephasing time of LSP excited by the PSP is longer than by the traditional laser source. We believe the results of this study can be used to improve the efficiency of remote catalytic reactions and provide new ways to prolong the dephasing time.
Realizing multiple beam shaping functions in a single plasmonic device is the key to realizing photonic integration. Surface Plasmon Polariton (SPP) Bessel-like Beams and Bottle Beams have potential applications in nanophotonics, especially in near-field optical trapping, micro-manipulation, and on-chip interconnect circuits. Thus, it is very interesting to find new approaches for simultaneous generation of surface plasmon polariton Bessel-like beams and bottle beams in a single photonic device. The wavelength-manipulated Bessel SPP beam and Bottle beam emitters devices are composed of four and five compact coupling elements with a specific spatial distribution, respectively. Besides, as the wavelength of the incident light is changed, the generated Bessel-like SPP beam and SPP Bottle beam can be directionally excited on one side of the transmitter and the launching direction can be dynamically selected. The design scheme of the proposed device provides a new means for constructing plasmonic devices with multiple beam shaping functionalities.
Accurately capturing the spatiotemporal information of surface plasmon polaritons (SPPs) is the basis for expanding SPPs applications. We record time-resolved nonlinear photoemission electron microscopy (TR-PEEM) images of weakly excited femtosecond SPPs launched from a rectangular trench milled into a flat silver film. Experimental results show that the overall photoelectron yield is greatly enhanced (typically 6-fold enhancement with the comparison of that without 400nm pulse) in this configuration. The spatiotemporal evolution of SPP can be easily investigated with its carrier wavelength, group velocityand phase velocity. The improvement of photoemission yield is interpreted by the quantum pathway for two-color with changing the nonlinear for photoemission. It is found that the contrast between bright and dark fringes of SPPs is significantly improved compared to the single-color scheme due to the improvement of quantum pathway channels for photoemission. These findings complete the underlying physics of two-color PEEM optimized SPP spatiotemporal imaging.
Turbulence is ubiquitous in ambient air and has always posed a great challenge for realizing optical applications in the atmosphere. The refractive index of turbulent air fluctuates randomly has obviously influences on the nonlinear process and characteristics of filamentation, which is a crucial role for many practical applications in free-space atmosphere. In the numerical simulation, we have investigated the filamentation and supercontinuum generation of femtosecond laser pulse with strong atmospheric turbulence in the path of whole the propagation distance. The spatial profile of intensity is analyzed in detail by changing perturbation strength of turbulence. The results show that the laser energy flow distribution presents stronger optical modulation instability with the increase of turbulence intensity in air. Complex spatial multifilament structures are generated by nonlinear propagation in strong turbulent air. The optical intensity along propagation distance significantly decrease as the strength of turbulence increases. The supercontinuum spectrum of filamentation in different strengths turbulence is also investigated. The physical mechanism of supercontinuum intensity change with different strengths of turbulence is discussed. Our results are valuable for realizing the free-space applications based on filamentation and supercontinuum in extreme weather with strong turbulence.
Multiple filaments generation and manipulation are crucial to numerous applications. The refractive index of medium has essentially influenced nonlinear effects in the ultrafast optical process of femtosecond laser filamentation, which is an effective method to tailor and manipulate filamentation. We have proposed a new method that using a periodic gas lattice as transmission medium to generate multiple filaments of femtosecond laser pulse. The periodic gas lattice is consisted of air waveguides, which has the special refractive index change distribution with alternating positive and negative. Our results show that the properties of multi-filament can be well controlled by changing the parameters of the gas lattice. A multi-filament array is formed by the gas lattice along the beam propagation direction, which are systemically investigated with different parameters of lattice at filamentation stages. The underlying physical mechanism of the multi-filament array formation is discussed, which has been demonstrated that the effects of extra focusing and discrete diffraction introduced by the gas lattice medium to promote a new competition and balance in the nonlinear filamentation process. The findings in this work provide a new way for many potential and promising applications that based on the controlled and optimized the femtosecond laser multi-filament array.
Accurately grasping and controlling the plasmon dynamics and dephasing time is a prerequisite for the application of plasmons. Here, we report on the investigation of dynamics and dephasing time of different plasmonic hot spots in a single bowtie structure under varied light polarization using time-resolved photoemission electron microscopy (PEEM). In contrast to those previous global-parameter descriptions, we here report the experimental observation of apparently spatially diverse plasmon dynamic characteristics and spatially different dephasing time within a plasmonic bowtie. We experimentally obtain different plasmon dynamics in the tips of the bowtie nanostructure with different light polarization and actively control dephasing time by changing the light polarization which transforms the plasmon mode. Experimental results got the minimum dephasing time of 8.5fs and the maximum dephasing time of 17fs, which has a large adjustment range. In addition, we found that structural defects can prolong the dephasing time, and we analyzed its role in the influence of plasmon dynamics and dephasing time.
The plasmon effect is of great significance for photoemission in metallic nanostructure. We introduced the photoemission electron microscope (PEEM) in detail, and used it to study the effects of polarization on the far-field and near-field of the plasmon. We further investigate the photoelectron energy spectrum obtained by PEEM and demonstrated the spatial distribution of photoelectrons with different energies. These experimental results help us to further understand the mechanism of photoemission and laid the foundation for the future development of plasmon device and technology.
Radio frequency (RF) emissions in the range of 30 ~ 800 MHz from laser induced air plasma by a 532 nm nanosecond laser are investigated. The RF emissions from air plasma induced by different laser energies and laser polarization are obtained. It is noted there is no consistency of the radio frequency emission with the change of laser energy. Unlike the optical emissions from plasma, which origins from electron transition between energy levels, RF radiation generates from oscillation of electric dipoles in plasma. The space distribution of the electric dipoles in plasma is not symmetrical along the laser propagation direction. As the laser parameters change, the distribution of the electric dipoles varies, so the radio frequency emissions do not change continuous. The RF signal of air plasma is found to depend on laser polarization directions and laser energy. The amplitudes of RF emissions are observed first increase and then decrease with further increase of laser energy, which is due to higher of ionization degree and electron density at larger laser energy, thus made the RF radiation quickly decay. The dominant frequencies and amplitude of RF emissions were observed vary with the laser polarization direction, and it is found that the maximum amplitude of the output of RF emissions were detected when the polarization direction of laser beam is along the axis of the antenna and minimum when the polarization direction of the laser beam is perpendicular to the axis of the antenna. Potential physical mechanism responsible for laser parameter dependent on RF emission, rich emission lines of air plasma was discussed.
The anti-icing surface of stainless steel with hydrophobic properties can be used in many fields: aircraft, power grid systems and wind power plants, etc. In this paper, the surface of stainless steel with micro-nanostructure morphologies was prepared by femtosecond laser and the surface chemical modification method was used to obtain hydrophobicity. We systematically studied the effect of low temperature environment on the anti-icing performance of micro-nanostructured surface. The results show that these structured surfaces can effectively delay the formation of ice. The formation of this phenomenon is related to its surface wetting properties and micro-nanostructure.
The precise understanding of the spatiotemporal characteristics of ultrafast surface plasmons is a prerequisite for applications of plasmonics. Here, we report on the investigation of near-field imaging and dynamics of propagating and localized surface plasmons (PSPs and LSPs) using photoemission electron microscopy (PEEM) of the trench on the silver film and gold bowtie nanostructure. The actual propagation direction of PSPs is directly obtained by reading PEEM images via the non-collinear exciting method by the trench. The results have demonstrated that the trench structure is potential as a 2D plasmonic dispersion element. Moreover, we experimentally obtain different LSPs dephasing times in the tips of the bowtie nanostructure by interferometric time-resolved PEEM. Experimental result reveals the dynamics of the LSP field initially oscillate at the laser field frequency and finally develop into its eigenfrequency after experiencing a few periods of frequency fluctuation.
Subwavelength imaging and control of localized near-field distribution under off-resonant excitation within identical gold bowtie structure, and of dark mode distribution within nanoring were demonstrated. The near-field control was established by coherent control of two orthogonally polarized fs laser pulses in bowtie and by varying polarization direction and wavelength of single femtosecond laser beam in the nanoring structure. We found that the hot spot under off-resonant wavelength illumination mainly distributed along the edges of the nanoprism in the bowtie and quadruple mode formation in the nanoring. The obtained results show that the PEEM images correspond generally to the simulated patterns of the plasmonic modes for the both structures and difference exists between experimental and simulated images. The responsible reasons for difference are discussed in terms of band structure near Fermi level and of surface imperfects of the structure. Our finding for the near field control of the nanostructure provides a fundamental understanding of the non-radiative optical near field and will pave the ways for the applications such as sensing, SERS, biomedicine and plasmonic devices.
Strong coupling of metallic nanoparticles results in interaction of the plasmonic properties of individual nanoparticles
and forms a new hybridized response that can be controlled through nanoparticle geometry and excitation field
parameters. In this report, we show controlled excitation and enhancement of gap plasmon responses in closely spaced
and differently aligned gold nanoparticles of various sizes and shapes. Our numerical results reveal that the spectral,
spatial, and temporal intensities of coupled nanoparticles can be hugely enhanced by controlling the geometry,
morphology, and alignment of the nanoparticles. Besides, shaping the temporal profiles of the excitation field gives an
unprecedented control over the spectral and temporal responses of the gap plasmons. These findings might have
implications for designing and fabrication of metallic nanoparticles for surface-based applications.
KEYWORDS: Near field, Ultrafast phenomena, Plasmons, Electron microscopy, Near field optics, Gold, Ultrafast imaging, Polarization, Femtosecond phenomena, Plasmonics
We report the direct imaging of plasmon on the tips of nano-prisms in a bowtie structure excited by 7 fs laser pulses and probing of ultrafast plasmon dynamics by combining the pump-probe technology with three-photon photoemission electron microscopy. A series of images of the evolution of local surface plasmon modes on different tips of the bowtie are obtained by the time-resolved three-photon photoemission electron microscopy, and the result discloses that plasmon excitation is dominated by the interference of the pump and probe pulses within the first 13 fs of the delay time, and thereafter the individual plasmon starts to oscillate on its own characteristic resonant frequencies. On the other hand, control of the near-field distribution was realized by variation of the phase delay of two orthogonally polarized 200fs laser pulses. The experimental results of the optical near-field distribution control are well reproduced by finite-difference time-domain simulations and understood by linear combination of electric charge distribution of the bowtie by s- and p- polarized light illumination. In addition, an independent shift of the excitation position or the phase of the near-field can be realized by coherent control of two orthogonally polarized fs laser pulses.
In this paper, the laser damage thresholds of the quartz glass with/without HF acid etching are investigated induced by the wavelength of the 355nm and 1064nm respectively. Laser-induced damage threshold of the quartz glass can be improved by optimizing the HF concentration and etched time. The experimental results shown that laser induced damage thresholds of quartz glass for 355nm and 1064nm were 7.1×108 W/cm2 and 1.15×109 W/cm2 respectively, after HF acid treatment with the 10% HF concentration and etched time 15 minutes, laser induced damage thresholds of quartz glass for 355nm changed to 1.29×109 W/cm2 and improved 81.7%, while for 1064nm changed to 1.73×109 W/cm2 and improved 50.4%. The surface damage morphologies of quartz glass induced by the 355nm and 1064nm with/without HF acid etching were comparative analyzed. Finally, the laser induced damaged mechanisms of quartz glass for 355nm and 1064nm were given.
A hollow cylindrical plasma waveguide, which cladding consists of a large number of a chaotically distributed plasma filaments induced by the propagation of femtosecond (fs) laser pulses in air, is shown to support guided modes of pulsed infrared (IR) laser radiation. Taking into account the discontinuity and the finiteness of the waveguide cladding, the loss coefficient loss of the laser radiation is calculated for different spatial configurations. We report how the waveguide loss depends on its structural parameters like normalized plasma diameter, distance between filaments, core-radius, cladding’s thickness, and filaments’ electron density. For typical plasma parameters, the loss of the fs plasma waveguide is found to be lower than that of freely propagating IR laser beams to distances in the order of the filamentation length. This fact allows the delivery of collimated pulsed laser light over long distances in atmospheric air, which is necessary for optical-based remote sensing and the detection of chemical and biological agents.
We have been developing a debris-free laser plasma light source with a gas-puff target system whose nozzle is driven by a piezoelectric crystal membrane. The gas-puff target system can utilize gases such as CO2, O2 or some gas mixture according to different experiments. Therefore, in comparison with soft X-ray source using a metal target, after continuously several-hour laser interaction with gas from the gas-puff target system, no evidences show that the light source can produce debris. The debris-free soft X-ray source is prepared for soft X-ray projection lithography research at State Key Laboratory of Applied Optics. Strong emission from CO2, O2 and Kr plasma is observed.
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