Incident Pulse Width Research Articles

Dual-band waveform-selective metasurfaces for reflection suppression

In this study, we present a design method to realize waveform-selective metasurface absorbers operating in more than one frequency band, which is validated both numerically and experimentally. The waveform-selective metasurface absorbers could preferentially absorb target electromagnetic waves of the same frequency in accordance with the incident waveform, more specifically, the pulse width. Although such waveform selectivity is expected to offer additional selectivity at a fixed frequency, thus far, its operation has been limited to a single frequency band. Our design method enables waveform-selective metasurface absorbers to suppress reflection depending on the incident pulse width at two independent frequencies. Importantly, the dual-band approach is enhanced by varying the spatial ratio of unit cells assigned to the two frequencies. Thus, our study opens up possibilities for the utilization of waveform-selective metasurfaces in diverse frequency bands, providing a valuable and versatile solution to address challenges spanning various applications, such as wireless communications, sensing, and electromagnetic shielding.

Design and analysis for the SPICE parameters of waveform-selective metasurfaces varying with the incident pulse width at a constant oscillation frequency

In this study, we numerically demonstrate how the response of recently reported circuit-based metasurfaces is characterized by their circuit parameters. These metasurfaces, which include a set of four diodes as a full wave rectifier, are capable of sensing different waves even at the same frequency in response to the incident waveform, or more specifically the pulse width. This study reveals the relationship between the electromagnetic response of such waveform-selective metasurfaces and the SPICE parameters of the diodes used. In particular, we draw conclusions about how the SPICE parameters are related to (1) the high-frequency operation, (2) input power requirement and (3) dynamic range of waveform-selective metasurfaces with supporting simulation results. First, we show that reducing a parasitic capacitive component of the diodes is important for realization of the waveform-selective metasurfaces in a higher frequency regime. Second, we report that the operating power level is closely related to the saturation current and the breakdown voltage of the diodes. Moreover, the operating power range is found to be broadened by introducing an additional resistor into the inside of the diode bridge. Our study is expected to provide design guidelines for circuit-based waveform-selective metasurfaces to select/fabricate optimal diodes and enhance the waveform-selective performance at the target frequency and power level. Our results are usefully exploited to ensure the selectivity based on the pulse duration of the incident wave in a range of potential applications including electromagnetic interference, wireless power transfer, antenna design, wireless communications, and sensing.

Pulse-driven self-reconfigurable meta-antennas

Wireless communications and sensing have notably advanced thanks to the recent developments in both software and hardware. Although various modulation schemes have been proposed to efficiently use the limited frequency resources by exploiting several degrees of freedom, antenna performance is essentially governed by frequency only. Here, we present an antenna design concept based on metasurfaces to manipulate antenna performances in response to the time width of electromagnetic pulses. We numerically and experimentally show that by using a proper set of spatially arranged metasurfaces loaded with lumped circuits, ordinary omnidirectional antennas can be reconfigured by the incident pulse width to exhibit directional characteristics varying over hundreds of milliseconds or billions of cycles, far beyond conventional performance. We demonstrate that the proposed concept can be applied for sensing, selective reception under simultaneous incidence and mutual communications as the first step to expand existing frequency resources based on pulse width.

Study on the mechanism of surface pressure of optical films formed by laser plasma shock wave

In a high-power laser system, when the surface pressure of the optical film caused by laser plasma shock wave is greater than the adhesion per unit area of the film layer, the film will produce mechanical damage, and in serious cases, the whole system may not work. Therefore, studying the formation mechanism of optical film surface pressure caused by laser plasma shock wave and calculating the pressure is the key to ensure the normal operation of high power laser system. In this paper, by studying the relaxation process of shock wave on optical film surface pressure, a theoretical calculation model of shock wave on optical film pressure is established, and the variation law of pressure with different parameters is obtained, which reveals the mechanism of forming the optical film surface pressure. The calculation and simulation results show that the maximum pressure is 108 N m−2 during the laser pulse, and the pressure decreases with the increase of laser pulse time after the pulse, and the total action time of laser plasma and shock wave on the film is in the order of microseconds. The pressure increases with the increase of incident laser energy, focal length of focusing lens and incident laser pulse width, which increases with the decrease of the distance between the film surface and the focal plane of the focusing lens. The pressure changes more obviously with the incident laser energy and the distance between the film surface and the focal plane of the focusing lens than with the focal length of the focusing lens and the incident laser pulse width.

Brillouin frequency shift error estimation formula for distributed optical fiber sensing technology based on Brillouin scattering.

In this work, the problems in the existing Brillouin frequency shift (BFS) error estimation formulas for distributed optical fiber sensing technology based on Brillouin scattering are discussed. Based on the analysis, a new, to the best of our knowledge, BFS error estimation formula is proposed. To validate the proposed formula, a large number of Brillouin spectra with different frequency sweep spans, signal-to-noise ratios, linewidths, and frequency steps are numerically generated, and at the same time, Brillouin spectra with different values of incident light pulse widths and frequency sweep spans are measured with a Brillouin optical time domain reflectometer. Based on those Brillouin spectra, the errors of the proposed formula and existing formulas are systematically compared. The results reveal that the proposed formula generally has a higher accuracy than the existing typical formulas, especially when the frequency sweep span or incident light pulse width is large. Therefore, it has a much wider application range.

Effect of PMT output electron flow pulse pile-up on photon counting ranging method

Photomultiplier tubes (PMT) have single photon level sensitivity, low dark count, low after pulse probability, and are widely used in photon-counting lidar in visible spectrum. PMT has no photon detection dead time, for every photon it responds to, it sends out a electron flow pulse, these pulses of electron flow have the po·tential to pile up into larger pulses. When using threshold identification method to identify photon-events, stacked pulse will introduce additional pulse walking error, in the practical application of laser ranging, will directly affect the ranging precision of photon-counting ranging method. Considering the influence of pulse pile-up, a new theoretical model of PMT photon detection was established to describe the influence of pulse pile-up on the detection probability of photon-events by analyzing the relationship between the detection time of photon and the identification time of the PMT final output photon-events. Through Monte Carlo simulation, the relationship among the ranging walking error, ranging accuracy, incident laser pulse width, PMT output electron flow pulse width and photon-events identification threshold is obtained. In order to verify the correctness of the theory, a PMT-based photon-counting lidar system is built. The comparison experiment with GM-APD proves that the influence of pulse pile-up on PMT photon-counting ranging method can not be ignored, and the experimental results are in good agreement with the theoretical model. The PMT photon detection model based on pulse pile-up can guide the design of PMT photon-counting radar and improve the ranging accuracy and precision of the ranging system.

Effect of pile-up of electron flow pulse from photomultiplier tube on ranging by photon counting

Photomultiplier tube (PMT) features single photon level sensitivity, low dark count, and low afterpulse probability, and are widely used in photon-counting lidar in the visible spectrum. The PMT has no photon detection dead time, for every photon it responds to, it can output an electron flow pulse, these pulses of electron flow are likely to pile up into larger pulses. When using threshold identification method to identify photon-events, the stacked pulse will introduce additional pulse walking error, directly affecting the ranging precision of photon-counting ranging method in the practical application of laser ranging. Considering the influence of pulse pile-up, a new theoretical model of PMT photon detection is established to describe the influence of pulse pile-up on the detection probability of photon-events by analyzing the relationship between the detection time of photon and the identification time of the PMT final output photon-events. Through Monte Carlo simulation, the relationship among the ranging walking error, ranging accuracy, incident laser pulse width, PMT output electron flow pulse width and photon-events identification threshold is obtained. In order to verify the correctness of the theory, a PMT-based photon-counting lidar system is built. The comparative experiment with GM-APD proves that the influence of pulse pile-up on PMT photon-counting ranging method cannot be ignored, and that the experimental results are in good agreement with results from the theoretical model. The PMT photon detection model based on pulse pile-up can guide the design of PMT photon-counting radar and improve the ranging accuracy and precision of the ranging system.

Laser-induced deformation and fragmentation of droplets in an array

Droplet–droplet interactions is ubiquitous in various applications ranging from medical diagnostics to enhancing and optimizing liquid jet propulsion. We employ an experimental technique where the laser pulse interacts with a micron-sized droplet and causes optical breakdown. The interaction of a nanosecond laser pulse and an isolated spherical droplet is accurately controlled and manipulated to influence the deformation and fragmentation of an array of droplets. We elucidate how the fluid dynamic response (such as drop–drop and shock–drop interactions) of an arrangement of droplets can be regulated and optimally shaped by laser pulse energy and its interplay with the optical density of liquid target. A new butterfly type breakup is revealed, which is found to result in controlled and efficient fragmentation of the outer droplets in an array. The spatio-temporal characteristics of a laser-induced breakdown dictate how shock wave and central droplet fragments can influence outer droplets. The incident laser energy and pulse width employed in this work are representative of diverse industrial applications such as surface cleaning, nano-lithography, microelectronics, and medical procedures such as intraocular microsurgery.

Vascular damage mechanism and parameter optimization under alexandrite laser irradiation: a theoretical study.

The 755-nm Alexandrite Laser has a good clinical effect in treating resistant port wine stain, without causing thermal damage of normal tissue and side effects such as purpura. However, little is known about the mechanism of vascular damage induced by 755-nm laser irradiation, which restricts the optimization of laser parameters. In this work, the thermal damage model and the pressure damage model were used to study the damage mechanism of 755-nm laser irradiation on vessels, and the incident energy density and pulse width required for vascular damage were determined according to the damage mode. Under the irradiation of 755-nm laser, the vascular injury pattern was the co-occurrence of vessel rupture and vessel constriction, and the energy density required for the treatment of vessels with a diameter of 200μm to reach the damage threshold was the lowest.

Temporal visualization of femtosecond laser pulses with single-edge transport in turbid media via Monte Carlo simulation.

We study the propagation of femtosecond laser pulses with a single (front or rear) edge or dual edge through turbid media via Monte Carlo simulation. The results show that both the transmitted pulses spread on the basis of the incident pulse width ${t_{p}} = {{100}}\;{\rm{fs}}$, arising from the scattering effect. Further, the broadening width of the incident laser with a dual-edge pulse is wider than that of the incident laser width a single-edge pulse. The effect of the scattering particles on the front edge and the rear edge of the femtosecond laser can be distinguished in the time domain for femtosecond laser pulses through turbid media with the optical depth (OD) less than 10. In this scattering regime, the front-edge pulse scattered by the particles contributes more to diffused photons, but the effect of the scattering particles on the front edge and the rear edge of the femtosecond laser cannot be discriminated in turbid media with the OD more than 10, where the scattering is dominated by multiple scattering.

Single-photon pulse transport in a waveguide coupled with a quantum system

We study the transmission and reflection properties of a single-photon pulse in a one-dimensional waveguide side coupled with a Jaynes-Cummings system, namely, a multi-cavity with an embedded Rydberg atom. A Single-photon pulse is applied to tune the atoms energy spectrum, and varying it can bring the atomic qubit transition on or off-resonance for the cavity. The transmission and reflection behavior are systematically explored by a real-space model Hamiltonian and we find that the coupling strength between the waveguide and the side optical cavity exerts a tremendous impact on it. The influence of cavity and atomic dissipations and the incident pulse width on the transport properties of the single-photon is also investigated simultaneously. Based on these phenomena, we point out the possibility of utilizing this system as a single-photon transport in practice and has potential applications in quantum information.

Picosecond evolution of pulsed and CW alkali vapor lasers: laser oscillation buildup.

Investigation of the kinetic principle of a diode-pumped alkali vapor laser (DPAL) is key to achieve multifunctional DPALs. In this work, we propose a spatiotemporal model, which combines the time-dependent rate equations of population densities and propagation equations of energies to study the dynamic processes from turn-on to steady-state in DPALs. Time evolution of population densities and pump and laser intensity are resolved on a picosecond time scale to study the build-up process of laser oscillations. For nanosecond-pulse pumping, we obtain a laser pulse of 1.6 ns and a delay time of 2.6 ns at an incident pulse width of 2 ns. This pulse can be stretched by increasing the pump pulse width and delayed by applying a more extended cavity. For CW operation, spiking and relaxation oscillations resulting from a dynamic balance of gain and losses are demonstrated to be much faster than other types of lasers.

Nonlinear Optical Properties of Materials Based on Graphene Oxide: A Review

Background: Nonlinear optical properties of Graphene and Graphene Oxide have been widely used in industry and academia. Graphene oxide disperses easily in water and has easier interaction with other materials because of the presence of oxygen groups. So, this feature of Graphene oxide enables us to manipulate its nonlinear optical properties by combining it with other nanoparticles. Objective: We introduced recent advances in the nonlinear optical properties of materials based on Graphene oxide. Methods: Nonlinear optical properties and optical limiting of Graphene oxide and/or its composites with various nanoparticles, considering the wavelength and the incident pulse width, are investigated in this review. Conclusion: At low intensities and in all pulse regimes, saturation absorption seems to be the dominant mechanism of nonlinear absorption in Graphene oxide, while at higher intensities, the main mechanism is the reverse saturation absorption. In the regime of very short pulses of picoseconds and femtoseconds, the dominant mechanisms of two-photon and multiphoton absorption lead to reverse saturation. In the nanosecond pulse regime, long laser pulses and short pulses with high pulse repetition rates, excited-state absorption and nonlinear scattering due to thermal effects are causing the nonlinear process.

Tip-Enhanced Thermal Expansion Force for Nanoscale Chemical Imaging and Spectroscopy in Photoinduced Force Microscopy

We investigate the tip-enhanced thermal expansion force for nanoscale chemical imaging and spectroscopy in the tip-sample junction. It is found, both theoretically and experimentally, that the tip-enhanced absorption of the near-field at the tip followed by sample expansion shows characteristic behaviors with respect to the sample thickness and the incident laser pulse width. The van der Waals interaction plays a major role in exerting a force on the tip from the thermally expanded sample. The force behavior of the photoinduced force microscope (PiFM) is compared with that of the existing photothermal-induced resonance technique (PTIR) to unravel the ambiguous thermal expansion force mechanism. The present study opens up new opportunities for enhancing the performance of optical nanoscopy and spectroscopy such as chemical imaging of nanobiomaterials and the local field mapping of photonic devices, including surface polaritons on van der Waals materials with the assistance of the thermal expansion of a functionalized tip.

Direct numerical simulation of the initial stage of a thermally induced microcavitation in a water-rich biotissue triggered by a nanosecond pulsed laser.

A numerical analysis capable of describing the early stage of a thermal microcavitation process in a water-rich biotissue without avalanche breakdown was developed. The analysis successfully reproduced the laser-induced heating, vapor bubble formation, bubble expansion, and shockwave propagation inside a water-rich biotissue during a thermal microcavitation process. Based on the analysis, it was determined that the evolution of the temperature, pressure, and laser-induced shockwave is dependent on the incident laser energy and laser pulse width. On the other hand, the early stage dynamics of the microcavitation process showed little dependence on the elastic modulus of the biotissue for the laser and tissue conditions studied.

Impact of two-photon absorption on soliton trapping in silicon-on-insulator waveguides

In this paper, we present a numerical simulation study of the effects of two-photon absorption on soliton trapping occurring between two components decomposed from a soliton pulse. The two pulse components polarize along the polarization directions of the quasi-TE mode and quasi-TM mode in a silicon-on-insulator waveguide. The effects of free carriers generated by two-photon absorption are also investigated, leading to free-carrier absorption and free-carrier dispersion. The simulation image of the propagation of solitons is provided by solving the coupled nonlinear Schrödinger equation satisfied by the quasi-TE mode component and the quasi-TM mode component utilizing the split-step Fourier method with MATLAB. The simulation shows that two-photon absorption and free carriers will hinder the formation of soliton trapping. As the incident pulse power increases, the soliton trapping will occur, inspite of the presence of two-photon absorption and free carriers. The simulation also indicates that if the value of the free-carrier’s lifetime is comparable to the value of the pulse width, the formation of soliton trapping is susceptible to the lifetime of free carriers. With the incident pulse width increasing, soliton trapping forms between the mean peak of two polarization components, whereas it originally formed between the mean peak of TE mode and secondary peak of TM mode at small pulse width.

Design of nonlinear optical response of multipole-type excitons by film thickness and incident pulse width

We theoretically investigate the nonlinear optical pulse responses of excitons in a thin film where the excitonic center-of-mass motion is confined. A large interaction volume between excitons and radiation yields particular coupled states with radiative decay times reaching several femto-seconds. By considering two polarization directions of light, we reveal that these fast-decay modes dominantly survive in an optical Kerr spectra even under a massive nonradiative damping $\Gamma=30$ meV. The results clearly show that there is an optimal combination of the incident pulse width and the film thickness for maximizing the integrated intensity of nonlinear signals.

Experimental study on SBS-PCM at 200 Hz repetition rate pumped with joule-level energy

In this paper, an experiment is designed to research the properties of the stimulated Brillouin scattering phase conjugation mirror (SBS-PCM) working under joule-level pump energy and 200 Hz repetition rate. The nonlinear medium FC-770 is chosen as the SBS medium working in high-power condition. Due to the excellent preparation process and ultra-filtration technology for SBS-PCM, there is no optical breakdown phenomenon under the reflectivity maintained at 98% under 200 Hz and pump energy from 500 mJ to 1100 mJ. Under 800 mJ pump energy, the diffraction fringes of the near-field pattern are reduced and the beam quality of the laser beam has been improved to 1.42 times diffraction limited after the double-pass amplification from the single-pass 2.01 times diffraction limited. The incident and reflected pulse width are becoming wider with pump energy increasing. When the pump energy reaches a certain extent, the reflected pulse width will become nearly approximate to the incident pulse width. When the pump energy is 800 mJ, the pump pulse width is 40.3 ns and the output pulse width is 38.8 ns.

Research on MeV pulsed radiation detection based on refractive index modulaiton

High time resolution detecting systems for MeV pulsed radiation are essential for inertial confinement fusion diagnostics. Traditional detection of system time resolution is restricted by cable bandwidth. Based on recording excess carrier dynamics in semiconductors, a new detecting mechanism, called RadOptic, was developed by Lawrence Livermore National Laboratory (LLNL). The variation of intensity of pulsed radiation with time was converted into the variation of intensity of infrared laser probe by using this mechanism. The sensing material was InGaAsP quantum wells with severalmicrometer thickness. Picosecond time resolution for several keV pulsed radiation has been demonstrated. The reported system is not suitable for MeV pulses due to its low efficiency to MeV photons. Multiple cascaded structure for MeV photon to electron transformation was proposed by LLNL. Applying bulk material with several-hundredmicrometer thickness is an alternative. Based on transient free carrier absorption, a system recording bulk materials' instantaneous refractive index change is established. The system consists of a probe laser, an interferometer module, a signal transmission module and a signal recording module. The probe is a tunable infrared continuous wave laser whose wavelength is ~1453 nm, guided by single mode fiber to the interferometer. The interferometer consists of a single mode fiber head coupled directly with the polished face of a bulk semiconductor. The interference pattern forms by multiple beams reflected from the front face and the back face of the bulk. Part of interference light is coupled to the single mode fiber and forms the output signal. Pulsed radiation will deposit energy and generate excess carriers in the bulk material. The refractive index of the bulk material changes therewith according to the Drude model. The interference pattern and the light coupled to the single mode fiber also change therewith. The signal is transmitted by a long single mode fiber. The signal recording module consists of photoelectric detectors and a digital oscilloscope. The signal generation process and the time resolution of the system are analyzed. Intrinsic GaAs refractive index change is exploited under electron pulses and X ray pulses. The analysis of signal generation process shows that when the excess carriers recombine much faster/much slower than the pulse width, the output signal/output signal differential can be viewed as a measure of intensity variation with time of the incident pulse. For this prototype system, the time resolution is restricted by the digital oscilloscope to 1 GHz. Bulk intrinsic GaAs demonstrates 30 ns refractive index response time, which is longer than the incident pulse width. The differential signal can be viewed as a measure of incident pulse intensity when GaAs is exposed to 1 ns~0.2 MeV electrons pulses. The differential signal width is shorter than the pulse width when GaAs is exposed to 5 ns~0.2 MeV electrons pulses. Auger recombination process may occur in the pulse duration under this situation. The differential signal width is longer than the pulse width when GaAs is exposed to 1 ns~0.2 MeV X ray pulses. The poor signal to noise ratio affects the signal. The excess carrier generation process may be longer than theoretically estimated one under X ray pulse incident situation. The generation process and recombination process of excess carriers in GaAs show very different characteristics compared with optical excitation. The relationship between the system output signal and the incident pulsed radiation depends on the type of the incident radiation. With carefully considering the effects from incident pulse type and transient carriers density, the system can be used to detect ~MeV pulsed radiation. With an upgraded recording module, the system would demonstrate much higher time resolution.

High Resolution Numerical Simulation of Shock-to-Detonation Transition of Condensed-Phase Explosives

In this paper, shock-to-detonation transition for condensed phase explosives is numerically simulated by adopting high resolution numerical scheme. Fifth-order WENO scheme and third-order TVD Runge-Kutta method are employed to discretize Euler equations with chemical reaction source in space and time respectively, and parallel high resolution code is developed. Applying this code, the influence of incident pressure and pulse width on the run distance to detonation is investigated. The numerical results show that incident pressure and pulse width govern the initiation process. If appropriate values are taken for incident pressure and pulse width, the pressure will increase with the enlarging of the shock wave propagation distance, and finally the explosives reach steady detonation. The run distance to detonation is also influenced by those two factors, and it gets shorter with the increase of pulse width and incident pressure. When the incident pressure and the pulse width are small enough, the retonation phenomenon can be observed, and it becomes obvious along with the decreasing of incident pressure and pulse width.