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A linear state space model for photoacoustic imaging in an acoustic attenuating media
摘要： In photoacoustic imaging, ultrasound waves generated by a temperature rise after illumination of light absorbing structures are measured on the sample surface. These measurements are then used to reconstruct the optical absorption. We develop a method for reconstructing the absorption inside the sample based on a discrete linear state space reformulation of a partial differential equation that describes the propagation of the ultrasound waves. Fundamental properties of the corresponding state space model such as stability, observability and controllability are also analyzed. By using Stokes’ equation, the frequency dependent attenuation of the ultrasound waves is incorporated into our model, therefore the proposed method is of general nature. This approach allows for inhomogeneous probes with arbitrary absorption profiles and it accounts for the decrease in laser intensity due to absorption. Furthermore, it provides a method for optimizing the laser modulation signal such that the accuracy of the estimated absorption profile is maximized. Utilizing the optimized laser modulation signal yields an increase in reconstruction accuracy compared to short laser pulses as well as chirp modulation in many scenarios.
关键词： optimal temporal laser excitation,photoacoustic image reconstruction,acoustic attenuation,linear state space model
更新于2025-09-23 15:22:29
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Extraordinarily Large Third-Order Optical Nonlinearity in Au Nanorods under Nanowatt Laser Excitation
摘要： Size and operating energy are two key parameters that constrain the integration capability of a nonlinear optical device. Here, we report a 60-nm thin film made of randomly distributed gold nanorods that demonstrates extraordinarily large third-order nonlinear optical absorption under an excitation power as low as 100 nanowatts. Where the effective nonlinear optical absorption (NLA) coefficient is calculated to be 8x109 cm/GW with the corresponding optical susceptibility (Imχ(3)) of 7.6x10-2 esu, which is 3-4 orders larger than that of other metallic nanostructures. The sign of the NLA could be controlled by changing the excitation wavelength or varying the pump power. The optical Kerr effect measurement also shows that the intrinsic χ(3) of our sample is quite large compared to other nanostructures. Our work eliminates the requirements of strong operation power and large physical size of nonlinear optical materials and offers new opportunities for plasmonic nanostructures in applications of on-chip nonlinear optical devices.
关键词： Nanowatt Laser Excitation,and Hybrid Materials,Optical,Plasmonics,Magnetic,Au Nanorods,Third-Order Optical Nonlinearity
更新于2025-09-19 17:13:59
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Decay Kinetics of Thermal Radiation Emitted by Surface Layers of Carbon Materials under Pulsed Laser Excitation
摘要： The decay of thermal radiation emitted by surface layers of carbon materials excited by pulses of a Q-switched neodymium laser is investigated experimentally and theoretically. It is discovered that the decay curves can be approximated with satisfactory accuracy by a sum of two exponential components with decay times of about 10 and 100 ns. Changes in the decay curves under sample irradiation by a sequence of laser pulses can be interpreted as being the result of redistribution of intensities of these two components. Based on the results of computer simulation, the conclusion is drawn that the glow-decay time is determined by the ratio of the penetration depth of the laser radiation and the thermal-diffusion length, which creates an opportunity to determine the coefficient of temperature conductivity in a thin surface layer of the studied material at high temperatures (thousands of Kelvins).
关键词： decay kinetics,temperature conductivity,pulsed laser excitation,carbon materials,thermal radiation
更新于2025-09-16 10:30:52
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Direct observation of elastic softening immediately after femtosecond-laser excitation in a phase-change material
摘要： The generation and propagation of photoexcited elastic waves in crystalline Ge2Sb2Te5 were analyzed by picosecond time-resolved x-ray diffraction using a femtosecond-laser pump and an x-ray free-electron laser probe technique. The crystalline lattice anisotropically expanded initially in approximately 20 ps after the excitation. This was followed by a periodic oscillation of the lattice strain. The elastic stiffness along the cubic [111] direction had significantly softened during the initial expansion, and the strain magnitude was the largest in the [100] and [110] directions. This indicates that femtosecond-laser excitation creates a shallower interlayer potential between the Te and Ge-Sb layers and eventually leads to softening of the elastic stiffness along the cubic [111] direction. Furthermore, this softened state increases the system’s sensitivity to an external stress field. This residual internal stress in a thin film enhances the selective formation of a particular type of variant during the symmetry change from cubic to rhombohedra. This causes the subsequent anisotropic expansion. These phenomena are quite interesting and align with the ultrafast amorphization of this material.
关键词： elastic stiffness,femtosecond-laser excitation,elastic waves,x-ray diffraction,phase-change material
更新于2025-09-16 10:30:52
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AIP Conference Proceedings [AIP Publishing ADVANCES IN BASIC SCIENCE (ICABS 2019) - Bahal, India (7–9 February 2019)] ADVANCES IN BASIC SCIENCE (ICABS 2019) - Upconversion emission of Ag nanoparticles with Er3+/Yb3+ ions in tellurite glasses under 980nm laser excitation
摘要： The light upconversion in Er3+ - Yb3+ doped TeO2 -WO3- Li2O (TWLEY) doped glass containing silver nanoparticles (NPs) has been reported. Tellurite based glasses have been prepared by melt quenching technique with small amount Ag nanoparticles(NPs). The optical excitation is made at 980 nm in resonance with the 2F5/2→2F7/2 transition of Yb3+ ions in glass system. Intense emission bands centered at 525nm, 550nm, and 654 nm were observed corresponding to Er3+ ions transitions. Upconversion fluorescence Intensity of the glasses samples are also observed at 980nm femtosecond laser excitation using two photons microscope. For this green and red colour intensity measurement of the TWLEY with Ag NPs glass sample has been analyzed by intensity histograms. Results show that plasmon enhances upconversion of silver and rare-earth doped glasses may be utilized to explore its potential for multiphoton microscopy.
关键词： Tellurite glasses,Ag nanoparticles,Er3+/Yb3+ ions,980nm laser excitation,Upconversion emission
更新于2025-09-16 10:30:52
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<i>Ab initio</i> study of temperature- and laser-induced phase transitions in
摘要： We analyze the bulk phase transitions of the three titanium dioxide polymorphs anatase, rutile, and brookite in thermodynamical equilibrium and after ultrafast laser excitation using the generalized solid-state nudged elastic band (G-SSNEB) method. For calculating the forces and stresses needed for G-SSNEB, we use electronic-temperature dependent density functional theory calculations. Our results show that the anatase-rutile and brookite-rutile phase transitions have a considerably lower energy barrier than the anatase-brookite phase transition in the thermodynamical equilibrium and upon laser excitation. The obtained transition temperatures in thermodynamical equilibrium are consistent with experimentally observed ones. Furthermore, our calculations show that the pathways of the phase transition are not affected considerably by the applied laser pulse. Although the pathways do not change considerably we see that the rutile phase is stabilized by the laser pulse.
关键词： phase transitions,density functional theory,G-SSNEB method,titanium dioxide,laser excitation
更新于2025-09-12 10:27:22
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Ultrafast Broadband Diagnostics of the Filling of the s Band at the Two-Photon Femtosecond Laser Excitation of a Gold Film
摘要： The transmission of near infrared femtosecond laser pulses (wavelength of 800 nm) and supercontinuum radiation (300–750 nm) generated by them through a 50-nm gold film immersed in water has been experimentally studied at the power of focused laser radiation in the range of 1–10 GW. An increase in the excitation intensity is accompanied by the blueshift of the edge of the extinction band of the film, which is attributed to the filling of electronic states of the s band with a low density of states caused by two-photon interband transitions from d bands with a high density of states.
关键词： supercontinuum radiation,gold film,ultrafast broadband diagnostics,s band filling,two-photon femtosecond laser excitation
更新于2025-09-11 14:15:04
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Broad-range ultrafast all-optical red-shifting of EUV surface plasmons: Proof-of-principle and advanced surface nanotexturing in aluminum
摘要： Using aluminum as an example, ultrafast sub-ablative broad-range (EUV-IR) spectral tuning of surface plasmon resonance in metals by IR femtosecond laser pulses was demonstrated by theoretical modeling of prompt surface optics for strongly photoexcited materials and of their surface plasmon-polariton (SPP) dispersion curves, as well as by experimental multi-shot laser imprinting of surface plasmon-polaritons as large-scale regular surface ripples with minimal sub-di?raction periods down to 200 nm. According to laser-pump self-re?ection and charge emission experiments, the key issue in the prompt optical tuning (spectral red-shifting) of EUV surface plasmon resonance in aluminum by the IR laser pulses is related to ultrafast surface charging of its surface via intense electron emission during the laser pump pulse, depleting its surface electron density and simultaneously decreasing its surface/bulk plasma frequencies, at the corresponding negligible surface ablative etching.
关键词： Surface nanoripples,Femtosecond laser excitation,Spectral tuning,EUV surface plasmon resonance,Electronic heating and charging,Aluminum
更新于2025-09-10 09:29:36