Impinging Laser Research Articles

Plasmon Coupling—The Root Cause of Raman Anomaly and Laser Cooling in Nanocrystal Ge

Laser cooling of matter through anti‐Stokes photoluminescence, where the emitted frequency of light exceeds that of the impinging laser light by virtue of absorption of thermal vibrational energy, has been successfully realized in condensed media, and in particular with rare‐earth‐doped systems achieving sub‐100 K solid‐state optical refrigeration. Studies suggest that laser cooling in semiconductors has the potential of achieving temperatures down to ≈10 K and that its direct integration can usher in unique high‐performance nanostructured semiconductor devices. While laser cooling of nanostructured II–VI semiconductors has been reported recently, laser cooling of indirect bandgap semiconductors such as group IV silicon and germanium remains a major challenge. Herein, the anomalous observation of dominant anti‐Stokes photoluminescence in germanium nanocrystals principally associated with plasmon coupling is reported. Specifically, this Raman anomaly to the confluence of ultrahigh‐purity nanocrystal germanium, generation of high density of electron–hole plasma, the inherent degeneracy of longitudinal and transverse optical phonons in nonpolar indirect bandgap semiconductors, and simultaneous spatial confinement effects are attributed. At high laser intensities, plasmon‐assisted laser cooling with lattice temperature as low as ≈50 K is inferred.

Raman anisotropy in serpentine minerals, with a caveat on identification

AbstractThe serpentine minerals lizardite, polyhedral serpentine, chrysotile, antigorite and 15‐sector and 30‐sector polygonal serpentine have been studied by micro‐Raman spectroscopy, using selected samples, that had been previously characterized. The appropriate crystal orientations were determined by optical microscopy of petrographic sections. Oriented spectra, obtained using Nd‐YAG green laser radiation (532 nm), were deconvolved, extracting wavenumber and intensity values for the peaks, possibly overlapping and forming complex bands. Relevant Raman anisotropy is common and relevant in serpentine. Depending upon the orientation of the impinging laser, significant wavenumber shifts occur (up to 10 cm−1, mostly in polyhedral serpentine and lizardite). Furthermore, also, important intensity variations (up to 1 order of magnitude) occur in polyhedral serpentine, lizardite, chrysotile and antigorite as well. On the one hand, the possibly characteristic peaks have been identified and discussed. On the other hand, caution is suggested as far as the micro‐Raman characterization of polyphasic, variably oriented serpentine minerals is concerned.

Anomalous violation of the local constant field approximation in colliding laser beams

The local constant field approximation (LCFA) is widely used in the study of QED effects in laser-matter interactions, with the justification that the classical strong-field parameter of the impinging laser beam is large. Here, the failure of this conjecture is demonstrated for an electron interacting with strong counterpropagating laser waves due to the emergence of an additional small time scale in the electron dynamics. Moreover, we identify a class of anomalous LCFA violation in which electrons turn sharply and leave the radiation formation zone much earlier than in the LCFA estimation. In contrast to previous observations of LCFA violation in a single laser beam, resulting in new low-harmonic peaks in the spectrum, here deviations from LCFA results are seen across the whole spectrum. A similar phenomenon is also demonstrated for an electron colliding with an ultrashort laser pulse. These results indicate the necessity of amending laser-plasma kinetic simulations in multiple beam laser configurations.Received 13 March 2020Revised 8 June 2020Accepted 9 February 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.013214Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHigh intensity laser-plasma interactionsQuantum electrodynamicsStrong electromagnetic field effectsAccelerators & BeamsPlasma PhysicsGeneral Physics

Chalcogenide Films: Laser Generation of Sub‐Micrometer Wrinkles in a Chalcogenide Glass Film as Physical Unclonable Functions (Adv. Mater. 38/2020)

“Pop-art” like patterns resulting from the interaction of a single laser pulse focused on a-Ge based chalcogenide thin film capped with a SiN layer are presented by Jérôme Gaudin and co-workers. These nondeterministic surface patterns, described in article number 2003032, are wrinkles with a height of less than 100 nm and sub-micrometer periodicity, which depends on the impinging laser fluence. Application as physical unclonable functions is demonstrated using a fast recognition algorithm.

Laser Generation of Sub-Micrometer Wrinkles in a Chalcogenide Glass Film as Physical Unclonable Functions.

Laser interaction with solids is routinely used for functionalizing materials' surfaces. In most cases, the generation of patterns/structures is the key feature to endow materials with specific properties like hardening, superhydrophobicity, plasmonic color-enhancement, or dedicated functions like anti-counterfeiting tags. A way to generate random patterns, by means of generation of wrinkles on surfaces resulting from laser melting of amorphous Ge-based chalcogenide thin films, is presented. These patterns, similar to fingerprints, are modulations of the surface height by a few tens of nanometers with a sub-micrometer periodicity. It is shown that the patterns' spatial frequency depends on the melted layer thickness, which can be tuned by varying the impinging laser fluence. The randomness of these patterns makes them an excellent candidate for the generation of physical unclonable function tags (PUF-tags) for anti-counterfeiting applications. Two specific ways are tested to identify the obtained PUF-tag: cross-correlation procedure or using a neural network. In both cases, it is demonstrated that the PUF-tag can be compared to a reference image (PUF-key) and identified with a high recognition ratio on most real application conditions. This paves the way to straightforward non-deterministic PUF-tag generation dedicated to small sensitive parts such as, for example, electronic devices/components, jewelry, or watchmak.

Time-resolved characterization of ultrafast electrons in intense laser and metallic-dielectric target interaction.

High-intensity ultrashort laser pulses interacting with thin solid targets are able to produce energetic ion beams by means of extremely large accelerating fields set by the energetic ejected electrons. The characterization of such electrons is thus important in view of a complete understanding of the acceleration process. Here, we present a complete temporal-resolved characterization of the fastest escaping hot electron component for different target materials and thicknesses, using temporal diagnostics based on electro-optical sampling with 100 fs temporal resolution. Experimental evidence of scaling laws for ultrafast electron beam parameters have been retrieved with respect to the impinging laser energy (0.4-4 J range) and to the target material, and an empirical law determining the beam parameters as a function of the target thickness is presented.

Ultranarrow Second-Harmonic Resonances in Hybrid Plasmon-Fiber Cavities.

We demonstrate second-harmonic generation with ultranarrow resonances in hybrid plasmon-fiber cavities, formed by depositing single-crystalline gold nanorods onto the surface of tapered microfibers with diameters in the range of 1.7-1.8 μm. The localized surface plasmon mode of the single gold nanorod efficiently couples with a whispering gallery mode of the fiber, resulting in a very narrow hybrid plasmon-fiber resonance with a high quality factor Q of up to 250. When illuminated with a tunable 100 fs laser, a sharp SHG peak narrower than half of the spectral width of the impinging laser emerges, superimposed on a broad multiphoton photoluminescence background. The enhancement of the SHG peak of the hybrid system is typically 1000-fold when compared to that of a single gold nanorod alone. Tuning the laser over the hybrid resonance enables second-harmonic spectroscopy and yields an ultranarrow line width as small as 6.4 nm. We determine the second-harmonic signal to rise with the square of the laser power, while the multiphoton photoluminescence background rises with powers between 4 and 6, indicating a very efficient higher-order process. A coupled anharmonic oscillator model is able to describe the linear as well as second-harmonic resonances very well. Our work will open the door to the simultaneous utilization of narrow whispering gallery resonances together with high plasmonic near-field enhancement and should allow for nonlinear sensing and extremely efficient nonlinear light generation from ultrasmall volumes.

Field-Dependent Measurement of GaAs Composition by Atom Probe Tomography.

The composition of GaAs measured by laser-assisted atom probe tomography may be inaccurate depending on the experimental conditions. In this work, we assess the role of the DC field and the impinging laser energy on such compositional bias. The DC field is found to have a major influence, while the laser energy has a weaker one within the range of parameters explored. The atomic fraction of Ga may vary from 0.55 at low-field conditions to 0.35 at high field. These results have been interpreted in terms of preferential evaporation of Ga at high field. The deficit of As is most likely explained by the formation of neutral As complexes either by direct ejection from the tip surface or upon the dissociation of large clusters. The study of multiple detection events supports this interpretation.

Magneto-optic measurements on uneven magnetic layers on cardboard

Measurements of magnetic hysteresis loops by magneto-optic Kerr effect (MOKE) are usually performed on even surfaces which reflect the impinging laser beam without any disturbance. Alternatively, such measurements can be done on regularly structured samples, resulting in the possibility to investigate different diffraction orders who deliver different information about the magnetism in the magnetic particles. Rough magnetic surfaces, however, occur when rough substrates are coated with a magnetic layer, or when large magnetic particles are placed on a base material due to practical reasons. The article depicts the possibility to measure magnetic hysteresis loops on surfaces with a roughness about one order of magnitude higher than the light wavelength. This enables applied measurements of magnetic parameters on biological samples, textiles, irregular magnetic nanofibers etc.

Self-phase modulation effects as laser produced plasma diagnostics

High intensity laser radiation propagating in a plasma suffers deep changes concerning both its spatial and spectral distribution. These changes can give information about the kind of the developed interaction. In particular the Self Phase Modulation (SPM) of the impinging laser radiation is currently used to evidence fast variation of the electron plasma density, produced for example by the target ionization or ponderomotive forces. The SPM effects dramatically increase as the laser pulse decreases and this is the reason for why they have to be specially considered in the experiments in which femtosecond laser pulses are used.

Nonequilibrium properties of trapped ions under sudden application of a laser

Coherent quantum-state manipulation of trapped ions using classical laser fields is a trademark of modern quantum technologies. In this work, we study aspects of work statistics and irreversibility in a single trapped ion due to sudden interaction with the impinging laser. This is clearly an out-of-equilibrium process where work is performed through illumination of an ion by the laser. Starting with the explicit evaluation of the first moments of the work distribution, we proceed to a careful analysis of irreversibility as quantified by the nonequilibrium lag. The treatment employed here is not restricted to the Lamb-Dicke limit, which allows us to investigate the interplay between nonlinearities and irreversibility. We show that in these multiquantum or sideband regimes, variation of the Lamb-Dicke parameter causes a non-monotonic behavior of the irreversibility indicator. Counterintuitively, we find a working point where nonlinearity helps reversibility, making the sudden quench of the Hamiltonian closer to what would have been obtained quasistatically and isothermally.

SiC absorption of near-infrared laser radiation at high temperatures

We report on a theoretical and experimental investigation of the temperature-dependent optical absorption of nitrogen-doped 4H-SiC for a temperature range between room temperature and the decomposition point. The theoretical model is based on free carrier absorption including the temperature dependence of the electron mobility. With respect to laser material processing of silicon carbide, the analysis focusses on a near-infrared wavelength range. At room temperature, the calculated absorption is in excellent agreement to transmission and reflection measurements. For the experimental study of the absorption at higher temperatures induced by intense 1070-nm laser irradiation, a two-color pyrometer is employed with the thermal emission of the laser interaction zone being collected coaxial to the impinging laser. Exemplarily, the simulated temperature-dependent absorption is used to determine the heating of a 0.4-mm-thick 4H-SiC specimen during laser irradiation and compared to the experimentally determined temperature. In an initial time domain of the irradiation with an attained temperature below 1350 K, the simulated and measured temperatures are in good agreement. Above 1350 K, however, the measured temperature reveals a sharp and fast increase up to 2100 K which is not predicted by the model. This discrepancy is attributed to a strong additional absorption mechanism caused by carbonization at the surface which is confirmed by EDX analysis.

Influence of Polishing Orientation on the Generation of LIPSS on Stainless Steel

We report on the influence of different angles between the electrical field of the impinging laser and the polishing direction of linearly polished surfaces on the generation of low spatial frequency LIPSS on stainless steel. The electrical field is rotated in a range of 0° to 90° with respect to the polishing direction and its effect on the orientation and homogeneity of the LIPSS is determined. In addition, the influences of the initial surface roughness and laser parameters such as the laser fluence on the generation of LIPSS are investigated. It can be shown, that the formation of LIPSS is driven by the initial surface roughness. The experimental results lead to the assumption that LIPSS were attracted by the linear grooves caused by polishing. Depending on the used parameter set, the orientation of the generated LSFL formation derived up to a value of 45° against the common predictions. Furthermore, a dependency of the required fluence for LSFL on surface roughness and polishing direction is demonstrated. Particularly, LSFL generated with a low fluence are more attracted by the surface polishing. Continuatively, the results may contribute to a further understanding of the underlying mechanisms involved in the generation of LIPSS. Moreover the results can be useful for producing LIPSS in large-scale for possible applications.

Detection efficiency calibration of single-photon silicon avalanche photodiodes traceable using double attenuator technique

A highly accurate method for the determination of the detection efficiency of a silicon single-photon avalanche diode (Si-SPAD) is presented. This method is based on the comparison of the detected count rate of the Si-SPAD compared to the photon rate determined from a calibrated silicon diode using a modified attenuator technique, in which the total attenuation is measured in two attenuation steps. Furthermore, a validation of this two-step method is performed using attenuators of higher transmittance. The setup is a tabletop one, laser-based, and fully automated. The measurement uncertainty components are determined and analyzed in detail. The obtained standard measurement uncertainty is < 0.5%. Main contributions are the transmission of the neutral density filters used as attenuators and the spectral responsivity of the calibrated analog silicon diode. Furthermore, the dependence of the detection efficiency of the Si-SPAD on the mean photon number of the impinging laser radiation with Poissonian statistics is investigated.

Precise surface modification of polymethyl-methacrylate with near-infrared femtosecond laser

The fabrication of lines at the surface of polymethyl-methacrylate (PMMA) was studied. A femtosecond laser with pulse duration of 450fs and wavelength of 1027nm was used. The z-scan method was employed as a focusing procedure to control the sample position with respect to the beam waist through transmittance measurements. This allowed the production of continuous lines with a length of 2mm on the surface of the PMMA sample. The variation of the lines profiles and dimensions was studied for different values of the sample position with reference to the beam waist. Also effects on the lines fabricated at different values of incident energy and separation on the sample surface between consecutive impinging laser pulses were investigated. A range of positions where lines with similar features and sizes were produced was obtained. Submicrometric surface modifications were achieved as surface swelling in form of successive bumps. After inspecting the sample cross section, it was observed that depending on the relative position of the beam waist regarding the sample surface, inner modifications appear under the modified material surface.

The Effect of Standing Acoustic Waves on the Formation of Laser-Induced Air Plasmas

The expected location of an air plasma produced by a focused YAG laser pulse has been found to be influenced by the acoustics of the surrounding environment. In open air, the expected location of a laser-induced air plasma is centered close to the focal point of the lens focusing the laser beam. When confining the same beam coaxially along the interior of a quartz tube, the expected location of the air plasma shifts away from the focal point, toward the focusing lens, in a region of less laser fluence. This shift is caused by an interaction between standing acoustic waves (formed from sound waves produced by previous laser-induced plasmas) and the impinging laser pulse. Standing acoustic waves in a tube produce areas (antinodes) of slightly higher and slightly lower pressure than ambient atmospheric conditions, that in turn have a noticeable affect on the probability of creating an air plasma at a given location. This leads to two observed phenomena: Increased probability of air plasma formation before the optical focal point is reached, and the formation of distinct (separate) air plasmas at the antinodes themselves.

Radiation-pressure-dominant acceleration: Polarization and radiation reaction effects and energy increase in three-dimensional simulations

Polarization and radiation reaction (RR) effects in the interaction of a superintense laser pulse (I>10(23) W cm-2) with a thin plasma foil are investigated with three dimensional particle-in-cell (PIC) simulations. For a linearly polarized laser pulse, strong anisotropies such as the formation of two high-energy clumps in the plane perpendicular to the propagation direction and significant radiation reactions effects are observed. On the contrary, neither anisotropies nor significant radiation reaction effects are observed using circularly polarized laser pulses, for which the maximum ion energy exceeds the value obtained in simulations of lower dimensionality. The dynamical bending of the initially flat plasma foil leads to the self-formation of a quasiparabolic shell that focuses the impinging laser pulse strongly increasing its energy and momentum densities.

Optimization of laser printing of nanoparticle suspensions for microelectronic applications

Digital printing of interconnects for electronic devices requires processes capable of delivering controlled amounts of conductive inks in a fast and accurate way. Laser-induced forward transfer (LIFT) is an emerging technology that enables controlled printing of voxels of a wide range of inks with micrometer resolution. Its use with high solids content nanoparticle suspensions results in the deposition of voxels shaped as the impinging laser beam. This allows higher processing speeds, increasing the throughput of the technique. However, the optimum conditions for printing spot-like voxels have not been determined, yet. In this work, we perform a systematic study of the main experimental parameters, including laser pulse energy, laser beam dimensions, and gap distance, in order to understand the role that these parameters play in laser printing. Based on these results, we find that there is a narrow fluence range at distances close to the receiving substrate where spot-like voxels are deposited. We also provide a detailed discussion of the possible mechanisms that may lead to the observed features.

Photon self-induced spin-to-orbital conversion in a terbium-gallium-garnet crystal at high laser power

In this paper, we present experimental evidence of a third-order nonlinear optical process, self-induced spin-to-orbital conversion (SISTOC) of the photon angular momentum. This effect is the physical mechanism at the origin of the depolarization of very intense laser beams propagating in isotropic materials. The SISTOC process, like self-focusing, is triggered by laser heating leading to a radial temperature gradient in the medium. In this work we tested the occurrence of SISTOC in a terbium-gallium-garnet rod for an impinging laser power of about 100 W. To study the SISTOC process we used different techniques: polarization analysis, interferometry, and tomography of the photon orbital angular momentum. Our results confirm, in particular, that the apparent depolarization of the beam is due to the occurrence of maximal entanglement between the spin and orbital angular momentum of the photons undergoing the SISTOC process. This explanation of the true nature of the depolarization mechanism could be of some help in finding novel methods to reduce or to compensate for this usually unwanted depolarization effect in all cases where very high laser power and good beam quality are required.

Influence of temperature-dependent mobilities on the nanosecond response of organic solar cells and photodetectors

We investigate the impact of temperature on the transient current density characteristics of organic solar cells and photodetectors. This is done by both experimental measurements and numerical simulations. In the process, we investigate the photoresponse of the device to an impinging laser pulse at different temperatures. By fitting the experimental results with the correlated disorder model we are able to quantify the influence of temperature on charge carrier mobilities in organic bulk heterojunction solar cells. We determine an almost doubling of the electron mobility on increasing the temperature from 11 to 50 °C.