Laser Photon Energy Research Articles

Laser damage mechanisms in conductive widegap semiconductor films

Laser damage mechanisms of two conductive wide-bandgap semiconductor films - indium tin oxide (ITO) and silicon doped GaN (Si:GaN) were studied via microscopy, spectroscopy, photoluminescence (PL), and elemental analysis. Nanosecond laser pulse exposures with a laser photon energy (1.03 eV, 1064 nm) smaller than the conductive films bandgaps were applied and radically different film damage morphologies were produced. The laser damaged ITO film exhibited deterministic features of thermal degradation. In contrast, laser damage in the Si:GaN film resulted in highly localized eruptions originating at interfaces. For ITO, thermally driven damage was related to free carrier absorption and, for GaN, carbon complexes were proposed as potential damage precursors or markers.

Quantitative measurement of electron number in nanosecond and picosecond laser-induced air breakdown

Here we present quantitative measurements of total electron numbers in laser-induced air breakdown at pressures ranging from atmospheric to 40 barg by 10 ns and 100 ps laser pulses. A quantifiable definition for the laser-induced breakdown threshold is identified by a sharp increase in the measurable total electron numbers via dielectric-calibrated coherent microwave scattering. For the 10 ns laser pulse, the threshold of laser-induced breakdown in atmospheric air is defined as the total electron number of ∼106. This breakdown threshold decreases with an increase of pressure and laser photon energy (shorter wavelength), which is consistent with the theory of initial multiphoton ionization and subsequent avalanche processes. For the 100 ps laser pulse cases, a clear threshold is not present and only marginal pressure effects can be observed, which is due to the short pulse duration leading to stronger multiphoton ionization and minimal collisional avalanche ionization.

Effect of Mn doping on ultrafast carrier dynamics in thin films of the topological insulator Bi2Se3

Transient reflectivity (TR) measured at laser photon energy 1.51 eV from the indirectly intersurface-coupled topological insulator Bi2−xMnxSe3 films (12 nm thick) revealed a strong dependence of the rise-time and initial decay-time constants on photoexcited carrier density and Mn content. In undoped samples (x = 0), these time constants are exclusively governed by electron–electron and electron–phonon scattering, respectively, whereas in films with x = 0.013–0.27 ultrafast carrier dynamics are completely controlled by photoexcited electron trapping by ionized Mn2+ acceptors and their dimers. The shortest decay-time (~0.75 ps) measured for the film with x = 0.27 suggests a great potential of Mn-doped Bi2Se3 films for applications in high-speed optoelectronic devices. Using Raman spectroscopy exploiting similar laser photon energy (1.58 eV), we demonstrate that due to indirect intersurface coupling in the films, the photoexcited electron trapping in the bulk enhances the electron–phonon interaction strength in Dirac surface states.

Electron-impact elastic scattering of helium in the presence of a laser field: non perturbative approach

We report a detailed analysis of electron-helium scattering in the presence of a laser field; focusing on the elastic process of helium atoms from the ground state 11S. The process under investigation is dealt with a nonperturbative approach using the Volkov wave function to describe the incident and scattered electrons, while the laser–target interaction is treated by using the Floquet method. The interaction of the incident electron with the atomic target is treated within the first Born approximation. Our results are perfectly consistent with the experimental data of DeHarak et al and with the Kroll–Watson approximation results for both one and two photon emission. We have investigated the effect of nonresonant and near resonant laser field on the electron–helium elastic collision process. It was found that the differential cross section is sensitive to the intensity and the frequency of the laser field. In the case of a non resonant laser field, dressing effects are important at small scattering angles. For a near-resonant laser photon energy, those effects are strongly reduced in the forward direction.

Single- and double-core-hole ion emission spectroscopy of transient neon plasmas produced by ultraintense x-ray laser pulses

Single-core-hole (SCH) and double-core-hole (DCH) spectroscopy is investigated systematically for neon gas in the interaction with ultraintense x-ray pulses with photon energy from 937 eV to 2000 eV. A time-dependent rate equation, implemented in detailed level accounting approximation, is utilized to study the dynamical evolution of the level population and emission properties of the laser-produced highly transient plasmas. The plasma-density effects on level populations and charge-state distribution are demonstrated with an x-ray photon energy of 2000 eV. It is shown that atomic number density of relevant experiment is about 1 × 1018 cm−3, which is comparable to a recent experiment. At this density, we systematically investigate the emissivity of the transient neon plasmas. For laser photon energy in the range 937–1360 eV, resonant absorptions (RA) of ( 2) transitions play important roles in time evolution of the population and DCH emission spectroscopy. The RA effects are illustrated in detail for an x-ray pulse of 944 eV photon energy, which creates the RA from the SCH states (, 1s2s2p5, and 1s2p6) of Ne3+. After averaging over the space and time distribution of x-ray pulse, DCH emission spectroscopy is studied at x-ray photon energies of 937, 944, 955, 968, 980, and 990 eV, where there exist resonances from SCH states of Ne2+-Ne7+. The processes with producing DCH states are discussed. For x-ray photon energy larger than 1360 eV, no RA exist and transient plasmas show different features in the DCH spectroscopy.

Conditions for ponderomotive resonances in the Kapitza–Dirac effect

By applying a nonperturbative quantum electrodynamic theory, we study ponderomotive resonances when an electron beam is scattered by a standing photon wave. Our study shows that the ponderomotive parameter u p , the ponderomotive energy per laser-photon energy, for each of the two traveling laser modes possesses a minimum value ћω/(m e c 2). Ponderomotive resonances occur only when the ratio of the laser photon energy to the electron rest-mass energy is a fraction, where the denominator is twice the square of a positive integer and the numerator is the total ponderomotive number, which is also a positive integer.

Doping- and interference-free measurement of I2D/IG in suspended monolayer graphene blisters

We report on strong interference effects on the ratio of the integrated intensities of the Raman 2D- and G-mode features (herein denoted ) in suspended graphene monolayers. Free from substrate-induced doping and residual charge inhomogeneity, suspended samples are an ideal platform to study the intrinsic properties of graphene. Here, we demonstrate that , measured on a pressurized suspended graphene blister, depends very sensitively on the distance between the bulged graphene membrane and the underlying substrate. The data obtained on three different samples are fit to theoretically predicted Raman enhancement factors and allow to extract an intrinsic, i.e., doping- and interference-free, value of for undoped, unscreened graphene at a laser photon energy of .

Surface states resonances at the single-layer graphene/Cu(111) interface

By tuning the laser photon energy in the non-linear two-photons photoemission at a single-layer graphene/Cu(111) interface, it is possible to observe a strong resonance at hυ=3.5eV along with a weaker one at hυ=3.85eV. The main resonance photon energy is consistent with a direct optical transition between the occupied Cu(111) Shockley surface state and the n=1 image potential state, located in the real gap-space between the single-layer graphene and the Cu(111) surface. The large amplitude of this resonance unveils a high value of the electric dipole matrix element integral that governs this transition. Furthermore, the Lorentzian shape of this resonance implies that these two states are decoupled from the continuum of states and the lifetime of the image potential state can be estimated.

Silicon as a virtual plasmonic material: Acquisition of its transient optical constants and the ultrafast surface plasmon-polariton excitation

Ultrafast intense photoexcitation of a silicon surface is complementarily studied experimentally and theoretically, with its prompt optical dielectric function obtained by means of time-resolved optical reflection microscopy and the underlying electron-hole plasma dynamics modeled numerically, using a quantum kinetic approach. The corresponding transient surface plasmon-polariton (SPP) dispersion curves of the photo-excited material were simulated as a function of the electron-hole plasma density, using the derived optical dielectric function model, and directly mapped at several laser photon energies, measuring spatial periods of the corresponding SPP-mediated surface relief nanogratings. The unusual spectral dynamics of the surface plasmon resonance, initially increasing with the increase in the electron-hole plasma density but damped at high interband absorption losses induced by the high-density electron-hole plasma through instantaneous bandgap renormalization, was envisioned through the multi-color mapping.

Thickness tunable quantum interference between surface phonon and Dirac plasmon states in thin films of the topological insulator Bi2Se3

We report on a >100-fold enhancement of Raman responses from Bi2Se3 thin films if laser photon energy switches from 2.33 eV (532 nm) to 1.58 eV (785 nm), which is due to direct optical coupling to Dirac surface states (SS) at the resonance energy of ∼1.5 eV (a thickness-independent enhancement) and due to nonlinearly excited Dirac plasmon (a thickness-dependent enhancement). Owing to the direct optical coupling, we observed an in-plane phonon mode of hexagonally arranged Se-atoms associated with a continuous network of Dirac SS. This mode revealed a Fano lineshape for films <15 nm thick, resulting from quantum interference between surface phonon and Dirac plasmon states.

Femtosecond laser irradiation induced-high electronic excitation in band gap materials: A quantum-kinetic model based on Boltzmann equation

A quantum kinetic approach based on the Boltzmann equation is employed to describe the response of dielectric and semiconductor materials to high electronic excitation induced by laser irradiation. The formalism describes from the initial photo-ionization inter-band processes through free carrier absorption inducing additional impact ionization to the final heat up by electron–phonon coupling. Swift thermalization through electron–electron scattering, Auger recombination and formation of free excitons, their self-trapping and subsequent non-radiative decay are included. The energy exchange between the electrons and phonons are given by a separate equation for the lattice temperature where the rates of energy transfer from the electrons to the lattice per unit volume are defined quantum mechanically. As a result of our calculations the electron energy distribution function, average kinetic energy of the electron system and electron density are obtained as a function of laser intensity, laser photon energy (wavelength) and laser pulse duration. Examples of application in fs-laser irradiated-silica are discussed.

Modification of properties of yttria stabilized zirconia epitaxial thin films by excimer laser annealing.

This study focuses on the ultrafast improvement of surface wettability, electrical, and room temperature magnetic characteristics of cubic zirconia single crystalline thin films after laser annealing. The point defects generated by the laser treatment are envisaged to play a critical role in altering the above properties. Yttria stabilized zirconia (YSZ) thin films were epitaxially grown on Si(100) substrates by pulsed laser deposition technique and subsequently annealed by a KrF excimer laser beam (τ = 25 ns) using low-energy laser pulses. An atomically sharp interface, parallel to the film free surface, between laser annealed layer and the pristine region was observed. The single crystalline nature of thin films was preserved following the laser treatment. The laser-solid interaction with YSZ led to the introduction of point defects, i.e., oxygen vacancies, resulting in a strained structure which, in turn, resulted in the formation of a tetragonal-like zirconia. With the increase of number of laser pulses the laser treated films got highly disordered due to the high concentration of the point defects, while maintaining their crystalline nature. Although the surface of the pristine sample showed weak hydrophilic characteristics (contact angle ∼ 73°), the laser annealed samples exhibited significantly improved hydrophilic characteristics. It was found that there is an optimum number of laser pulses where the maximum hydrophilicity (contact angle ∼ 22°) is obtained. The carrier concentration in the sample with the highest hydrophilicity was determined to be higher by about 5 orders of magnitude compared to the pristine sample. This sample possessed the lowest electrical resistivity. The laser annealed YSZ epilayers showed a superior room-temperature ferromagnetic behavior, compared to the pristine samples. A 2-fold enhancement in the magnetization of the samples was observed following the laser treatment which is a clear demonstration of the key role of defects and their transient distribution throughout the lattice. All these observations were correlated with the formation of point defects due to the photon interaction with YSZ and absorption of energy of the KrF laser photons to produce defects.

Mid-infrared light emission from a Fe2+:ZnSe polycrystal using quantum cascade laser pumping

We report the realization of light emission from a Fe2+ doped ZnSe polycrystal with efficient pumping from a quantum cascade (QC) laser. The QC laser photon energy is near the absorption edge of the Fe2+:ZnSe with less than ∼60 meV Stokes' shift. The Fe2+:ZnSe polycrystal shows an absorption band of ∼2.2 – 5 μm in the mid-infrared (mid-IR) at room temperature, which narrows down to ∼2.4–4.2 μm at 80 K. Clear photoluminescence (PL) from 4.5 μm to 6 μm are observed throughout the temperature range of 300 K to 80 K. At room temperature, the luminescence lifetime is about 0.38 ± 0.1 μs which increases up to 101 ± 2 μs at 118 K. This system transfers energy stored in the upper state of a QC laser with ∼ps lifetime to the Fe2+:ZnSe with 105 to 108 times longer lifetime, at the cost of minute photon energy losses.

Energy calibration at high-energy photon colliders

Calibration of the absolute energy scale at high-energy photon (γγ, γe) colliders is discussed. The luminosity spectrum at photon colliders is broad and has a rather sharp high-energy edge, which can be used, for example, to measure the mass of the Higgs boson in the process γγ →H or masses of charged scalars by observing the cross-section threshold. In addition to the precise knowledge of the edge energy of the luminosity spectrum, it is even more important to have a way to calibrate the absolute energy scale of the detector. At first sight, Compton scattering itself provides a unique way to determine the beam energies and produce particles of known energies that could be used for detector calibration. The energy scale is given by the electron mass me and laser photon energy ω0. However, this does not work at realistic photon colliders due to large nonlinear effects in Compton scattering at the conversion region (ξ2 ∼ 0.3). It is argued that the process γe →eZ0 provides the best way to calibrate the energy scale of the detector, where the energy scale is given by mZ.

Photo-Detachment of a Noncollinear Triatomic Anion

Photo-detachment of a noncollinear triatomic anion is investigated by considering each atom of the molecular anion as a coherent source of detached-electron waves, originating in all possible directions. The waves traveling along three different trajectories result in a quantum interference that displays on a screen placed at a very large distance from the system. To explain this quantum interference, an analytical formula of detached-electron flux is derived using a collinear three-center model recently published. The detached-electron flux versus laser photon energy expression displays molecular geometry-dependent oscillatory structures on an observation plane. This oscillatory behavior in the result can be explained using the semiclassical closed-orbit theory. The outgoing electron waves produced from one coherent center are propagated in the vicinity of the sources at other two coherent centers which cause these oscillations. It is also observed that in a particular case, the noncollinear triatomic system reduces to the collinear three-center system.

Single photoelectron detection after selective excitation of electron heavy-hole and electron light-hole pairs in double quantum dots

We demonstrate the real-time detection of single photogenerated electrons in two different lateral double quantum dots made in AlGaAs/GaAs/AlGaAs quantum wells having a thin or a thick AlGaAs barrier layer. The observed incident laser power and photon energy dependences of the photoelectron detection efficiency both indicate that the trapped photoelectrons are, for the thin barrier sample, predominantly photogenerated in the buffer layer followed by tunneling into one of the two dots, whereas for the thick barrier sample they are directly photogenerated in the well. For the latter, single photoelectron detection after selective excitation of the heavy and light hole state in the dot is well resolved. This ensures the applicability of our quantum well-based quantum dot systems for the coherent transfer from single photon polarization to single electron spin states.

Beam energy measurements for an experimenton elastic e±p scattering at the VEPP-3 storage ring

A precise comparison of the cross sections for the elastic scattering of electrons and positrons on protons was performed recently at the VEPP-3 storage ring in Novosibirsk, Russia. To provide the proposed accuracy, the energies of the electron and positron beams used during the experiment needed to be carefully maintained. This paper describes the special beam energy measurement system used in this experiment. This system is based on measuring the maximum energy of laser photons backscattered from electron and positron beams. It provides continuous beam energy measurements with a total relative uncertainty of less than 4.4·10−5.

Laser-assisted nuclear photoeffect re-examined

In a recent article of Dadi and Muller the modification of nuclear photoeffect due to intense laser field is discussed. The paper is criticised from the point of view of gauge invariance. The S-matrix element and the cross-section of the laser-assisted nuclear photoeffect are recalculated in a gauge invariant manner taking into account the effect of the Coulomb field of the remainder nucleus. The γ-photon energy dependence of the laser-free cross-section obtained in the plane wave and long-wavelength Coulomb–Volkov approximations are compared. The laser-assisted partial cross-sections with laser photon energy 2 keV and some different polarisation states of γ-photon of energy 3 MeV are numerically investigated.

Syntheses of negative thermal expansion materials Sc2(MO4)3 (M=W, Mo) with a CO2 laser and their Raman spectra

Negative thermal expansion materials Sc2(MO4)3 (M={W}, Mo) are synthesized with a CO2 laser. It is shown that the synthesis of Sc2(WO4)3 or Sc2(MoO4)3 by laser sintering is a rapid process so that a sample can be synthesized within a few or tens of seconds and has the characteristic of rapid solidification. X-ray diffraction and Raman spectrum analyses demonstrate that the synthesized Sc2(MO4)3 (M={W}, Mo) are crystallized into orthorhombic structures and each have a high purity. Temperature dependent Raman spectrum analysis suggests that the synthesized samples do not have phase transitions above room temperature but possibly have weak hygroscopicities. According to the Raman analyses of MoO3, WO3, Sc2(MoO4)3, and Sc2(WO4)3, we draw a diagram describing their phonon energy levels and the photon energy of the laser, and then assess the mechanism of the synthesis by laser sintering. Transferring the laser photon energy to phonon energy is the channel of light-heat converting. The materials react in the molten pool and are solidified rapidly, forming the final products.

Effect of laser pulse energy and wavelength on the structure, morphology and optical properties of ZnO nanoparticles

In this work, the effects of the laser pulse energy and laser wavelength on the production of ZnO nanoparticles prepared by pulsed laser ablation of Zn metal plate in deionized water are investigated. The beam of a Q-switched Nd:YAG laser of 1064 and 532 nm wavelengths at 6 ns pulse width and different fluences is employed to irradiate the solid target in water. The ZnO nanoparticles were found to be hexagonal. The size distribution of generated ZnO nanoparticles is decreased by increasing the laser pulse energy. The rate of ZnO nanoparticles production is increased with increasing the laser pulse energy and is decreased with increasing the laser photon energy. ZnO nanoparticles were formed with different shapes depending on the laser pulse energy and laser wavelength. The bandgap energy for ZnO nanoparticles generated with 1064 nm laser pulse wavelength is calculated to be 3.59–3.89 eV.