Ultrafast Laser Interaction Research Articles

Inhibition and Enhancement of Quantized, Interference‐Driven, Ultrafast‐Laser Cleaving, and Intrafilm Ejection with Angle and Polarization Control

AbstractThe combination of optical interference and ultrafast laser interaction within microscale transparent films offers novel high resolution means for nonlinear confinement of dissipated energy to facilitate 3D nanostructuring. This approach relies on the formation of nanoscale (≈40 nm) plasma disks stacked on half‐wavelength spacings, λ/2nfilm (film refractive index, nfilm), opening directions for intrafilm cleaving and nanostructuring of free‐standing blisters or embedded nanocavities with controllable surface topography. Given a limited number of film‐substrate systems suitable for generating high contrast interference fringes, this paper introduces angle and polarization control to manipulate fringe visibility in SiOx thin‐films (1 µm thickness) with silicon substrates. An enhancement or diminishment of quantized intrafilm processing is definitively demonstrated according to s‐ and p‐polarization states, respectively. Modeling of Gaussian beam walk‐off effects further explores film interference in tight focusing limits, predicting new asymmetry that manipulates intrafilm cleaving morphology. This research opens a path to quantized structuring of previously unsuitable low‐contrast thin‐film systems, while improving the design and control over novel surface and intrafilm morphologies. The development of intrafilm structuring in SiOx thin‐films is relevant to lab‐in‐film opportunities for assessing cell or subcellular species in CMOS‐compatible microelectronics and improving functionality of LED, lab‐on‐a‐chip and MEMS devices.

Photophysical and photochemical effects in ultrafast laser patterning of CVD graphene

The micro-scale patterns in graphene on Si/SiO2 substrate were fabricated using ultrashort 515 nm laser pulses. For both picosecond and femtosecond laser pulses two competitive processes, based on photo-thermal (ablation) and photochemical (oxidation/etching) effects, were observed. The laser-induced etching of graphene starts just below the threshold energy of graphene ablation: 1.7 nJ per 280 fs pulse and 2.7 µJ per 30 ps pulse. Whilst etching is not sensitive to thermal properties of graphene and provides uniform patterns, the ablation, in contrast, is highly affected by defects in the graphene structure like wrinkles and bilayer islands. The mechanisms of ultrafast laser interaction with graphene are discussed.

Effect of fluence and ambient environment on the surface and structural modification of femtosecond laser irradiated Ti**Project supported by Österreichische Forschungsfödergesellschaft (FFG) (Grant No. 834325).

Under certain conditions, ultrafast pulsed laser interaction with matter leads to the formation of self-organized conical as well as periodic surface structures (commonly reffered to as, laser induced periodic surface structures, LIPSS). The purpose of the present investigations is to explore the effect of fsec laser fluence and ambient environments (Vacuum & O2) on the formation of LIPSS and conical structures on the Ti surface. The surface morphology was investigated by scanning electron microscope (SEM). The ablation threshold with single and multiple (N = 100) shots and the existence of an incubation effect was demonstrated by SEM investigations for both the vacuum and the O2 environment. The phase analysis and chemical composition of the exposed targets were performed by x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS), respectively. SEM investigations reveal the formation of LIPSS (nano & micro). FFT d-spacing calculations illustrate the dependence of periodicity on the fluence and ambient environment. The periodicity of nano-scale LIPSS is higher in the case of irradiation under vacuum conditions as compared to O2. Furthermore, the O2 environment reduces the ablation threshold. XRD data reveal that for the O2 environment, new phases (oxides of Ti) are formed. EDS analysis exhibits that after irradiation under vacuum conditions, the percentage of impurity element (Al) is reduced. The irradiation in the O2 environment results in 15% atomic diffusion of oxygen.

Multi-phase functionalization of titanium for enhanced photon absorption in the vis-NIR region.

Inadequate absorption of Near Infrared (NIR) photons by conventional silicon solar cells has been a major stumbling block towards the attainment of a high efficiency “full spectrum” solar cell. An effective enhancement in the absorption of such photons is desired as they account for a considerable portion of the tappable solar energy. In this work, we report a remarkable gain observed in the absorption of photons in the near infrared and visible region (400 nm–1000 nm) by a novel multi-phased oxide of titanium. Synthesised via a single step ultra-fast laser pulse interaction with pure titanium, characterisation studies have identified this oxide of titanium to be multi-phased and composed of Ti3O, (TiO.716)3.76 and TiO2 (rutile). Computed to have an average band gap value of 2.39 eV, this ultrafast laser induced multi-phased titanium oxide has especially exhibited steady absorption capability in the NIR range of 750–1000 nm, which to the best of our knowledge, was never reported before. The unique NIR absorption properties of the laser functionalised titanium coupled with the simplicity and versatility of the ultrafast laser interaction process involved thereby provides tremendous potential towards the photon sensitization of titanium and thereafter for the inception of a “full spectrum” solar device.

Rabi oscillations of Morris-Shore–transformedN-state systems by elliptically polarized ultrafast laser pulses

We present an experimental investigation of ultrafast-laser driven Rabi oscillations of atomic rubidium. Since the broadband spectrum of an ultrafast laser pulse simultaneously couples all the electronic hyperfine transitions between the excited and ground states, the complex excitation linkages involved with the D1 or D2 transition are energy degenerate. Here, by applying the Morris-Shore transformation, it is shown that this multi-state system is reduced to a set of independent two-state systems and dark states. In experiments performed by ultrafast laser interactions of atomic rubidium in the strong interaction regime, we demonstrate that the ultrafast dynamics of the considered multi-state system is governed by a sum of at most two decoupled Rabi oscillations when this system interacts with ultrafast laser pulses of any polarization state. We further show the implication of this result to possible controls of photo-electron polarizations.

Size-dependent generalized thermoelasticity using Eringen's nonlocal model

Abstract Ultrafast lasers, even the novel laser burst technology, have been widely using in numerous applications, especially in micro-machining. The mechanism of ultrafast laser and matter interaction, such as: heat transfer, deformation of nanostructure, has caught numerous theoretical and experimental research interests. However, in such cases the classical models of thermo-elasticity may be challenged to give accurate responses: firstly, Fourier's law of heat conduction law may break down under the high heat flux and low temperature conditions; secondly, classical elasticity may fail as the external characteristic length (or time) approaches to the internal characteristic length (or time). In this work, to simulate transient thermo-elastic responses of the nanostructure subjected to a sudden thermal loading, classical thermo-elastic models are extended in two aspects: in mechanical sense, Eringen's nonlocal elasticity (differential constitutive relations) is employed to depict the size-dependence; while the memory dependence of heat conduction is considered using Caputo fractional derivative and m emory d ependent d erivative (MDD). In a separated section, the concept of Nonlocal operator and Memory dependent operator are proposed by revisiting Eringen's integral-type nonlocal theory, and comparing fractional calculus, MDD, and a newly reported “Most nature fractional derivative and integral”. In the numerical part, a thermo-elastic medium subjected to a sudden heating at one end is considered, and an analytical technique based on Laplace transform is adopted. While the inverse Laplace transform is numerically implemented by using an efficient and pragmatic algorithm ‘NILT’. Numerical results, i.e., temperature vs. position, displacement vs. position and stress vs. position, are shown graphically, and the influences of nonlocal scale parameter on them are also evaluated. It is concluded that nonlocal scale parameter's effect on the deformation and stress are significant, which is excessively important is determining material's failure when subjected to ultrafast laser like heating, although its effect on the temperature is negligible.

Ultrafast Ramsey interferometry to implement cold atomic qubit gates.

Quantum computing is based on unitary operations in a two-level quantum system, a qubit, as the fundamental building block, and the ability to perform qubit operations in an amount of time that is considerably shorter than the coherence time is an essential requirement for quantum computation. Here, we present an experimental demonstration of arbitrary single-qubit SU(2) quantum gate operations achieved at a terahertz clock speed. Implemented by coherent control methods of tailored ultrafast laser interaction with cold rubidium atomic qubits, Bloch vector manipulation about all three rotational axes was successfully demonstrated. The dynamic evolution of the qubits was successfully measured by devised femtosecond Ramsey interferometry. We anticipate this demonstration to be a starting point to process quantum algorithm in a simplified manner by a programmed sequence of femtosecond laser pulses.

Pulsetrain-burst mode, ultrafast-laser interactions with 3D viable cell cultures as a model for soft biological tissues

A 3D living-cell culture in hydrogel has been developed as a standardized low-tensile-strength tissue proxy for study of ultrafast, pulsetrain-burst laser-tissue interactions. The hydrogel is permeable to fluorescent biomarkers and optically transparent, allowing viable and necrotic cells to be imaged in 3D by confocal microscopy. Good cell-viability allowed us to distinguish between typical cell mortality and delayed subcellular tissue damage (e.g., apoptosis and DNA repair complex formation), caused by laser irradiation. The range of necrosis depended on laser intensity, but not on pulsetrain-burst duration. DNA double-strand breaks were quantified, giving a preliminary upper limit for genetic damage following laser treatment.

Visual and Physiological Optics

Visual and Physiological Optics

Ultrafast electronic relaxation in superheated bismuth

Interaction of moving electrons with vibrating ions in the lattice forms the basis for many physical properties from electrical resistivity and electronic heat capacity to superconductivity. In ultrafast laser interaction with matter the electrons are heated much faster than the electron–ion energy equilibration, leading to a two-temperature state with electron temperature far above that of the lattice. The rate of temperature equilibration is governed by the strength of electron–phonon energy coupling, which is conventionally described by a coupling constant, neglecting the dependence on the electron and lattice temperature. The application of this constant to the observations of fast relaxation rate led to a controversial notion of ‘ultra-fast non-thermal melting’ under extreme electronic excitation. Here we provide theoretical grounds for a strong dependence of the electron–phonon relaxation time on the lattice temperature. We show, by taking proper account of temperature dependence, that the heating and restructuring of the lattice occurs much faster than were predicted on the assumption of a constant, temperature independent energy coupling. We applied the temperature-dependent momentum and energy transfer time to experiments on fs-laser excited bismuth to demonstrate that all the observed ultra-fast transformations of the transient state of bismuth are purely thermal in nature. The developed theory, when applied to ultrafast experiments on bismuth, provides interpretation of the whole variety of transient phase relaxation without the non-thermal melting conjecture.

LIAD-fs scheme for studies of ultrafast laser interactions with gas phase biomolecules

Laser induced acoustic desorption (LIAD) has been used for the first time to study the parent ion production and fragmentation mechanisms of a biological molecule in an intense femtosecond (fs) laser field. The photoacoustic shock wave generated in the analyte substrate (thin Ta foil) has been simulated using the hydrodynamic HYADES code, and the full LIAD process has been experimentally characterised as a function of the desorption UV-laser pulse parameters. Observed neutral plumes of densities >10(9) cm(-3) which are free from solvent or matrix contamination demonstrate the suitability and potential of the source for studying ultrafast dynamics in the gas phase using fs laser pulses. Results obtained with phenylalanine show that through manipulation of fundamental femtosecond laser parameters (such as pulse length, intensity and wavelength), energy deposition within the molecule can be controlled to allow enhancement of parent ion production or generation of characteristic fragmentation patterns. In particular by reducing the pulse length to a timescale equivalent to the fastest vibrational periods in the molecule, we demonstrate how fragmentation of the molecule can be minimised whilst maintaining a high ionisation efficiency.

Combined continuum-atomistic modeling of ultrashort-pulsed laser irradiation of silicon

Ultrafast thermomechanical responses of silicon thin films due to ultrashort-pulsed laser irradiation were investigated using an atomic-level hybrid method coupling the molecular dynamics and the ultrafast two-step energy transport model. The dynamic reflectivity and absorption were considered, and the effects of laser fluence and pulse duration on the thermomechanical response were studied. It was found that both the carrier temperature and number density rapidly increase to their maximum while the lattice temperature rises at a much slower rate. The ultrafast laser heating could induce a strong stress wave in the film, with the maximum compressive and tensile stress occurring near the front and back surfaces, respectively. For laser pulses of the same duration, the higher the laser fluence is, the higher the carrier temperature and density and lattice temperature are induced. For the same laser fluence, a longer pulse generally produces lower carrier density and temperatures and weaker stress shock strength. However, for the fluence of 0.2 J/cm2, the lowest lattice temperature was simulated for a 100-fs pulse compared to the 1-ps and 5-ps pulses, due to the increase of reflectivity by high carrier density. It is also shown that the optical properties as functions of lattice temperature usually employed are not suited for modeling ultrafast laser interactions with silicon materials.

Ultrafast laser melting of Au nanoparticles: atomistic simulations

In spite of the technological importance of laser modification and processing of nanoparticles, the interaction of laser energy with nanoparticles is not well understood. In this work, integrated molecular dynamics (MD) and two-temperature (TTM) computational models have been developed to study ultrafast laser interaction with free Au nanoparticles with sizes 2.44–6.14 nm. At low intensity, when surface premelting and solid–liquid phase transition dominate, a nonhomogeneous surface premelting mechanism was identified. The appearance of a contiguous surface liquid layer (complete surface premelting) is size dependent and is not related to surface premelting history. The effects of temporary superheating and stable supercooling of nanoparticles were found and examined.

Heat accumulation during high repetition rate ultrafast laser interaction: Waveguide writing in borosilicate glass

During high repetition rate (>200 kHz) ultrafast laser waveguide writing, visible heat modified zones surrounding the formed waveguide occur as a result of heat accumulation. The radii of the heat-modified zones increase with the laser net fluence, and were found to correlate with the formation of low-loss and cylindrically symmetric optical waveguides. A numerical thermal model based on the finite difference method is applied here to account for cumulative heating and diffusion effects. The model successfully shows that heat propagation and accumulation accurately predict the radius of the ‘heat modified’ zones observed in borosilicate glass waveguides formed across a wide range of laser exposure conditions. Such modelling promises better control of thermal effects for optimizing the fabrication and performance of three-dimensional optical devices in transparent materials.