Visible Laser Pulses Research Articles

Studies of inactivation mechanism of non-enveloped icosahedral virus by a visible ultrashort pulsed laser

BackgroundLow-power ultrashort pulsed (USP) lasers operating at wavelengths of 425 nm and near infrared region have been shown to effectively inactivate viruses such as human immunodeficiency virus (HIV), M13 bacteriophage, and murine cytomegalovirus (MCMV). It was shown previously that non-enveloped, helical viruses such as M13 bacteriophage, were inactivated by a USP laser through an impulsive stimulated Raman scattering (ISRS) process. Recently, enveloped virus like MCMV has been shown to be inactivated by a USP laser via protein aggregation induced by an ISRS process. However, the inactivation mechanism for a clinically important class of viruses – non-enveloped, icosahedral viruses remains unknown.Results and discussionsWe have ruled out the following four possible inactivation mechanisms for non-enveloped, icosahedral viruses, namely, (1) inactivation due to ultraviolet C (UVC) photons produced by non-linear optical process of the intense, fundamental laser beam at 425 nm; (2) inactivation caused by thermal heating generated by the direct laser absorption/heating of the virion; (3) inactivation resulting from a one-photon absorption process via chromophores such as porphyrin molecules, or indicator dyes, potentially producing reactive oxygen or other species; (4) inactivation by the USP lasers in which the extremely intense laser pulse produces shock wave-like vibrations upon impact with the viral particle. We present data which support that the inactivation mechanism for non-enveloped, icosahedral viruses is the impulsive stimulated Raman scattering process. Real-time PCR experiments show that, within the amplicon size of 273 bp tested, there is no damage on the genome of MNV-1 caused by the USP laser irradiation.ConclusionWe conclude that our model non-enveloped virus, MNV-1, is inactivated by the ISRS process. These studies provide fundamental knowledge on photon-virus interactions on femtosecond time scales. From the analysis of the transmission electron microscope (TEM) images of viral particles before and after USP laser irradiation, the locations of weak structural links on the capsid of MNV-1 were revealed. This important information will greatly aid our understanding of the structure of non-enveloped, icosahedral viruses. We envision that this non-invasive, efficient viral eradication method will find applications in the disinfection of pharmaceuticals, biologicals and blood products in the near future.

Two-photon absorption and sensitivity to DNA of dye molecule-driven CdSe quantum dots

Two photons are simultaneously absorbed by a molecule when their frequencies correspond exactly to the energy between two eigenstates. In a medium, however, homogeneous and inhomogeneous broadenings strongly hinder this two-photon absorption. By compensating for the impedance mismatch between visible wavelengths and intense laser pulses, CdSe quantum dots can enhance light-matter interactions over a broad frequency range. We used CdSe quantum dots to enhance two photon absorption statistics of dye molecules. Qualitative agreement between the measured enhancement and the theoretically forecast enhancement indicate that the two-photon absorption cross-section (σ) can be increased by 500 GM (1 GM = 1 × 10−50 cm4 s per photon). The interactions between the chromophore and calf thymus DNA were investigated by UV-visible absorption experiments. The results show that the chromophore could interact with DNA in an intercalation mode and could therefore potentially be used in cell imaging applications.

Surface dominant photoresponse of multiferroic BiFeO3 nanowires under sub-bandgap illumination

A surface dominant sub-bandgap photo-carrier generation has been observed in multiferroic BiFeO3 (BFO) nanowires, which is mainly attributed to the depopulation of surface states that exist within the bandgap. Mapping of surface potential using Kelvin probe force microscopy (KPFM) further supports the depopulation of surface states in BFO nanowires under sub-bandgap illumination. The mechanism of photovoltage generation in BFO nanowires is investigated by measuring the photoresponse with local illumination of visible laser pulses at different positions of the BFO nanowires. Interestingly, large photovoltage signals were observed when the laser spot was focused close to contact electrodes, showing a position dependent effect of photoresponse in the BFO nanowires. The sub-bandgap excitation of surface states in multiferroic nanowires offers potential new strategies for application in photovoltaic devices.

Electron- and phonon-coupling in femtosecond laser-induced desorption of CO from Ru(0001)

We studied femtosecond laser-induced desorption of CO from Ru(0001) using intense near-infrared and visible femtosecond laser pulses. We find a pronounced wavelength dependence with a factor 3–4 higher desorption yield at comparable fluence when desorption is induced via 400nm light, compared to 800nm and attribute this difference to the difference in penetration depth of the incident light. All our data can be described using empirical friction-modeling to determine the desorption mechanism with the same mechanism for both wavelengths. We find that both hot electrons and phonons contribute to the desorption process.

Nonlinear Optical Properties and Broadband Optical Power Limiting Action of Graphene Oxide Colloids

In this work we report on the preparation of some aqueous graphene oxide (GO) dispersions and the investigation of their nonlinear optical response under visible (532 nm) and infrared (1064 nm), picosecond and nanosecond laser excitation. The GO colloids were prepared under specific and well-defined conditions resulting in finely dispersed heavily oxidized large GO sheets. In all cases, GO colloids were found to present large nonlinear absorption and negligible nonlinear refraction. The physical mechanisms responsible for their nonlinear optical response are discussed. In addition, the so-prepared GO dispersions were found to exhibit large broadband optical power limiting action for both pulse durations, comparable to that of C60 for visible laser pulses and much superior for infrared ones.

Ultrafast AMO physics at the LCLS x-ray FEL

The Linac Coherent Light Source at the SLAC National Accelerator Laboratory, Stanford University, began operation in 2009 as the world's first hard x-ray free electron laser. Early experiments have concentrated on atomic physics, and have demonstrated several key features of the ultrafast high field x-ray-atom interaction. This paper reviews some of these early results. 1. INTRODUCTION: WORLD'S BRIGHTEST ULTRAFAST X-RAY SOURCE, LCLS The new class of so-called fourth generation synchrotron light sources are a revolutionary departure from storage ring-based electron synchrotrons. They are true x-ray lasers, which use relativistic electrons as a gain medium. The Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in Menlo Park California is the world's first hard x-ray free electron laser (XFEL) (1). It produces coherent x-ray fields that exceed the brightness of any previous source (measured in energy per unit time per unit solid angle per unit frequency interval) by a factor of one billion or more. The increase of peak brightness is due to a combination of increased pulses energy and decreased pulse duration. These novel lasers allow us to observe the fastest timescales in atoms and molecules, which are due to the interactions among electrons. The timescales involved can be characterized by electronic energy separations, which can be many electron volts. Any ultrafast laser source is limited to pulse durations no shorter than a single cycle of the electromagnetic field. For the conventional Ti : Sapphire laser ( = 800nm), this is about 2fs. But for x-rays, the pulse duration can be much shorter. Simulations show that the LCLS is capable of sub-femtosecond pulses, which could capture electronic coherences in molecules. X-rays can reveal multielectron processes inside atoms and molecules. Furthermore, these sources have not yet reached their intrinsic limits in pulse duration, where sub-femtosecond pulses are possible (2). Accelerator and x-ray laser scientists and engineers at SLAC have also begun to explore techniques that will one day provide routine operation with pulses whose time-bandwidth product is near the Fourier transform limit (3). Such control over x-ray fields could be exploited just as in the field of quantum optics with visible and ultraviolet laser pulses. This short review will highlight some of the first results, and the opportunities for further work in strong-field x-ray physics of atoms and molecules. The work described here was all done at the LCLS x-ray free electron laser by the Stanford PULSE Institute AMO group in collaboration with other AMO research groups around the world (4-9). Some of the experiments described form the thesis research of two Stanford students, Mike Glownia and James Cryan (10, 11).

Effect of spatial coherence on the focusability of annular laser beams

This paper presents a first comprehensive experimental study and theoretical analysis on the effect of spatial coherence on the focusability of annular laser beams. The circular apertures of annularity 0, 0.2, 0.4, 0.6 and 0.8 were studied experimentally for the far-field intensity distributions with three different partially coherent incident visible pulsed laser beams of spatial coherence fraction of 100%, 40% and 2% of full beam cross-section. These beams were generated from a copper vapor laser with different optical resonators. The spatial coherence fraction was computed from the interferograms recorded by a reversal shear interferometer. The resulting far-field patterns of focused annular beam were found to depend very critically on the spatial coherence of the incident beam. A theoretical analysis, based on Fourier transform of partially coherent incident light with coherent function taken as J1(r)/r, was carried out. The far-field diffraction patterns were estimated numerically for the annular beams involving correlation interval and obstruction ratio. The MATLAB based mathematical process was the Fourier transformation of the product of source autocorrelation function and correlation function across the aperture. Experimental and theoretical results are in good agreement.

Electron dynamics in transparent materials under high-intensity laser irradiation

The energy of a laser beam irradiating a surface is primarily absorbed by electrons within the solid. In actual transparent materials, absorption is low. High-intensity lasers may, however, be absorbed by initially bounded electrons through nonlinear processes. The increase of free-electron density leads eventually to dielectric breakdown, and the material becomes highly absorbing. We present theoretical studies on the dynamics of electrons in dielectrics under irradiation with a visible high-intensity laser pulse. We consider microscopic processes determining absorption, redistribution of the energy among electrons, and transfer of energy to the crystal lattice. We review different aspects of electronic excitation, studied with time-resolved models as the Boltzmann kinetic approach and the time and spatial resolved multiple rate equation. Furthermore, we investigate criteria for damage thresholds. Two concepts are compared, namely a critical free-electron density and the melting threshold of the lattice. We show that in dielectrics both criteria are fulfilled simultaneously. Optical parameters depend on the density of free electrons in the conduction band of the solid, so the free-electron density directly leads to an increased energy absorption causing material modification. We present results on the spatial dependence of dielectric breakdown.

Optical nonlinear refractive and limiting behavior of nickel complex dye doped solid-state matrix for both visible and near infra-red nanosecond excitations

The present work focuses on the investigations for laser induced third order nonlinear optical response in nickel-complex bis(4-dimethylaminodithiobenzyl)-nickel (BDN) dye encapsulated in poly(methylmethacrylate) (PMMA) matrix under excitation with nanosecond laser pulses at two different wavelengths of 532nm and 1064nm. Intensity dependent absorption coefficient, nonlinear refractive index and cubic nonlinear susceptibility are estimated using Z-scan technique. The observed nonlinearity with nanosecond excitation is of thermal origin. Nonlinear change in refraction attributes to self-defocusing due to two-photon absorption. Optical limiting behavior is characterized by the limiting threshold and dynamic range parameters. Existence of strong third order nonlinearity with quite low limiting threshold for both visible and near infra-red laser pulses confirms that BDN doped solid-state samples have a great potential for the applications as broad band optical limiter.

Generation and amplification of dual-color femtosecond pulses by a noncollinear optical parametric up-conversion process

Noncollinear optical parametric up-conversion generation and amplification are realized in a thick β-barium borate (BBO) crystal, and a couple of visible femtosecond up-conversion laser pulses can be achieved by a femtosecond pulse at 800 nm as the pump sources. The theoretical and experimental results indicate that there exist phase-matching conditions for dual-color noncollinear parametric up-conversion generation and amplification, and their wavelengths can be tuned by rotating the BBO crystal. This parametric up-conversion generation and amplification can be attributed to three and five-wave mixing in a thick BBO crystal, and it shows the potential application on optical parametric chirped pulse amplification (OPCPA) to generate multi-color ultraviolet or visible femtosecond laser pulses pumped directly by femtosecond fundamental laser pulses without frequency-doubling or tripling.

Time-resolved EUV photoelectron spectroscopy of dissociating I_2 by laser harmonics at 80 nm

Generation of single-order laser harmonics in extreme ultraviolet (EUV) and the application to the time-resolved photoelectron spectroscopy of I(2) are demonstrated. The EUV pulses at 80 nm were generated from Kr as the 5th order harmonics of intense 400 nm laser pulses and then separated from other harmonic orders by a thin indium foil. The pump-probe photoelectron spectroscopy of I(2) in the B (3)Π(0(u)(+)) and B" (1)Π(1(u)) states excited by visible laser pulses at 490 nm showed a rapid increase in the yield of atomic iodine (~400 fs), reflecting the dissociation dynamics evolving simultaneously in the two excited states.

Development of real-time vibrational spectroscopy of molecules in electronic excited states: toward mapping molecular potential energy hypersurfaces

We have developed an experimental system for real-time vibrational spectroscopy of molecules in a specific electronic excited state. A synchronized UV light source and a few-cycle visible laser pulse were utilized to prepare and investigate the ultrafast dynamics of molecules in the electronic excited states. A multichannel lock-in amplifier detection system operating in a tandem double lock-in detection mode was used to extract the signal correlated only to the UV and visible pump pulses. Real-time vibrational spectroscopy of chrysene in the triplet state and 1′, 3′-dihydro-1′, 3′, 3′-trimethyl-6-nitrospiro [2H-1-benzopyran-2, 2′-(2H)-indole] in a photochromic state was demonstrated.

Adsorption and degradation performance of Rhodamine B over BiOBr under monochromatic 532 nm pulsed laser exposure

The adsorption (under dark condition) and photodegradation behaviors of Rhodamine B molecules on BiOBr under visible (532 nm) pulsed laser exposure were studied and reported for the first time. The adsorption kinetics/isotherm behaviors and thermodynamic process (adsorption dynamical parameters) occurred on as-prepared BiOBr were investigated. The photodegradation mechanisms, effects of BiOBr dosage and pulsed laser energy on dye removal efficiency in the presence of BiOBr particles were studied as well. Furthermore, the photodegradation of other kinds of Rhodamine dyes such as Rhodamine 6G, Sulforhodamine B and Sulforhodamine 640 were initially investigated and compared under 532 nm pulsed laser exposure. The study also demonstrated that the dissolved oxygen plays a very significant role in the photo-catalytic decomposition of Rhodamine. The BiOBr catalyst was found intact even under acidified conditions showing good stability of the catalyst.

Ring-Closure and Isomerization Capabilities of Spiropyran-Derived Merocyanine Isomers

We report the photochemistry of two ring-open isomers, namely TTC and TTT, of a bidirectional photoswitchable spiropyran, 6,8-dinitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (6,8-dinitro BIPS). Both isomers are capable of ring closure after excitation with visible fs laser pulses, as disclosed by pump-wavelength-dependent transient absorption experiments in the visible spectral range. The main isomer TTC has its maximum absorption at 560 nm, whereas the minor isomer TTT is red-shifted (600 nm). The excited-state lifetimes differ strongly (τ ≈ 900 ps for TTT and τ ≈ 95 ps for TTC), nevertheless the quantum efficiencies for ring closure (40% for TTC and 35% for TTT) and isomerization (1-2% for TTC and 1-2% for TTT) are comparable. With regard to the bidirectional photoswitching capabilities, 6,8-dinitro BIPS is the first molecular switch based on a 6π-electrocyclic reaction where both ring-open isomers are capable of ring closure.

Ultrafast electronic and vibrational dynamics of a ruthenium porphyrin complex in intersystem crossing

Using sub-5fs visible laser pulses, ultrafast time-resolved absorption change was observed for RuII (TPP)(CO) (TPP=5,10, 15, 20 tetraphenylporphirine) solved in chloroform. Exponential fitting of the time traces estimated decay life times of intermediates as 230±70 fs, 1150±260 fs, 2150±360 fs, and ≫4.8 ps for 1Qx(1,0)/(π,π∗),1Qx(0,0)(π,π∗), 3(d,π∗), and 3(π,π∗), respectively. Real-time 4.8 change of instantaneous molecular vibration frequency was studied by spectrogram analysis, which reflects gradual vibration mode change in the intersystem crossing from singlet state to triplet state.

Enhanced Triplet Formation in Polycrystalline Tetracene Films by Femtosecond Optical-Pulse Shaping

Polycrystalline tetracene films have been explored using weak ∼ 30 fs visible laser pulses that excite the lowest singlet exciton as well as coherent vibrational motion. Transient difference spectra show a triplet absorption which arises following singlet fission (SF) and persists for 1.6 ns without decay. Adaptive pulse shaping identifies multipulse optimal fields which maximize this absorption feature by ∼ 20%. These are comprised of subpulses separated by time delays well correlated with the period of lattice vibrations suggesting such modes control the yield of SF photochemistry.

Generation of DNA photolesions by two-photon absorption of a frequency-doubled Ti:sapphire laser

The formation of spatially localized regions of DNA damage by multiphoton absorption of light is an attractive tool for investigating DNA repair. Although this method has been applied in cells, little information is available about the formation of lesions by multiphoton absorption in the absence of exogenous or endogenous sensitizing agents. Therefore, we have investigated DNA damage induced in vitro by direct two-photon absorption of frequency-doubled femtosecond pulses from a Ti:sapphire laser. We first developed a quantitative polymerase chain reaction assay to measure DNA damage, and determined that the quantum yield of lesions formed by one-photon absorption of 254 nm light is 7.86 × 10 −4. We then measured the yield of lesions resulting from exposure to the visible femtosecond laser pulses, which exhibited a quadratic intensity dependence. The two-photon absorption cross section of DNA has a value (per nucleotide) of 2.6 GM at 425 nm, 2.4 GM at 450 nm, and 1.9 GM at 475 nm. A comparison of these in vitro results to several in vivo studies of multiphoton photodamage indicates that the onset of DNA damage occurs at lower intensities in vivo; we suggest possible explanations for this discrepancy.

Polyethylene welding by pulsed visible laser irradiation

Laser welding of plastics is a relatively new process that induces locally a fast polymer heating. For most applications, the process involves directing a pulsed beam of visible light at the weld joint by going through one of the two parts. This is commonly referred to as “through transmission visible laser welding”. In this technique, the monochromatic visible light source uses a power ns pulsed laser in order to irradiate the joint through one part and the light is absorbed in the vicinity of the other part.In order to evaluate the mechanical resistance of the welded joint, mass quadrupole spectrometry, surface profilometry, microscopy techniques and mechanical shear tests were employed. The welding effect was investigated as a function of the laser irradiation time, nature of the polyethylene materials and temperature.

Ultrafast Spectroscopy of Oxyhemoglobin during Photodissociation

Ultrafast time-resolved pump-probe spectroscopy was studied to clarify the detailed mechanism in the photodissociation process of oxyhemoglobin in the visible spectral range. The photodissociation had not been time-resolved and only the upper limit of the time needed for the dissociation process was claimed to be faster than 50 fs; it was time-resolved to be 45 +/- 5 fs with 10 fs time resolution. The broadband spectrum of the visible laser pulses enabled us to observe the signal over a broad spectral range. A broadband multichannel detector array was used to simultaneously obtain the pump-probe signal at all probe frequencies. Thus, we obtained for the first time the spectra of the ultrashort lifetime electronic excited state of HbO(2) in the visible range from 523 nm (19 109 cm(-1)) to 719 nm (13 914 cm(-1)). After the photodissociation, subpicosecond time constant was found and attributed to the dynamics of the photolyzed Hb. Time-resolved difference absorption spectra in 0-100 fs showed oscillatory motion reflecting wavepacket motion in the potential energy surface of the photoexcited HbO(2) during the ultrafast photolysis.

Cluster generation under pulsed laser ablation of zinc oxide

Neutral and cationic Zn n O m clusters of various stoichiometry have been produced by nanosecond laser ablation of ZnO in vacuum and investigated by time-of-flight mass spectrometry. Particular attention was paid to the effect of laser wavelength (in the range from near-IR to UV) on cluster composition. Under 193-nm laser ablation, the charged clusters are essentially substoichiometric with $\mathrm{Zn}_{n}\mathrm{O}_{n-1}^{+}$ and $\mathrm{Zn}_{n}\mathrm{O}_{n-3}^{+}$ being the most abundant series. Both sub- and stoichiometric cationic clusters are generated in abundance at 532- and 1064-nm ablation whose composition depends on the cluster size. The reactivity of small stoichiometric $\mathrm{Zn}_{n}\mathrm{O}_{n}^{+}$ clusters (n<11) toward hydrogen is found to be high, while oxygen-deficient species are less reactive. The neutral plume particles are mainly stoichiometric with Zn4O4 tetramer being a magic cluster. It is suggested that the Zn4O4 loss is the dominant fragmentation channel of large zinc oxide clusters upon electron impact. Plume expansion conditions under ZnO ablation with visible and IR laser pulses are shown to be favorable for stoichiometric cluster formation.