Showing papers on "Femtosecond published in 2009"

Ultrafast active plasmonics

Abstract: Surface plasmon polaritons, propagating bound oscillations of electrons and light at a metal surface, have great potential as information carriers for next-generation, highly integrated nanophotonic devices [1,2]. Since the term 'active plasmonics' was coined in 2004 [3], a number of techniques for controlling the propagation of guided surface plasmon polariton signals have been demonstrated [4-7]. However, with sub-microsecond or nanosecond response times at best, these techniques are likely to be too slow for future applications in such fields as data transport and processing. Here we report that femtosecond optical frequency plasmon pulses can propagate along a metal-dielectric waveguide and that they can be modulated on the femtosecond timescale by direct ultrafast optical excitation of the metal, thereby offering unprecedented terahertz modulation bandwidth - a speed at least five orders of magnitude faster than existing technologies.

Photon-induced near-field electron microscopy

Abstract: In materials science and biology, optical near-field microscopies enable spatial resolutions beyond the diffraction limit, but they cannot provide the atomic-scale imaging capabilities of electron microscopy. Given the nature of interactions between electrons and photons, and considering their connections through nanostructures, it should be possible to achieve imaging of evanescent electromagnetic fields with electron pulses when such fields are resolved in both space (nanometre and below) and time (femtosecond). Here we report the development of photon-induced near-field electron microscopy (PINEM), and the associated phenomena. We show that the precise spatiotemporal overlap of femtosecond single-electron packets with intense optical pulses at a nanostructure (individual carbon nanotube or silver nanowire in this instance) results in the direct absorption of integer multiples of photon quanta (nhomega) by the relativistic electrons accelerated to 200 keV. By energy-filtering only those electrons resulting from this absorption, it is possible to image directly in space the near-field electric field distribution, obtain the temporal behaviour of the field on the femtosecond timescale, and map its spatial polarization dependence. We believe that the observation of the photon-induced near-field effect in ultrafast electron microscopy demonstrates the potential for many applications, including those of direct space-time imaging of localized fields at interfaces and visualization of phenomena related to photonics, plasmonics and nanostructures.

On the role of surface plasmon polaritons in the formation of laser-induced periodic surface structures upon irradiation of silicon by femtosecond-laser pulses

Abstract: The formation of nearly wavelength-sized laser-induced periodic surface structures (LIPSSs) on single-crystalline silicon upon irradiation with single or multiple femtosecond-laser pulses (pulse duration τ=130 fs and central wavelength λ=800 nm) in air is studied experimentally and theoretically. In our theoretical approach, we model the LIPSS formation by combining the generally accepted first-principles theory of Sipe and co-workers with a Drude model in order to account for transient intrapulse changes in the optical properties of the material due to the excitation of a dense electron-hole plasma. Our results are capable to explain quantitatively the spatial periods of the LIPSSs being somewhat smaller than the laser wavelength, their orientation perpendicular to the laser beam polarization, and their characteristic fluence dependence. Moreover, evidence is presented that surface plasmon polaritons play a dominant role during the initial stage of near-wavelength-sized periodic surface structures in fem...

Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex

Abstract: X-ray absorption spectroscopy is a powerful probe of molecular structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. We investigated the ultrafast formation of the lowest quintet state of aqueous iron(II) tris(bipyridine) upon excitation of the singlet metal-to-ligand-charge-transfer ( 1 MLCT) state by femtosecond optical pump/x-ray probe techniques based on x-ray absorption near-edge structure (XANES). By recording the intensity of a characteristic XANES feature as a function of laser pump/x-ray probe time delay, we find that the quintet state is populated in about 150 femtoseconds. The quintet state is further evidenced by its full XANES spectrum recorded at a 300-femtosecond time delay. These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which we identify as a simple 1 MLCT→ 3 MLCT→ 5 T cascade from the initially excited state. The time scale of the 3 MLCT→ 5 T relaxation corresponds to the period of the iron-nitrogen stretch vibration.

Coherent ultrafast magnetism induced by femtosecond laser pulses

Abstract: The quest for ultrafast magnetic processes has triggered a new field of research—femtomagnetism: using femtosecond laser pulses to demagnetize ferromagnetic metallic thin films. Despite being the subject of intense research for over a decade, the underlying mechanisms that govern the demagnetization remain unclear. Here, we investigate how an ultrashort laser pulse couples to the spin of electrons in ferromagnetic metals. It is shown that a single 50-fs laser pulse couples efficiently to a ferromagnetic film during its own propagation. This result indicates that the material polarization induced by the photon field interacts coherently with the spins. The corresponding mechanism has its origin in relativistic quantum electrodynamics, beyond the spin–orbit interaction involving the ionic potential. In addition, this coherent interaction is clearly distinguished from the incoherent ultrafast demagnetization associated with the thermalization of the spins. We forecast that the corresponding coherent self-induced processes are the dawn of a new era for future research in magnetism. Femtosecond laser pulses can demagnetize ferromagnetic metallic thin films on an ultrafast timescale. Studying how a single optical pulse interacts with a magnetic film now provides a better understanding of this so-called femtomagnetism.

Mapping GFP structure evolution during proton transfer with femtosecond Raman spectroscopy

Abstract: GFP, the green fluorescent protein from the jellyfish Aequorea victoria, is widely used in the life sciences as a gene expression marker because of its efficient bioluminescence. The atomistic details of the excited-state proton transfer that generates this bioluminescence are still not fully understood. Fang et al. have used femtosecond stimulated Raman spectroscopy of the GFP chromophore to obtain detailed time-resolved vibrational spectra that reveal how a low-frequency skeletal vibration poises the protein for the critical proton transfer process. As well as throwing light on a reaction central to countless biological studies, this method of making real-time observations of the structural changes of a molecule during a multidimensional chemical reaction should have broad application in the determination of reaction mechanisms. Tracing the transient atomic motions that lie at the heart of chemical reactions requires high-resolution structural information on the timescale of molecular vibrations. Femtosecond stimulated Raman spectroscopy is now shown to provide sufficiently detailed and time-resolved vibrational spectra of the electronically excited chromophore of green fluorescent protein to reveal skeletal motions involved in the proton transfer that produces the fluorescent form of the protein. Tracing the transient atomic motions that lie at the heart of chemical reactions requires high-resolution multidimensional structural information on the timescale of molecular vibrations, which commonly range from 10 fs to 1 ps. For simple chemical systems, it has been possible to map out in considerable detail the reactive potential-energy surfaces describing atomic motions and resultant reaction dynamics1, but such studies remain challenging for complex chemical and biological transformations2. A case in point is the green fluorescent protein (GFP)3,4,5 from the jellyfish Aequorea victoria, which is a widely used gene expression marker owing to its efficient bioluminescence. This feature is known to arise from excited-state proton transfer (ESPT)6,7,8, yet the atomistic details of the process are still not fully understood. Here we show that femtosecond stimulated Raman spectroscopy9,10 provides sufficiently detailed and time-resolved vibrational spectra of the electronically excited chromophore of GFP to reveal skeletal motions involved in the proton transfer that produces the fluorescent form of the protein. In particular, we observe that the frequencies and intensities of two marker bands, the C–O and C = N stretching modes at opposite ends of the conjugated chromophore, oscillate out of phase with a period of 280 fs; we attribute these oscillations to impulsively excited low-frequency phenoxyl-ring motions, which optimize the geometry of the chromophore for ESPT. Our findings illustrate that femtosecond simulated Raman spectroscopy is a powerful approach to revealing the real-time nuclear dynamics that make up a multidimensional polyatomic reaction coordinate.

Phase matching of high harmonic generation in the soft and hard X-ray regions of the spectrum

Abstract: We show how bright, tabletop, fully coherent hard X-ray beams can be generated through nonlinear upconversion of femtosecond laser light. By driving the high-order harmonic generation process using longer-wavelength midinfrared light, we show that, in theory, fully phase-matched frequency upconversion can extend into the hard X-ray region of the spectrum. We verify our scaling predictions experimentally by demonstrating phase matching in the soft X-ray region of the spectrum around 330 eV, using ultrafast driving laser pulses at 1.3-μm wavelength, in an extended, high-pressure, weakly ionized gas medium. We also show through calculations that scaling of the overall conversion efficiency is surprisingly favorable as the wavelength of the driving laser is increased, making tabletop, fully coherent, multi-keV X-ray sources feasible. The rapidly decreasing microscopic single-atom yield, predicted for harmonics driven by longer-wavelength lasers, is compensated macroscopically by an increased optimal pressure for phase matching and a rapidly decreasing reabsorption of the generated X-rays.

Micromachining of photonic devices by femtosecond laser pulses

Abstract: In this paper we review the micromachining of photonic devices in several materials by means of ultrashort laser pulses. The physical mechanisms underlying the refractive index modification and the different laser systems used to induce such modification are discussed. A thorough review of the photonic devices demonstrated with the femtosecond laser microfabrication technique is presented. In particular, this paper is focused on photonic devices based on optical waveguides. The devices are organized into two categories: passive and active devices. In the former category power splitters, directional couplers, interferometers and Bragg gratings are reviewed, while in the latter amplifiers and lasers are discussed. Finally, conclusions and future perspectives of femtosecond laser micromachining in photonics are provided.

Femtosecond laser-induced periodic surface structures revisited: A comparative study on ZnO

Abstract: Laser-induced periodic surface structures (LIPSS) (ripples) with different spatial characteristics have been observed after irradiation of single-crystalline zinc oxide surfaces with multiple linearly polarized femtosecond pulses (150–200 fs, 800 nm) in air. For normal incident laser radiation, low spatial frequency LIPSS (LSFL) with a period (630–730 nm) close to the wavelength and an orientation perpendicular to the laser polarization have been found in the fluence range between ∼0.7 and ∼0.8 J/cm2 and predominantly for pulse numbers up to N=100. For lower fluences (0.5–0.7 J/cm2), a sharp transition from the LSFL features toward the formation of high spatial frequency LIPSS (HSFL) appears at any given pulse number below N=100. The HSFL are always parallel to the LSFL, exhibit spatial periods between 200 and 280 nm, and completely substitute the LSFL for pulse numbers N>100. Additionally, the influence of the angle of incidence has been studied experimentally for both LIPSS types revealing a different b...

Coherent Polarization Control of Terahertz Waves Generated from Two-Color Laser-Induced Gas Plasma

Abstract: Electrons ionized from an atom or molecule by circularly or elliptically polarized femtosecond omega and 2omega pulses exhibit different trajectory orientations as the relative phase between the two pulses changes. Macroscopically, the polarization of the terahertz wave emitted during the ionization process was found to be coherently controllable through the optical phase. This new finding can be completely reproduced by numerical simulation and may enable fast terahertz wave modulation and coherent control of nonlinear responses excited by intense terahertz waves with controllable polarization.

Generation of Isolated Attosecond Pulses with 20 to 28 Femtosecond Lasers

Abstract: Isolated attosecond pulses are powerful tools for exploring electron dynamics in matter. So far, such extreme ultraviolet pulses have only been generated using high power, few-cycle lasers, which are very difficult to construct and operate. We propose and demonstrate a technique called generalized double optical gating for generating isolated attosecond pulses with 20 fs lasers from a hollow-core fiber and 28 fs lasers directly from an amplifier. These pulses, generated from argon gas, are measured to be 260 and 148 as by reconstructing the streaked photoelectron spectrograms. This scheme, with a relaxed requirement on laser pulse duration, makes attophysics more accessible to many laboratories that are capable of producing such multicycle laser pulses.

Femtosecond continuum generation in bulk laser host materials with sub-μJ pump pulses

Abstract: We report a comprehensive investigation of femtosecond continuum generation in single crystals of several common laser host materials. The absolute spectral energy density, pulse-to-pulse stability, pump threshold, and beam profile are studied in dependence on the focusing conditions, crystal thickness, pump pulse energy, and pump wavelength (775–1600 nm). Continuum generation is shown at repetition rates of up to 80 MHz and for pump pulse durations of up to 350 fs. In yttrium aluminum garnet (YAG), yttrium vanadate (YVO4), gadolinium vanadate (GdVO4), and potassium-gadolinium tungstate (KGW) thresholds below 50 nJ, plateau-like visible and infrared spectra, and higher infrared photon flux as compared to conventional materials like sapphire are found. We discuss the particular advantages of these materials for application in parametric amplification, femtosecond spectroscopy, and carrier-envelope phase stabilization.

Ultrabroadband supercontinuum generation from ultraviolet to 6.28 μm in a fluoride fiber

Abstract: Ultrabroadband supercontinuum light expanding from ultraviolet to 6.28 μm is generated in a centimeter-long fluoride fiber pumped by a 1450 nm femtosecond laser. The spectral broadening in the fluoride fiber is caused by self-phase modulation, Raman scattering and four-wave mixing. The experimental and simulated results show that fluoride fiber is a promising candidate for generating the midinfrared supercontinuum light up to 8 μm.

Generation of double-scale femto/pico-second optical lumps in mode-locked fiber lasers

Abstract: We observed generation of stable picoseconds pulse train and double-scale optical lumps with picosecond envelope and femtosecond noise-like oscillations in the same Yb-doped fiber laser with all-positive-dispersion cavity mode-locked due to the effect of non-linear polarization evolution. In the noise-like pulse generation regime the auto-correlation function has a non-usual double (femto- and picosecond) scale shape. We discuss mechanisms of laser switching between two operation regimes and demonstrate a good qualitative agreement between experimental results and numerical modeling based on modified nonlinear Schrodinger equations.

Sub-50 fs broadband absorption spectroscopy with tunable excitation: putting the analysis of ultrafast molecular dynamics on solid ground

Abstract: We give a full description of a state-of-the-art femtosecond transient spectrometer. The setup has been put together under full consideration of all technical and conceptual developments that became available in the last few years. Particular care was taken to avoid any unneeded components and modules.

Spin polarization in half-metals probed by femtosecond spin excitation.

Abstract: Knowledge of the spin polarization is of fundamental importance for the use of a material in spintronics applications. Here, we used femtosecond optical excitation of half-metals to distinguish between half-metallic and metallic properties. Because the direct energy transfer by Elliot-Yafet scattering is blocked in a half-metal, the demagnetization time is a measure for the degree of half-metallicity. We propose that this characteristic enables us vice versa to establish a novel and fast characterization tool for this highly important material class used in spin-electronic devices. The technique has been applied to a variety of materials where the spin polarization at the Fermi level ranges from 45 to 98%: Ni, Co(2)MnSi, Fe(3)O(4), La(0.66)Sr(0.33)MnO(3) and CrO(2).

Surgical applications of femtosecond lasers

Abstract: Femtosecond laser ablation permits non-invasive surgeries in the bulk of a sample with submicrometer resolution. We briefly review the history of optical surgery techniques and the experimental background of femtosecond laser ablation. Next, we present several clinical applications, including dental surgery and eye surgery. We then summarize research applications, encompassing cell and tissue studies, research on C. elegans, and studies in zebrafish. We conclude by discussing future trends of femtosecond laser systems and some possible application directions.

Impulsive orientation and alignment of quantum-state-selected NO molecules

Abstract: A technique that produces significant alignment of molecules in a beam should aid a wide range of experiments geared towards understanding and controlling molecular processes in the gas phase. Manipulation of the molecular-axis distribution is an important ingredient in experiments aimed at understanding and controlling molecular processes1,2,3,4,5,6. Samples of aligned or oriented molecules can be obtained following the interaction with an intense laser field7,8,9, enabling experiments in the molecular rather than the laboratory frame10,11,12. However, the degree of impulsive molecular orientation and alignment that can be achieved using a single laser field is limited13 and crucially depends on the initial states, which are thermally populated. Here we report the successful demonstration of a new technique for laser-field-free orientation and alignment of molecules that combines an electrostatic field, non-resonant femtosecond laser excitation14 and the preparation of state-selected molecules using a hexapole2. As a unique quantum-mechanical wavepacket is formed, a large degree of orientation and alignment is observed both during and after the femtosecond laser pulse, which is even further increased (to 〈cosθ〉=−0.74 and 〈cos2θ〉=0.82, respectively) by tailoring the shape of the femtosecond laser pulse. This work should enable new applications such as the study of reaction dynamics or collision experiments in the molecular frame, and orbital tomography11 of heteronuclear molecules.

Paradigm of the time-resolved magneto-optical Kerr effect for femtosecond magnetism

Abstract: Conventional understanding of the magneto-optical Kerr effect, in which changes in the magnetization of a material cause changes in the polarization of reflected light, assumes that this incident light is continuous. However, first-principles simulations of nickel show that this assumption breaks down for femtosecond pulses of light, and establishes a firm foundation for understanding the dynamics of femtomagnetism. The magneto-optical Kerr effect (MOKE) is a powerful tool for studying changes in the magnetization of ferromagnetic materials. It works by measuring changes in the polarization of reflected light. However, because the conventional theoretical basis for interpreting a MOKE signal assumes measurement with continuous-wave light1,2, its use for understanding high-speed magnetization dynamics of a material probed with femtosecond optical pulses3,4 has been controversial5,6,7,8,9,10. Here we establish a new paradigm for interpreting time-resolved MOKE measurements, through a first-principles investigation of ferromagnetic nickel. We show that the time-resolved optical and magnetic responses energetically follow their respective optical and magneto-optical susceptibilities. As a result, the one-to-one correspondence between them sensitively depends on the incident photon energy. In nickel, for photon energies below 2 eV the magnetic response is faithfully reflected in the optical response, but above 2 eV they decouple. By constructing a phase-sensitive polarization versus magnetization plot, we find that for short pulses the magnetic signals are delayed by 10 fs with respect to the optical signals. For longer pulses, the delay shortens and the behaviour approaches the continuous-wave response. This finally resolves the long-standing dispute over the interpretation in the time-resolved MOKE measurements and lays a solid foundation for understanding femtomagnetism3,4.

Field-free orientation of CO molecules by femtosecond two-color laser fields.

Abstract: We report the first experimental observation of nonadiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction, and molecular frame photoelectron emission.

10-GHz Self-Referenced Optical Frequency Comb

Abstract: The femtosecond laser–based frequency comb has played a key role in high-precision optical frequency metrology for a decade. Although often referred to as a precise optical frequency ruler, its tick marks are in fact too densely spaced for direct observation and individual use, limiting important applications in spectroscopy, astronomy, and ultrafast electromagnetic waveform control. We report on a femtosecond laser frequency comb with a 10-gigahertz repetition rate that creates a stabilized output spectrum with coverage from 470 to 1130 nanometers. The individual modes can be directly resolved with a grating spectrometer and are visible by eye.

Three-dimensional microfabrication of materials by femtosecond lasers for photonics applications

Abstract: Femtosecond laser fabrication of three-dimensional structures for photonics applications is reviewed. Fabrication of photonic crystal structures by direct laser writing and holographic recording by multiple beam interference techniques are discussed. The physical mechanisms associated with structure formation and postfabrication are described. The advantages and limitations of various femtosecond laser microfabrication techniques for the preparation of photonic crystals and elements of microelectromechanical and micro-optofluidic systems are discussed.

Optimally chirped multimodal CARS microscopy based on a single Ti:sapphire oscillator.

Abstract: We demonstrate high performance coherent anti-Stokes Raman scattering (CARS) microscopy of live cells and tissues with user-variable spectral resolution and broad Raman tunability (2500 - 4100 cm-1), using a femtosecond Ti:Sapphire pump and photonic crystal fiber output for the broadband synchronized Stokes pulse. Spectral chirp of the fs laser pulses was a user-variable parameter for optimization in a spectral focussing implementation of multimodal CARS microscopy. High signal-to-noise, high contrast multimodal imaging of live cells and tissues was achieved with pixel dwell times of 2-8 μs and low laser powers (< 30 mW total).

Femtosecond carrier dynamics and saturable absorption in graphene suspensions

Abstract: Nonlinear optical properties and carrier relaxation dynamics in graphene, suspended in three different solvents, are investigated using femtosecond (80 fs pulses) Z-scan and degenerate pump-probe spectroscopy at 790 nm. The results demonstrate saturable absorption property of graphene with a nonlinear absorption coefficient, beta of (similar to 2-9) x 10(-8) cm/W. Two distinct time scales associated with the relaxation of photoexcited carriers, a fast one in the range of 130-330 fs (related to carrier-carrier scattering) followed by it slower one in 3.5-4.9 ps range (associated with carrier-phonon scattering) are observed. (C) 2009 American Institute of Physics.

Single-shot diffractive imaging with a table-top femtosecond soft x-ray laser-harmonics source.

Abstract: Coherent x-ray diffractive imaging is a powerful method for studies on nonperiodic structures on the nanoscale. Access to femtosecond dynamics in major physical, chemical, and biological processes requires single-shot diffraction data. Up to now, this has been limited to intense coherent pulses from a free electron laser. Here we show that laser-driven ultrashort x-ray sources offer a comparatively inexpensive alternative. We present measurements of single-shot diffraction patterns from isolated nano-objects with a single 20 fs pulse from a table-top high-harmonic x-ray laser. Images were reconstructed with a resolution of 119 nm from the single shot and 62 nm from multiple shots.

Filamentation of femtosecond laser Airy beams in water.

Abstract: We report experiments on the propagation of intense, femtosecond, self-bending Airy laser beams in water. The supercontinuum radiation generated along the curved beam path is angularly resolved in the far field. Spectral maps of this radiation reveal the changing character of the laser-pulse evolution on propagation.

Dynamics of chemical bonding mapped by energy-resolved 4D electron microscopy.

Abstract: Chemical bonding dynamics are fundamental to the understanding of properties and behavior of materials and molecules. Here, we demonstrate the potential of time-resolved, femtosecond electron energy loss spectroscopy (EELS) for mapping electronic structural changes in the course of nuclear motions. For graphite, it is found that changes of milli–electron volts in the energy range of up to 50 electron volts reveal the compression and expansion of layers on the subpicometer scale (for surface and bulk atoms). These nonequilibrium structural features are correlated with the direction of change from sp^2 [two-dimensional (2D) graphene] to sp^3 (3D-diamond) electronic hybridization, and the results are compared with theoretical charge-density calculations. The reported femtosecond time resolution of four-dimensional (4D) electron microscopy represents an advance of 10 orders of magnitude over that of conventional EELS methods.

Ultrafast laser written active devices

Abstract: t Direct-write optical waveguide device fabrication is probably the most widely studied application of femtosecond laser micromachining in transparent dielectrics at the present time. Devices such as buried waveguides, power splitters, couplers, gratings, optical amplifiers and laser oscillators have all been demonstrated. This paper reviews the application of the femtosecond laser direct-write technique to the fabrication of active waveguide devices in bulk glass materials.

Femtosecond and picosecond laser drilling of metals at high repetition rates and average powers

Abstract: The influence of pulse duration on the laser drilling of metals at repetition rates of up to 1 MHz and average powers of up to 70 W has been experimentally investigated using an ytterbium-doped-fiber chirped-pulse amplification system with pulses from 800 fs to 19 ps. At a few hundred kilohertz particle shielding causes an increase in the number of pulses for breakthrough, depending on the pulse energy and duration. At higher repetition rates, the heat accumulation effect overbalances particle shielding, but significant melt ejection affects the hole quality. Using femtosecond pulses, heat accumulation starts at higher repetition rates, and the ablation efficiency is higher compared with picosecond pulses.