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Showing papers on "Femtosecond published in 2000"


Journal ArticleDOI
28 Apr 2000-Science
TL;DR: The carrier-envelope phase of the pulses emitted by a femtosecond mode-locked laser is stabilized by using the powerful tools of frequency-domain laser stabilization to perform absolute optical frequency measurements that were directly referenced to a stable microwave clock.
Abstract: We stabilized the carrier-envelope phase of the pulses emitted by a femtosecond mode-locked laser by using the powerful tools of frequency-domain laser stabilization. We confirmed control of the pulse-to-pulse carrier-envelope phase using temporal cross correlation. This phase stabilization locks the absolute frequencies emitted by the laser, which we used to perform absolute optical frequency measurements that were directly referenced to a stable microwave clock.

2,499 citations


Journal ArticleDOI
17 Aug 2000-Nature
TL;DR: Computer simulations are used to investigate the structural information that can be recovered from the scattering of intense femtosecond X-ray pulses by single protein molecules and small assemblies and predict that ultrashort, high-intensity X-rays from free-electron lasers that are currently under development will provide a new approach to structural determinations with X- rays.
Abstract: Sample damage by X-rays and other radiation limits the resolution of structural studies on non-repetitive and non-reproducible structures such as individual biomolecules or cells(1). Cooling can slow sample deterioration, but cannot eliminate damage-induc

1,770 citations


Journal ArticleDOI
TL;DR: A great simplification in the long-standing problem of measuring optical frequencies in terms of the cesium primary standard is demonstrated, enabling us to measure the 282 THz frequency of an iodine-stabilized Nd:YAG laser directly in Terms of the microwave frequency that controls the comb spacing.
Abstract: We demonstrate a great simplification in the long-standing problem of measuring optical frequencies in terms of the cesium primary standard. An air-silica microstructure optical fiber broadens the frequency comb of a femtosecond laser to span the optical octave from 1064 to 532 nm, enabling us to measure the 282 THz frequency of an iodine-stabilized Nd:YAG laser directly in terms of the microwave frequency that controls the comb spacing. Additional measurements of established optical frequencies at 633 and 778 nm using the same femtosecond comb confirm the accepted uncertainties for these standards.

1,072 citations


Journal ArticleDOI
TL;DR: A classical-oscillator description of these spectroscopies in terms of interacting quasiparticles (rather than transitions among global eigenstates) sets the stage for designing new pulse sequences and inverting the multidimensional signals to yield molecular structures.
Abstract: Femtosecond visible and infrared analogues of multiple-pulse nuclear magnetic resonance techniques provide novel snapshot probes into the structure and electronic and vibrational dynamics of complex molecular assemblies such as photosynthetic antennae, proteins, and hydrogen-bonded liquids. A classical-oscillator description of these spectroscopies in terms of interacting quasiparticles (rather than transitions among global eigenstates) is developed and sets the stage for designing new pulse sequences and inverting the multidimensional signals to yield molecular structures. Considerable computational advantages and a clear physical insight into the origin of the response and the relevant coherence sizes are provided by a real-space analysis of the underlying coherence-transfer pathways in Liouville space.

818 citations


Journal ArticleDOI
TL;DR: In this article, the shape transformations of gold nanorods were investigated by using visible absorption spectroscopy and transmission electron microscopy (TEM) in order to analyze the final shape and size distribution.
Abstract: Gold nanorods have been found to change their shape after excitation with intense pulsed laser irradiation. The final irradiation products strongly depend on the energy of the laser pulse as well as on its width. We performed a series of measurements in which the excitation power was varied over the range of the output power of an amplified femtosecond laser system producing pulses of 100 fs duration and a nanosecond optical parametric oscillator (OPO) laser system having a pulse width of 7 ns. The shape transformations of the gold nanorods are followed by two techniques: (1) visible absorption spectroscopy by monitoring the changes in the plasmon absorption bands characteristic for gold nanoparticles; (2) transmission electron microscopy (TEM) in order to analyze the final shape and size distribution. While at high laser fluences (∼1 J cm-2) the gold nanoparticles fragment, a melting of the nanorods into spherical nanoparticles (nanodots) is observed when the laser energy is lowered. Upon decreasing the...

754 citations


Journal ArticleDOI
TL;DR: In this paper, a supercontinuum light with a spectrum more than two octaves broad (370-1545 nm at the 20-dB level) was generated in a standard telecommunications fiber by femtosecond pulses from an unamplified Ti:sapphire laser.
Abstract: Supercontinuum light with a spectrum more than two octaves broad (370–1545 nm at the 20-dB level) was generated in a standard telecommunications fiber by femtosecond pulses from an unamplified Ti:sapphire laser. The fiber had been tapered to a diameter of ∼2 µm over a 90-mm length. The pulse energy was 3.9 nJ (average power, 300 mW). This source of high-intensity single-mode white light should find widespread applications in frequency metrology and spectroscopy, especially since no unconventional fibers are needed.

711 citations


Journal ArticleDOI
TL;DR: A strong dependence of the plasmon lifetime and resonance wavelength on the grating constant is found and it is found that gold nanoparticles arranged in regular two-dimensional patterns have regular grating effects.
Abstract: We probe the influence of grating effects on plasmon excitations in gold nanoparticles arranged in regular two-dimensional patterns. Samples produced by electron-beam lithography are investigated by femtosecond time-resolved and spectroscopic methods. We find a strong dependence of the plasmon lifetime and resonance wavelength on the grating constant.

575 citations


Journal ArticleDOI
24 Mar 2000-Science
TL;DR: Femtosecond synchrotron pulses were generated directly from an electron storage ring and offer the possibility of applying x-ray techniques on an ultrafast time scale to investigate structural dynamics in condensed matter.
Abstract: Femtosecond synchrotron pulses were generated directly from an electron storage ring. An ultrashort laser pulse was used to modulate the energy of electrons within a 100-femtosecond slice of the stored 30-picosecond electron bunch. The energy-modulated electrons were spatially separated from the long bunch and used to generate ∼300-femtosecond synchrotron pulses at a bend-magnet beamline, with a spectral range from infrared to x-ray wavelengths. The same technique can be used to generate ∼100-femtosecond x-ray pulses of substantially higher flux and brightness with an undulator. Such synchrotron-based femtosecond x-ray sources offer the possibility of applying x-ray techniques on an ultrafast time scale to investigate structural dynamics in condensed matter.

571 citations


Journal ArticleDOI
TL;DR: Two-dimensional infrared spectra of peptides are introduced that are the direct analogues of two- and three-pulse multiple quantum NMR that show great promise to measure structure changes in biology on a wide range of time scales.
Abstract: Two-dimensional infrared spectra of peptides are introduced that are the direct analogues of two- and three-pulse multiple quantum NMR. Phase matching and heterodyning are used to isolate the phase and amplitudes of the electric fields of vibrational photon echoes as a function of multiple pulse delays. Structural information is made available on the time scale of a few picoseconds. Line narrowed spectra of acyl-proline-NH(2) and cross peaks implying the coupling between its amide-I modes are obtained, as are the phases of the various contributions to the signals. Solvent-sensitive structural differences are seen for the dipeptide. The methods show great promise to measure structure changes in biology on a wide range of time scales.

487 citations


Journal ArticleDOI
TL;DR: This work achieved a first complete identification by explicitly measuring the time-resolved Kerr ellipticity and rotation, as well as its temperature and magnetic field dependence in epitaxially grown Cu/Ni/Cu wedges.
Abstract: Several magnetic and optical processes contribute to the magneto-optical response of nickel thin films after excitation by a femtosecond laser pulse. We achieved a first complete identification by explicitly measuring the time-resolved Kerr ellipticity and rotation, as well as its temperature and magnetic field dependence in epitaxially grown (111) and (001) oriented Cu/Ni/Cu wedges. The first hundreds of femtoseconds the response is dominated by state filling effects. The true demagnetization takes approximately 0.5-1 ps. At the longer (sub-ns) time scales the spins are found to precess in their anisotropy field. Simple and transparent models are introduced to substantiate our interpretation.

459 citations


Journal ArticleDOI
TL;DR: The phase shift of the optical beat component between longitudinal modes of a mode-locked laser is measured and an absolute mechanical distance of 240 m was obtained to within 8-ppm accuracy by use of a series of beat frequencies with the advantage of a wide range of intermode frequency.
Abstract: A high-accuracy optical distance meter with a mode-locked femtosecond laser is proposed for distance measurements in a 310-m-long optical tunnel. We measured the phase shift of the optical beat component between longitudinal modes of a mode-locked laser. A high resolution of 50 microm at 240-m distance was obtained without cyclic error correction. The group refractive index of air is automatically extracted to an accuracy of 6 parts per million (ppm) by two-color measurement with the pulses of fundamental and second-harmonic wavelengths. Finally, an absolute mechanical distance of 240 m was obtained to within 8-ppm accuracy by use of a series of beat frequencies with the advantage of a wide range of intermode frequency, together with the results of the two-color measurement.

Journal ArticleDOI
TL;DR: In this paper, the central frequency of the transients is continuously tunable over a wide interval extending from 41 THz (λ = 7μm) to the far-infrared.
Abstract: Bandwidth-limited infrared pulses as short as 50 fs are generated in thin GaSe crystals by phase-matched optical rectification of 10 fs laser pulses. The central frequency of the transients is continuously tunable over a wide interval extending from 41 THz (λ=7 μm) to the far-infrared. The electric field of the THz transients is directly monitored via ultrabroadband free-space electro-optic sampling. A simulation of the spectra based on a plane-wave model shows excellent agreement with the experiment.

Journal ArticleDOI
TL;DR: In this article, it was shown that short-pulse laser ablation can be interpreted in terms of transient thermal processes involving states of matter with unusual thermodynamic, hydrodynamic and optical properties.

Journal ArticleDOI
TL;DR: In this article, an intense mid-infrared light source that provides femtosecond pulses on a microjoule energy level, broadly tunable in the 3-20-µm wavelength range with pulse durations as short as 50 fs at 5 µm.
Abstract: We report on an intense mid-infrared light source that provides femtosecond pulses on a microjoule energy level, broadly tunable in the 3–20-µm wavelength range with pulse durations as short as 50 fs at 5 µm. The pulses are generated by phase-matched difference-frequency mixing in GaSe of near-infrared signal and idler pulses of a parametric device based on a 1-kHz Ti:sapphire amplifier system. Pulse durations are characterized with different techniques including autocorrelation measurements in AgGaS2, two-photon absorption in InSb, and cross-correlation measurements with near-infrared pulses in a thin GaSe crystal. A subsequent zero-dispersion stretcher of high transmission allows for optimum pulse compression, a more detailed amplitude and phase characterization and, ultimately, amplitude shaping of the mid-infrared pulses.

Journal ArticleDOI
TL;DR: In this article, femtosecond optical pulses are propagated through a periodically poled lithium-niobate crystal, where the domain length is matched to the walk-off length between the optical and THz pulses.
Abstract: We demonstrate a promising technique for generating narrow-band terahertz electromagnetic radiation. Femtosecond optical pulses are propagated through a periodically poled lithium-niobate crystal, where the domain length is matched to the walk-off length between the optical and THz pulses. The bandwidth of the THz wave forms is 0.11 at 1.7 THz. Optical rectification gives rise to a THz wave form which corresponds to the domain structure of the periodically poled lithium niobate.

Journal ArticleDOI
TL;DR: An experimental and theoretical study of intrinsic correlations and noise-suppression mechanisms in two-stage femtosecond mid-IR light sources is presented and the noise suppression is found to stem from the interplay between dispersion and pump depletion.
Abstract: An experimental and theoretical study of intrinsic correlations and noise-suppression mechanisms in two-stage femtosecond mid-IR light sources is presented. The setup, based on parametric amplification in BBO and subsequent difference-frequency mixing in AgGaS(2), delivers approximately 100-fs mid-IR pulses with 1-2-muJ energy. Exceptionally low pulse-energy fluctuations of only 0.2% in the mid-IR (lambda approximately 3-6 mum) are found, which are much smaller than the Ti:sapphire amplifer noise. The noise suppression is analyzed and found to stem from the interplay between dispersion and pump depletion.

Journal ArticleDOI
TL;DR: Femtosecond photodisruption has the potential to become an attractive tool for intrastromal refractive surgery by generating micro-plasmas inside the corneal stroma with fs pulses.
Abstract: · Background: Recently, laser systems have become available which generate ultrashort laser pulses with a duration of 100–200 femtoseconds (fs). By generating micro-plasmas inside the corneal stroma with fs pulses, it is possible to achieve a cutting effect inside the tissue while leaving the anterior layers intact. The energy threshold to generate a micro-plasma with fs pulses is some orders of magnitude lower than it is for picosecond or nanosecond pulses. This results in a strong reduction of the thermal and mechanical damage of the surrounding tissue. · Methods: With a titanium:sapphire fs laser system, the cutting effect on corneal tissue from freshly enucleated porcine eye globes was investigated with different pulse energies. The irradiated samples were examined by light and electron microscopy. The laser-induced pressure transients and the laser-induced bubble formation were analysed with a broadband acoustic transducer and by flash photography. · Results: With fs laser pulses, the extent of thermal and mechanical damage of the adjacent tissue is in the order of 1 µm or below and therefore comparable with the tissue alterations after ArF excimer laser ablation. Using pulse energies of approximately 1–2 µJ and a spot diameter of 5–10 µm, intrastromal cuts can be performed very precisely in order to prepare corneal flaps and lenticules. · Conclusion: Femtosecond photodisruption has the potential to become an attractive tool for intrastromal refractive surgery.

Journal ArticleDOI
TL;DR: In this article, the density and decay kinetics of a plasma channel created in air by a self-guided intense ultra-short laser pulse are determined by a new electric cross-conductivity method and by diffraction interferometry.

Journal ArticleDOI
TL;DR: The spectral behavior of a white-light continuum generated in air by 2-TW femtosecond laser pulses at 800 nm shows that the continuum's intensity increases strongly with the laser energy and depends on the initial chirp.
Abstract: We investigated the spectral behavior of a white-light continuum generated in air by 2-TW femtosecond laser pulses at 800 nm. The spectrum extends at least from 300 nm to 4.5 mum. From 1 to 1.6 mum the continuum's intensity increases strongly with the laser energy and depends on the initial chirp.

Journal ArticleDOI
TL;DR: The internal thermalization dynamics of the conduction electrons is investigated in silver nanoparticles with radius ranging from 13 to 1.6 nm using a femtosecond IR pump-UV probe absorption saturation technique, consistent with electron-electron scattering acceleration due to surface induced reduction of the Coulomb interaction screening.
Abstract: The internal thermalization dynamics of the conduction electrons is investigated in silver nanoparticles with radius ranging from 13 to 1.6 nm using a femtosecond IR pump--UV probe absorption saturation technique. A sharp increase of the electron energy exchange rate is demonstrated for nanoparticles smaller than 5 nm. The results are consistent with electron-electron scattering acceleration due to surface induced reduction of the Coulomb interaction screening by the conduction and core electrons.

Journal ArticleDOI
TL;DR: In this paper, the femtosecond dynamics of colloidal gold nanorods encapsulated in micelles after excitation with 400 nm pulses of 100 fs duration were studied.
Abstract: We studied the femtosecond dynamics of colloidal gold nanorods encapsulated in micelles after excitation with 400 nm pulses of 100 fs duration. It is found that the laser heating of the electron gas of gold nanorods with an average aspect ratio of 3.8 leads to the bleaching of both the transverse and longitudinal mode of the surface plasmon oscillation at 520 and 750 nm. The bleach recovers with the same time constant for both the transverse and longitudinal oscillation, for gold nanodots prepared by photothermal reshaping of the rods as well as for nanodots synthesized chemically by citrate reduction (and known to have twin boundaries and surface defects). Since the bleach recovery on the 3 ps time scale is assigned to electron-phonon relaxation processes, these results suggest that phonon dependent relaxation processes in gold nanoparticles are independent of the shape, size, type of the surfaces, or the mode of the surface plasmon, oscillation excited. The fact that the mean free path of the electron in metallic gold is in the nanometer length scale (\ensuremath{\sim}50 nm) raised the question of the importance of surface scattering to the electron-phonon relaxation process in gold nanoparticles. Our previous studies showed little dependence of the relaxation rate of the size of gold nanodots (from 9 to 48 nm). In the present study, the electron-phonon relaxation is measured in gold nanorods, which have different facets from those of gold nanodots.

Journal ArticleDOI
TL;DR: The vibration dynamics of ellipsoidal silver nanoparticles are investigated, using time-resolved optical pump-probe spectroscopy, and it is shown that these vibrations are triggered by the thermal expansion of the optically heated particles.
Abstract: We investigate the vibration dynamics of ellipsoidal silver nanoparticles, using time-resolved optical pump-probe spectroscopy When excited with femtosecond laser pulses, the particles execute anisotropic shape oscillations We show that these vibrations are triggered by the thermal expansion of the optically heated particles The time dependence of the vibrations indicates that this expansion is caused by two mechanisms: The lattice anharmonicity and the extremely large pressure of the hot conduction electrons

Journal ArticleDOI
TL;DR: In this paper, femtosecond infrared spectroscopy was used to study the electron injection dynamics of Re(CO)3Cl(dcbpy) (ReC0A) and related derivatives on the surface of TiO2 nanocrystalline thin films.
Abstract: Femtosecond infrared spectroscopy was used to study the electron injection dynamics of Re(CO)3Cl(dcbpy) (ReC0A) and related derivatives on the surface of TiO2 nanocrystalline thin films. We prepared two derivatives of ReC0A by inserting 1 and 3 CH2 spacers (ReC1A and ReC3A respectively) between the bipyridine rings and the carboxylate anchoring groups. We directly observed electron injection from the excited states of Re dyes to TiO2 by simultaneously measuring the broad absorption of injected electrons and the vibrational spectrum of the complex in its ground, excited, and oxidized states. Electron injection from ReC0A to TiO2 was found to occur on the 200 from ReC0A to ReC1A. This decrease is much larger than the predicted change fo...

Journal ArticleDOI
TL;DR: In this article, a double pump/reflectivity probe using femtosecond laser pulses was used to investigate nonequilibrium electron dynamics at metal surfaces by measuring the equilibrated surface temperature.
Abstract: An experimental scheme (double pump/reflectivity probe using femtosecond laser pulses) enables the investigation of nonequilibrium electron dynamics at metal surfaces by measuring the equilibrated surface temperature. The competition between electron-phonon coupling and hot-electron transport gives rise to a reduced equilibrated temperature when the two pump pulses overlap in time, and provides a way of accurately determining the electron-phonon coupling constant. These observations have important consequences for femtosecond photochemical investigations.

Journal ArticleDOI
TL;DR: Preliminary measurements of two-photon absorption indicate the novel fluorene derivatives exhibit high two-Photon absorptivity, affirming their potential as two-PHoton fluorophores in two- photon fluorescence microscopy.
Abstract: The Ullmann amination reaction was utilized to provide access to a number of fluorene analogues from common intermediates, via facile functionalization at positions 2, 7, and 9 of the fluorene ring. Through variation of amine or iodofluorene derivative, analogues bearing substitutents with varying electron-donating and electron-withdrawing ability, e.g., diphenylamino, bis-(4-methoxyphenyl)amine, nitro, and benzothiazole, were synthesized in good yield. The novel fluorene derivatives were fully characterized, including absorption and emission spectra. Didecylation at the 9-position afforded remarkably soluble derivatives. Target compounds 4, 5, and 9 are potentially useful as fluorophores in two-photon fluorescence microscopy. Their UV-vis spectra display desirable absorption in the range of interest suitable for two-photon excitation by near-IR femtosecond lasers. Preliminary measurements of two-photon absorption indicate the derivatives exhibit high two-photon absorptivity, affirming their potential as two-photon fluorophores. For example, using a 1,210 nm femtosecond pump beam, diphenylaminobenzothiazolylfluorene 4 exhibited nondegenerate two-photon absorption, with two-photon absorptivity (delta) of ca. 820 x 10(-50) cm(4) s photon(-1) molecule(-1) at the femtosecond white light continuum probe wavelength of 615 nm.

Journal ArticleDOI
TL;DR: In this article, femtosecond table-top laser system was used to convert low-energy photons into antiparticles, namely positrons, and the average intensity of this source of positrons was estimated to be equivalent to 2×108
Abstract: Utilizing a femtosecond table-top laser system, we have succeeded in converting via electron acceleration in a plasma channel, low-energy photons into antiparticles, namely positrons. The average intensity of this source of positrons is estimated to be equivalent to 2×108 Bq and it exhibits a very favorable scaling for higher laser intensities. The advent of positron production utilizing femtosecond laser pulses may be the forerunner to a table-top positron source appropriate for applications in material science, and fundamental physics research like positronium spectroscopy.

Journal ArticleDOI
TL;DR: Two different previously unobserved decay channels of the collective excitation have been identified, namely, decay into one or several single-particle excitations of the surface plasmon in silver nanoparticles on graphite.
Abstract: Multiphoton photoelectron spectroscopy reveals the multiple excitation of the surface plasmon in silver nanoparticles on graphite. Resonant excitation of the surface plasmon with 400 nm femtosecond radiation allows one to distinguish between photoemission from the nanoparticles and the substrate. Two different previously unobserved decay channels of the collective excitation have been identified, namely, decay into one or several single-particle excitations.

Journal ArticleDOI
TL;DR: In this paper, the spectral hole-burning and carrier heating were investigated in detail in a set of InAs-InGaAs-GaAs quantum-dot (QD) amplifiers with femtosecond resolution.
Abstract: The ultrafast gain and index dynamics in a set of InAs-InGaAs-GaAs quantum-dot (QD) amplifiers are measured at room temperature with femtosecond resolution. The role of spectral hole-burning (SHB) and carrier heating (CH) in the recovery of gain compression is investigated in detail. An ultrafast recovery of the spectral hole within /spl sim/100 fs is measured, comparable to bulk and quantum-well amplifiers, which is contradicting a carrier relaxation bottleneck in electrically pumped QD devices. The CH dynamics in the QD is quantitatively compared with results on an InGaAsP bulk amplifier. Reduced CH for both gain and refractive index dynamics of the QD devices is found, which is a promising prerequisite for high-speed applications. This reduction is attributed to reduced free-carrier absorption-induced heating caused by the small carrier density necessary to provide amplification in these low-dimensional systems.

Journal ArticleDOI
TL;DR: In this paper, the ultrafast dynamics of gain and refractive index in an electrically pumped InAs-InGaAs quantum-dot (QD) optical amplifier are measured at room temperature using differential transmission with femtosecond time resolution.
Abstract: The ultrafast dynamics of gain and refractive index in an electrically pumped InAs-InGaAs quantum-dot (QD) optical amplifier are measured at room temperature using differential transmission with femtosecond time resolution. Both absorption and gain regions are investigated. While the absorption bleaching recovery occurs on a picosecond time scale, the gain compression recovers with /spl sim/100-fs time constant, making devices based on such dots promising for high-speed optical communications.

Journal ArticleDOI
TL;DR: In this article, a near-IR femtosecond laser was used to directly write a 1 cm long optical waveguide in Nd-doped silicate glass, achieving a gain of 1.5 dB/cm at a signal wavelength of 1054 nm.
Abstract: A near-IR (775 nm) femtosecond laser has been used to directly write a 1 cm long optical waveguide in Nd-doped silicate glass. A gain of 1.5 dB/cm was obtained at a signal wavelength of 1054 nm for /spl sim/346 mW of 514 nm pump power, in front of the input coupling objective.