Showing papers on "Femtosecond published in 1989"

Femtosecond pulse generation in a laser with a nonlinear external resonator.

Abstract: A simple model is presented to describe mode locking in a laser coupled to a nonlinear resonator. It reveals a new mechanism for pulse shortening and shows that shortening does not rely on dispersion in the auxiliary cavity. Experimental results are given to support the basic predictions of the model.

Corneal Ablation by Nanosecond, Picosecond, and Femtosecond Lasers at 532 and 625 nm

Abstract: • We produced corneal excisions with nanosecond (ns)-, picosecond-, and femtosecond (fs)-pulsed lasers at visible wavelengths. The threshold energy for ablation was proportional to the square root of the pulse duration and varied from 2.5 microjoules (μJ) at 100 fs to 500 μJ at 8 ns. Excisions made with picosecond and femtosecond lasers were ultrastructurally superior to those made with nanosecond lasers and, at pulse energies near threshold, showed almost as little tissue damage as excisions made with excimer lasers at 193 nm. We conclude that ultra-short-pulsed lasers at visible and near-infrared wavelengths are a possible alternative to excimer lasers for corneal surgery and might have advantages over conventional ophthalmic neodymium-YAG lasers for some intraocular applications.

Distortion of femtosecond laser pulses in lenses

Abstract: Large temporal front distortion of femtosecond pulses occurs in lenses having chromatic aberration. The effect is due to the difference between the phase and group velocities. Equations describing the pulse-front delay in singlet lenses, achromats, and compound lenses are presented. The pulse-front delay is several orders of magnitude larger than the broadening caused by group-velocity dispersion in the lens material. Delays occurring in Fresnel-type zone plates are also described.

Femtosecond photon echoes from molecules in solution.

Abstract: Two pulse photon echoes were observed in organic dye molecules in solution using 6-fs optical pulses. The results show an initial rapid dephasing of the echo due to level multiplicity followed by quantum beaks. The temporal decay of the quantum beats is characterized by a ${\mathrm{T}}_{2}$ of about 60 fs.

Achromatic phase matching for second harmonic generation of femtosecond pulses

Abstract: A scheme is proposed by which the phase matching bandwidth can be increased by adjusting higher-order terms in frequency shift in the phase-matching condition. This is achieved by introducing a spectral angular dispersion so that the different spectral components propagate at their phase-matching angles. This is equivalent to canceling the group velocity mismatch. The equations describing the requirements on the angular dispersion are discussed and applied to the particular case of type I phase matching. A possible experimental setup is shown that would meet all the conditions required. >

Femtosecond passively mode-locked Ti:Al(2)O(3) laser with a nonlinear external cavity.

Abstract: Ultrashort pulses are generated in a Ti:Al2O3 laser by using a coupled nonlinear external cavity. The external cavity uses self-phase modulation in an optical fiber to achieve passive mode locking without the need for synchronous pumping or acousto-optic modulation. A stable train of chirped 1.4-psec pulses is generated. After dispersive compensation, pulses as short as 200 fsec are obtained.

Time resolved propagation of ultrashort laser pulses within turbid tissues

Abstract: The time resolved propagation of femtosecond and picosecond laser pulses within turbid tissues is simulated by a Monte Carlo model. The internal distribution of irradiance for an impulse vs a 4-ps pulse is specified at different times for various scattering coefficients and scattering phase functions. Such simulations provide time resolved dosimetry for predicting the distribution of single- and two-photon chemical reactions in turbid tissues. For femtosecond pulses in highly scattering tissues, two-photon reactions are dominated by the initial primary (unscattered, unabsorbed) pulse, and single-photon reactions are dominated by the scattered diffuse irradiance. For picosecond pulses in highly scattering tissues, both single- and two-photon reactions are dominated by the scattered irradiance.

Femtosecond distributed soliton spectrum in fibers

Abstract: By pumping a fiber in the anomalous group-velocity dispersion regime with a color-center laser, we generate a femtosecond distributed soliton spectrum with τ ≳ 100 fsec covering a wide spectral range between 1.55 < λ < 1.85 μm. We present a theory to explain the spectrum and use autocorrelations and cross correlations to verify the results experimentally. Modulation instability and the soliton self-frequency shift effect initiate a multisoliton collision process that results in narrow, high-intensity solitons. An ensemble average over these solitons, which start from noise and frequency shift by different amounts, yields the observed broad spectra. We use the fiber output in pump–probe experiments, measuring the exciton ionization time in InGaAs/InP multiple quantum wells for the first time to our knowledge.

A quantum mechanical study of predissociation dynamics of NaI excited by a femtosecond laser pulse

Abstract: This work was stimulated by the experiments of Rosker, Rose, and Zewail (RRZ) who used a pump–probe sequence of femtosecond pulses to study the predissociation of NaI. We calculate quantum mechanically the nuclear wave function created by the pump pulse and its subsequent evolution. To make contact with the experiments we assume that, depending on its wavelength, the probe pulse is absorbed by either the free sodium atoms or by the bound molecule in the neutral state, and that the measured fluorescence induced by the probe pulse is proportional to the populations of these two ‘‘species.’’ The similarity between computed populations and the observed signal confirms this conjecture. We study how various factors, such as the pulse length and shape, the initial vibrational state, the temperature, and the diabatic coupling strength affect the populations (and thus the LIF signal). We also show that the RRZ analysis based on classical trajectories and the Landau–Zener formula agrees semiquantitatively with the exact quantum results.

Femtosecond optical nonlinearities of CdSe quantum dots

Abstract: Femtosecond differential absorption measurements of the quantum-confined transitions in CdSe microcrystallites are reported. Spectral hole burning is observed, which is accompanied by an induced absorption feature on the high-energy side. The spectral position of the burned hole depends on the excitation wavelength. For excitation on the low-energy side of the lowest quantum-confined transition, a slight shift of the hole towards the line center is observed. The hole width increases with pump intensity and the magnitude of the induced transparency saturates at the highest excitation level. The results are consistently explained by bleaching of one-pair states and induced absorption caused by the photoexcited two electron-hole pair states. It is concluded that the presence of one electron in the excited state prevents further absorption of photons at the pair-transition energy and accounts for the major portion of the bleaching of the transition. >

Ultrafast pump-probe spectroscopy: femtosecond dynamics in Liouville space

Abstract: A theory for ultrafast pump-probe spectroscopy is developed using a correlation function description of molecular nonlinear optical processes which is based on the density matrix and its evolution in Liouville space. The Liouville space description applies to isolated small molecules as well as complex systems in condensed phases. We identify a condition, called ultrafast dephasing, which allows the probe absorption to be written in terms of an intuitive picture of instantaneous preparation, field-free evolution, and instantaneous detection. The probe absorption is calculated by following a phase-space doorway function which is prepared by the pump, propagates for a specified delay, and is finally projected into a windowfunction which depends on the probe frequency. We find that the doorway and window functions have well-known classical limits: the delta functions in coordinate appearing in the classical Condon approximation. The deviation of the doorway and window functions from their classical limits is a direct and visual measure of the importance of quantum corrections in pumpprobe spectroscopy. The signal consists of a sequential term and a tunneling (coherent) term which are formally analogous to fluorescence and Raman line shapes, respectively. Application is made to the femtosecond photodissociation of ICN, where the importance of quantum corrections and the tunneling term is assessed. The role of dephasing processes and the relation to other spectroscopic techniques related to the nonlinear susceptibility x ( ~ ) are discussed.

Theory for the etching of organic materials by ultraviolet laser pulses

Abstract: A theoretical description of the ultraviolet laser etching process is developed. The threshold for laser ablation is reached when the density of absorbed photons is approximately equal to the density of chromophores in the material. Saturation of the absorption coefficient, absorption by the plume of ablated products, and multiphoton effects are considered. Agreement with all available experimental etch data, including femtosecond ultraviolet laser ablation, is found. The description is based on an analysis of the radiation transport at high intensities and is independent of the question as to whether ultraviolet laser ablation is photochemical or thermal.

Femtosecond pulse generation from a titanium-doped sapphire laser using nonlinear external cavity feedback.

Abstract: A continuous-wave, argon-ion-pumped, titanium-doped sapphire laser has been constructed. Pulses of 80-psec duration obtained through active mode locking have been compressed to less than 800 fsec using nonlinear external cavity feedback.

Femtosecond real‐time probing of reactions. III. Inversion to the potential from femtosecond transition‐state spectroscopy experiments

Abstract: Femtosecond transition-state spectroscopy (FTS) of elementary reactions [M. Dantus, M. J. Rosker, and A. H. Zewail, J. Chem. Phys. 87, 2395 (1987)] provides real-time observations of photofragments in the process of formation. A classical mechanical description of the time-dependent absorption of fragments during photodissociation [R. Bersohn and A. H. Zewail, Ber. Bunsenges. Phys. Chem. 92, 373 (1988)] forms the basis for the present scheme for relating observations to the potential energy surface. A direct inversion scheme is presented that allows the difference in the two relevant excited-state potential curves to be deduced from observed transients at different probe wavelength tunings. In addition, from the shape and dependence of the transients on pump wavelength, information on the lower of the two potential curves (i.e., that of the dissociating molecule) is obtained. The methodology is applied to the experimental FTS data (Dantus et al.) on the CN photofragment from the ICN photodissociation.

Theoretical studies on the femtosecond real‐time measurement of ultrafast electronic decay in polyatomic molecules

Abstract: We present a computer simulation of the real‐time detection of ultrafast electronic decay dynamics in polyatomic molecules with femtosecond laser pulses. The intramolecular non‐Born‐Oppenheimer quantum dynamics is treated numerically exactly for a two‐state three‐mode vibronic coupling model representing the conically intersecting S1 and S2 excited states of pyrazine. The pump–probe signal is evaluated in lowest order perturbation theory with respect to the radiation–matter interaction by numerical integration over the pump and probe pulses. We discuss in some detail the dependence of the pump–probe signal on the properties of the laser pulses (frequencies and pulse durations). The calculations predict a dramatic (∼12 000 cm−1) and ultrafast (∼20 fs) red shift of the stimulated‐emission signal as well as distinctive quantum beats in the pump–probe signal as a function of the delay time. Both effects are very pronounced and should therefore be relatively easily detectable experimentally. They are expected ...

Femtosecond-resolution pulse-front distortion measurement by time-of-flight interferometry.

Abstract: The pulse-front distortion occurring in lenses and lens systems has been measured by a Michelson interferometer. In this technique the plane pulse front from one arm of the interferometer is used as a temporal reference level to map contour lines of equal propagation time on the pulse-front surface. The experimental arrangement is capable of detecting pulse-front distortion with a resolution of 20 fsec, and this can be improved to approximately 1 fsec. The measured value of pulse-front distortion in a telescope (1.1 psec) is in good agreement with the calculated data.

Hot-electron recombination at neutral acceptors in GaAs: A cw probe of femtosecond intervalley scattering.

Abstract: Etude de la diffusion d'electrons chauds de la vallee M vers les vallees L et X dans GaAs en fonction de l'energie de l'electron

Femtosecond studies of electron-cation geminate recombination in water

Abstract: Understanding the properties of water is one of the most fundamental and challenging problems in science. A very interesting example of this is the behavior of electrons in water. This has been a subject of great interest and controversy both experimentally and theoretically for many years. For example, attempts to predict the absorption spectrum of the solvated electron have only been partially successful.' With the development of femtosecond lasers, the dynamics of an electron solvating in neat water have been measured.* Another important phenomena common to all condensed media is geminate recombination of an electrori-cation pair following i~niza t ion .~ In a previous report, we discussed the first femtosecond time-resolved measurements of geminate recombination in a neat alkane at room t e m p e r a t ~ r e . ~ In this Letter, we will discuss what we believe is the first observation of geminate electron-cation recombination in any polar solvent, in this case neat water. The measurements reported here were performed using an amplified colliding pulse mode-locked dye laser (CPM)5 operating at 10 Hz, 625 nm, 300 pJ/pulse, 8C-100 fs fwhm. The 312.5-nm (3.94-eV) pump beam was created by frequency doubling the CPM beam with a 1-mm KD*P crystal. Since the ionization potential for liquid water is 6.5 eV, the neat water can be twophoton ionized! The probe beam was either a small fraction of the remaining fundamental beam or part of a continuum pulse. The signal recorded was the difference of the probe beam and a reference divided by the reference. The water used in these experiments was HPLC grade from Aldrich. Further details will be published at a later date. There have been many experimental studies of electron solvation in watera7 However, until recently, all attempts to measure the solvation dynamics were instrument limited. Wiesenfeld and Ippen obtained an upper limit of 0.3 ps for the electron solvation time in water.* Migus et al. were the first to directly measure the solvation dynamics of an electron in water by photoionizing the solvent. Their work supports the notion of a dry, wet, and solvated electron in water. This is characterized by the appearance of a delayed IR absorption which shifts to the visible with an absorption peak at 720 nm. This entire process occurs in approximately 350 fs. Our measurements at early times are in agreement with their results. However, a t longer times (5-100 ps) our data show dramatic and previously unreported events. There is a decay in the absorption signal which slows down after approximately 60 ps, and it appears that the curve levels off a t approximately 50-60% of the original signal height. Typical data are shown in Figure la-c. Measurements at probe wavelengths of 550,625, and 700 nm show the same behavior. The available evidence suggests that we are monitoring only the solvated electron. First, the signal occurs with a rise time consistent with the solvation time for an electron in water. If there was an excited-state absorption present, it would probably appear instantaneously. Second, the signals at all three probe wavelengths have an identical time dependence. This eliminates the possibility of having another species contributing to the signal unless it obeyed the same rise and decay kinetics as the electron. As the probe wavelength is varied from 550 to 700

Dynamic Stark effect of exciton and continuum states in CdS

Abstract: The first observation of the optical Stark effect of electron-hole continuum states in addition to the bound-exciton states is reported under femtosecond excitation conditions chosen to minimize the generation of real carriers. The experimental results agree well with calculations using the generalized semiconductor Bloch equations. The theory shows that the commonly used adiabatic approximation is correct only for pulses which are longer than the coherence decay time. For shorter pulses coherent dynamic effects strongly influence the spectral changes.

Femtosecond continua produced in gases

Abstract: Self-focusing and continuum generation are explored in gases using ultrashort 625-nm pulses. Observations of spectral broadening and beam propagation in gases were made both below and above the self-focusing threshold. It is shown that the nonlinearity responsible for self-focusing and self-phase modulation saturates at an intensity of approximately 10/sup 13/ W/cm/sup 2/ in xenon. It was found that a diffraction-limited beam can be almost totally reconstructed spatially after passing through the focal region, even though self-focusing has been initiated. The spectrum, however, is catastrophically changed by the process. For intensities significantly above the self-focusing threshold, conical emission was observed. Most of the observations described herein were made using an approximately 90-fs, 625-nm pulse with a maximum energy of approximately 500 mu J. >

Femtosecond all‐optical switching in AlGaAs waveguides using a time division interferometer

Abstract: All‐optical switching of femtosecond pulses in AlGaAs waveguides is investigated using a novel time division interferometric technique which eliminates thermal imbalances. In addition to an instantaneous refractive index nonlinearity, free‐carrier generation via two‐photon absorption produces a response of several hundred picoseconds duration.

Storage and time reversal of femtosecond light signals via persistent spectral hole burning holography

Abstract: Time and space domain holography by persistent spectral hole burning in photochemically active media is shown to permit storage, recall, and conjugation of temporal profiles of light signals as short as 100 fs. This limit for the temporal resolution is set by the finite spectral width of the impurity absorption bands of presently available recording materials.