Femtosecond transform-limited Kerr-lens mode-locked dye lasers

Using SF6 glass plates as intracavity Kerr lenses and double-prism pairs for dispersion compensation, we achieve tunable femtosecond passive mode locking in rhodamine 590 (R6G) and 4-dicyanomethylene-2-methyl-16-p-dimethylaminostyryl-4H-pyran (DCM) dye lasers. The R6G laser produces transform limited 240–500 fs pulses between 577 and 606 nm, and the DCM laser produces 150 fs transform-limited pulses between 650 and 671 nm. We use dilute intracavity saturable-absorber jets to make the mode locking self-starting. Characteristics of the pulses and the stability regions of the lasers agree with general theories of passive mode locking.     

Femtosecond Kerr-lens mode locking with negative nonlinear phase shifts

We report a Kerr-lens mode-locked Cr:forsterite laser operated with negative nonlinear phase shift. The nonlinear phase shift is induced by the cascade chi((2;)):chi((2)) process in a lithium triborate crystal. Employing the cascade process at large phase mismatch produces a nearly linear frequency chirp. Transform-limited pulses as short as 60 fs are generated with positive cavity dispersion.     

Femtosecond autocorrelation measurements based on two-photon photoconductivity in ZnSe

Two-photon photoconductivity in ZnSe is used to record femtosecond autocorrelation functions. This technique requires <100 muW of average power of a typical mode-locked femtosecond Ti:sapphire laser and distinguishes itself by a dynamic range over several decades and great conversion bandwidth, permitting the sensitive correlation of pulses of a few femtoseconds.     

Femtosecond ultraviolet autocorrelation measurements based on two-photon conductivity in fused silica

We utilize the two-photon conductivity of a fused-silica substrate to produce a photoconductive switch for use in an intensity autocorrelator for ultraviolet ultrashort pulses. We perform measurements at 267 nm with pulse durations in the range of 110-330 fs and with energies as weak as 10 nJ. Based on the bandgap of fused silica, this device can potentially operate in the wavelength range of 140-280 nm.     

Femtosecond first-order autocorrelation measurement based one-photon induced photocurrent in Si Schottky diodes

Si Schottky diodes promise to provide cheap, reliable, and linear detectors for use in femtosecond and picosecond pulse width measurement. At low pulse excitation density (<100 pJ/cm²), the pulse width can be deduced from the first-order autocorrelation, and the measured results are in agreement with the theoretical ones. When pulse excitation density is larger than 1 nJ/cm², the pulse trailing edge is distorted due to bandgap reduction in semiconductor Si and the pulse width cannot be measured accurately by means of the first-order autocorrelation. The interference fringes presented in the first-order autocorrelation measurement can provide a way of calibrating the delay, and the pulse width can also be obtained by calculating the number of interference fringes.     

Kerr-lens mode-locking of Nd:glass laser

We report what is to our best knowledge the first Kerr-lens mode-locking of a Nd:silicate glass laser. Pulses as short as 64 fs were generated. For a broadband inhomogeneously broadened laser, the formation of the soliton-like pulses requires a minimum amount of negative group velocity dispersion (GVD), and more negative GVD is needed to have stable, self-sustained mode locking.     

Blue-lines pumped Kerr-lens mode-locked Cr:LiSGaF laser

We report a blue-lines pumped femtosecond Kerr-lens mode-locked Cr:LiSGaF laser based on a tight-focusing geometry. With a combination of 488, 476 and 458 nm lines as a pump source, 58 fs pulses centered at 845 nm were generated. The average output power is 30 mW, and time–band product is 0.331, assuming a sec h² pulse shape. Neither acousto-modulator nor physical aperture or slit were used in this cavity.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Ultrashort-pulse wave-front autocorrelation

Combined spatially resolved collinear autocorrelation and Shack-Hartmann wave-front sensing of femtosecond laser pulses is demonstrated for the first time to our knowledge. The beam is divided into multiple nondiffracting subbeams by thin-film micro-optical arrays. With hybrid refractive-reflective silica/silver microaxicons, wave-front autocorrelation is performed in oblique-angle reflection. Simultaneous two-dimensional detection of local temporal structure and wave-front tilt of propagating few-cycle wave packets is demonstrated.     

Dynamical gain saturation in Kerr-lens mode-locked CW solid- state lasers

It is shown that dynamical gain saturation, usually neglected for solid-state lasers, can contribute significantly to the pulse-shaping mechanism in Kerr-lens mode-locked lasers. In particular, in combination with self-phase modulation it can cause red shifts and blue shifts of the laser spectrum. A control of the reciprocal gain and loss saturation rates using these shifts leads to improved mode- locking stability.     

High-power 200 fs Kerr-lens mode-locked Yb:YAG thin-disk oscillator

We demonstrate a power-scalable Kerr-lens mode-locked Yb:YAG thin-disk oscillator. It delivers 200 fs pulses at an average power of 17 W and a repetition rate of 40 MHz. At an increased (180 W) pump power level, the laser produces 270 fs 1.1 μJ pulses at an average power of 45 W (optical-to-optical efficiency of 25%). Semiconductor-saturable-absorber-mirror-assisted Kerr-lens mode locking (KLM) and pure KLM with a hard aperture show similar performance. To our knowledge, these are the shortest pulses achieved from a mode-locked Yb:YAG disk oscillator and this is the first demonstration of a Kerr-lens mode-locked thin-disk laser.     

Diode-pumped Kerr-lens mode-locked femtosecond Yb:YAG ceramic laser

We experimentally demonstrated a diode-pumped Kerr-lens mode-locked femtosecond laser based on an Yb:YAG ceramic. Stable laser pulses with 97-fs duration, 2.8-nJ pulse energy, and 320-mW average power were obtained. The femtosecond oscillator operated at a central wavelength of 1049 nm and a repetition rate of 115 MHz. To the best of our knowledge, this is the first demonstration of a Kerr-lens mode-locked operation in a diode-pumped Yb:YAG ceramic laser with sub-100 fs pulse duration.     

Directly diode-pumped Kerr-lens mode-locked Cr4+:YAG laser

A directly diode-pumped Kerr-lens mode-locked Cr4+:YAG laser is demonstrated for what is to our knowledge the first time. Pulses as short as 65 fs with up to 30 mW of average output power, at a central wavelength of 1569 nm, were obtained at a repetition rate of 100 MHz. Low-loss chirped mirrors have been used for dispersion compensation up to the third order. Comparison with an Yb-fiber-pumped configuration shows good prospects for improvement.     

Sub-two-cycle pulses from a Kerr-lens mode-locked Ti:sapphire laser

Pulses shorter than two optical cycles with bandwidths in excess of 400 nm have been generated from a Kerr-lens mode-locked Ti:sapphire laser with a repetition rate of 90 MHz and an average power of 200 mW. Low-dispersion prisms and double-chirped mirrors provide broadband controlled dispersion and high reflectivity. These pulse durations are to our knowledge the shortest ever generated directly from a laser oscillator.     

Diode-pumped Kerr-lens mode-locked Ti: sapphire laser with broad wavelength tunability

We report a direct blue-diode-pumped wavelength tunable Kerr-lens mode-locked Ti: sapphire laser.Central wavelength tunability as broad as 89 nm(736–825 nm) is achieved by adjusting the insertion of the prism.Pulses as short as 17 fs are generated at a central wavelength of 736 nm with an average output power of 31 mW.The maximum output power is 46.8 mW at a central wavelength of 797 nm with a pulse duration of 46 fs.     

Femtosecond X-Ray fluorescence

Using few-cycle-driven coherent laser harmonics, K-shell vacancies have been created in light elements, such as boron (E(B) = 188 eV) and carbon (E(B) = 284 eV), on a time scale of a few femtoseconds for the first time. The capability of detecting x-ray fluorescence excited by few-femtosecond radiation with an accuracy of the order of 1 eV paves the way for probing the evolution of the microscopic environment of selected atoms in chemical and biochemical reactions on previously inaccessible time scales (<100 fs) by tracing the temporal evolution of the "chemical shift" of peaks associated with inner-shell electronic transitions in time-resolved x-ray fluorescence and photoelectron spectra.     

Sub-10-fs Ti:sapphire Kerr-lens Mode Locked Oscillator

Assembly of a sub-10-fs Ti:sapphire oscillator using only optical components commercially available in catalogues is reported. It was demonstrated that stable sub-10-fs optical pulses near 800 nm can be generated from a Kerr-lens mode locked oscillator equipped with a pair of prisms to compensate group delay dispersions (GDD) in the cavity. For the GDD control in a 10fs-pulse oscillator it was concluded that chirped mirrors are not always necessary, and a pair of prisms is still available.     

Femtosecond pulse processing

We discuss Fourier methods for shaping and processing femtosecond optical signals. Examples of applications in optical communications and in generation of terahertz radiation are presented.     

Femtosecond photoemission microscopy

We developed a novel experiment for time-resolved photoemission microscopy by combining a commercial photoemission electron microscope (PEEM) with a pulsed Ti:sapphire laser oscillator. The laser system, the setup of the delay stage for pump-probe experiments, and the interface between the PEEM and the laser system are discussed. We use self-organization of Ag islands and nanowires on Si(1 1 1) and 4° vicinal Si(0 0 1) to generate structures with a plasmon resonance that matches the photon energy of our laser (?ω = 3.1 eV after frequency doubling). In two-photon photoemission (2PPE) the photoemission yield then directly visualizes the plasmons in the nanostructures. Accordingly, the photoemission yield depends on the size and shape of the nanostructures, and on the polarization of the laser pulses as well. In Ag nanowires, we observe surface plasmon polariton (SPP) waves by a beating that is formed by interference of the SPP wave and the incident laser light. In a pump-probe experiment, we can directly visualize the propagation of the SPP on a femtosecond time scale.     

Serial Femtosecond Crystallography

X-ray free-electron lasers produce brief flashes of X-rays that are of about a billion times higher peak brightness than achievable from storage ring sources. Such a tremendous jump in X-ray source capabilities, which came in 2009 when the Linac Coherent Light Source began operations, was unprecedented in the history of X-ray science. Protein structure determination through the method of macromolecular crystallography has consistently benefited from the many increases in source performance from rotating anodes to all generations of synchrotron facilities. But when confronted with the prospects of such bright beams for structural biology, enthusiastic proposals were tempered by trepidation of the effects of such beams on samples and challenges to record data [1 M. Wilmanns, J. Synchr. Rad. 7, 41 (2000).[Crossref], [PubMed] , [Google Scholar]]. A decade after these discussions (and others in the USA) on the applications of X-ray FELs for biology, the first experiments took place at LCLS, giving results that fulfilled many of the dreams of the early visionaries. In particular, the concept that diffraction representing the pristine object could be recorded before the X-ray pulse completely vaporizes the object was validated [2 H.N. Chapman, Nature 470, 73 (2011).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]], confirming predictions [3 R. Neutze, Nature 406, 753 (2000).[Crossref], [Web of Science ®] , [Google Scholar]] that established dose limits could be vastly exceeded using femtosecond-duration pulses. The first experiments illuminated a path to achieve room-temperature structures free of radiation damage, from samples too small to provide useful data at synchrotron facilities, as well as providing the means to carry out time-resolved crystallography at femtoseconds to milliseconds. In the five years since, progress has been substantial and rapid, invigorating the field of macromolecular crystallography [4 J.C.H. Spence and H.N. Chapman, Phi. Trans. Roy. Soc. B 369, 20130309 (2014).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar], 5 I. Schlichting, IUCrJ 2, 246 (2015).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]]. This phase of development is far from over, but with both the LCLS and the SPring-8 Ångström Compact Free-electron Laser (SACLA) providing facilities for measurements, the benefits of X-ray FELs are already being translated into new biological insights.