Transport Coefficients from Large Deviation Functions

We describe a method for computing transport coefficients from the direct evaluation of large deviation function. This method is general, relying on only equilibrium fluctuations, and is statistically efficient, employing trajectory based importance sampling. Equilibrium fluctuations of molecular currents are characterized by their large deviation functions, which is a scaled cumulant generating function analogous to the free energy. A diffusion Monte Carlo algorithm is used to evaluate the large deviation functions, from which arbitrary transport coefficients are derivable. We find significant statistical improvement over traditional Green-Kubo based calculations. The systematic and statistical errors of this method are analyzed in the context of specific transport coefficient calculations, including the shear viscosity, interfacial friction coefficient, and thermal conductivity.Comment: 11 pages, 5 figure

Media 1: All-optical Q-switching limiter for high-power gigahertz modelocked diode-pumped solid-state lasers

Originally published in Optics Express on 06 April 2015 (oe-23-7-8532

Ultrafast 550-W average-power thin-disk laser oscillator

We present a SESAM modelocked ultrafast thin-disk laser oscillator providing 550 W of average output power with 852 fs pulses at 5.5 MHz repetition rate. To the best of our knowledge, this represents the highest average output power ever achieved from a modelocked oscillator. To reach this significant power scaling, a new replicating cavity design for modelocked oscillators is utilized. The oscillator delivers 103 MW of peak power with a pulse energy of 100 µJ at a beam quality of M²< 1.2, with a high optical-to-optical efficiency of 35%. As well as providing record average output power, this oscillator thus also provides, to the best of our knowledge, the highest pulse energy from any modelocked oscillator and the highest peak power from any SESAM modelocked oscillator. Advances in SESAM design and manufacturing that enabled this result are discussed, as well as practical challenges when scaling oscillators to the kW-class

Supplement 1: Compact extreme ultraviolet source at megahertz pulse repetition rate with a low-noise ultrafast thin-disk laser oscillator

Originally published in Optica on 20 November 2015 (optica-2-11-980

Supplementary document for Ultrafast Yb:YAG laser oscillator with gigahertz repetition rate - 6577590.pdf

Overview of gigahertz solid-state laser

Ultrafast Relaxation Dynamics of the Ethylene Cation C₂H₄⁺

We present a combined experimental and computational study of the relaxation dynamics of the ethylene cation. In the experiment, we apply an extreme-ultraviolet-pump/infrared-probe scheme that permits us to resolve time scales on the order of 10 fs. The photoionization of ethylene followed by an infrared (IR) probe pulse leads to a rich structure in the fragment ion yields reflecting the fast response of the molecule and its nuclei. The temporal resolution of our setup enables us to pinpoint an upper bound of the previously defined ethylene–ethylidene isomerization time to 30 ± 3 fs. Time-dependent density functional based trajectory surface hopping simulations show that internal relaxation between the first excited states and the ground state occurs via three different conical intersections. This relaxation unfolds on femtosecond time scales and can be probed by ultrashort IR pulses. Through this probe mechanism, we demonstrate a route to optical control of the important dissociation pathways leading to separation of H or H₂

Ultrafast Relaxation Dynamics of the Ethylene Cation C₂H₄⁺

We present a combined experimental and computational study of the relaxation dynamics of the ethylene cation. In the experiment, we apply an extreme-ultraviolet-pump/infrared-probe scheme that permits us to resolve time scales on the order of 10 fs. The photoionization of ethylene followed by an infrared (IR) probe pulse leads to a rich structure in the fragment ion yields reflecting the fast response of the molecule and its nuclei. The temporal resolution of our setup enables us to pinpoint an upper bound of the previously defined ethylene–ethylidene isomerization time to 30 ± 3 fs. Time-dependent density functional based trajectory surface hopping simulations show that internal relaxation between the first excited states and the ground state occurs via three different conical intersections. This relaxation unfolds on femtosecond time scales and can be probed by ultrashort IR pulses. Through this probe mechanism, we demonstrate a route to optical control of the important dissociation pathways leading to separation of H or H₂

Shot-noise limited dual-comb supercontinuum

Dual-comb supercontinuum (SC) sources are promising for metrology and spectroscopy applications as their broad bandwidth supports the detection of multiple spectral features simultaneously. However, the limited sensitivity inherent to their high relative intensity noise (RIN) and low power per comb line so far hindered their huge potential in those fields. In this work, we overcome both of these issues with the first shot-noise limited dual-comb SC with gigahertz pulse repetition rate and >1 W output power. It is based on a high-power single-cavity dual-comb Yb:CALGO oscillator centered at 1053 nm, combined with a single polarization-maintaining all-normal-dispersion (ANDi) fiber for spectral broadening. The resulting SC spans 820 nm-1280 nm and has a gigahertz pulse repetition rate enabling high power per comb line and sufficient resolution in the optical domain for most spectroscopy applications. The SC exhibits a shot-noise limited spectrally-resolved RIN power spectral density for all spectral bands, including the spectral wings of the SC and record-low integrated RIN down to 2.7 × 10⁵ for the spectral band at 1100 nm ± 8 nm for the integration range [1 kHz, 10 MHz]. This exceptional performance originates from the pump laser's low noise properties and its high output power which is sufficient to drive the SC process directly without amplification, in combination with the unprecedented noise-suppression in the ANDi fiber reaching up to >20 dB around the oscillator wavelength. To better understand the observed noise-suppression mechanisms, we perform a numerical simulation of the RIN which is in excellent agreement with our measurements. We further analyze a dual-comb interferometry measurement with this source at a repetition rate difference of ~3.95 kHz, which supports the resolution of the entire SC spectrum in parallel without spectral aliasing. The dual-comb spectroscopy figure of merit (FOM) is >1.1 × 10⁷ Hz½ for all spectral bands, making it suitable for high-sensitivity applications. Our measurements further show that recording the entire SC spectrum at once with around 30 parallel detectors would yield an exceptionally high FOM of around 5 × 10⁸ Hz½

Supplement 1: Energy-dependent photoemission delays from noble metal surfaces by attosecond interferometry

Originally published in Optica on 20 May 2015 (optica-2-5-405

Media 1: Ultrafast resolution of tunneling delay time

Originally published in Optica on 20 November 2014 (optica-1-5-343