R. Holzwarth

Attosecond control of electronic processes by intense light fields.

The amplitude and frequency of laser light can be routinely measured and controlled on a femtosecond (10-15 s) timescale. However, in pulses comprising just a few wave cycles, the amplitude envelope and carrier frequency are not sufficient to characterize and control laser radiation, because evolution of the light field is also influenced by a shift of the carrier wave with respect to the pulse peak. This so-called carrier-envelope phase has been predicted and observed to affect strong-field phenomena, but random shot-to-shot shifts have prevented the reproducible guiding of atomic processes using the electric field of light. Here we report the generation of intense, few-cycle laser pulses with a stable carrier envelope phase that permit the triggering and steering of microscopic motion with an ultimate precision limited only by quantum mechanical uncertainty. Using these reproducible light waveforms, we create light-induced atomic currents in ionized matter; the motion of the electronic wave packets can be controlled on timescales shorter than 250 attoseconds (250 × 10-18 s). This enables us to control the attosecond temporal structure of coherent soft X-ray emission produced by the atomic currents—these X-ray photons provide a sensitive and intuitive tool for determining the carrier-envelope phase.

Attosecond spectroscopy in condensed matter

Comprehensive knowledge of the dynamic behaviour of electrons in condensed-matter systems is pertinent to the development of many modern technologies, such as semiconductor and molecular electronics, optoelectronics, information processing and photovoltaics. Yet it remains challenging to probe electronic processes, many of which take place in the attosecond (1 as = 10-18 s) regime. In contrast, atomic motion occurs on the femtosecond (1 fs = 10-15 s) timescale and has been mapped in solids in real time using femtosecond X-ray sources. Here we extend the attosecond techniques previously used to study isolated atoms in the gas phase to observe electron motion in condensed-matter systems and on surfaces in real time. We demonstrate our ability to obtain direct time-domain access to charge dynamics with attosecond resolution by probing photoelectron emission from single-crystal tungsten. Our data reveal a delay of approximately 100 attoseconds between the emission of photoelectrons that originate from localized core states of the metal, and those that are freed from delocalized conduction-band states. These results illustrate that attosecond metrology constitutes a powerful tool for exploring not only gas-phase systems, but also fundamental electronic processes occurring on the attosecond timescale in condensed-matter systems and on surfaces.

Frequency Comparison and Absolute Frequency Measurement of I2-stabilized Lasers at 532 nm

We present a frequency comparison and an absolute frequency measurement of two independent I2-stabilized frequency-doubled Nd:YAG lasers at 532 nm, one set up at the Institute of Laser Physics, No vosibirsk, Russia, the other at the Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. The absolute frequency of the I2-stabilized lasers was determined using a CH4-stabilized He-Ne laser as a reference. This laser had been calibrated prior to the measurement by an atomic cesium fountain clock. The frequency chain linking phase-coherently the two frequencies made use of the frequency comb of a Kerr-lens mode-locked Ti:sapphire femtosecond laser where the comb mode separation was controlled by a local cesium atomic clock. A new value for the R(56)32-0:a10 component, recommended by the Comite International des Poids et Mesures (CIPM) for the realization of the metre [1], was obtained with reduced uncertainty. Absolute frequencies of the R(56)32-0 and P(54)32-0 iodine absorp tion lines together with the hyperfine line separations were measured.

Imaging ex vivo and in vitro brain morphology in animal models with ultrahigh resolution optical coherence tomography

The feasibility of ultrahigh resolution optical coherence tomography (UHR OCT) to image ex vivo and in vitro brain tissue morphology on a scale from single neuron cells to a whole animal brain was investigated using a number of animal models. Sub-2-microm axial resolution OCT in biological tissue was achieved at different central wavelengths by separately interfacing two state-of-the-art broad bandwidth light sources (titanium:sapphire, Ti:Al2O3 laser, lambdac=800 nm, Deltalambda=260 nm, Pout=50 mW and a fiber laser light source, lambdac=1350 nm, Deltalambda=470 nm, Pout=4 mW) to free-space or fiber-based OCT systems, designed for optimal performance in the appropriate wavelength regions. The ability of sub-2-microm axial resolution OCT to visualize intracellular morphology was demonstrated by imaging living ganglion cells in cultures. The feasibility of UHR OCT to image the globular structure of an entire animal brain as well as to resolve fine morphological features at various depths in it was tested by imaging a fixed honeybee brain. Possible degradation of OCT axial resolution with depth in optically dense brain tissue was examined by depositing microspheres through the blood stream to various depths in the brain of a living rabbit. It was determined that in the 1100 to 1600-nm wavelength range, OCT axial resolution was well preserved, even at depths greater than 500 microm, and permitted distinct visualization of microspheres 15 microm in diameter. In addition, the OCT image penetration depth and the scattering properties of gray and white brain matter were evaluated in tissue samples from the visual cortex of a fixed monkey brain.

Absolute frequency measurement of the 115In+ 5S2 1S0-5S5p 3P0 transition

Abstract We have measured the absolute frequency of the 115 In+ 5s 2 1 S0–5s5p 3 P0 clock transition at 236.5 nm with an accuracy of 3.3 parts in 1011. For this measurement, a frequency synthesis chain was used which links the indium clock transition to a methane-stabilized He–Ne laser at 3.39 μm and a Nd:YAG laser at 1064 nm whose second harmonic was locked to a hyperfine component in molecular iodine. A frequency gap in the chain of 1.43 THz at 850 nm was bridged with the help of an optical frequency comb generator. The frequency of the 115 In+ clock transition was determined to 1 267 402 452 914 (41) kHz, where the accuracy is limited by the uncertainty of the iodine reference. This measurement represents an improvement of more than three orders of magnitude in accuracy compared to previous measurements of the line.

Performance of a laser frequency comb calibration system with a high-resolution solar echelle spectrograph

Laser frequency combs (LFC) provide a direct link between the radio frequency (RF) and the optical frequency regime. The comb-like spectrum of an LFC is formed by exact equidistant laser modes, whose absolute optical frequencies are controlled by RF-references such as atomic clocks or GPS receivers. While nowadays LFCs are routinely used in metrological and spectroscopic fields, their application in astronomy was delayed until recently when systems became available with a mode spacing and wavelength coverage suitable for calibration of astronomical spectrographs. We developed a LFC based calibration system for the high-resolution echelle spectrograph at the German Vacuum Tower Telescope (VTT), located at the Teide observatory, Tenerife, Canary Islands. To characterize the calibration performance of the instrument, we use an all-fiber setup where sunlight and calibration light are fed to the spectrograph by the same single-mode fiber, eliminating systematic effects related to variable grating illumination.

Comment on: “Lorentz violation in high-energy ions” by Santosh Devasia

In an article ”Missing Transverse-Doppler Effect in Time-Dilation Experiments with High-Speed Ions” by S. Devasia [arXiv:1003.2970v1], our recent Doppler shift experiments on fast ion beams are reanalyzed. Contrary to our analysis, Devasia concludes that our results provide an ”indication of Lorentz violation”. We argue that this conclusion is based on a fundamental misunderstanding of our experimental scheme and reiterate that our results are in excellent agreement with Special Relativity. We have performed experiments of the Ives-Stilwell (IS) type [1] that test time dilation of Special Relativity (SR) via the relativistic Doppler shift [2,3,4,5]. A beam of ions, which exhibit an optical transition with a frequency ν0 in their rest frame, is stored at velocity β = v/c in a storage ring. To resonantly excite these ions by a laser at rest in the laboratory frame, the frequency ν of the laser needs to be Doppler shifted according to ν = ν0/γ(1 − β cos θ), where θ is the angle between the laser and the ion beam, measured in the laboratory frame, and γ governs time dilation. For a parallel (θp = 0) or an antiparallel (θa = π) laser beam the frequencies required are νp,a = ν0/γ(1 ∓ β), respectively. Multiplying these two frequencies and using γ = (1 − β) as predicted by SR results in νpνa/ν 2 0 = 1, (1) i.e. the geometric mean of the Doppler shifted frequencies equals the rest frame frequency for all velocities β. In one of our implementations of the IS experiment saturation spectroscopy is used by overlapping simultaneously a parallel and antiparallel laser beam with the ion beam to select a narrow velocity class β0 within the ions’ velocity distribution. The parallel laser is held fixed at the laser frequency νp = ν0/γ(1 − β0) and is resonant with ions at β0, while the other laser is scanned over the velocity distribution. The fluorescence yield, measured with a photomultiplier (PMT) located around 90 degree with respect to the ion beam, will exhibit a minimum (a Lamb dip) when the antiparallel laser talks to the same velocity class β0, i.e. when its frequency is at νa = ν0/γ(1 + β0). SR thus predicts the Lamb dip to occur when Eq. 1 is fulfilled, which is shown to be confirmed by our experiments to an accuracy of < 2 × 10 on Li ions at β0 = 0.03 and β0 = 0.06 [3]. S. Devasia [6] claims that the Doppler shift of the emitted light has to be taken into account and replaces ν0 in Eq. 1 by γν0, i.e. by the frequency of the light detected exactly at θ = π/2. This is a misconception of our experimental measurement scheme. While it is true that the detected light is Doppler-shifted, this Doppler shift is irrelevant for the analysis. Neither do we measure the frequency ν0 β 0 PMT ν a =ν 0 /γ(1+β 0 ) IF ν p =ν 0 /γ(1−β 0 ) of the emitted light nor do we intend to observe at exactly right angle. We only record the number of re-emitted photons as a function of the scanning laser frequency to monitor the Lamb dip caused by the simultaneous resonance of both lasers with the same ions. Thus the angle of detection is irrelevant but θ ≈ π/2 helps to separate fluorescence from laser stray light. In fact, stray light suppression is the only reason for using an interference filter (IF) in front of the PMT; its transmission width of 10 nm corresponds to 10 THz, about 10 times broader than the width of the Lamb dip, and a factor of 10 larger than the transverse Doppler shift (at β = 0.064). None of the filters employed in our experiments [2,3,4,5] to improve the signal-to-noise ratio in the fluorescence light detection are affecting the shape and position of the signal indicating the resonance of the parallel and antiparallel laser with the same velocity class β0. The frequency ν0 occurring in Eq. 1 has nothing to do with the frequency of the emitted light in our experiment, but is the rest frame frequency ν0 deduced from experiments at smaller ion velocities [3,7]. In conclusion, SR predicts Eq. 1 as the outcome of our experiments, which is confirmed with high accuracy.

Phase-coherent frequency comparison of optical clocks using a telecommunication fiber link

We have explored the performance of two ldquodark fibersrdquo of a commercial telecommunication fiber link for a remote comparison of optical clocks. These fibers establish a network in Germany that will eventually link optical frequency standards at PTB with those at the Institute of Quantum Optics (IQ) at the Leibniz University of Hanover, and the Max Planck Institutes in Erlangen (MPL) and Garching (MPQ). We demonstrate for the first time that within several minutes a phase coherent comparison of clock lasers at the few 10-15 level can also be accomplished when the lasers are more than 100 km apart. Based on the performance of the fiber link to the IQ we estimate the expected stability for the link from PTB to MPQ via MPL that bridges a distance of approximately 900 km.

erratum: Attosecond control of electronic processes by intense light fields

This corrects the article DOI: nature01414

Terabit/s data transmission using optical frequency combs

Terabit/s interconnects rely on advanced wavelength-division multiplexing (WDM) schemes. However, while efficient photonic-electronic interfaces can be efficiently realized on silicon-on-insulator chips, dense integration of WDM laser sources still represents a major challenge. Chip-scale frequency comb sources are an attractive alternative for providing optical carriers for WDM transmission. In this paper we give an overview on our recent work towards terabit/s data transmission using optical frequency combs. We demonstrate transmission of a 32.5 Tbit/s data stream using a modelocked solid-state laser as an optical source. Our current experiments aim at transmission schemes that exploit Kerr nonlinearities in high-Q microresonators for frequency comb generation.