Attosecond science is based on steering electrons with the electric field of well controlled femtosecond laser pulses. It has led to the generation of extreme-ultraviolet pulses with a duration of less than 100 attoseconds (ref. 3; 1 as = 10(-18) s), to the measurement of intramolecular dynamics (by diffraction of an electron taken from the molecule under scrutiny) and to ultrafast electron holography. All these effects have been observed with atoms or molecules in the gas phase. Electrons liberated from solids by few-cycle laser pulses are also predicted to show a strong light-phase sensitivity, but only very small effects have been observed. Here we report that the spectra of electrons undergoing photoemission from a nanometre-scale tungsten tip show a dependence on the carrier-envelope phase of the laser, with a current modulation of up to 100 per cent. Depending on the carrier-envelope phase, electrons are emitted either from a single sub-500-attosecond interval of the 6-femtosecond laser pulse, or from two such intervals; the latter case leads to spectral interference. We also show that coherent elastic re-scattering of liberated electrons takes place at the metal surface. Owing to field enhancement at the tip, a simple laser oscillator reaches the peak electric field strengths required for attosecond experiments at 100-megahertz repetition rates, rendering complex amplified laser systems dispensable. Practically, this work represents a simple, extremely sensitive carrier-envelope phase sensor, which could be shrunk in volume to about one cubic centimetre. Our results indicate that the attosecond techniques developed with (and for) atoms and molecules can also be used with solids. In particular, we foresee subfemtosecond, subnanometre probing of collective electron dynamics (such as plasmon polaritons) in solid-state systems ranging in scale from mesoscopic solids to clusters and to single protruding atoms.
We present energy-resolved measurements of electron emission from sharp metal tips driven with low energy pulses from a few-cycle laser oscillator. We observe above-threshold photoemission with a photon order of up to 9. At a laser intensity of ∼ 2 × 10(11) W/cm2 the suppression of the lowest order peak occurs, indicating the onset of strong-field effects. We also observe peak shifting linearly with intensity, with a slope of around -1.0 eV/(10(12) W/cm2). We attribute the magnitude of the laser field effects to field enhancement taking place at the tip's surface.
Nanometre-scale metal tips irradiated by femtosecond laser pulses represent ultrafast electron sources. The combination of the laser pulse and the tip offers the possibility of extending attosecond science from atomic or molecular gases to surfaces of solid nanoemitters. We first review this emerging research field focusing on electron rescattering at sharp metal tips. In particular, we investigate the carrier–envelope phase effects that reveal attosecond emission dynamics. Furthermore, we present detailed theory models that support this interpretation.
We present a method which delivers a continuous, high-density beam of slow and internally cold polar molecules. In our source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electrostatic quadrupole guide. For ND3 the source produces fluxes up to (7± 7 4 ) × 10 10 molecules/s with peak densities up to (1.0± 1.0 0.6 ) × 10 9 molecules/cm 3 . For H2CO the population of rovibrational states is monitored by depletion spectroscopy, resulting in single-state populations up to (82 ± 10)%.
We discuss the interaction of ultrashort near-infrared laser pulses with sharp metal tips at moderate nominal intensities (I 0 ∼ 10 11 W cm −2 ). As external electric fields are strongly enhanced at such tips (enhancement factor ∼10) our system turns out to be an ideal miniature laboratory to investigate strong-field effects at solid surfaces. We analyse the electron-energy spectra as a function of the strength of the laser field and the static extraction field and present an intuitive model for their interpretation. The size of the effective field acting on the metal electrons can be determined from the electron spectra. The latter are also reproduced by time-dependent density functional theory (TDDFT) simulations.The emerging field of attosecond science is based on the ability to control electronic wavepackets with the help of well-controlled few-cycle laser pulses [1]. The key effect is the recollision of an electronic wavepacket with the parent matter within a fraction of the laser cycle after its generation through ionization [2,3]. Prominent dynamical processes based on recollision are high-harmonic generation (HHG) and high-order above-threshold ionization (ATI), observed with atomic and molecular gases. The exquisite control over the recolliding wavepacket allows one to take, for example, diffraction images of the parent molecule [4,5] and to achieve attosecond time resolution for electronic processes [6][7][8]. Recently, laser-electricfield-driven recollision processes were also observed with solid-state systems, namely at dielectric nanoparticles [9] and at sharp metal tips [10,11].For versatile imaging techniques applied to atomic, molecular and solid-state systems it would be of great interest to vary the parameters of the returning electron over a large energy range. The same holds true in the process of the generation of high-harmonic photons, which is currently being optimized to match certain flux and energy requirements (see, e.g., [12,13]). Improving and extending the ability to control the motion of the electronic wavepacket requires additional control knobs for the strong fields driving the wavepacket. One obvious choice would be the addition of a static (dc) electric field. Theory suggests [14][15][16] that the application of relatively small static fields, one to four orders of magnitude weaker than the maximum laser field, already leads to a modification of the path of the recolliding wavepacket and, hence, to changes of the resulting HHG spectrum. For example, the maximum HHG energy could be tripled if a static field with a field strength 10% that of the maximum laser field was applied [16]. Although considerably smaller than the amplitude of the moderately strong laser itself, application of such a (macroscopic) static field with a strength of a fraction of a GV m −1 is unfeasible, rendering this control tool exceedingly difficult to employ in laser-atom or laser-molecule interactions.When nanostructures are irradiated with laser light, optical field enhancement takes place. The laser ele...
We present a new method of measuring optical near-fields within ∼1 nm of a metal surface, based on rescattering of photoemitted electrons. With this method, we precisely measure the field enhancement factor for tungsten and gold nanotips as a function of tip radius. The agreement with Maxwell simulations is very good. Further simulations yield a field enhancement map for all materials, which shows that optical near-fields at nanotips are governed by a geometric effect under most conditions, while plasmon resonances play only a minor role. Last, we consider the implications of our results on quantum mechanical effects near the surface of nanostructures and discuss features of quantum plasmonics.The excitation of enhanced optical near-fields at nanostructures allows the localization of electromagnetic energy on the nanoscale [1,2]. At nanotips, this effect has enabled a variety of applications, most prominent amongst them are scanning near-field optical microscopy (SNOM) [3][4][5][6][7], which has reached a resolving power of 8 nm [8], and tip-enhanced Raman spectroscopy (TERS) [3,9]. Because of the intrinsic nanometric length scale, measuring and simulating the tips' near-field has proven hard and led to considerably diverging results (see Refs. [1,7] for overviews). Here we demonstrate a nanometric field sensor based on electron rescattering, a phenomenon well known from attosecond science [10]. It allows measurement of optical near-fields, integrating over only 1 nm right at the structure surface, close to the length scale where quantum mechanical effects become relevant [11][12][13][14][15]. Hence, this method measures near-fields on a scale that is currently inaccessible to other techniques (such as SNOM or plasmonic methods in electron microscopy [16][17][18]), and reaches down to the minimum length scale where one can meaningfully speak about a classical field enhancement factor. In the future, the method will allow tomographic reconstruction of the optical near-field and potentially the sensing of fields in more complex geometries such as bow-tie or split-ring antennas.In general, three effects contribute to the enhancement of optical electric fields at structures that are smaller than the driving wavelength [7,[20][21][22]. The first effect is geometric in nature, similar to the electrostatic lightning rod effect: the discontinuity of the electric field at the material boundary and the corresponding accumulation of surface charges lead to an enhanced near-field at any sharp protrusion or edge. This effect causes singularities in the electric field at ideal edges of perfect conductors. For real materials at optical frequencies, the electric field is not as strongly enhanced and remains finite [23]. The second effect occurs at structures whose size is an odd multiple of half the driving wavelength: optical antenna resonances can be observed there. The third effect concerns only plasmonic materials like gold and silver, where an enhanced electric field can arise due to a localized surface plasmon resonance. ...
We present measurements of the internal state distribution of electrostatically guided formaldehyde. Upon excitation with continuous tunable ultraviolet laser light the molecules dissociate, leading to a decrease in the molecular flux. The population of individual guided states is measured by addressing transitions originating from them. The measured populations of selected states show good agreement with theoretical calculations for different temperatures of the molecule source. The purity of the guided beam as deduced from the entropy of the guided sample using a source temperature of 150 K corresponds to that of a thermal ensemble with a temperature of about 30 K.
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