Using a simple model of strong-field ionization of atoms that generalizes the well-known 3-step model from 1D to 3D, we show that the experimental photoelectron angular distributions resulting from laser ionization of xenon and argon display prominent structures that correspond to electrons that pass by their parent ion more than once before strongly scattering. The shape of these structures can be associated with the specific number of times the electron is driven past its parent ion in the laser field before scattering. Furthermore, a careful analysis of the cutoff energy of the structures allows us to experimentally measure the distance between the electron and ion at the moment of tunnel ionization. This work provides new physical insight into how atoms ionize in strong laser fields and has implications for further efforts to extract atomic and molecular dynamics from strong-field physics. DOI: 10.1103/PhysRevLett.109.073004 PACS numbers: 32.80.Fb, 32.80.Rm, 34.80.Qb When an atom or molecule is illuminated with a moderately intense femtosecond laser field ($ 10 14 W=cm 2 ), an electron wave packet will tunnel ionize and accelerate in the field before being turned around by the field and returning to the parent ion. The returning electron can either recombine with the parent ion, releasing its kinetic energy as a high-energy photon [1-3], or can elastically scatter from the potential of the ion. The photons and electrons generated by these strong-field processes have the potential to probe the dynamic structure of molecules and materials on the subnanometer length scale and femtosecond-to-attosecond time scale. Several recent papers have suggested that structures seen in angledependent photoelectron spectra may be useful for determining time-resolved molecular structures [4], characterizing attosecond electron wave packets [5], and studying the dynamics of electron wave packet propagation [6]. However, despite extensive analyses [7][8][9][10][11], many features observed in angle-resolved photoelectron spectra still lack a simple physical explanation.The recent development of midinfrared (mid-IR) femtosecond lasers [12] and angle-resolved detection schemes [13] has enabled new advances in visualizing strong-field physics. Electrons that are ionized in a mid-IR laser field reach higher velocities because of the larger ponderomotive energy, given by U P / I 2 , where I is the intensity and is the wavelength. The possibility of harnessing the high-energy electrons that are first ionized and then driven back to a molecule by a strong laser field has inspired several theoretical and experimental efforts to use strongfield ionization to probe molecular structure [4,[14][15][16]. Recently, Huismans and co-workers [17] used 7 m mid-IR lasers, in combination with angle-resolved detection, to observe angular interference structures in the photoelectron spectra. They presented a theoretical model that explains these structures based on the difference in the phase between two different paths that electrons can take to...

The ability to visualize neurons inside living brain tissue is a fundamental requirement in neuroscience and neurosurgery. Especially the development of a noninvasive probe of brain morphology with micrometer-scale resolution is highly desirable, as it would provide a noninvasive approach to optical biopsies in diagnostic medicine. Two-photon laser-scanning microscopy (2PLSM) is a powerful tool in this regard, and has become the standard for minimally invasive high-resolution imaging of living biological samples. However, while 2PLSM-based optical methods provide sufficient resolution, they have been hampered by the requirement for fluorescent dyes to provide image contrast. Here we demonstrate high-contrast imaging of live brain tissue at cellular resolution, without the need for fluorescent probes, using optical third-harmonic generation (THG). We exploit the specific geometry and lipid content of brain tissue at the cellular level to achieve partial phase matching of THG, providing an alternative contrast mechanism to fluorescence. We find that THG brain imaging allows rapid, noninvasive label-free imaging of neurons, white-matter structures, and blood vessels simultaneously. Furthermore, we exploit THG-based imaging to guide micropipettes towards designated neurons inside live tissue. This work is a major step towards label-free microscopic live brain imaging, and opens up possibilities for the development of laser-guided microsurgery techniques in the living brain.

Precision spectroscopy at ultraviolet and shorter wavelengths has been hindered by the poor access of narrow-band lasers to that spectral region. We demonstrate high-accuracy quantum interference metrology on atomic transitions with the use of an amplified train of phase-controlled pulses from a femtosecond frequency comb laser. The peak power of these pulses allows for efficient harmonic upconversion, paving the way for extension of frequency comb metrology in atoms and ions to the extreme ultraviolet and soft x-ray spectral regions. A proof-of-principle experiment was performed on a deep-ultraviolet (2 Â 212.55 nanometers) two-photon transition in krypton; relative to measurement with single nanosecond laser pulses, the accuracy of the absolute transition frequency and isotope shifts was improved by more than an order of magnitude.In recent years, the invention of the femtosecond frequency comb laser (1-3) has brought about a revolution in metrology. A frequency comb acts as a bridge between the radio frequency (RF) domain (typically tens of MHz) and the optical frequency domain (typically hundreds of THz). Thus, in precision spectroscopy, the optical cycles of a continuous wave (CW) ultrastable laser can be phase-locked and counted directly with respect to an absolute frequency standard such as an atomic clock (4, 5). The resultant frequency measurements approach a precision of 1 part in 10 15 in certain cases, potentially enabling the detection of possible drift in the fundamental constants (6, 7), among other quantum mechanical applications.Here, we perform precision metrology without the use of a CW laser. Instead, an atomic transition is excited directly with amplified and frequency-converted pulses from a femtosecond frequency comb laser. As a result of quantum interference effects in the atomic excitation process, we can achieve an accuracy that is about six orders of magnitude higher than the optical bandwidth of the individual laser pulses.The method used is related to Ramsey_s principle of separated oscillatory fields (8), which probes the phase evolution of an atom in spatially separated interaction zones. This technique is widely used in the RF domain for atomic fountain clocks (9). By extension, in the optical domain, excitation can be performed by pulses separated in time (rather than in space) to maintain phase coherence between the excitation contributions. Several experiments have been performed to investigate Ramsey-type quantum interference fringes in the optical domain (10 -14) and phase-stable amplification of single pulses (15). Actual quantitative spectroscopy with phase-coherent oscillator pulses has been limited to a few relative frequency measurements on fine and hyperfine structure of atoms (13,14,16) and relative and absolute measurements on rubidium (17); absolute frequency measurements with amplified pulses have been frustrated by an unknown phase difference between the pulses or by limited resolution.We generate powerful laser pulses with a precise phase relationship by amplifyin...