204 research outputs found
Ultra-Short Optical Pulse Generation with Single-Layer Graphene
Pulses as short as 260 fs have been generated in a diode-pumped low-gain
Er:Yb:glass laser by exploiting the nonlinear optical response of single-layer
graphene. The application of this novel material to solid-state bulk lasers
opens up a way to compact and robust lasers with ultrahigh repetition rates.Comment: 6 pages, 3 figures, to appear in Journal of Nonlinear Optical Physics
& Material
VUV frequency combs from below-threshold harmonics
Recent demonstrations of high-harmonic generation (HHG) at very high
repetition frequencies (~100 MHz) may allow for the revolutionary transfer of
frequency combs to the vacuum ultraviolet (VUV). This advance necessitates
unifying optical frequency comb technology with strong-field atomic physics.
While strong-field studies of HHG have often focused on above-threshold
harmonic generation (photon energy above the ionization potential), for VUV
frequency combs an understanding of below-threshold harmonic orders and their
generation process is crucial. Here we present a new and quantitative study of
the harmonics 7-13 generated below and near the ionization threshold in xenon
gas. We show multiple generation pathways for these harmonics that are
manifested as on-axis interference in the harmonic yield. This discovery
provides a new understanding of the strong-field, below-threshold dynamics
under the influence of an atomic potential and allows us to quantitatively
assess the achievable coherence of a VUV frequency comb generated through below
threshold harmonics. We find that under reasonable experimental conditions
temporal coherence is maintained. As evidence we present the first explicit VUV
frequency comb structure beyond the 3rd harmonic.Comment: 16 pages, 4 figures, 1 tabl
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All polarization-maintaining Er fiber-based optical frequency combs with nonlinear amplifying loop mirror.
A fully stabilized all polarization-maintaining Er frequency comb with a nonlinear amplifying loop mirror with below 0.2 rad carrier-envelope-offset frequency phase noise is demonstrated. The integrated timing jitter is measured as 40 attosecond from 10 kHz to 10 MHz, which is the lowest value of any Er fiber frequency comb to date
Testing ultrafast mode-locking at microhertz relative optical linewidth
We report new limits on the phase coherence of the ultrafast mode-locking
process in an octave-spanning Ti:sapphire comb. We find that the mode-locking
mechanism correlates optical phase across a full optical octave with less than
2.5 micro Hz relative linewidth. This result is at least two orders of
magnitude below recent predictions for quantum-limited individual comb-mode
linewidths, verifying that the mode-locking mechanism strongly correlates
quantum noise across the comb spectrum.Comment: Expanded discussion to include correlated noise terms, made minor
formatting changes, added a reference, and fixed typographical errors. 10
pages, 5 figure
Frequency Comb Assisted Diode Laser Spectroscopy for Measurement of Microcavity Dispersion
While being invented for precision measurement of single atomic transitions,
frequency combs have also become a versatile tool for broadband spectroscopy in
the last years. In this paper we present a novel and simple approach for
broadband spectroscopy, combining the accuracy of an optical fiber-laser-based
frequency comb with the ease-of-use of a tunable external cavity diode laser.
This scheme enables broadband and fast spectroscopy of microresonator modes and
allows for precise measurements of their dispersion, which is an important
precondition for broadband optical frequency comb generation that has recently
been demonstrated in these devices. Moreover, we find excellent agreement of
measured microresonator dispersion with predicted values from finite element
simulations and we show that tailoring microresonator dispersion can be
achieved by adjusting their geometrical properties
Nonlinear Femtosecond Pulse Reshaping in Waveguide Arrays
We observe nonlinear pulse reshaping of femtosecond pulses in a waveguide
array due to coupling between waveguides. Amplified pulses from a mode-locked
fiber laser are coupled to an AlGaAs core waveguide array structure. The
observed power-dependent pulse reshaping agrees with theory, including
shortening of the pulse in the central waveguide
Prospects for precision measurements of atomic helium using direct frequency comb spectroscopy
We analyze several possibilities for precisely measuring electronic
transitions in atomic helium by the direct use of phase-stabilized femtosecond
frequency combs. Because the comb is self-calibrating and can be shifted into
the ultraviolet spectral region via harmonic generation, it offers the prospect
of greatly improved accuracy for UV and far-UV transitions. To take advantage
of this accuracy an ultracold helium sample is needed. For measurements of the
triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap
metastable 2^3S state atoms. We analyze schemes for measuring the two-photon
interval, and for resonant two-photon excitation to high
Rydberg states, . We also analyze experiments on the
singlet-state spectrum. To accomplish this we propose schemes for producing and
trapping ultracold helium in the 1^1S or 2^1S state via intercombination
transitions. A particularly intriguing scenario is the possibility of measuring
the transition with extremely high accuracy by use of
two-photon excitation in a magic wavelength trap that operates identically for
both states. We predict a ``triple magic wavelength'' at 412 nm that could
facilitate numerous experiments on trapped helium atoms, because here the
polarizabilities of the 1^1S, 2^1S and 2^3S states are all similar, small, and
positive.Comment: Shortened slightly and reformatted for Eur. Phys. J.
Fixed-point attractor for chirp in nonlinear waveguide arrays
The propagation of ultrashort optical pulses in an AlGaAs waveguide array is studied using frequency-resolved optical gating measurements. In the nonlinear regime, the measurements show that the pulses at the output of the array evolve toward a set chirp value that is independent of the input chirp. Simulations reproduce the experimental results. The observations can be described as a fixed-point attractor on a chirp-intensity map
Topology of the C-Terminal Tail of HIV-1 gp41: Differential Exposure of the Kennedy Epitope on Cell and Viral Membranes
The C-terminal tail (CTT) of the HIV-1 gp41 envelope (Env) protein is increasingly recognized as an important determinant of Env structure and functional properties, including fusogenicity and antigenicity. While the CTT has been commonly referred to as the “intracytoplasmic domain” based on the assumption of an exclusive localization inside the membrane lipid bilayer, early antigenicity studies and recent biochemical analyses have produced a credible case for surface exposure of specific CTT sequences, including the classical “Kennedy epitope” (KE) of gp41, leading to an alternative model of gp41 topology with multiple membrane-spanning domains. The current study was designed to test these conflicting models of CTT topology by characterizing the exposure of native CTT sequences and substituted VSV-G epitope tags in cell- and virion-associated Env to reference monoclonal antibodies (MAbs). Surface staining and FACS analysis of intact, Env-expressing cells demonstrated that the KE is accessible to binding by MAbs directed to both an inserted VSV-G epitope tag and the native KE sequence. Importantly, the VSV-G tag was only reactive when inserted into the KE; no reactivity was observed in cells expressing Env with the VSV-G tag inserted into the LLP2 domain. In contrast to cell-surface expressed Env, no binding of KE-directed MAbs was observed to Env on the surface of intact virions using either immune precipitation or surface plasmon resonance spectroscopy. These data indicate apparently distinct CTT topologies for virion- and cell-associated Env species and add to the case for a reconsideration of CTT topology that is more complex than currently envisioned
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