Biomedical subjects

T M Fortier

Publications and source records attributed to T M Fortier.

9 recordsLinked to original sources

PubMed2006

Kilohertz-resolution spectroscopy of cold atoms with an optical frequency comb.

We have performed sub-Doppler spectroscopy on the narrow intercombination line of cold calcium atoms using the amplified output of a femtosecond laser frequency comb. Injection locking of a 657-nm diode laser with a femtosecond comb allows for two regimes of amplification, one in which many lines of the comb are amplified, and one where a single line is predominantly amplified. The output of the laser in both regimes was used to perform kilohertz-level spectroscopy. This experiment demonstrates the potential for high-resolution absolute-frequency spectroscopy over the entire spectrum of the frequency comb output using a single high-finesse optical reference cavity.

Journal Article↗

PubMed2006

Single-atom optical clock with high accuracy.

For the past 50 years, atomic standards based on the frequency of the cesium ground-state hyperfine transition have been the most accurate time pieces in the world. We now report a comparison between the cesium fountain standard NIST-F1, which has been evaluated with an inaccuracy of about 4 x 10(-16), and an optical frequency standard based on an ultraviolet transition in a single, laser-cooled mercury ion for which the fractional systematic frequency uncertainty was below 7.2 x 10(-17). The absolute frequency of the transition was measured versus cesium to be 1,064,721,609,899,144.94 (97) Hz, with a statistically limited total fractional uncertainty of 9.1 x 10(-16) the most accurate absolute measurement of an optical frequency to date.

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PubMed2006

Octave-spanning Ti:sapphire laser with a repetition rate >1 ghz for optical frequency measurements and comparisons.

We demonstrate a self-referenced, octave-spanning, mode-locked Ti:sapphire laser with a scalable repetition rate (550 MHz - 1.35 GHz). We use the frequency comb output of the laser, without additional broadening in optical fiber, for simultaneous measurements against atomic optical standards at 534, 578, 563, and 657 nm and to stabilize the laser offset frequency.

Journal Article↗

PubMed2005

Observation and absolute frequency measurements of the 1S0-3P0 optical clock transition in neutral ytterbium.

We report the direct excitation of the highly forbidden (6s2) 1S0 <--> (6s6p) 3P0 optical transition in two odd isotopes of neutral ytterbium. As the excitation laser frequency is scanned, absorption is detected by monitoring the depletion from an atomic cloud at approximately 70 microK in a magneto-optical trap. The measured frequency in 171Yb (F=1/2) is 518,295,836,591.6 +/- 4.4 kHz. The measured frequency in 173Yb (F=5/2) is 518,294,576,847.6 +/- 4.4 kHz. Measurements are made with a femtosecond-laser frequency comb calibrated by the National Institute of Standards and Technology cesium fountain clock and represent nearly a 10(6)-fold reduction in uncertainty. The natural linewidth of these J=0 to J=0 transitions is calculated to be approximately 10 mHz, making them well suited to support a new generation of optical atomic clocks based on confinement in an optical lattice.

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PubMed2005

Solid-state carrier-envelope phase stabilization via quantum interference control of injected photocurrents.

We demonstrate carrier-envelope phase stabilization of a mode-locked Ti:sapphire laser by use of quantum interference control of injected photocurrents in a semiconductor. No harmonic generation is required for this stabilization technique. Instead, interference between coexisting single- and two-photon absorption pathways in the semiconductor provides a phase comparison between different spectral components. The phase comparison, and the detection of the photocurrent that it produces, both occur within a single low-temperature-grown gallium arsenide sample. The carrier-envelope offset beat note fidelity is 30 dB in a 10-kHz resolution bandwidth. The out-of-loop phase-noise level is essentially identical to the best previous measurements with the standard self-referencing technique.

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PubMed2004

Nonlinear polarization evolution of ultrashort pulses in microstructure fiber.

We present experimental and numerical results for nonlinear polarization evolution of femtosecond pulses during propagation in microstructure fiber. Numerical modeling shows that fiber dispersion permits a long interaction length between the components polarized along the two principal axes, thereby enhancing the effective nonlinear polarization evolution in microstructure fiber.

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PubMed2004

Carrier-envelope phase-controlled quantum interference of injected photocurrents in semiconductors.

We demonstrate quantum interference control of injected photocurrents in a semiconductor using the phase stabilized pulse train from a mode-locked Ti:sapphire laser. Measurement of the comb offset frequency via this technique results in a signal-to-noise ratio of 40 dB (10 Hz resolution bandwidth), enabling solid-state detection of carrier-envelope phase shifts of a Ti:sapphire oscillator.

Journal Article↗

PubMed2003

Phase stabilization of an octave-spanning Ti:sapphire laser.

We obtain direct stabilization of the carrier-envelope phase for a standard x-folded geometry, octave-spanning, Ti:sapphire laser. The in-loop accumulated carrier-envelope phase error is 0.175 rad (1.65 mHz to 102 kHz). Intracavity continuum generation, which is responsible for the octave bandwidth, is characterized through measurement of the beam parameters.

Journal Article↗

PubMed1997

Mitosis in vertebrate somatic cells with two spindles: implications for the metaphase/anaphase transition checkpoint and cleavage.

During mitosis an inhibitory activity associated with unattached kinetochores prevents PtK1 cells from entering anaphase until all kinetochores become attached to the spindle. To gain a better understanding of how unattached kinetochores block the metaphase/anaphase transition we followed mitosis in PtK1 cells containing two independent spindles in a common cytoplasm. We found that unattached kinetochores on one spindle did not block anaphase onset in a neighboring mature metaphase spindle 20 microm away that lacked unattached kinetochores. As in cells containing a single spindle, anaphase onset occurred in the mature spindles x = 24 min after the last kinetochore attached regardless of whether the adjacent immature spindle contained one or more unattached kinetochores. These findings reveal that the inhibitory activity associated with an unattached kinetochore is functionally limited to the vicinity of the spindle containing the unattached kinetochore. We also found that once a mature spindle entered anaphase the neighboring spindle also entered anaphase x = 9 min later regardless of whether it contained monooriented chromosomes. Thus, anaphase onset in the mature spindle catalyzes a "start anaphase" reaction that spreads globally throughout the cytoplasm and overrides the inhibitory signal produced by unattached kinetochores in an adjacent spindle. Finally, we found that cleavage furrows often formed between the two independent spindles. This reveals that the presence of chromosomes and/or a spindle between two centrosomes is not a prerequisite for cleavage in vertebrate somatic cells.

Animals↗