129 research outputs found
Two-photon coherent control of femtosecond photoassociation
Photoassociation with short laser pulses has been proposed as a technique to
create ultracold ground state molecules. A broad-band excitation seems the
natural choice to drive the series of excitation and deexcitation steps
required to form a molecule in its vibronic ground state from two scattering
atoms. First attempts at femtosecond photoassociation were, however, hampered
by the requirement to eliminate the atomic excitation leading to trap
depletion. On the other hand, molecular levels very close to the atomic
transition are to be excited. The broad bandwidth of a femtosecond laser then
appears to be rather an obstacle. To overcome the ostensible conflict of
driving a narrow transition by a broad-band laser, we suggest a two-photon
photoassociation scheme. In the weak-field regime, a spectral phase pattern can
be employed to eliminate the atomic line. When the excitation is carried out by
more than one photon, different pathways in the field can be interfered
constructively or destructively. In the strong-field regime, a temporal phase
can be applied to control dynamic Stark shifts. The atomic transition is
suppressed by choosing a phase which keeps the levels out of resonance. We
derive analytical solutions for atomic two-photon dark states in both the
weak-field and strong-field regime. Two-photon excitation may thus pave the way
toward coherent control of photoassociation. Ultimately, the success of such a
scheme will depend on the details of the excited electronic states and
transition dipole moments. We explore the possibility of two-photon femtosecond
photoassociation for alkali and alkaline-earth metal dimers and present a
detailed study for the example of calcium
NIR Femtosecond Control of Resonance-Mediated Generation of Coherent Broadband UV Emission
We use shaped near-infrared (NIR) pulses to control the generation of
coherent broadband ultraviolet (UV) radiation in an atomic resonance-mediated
(2+1) three-photon excitation. Experimental and theoretical results are
presented for phase controlling the total emitted UV yield in atomic sodium
(Na). Based on our confirmed understanding, we present a new simple scheme for
producing shaped femtosecond pulses in the UV/VUV spectral range using the
control over atomic resonance-mediated generation of third (or higher order)
harmonic.Comment: 14 pages, 4 figure
Transform-limited pulses are not optimal for resonant multiphoton transitions
Maximizing nonlinear light-matter interactions is a primary motive for
compressing laser pulses to achieve ultrashort transform limited pulses. Here
we show how, by appropriately shaping the pulses, resonant multiphoton
transitions can be enhanced significantly beyond the level achieved by
maximizing the pulse's peak intensity. We demonstrate the counterintuitive
nature of this effect with an experiment in a resonant two-photon absorption,
in which, by selectively removing certain spectral bands, the peak intensity of
the pulse is reduced by a factor of 40, yet the absorption rate is doubled.
Furthermore, by suitably designing the spectral phase of the pulse, we increase
the absorption rate by a factor of 7.Comment: 4 pages, 3 figure
Shot noise limited characterization of femtosecond light pulses
Probing the evolution of physical systems at the femto- or attosecond
timescale with light requires accurate characterization of ultrashort optical
pulses. The time profiles of such pulses are usually retrieved by methods
utilizing optical nonlinearities, which require significant signal powers and
operate in a limited spectral
range\cite{Trebino_Review_of_Scientific_Instruments97,Walmsley_Review_09}. We
present a linear self-referencing characterization technique based on time
domain localization of the pulse spectral components, operated in the
single-photon regime. Accurate timing of the spectral slices is achieved with
standard single photon detectors, rendering the technique applicable in any
spectral range from near infrared to deep UV. Using detection electronics with
about ps response, we retrieve the temporal profile of a picowatt pulse
train with fs resolution, setting a new scale of sensitivity in
ultrashort pulse characterization.Comment: Supplementary information contained in raw dat
Photonics - An atomic dimmer switch
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62775/1/396217a0.pd
Coherent control using adaptive learning algorithms
We have constructed an automated learning apparatus to control quantum
systems. By directing intense shaped ultrafast laser pulses into a variety of
samples and using a measurement of the system as a feedback signal, we are able
to reshape the laser pulses to direct the system into a desired state. The
feedback signal is the input to an adaptive learning algorithm. This algorithm
programs a computer-controlled, acousto-optic modulator pulse shaper. The
learning algorithm generates new shaped laser pulses based on the success of
previous pulses in achieving a predetermined goal.Comment: 19 pages (including 14 figures), REVTeX 3.1, updated conten
Laser phase modulation approaches towards ensemble quantum computing
Selective control of decoherence is demonstrated for a multilevel system by
generalizing the instantaneous phase of any chirped pulse as individual terms
of a Taylor series expansion. In the case of a simple two-level system, all odd
terms in the series lead to population inversion while the even terms lead to
self-induced transparency. These results also hold for multiphoton transitions
that do not have any lower-order photon resonance or any intermediate virtual
state dynamics within the laser pulse-width. Such results form the basis of a
robustly implementable CNOT gate.Comment: 10 pages, 4 figures, PRL (accepted
Ultrashort-pulse laser with an intracavity phase shaping element
A novel ultrashort-pulse laser cavity configuration that incorporates an intracavity deformable mirror as a phase control element is reported. A user-defined spectral phase relation of 0.7 radians relative shift could be produced at around 1035 nm. Phase shaping as well as pulse duration optimization was achieved via a computer-controlled feedback loop
Problems and Aspects of Energy-Driven Wavefunction Collapse Models
Four problematic circumstances are considered, involving models which
describe dynamical wavefunction collapse toward energy eigenstates, for which
it is shown that wavefunction collapse of macroscopic objects does not work
properly. In one case, a common particle position measuring situation, the
apparatus evolves to a superposition of macroscopically distinguishable states
(does not collapse to one of them as it should) because each such
particle/apparatus/environment state has precisely the same energy spectrum.
Second, assuming an experiment takes place involving collapse to one of two
possible outcomes which is permanently recorded, it is shown in general that
this can only happen in the unlikely case that the two apparatus states
corresponding to the two outcomes have disjoint energy spectra. Next, the
progressive narrowing of the energy spectrum due to the collapse mechanism is
considered. This has the effect of broadening the time evolution of objects as
the universe evolves. Two examples, one involving a precessing spin, the other
involving creation of an excited state followed by its decay, are presented in
the form of paradoxes. In both examples, the microscopic behavior predicted by
standard quantum theory is significantly altered under energy-driven collapse,
but this alteration is not observed by an apparatus when it is included in the
quantum description. The resolution involves recognition that the statevector
describing the apparatus does not collapse, but evolves to a superposition of
macroscopically different states.Comment: 17 page
Stable mode-locked pulses from mid-infrared semiconductor lasers
We report the unequivocal demonstration of mid-infrared mode-locked pulses
from a semiconductor laser. The train of short pulses was generated by actively
modulating the current and hence the optical gain in a small section of an
edge-emitting quantum cascade laser (QCL). Pulses with pulse duration at
full-width-at-half-maximum of about 3 ps and energy of 0.5 pJ were
characterized using a second-order interferometric autocorrelation technique
based on a nonlinear quantum well infrared photodetector. The mode-locking
dynamics in the QCLs was modelled and simulated based on Maxwell-Bloch
equations in an open two-level system. We anticipate our results to be a
significant step toward a compact, electrically-pumped source generating
ultrashort light pulses in the mid-infrared and terahertz spectral ranges.Comment: 26 pages, 4 figure
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