[show abstract][hide abstract] ABSTRACT: We study infrared backward cooperative emission in a rubidium vapor induced by ultrafast twophoton
optical excitations. The laser coherent control of the backward emission is demonstrated by
using a pair of 100 fs pulses with a variable time delay. The temporal variation (quantum beat) of
the backward beam intensity due to interference of atomic transitions in the rubidium atomic level
system 5S-5P-5D is produced and controlled. Based on the obtained experimental results, we
discuss possible applications of the developed approach for creation of an effective “guide star” in
the sodium atomic layer in the upper atmosphere (mesosphere).
[show abstract][hide abstract] ABSTRACT: Sub-surface analysis of chemical species is imperative for biomedical diagnostics and imaging, homeland security, pharmaceutical and other industries; however, the access to the object of interest is often obscured by an optically scattering medium which limits the ability to inspect the chemical composition of the sample. In this report, we employ coherent Raman microspectroscopy in a combination with a hierarchical cluster analysis to mitigate the effect of scattering and demonstrate the identification of multiple chemical species.
[show abstract][hide abstract] ABSTRACT: We study high-order sideband generation of coherent short pulse emission
without population inversion in the transient regime. We use a universal method
to study the propagation of a pulse in various spectral regions through the gas
medium strongly driven on a low-frequency transition on a time scale shorter
than the decoherence time. The results show that gain on the high-order
sidebands can be produced even if there is no initial population inversion
prepared. This method has the potential to make high frequency lasers (such as
in the extreme ultraviolet and x-ray spectral regions).
[show abstract][hide abstract] ABSTRACT: A laser generates light through stimulated emission of radiation and requires population inversion. Quantum interference can yield lasing without inversion. However, such phase-sensitive quantum amplification still requires some atomic population in the excited state. Here, we present a new kind of quantum amplifier based on collective superradiant emission which does not need any population in the excited state. We show that parametric resonance between the driving (e.g., infrared) field and collective superradiant oscillations of the atomic polarization can yield light amplification at high (e.g., XUV) frequencies. To achieve gain, one must suppress a time-dependent Stark shift caused by the driving field. The resulting superradiant amplifier is many orders of magnitude more efficient than the usual nonlinear multiphoton excitation and holds promise for a new kind of generator of high-frequency coherent radiation. In addition to a detailed analytical analysis, confirmed by numerical simulations, we provide a physically appealing explanation of the quantum amplification by superradiant emission of radiation (QASER) operation in terms of coupled classical oscillators. We also present an experiment that demonstrates the QASER amplification mechanism in an electronic circuit, which, to the best of our knowledge, is the first experimental demonstration of the difference combination resonance.
[show abstract][hide abstract] ABSTRACT: We investigate surface plasmon amplification in a silver nanoparticle coupled to an externally driven three-level gain medium, and show that quantum coherence significantly enhances the generation of surface plasmons. Surface plasmon amplification by stimulated emission of radiation is achieved in the absence of population inversion on the spasing transition, which reduces the pump requirements. The coherent drive allows us to control the dynamics, and holds promise for quantum control of nanoplasmonic devices.
[show abstract][hide abstract] ABSTRACT: It has been shown that efficiency of a photovoltaic cell can be enhanced
in the presence of intermediate levels in a semiconductor band gap.
However the practical realization of this concept is difficult due to
low absorption by the intermediate levels. We show that it is possible
to significantly increase photon absorption and generate more power
utilizing noise induced quantum coherence between intermediate
electronic states which is created due to interference among the
absorption pathways. We discuss possible experimental demonstration of
this effect using confined electronic-hole states in self assembled
[show abstract][hide abstract] ABSTRACT: It has been known for two decades that coherence can yield lasing without inversion (LWI), which could be useful for making lasers at shorter wavelength. However, excitation of extreme ultraviolet and x-ray atomic transitions typically requires a plasma medium with rapid collisions, which destroy atomic coherence. Here we demonstrate LWI on a time scale shorter than the decoherence time. We show that in such a regime LWI is possible in a V -scheme with a strong coherent drive on the low-frequency transition and obtain an analytical expression for the gain of the laser pulse at high frequency. We propose an experiment in which such LWI can be realized in He plasma.
New Journal of Physics 05/2013; 15(5):053044. · 4.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: We show that the momentum of light can be reversed via the atomic coherence
created by another light with one or two orders of magnitude lower frequency.
Both the backward retrieval of single photons from a time-ordered Dicke state
and the reflection of continuous waves by high-order photonic band gaps are
analyzed. A proof-of-principle experiment with thermal Rb vapor is proposed
based on presently available techniques. This holds promise for X-ray
reflectors controlled by low-frequency light.
[show abstract][hide abstract] ABSTRACT: Two commonly used techniques that provide species-specific spectroscopic
signals in the form of vibrational fingerprints are surface-enhanced
Raman scattering (SERS) and coherent anti-Stokes Raman scattering (CARS)
spectroscopies. In order to enhance the signal, SERS takes advantage of
the electromagnetic near-field enhancement while CARS employs molecular
coherence. We have combined these two techniques to achieve
best-of-both-worlds maximum signal enhancement by using optimized laser
pulse shaping and time-resolved detection. We applied this new
time-resolved surface-enhanced coherent anti-Stokes Raman scattering
(tr-SECARS) technique to investigate various molecular complexes in a
vicinity of gold nanoparticles. While large signal enhancement has
previously been achieved in SERS, surfaced-enhanced coherent signals
have shown lower values. We investigate the mechanisms of these effects
by analyzing the spatial dependence of the coherent Raman spectra for
different hot spots in aggregated plasmonic nanoparticles. Understanding
coherence effects in surface-enhanced Raman scattering may lead to
improved nanoscale sensors.
[show abstract][hide abstract] ABSTRACT: Quantum mechanics and thermodynamics have deep connections which govern
the behavior of laser and photocell quantum heat engines (QHEs). We
describe QHEs inspired by photosynthesis that operate under the natural
conditions of incoherent excitation by sunlight. We investigate
parameter regimes where large electric current yield enhancement and/or
population oscillations are observed and identify noise-induced quantum
coherence as the common origin of these effects. Quantum coherence plays
a role in enhancing energy and charge transfer efficiencies and holds
promise for improving the design and boosting the efficiencies of
light-harvesting devices. A broad range of parameter regimes provides
flexibility in designs and materials.
[show abstract][hide abstract] ABSTRACT: We present a detailed theoretical study of a recent experiment [A. J. Traverso et al., Proc. Natl. Acad. Sci. USA 109, 15185 (2012)] in which a laserlike source is created in air by pumping with a nanosecond pulse. The source generates radiation in the forward and backward directions. The temporal behavior of the emitted pulses is investigated for different pump shapes and durations. Our analysis indicates that the spiky emission is due to quantum coherence via cooperation between atoms of an ensemble, which leads to strong-oscillatory superfluorescence. We show that these cooperative nonadiabatic coherence effects cannot be described by rate equations and instead a full set of the Maxwell-Bloch equations must be used. We consider a range of parameters and study transitions between various regimes. Understanding these coherence-brightened processes in air should lead to improvements in environmental, atmospheric remote sensing and other applications.
Physical Review A 02/2013; 87(2). · 3.04 Impact Factor
[show abstract][hide abstract] ABSTRACT: We propose two atom lithography techniques with subwavelength resolution based on position-dependent Rabi oscillations. Our method either uses neutral or ionized atoms to write subwavelength patterns. We illustrate our proposal by numerical simulations of an experimental setup using rubidium Rydberg atoms. We show that, for a microwave wavelength of 1.4 cm, a spacing of a few hundred nanometers is possible.
Physical Review A 02/2013; 87(2). · 3.04 Impact Factor
[show abstract][hide abstract] ABSTRACT: Two seemingly unrelated effects attributed to quantum coherence have been reported recently in natural and artificial light-harvesting systems. First, an enhanced solar cell efficiency was predicted and second, population oscillations were measured in photosynthetic antennae excited by sequences of coherent ultrashort laser pulses. Because both systems operate as quantum heat engines (QHEs) that convert the solar photon energy to useful work (electric currents or chemical energy, respectively), the question arises whether coherence could also enhance the photosynthetic yield. Here, we show that both effects arise from the same population-coherence coupling term which is induced by noise, does not require coherent light, and will therefore work for incoherent excitation under natural conditions of solar excitation. Charge separation in light-harvesting complexes occurs in a pair of tightly coupled chlorophylls (the special pair) at the heart of photosynthetic reaction centers of both plants and bacteria. We show the analogy between the energy level schemes of the special pair and of the laser/photocell QHEs, and that both population oscillations and enhanced yield have a common origin and are expected to coexist for typical parameters. We predict an enhanced yield of 27% in a QHE motivated by the reaction center. This suggests nature-mimicking architectures for artificial solar energy devices.
Proceedings of the National Academy of Sciences 01/2013; · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: We investigate surface plasmon amplification in a silver nanoparticle coupled
to an externally driven three-level gain medium, and show that quantum
coherence significantly enhances the generation of surface plasmons. Surface
plasmon amplification by stimulated emission of radiation is achieved in the
absence of population inversion on the spasing transition, which reduces the
pump requirements. The coherent drive allows us to control the dynamics, and
holds promise for quantum control of nanoplasmonic devices.
[show abstract][hide abstract] ABSTRACT: Nanoscale real-time molecular sensing requires large signal enhancement, small background, short detection time and high spectral resolution. We demonstrate a new vibrational spectroscopic technique which satisfies all of these conditions. This time-resolved surface-enhanced coherent anti-Stokes Raman scattering (tr-SECARS) spectroscopy is used to detect hydrogen-bonded molecular complexes of pyridine with water in the near field of gold nanoparticles with large signal enhancement and a fraction of a second collection time. Optimal spectral width and time delays of ultrashort laser pulses suppress the surface-enhanced non-resonant background. Time-resolved signals increase the spectral resolution which is limited by the width of the probe pulse and allow measuring nanoscale vibrational dephasing dynamics. This technique combined with quantum chemistry simulations may be used for the investigation of complex mixtures at the nanoscale and surface environment of artificial nanostructures and biological systems.
[show abstract][hide abstract] ABSTRACT: This work has been stimulated by the quenching of spontaneous emission in an
optically thin high-density plasma reported by Suckewer and coworkers
[Phys.Rev.Lett. 60,1122(1988)]. Our work is based on the collision-induced
coherence of two decay channels along two optical transitions.The quantum
interference of pumping processes creates the dark state and the more atoms are
pumped in this collision-induced dark state the stronger the suppression of the
spontaneous emission. The efficiency of this suppression is quantified by
putting it in comparison with the spontaneous emission on the ultraviolet
transition which proceeds in a regular fashion. The branching ratio of these
two(visible and ultraviolet) transitions is introduced as the effective measure
of the degree of the suppression of the spontaneous emission on the visible
transition. Our preliminary calculations show that a significant decrease of
the branching ratio with increase of electron densities is reproduced in the
theory. More experiments and more elaborated theories are needed to conclude
about the role of quantum coherence and interference in such manifestly
incoherent media as plasmas.
[show abstract][hide abstract] ABSTRACT: We have studied coherent emission from ambient air and demonstrated efficient generation of laser-like beams directed both forward and backward with respect to a nanosecond ultraviolet pumping laser beam. The generated optical gain is a result of two-photon photolysis of atmospheric O(2), followed by two-photon excitation of atomic oxygen. We have analyzed the temporal shapes of the emitted pulses and have observed very short duration intensity spikes as well as a large Rabi frequency that corresponds to the emitted field. Our results suggest that the emission process exhibits nonadiabatic atomic coherence, which is similar in nature to Dicke superradiance where atomic coherence is large and can be contrasted with ordinary lasing where atomic coherence is negligible. This atomic coherence in oxygen adds insight to the optical emission physics and holds promise for remote sensing techniques employing nonlinear spectroscopy.
Proceedings of the National Academy of Sciences 09/2012; 109(38):15185-90. · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: Using femtosecond transient absorption spectroscopy on excited-state helium atoms in a plasma created through optical field ionization, we measured the decay of 23S-23P excitation with sub-ps temporal resolution. The population evolution shows that initial decay is significantly faster than the electron-atom collisions and three orders of magnitude faster than the single atom spontaneous decay rate. This indicates on superradiant coherent behavior of the atomic system inside the plasma.
[show abstract][hide abstract] ABSTRACT: Ultralow-power diode-laser radiation is employed to induce photodesorption of
cesium from a partially transparent thin-film cesium adsorbate on a solid
surface. Using resonant Raman spectroscopy, we demonstrate that this
photodesorption process enables an accurate local optical control of the
density of dimer molecules in alkali-metal vapors.