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We have studied the pump-probe spectroscopy of graphene oxide quantum dots. In the presence of gold nanoparticles, strong modifications of pump-probe spectra have been observed. We develop a theoretical model to explain the observed features of the pump-probe spectra. The model takes into account self-consistently the interaction of exciton with surface plasmonic waves excited in the gold nanoparticles, and it provides a qualitative agreement with the observed pump-probe spectra. We have demonstrated that the using gold nanoparticles can increase sensitivity, and potentially to have a broader range of applications to both spectroscopy and microscopy.

For most three centuries after Hooke introduced optical microscopy, refinement of the instrumentation made microscopes more convinient than anything else. As in modern microscopy, a light source, an objective lens, and an eyepiece were used to project an image magnified from a 100-fold to 1,000-fold into a human eye. This range of magnifications and resolution (of the order of 200 nm) has brought cellular morphology and tissue structure into view and made optical microscopy the perfect partner for biological investigations. A resolution constraint in optical imaging is imposed by the diffraction limit discovered by Lord Rayleigh in the 19th century. Fundamentally, this restricts the ability to resolve two distant point sources, such as stars or planets, with an angular resolution α smaller than the wavelength of light λ divided by the numerical aperture of the imaging system

Spontaneous emission from individual atoms in vapor lasts nanoseconds, if not microseconds, and beatings in this emission involve only directly excited energy sublevels. In contrast, the superfluorescent emissions burst on a much-reduced timescale and their beatings involve both directly and indirectly excited energy sublevels. In this work, picosecond and femtosecond superfluorescent beatings are observed from a dense cesium atomic vapor. Cesium atoms are excited by 60-femtosecond long, 800 nm laser pulses via two-photon processes into their coherent superpositions of the ground 6S and excited 8S states. As a part of the transient four wave mixing process, the yoked superfluorescent blue light at lower transitions of 6S - 7P are recorded and studied. Delayed buildup time of this blue light is measured as a function of the input laser beam power using a high-resolution 2 ps streak camera. The power dependent buildup delay time is consistently doubled as the vapor temperature is lowered to cut the number of atoms by half. At low power and density, a beating with a period of 100 picoseconds representing the ground state splitting is observed. The autocorrelation measurements of the generated blue light exhibit a beating with a quasi-period of 230 fs corresponding to the splitting of the 7P level primarily at lower input laser power. Understanding and, eventually, controlling the intriguing nature of superfluorescent beatings may permit a rapid quantum operation free from the rather slow spontaneous emission processes from atoms and molecules., Comment: 7 pages, 5 figures

Using quantum coherence effects, we have developed a new technique of detection of motion of single atoms or ions in an optical cavity. We have theoretically demonstrated that the presence of a sin...

The absorption in quantum dots (QDs) embedded within a semiconductor matrix can be manipulated by resonant optical excitation of phonons. The far-field interaction of the light leading to dressed phonon states within reduced graphene oxide quantum dots has been coherently modified by a near-field optical driving field at the nanoscale limit. The near-field optical excitation was introduced by resonant excitation of localized plasmons in metal nanoparticles coupled to QDs for ultrafast light manipulation. The coherent interaction of photons due to localized plasmon induced a change in the transient absorption of graphene oxide quantum dots conjugated to silver nanoparticles. Resonant pumping of plasmons and phonons with 400 nm pump photons induces a coherent change in excitonic absorption within QDs which results in phonon-assisted plasmon induced transparency at room temperature. This novel effect can be related to the appearance of the coherent effects such as forming dark states and coherent population ...

We have considered the Raman scattering in molecular media. Applying two laser fields in a two-photon resonance with vibrational transition, we have studied the role of rotational levels for excitation of vibrational coherence. It is shown that the molecular vibrational coherence strongly depends on the effect of coherent population trapping for rotational levels. The obtained results are important for applications of Raman spectroscopy to molecular detection in engineering, chemical, and biological applications.

Using the Magnus expansion to the fourth order, we obtain analytic expressions for the atomic state of a two-level system driven by a laser pulse of arbitrary shape with small pulse area. We also determine the limitation of our obtained formulas due to limited range of convergence of the Magnus series. We compare our method to the recently developed method of Rostovtsev et al. (PRA 2009, 79, 063833) for several detunings. Our analysis shows that our technique based on the Magnus expansion can be used as a complementary method to the one in PRA 2009., 12 pages, 4 figures

We present a study of the interaction of coherent field effects on population of two bound atomic states of a dense atomic gas interacting in a cooperative regime. We provide a theoretical description of this phenomenon using density matrix equations. We discuss the implications and applications of the present studies to quantum amplification by superradiant emission of radiation action. A classical model is developed to gain physical insights into qaser physics.

We investigate cooperative emission from a rubidium vapor, and demonstrate a controlled transition from yoked superfluorescence to three-photon-induced superradiance by driving the medium with co-propagating ultrashort laser pulses. We study temporal emission profiles and time delays on a picosecond time scale and compare the measured pulse shapes with simulations. Our results suggest strategies to improve efficiency of mirrorless lasers and superradiant light sources.

The absorption due to phonon vibrations in graphene oxide quantum dots (GO-QDs) has been modified by localized plasmons induced by Ag nanoparticles (NPS). The interference of phonons and plasmons results in coherent interaction that leads to the formation of "dark states" and "coherent population trapping" related to Fano like effect. The localized electric field effectively drives the phonons which in turn modifies the coherent interaction in exciton absorption states of graphene oxide. A quantum optical theory is used to qualitatively explain the differential absorption spectra measured using pump-probe spectroscopy demonstrating the formation of dark states.

The vibrational molecular excitation by strong laser fields have been studied. Starting with the molecular Hamiltonian, we derive the vibrational excitation rate due to a gradient of laser field and show that the rate can be two to three orders of magnitude larger than the regular Raman rates. The developed theory contributes to understanding of interaction of powerful laser pulses with molecular systems, and it might be used in developing quantitative coherent Raman spectroscopy. Copyright © 2013 John Wiley & Sons, Ltd.

The absorption due to phonon vibrations in graphene oxide quantum dots (GO-QDs) has been modified by localized plasmons induced by Ag nanoparticles (NPS). The interference of phonons and plasmons results in coherent interaction that leads to the formation of “dark states” and “coherent population trapping” related to Fano like effect. A quantum optical theory is used to qualitatively explain the differential absorption spectra measured using pump-probe spectroscopy demonstrating the formation of dark states.

Quantum optics with X-rays has long been a somewhat exotic activity, but it is now rapidly becoming relevant as precision x-ray optics and novel X-ray light sources, and high-intensity lasers are becoming available. This article gives an overview of the current state of the field and an outlook to future prospects.

It was shown some time ago that when the excitation of an ensemble of two-level atoms is swept in the direction of lasing, so that atoms are prepared in the excited state just as the radiation from previously excited atoms reaches them, the resulting laser amplifier is “highly anomalous” and yields superradiant emission without population inversion. We here show that transient gain in a three-level system has common features with Dicke superradiance and can yield strong extreme ultraviolet lasing in, for example, He atoms (at 58 nm) or He-like ions such as B3+ (at 6.1 nm).

We have studied electromagnetically induced transparency (EIT) in diatomic cesium molecules in a vapor cell by using tunable diode lasers. We have observed a sub-natural Lambda-resonance in an absorption molecular band at different cesium vapor pressures. The width of the EIT resonance shows a linear dependence on cesium vapor pressure. Narrow Lambda-resonances in molecules can be used as frequency references for femtosecond laser frequency combs.

Motivated by the recent experiment [Sautenkov, V.A.; Rostovtsev, Yu.V.; Scully, M.O. Phys. Rev. A 2005, 72, 065801], we study the field intensity fluctuations due to interaction between a laser with a finite bandwidth and a dense atomic medium. The intensity–intensity cross-correlation of two orthogonal, circular polarized beams can be controlled by the applied external magnetic field. A smooth transition from perfect correlations to anti-correlations (at zero delay time) of the outgoing beams is observed.

Nuclear quantum optics deals with the resonant interaction of gamma-radiation and nuclei, generally incorporated in a solid-state material. Single gamma-rays interact with an ensemble of nuclei. The properties of the resonantly forward-scattered gamma-radiation depend on the details of the interaction. An external modification of the nuclear energies will influence the interactions, and hence the properties of the radiation. Several techniques are proposed to reduce the resonant absorption of the gamma-rays, creating a nuclear medium that is partially transparent: nuclear-level-mixing induced transparency, modification of nuclear levels by means of an optical laser, phase-shift-induced transparency. Interference is crucial in all this. The partial transparency could be exploited to study special effects such as slow gamma-radiation and gain without population inversion. It could alsa be of importance for the realization of a gamma-ray laser.

We have studied the intensity correlations between two orthogonally linearly polarized components of a laser field propagating through a resonant atomic medium. These experiments have been performed in a rubidium atomic vapor. We observe that the correlations between the orthogonally polarized components of the laser beam are maximal in the absence of a magnetic field. The magnitude of the correlations depends on the applied magnetic field, and the magnitude first decreases and then increases with increasing magnetic field. Minimal correlations and maximal rotation angles are observed at the same magnetic fields. The width of the correlation function is directly proportional to the excited state lifetime and inversely proportional to the Rabi frequency of laser field.

We study Raman scattering in molecular media by applying two laser fields in a two-photon resonance with vibrational transition. The role of rotational levels has been investigated. It is shown that the molecular vibrational coherence strongly depends on the effect of coherent population trapping for rotational levels. The obtained results are important for application of Raman spectroscopy to molecular detection for engineering, chemical, and biological applications.

We introduce a hybrid technique that combines the robustness of frequency-resolved coherent anti-Stokes Raman scattering (CARS) with the advantages of time-resolved CARS spectroscopy. Instantaneous coherent broadband excitation of several characteristic molecular vibrations and the subsequent probing of these vibrations by an optimally shaped time-delayed narrowband laser pulse help to suppress the nonresonant background and to retrieve the species-specific signal. We used this technique for coherent Raman spectroscopy of sodium dipicolinate powder, which is similar to calcium dipicolinate (a marker molecule for bacterial endospores, such as Bacillus subtilis and Bacillus anthracis ), and we demonstrated a rapid and highly specific detection scheme that works even in the presence of multiple scattering.

We report the first observation of a laser-produced vibration with the aid of Mossbauer techniques. Thin platelets of MgO single crystals were doped by diffusion of 57Fe atoms. The illumination of the MgO:57Fe sample with a pulsed Nd:YAG laser produced a significant broadening of the Mossbauer spectrum. In order to find out what caused these changes, we performed a series of time-domain experiments, in which the Mossbauer spectra were collected only during a 2.5 μs gate interval. This gate interval was swept from 5 μs to 190 μs over the time interval between the two laser pulses. After laser irradiation, the position of the Mossbauer line was found to be changing in time as a decaying oscillations of well-defined frequency, which can be due to the vibration of the sample induced by the laser pulse.

Maximizing the molecular response to coherent anti-Stokes Raman spectroscopy (CARS) requires optimizing the preparation of a vibrational quantum coherence. We simulate the amount of laser-induced coherence prepared in a three-level system modeled after dipicolinic acid (DPA), a marker molecule for bacterial spores. The level spacings and decoherence times were chosen to agree with experimental data observed from DPA. Nearly-maximal vibrational coherences can be induced for 150 fs optical pulses, despite the very fast dephasing rates of the electronic transitions. It is also shown that pulse propagation effects play an important role in the development of coherence throughout an extended sample, due to nonlinear index effects of the molecule at the laser frequencies.

We show that it is possible to improve the efficiency of a laser system by adding a classical heat engine cycle to the laser gas which can remove both thermal and vibrational energy from the gas. Rapid pulsed CO 2 laser-gas expansion cycle engine and a continuous CO 2 laser-Brayton cycle engine are analysed to determine practicality. Combined laser-mechanical efficiencies can approach 80% for operational systems.

A ‘phased’ optical klystron drift region developed to obtain free-electron lasing (FEL) without inversion in the small-gain small-signal regime is here applied to the high-gain regime. Numerical simulations show improvement in the gain and the tolerance to electron beam energy spread compared to conventional FEL. Longitudinal phase space dynamics has been studied to show the effect of the drift region on the electron beam.

SCOPUS: re.j, info:eu-repo/semantics/published

Laser induced line narrowing effect, discovered more than thirty years ago, can also be applied to recent studies in high resolution spectroscopy based on electromagnetically induced transparency. In this paper we first present a general form of the transmission width of electromagnetically induced transparency in a homogeneously broadened medium. We then analyze a Doppler broadened medium by using a Lorentzian function as the atomic velocity distribution. The dependence of the transmission linewidth on the driving field intensity is discussed and compared to the laser induced line narrowing effect. This dependence can be characterized by a parameter which can be regarded as ``the degree of optical pumping''., Comment: 8 pages, 5 figures

The laser-induced line narrowing effect in Doppler broadened systems was discovered thirty years ago. We have revisited this effect to determine its role for dense gases where recent experimental studies have found many intriguing atomic coherence effects. Using the density matrix approach, we study the width of electromagnetically induced transparency under different regimes of broadening.

We study a force acting on a three-level Λ-type atom in an inhomogeneous coherent electromagnetic field. We show that the magnitude of the force as well as incoherent scattering of the field is comparable with that of a two-level atom. Using Λ-type three-level atoms, one is able to realize a new kind of quantum nondemolition measurement of a photon number of modes of an open optical cavity.

New spectroscopic technique based on modification of Mossbauer spectra via optical coherent driving of atomic transitions is suggested. Possible experimental realizations are discussed.

A significant reduction of absorption of single gamma photons has been observed using the Mossbauer spectra of 57Fe in a FeCO3 crystal. The absorption deficit can be ascribed to partially destructive interference for absorption because of two indistinguishable absorption paths. The necessary coherence is created by means of level mixing produced by a suitable combination of a magnetic dipole and electric quadrupole interaction.

Various effects leading to modifications of Mossbauer spectra as a result of interaction of nuclei with external electromagnetic fields of different frequency ranges (dc, radio-frequency, microwave and optical) are reviewed. Special attention is paid to the coherent effects, where interference of nuclear transitions plays a decisive role. Possible applications of these effects in gamma-ray spectroscopy, quantum computations, and resolution of the gamma-ray laser problem are discussed.

We give a comparative analysis of basic schemes of optical control of Mossbauer spectra providing an elimination of resonant absorption by a gamma-ray nuclear transition and suggest a novel scheme of frequency non-selective optical pumping which turns out to be the most promising for a realization of induced gain in the gamma-ray range.

Considering a propagation of electromagnetic (EM) waves through overdense homogeneous plasmas, a new regime of amplification of EM waves with frequencies below the plasma frequency has been found. An application to free electron lasers is suggested.

The small-gain regime of free-electron lasers (FEL) without inversion is considered and seeming contradictions with the traditional theory of FEL are resolved. As a result, a generalized Madey's theorem is obtained for the case of a phase shift given to electrons between the two wigglers. It explicitly demonstrates the contribution of interference of radiation from the wigglers. It is shown, by considering the motion in the phase space, that Liouville's theorem applied to the motion of electrons in a two-dimensional (rather than one-dimensional) real space is consistent with a nonzero integral of the gain over electron energies. These conclusions are crucial for the achievement of orders-of-magnitude improvement in the FEL gain in the case of a large electron energy spread.

We propose an experiment to demonstrate a strong coherent backward and forward microwave generation using forward-propagating optical fields only. This is achieved by applying laser fields to an ultradispersive medium with properly chosen detunings and/or Rabi frequencies to excite a spin coherence that generates a forward- or backward-propagating microwave. Applications to coherent scattering and remote sensing are discussed.

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.

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.

We pump ambient air by loosely-focused ultraviolet pulses and generate a backward laser-like beam for use in remote sensing. We find that this efficient emission is likely linked to non-adiabatic coherence effects in atomic oxygen.

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., 4 pages, 4 figures

Electromagnetically induced transparency [1, 2] or EIT is an important nonlinear optical effect with wide ranging applications from slow light [3, 4] to sub-diffraction-limited optical imaging [5]. The most striking feature is the presence of zero (or low) absorption of a probe laser when it is exactly on resonance. In addition to the greatly reduced absorption is greatly enhanced normal dispersion on resonance. It is this steep dispersion coupled with low absorption that makes the system so rich.

We have theoretically studied the pulsed regime of electromagnetically induced transparency. In particular, simulations of propagation of Gaussian and $0\ensuremath{\pi}$ copropagating laser pulses in a medium consisting of three-level $\ensuremath{\Lambda}$ atoms have been performed. It has been found that even at the two-photon resonance, the length of propagation for the $0\ensuremath{\pi}$ pulses is much smaller than the one for the Gaussian probe pulses. Using the dark and bright basis, we explain this behavior. Some possible applications are discussed.

We study the superfluorescence (SF) from a gas of rubidium atoms. The atoms of a dense vapor are excited to the 5D state from the 5S state by a two-photon process driven by 100-fs laser pulses. The atoms decay to the 6P state and then to the 5S state. The SF emission at 420 nm on the 6P-5S transition is recorded by a streak camera with picosecond time resolution. The time duration of the generated SF is tens of picoseconds, which is much shorter than the time scale of the usual relaxation processes, including spontaneous emission and atomic coherence dephasing. The dependence of the time delay between the reference input pulse and SF is measured as a function of laser power. The experimental data are described quantitatively by a simulation based on the semiclassical atom-field interaction theory. The observed change in scaling laws for the peak intensity and delay time can be elucidated by an SF theory in which the sample length is larger than the cooperation length.

We study the dynamics of a two-level system driven by an off-resonant few-cycle pulse which has a phase jump $\phi$ at $t=t_{0}$, in contrast to many cycle pulses, under non rotating-wave approximation (NRWA). We give a closed form analytical solution for the evolution of the probability amplitude $|C_{a} (t)|$ for the upper level. Using the appropriate pulse parameters like phase-jump $\phi$, jump time $t_{0}$, pulse width $\tau$, frequency $\nu$ and Rabi frequency $\Omega_{0}$ the population transfer, after the pulse is gone, can be optimized and for the pulse considered here, enhancement of $10^{6}-10^{8}$ factor was obtained., Comment: 5 Pages, 7 Figures

We present analytical solutions for the problem of a two-level atom driven by a class of chirped pulses. The solutions are given in terms of Heun functions. Using appropriate chirping parameters an enhancement of four-orders of magnitudes in the population transfer is obtained., Comment: 5 pages, 5 figures

We study population transfer and excitation of quantum coherence in atomic and nuclear systems interacting with strong ultra-short laser pulses. We discuss CEP effects and possible applications to cooperative generation of XUV and nuclear radiation.

We study dynamics of a two-level system driven by a strong off-resonant electromagnetic field. We derive an analytical solution for arbitrary pulse shape. We discuss possible applications and presented experimental demonstration of the results obtained.

We have experimentally studied the reflection of a weak probe beam from a dense atomic potassium vapor in the presence of a strong laser field tuned to the atomic resonance transition. We have observed an Autler-Townes doublet under hitherto unexplored conditions, namely that the Rabi frequency induced by the strong laser field is much smaller than the self-broadened width of the resonance transition of the unexcited vapor. We attribute our observation to a reduction of the atomic decoherence by the strong drive field. We present a theoretical model of nonlinear processes in a dense atomic gas to explain the observed results.

We demonstrate a strong coherent backward wave oscillation using forward propagating fields only. This is achieved by applying laser fields to an ultradispersive medium with proper chosen detunings to excite a molecular vibrational coherence that corresponds to a backward propagating wave. The physics then has much in common with the propagation of ultraslow light. Applications to coherent scattering and remote sensing are discussed.

We have shown that a coherently driven solid state medium can potentially produce strong controllable short pulses of THz radiation. The high efficiency of the technique is based on excitation of maximal THz coherence by applying resonant optical pulses to the medium. The excited coherence in the medium is connected to macroscopic polarization coupled to THz radiation. We have performed detailed simulations by solving the coupled density matrix and Maxwell equations. By using a simple $V$-type energy scheme for ruby, we have demonstrated that the energy of generated THz pulses ranges from hundreds of pico-Joules to nano-Joules at room temperature and micro-Joules at liquid helium temperature, with pulse durations from picoseconds to tens of nanoseconds. We have also suggested a coherent ruby source that lases on two optical wavelengths and simultaneously generates THz radiation. We discussed also possibilities of extension of the technique to different solid-state materials.