Resonance Fluorescence Spectrum Research Articles

Влияние температуры на интенсивности вибронных полос в спектрах поглощения и резонансной флуоресценции

Влияние температуры на интенсивности вибронных полос в спектрах поглощения и резонансной флуоресценции

DETUNING-DEPENDENT NARROWING OF MOLLOW TRIPLET LINES OF DRIVEN QUANTUM DOTS

We study the two-time correlation function and the resonance fluorescence spectrum of a semiconductor quantum dot excited by a strong off-resonant laser pulse. The obtained analytical expressions exhibit a specific detuning-dependent damping of Rabi oscillations of the dressed quantum dot as well as a detuning-dependent width of Mollow-triplet lines. In the absence of pure dephasing, the central peak of the triplet is broadened, upon increasing detuning, but the blue and red side peaks are narrowed. We demonstrate that pure dephasing processes can invert these dependences. A crossover between the regimes of detuning-dependent narrowing and broadening of the side and central peaks is identified. The predicted effects are consistent with resent experimental results and numerical calculations.

Effects of counter-rotating-wave terms of the driving field on the spectrum of resonance fluorescence

We investigate the fluorescence spectrum of a two-level system driven by a monochromatic classical field by the Born-Markovian master equation based on a unitary transformation. The main purpose is to understand the effects of counter-rotating-wave terms of the driving on spectral features of the fluorescence. We have derived an analytical expression for the fluorescence spectrum, which is different from Mollow's theory, while Mollow's result on resonance is the limiting case of ours in moderately weak driving regimes. Our results demonstrate precisely that the counter-rotating-wave terms of the driving play an important role in the fluorescence spectrum for intense driving: (i) the counter-rotating coupling suppresses the red sideband in the Mollow triplet and it enhances the blue one in explicitly contrast to the well-known equal intensity of the sideband in Mollow's theory, (ii) the higher-order Mollow triplets appear as a characteristic spectral feature arising from counter-rotating-wave terms of the driving, and (iii) a significant frequency shift of the sidebands is observed, which depends on both the detuning and driving strength.

Resonance fluorescence spectrum of ap-doped quantum dot coupled to a metallic nanoparticle

The resonance fluorescence spectrum (RFS) of a hybrid system consisting of a $p$-doped semiconductor quantum dot (QD) coupled to a metallic nanoparticle (MNP) is analyzed. The quantum dot is described as a four-level atomlike system using the density matrix formalism. The lower levels are Zeeman-split hole spin states and the upper levels correspond to positively charged excitons containing a spin-up, spin-down hole pair and a spin electron. A linearly polarized laser field drives two of the optical transitions of the QD and produces localized surface plasmons in the nanoparticle, which act back upon the QD. The frequencies of these localized plasmons are very different along the two principal axes of the nanoparticle, thus producing an anisotropic modification of the spontaneous emission rates of the allowed optical transitions, which is accompanied by very minor local field corrections. This manifests into dramatic modifications in the RFS of the hybrid system in contrast to the one obtained for the isolated QD. The RFS is analyzed as a function of the nanoparticle's aspect ratio, the external magnetic field applied in the Voigt geometry, and the Rabi frequency of the driving field. It is shown that the spin of the QD is imprinted onto certain sidebands of the RFS, and that the signal at these sidebands can be optimized by engineering the shape of the MNP.

Resonance Fluorescence of Optically Dense Ensembles of Three-Level Resonant Centers Under Conditions of Energy-Level Population Auto-Oscillations*

A consistent theoretical approach for analyzing the dynamics of three-level resonant centers interacting locally with a laser field was presented. The formalism was based on the behavior of a density matrix for an optically dense ensemble of multi-level resonant centers in an external field. It was shown that the influence of the local field on the ensemble dynamics in an external field of constant intensity that resulted in the well-known intrinsic (non-cavity) optical bistability could also be a reason for auto-oscillations of the level populations. As a result, new components appeared in the resonance fluorescence spectra as compared with the spectrum of a single resonant center.

Jet-Cooled Spectroscopy of the α-Methylbenzyl Radical: Probing the State-Dependent Effects of Methyl Rocking Against a Radical Site

The state-dependent spectroscopy of α-methylbenzyl radical (α-MeBz) has been studied under jet-cooled conditions. Two-color resonant two-photon ionization (2C-R2PI), laser-induced fluorescence, and dispersed fluorescence spectra were obtained for the D0-D1 electronic transition of this prototypical resonance-stabilized radical in which the methyl group is immediately adjacent to the primary radical site. Extensive Franck-Condon activity in hindered rotor levels was observed in the excitation spectrum, reflecting a reorientation of the methyl group upon electronic excitation. Dispersed fluorescence spectra from the set of internal rotor levels are combined with the excitation spectrum to obtain a global fit of the barrier heights and angular change of the methyl group in both D0 and D1 states. The best-fit methyl rotor potential in the ground electronic state (D0) is a flat-topped 3-fold potential (V3" = 151 cm(-1), V6" = 34 cm(-1)) while the D1 state has a lower barrier (V3' = 72 cm(-1), V6' = 15 cm(-1)) with Δφ = ± π/3, π, consistent with a reorientation of the methyl group upon electronic excitation. The ground state results are compared with calculations carried out at the DFT B3LYP level of theory using the 6-311+G(d,p) basis set, and a variety of excited state calculations are carried out to compare against experiment. The preferred geometry of the methyl rotor in the ground state is anti, which switches to syn in the D1 state and in the cation. The calculations uncover a subtle combination of effects that contribute to the shift in orientation and change in barrier in the excited state relative to ground state. Steric interaction favors the anti conformation, while hyperconjugation is greater in the syn orientation. The presence of a second excited state close by D1 is postulated to influence the methyl rotor properties. A resonant ion-dip infrared (RIDIR) spectrum in the alkyl and aromatic CH stretch regions was also recorded, probing in a complementary way the state-dependent conformation of α-MeBz. Using a scheme in which infrared depletion occurs between excitation and ionization steps of the 2C-R2PI process, analogous infrared spectra in D1 were also obtained, probing the response of the CH stretch fundamentals to electronic excitation. A reduced-dimension Wilson G-matrix model was implemented to simulate and interpret the observed infrared results. Finally, photoionization efficiency scans were carried out to determine the adiabatic ionization threshold of α-MeBz (IP = 6.835 ± 0.002 eV) and provide thresholds for ionization out of specific internal rotor levels, which report on the methyl rotor barrier in the cation state.

Plasmonic luminescent core–shell nanocomposites-enhanced chemiluminescence arising from the decomposition of peroxomonosulfite

A core–shell structure of plasmonic luminescent nanocomposite, Ni@SiO2@FITC@SiO2 (NSFS) combining the stable luminescence of fluorophore with the excellent plasmonic property of metal nanomaterials, has been synthesized through layer-by-layer assembly. The effect of NSFS on the ultraweak chemiluminescence (CL) reaction of hydrogen peroxide (H2O2) and sodium bisulfite (NaHSO3) was explored for the first time. It was found that the CL intensity from the decomposition of peroxomonosulfite was significantly enhanced by NSFS. The mechanism of the nanocomposite-enhanced CL was revealed as the coupling of chemically induced excited states of fluorescein isothiocyanate (FITC) with surface plasmons of Ni nanoparticles based on studies of CL emission spectra, electron spin resonance spectra, extinction spectra and fluorescence spectra. The work sheds new light on the characteristics of the versatile materials and gives us new insight into the optical properties of fluorophores.

Cooperative fluorescence from a strongly driven dilute cloud of atoms

We investigate cooperative fluorescence in a dilute cloud of strongly driven two-level emitters. Starting from the Heisenberg equations of motion, we compute the first-order scattering corrections to the saturation of the excited-state population and to the resonance-fluorescence spectrum, which both require going beyond the state-of-the-art linear-optics approach to describe collective phenomena. A dipole blockade is observed due to long range dipole-dipole coupling that vanishes at stronger driving fields. Furthermore, we compute the inelastic component of the light scattered by a cloud of many atoms and find that the Mollow triplet is affected by cooperativity. In a lobe around the forward direction, the inelastic Mollow triplet develops a spectral asymmetry, observable under experimental conditions.

Modification of the resonance fluorescence spectrum of a two-level atom in the near field of a plasmonic nanoparticle

The resonance fluorescence spectrum of an atom located in the near field of a plasmonic nanoparticle is considered. It is shown that, as the atom gets close to the nanoparticle, the high-frequency peak of the Mollow triplet disappears and the spectrum has a Fano resonance shape. The low-frequency peak also disappears as the distance between the atom and the nanoparticle decreases further. For small distances, when the atom interacts with the nanoparticle much more strongly than with the external field, the spectrum represents a Lorentzian line whose width is proportional to the square of the atom-nanoparticle coupling constant.

Accessing quantum nanoplasmonics in a hybrid quantum dot–metal nanosystem: Mollow triplet of a quantum dot near a metal nanoparticle

We present a theoretical study of the resonance fluorescence spectra of an optically driven quantum dot placed near a single metal nanoparticle. The metallic reservoir coupling is calculated for an 8-nm metal nanoparticle using a time-convolutionless master equation approach where the exact photon reservoir function is included using Green function theory. By exciting the system coherently near the nanoparticle dipole mode, we show that the driven Mollow spectrum becomes highly asymmetric due to internal coupling effects with higher-order plasmons. We also highlight regimes of resonance squeezing and broadening as well as spectral reshaping through light propagation. Our master equation technique can be applied to any arbitrary material system, including lossy inhomogeneous structures, where mode expansion techniques are known to break down.

Resonance fluorescence spectra near a photonic bandgap

Optical radiation properties of a single atom have been among the most popular research problems in quantum optics since the early years. In contrast to classical intuition, the quantum theory asserts that the optical radiation properties are not wholly determined by the intrinsic characteristics of the atom but also depend on the electromagnetic mode structure seen by the atom even when these modes are empty (i.e., vacuum states). The emission frequency will be slightly modified through the interaction with these vacuum states (the Lamb shift), and the spontaneous emission rate can be enhancedwhen the atom is placed inside a resonant cavity (the Purcell effect). These are the two well-known examples dated back to the 1940s. Another fundamental example is the resonance fluorescence from a two-level atom. If the twolevel atom is excited on resonance by an intense optical field, the fluorescence spectrum can exhibit three peaks (the Mollow triplet), which was theoretically predicted in 1969 [1] and experimentally observed in 1975 by using the sodium atoms [2]. The coupling of the optical pump field with the two atomic levels produces Rabi oscillation in the time domain or equivalently the energy level splitting in the frequency domain. From such a dressed atom picture, it is easy to understand that the down transition from the upper two sublevels to the lower two sub-levels will exhibit three transmission frequencies since the frequency splitting of ω

X-ray laser spectroscopy with an electron beam ion trap at the free electron laser LCLS

We present a first laser spectroscopy experiment in the keV energy regime, performed at the Free-Electron Laser LCLS at Stanford. An electron beam ion trap was used to provide a target of highly charged O, F and Fe ions. The resonant fluorescence spectra obtained for various transitions were calibrated to simultaneously measured Lyman lines of hydrogenic ions.

The resonance fluorescence spectra of N two-level quantum systems located around the spherical nanoantenna

Modification of the resonance fluorescence spec-trum of a two-level atom driving by a lasermonochromatic field in the close proximity of aplasmonic nanostructure (metal sphere) is studiedin detail. It is shown that one can control thisspectrum varying four key parameters: distancebetween the atom and the nanosphere, atom’s lo-cation around the nanosphere, the radius of thenanosphere, and polarization of the incident ra-diation. These parameters affect the local fieldenchancement, transition frequency shift and themodification of the radiative decay rate of the atominteracting with the nanosphere, which lead tomodification of the resonance fluorescence spec-trum of the atom (frequency shift of the satellitelines in the Mollow-type triplet, widths of the lines,the spectrum intensity) by contrast with that onein free space. The permittivity of the metal thenanosphere is made of is also an additional param-eter, which defines the nonradiative decay. The lat-ter in combination with other parameters allows tocontinuously control the transition from resonancefluorescence enhancement to its quenching. Thecalculation results are generalized to the case ofN two-level atoms, distributed around the nanopar-ticle in the close proximity of its surface. Thecalculations were performed for different positionsof the detector relative to the system nanoparticle-atom(s).

Resonance fluorescence in ultrafast and intense x-ray free-electron-laser pulses

The spectrum of resonance fluorescence is calculated for a two-level system excited by an intense, ultrashort x-ray pulse made available, for instance, by free-electron lasers such as the Linac Coherent Light Source. We allow for inner-shell hole decay widths and destruction of the system by further photoionization. This two-level description is employed to model neon cations strongly driven by x rays tuned to the $1s2{p}^{\ensuremath{-}1}\ensuremath{\rightarrow}1{s}^{\ensuremath{-}1}2p$ transition at $848\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$; the x rays induce Rabi oscillations which are so fast that they compete with Ne $1s$-hole decay. We predict resonance fluorescence spectra for two different scenarios: first, chaotic pulses based on the self-amplified spontaneous emission principle, like those presently generated at x-ray free-electron-laser facilities and, second, Gaussian pulses which will become available in the foreseeable future with self-seeding techniques. As an example of the exciting opportunities derived from the use of seeding methods, we predict, in spite of the above obstacles, the possibility to distinguish at x-ray frequencies a clear signature of Rabi flopping in the spectrum of resonance fluorescence.

Observation of linewidth narrowing due to a spontaneously generated coherence effect

We investigate the resonance fluorescence spectrum of an atomic three-level ladder system driven by two laser fields. We show that such a system emulates to a large degree a V-type atom with parallel dipole moments-the latter being a system that exhibits spontaneously generated coherence and can display ultrasharp spectral lines. We find a suitable energy scheme in a 85Rb atom and experimentally observe the narrowing of the central peak in a rubidium atomic beam. The corresponding spectrum can convincingly demonstrate the existence of spontaneously generated coherence.

Anomalous phonon-mediated damping of a driven quantum dot embedded in a high-Q semiconductor microcavity

We describe and employ a recently developed polaron master equation model to study the fluorescence spectra of a coherently driven quantum dot (QD) placed within a high-Q semiconductor microcavity (with Q the quality factor). We investigate phonon-induced damping in a regime where many cavity photons are required, and we also compare the resonance fluorescence spectra obtained using an effective phonon master equation in Lindblad form where simple analytical expressions are identified for various phonon-induced scattering rates. We consider two separate continuous-wave pumping scenarios, where either the system is driven through exciton pumping or the system is driven via the cavity. The cavity-QED (quantum electrodynamics) system is pumped sufficiently strongly such that the low-energy sideband of the Mollow triplet is tuned across the cavity mode resonance which is negatively detuned from the QD. For comparison, we also consider the case where the QD-cavity detuning is large enough such that the Mollow triplets do not spectrally overlap with the cavity mode. We find that the full width at half maximum (FWHM) of the high-energy Mollow sideband shows a pronounced nonlinear dependence on the pump intensity when the low-energy component of the triplet overlaps with the cavity mode (or vice versa), and can even be reduced with increased pumping. However, the FWHM depends linearly on the pump intensity when the Mollow triplets are far from the cavity resonance. We observe similar fluorescence spectra for both the exciton-driven system and the cavity-driven system.

Modification of two-level-atom resonance fluorescence near a plasmonic nanostructure

Modification of the resonance fluorescence spectrum of a two-level atom driven by a monochromatic field in the close proximity of a plasmonic nanostructure (metal sphere) is studied in detail. It is shown that one can control this spectrum by varying several key parameters: (i) the radius of the nanosphere, (ii) polarization of the incident radiation, and (iii) the atom's location around the nanosphere (its radial coordinate and polar angle in the spherical coordinate system). These parameters affect the local field enhancement and the modification of the radiative decay rate of the atom interacting with the nanosphere, which leads to modification of the resonance fluorescence spectrum of the atom (e.g., frequency shift of the satellite lines in the Mollow-type triplet, widths of the lines, and the spectrum intensity) by contrast with that in free space. The permittivity of the metal the nanosphere is made of is also an additional parameter, which defines the nonradiative decay. The latter in combination with other parameters allows continuous control of the transition from resonance fluorescence enhancement to its quenching.

Polaron master equation theory of the quantum-dot Mollow triplet in a semiconductor cavity-QED system

We present a comprehensive theoretical study of the resonance fluorescence spectra of an coherently-driven quantum dot (QD) placed inside a high-$Q$ semiconductor cavity and interacting with an acoustic-phonon bath. We derive a quantum master equation (ME) in the polaron frame, which includes exciton-phonon and exciton-cavity coupling to all orders. This work details and extends the theory used in a recent paper [C. Roy and S. Hughes, Phys. Rev. Lett. 106, 247403 (2011)] to describe the QD Mollow triplet in the regime of semiconductor cavity QED. Here, we introduce two ME forms, Nakajima-Zwanzig and time convolutionless (TC), both to second order in the system--phonon-reservoir perturbation. In the polaron frame, these two ME forms are shown to yield equivalent population dynamics and fluorescence spectra for a continuous wave (cw) driving field. We also demonstrate that a Markov approximation is valid for computing the incoherent scattering processes, and we subsequently exploit the TC ME to explore the resonance fluorescence spectra of an exciton-driven QD. Both cavity-emitted and exciton-emitted spectra are studied, and these are found to have qualitatively different spectral features. Using a coherent driving field, the well-known characteristics of the atomic Mollow triplet are shown to be considerably modified with electron--acoustic-phonon scattering, and we highlight the key effects arising from both cavity coupling and electron-phonon coupling. Regimes of pronounced cavity feeding and anharmonic cavity QED are exemplified, and we find that the cavity coupling depends sensitively on the exciton-cavity detuning and the temperature of the phonon bath. We show how the full width at half maximum (linewidth) of the Mollow-triplet sidebands varies as a function of the square of the Rabi frequency of the cw pump. Phonon-mediated cavity coupling also contributes to the spectral broadening of the Mollow triplet, depending upon the exciton-cavity detuning and the strength of the exciton-cavity coupling rate. Finally, we calculate the fluorescence spectra for off-resonance cw driving and investigate the resulting Mollow-triplet linewidths.

Resonance-fluorescence-localization microscopy with subwavelength resolution

We evaluate the resonance-fluorescence spectrum of a bunch of two-level atoms driven by a gradient coherent laser field. The result shows that we can determine the positions of atoms from the spectrum even when the atoms locate within the subwavelength range and the dipole-dipole interaction is significant. This far-field resonance-fluorescence-localization microscopy method does not require point-by-point scanning, and it may be more time efficient. We also give a possible scheme to extract the position information in an extended region without requiring more peak power of the laser. Finally, we also briefly discuss how to perform two-dimensional imaging based on our scheme.

Signatures of single-site addressability in resonance fluorescence spectra

Pioneering methods in recent optical lattice experiments allow to focus laser beams down to a spot size that is comparable to the lattice constant. Inspired by this achievement, we examine the resonance fluorescence spectra of two-level atoms positioned in adjacent lattice sites and compare the case where the laser hits only one atom (single site addressing) with cases where several atoms are illuminated. In contrast to the case where the laser hits several atoms, the spectrum for single site addressing is no longer symmetric around the laser frequency. The shape of the spectrum of fluorescent light can therefore serve as a test for single site addressing. The effects we find can be attributed to a dipole-dipole interaction between the atoms due to mutual exchange of photons.