Resonance Fluorescence Spectrum Research Articles

Nonlinear refraction and nonlinear absorption of CdSe0.3S0.7/ZnS quantum dots

Nonlinear optical properties of CdSe0.3S0.7/ZnS quantum dots(QDs) were investigated by Z-scan technique using the Nd:YAG laser second harmonic radiation (λ=532nm, t=35ps). The Z-scan results show that the CdSe0.3S0.7 QDs have strong reverse saturable absorption(RSA). Based on the linear absorption spectrum and one-photon resonant fluorescence spectrum, it is reasonable for us to infer that the nonlinear absorption arises from free carrier absorption(FCA). Based on standard Z-scan theory, we obtain the FCA cross section and nonlinear refractive index of CdSe0.3S0.7/ZnS QDs. These QDs have average absorption cross section of 2.8×10−16cm2 and nonlinear refractive index on order of 10−7esu. The strong nonlinear absorption and large nonlinear refractive index perhaps allow them to be applicable in the optoelectronic field.

A study on resonance Rayleigh scattering spectra of Humic Acid (HA)

This paper deals with the influences of pH, acidity and ionic intensity of the solutions on the resonance Rayleigh scattering spectra and fluorescence spectra of humic acid. When the pH value is low and the acidity and ionic intensity are high, the resonance Rayleigh spectra and fluorescence spectra both show a tendency of increasing, though the former’s intensity is much higher. In combination with the transmission electron microscope data, the factors leading to the occurrence and enhancement of the resonance Rayleigh scattering spectra of humic acid were explored. It is considered that particle enlargement caused by aggregation, the increase of heterogeneity, the increase of hydrophobility, the formation of interface, etc., are the factors leading to the occurrence and enhancement of the resonance Rayleigh scattering spectra of humic acid. As the intensity of the resonance Rayleigh scattering spectra of humic acid is much higher, resonance Rayleigh scattering spectroscopy can be used as a newly developed spectrum technology, which is more sensitive and simpler, to study humic acid and its complicated behaviors.

Resonance Fluorescence from Semiconductor Quantum Dots: Beyond the Mollow Triplet

We show that the resonance fluorescence spectrum of a quantum dot excited by a strong optical pulse contains multiple peaks beyond those of the Mollow triplet. We show that as the area of the optical pulse is increased, new side peaks split off the central peak and shift in frequency. A simple analytical theory has been derived, which quantitatively accounts for the appearance and position of the peaks. This theory explains the physics responsible for the multiple peaks. By considering the time-dependent spectrum we demonstrate a time ordering of the side peaks, which is further evidence for the suggested physical explanation.

Resonance fluorescence of a two-level atom in a feedback loop

The resonance fluorescence spectrum of a two-level atom in a classical light field, whose phase is changed by a feedback system by π after the detection of each spontaneous photon, has been calculated. The spectrum resembles the well-known Rautian-Mollow triplet. However, the sidebands become sharply asymmetric at a nonzero frequency detuning of the light field from the atomic resonance transition. In addition, the spectrum retains the pronounced triplet shape even beyond the secular approximation, when the sidebands of the Rautian-Mollow structure become unresolved in the absence of the feedback.

Geometric Phase of Mixed States in the Resonance Fluorescence Spectrum

Berry phase of mixed state is investigated for modified Bloch equation with constant terms, which was used to explain sidebands in the spectrum of fluorescent light. The results show that for the physical phenomenon of sidebands, the Berry phases under the quasicyclic evolution exhibit as a geometric phase transition, where the transition point and region depend fully on the dynamics of population inversion and mixed degree. We find that, furthermore, the transition position is correlated to photon number. Thus the open quantum system preserves indeed a memory of its evolution in terms of the Berry phase, which may provide another clue for looking for devices of quantum memory in terms of geometric sideband approach.

Dissipation in a spin bath: Thermally induced coherent intensity and spectral splitting

We have explored the role of dissipation in the scattered intensity and spectrum of resonance fluorescence from an excited two-level system in a spin bath. It has been shown that depending on the field strength a crossover temperature sets up a boundary between the coherent and incoherent intensity regimes, and at low field the scattered intensity may be coherent even at high temperature. We demonstrate the formation of a thermally induced Mollow triplet in the low-field regime.

Subwavelength optical microscopy in the far field

We present a procedure for subwavelength optical microscopy. The identical atoms are distributed on a plane and shined with a standing wave. We rotate the plane to different angles and record the resonant fluorescence spectra in the far field, from which we can obtain their distance and location information. This procedure also works for atomic separation above one wavelength and therefore provides a seamless microscopy.

Phonon-Dressed Mollow Triplet in the Regime of Cavity Quantum Electrodynamics: Excitation-Induced Dephasing and Nonperturbative Cavity Feeding Effects

We study the resonance fluorescence spectra of a driven quantum dot placed inside a high-Q semiconductor cavity and interacting with an acoustic phonon bath. The dynamics is calculated using a time-convolutionless master equation in the polaron frame. We predict pronounced spectral broadening of the Mollow sidebands through off-resonant cavity emission which, for small cavity-coupling rates, increases quadratically with the Rabi frequency in direct agreement with recent experiments using semiconductor micropillars [S. M. Ulrich et al., preceding Letter, Phys. Rev. Lett. 106, 247402 (2011)]. We also demonstrate that, surprisingly, phonon coupling actually helps resolve signatures of the elusive second rungs of the Jaynes-Cummings ladder states via off-resonant cavity feeding. Both multiphonon and multiphoton effects are shown to play a qualitatively important role on the fluorescence spectra.

Quantum jumps induced by the center-of-mass motion of a trapped atom

We theoretically study the occurrence of quantum jumps in the resonance fluorescence of a trapped atom. Here, the atom is laser cooled in a configuration of level such that the occurrence of a quantum jump is associated to a change of the vibrational center-of-mass motion by one phonon. The statistics of the occurrence of the dark fluorescence period is studied as a function of the physical parameters and the corresponding features in the spectrum of resonance fluorescence are identified. We discuss the information which can be extracted on the atomic motion from the observation of a quantum jump in the considered setup.

Mollow Triplet under Incoherent Pumping

A counterpart of the Mollow triplet (luminescence line shape of a two-level system under coherent excitation) is obtained for the case of incoherent excitation in a cavity. The system acquires coherence through the strong-coupling between the cavity and the emitter. Analytical expressions, in excellent agreement with numerical results, pinpoint analogies and differences between the conventional resonance fluorescence spectrum and its cavity QED analogue under incoherent excitation. Most notably, the satellites broaden and split sublinearly with increasing incoherent pumping.

SQUEEZING SPECTRUM OF RESONANCE FLUORESCENCE FOR A CAVITY COUPLED TWO-LEVEL ATOM

We investigate the modification of squeezing spectrum of resonance fluorescence of a weakly driven two-level atom coupled with a frequency-tunable cavity mode with finite bandwidth. For atom-laser resonance or small atom-laser detuning, as the cavity resonant frequency adjusted, the squeezing switches between the phase quadratures. When the coupling between the atom and the cavity increases, the degree of squeezing in the out-of-phase quadrature decreases, while the noise in the in-phase quadrature decreases and can be squeezed. For proper atom-laser detuning, with the cavity-laser detuning increasing, the degree of squeezing in the in-phase quadrature increases and the squeezing spectrum exhibits two-mode squeezing.

Chemiluminescence Arising from the Decomposition of Peroxymonocarbonate and Enhanced by CdTe Quantum Dots

CdTe quantum dots (QDs) capped with mercaptoacetic acid were applied to the hydrogen peroxide-sodium hydrogen carbonate chemiluminescence (CL) system. The CL emission intensity was significantly enhanced by different sizes of CdTe QDs. Peroxymonocarbonate (HCO(4)(-)) was formed in the reaction of hydrogen peroxide and sodium hydrogen carbonate, which was a reactive oxygen species. Decomposition of HCO(4)(-) generated superoxide ion radical (·O(2)(-)) and hydroxide radical (·OH). The enhanced CL was induced by the excited CdTe QDs, which could be produced from the combination of hole (oxidized QDs (h(+))) and electron (reduced QDs (e(-))) injected QDs. Radical scavengers and organic reagents such as nitro blue tetrazolium chloride (NBT), cytochrome c, sodium azide, ascorbic acid, thiourea, tert-butanol, and dimethyl sulphoxide were used to study the emitting species. The intermediate hydroxide radical and superoxide ion were key species for producing hole and electron-injected QDs. Four emitters such as (1)O(2), (O(2))(2)*, (CO(2))(2)* and CdTe* were detected in the CL system. The mechanism was discussed based on the CL emission spectra, electron spin resonance spectra, fluorescence spectra, and UV-vis absorption spectra. The CL properties of CdTe QDs will be helpful to study semiconductor nanocrystals and will open new avenues for the application of QDs in many fields, such as chemistry, biology, microbiology, and biochemistry.

Control of resonance fluorescence in coupled quantum dots

Spectral and squeezing properties of the resonance fluorescence of two coupled quantum dots forming an effective Λ-type three-level structure and connected with two normal metal leads are studied. We assume that one of the dots is driven by a strong light field and find that by modulating the bias voltage and the dot-lead coupling strengths, the electron can be selectively trapped in one of the two dressed states produced by the light field. The trapped electron possesses large coherence, and the resulting unbalanced dressed-state populations can lead to a significant enhancement or suppression of the sidebands in the resonance fluorescence spectrum. Moreover, a wideband squeezing can be generated that may be significantly enhanced by modulating the gate voltage.

Interplay of vacuum-mediated inter- and intra-atomic couplings in a pair of atoms

The resonance fluorescence emitted by a system of two dipole-dipole interacting nearby four-level atoms in a $J=1/2\ensuremath{\leftrightarrow}J=1/2$ configuration is studied. This setup is the simplest realistic model system which provides a complete description of the (inter-atomic) dipole-dipole interaction for arbitrary orientation of the inter-atomic distance vector, and at the same time allows for intra-atomic spontaneously generated coherences. Our main interest is the interplay of both these different coupling mechanisms. We discuss different methods to analyze the contribution of the various vacuum-induced coupling constants to the total resonance fluorescence spectrum. These allow us to find a dressed state interpretation of the contribution of the different inter-atomic dipole-dipole couplings to the total spectrum. We further study the role of the spontaneously generated coherences, and identify two different contributions to the single-particle vacuum-induced couplings. We show that they have a noticeable impact on the total resonance fluorescence spectrum down to small inter-atomic distances, even though the dipole-dipole coupling constants then are much larger in magnitude than the the single-particle coupling constants. Interestingly, we find that the inter-atomic couplings can induce an effect of the intra-atomic spontaneously generated coherences on the observed spectra which is not present in single-atom systems.

Chemiluminescence behavior of sodium hydrogen carbonate in the potassium permanganate-hydrogen peroxide reaction

Chemiluminescence (CL) phenomenon of hydrogen peroxide with potassium permanganate in the presence of sodium hydrogen carbonate was reported. Effects of the surfactant on the CL system were investigated. Nonionic surfactants could effectively increase the CL signal. Radical scavengers and organic reagents such as nitro blue tetrazolium chloride (NBT), cytochrome c, sodium azide, ascorbic acid, thiourea, tert-butanol and dimethyl sulphoxide were used to study the emitting species. CL emission spectrum was recorded and the results showed that the maximal emission wavelengths of NaHCO3-H2O2-KMnO4 system were 440 and 634 nm. The mechanism was discussed based on electron spin resonance (ESR) spectra, fluorescence spectra and UV-vis absorption spectra. The addition of rhodamine B or uranine into this CL system enhanced the CL signal. It was due to part of the energy transfer from singlet oxygen and excited triplet dimers of two CO2 molecules to rhodamine B or uranine. The CL could be induced by excited rhodamine B or uranine.

Nonclassical properties of atomic radiation field in a nonlinear photonic crystal

We study spectral properties and photon statistical characteristics of a strongly driven two-level atom produced within a nonlinear photonic crystal. This study reveals that when a large discontinuity in the local photon density of states and the cavity field mode is resonant with the central component of the Mollow spectrum of atomic resonance fluorescence, there is squeezing of the cavity field below the quantum shot noise limit and the peak of the cavity field spectrum that is achieved in the nonlinear photonic crystal is higher than that in the linear photonic crystal. Furthermore, we can see the statistics of the photons emitted by the atom into the microcavity is sub-Poissonian and close to Poissonian when the frequency of the driving field is high.

Effects of spontaneously generated coherence on the resonance fluorescence spectrum of a laser-driven four-level diamond-shape atomic system

The resonance fluorescence spectra of a laser-driven four-level closed-loop diamond-shape atomic system are investigated in this paper. The four decay channels in the system interfere with each other in different ways according to several distinct arrangements of the two intermediate atomic levels, and hence produce the spontaneously generated coherence (SGC) effects. The numerical calculation shows that the amplitude, the linewidth and the number of sidebands of the resonance fluorescence spectrum are deeply modified by the SGC effects; accordingly, interesting effects such as the spectral-line narrowing, spectral-line suppression, spectral-line enhancement and even the complete quenching of fluorescence are generated under certain conditions. Moreover, we propose a new kind of level structure, the seesaw structure, in which the SGC effect can also be attained without the rigorous conditions of degenerate levels. In the end, we discuss the possibility of demonstrating our scheme in real atomic systems and summarize our results.

Applications of the master equation of a two-level atom in a narrow-bandwidth squeezed vacuum

In this paper, we derive the time-dependent solution of the effective master equation for the reduced density matrix operator of a two-level atom driven by a coherent laser field and damped by a finite-bandwidth squeezed vacuum. The master equation has the standard form known from the broadband squeezing approaches but with new effective squeezing parameters N ˜ and M ˜ and new rate coefficients. There are also new terms, proportional to β , which are essentially narrow-bandwidth modifications determined by the parameters γ n , δ n and the shifts δ N and δ M . These parameters become zero when the squeezing bandwidth tends to infinity. The effects of the detuning parameter Δ of the driving laser field frequency ω L from the atomic resonance ω A and the squeezing parameters N ˜ and M ˜ on the atomic inversion, the von Neumann entropy and the resonance fluorescence spectrum are discussed.

Reply to ‘Comment on “Discrepancies in the resonance-fluorescence spectrum calculated with two methods” ’

Reply to ‘Comment on “Discrepancies in the resonance-fluorescence spectrum calculated with two methods” ’

Comment on “Discrepancies in the resonance-fluorescence spectrum calculated with two methods”

There are two alternative methods used in the literature to calculate the incoherent part of the spectrum of light scattered by an atomic system. In the first, one calculates the spectrum of the total light scattered by the system and obtains the incoherent part by subtracting the coherent part. In the second method, one introduces the fluctuation operators and obtains the incoherent part of the spectrum by taking the Fourier transform of the two-time correlation function of the fluctuation operators. These two methods have been recognized for years as completely equivalent for evaluating the incoherent part of the spectrum. In a recent paper, Xu et al. [Phys. Rev. A 78, 013407 (2008)] showed that there are discrepancies between the incoherent parts of the stationary spectrum of a three-level $\ensuremath{\Lambda}$-type system calculated with these two methods. The predicted discrepancies can be severe that over a wide range of the Rabi frequencies and atomic decay rates, the spectrum calculated with the variance method can have negative values. In this Comment, we show that there are no discrepancies between these two methods. We show the equivalence of these two methods that leads to the same incoherent spectra which are positive for all frequencies independent of values of the parameters involved. We also identify the source of the discrepancy, that is, in an incorrect treatment of the incoherent part of the spectrum calculated with the two-time correlation function of the fluctuation operators.