Bichromatic Excitation Research Articles

Coherent Control of Absorption Resonances via Antirotating Coupling

Effects of the antirotating coupling on the absorption resonances arepresented for a microwave driven three-level system. It is shown thatnonlinear sideband generation, selective suppression and switching betweenpeaks and dips in the absorption spectrum are obtained as the microwave Rabifrequency is varied. A physical explanation is given in terms of thecoherent superposition of an infinite set of transitions that are associatedwith an infinite set of dressed states, which are created by the rotatingand nonrotating couplings as the equivalent bichromatic excitation.

Phase-sensitive electric modulation of photoluminescence upon bichromatic excitation of atoms

A new type of modulation of the photoluminescence intensity of atoms excited by a bichromatic laser radiation with the frequency ratio 1:2 is proposed and analysed. The modulation is produced by alternating electric field acting on atoms and occurs due to the quantum interference of the amplitudes of two excitation channels of an atom, which proves to be possible because the applied electric field removes the parity selection rule for one of the excitation channels. An important feature of this process is the dependence of photoluminescence on the phase difference of monochromatic components of exciting radiation. The calculation was performed for an alkali metal atom excited at the s—s transition taking the saturation effect into account.

Phase control of spontaneous emission by cascade bichromatic excitation

We study spontaneous emission of a three-level cascade atom driven by a pair of bichromatic fields of equal frequency differences. The fluorescence spectra from the cascade transitions depend strongly on the difference between the relative phases of two bichromatic fields. Selective and almost complete elimination of fluorescence spectra is achieved simply by varying the difference phase.

Bichromatic and trichromatic manipulation of spontaneous emission in a three-level Λ system

Bichromatic and trichromatic manipulation of spontaneous emission in a three-level system in Λ configuration is studied on the basis of density matrix equation and quantum regression theorem. The spontaneous emission spectrum is numerically calculated by using harmonic expansion and matrix inversion. Two characteristic features are shown. Firstly, the central resonance peak, which is absent in the case of monochromatic excitation, is recovered for the bichromatic or trichromatic excitation. Secondly, selective elimination of the spectral lines is obtained by varying the amplitudes and phases of the trichromatic components. For the phase dependence, it is the sum of the relative phases of the two sideband components to the central component that plays a crucial role. The spontaneous emission spectrum is drastically modified once the sum phase is changed, but is kept unchanged regardless of the respective phases when the sum phase is fixed.

Electromagnetically induced diffraction in sodium vapor

By using a sodium atomic vapor we have observed a honeycomb-type multiple-diffraction pattern that was due to a grating induced optically by three noncollinear bichromatic excitation beams. In particular, when the two incident frequencies satisfy the two-photon resonance condition and the atomic density is high, the diffraction is enhanced by electromagnetically induced transparency, and strong Stokes and anti-Stokes components are generated in well-defined directions.

Preparation of Motional Mesoscopic Superpositions of SqueezedCoherent States of N Trapped Ions

A scheme is proposed to generate arbitrary, discretesuperpostions of squeezed coherent states of the squeezed center of mass ofN trapped ions along a straight line in phase space.The scheme is based on aresonant bichromatic excitation of each trapped ion that generatesdisplacement and squeezing in the vibrational motion conditioned to eachinternal state. In this paper, we also show that such a method can beused for the engineering of motional quantum states.

Terahertz generation by difference-frequency mixing of exciton Wannier–Stark ladder states in biased semiconductor superlattices

We report the generation of terahertz (THz) radiation in a biased semiconductor superlattice by difference-frequency mixing of interband transitions. The Wannier–Stark spectrum of the superlattice was selectively excited by two spectrally narrow laser lines. The emitted THz radiation was measured as a function of the energetic splitting and spectral position of the bichromatic excitation. The generation of tunable THz radiation is verified. The results clearly show enhanced THz emission when either continuum or 1s exciton wave packets are excited. Good agreement is obtained between the experiment and the results of an exciton model of the nonlinear coherent dynamics.

Subkilohertz linewidths measured by heterodyne-detected coherent population trapping in sodium vapor.

Subkilohertz spectroscopy by coherent population trapping (CPT) has been performed in Na atomic vapor. The system employs a frequency-swept acousto-optic modulator for bichromatic excitation as well as a heterodyne-lock-in detection technique that permits the direct measurement of the CPT dispersion curves at a good signal-to-noise ratio. A CPT linewidth of -400 Hz was measured as well as a steep refractive-index dispersion of as much as 3.3 x 10(-10) Hz(-1), corresponding to a group velocity of 1800 m/s.

On the bichromatic excitation of a two-level atom with squeezed light

Analytical results for the dynamical evolution of a single two-level atom coupled to an ordinary heat bath and bichromatically excited with two finite bandwidth squeezed fields are presented by solving the Heisenberg equations of motion. Photon statistics of the system in terms of the second-order intensity-intensity correlation function are also discussed. Transient fluorescent intensity as well as intensity correlation function exhibit oscillatory phenomenon even in the weak field limit. The latter also shows enhanced delayed bunching effect. All these effects are sensitive to the bandwidth of squeezed light.

Entangled coherent states and squeezing in N trapped ions

We consider a resonant bichromatic excitation of N trapped ions that generates displacement and squeezing in their collective motion conditioned to their ionic internal state, producing eventually Scrhodinger cat states and entangled squeezing. Furthermore, we study the case of tetrachromatic illumination or producing the so called entangled coherent states in two motional normal modes.

Dark state in ruby: Analysis of the feasibility

Bichromatic excitation of the R1 line in ruby at liquid-helium temperature is considered. The frequency difference of the spectral components of the driving field is resonant with the energy gap between the anticrossed spin sublevels 11/2 and 13/2 in the ground state 4 A2. The condition of the spin coherence trapping in the nonabsorbing state is analyzed. Population trapping is revealed in the reduced spontaneous emission from the excited state and in large spin magnetization of the ground state Cr 31 precessing with the frequency difference of the spectral components of the driving field. This magnetization can be detected by a pickup coil. The lifetime of the induced magnetization is strongly dependent on the ratio of the lifetime of the excited state Eand the spin coherence decay time (T2) s pin .

Bichromatic two-photon coherent excitation of sodium to provide a dual-wavelength guidestar

A bichromatic sodium guide star based on two-photon coherent excitation followed by cascade emission is considered. Results of density matrix computations, including Doppler averaging, are presented for both a three-level model and the full hyperfine structure of the sodium 3S, 3P, and 4S levels. These results indicate that the proposed scheme for a bichromatic guide star is feasible.

Multiphoton coincidence spectroscopy

We extend the analysis of photon coincidence spectroscopy beyond bichromatic excitation and two-photon coincidence detection to include multichromatic excitation and multiphoton coincidence detection. Trichromatic excitation and three-photon coincidence spectroscopy are studied in detail, and we identify an observable signature of a triple resonance in an atom-cavity system.

Ringing in the resonance fluorescence spectrum of a driven two-level atom under bichromatic excitation in a broadband squeezed vacuum

We present analytical results for the steady-state resonance fluorescence spectrum produced by a driven two-level atom excited under two strong coherent fields in the presence of a broadband squeezed vacuum. Here the central frequency of the squeezed vacuum is considered to be resonant as well as off-resonant with the average driving field frequency while maintaining resonance between the atomic transition frequency and the average driving field frequency. We observe a ringing phenomenon in the fluorescence spectrum due to the small frequency difference in the bichromatic field and this phenomenon is found to be sensitive to the detuning of the carrier frequency of squeezed vacuum with respect to the central frequency of the bichromatic field.

Anomalous fluorescence spectra in an extremely weak, broadband squeezed vacuum under monochromatic and bichromatic excitation

We show that a two-level atom interacting with an extremely weak squeezed vacuum can display resonance fluorescence spectra that are qualitatively different to those that can be obtained using fields with a classical analogue. We consider first the free space situation with monochromatic excitation, and then discuss a bichromatically driven two-level atom in a cavity as a practical scenario for experimentally detecting the anomalous features predicted. We show that in the bad cavity limit, the anomalous spectral features appear for a weak squeezed vacuum and large frequency differences of the bichromatic field, conditions which are easily accessible in laboratories. The advantage of bichromatic, as opposed to monochromatic, excitation is that there is no coherent scattering at line centre which could obscure the observations. A scaling law is derived, N∼Ω 4 which relates the squeezed photon number to the Rabi frequency at which the anomalous features appear.

Bichromatic excitation of coherent Raman beats in rare-earth solids.

In coherent Raman beat experiments a laser field coherently scatters off a material excitation that evolves under the internal Hamiltonian of the system. The Raman signal, which is observed as a beat signal between the test laser and the scattered Raman field, provides information on energy-level splittings and dephasing rates. We propose a modification of this technique, which allows, in contrast to the conventional method, the excitation of coherent Raman beats also in systems with large sublevel splittings and small oscillator strengths. For this purpose, we use a bichromatic laser field to excite the coherent superposition in the medium. We calculate the evolution of the coherence during the laser excitation analytically and discuss its dependence on various experimental parameters. We compare the theoretical results with experimental data from ${\mathrm{Pr}}^{3+}$:${\mathrm{YAlO}}_{3}$.

Optical resonance and coherent transients in dressed atomic systems.

We experimentally investigate the transient dynamics of two-level atoms driven by a bichromatic optical excitation field when the bichromatic field is configured to contain a strong resonant component and a weaker nonresonant perturber component. We concentrate on the experimental situation in which the two-level atoms are prepared in an eigenstate (dressed state) of the coupled atom-resonant field system prior to exposure to the perturber field. We experimentally demonstrate perturber-field-induced, optical nutation of the dressed-state populations. We show how nutation of the dressed-state populations manifests itself in the fluorescence signal of the transiently driven atoms. We observe that the detuning of the perturber field from transitions between the dressed states of the atom-resonant field system plays the same role in determining the character of the dressed-state optical nutation as the atom-field detuning in the case of the ordinary optical nutation of atomic populations. Our experiments demonstrate the utility of the dressed-atom picture in understanding the complex dynamics of bichromatically driven atoms in a variety of circumstances.

Phase-sensitive dynamics of bichromatically driven two-level atoms.

Observations of qualitatively different coherent transient phenomena associated with bichromatic optical excitation are reported. The effects demonstrated include the control of atomic dynamics through a variation of the initial relative phase of the driving fields and the polarization of population within atom-field dressed states. The dynamics observed are fundamentally more complex than those characteristic of monochromatically driven atoms.

Resonance-fluorescence and absorption spectra of a two-level atom driven by a strong bichromatic field.

We examine the radiative properties of a two-level atom driven by a strong bichromatic field with frequencies ${\mathrm{\ensuremath{\omega}}}_{1}$=${\mathrm{\ensuremath{\omega}}}_{0}$-${\mathrm{\ensuremath{\delta}}}_{1}$ and ${\mathrm{\ensuremath{\omega}}}_{2}$=${\mathrm{\ensuremath{\omega}}}_{0}$+${\mathrm{\ensuremath{\delta}}}_{2}$, which can be asymmetrically placed about the atomic transition frequency ${\mathrm{\ensuremath{\omega}}}_{0}$ and can have different Rabi frequencies ${\mathrm{\ensuremath{\Omega}}}_{1}$ and ${\mathrm{\ensuremath{\Omega}}}_{2}$. Applying the optical Bloch equations for the bichromatic excitation, we derive an infinite set of equations of motion for the time evolution of the atomic variables. The equations are solved numerically by matrix inversion and Laplace transforms. Using the quantum regression theorem, we then solve for the steady-state total fluorescence intensity, and the resonance-fluorescence and absorption spectra. The spectra are found to depend on the frequency difference 2\ensuremath{\delta}=${\mathrm{\ensuremath{\delta}}}_{1}$+${\mathrm{\ensuremath{\delta}}}_{2}$, the average detuning \ensuremath{\Delta}=${\mathrm{\ensuremath{\omega}}}_{0}$-1/2(${\mathrm{\ensuremath{\omega}}}_{1}$+${\mathrm{\ensuremath{\omega}}}_{2}$), and the Rabi frequencies of the driving fields. For ${\mathrm{\ensuremath{\Omega}}}_{1}$=${\mathrm{\ensuremath{\Omega}}}_{2}$=\ensuremath{\Omega}, the total fluorescent intensity displays a series of maxima for \ensuremath{\Delta}=\ifmmode\pm\else\textpm\fi{}n\ensuremath{\delta}\ensuremath{\mp}${\mathrm{\ensuremath{\Omega}}}^{2}$/4\ensuremath{\delta}, where the n are the odd integers. The intensity-dependent shift from the resonances n\ensuremath{\delta} is explained as an analog of the generalized Bloch-Siegert shift. The resonance-fluorescence spectrum for \ensuremath{\Delta}\ensuremath{\ne}0 appears to contain more peaks than that for \ensuremath{\Delta}=0.This is due to the splitting of the central peak and the even sidebands into doublets. The absorption spectrum of a weak-probe beam for \ensuremath{\Delta}=0 and equal Rabi frequencies consists of a symmetric series of dispersionlike sidebands separated from ${\mathrm{\ensuremath{\omega}}}_{0}$ by integer multiples of \ensuremath{\delta}, together with a central absorption peak at ${\mathrm{\ensuremath{\omega}}}_{0}$, whose amplitude oscillates with \ensuremath{\Omega}. For \ensuremath{\Delta}\ensuremath{\ne}0 and/or ${\mathrm{\ensuremath{\Omega}}}_{1}$\ensuremath{\ne}${\mathrm{\ensuremath{\Omega}}}_{2}$, the odd sidebands remain dispersionlike, while the central peak and the even sidebands split into absorption-emission doublets. A simple physical interpretation of the spectral features is given in terms of the dressed-atom model.

Bichromatic excitation of long‐period Rayleigh and air waves by the Mount Pinatubo and El Chichon volcanic eruptions

We have detected a new powerful source of low‐frequency seismic energy which is associated with some violent volcanic eruptions. For the two eruptions for which we have identified this new source the spectrum is bichromatic, consisting of two narrow, phase coherent spectral lines. These bands were centered on 3.76 and 5.14 mHz for the April 4, 1982 El Chichón eruption, and on 3.68 and 4.44 mHz for the June 15, 1991 Mt. Pinatubo eruption. During the latter, 8 hour long eruption the resonance frequencies varied by less than 20 μHz. The strength of the source is sufficient to excite globe circling surface waves. We conclude from these observations that volcanoes can excite “harmonic tremors” at much longer periods than previously observed. Our preferred source model involves feedback between local atmospheric oscillations and the eruption process. The physics responsible for this feedback remains subject to speculations.