Two-photon Resonance Fluorescence Research Articles

Two-photon resonance fluorescence of two interacting nonidentical quantum emitters

We study a system of two interacting, nonidentical quantum emitters driven by a coherent field. We focus on the particular condition of two-photon resonance and obtain analytical expressions for the stationary density matrix of the system and observables of the fluorescent emission. Importantly, our expressions are valid for the general situation of nonidentical emitters with different transition energies. Our results allow us to determine the regime of parameters in which coherent two-photon excitation, enabled by the coherent coupling between emitters, is dominant over competing, first-order processes. Using the formalism of quantum parameter estimation, we show that the features imprinted by the two-photon dynamics into the spectrum of resonance fluorescence are particularly sensitive to changes in the distance between emitters, making two-photon excitation the optimal driving regime for the estimation of interemitter distance. This can be exploited for applications such as super-resolution imaging of pointlike sources.

Resonant excitation of nanowire quantum dots

GaAs quantum dots in nanowires are one of the most promising candidates for scalable quantum photonics. They have excellent optical properties, can be frequency-tuned to atomic transitions, and offer a robust platform for fabrication of multi-qubit devices that promise to unlock the full technological potential of quantum dots. Coherent resonant excitation is necessary for virtually any practical application because it allows, for instance, for on-demand generation of single and entangled photons, photonic clusters states, and electron spin manipulation. However, emission from nanowire structures under this excitation scheme has never been demonstrated. Here we show, for the first time, biexciton–exciton cascaded emission via resonant two-photon excitation and resonance fluorescence from an epitaxially grown GaAs quantum dot in an AlGaAs nanowire. We also report that resonant excitation schemes, combined with above-bandgap excitation, can be used to clean and enhance the emission of nanowire quantum dots.

Two-photon resonance fluorescence of a ladder-type atomic system

Multiphoton emitters are a sought-after resource in quantum photonics. Nonlinear interactions between a multilevel atomic system and a coherent drive can lead to resonant two-photon emission, but harvesting light from this process has remained a challenge due to the small oscillator strengths involved. Here we present a study of two-photon resonance fluorescence at microwave frequencies, using a superconducting, ladder-type artificial atom, a transmon, strongly coupled to a waveguide. We drive the two-photon transition between the ground and second-excited states at increasingly high powers and observe a resonance fluorescence peak whose intensity becomes comparable to single-photon emission until it splits into a Mollow-like triplet. We measure photon correlations of frequency-filtered spectral lines and find that while emission at the fundamental frequency stays antibunched, the resonance fluorescence peak at the two-photon transition is superbunched. Our results provide a route towards the realization of multiphoton sources in the microwave domain.

Optical control of nonlinearly dressed states in an individual quantum dot

We report two-photon resonance fluorescence of an individual semiconductor artificial atom. By non-linearly driving a single quantum dot via a two-photon transition, we probe the linewidth of the two-photon processes and show that, similar to their single-photon counterparts, they are close to being Fourier limited at low temperatures. The evolution of the population of excitonic states with the Rabi frequency exhibits a clear s-shaped behavior, indicative of the non-linear response via the two-photon excitation process. We model the non-linear response using a 4-level atomic system representing the manifold of excitonic and biexcitonic states of the quantum dot and show that quantitative agreement is obtained only by including the interaction with LA-phonons in the solid state environment. Finally, we demonstrate the formation of dressed states emerging from a two-photon interaction between the artificial atom and the excitation field. The non-linear optical dressing induces a mixing of all four excitonic states that facilitates the tuning of the polarization selection rules of the artificial atom.

Analysis of Low-Lying Gerade Rydberg States of Acetylene Using Two-Photon Resonance Fluorescence Excitation Spectroscopy

The np gerade Rydberg states of acetylene were analyzed using two-photon resonance fluorescence excitation spectroscopy in the 72,000-93,000 cm(-1) energy region. The npπ(1)Σ(g)(+) and npπ(1)Δ(g) Rydberg series (n = 3-5) were identified in the fluorescence excitation spectrum measured by monitoring the C(2) d(3)Π(g)-a(3)Π(u) Swan system. Some vibronic bands were assigned to the npπ(1)Δ(g)-X̃(1)Σ(g)(+) transition on the basis of rotational analysis. The 5pσ(1)Π(g) state was observed, which is the first such observation in an npσ(1)Π(g) series. Rotational analysis of the 5pσ(1)Π(g)-X̃(1)Σ(g)(+) transition showed e/f-symmetry dependent predissociation of acetylene in the 5pσ(1)Π(g) state. The 0(0)(0) band of the deuterated acetylene (C(2)D(2)) 4pπ(1)Σ(g)(+)-X̃(1)Σ(g)(+) transition exhibits an atypical structure, which was satisfactorily reproduced by a simple model of quantum interference between the discrete and quasi-continuum states. The predissociative lifetimes of the npπgerade Rydberg states were estimated from the spectral profiles. The predissociation mechanism of acetylene in the Rydberg states is discussed.

Voigt spectral profiles in two-photon resonance fluorescence

A recent work on two-photon fluorescence is extended by considering the pump field to be a coherent state, which represents a laser field operating well above threshold. The dynamical conditions are investigated under which the two-photon spectrum gives rise, in addition to a Lorentzian line shape at the pump frequency, to two Voigt spectral sideband profiles. Additional conditions are found under which the Voigt profile behaves like either a Gaussian or a Lorentzian line shape.

Two-photon resonance fluorescence

We present a theory of two-photon resonance fluorescence of an atom or molecule in which the excitation by an external electromagnetic field as well as fluorescence emission is mediated by two-photon processes. The treatment is based on first dressing the atom or molecule by the external field and then evaluating perturbatively the effect of the interaction with the vacuum or fluorescent field and so resonance fluorescence can be considered as spontaneous emission from the dressed atom. The introduction of the combined system of atom and external field via dressed states leads to simpler calculations and more transparent physics. The fluorescence spectrum derived by us has similarities as well as differences with that of one-photon resonance fluorescence and earlier theoretical predictions for the two-photon case.

Direct measurement of Gag-Gag interaction during retrovirus assembly with FRET and fluorescence correlation spectroscopy.

During retrovirus assembly, the polyprotein Gag directs protein multimerization, membrane binding, and RNA packaging. It is unknown whether assembly initiates through Gag–Gag interactions in the cytosol or at the plasma membrane. We used two fluorescence techniques—two-photon fluorescence resonance energy transfer and fluorescence correlation spectroscopy—to examine Rous sarcoma virus Gag–Gag and –membrane interactions in living cells. Both techniques provide strong evidence for interactions between Gag proteins in the cytoplasm. Fluorescence correlation spectroscopy measurements of mobility suggest that Gag is present in large cytosolic complexes, but these complexes are not entirely composed of Gag. Deletion of the nucleocapsid domain abolishes Gag interactions and membrane targeting. Deletion of the membrane-binding domain leads to enhanced cytosolic interactions. These results indicate that Gag–Gag interactions occur in the cytosol, are mediated by nucleocapsid domain, and are necessary for membrane targeting and budding. These methods also have general applicability to in vivo studies of protein–protein and –membrane interactions involved in the formation of complex macromolecular structures.

Amplitude-squared squeezing in two-photon resonance fluorescence

Interaction of single-mode coherent light with dipole-forbidden two-level atoms via non-resonant virtual level is studied in this paper. It is shown that, in the case of two-photon resonance fluorescence, the atomic system generates three bands of correlated photon pairs with the carrier frequencies ω k 0 , ω k 0 ± Γ, where Γ=[Ω 2+(Δ/2) 2] 1/2 , 2Ω is the two-photon Rabi frequency and Δ is the detuning from the two-photon resonance. It is established that amplitude-squared squeezing occurs from the sidebands of the two-photon resonance fluorescence spectrum. When the atom number increases, the amplitude-squared squeezing also increases.

Cooperative atomic effects in two-photon spontaneous emission and resonance fluorescence.

In this paper, we study cooperative two-photon transitions in a system of two (nonoverlapping) atoms for two processes: two-photon spontaneous emission and two-photon resonance fluorescence. Expressions are obtained for the energy shifts and decay rates due to the interaction of the atoms with and via the field. The altered decay rates signal the existence of superradiance and subradiance in the two-atom system. The energy shifts of the single-excitation states are found to be proportional to ${\mathit{R}}^{\mathrm{\ensuremath{-}}6}$ for small atomic separations R, and to ${\mathit{R}}^{\mathrm{\ensuremath{-}}2}$ for large separations.

Heitler-Ma treatment of two-photon spontaneous emission and resonance fluorescence

The Heitler-Ma equation is adapted so that it can deal with two-photon spontaneous emission and resonance fluorescence processes. The authors introduce two types of effective Hamiltonian, depending on whether all the intermediate states are non-resonant or one of them is resonant. The spectra for spontaneous emission, and for resonance fluorescence in both weak and strong excitation fields, are discussed.

Quantum theory of multiwave mixing. V. Two-photon two-level model.

We extend our theoretical formalism describing how one strong classical wave and one or two weak quantum-mechanical waves interact in a nonlinear medium to the ``two-photon two-level'' model. In this model, field transitions between two levels not connected by an electric dipole occur by means of a two-photon transition using off-resonant intermediate levels. This model has many similarities with, as well as significant differences from, the corresponding one-photon two-level model that has been treated in the previous papers in this series. The theory yields the first detailed calculation of the two-photon resonance fluorescence spectrum, as well as the complete set of four coefficients that describe the quantum theory of two-, three-, and four-wave mixing.

Theory of two-photon resonance fluorescence

We present calculations of the spontaneous-emission spectrum from a two-photon two-level system in the presence of an arbitrarily intense monochromatic field at half of the two-photon transition frequency. The single-photon counterpart to this is called resonance fluorescence. Because of the complexity of the two-photon two-level model, many effects arise that are absent in the one-photon problem. In particular, we show how the Rayleigh scattering from this system is markedly different and discuss the role of Stark shifts on the emission spectrum.

Quantum beats in the two-photon resonance flourescence of a three-level atom

The low-frequency modes pertaining to quantum beats in the two-photon resonance fluorescence of a three-level atom in the limit of high photon densities of the pump field is investigated without making the “rotating-wave approximation” in the coupling between the atom and the pump field. A somewhat different spectral function for the low-frequency modes from that predicted before is obtained.

Dynamic Stark splitting in two-photon resonance fluorescence

The excitation spectrum arising from resonant two-photon interaction between a strong pump field and a two-level model is investigated by means of the Green's function formalism. An atom-field interaction potential which is quadratic in photon creation and annihilation operators is used. The results, valid for high photon densities of the pump field, reveal a three-peak spectrum similar to the one-photon case. The energy shift is found to be linearly dependent on the intensity of the pump beam.

Model microfield theory of redistribution of laser radiation

A theoretical study is presented which permits the analysis of multiphoton processes in the presence of homogeneous, inhomogeneous, and power-broadening mechanisms. A method derived from the Markov model microfield theory is used. This is based on the statistics of the fluctuations of the interaction between the radiator and a perturbing environment. Application is made to velocity-changing collisions in Doppler-free spectroscopy and to homogeneous and inhomogeneous pressure broadening in two-photon absorption or resonance fluorescence.

Two-photon resonance fluorescence. II numerical results

Some numerical results and graphs are presented for the excitation spectrum of a two level atom whose transition frequency ω 21 is in resonance with twice the frequency of a strong laser field. These results are computed directly from the Green function and are in agreement with earlier theoretical predictions.

One- and two-photon resonance fluorescence of a three-level atom at high photon densities

We consider the possible physical processes that may arise in a three-level atom when only two of its levels interact with a strong electromagnetic field and when the atomic transition frequency is nearly equal to once and twice the frequency of the laser field, respectively. There have been found pronounced cooperative effects in the spectrum of the two-level system, which is in resonance with the laser field, arising from the presence of the third level. The excitation spectra describing the transitions from the first excited state into the second excited state and from that to the ground state consist, apart from the two central peaks, of two pairs of sidebands which are induced by the laser field of the neighbouring system. Detailed expressions of the spectral functions for the physical processes of one- and two-photon resonance fluorescence have been derived and discussed in the limit of high photon densities. The excitation spectrum of the low frequency modes has been considered and discussed in det...

Cooperative two-photon resonance fluorescence of a three-level atom

We have calculated the Green function describing the physical process where a three level atom interacts with a strong pump field as well as with a weak perturbing signal field in the limit of high photon densities. The theory is then used to describe the cooperative two-photon resonance fluorescence which occurs when the sum of the two atomic transition frequencies is nearly twice the frequency of the pump field. Our atomic system is equivalent to a two-level atom when the degeneracy of the ground state is removed by a Stark field. The excitation spectrum has been found to consist of new bands and sidebands which exist only in the presence of the Stark field. The resonance process which occurs when the Stark splitting is in the neighbourhood of the energy shift induced by the laser field has been discussed in detail.

Quantum beats in two-photon resonance fluorescence

The two-photon resonance fluorescence spectrum of a three-level atom is shown to consist of the low frequency modes in addition to the high frequency ones in the limit of high photon densities. The spectral function for the low frequency modes consists of two lorentzian lines describing: the peak occuring at the renormalized beat frequency Δ + and that of the zero-photon excitation at the frequency Δ -, where Δ ±=Δ-3Ω 2/2ω a ±Ω 2 u/2ω a , u 2=1+(2Δω a /Ω 2) 2. Here, 2Δ is the energy splitting between the two excited states, ω a is the photon energy of the pump field and Ω is the Rabi frequency. The peak at the renormalized beat frequency Δ + occurs provided that the condition (2Δω a /Ω 2) 2 > 1 is satisfied. The two-photon laser spectroscopy is expected to be a useful tool for the observation of the low frequency modes in question.