Resonant Two-photon Transitions Research Articles

Pump-probe response and four-wave mixing in quantum dot exciton-biexciton – metal nanoparticle hybrid

We investigate theoretically the pump-probe response and the four-wave mixing spectrum in a hybrid system composed of a semiconductor quantum dot coupled to a spherical metal nanoparticle including both ground to exciton and exciton-biexciton transitions. We use a density matrix methodology for the calculation of the absorption/gain, dispersion and four-wave mixing spectra and examine their spectral characteristics, for different interparticle distances and under different pump field detuning conditions with respect to the one-photon and the two-photon resonance transitions. We also analyze the form of the spectra using a metastate theory, a description based on the coupling between the excitonic and the plasmonic transitions, while the dressed-state picture is applied in order to predict the positions at which the spectral resonances arise on the spectra.

Estimating phase parameters of a three-level system interacting with two classical monochromatic fields in simultaneous and individual metrological strategies

Recently, the Hilbert–Schmidt speed, as a special class of quantum statistical speed, has been reported to improve interferometric phase in a single-parameter quantum estimation. Here, we test this concept in the multiparameter scenario where two laser phases are estimated in a theoretical model consisting of a three-level atom interacting with two classical monochromatic fields. When the atom is initially prepared in the lower bare state, taking into account the detuning parameters, we extract an exact analytical solution of the atomic density matrix in the case of a two-photon resonant transition. Further, we compare the performance of laser phase parameter estimation in individual and simultaneous metrological strategies and explore the role of quantum coherence in improving the efficiency of unknown multiphase shift estimation protocols. The obtained results show that the Hilbert–Schmidt speed detects the lower bound on the statistical estimation error as well as the optimal estimation regions, where its maximal corresponds to the maximal quantum Fisher information; further, the performance of simultaneous multiparameter estimation with individual estimation inevitably depends on the detuning parameters of the three-level atom. Aside from the quantum entanglement, the quantum coherence is also a crucial resource to improve the accuracy of a metrological protocol.

Addition to and revision of the HI Rydberg states energy region

Mass resolved REMPI data was recorded for jet cooled HI(g) for two-photon excitation in the region of 74 200 – 80 300 cm−1. REMPI spectra of the ions (HI+, I+ and H+) were derived from the data. The spectra revealed dense and overlapping rotational peak structure due to two-photon resonant transitions to number of Rydberg and ion-pair molecular vibrational states as well as iodine atom (2 + 1) REMPI peaks. A total of 16 Rydberg state spectra and 9 ion-pair state spectra were identified and simulated to derive spectroscopic constants (band origin (ν0) and 1st and 2nd order rotational constants (B′, D′)) for the excited states. Further analysis of the results for the Rydberg states and inspection of the spectral structure allowed more thorough characterization of the states as well as reassignment of some previously reported spectra. Detailed quantum defect analyses were particularly useful in that respect. The states could be specified by term symbols, ion core symmetries, Rydberg electron principal quantum numbers and configurations as well as vibrational quantum numbers. An attempt was made to assign the ion-pair spectra to vibrational quantum numbers. Relevant results on Rydberg and ion-pair states of HI, from the literature are summarized in the paper.

Ground-state resonant two-photon transitions in wurtzite GaN/AlN quantum dots

Two-photon transition rates are investigated in resonance to the ground state in wurtzite GaN/AlN quantum dots. The ground state transition is two-photon allowed because of the electron-hole separation inherent to polar wurtzite III-nitride heterostructures. We show that this built-in parity breaking mechanism can allow deterministic triggering of single-photon emission via coherent two-photon excitation. Radiative lifetimes obtained for single-photon relaxation are in good agreement with available time-resolved micro-photoluminescence experiments, indicating the reliability of the employed computational framework based on 8-band k.p-wavefunctions. Two-photon singly-induced emission is explored in terms of possible cavity and non-degeneracy enhancement of two-photon processes.

Resonant anisotropic emission in two-photon interferometric spectroscopy

We theoretically explore a variant of RABBITT spectroscopy in which the attosecond-pulse train comprises isolated pairs of consecutive harmonics of the fundamental infrared probe frequency. In this scheme, one-photon and two-photon amplitudes interfere resulting in an asymmetric photoelectron emission. This interferometric principle has the potential of giving access to the time-resolved ionization of systems that exhibit autoionizing states, since it imprints the group delay of both one-photon and two-photon resonant transitions in the energy-resolved photoelectron anisotropy as a function of the pump-probe time delay. To bring to the fore the connection between the pump-probe ionization process and its perturbative analysis, on the the one side, and the underlying field-free scattering observables as well as the radiative couplings in the target system, on the other side, we test this scheme with an exactly solvable analytical one-dimensional model that supports both bound states and shape-resonances. The asymmetric photoelectron emission near a resonance is computed using perturbation theory as well as solving the time-dependent Sch\"odinger equation; the results are in excellent agreement with the field-free resonant scattering properties of the model.

Luminescence and nonlinear optical properties of colloidal Ag2S quantum dots

The relationship of the spectral luminescent and nonlinear optical properties of Ag2S quantum dots (QDs), passivated with thioglycolic acid (Ag2S QDs/TGA) and Ag2S QDs, stabilized by gelatin (Ag2S QDs/Gel) with average sizes from 1.5 nm to 2.1 nm is found. Colloidal Ag2S QDs/TGA with different sizes, having the exciton (620 nm) and recombination (660 nm, 880 nm and 1205 nm) luminescence were studied using the methods of luminescent spectroscopy, luminescence decay, and Z-scans technique at the wavelengths of 532 nm using 10 ns pulses. Resonant two-photon transitions of reverse saturated absorption (RSA) were found for samples with recombination luminescence. In addition, the nonlinear refraction of thermal nature was observed using Z-scan method. Predominantly weak two-photon absorption (TPA) and nonlinear refraction (NR) of non-thermal nature occur for Ag2S/TGA quantum dots with exciton luminescence. The luminescence lifetimes support this conclusion. The differences in luminescence lifetimes of samples are associated with different state of interfaces for colloidal Ag2S QDs/TGA with recombination and exciton luminescence. This conclusion was confirmed by FTIR spectroscopy of colloidal Ag2S QDs/TGA solutions, synthesized under different conditions. RSA is characteristic of samples with a noticeable concentration of trap states acting as recombination luminescence centers (660 nm). The maximum effect was noted for Ag2S QDs with recombination luminescence in the region of 660 nm, 880 nm, 1205 nm. The fast component of this luminescence decay has a time constant of about 0.8 ns. It decay 5 times for the first 10 ns. In the case of Ag2S QDs/TGA with exciton luminescence (620 nm) there are no appropriate conditions for RSA. The fast component of luminescence decay has a time constant of 0.8 ns. The luminescence intensity decreases 50 times for the first 10 ns. In this case, TPA and nonlinear refraction of non-thermal nature take place, and RSA is absent.

Probing the predissociated levels of the S1 state of acetylene via H-atom fluorescence and photofragment fluorescence action spectroscopy.

We report two new experimental schemes to obtain rotationally resolved high-resolution spectra of predissociated S1 acetylene levels in the 47 000-47 300 cm-1 energy region (∼1200 cm-1 above the predissociation threshold). The two new detection schemes are compared to several other detection schemes (employed at similar laser power, molecular beam temperature, and number of signal averages) that have been used in our laboratory to study predissociated S1 acetylene levels, both in terms of the signal-to-noise ratio (S/N) of the resultant spectra and experimental simplicity. In the first method, H-atoms from the predissociated S1 acetylene levels are probed by two-photon laser-induced fluorescence (LIF). The H-atoms are pumped to the 3d level by the two-photon resonance transition at 205.14 nm. The resulting 3d-2p fluorescence (654.5 nm) is collected by a photomultiplier. The S/N of the H-atom fluorescence action spectrum is consistently better by ∼3× than that of the more widely used H-atom resonance-enhanced multiphoton ionization (REMPI) detection. Laser alignment is also considerably easier in H-atom fluorescence detection than H-atom REMPI detection due to the larger number-density of molecules that can be used in fluorescence vs. REMPI detection schemes. In the second method, fluorescence from electronically excited C2 and C2H photofragments of S1 acetylene is detected. In contrast to the H-atom detection schemes, the detected C2 and C2H photofragments are produced by the same UV laser as is used for the acetylene excitation. As a result, laser alignment is greatly simplified for the photofragment fluorescence detection scheme, compared to both H-atom detection schemes. Using the photofragment fluorescence detection method, we are able to obtain action spectra of predissociated S1 acetylene levels with S/N ∼2× better than the HCCH REMPI detection and ∼10× better than H-atom and HCCH LIF detection schemes.

Method of XUV attosecond pulse characterization using two-photon resonant Raman-type transition

We suggest a new method of attosecond pulse train measurement based on two-photon Raman-type resonant interaction of the XUV with an atom (Attosecond pulse reconstruction using Raman-type Transitions, ART). We show analytically and numerically that the attosecond pulse auto-correlation function and chirp can be measured using this process. Its resonant nature allows obtaining reasonable signal even for relatively low XUV intensity. The interpretation of the results is remarkably simple for the XUV generated by bi-circular field. The experimental realization of the method can be based either on measuring the excitation of the gas or on four-wave mixing fully optical scheme.

Resonant Two-Photon Transitions

We have developed a theory for a two-photon transition when the frequencies of the absorbed or emitted radiation are in resonance with transitions to the same intermediate level in the medium. We have determined the conditions under which such resonant two-photon transitions can play an important role.

Coherent Electron Transfer at the Ag/Graphite Heterojunction Interface

Charge transfer in transduction of light to electrical or chemical energy at heterojunctions of metals with semiconductors or semimetals is believed to occur by photogenerated hot electrons in metal undergoing incoherent internal photoemission through the heterojunction interface. Charge transfer, however, can also occur coherently by dipole coupling of electronic bands at the heterojunction interface. Microscopic physical insights into how transfer occurs can be elucidated by following the coherent polarization of the donor and acceptor states on the time scale of electronic dephasing. By time-resolved multiphoton photoemission spectroscopy (MPP), we investigate the coherent electron transfer from an interface state that forms upon chemisorption of Ag nanoclusters onto graphite to a σ symmetry interlayer band of graphite. Multidimensional MPP spectroscopy reveals a resonant two-photon transition, which dephases within 10fs completing the coherent transfer.

Multiphoton Rydberg and valence dynamics of CH3Br probed by mass spectrometry and slice imaging.

The multiphoton dynamics of CH3Br were probed by Mass Resolved MultiPhoton Ionization (MR-MPI), Slice Imaging and Photoelectron Imaging in the two-photon excitation region of 66 000 to 80 000 cm-1. Slice images of the CH3+ and Br+ photoproducts of ten two-photon resonant transitions to np and nd Rydberg states of the parent molecule were recorded. CH3+ ions dominate the mass spectra. Kinetic energy release spectra (KERs) were derived from slice and photoelectron images and anisotropy parameters were extracted from the angular distributions of the ions to identify the processes and the dynamics involved. At all wavelengths we observe three-photon excitations, via the two-photon resonant transitions to molecular Rydberg states, forming metastable, superexcited (CH3Br#) states which dissociate to form CH3 Rydberg states (CH3**) along with Br/Br*. A correlation between the parent Rydberg states excited and CH3** formed is evident. For the three highest excitation energies used, the CH3Br# metastable states also generate high kinetic energy fragments of CH3(X) and Br/Br*. In addition for two out of these three wavelengths we also measure one-photon photolysis of CH3Br in the A band forming CH3(X) in various vibrational modes and bromine atoms in the ground (Br) and spin-orbit excited (Br*) states.

Control of photoemission delay in resonant two-photon transitions

SynopsisIn contrast to one-photon transitions and non-resonant multiphoton transitions, time delay in resonant multi-photon electron emission can exhibit large positive and negative values that have no scattering equivalent, due to the interference of multiple ionization paths.

Control of photoemission delay in resonant two-photon transitions

The photoelectron emission time delay $\ensuremath{\tau}$ associated with one-photon absorption, which coincides with half the Wigner delay ${\ensuremath{\tau}}_{\mathrm{W}}$ experienced by an electron scattered off the ionic potential, is a fundamental descriptor of the photoelectric effect. Although it is hard to access directly from experiment, it is possible to infer it from the time delay of two-photon transitions, ${\ensuremath{\tau}}^{(2)}$, measured with attosecond pump-probe schemes, provided that the contribution of the probe stage can be factored out. In the absence of resonances, $\ensuremath{\tau}$ can be expressed as the energy derivative of the one-photon ionization amplitude phase, $\ensuremath{\tau}={\ensuremath{\partial}}_{E}arg{D}_{Eg}$, and, to a good approximation, $\ensuremath{\tau}={\ensuremath{\tau}}^{(2)}\ensuremath{-}{\ensuremath{\tau}}_{\mathrm{cc}}$, where ${\ensuremath{\tau}}_{\mathrm{cc}}$ is associated with the dipole transition between Coulomb functions. Here we show that, in the presence of a resonance, the correspondence between $\ensuremath{\tau}$ and ${\ensuremath{\partial}}_{E}arg{D}_{Eg}$ is lost. Furthermore, while ${\ensuremath{\tau}}^{(2)}$ can still be written as the energy derivative of the two-photon ionization amplitude phase, ${\ensuremath{\partial}}_{E}arg{D}_{Eg}^{(2)}$, it does not have any scattering counterpart. Indeed, ${\ensuremath{\tau}}^{(2)}$ can be much larger than the lifetime of an intermediate resonance in the two-photon process or more negative than the lower bound imposed on scattering delays by causality. Finally, we show that ${\ensuremath{\tau}}^{(2)}$ is controlled by the frequency of the probe pulse, ${\ensuremath{\omega}}_{\mathrm{IR}}$, so that by varying ${\ensuremath{\omega}}_{\mathrm{IR}}$, it is possible to radically alter the photoelectron group delay.

Frequency comb assisted two-photon vibrational spectroscopy.

We report a setup for high-resolution two-photon spectroscopy using a mid-infrared continuous wave optical parametric oscillator (CW-OPO) and a near-infrared diode laser as the excitation sources, both of which are locked to fully stabilized optical frequency combs. The diode laser is directly locked to a commercial near-infrared optical frequency comb using an optical phase-locked loop. The near-infrared frequency comb is also used to synchronously pump a degenerate femtosecond optical parametric oscillator to produce a fully stabilized mid-infrared frequency comb. The beat frequency between the mid-infrared comb and the CW-OPO is then stabilized through frequency locking. We used the setup to measure a double resonant two-photon transition to a symmetric vibrational state of acetylene with a sub-Doppler resolution and high signal-to-noise ratio.

Enhanced optical confinement of dielectric nanoparticles by two-photon resonance transition

Two-photon resonance enhances optical confinement of nanoparticles.

Quantum phase transition in arrays of dissipative cavities with two-photon process

In this paper, we employ a new kind of quasi-boson approach and the mean field theory to study analytically the Hamiltonian of an array of cavities with a three-level atom embedded in each cavity in the process of two-photon resonant transition under the influence of a bosonic bath. The superfluid order parameter of the system is obtained analytically and then analyzed numerically to investigate the effects of dissipation on the quantum phase transition from the superfluid to the Mott-insulator phase. It is shown that when the two-photon resonance is achieved one can have the superfluid phase at (ZJ/)= (ZJ/)c' 0.34 in the related ideal case. Furthermore, the system while in the two-photon resonant process has a larger dissipation rate as compared with that in the one-photon resonant process, thus leading to the suppression of the long-range coherence time and enhancement of the critical hopping rate for restoring coherence.

Modulation of Attosecond Beating by Resonant Two-Photon Transition

We present an analytical model that characterizes two-photon transitions in the presence of autoionising states. We applied this model to interpret resonant RABITT spectra, and show that, as a harmonic traverses a resonance, the phase of the sideband beating significantly varies with photon energy. This phase variation is generally very different from the π jump observed in previous works, in which the direct path contribution was negligible. We illustrate the possible phase profiles arising in resonant two-photon transitions with an intuitive geometrical representation.

Coherent control of resonant two-photon transitions by counterpropagating ultrashort pulse pairs

We describe optimized coherent control methods for two-photon transitions in atoms of a ladder-type three-state energy configuration. Our approach is based on the spatial coherent control scheme which utilizes counter-propagating ultrashort laser pulses to produce complex excitation patterns in an extended space. Since coherent control requires constructive interference of constituent transition pathways, applying it to an atomic transition with a specific energy configuration requires specially designed laser pulses. Here, we show, in an experimental demonstration, that the two-photon transition with an intermediate resonant energy state can be coherently controlled and retrieved out from the resonance-induced background, when phase-flipping of the laser spectrum near the resonant intermediate transition is used. A simple reason for this behavior is the fact that the transition amplitude function (to be added to give an overall two-photon transition) changes its sign at the intermediate resonant frequency, thus, by a proper spectral-phase programming, the excitation patterns (or the position-dependent interference of the transition given as a consequence of the spatial coherent control) are well isolated in space along the focal region of the counter-propagating pulses.

A two-component phase-modulated soliton in a medium with a two-photon resonant transition

A theory of optical self-induced transparency for a two-component phase-modulated soliton in a medium with a two-photon resonant transition has been developed. It is shown that a two-photon two-component soliton oscillating at the sum and difference frequencies in the vicinity of the carrier frequency can be formed under these conditions. Explicit analytical expressions are given for the form and parameters of non-linear wave under experimental conditions.

Optical Bistability and Multistability in an EIT Medium with Two-Photon Resonance Transitions

Intensity threshold of optical bistability (OB) and optical multistability (OM) can be controlled by amplitude and phase control of microwave driven field in the two-photon resonance transitions in a parametric region. It is found that in two-photon resonance case, the weak microwave field can reduce the threshold of optical bistability and strong microwave field can lead to optical multistability. The effect of relative phase between applied fields is also discussed. It is shown that for weak and strong microwave field, intensity threshold of OB and OM can be modified. Moreover, it is found that for strong microwave field, relative phase has an essential role for switching between OB and OM or vice versa.