Coherent Two-photon Excitation Research Articles

Efficient Quadrature Squeezing from Biexcitonic Parametric Gain in Atomically Thin Semiconductors.

Modification of electromagnetic quantum fluctuations in the form of quadrature squeezing is a central quantum resource, which can be generated from nonlinear optical processes. Such a process is facilitated by coherent two-photon excitation of the strongly bound biexciton in atomically thin semiconductors. We show theoretically that interfacing an atomically thin semiconductor with an optical cavity makes it possible to harness this two-photon resonance and use the biexcitonic parametric gain to generate squeezed light with input power an order of magnitude below current state-of-the-art devices with conventional third-order nonlinear materials that rely on far off-resonant nonlinearities. Furthermore, the squeezing bandwidth is found to be in the range of several meV. These results identify atomically thin semiconductors as a promising candidate for on-chip squeezed-light sources.

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.

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.

On-Demand Semiconductor Source of Entangled Photons Which Simultaneously Has High Fidelity, Efficiency, and Indistinguishability.

An outstanding goal in quantum optics and scalable photonic quantum technology is to develop a source that each time emits one and only one entangled photon pair with simultaneously high entanglement fidelity, extraction efficiency, and photon indistinguishability. By coherent two-photon excitation of a single InGaAs quantum dot coupled to a circular Bragg grating bull's-eye cavity with a broadband high Purcell factor of up to 11.3, we generate entangled photon pairs with a state fidelity of 0.90(1), pair generation rate of 0.59(1), pair extraction efficiency of 0.62(6), and photon indistinguishability of 0.90(1) simultaneously. Our work will open up many applications in high-efficiency multiphoton experiments and solid-state quantum repeaters.

Effects of laser linewidth on an effective method for excitation in three-level atomic systems by two optimally detuned counterpropagating pulsed lasers

Population transfer in three-level atomic systems with high efficiency and selectivity is desirable for practical applications in laser spectroscopy, trace analysis, and isotope separation. The effective excitation method for achieving this population transfer in three-level atomic systems using coherent two-photon excitation with two optimally detuned counterpropagating pulsed lasers, initially investigated analytically assuming monochromatic lasers and Doppler-free systems, is studied numerically by solving the density matrix equations incorporating finite laser linewidth and effective Doppler width. A three-level Yb system is chosen for illustration of numerical results. The effects of laser linewidth and effective Doppler width on the values of optimal detunings and maximum third-level population are discussed.

Magnetic Resonance Studies of Cold Atomic Hydrogen Gas

We present magnetic resonance studies of two-dimensional atomic hydrogen gas (H↓) in 4.6 T field at temperatures 70–100 mK i.e. well in the quantum gas or cold collision regime. Electron spin resonance at 128 GHz and nuclear magnetic resonance at 910 MHz were used to detect and manipulate populations of the hyperfine manifold of the electronic ground state. Using a coherent two-photon excitation we demonstrated effects of electromagnetically induced transparency and absorption as well as coherent population trapping in the 2D H gas. We performed a measurement of the magnetic resonance line shifts associated with exchange and dipolar interactions. The cold collision (clock) shift measured for the 2D case is two orders of magnitude smaller than predicted by the mean field theory and is of the same order as the internal dipolar field.

Lineshapes in coherent two-photon excitation

We compare lineshapes obtained in laser-induced fluorescence and resonantly enhanced multi-photon ionisation, resulting from coherent pulsed excitation of a two-photon transition in xenon atoms. We find that excitation profiles probed by ionisation show power broadening as expected. Here ionisation occurs simultaneously with the coherent excitation. The profiles observed by fluorescence show almost no broadening, because of coherent population return, since the fluorescence occurs predominantly after the excitation pulse is over. Thus the observed lineshape depends characteristically upon the nature of the probing process.

Measurement of theRb(5D5/2)photoionization cross section using trapped atoms

We report on measurements of the cross section for photoionization of Rb atoms in the highly excited ${5D}_{5/2}$ level at wavelengths ranging from 1064 to 532 nm (photoelectron energies of 0.175 to 1.34 eV). We efficiently populate the ${5D}_{5/2}$ level using coherent two-photon excitation of trapped atoms with pulses in the counterintuitive order. The absolute photoionization cross sections are then measured via the increased loss of atoms from the magneto-optical trap when it is illuminated by the photoionizing light. Our results are in good agreement with new calculations based on the valence-electron parametric potential method.

Laser‐induced fluorescence of coumarin derivatives in aqueous solution: Photochemical aspects for single molecule detection

The efficiency of detecting a single fluorescent coumarin dye molecule in aqueous solution by one-photon excitation (OPE) at 350 nm as well as by coherent two-photon excitation (TPE) at 700 nm is studied. The photostability, which is crucial for single molecule detection (SMD), is determined at a low irradiance for various coumarin derivatives using a ‘cell-bleaching’ method. The yields of photobleaching for these coumarins in aqueous solution are in the order of 10−3 to 10−4. Thus, most of the dyes are sufficiently stable to allow SMD. However, for SMD in a fluorescence microscope a high quasi-CW irradiance (at least 104 W cm−2) is necessary for efficient OPE by a pulsed, frequency doubled titanium:sapphire laser. Detailed investigations on the dye Coumarin-120 using fluorescence correlation spectroscopy (FCS), different repetition rates of the laser and transient absorption spectroscopy (TRABS) gave clear evidence that OPE at a high irradiance results in two-step photolysis via the first electronic excited singlet and triplet state, S1 and T1, producing dye radical ions and solvated electrons. Hence, this additional photobleaching pathway limits the applicable irradiance for OPE. Using coherent TPE for single molecule detection, saturation of the fluorescence was observed for a high quasi-CW irradiance (108 W cm−2), which may also be caused by photobleaching. Furthermore, TPE is deteriorated by other competing nonlinear processes (e.g. continuum generation in the solvent), which only occur above a threshold irradiance (7 × 107 W cm−2). Nevertheless, TPE allows an efficient detection of single Coumarin-120 molecules in water. Using a maximum likelihood estimator, we are also able to identify single dye molecules via their characteristic fluorescence lifetime of 4.8 ± 1.2 ns.

Selective photoionization of the ytterbium atom by coherent two-photon excitation

Highly selective photoionization of the ytterbium atom by coherent two-photon excitation was demonstrated experimentally. The Yb atom was excited by two single-mode lasers and ionized by a time-delayed broadband laser. The isotope ratio of the ions was analyzed by a time-of-flight mass spectrometer. The ion yield and the selectivity were recorded as a function of detuning of the first exciting laser. The maximum of ion yield was observed when the laser was detuned from resonance. The selectivity of $^{168}\mathrm{Yb}$ was increased higher than 20 000 when the laser was blue-detuned to the most efficient position. Also, numerical analysis was performed to explain the experimental results. \textcopyright{} 1996 The American Physical Society.

Triplet Rydberg states of the imidogen radical characterized via two-photon resonance-enhanced multiphoton ionization spectroscopy

Five new triplet excited states of the ND radical (three in the case of NH) in the wave-number range 85 000–91 000 cm−1 have been identified through analysis of the two-photon resonance enhancements they provide to the wavelength-resolved multiphoton ionization spectrum of X 3Σ− state NH(ND) radicals. The lowest energy of these, the B 3Π state, is found to be a ‘‘regular’’ Rydberg state which, on the basis of its observed quantum defect and its deduced rotational and spin–orbit coupling constant, is surmised to be the 3Π state derived from a 3pσ electron built on the 2Π ground-state ion core. Perturbations are evident in the B 3Π–X 3Σ− origin bands of both NH and ND. In the case of ND the perturbing state provides its own resonance enhancements, the analysis of which enables its definitive identification as the C 3Σ− state. The very small spin–orbit splitting found for the D 3Π state is taken to indicate that (at least in the Franck–Condon region) its wave function is dominated by the configuration involving one 3pπ Rydberg electron and a 4Σ− ion core. To still higher wave number we identify two more 3Σ− excited states, the upper of which (the F 3Σ− state) has a very small rotational constant which we take to imply that it has substantial valence character. Further indications that the F 3Σ− (and C 3Σ−) states possess significant valence character is provided by the observation that both parent and daughter (N+) ions contribute to the overall ion yield when the multiphoton ionization proceeds via these two states. Daughter-ion formation is considered to occur via an overall four-photon excitation process in which the coherent two-photon excitation to the 3Σ− state of interest is followed by a one-photon excitation to a ‘‘superexcited’’ state of the neutral. This is then presumed to absorb a further photon to yield the observed N+ ions and/or to predissociate, yielding highly excited N* atoms which then undergo a direct one-photon ionization.

Time-resolved investigation of exciton-polariton dephasing in CuCl.

The intrinsic dephasing time of an exciton polariton is investigated in the time domain by use of a time-resolved coherent two-photon excitation and probe technique. This method allows discrimination of the elementary interaction processes of the exciton polariton in semiconductors. The results obtained in CuCl show the role of the different phonons in the exciton-polariton dephasing

Coherent effects in hyper-Raman scattering of a 2nπ pump pulse

A new effect is predicted in the case of coherent two-photon excitation of a medium consisting of four-level atoms by a 2πn pump pulse: such excitation generates radiation at the hyper-Raman frequency. This takes the form of n successive pulses whose intensity increases toward the trailing edge of the pump pulse.

Fluorescence excitation and emission spectroscopy of the [formula omitted] Rydberg state of ammonia: Assignment of the “Schuster bands of ammonia”

Low vibronic levels of the C ̃ ′ 1A′ 1 electronic states of NH 3 and ND 3 have been populated by coherent two-photon excitation. New information about the spectroscopy and the predissociation behavior of these levels has been obtained through analysis of their subsequent C ̃ ′- A ̃ fluorescence. ν 1 is very low in the A ̃ state, being 2300 ± 50 cm −1 for NH 3 and 1790 ± 50 cm −1 for ND 3. Some of the emissions observed from ammonia discharges (the “Schuster bands”), and attributed previously to the ammonium radical, are demonstrated to arise from the C ̃ ′- A ̃ Rydberg-Rydberg transition in ammonia, as recently proposed by Watson.

Photochemistry with high power ultraviolet lasers

Rotationally resolved photofragment fluorescence from OH(A2Σ+) following the coherent two-photon excitation of H2O with a KrF laser (248 nm), is reported and the dynamics of the dissociation process are discussed. Fluorescence from CN(B2Σ+) following two-photon excitation of ICN is also described. In both cases the energy distribution in the photofragments is shown to differ significantly from that observed with single-photon excitation at closely similar energies.Two further examples of multiphoton excitation, involving CS2 and SO2, are briefly discussed. In these cases absorption of a further photon, by fragments produced in the primary step, gives rise to strong laser-induced fluorescence.

Coherent Two-Photon Excitation by Multiple Light Pulses

We have studied Doppler-free two-photon excitation of atoms with a train of phase-coherent standing-wave light pulses, originating from the same laser pulse. Quantum interference effects produce narrow spectral fringes which have a physical origin similar to that of Ramsey fringes. Linewidths much less than the Fourier-transform limit of an individual light pulse have been observed for the sodium $3S\ensuremath{-}5S$ transition; and a dramatic enhancement of the resonant signal is possible.

Intense spontaneous violet emission due to coherent two-photon excitation in potassium

This paper describes the measured result of intense violet emission at 4048 A that is due to the strong coherent two-photon excitation in potassium vapor. Anomalous delay and narrowing of the emitted pulse was observed in contrast to the incident pulse. The intensity dependence on the atomic density also is reported.