Resonant Two-photon Excitation Research Articles

Quantum-confined biexcitons in CuCl quantum dots and their unconventional opticalproperties

The optical properties of quantum-confined biexcitons in CuCl quantum dots (QDs) underthe novel method of two-photon resonance excitation of biexcitons are reviewed. First, webriefly overview the optical properties of excitons and biexcitons in bulk CuCl singlecrystals and also those in quantum dots, and then we cover the biexcitonic properties ofCuCl quantum dots in NaCl matrices in detail: realization of two-photon resonantexcitation of confined biexcitons with different angular momenta, studies on theenergy-level structure of biexcitonic states by size-selective two-photon resonant excitationof biexcitons, on subsequent efficient lasing of biexcitonic luminescence even inlow-Q cavity, on ultrafast radiative decay of biexcitons, and, finally, the observation of excitedstates of confined biexcitons studied by infrared transient absorption. All theseunconventional features are inherent in the biexcitons in CuCl QDs affected by quantumconfinement effects.

Prospects for precision measurements of atomic helium using direct frequency comb spectroscopy

We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral region via harmonic generation, it offers the prospect of greatly improved accuracy for UV and far-UV transitions. To take advantage of this accuracy an ultracold helium sample is needed. For measurements of the triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap metastable 2^3S state atoms. We analyze schemes for measuring the two-photon $2^3S \to 4^3S$ interval, and for resonant two-photon excitation to high Rydberg states, $2^3S \to 3^3P \to n^3S,D$. We also analyze experiments on the singlet-state spectrum. To accomplish this we propose schemes for producing and trapping ultracold helium in the 1^1S or 2^1S state via intercombination transitions. A particularly intriguing scenario is the possibility of measuring the $1^1S \to 2^1S$ transition with extremely high accuracy by use of two-photon excitation in a magic wavelength trap that operates identically for both states. We predict a ``triple magic wavelength'' at 412 nm that could facilitate numerous experiments on trapped helium atoms, because here the polarizabilities of the 1^1S, 2^1S and 2^3S states are all similar, small, and positive.

Neutral photodissociation of superexcited states of molecular iodine

The formation of high-n Rydberg atoms from the neutral dissociation of superexcited states of I(2) formed by resonant two-photon excitation of molecular iodine using an ArF laser has been investigated. The high-n Rydberg atoms I* are formed by predissociation of the optically excited molecular Rydberg states I*(2)[R(B (2)Sigma(g) (+))] converging on the I(2) (+)(B (2)Sigma(g) (+)) state of the ion. Measurement of the kinetic energy release of the Rydberg I* fragments allowed the identification of the asymptotic channels as I*[R((3)P(J))]+I((2)P(32)), where the I*[R((3)P(J))] are Rydberg atoms converging on the I(+)((3)P(J)) states of the ion with J=2, 1, and 0. In the case of the I*[R((3)P(2))] fragments, the average Rydberg lifetime is observed to be 325+/-25 micros. Based on experiments on the variation of the Rydberg atom signal with the field ionizing strength, the distribution of Rydberg levels peaks at about 25-50 cm(-1) below the ionization limit.

Electron Photodetachment Dissociation of DNA Anions with Covalently or Noncovalently Bound Chromophores

Double stranded DNA multiply charged anions coupled to chromophores were subjected to UV-Vis photoactivation in a quadrupole ion trap mass spectrometer. The chromophores included noncovalently bound minor groove binders (activated in the near UV), noncovalently bound intercalators (activated with visible light), and covalently linked fluorophores and quenchers (activated at their maximum absorption wavelength). We found that the activation of only chromophores having long fluorescence lifetimes did result in efficient electron photodetachment from the DNA complexes. In the case of ethidium-dsDNA complex excited at 500 nm, photodetachment is a multiphoton process. The MS(3) fragmentation of radicals produced by photodetachment at lambda = 260 nm (DNA excitation) and by photodetachment at lambda > 300 nm (chromophore excitation) were compared. The radicals keep no memory of the way they were produced. A weakly bound noncovalent ligand (m-amsacrine) allowed probing experimentally that a fraction of the electronic internal energy was converted into vibrational internal energy. This fragmentation channel was used to demonstrate that excitation of the quencher DABSYL resulted in internal conversion, unlike the fluorophore 6-FAM. Altogether, photodetachment of the DNA complexes upon chromophore excitation can be interpreted by the following mechanism: (1) ligands with sufficiently long excited-state lifetime undergo resonant two-photon excitation to reach the level of the DNA excited states, then (2) the excited-state must be coupled to the DNA excited states for photodetachment to occur. Our experiments also pave the way towards photodissociation probes of biomolecule conformation in the gas-phase by Förster resonance energy transfer (FRET).

The controlled excitation of forbidden transitions in the two-photon spectrum of strontium by using collisions and electric fields

We present experimental results involving controlled configuration mixing in two-photon spectroscopy of highly-excited states by exploiting a weak external electric field and collisions. The method has allowed new extensions to high members of the two-photon forbidden J = 3 odd-parity 5snf 1F 3 and the J = 0, even-parity 5sns 1S 0 Rydberg series of neutral strontium to be observed. We achieve resonant two-photon transverse excitation of a high density atomic jet by using a narrow bandwidth tunable dye laser in a heat pipe setup with sensitive ionization detection. Experimental term values are extended for the 5sns 1S 0 series up to n = 46. By suitable exploitation of the composition and pressure of the buffer gases in conjunction with the electric field strength in the excitation region and the exciting laser beam intensity we have also extended observations up to n = 44 for the 5snf 1F 3 series and up to n = 46 for the 5snp 1P 1 series. Our results demonstrate a novel and remarkably simple experimental method to access high Rydberg states to which transitions are forbidden from the ground state by parity and other selection rules.

Radiative emission from multiphoton-excited semiconductor quantum dots

Optical transitions in CdS semiconductor quantum dots (QDs) have been studied by the Monte Carlo method based on probability calculations of the time-dependent Schrödinger equation. It has been demonstrated that excited by a continuous-wave laser, an assembly of CdS QDs, whose radii range from 2to5nm centered at 3.7nm, shows an emission peak around 2.65eV in the optical emission spectrum, which corresponds to optical transitions among degenerate sublevels close to the ground sublevels in the conduction and valence bands of a CdS QD having a radius of 3.7nm. For resonant one-photon excitation, the emission peak is very sharp, while for resonant two-photon excitation, the emission peak becomes blueshifted and broadened. The inclusion of the nonradiative electron-phonon processes makes the two-photon excitation peak significantly sharper and shows a better agreement with experimental work, thus demonstrating the upconversion luminescence of the QDs required for many applications including bioimaging.

Optical studies of Si/SiO 2 interfaces by second-harmonic generation spectroscopy of silicon interband transitions

Optical second-harmonic generation (SHG) studies of Si/SiO2 interfaces by resonant two-photon excitation of Si interband transitions are reviewed. Three different types of interband resonances at Si interfaces are identified in SHG spectra: (i) E1 and E2 critical-point bulk interband transitions in the space-charge region of charged Si interfaces, (ii) E1- and E2-type transitions associated with Si atoms at the interface in bulk-like tetrahedral coordination, and (iii) transitions related to Si interface atoms with bonding properties specific of the atomic structure of the interface. The effects of the polarity of space-charge fields on SHG spectra of Si(100)/SiO2 interfaces are also discussed.

Nonlinear absorption and refraction of light in a colloidal solution of CdSe/ZnS quantum dots upon two-photon resonant excitation

This paper reports on the results of an investigation into the nonlinear transmission of individual ultrashort pulses of a train generated by a mode-locked laser through a colloidal solution of CdSe/ZnS quantum dots in toluene upon two-photon resonant excitation of the fundamental optical transition. The specific features of the nonlinear transmission are explained in terms of the two-photon absorption and self-defocusing processes. Analysis of the experimental results makes it possible to separate the self-defocusing processes that are governed by the inertialess change in the refractive index due to the interaction of high-power optical pulses with bound electrons and those determined by the nonlinear change in the refractive index under the action of two-photon-excited carriers in a quantum dot.

Collisions between supercooled excitons inCu2Ostudied by time-resolved Lyman spectroscopy

Supercooled orthoexcitons are generated in ${\mathrm{Cu}}_{2}\mathrm{O}$ by resonant two-photon excitation with a femtosecond laser pulse. The excitonic Lyman series is systematically analyzed by time-resolved induced-absorption measurements. The decay time of orthoexcitons as well as the buildup and decay times of paraexcitons are found to be dependent on their respective densities. With these results, the two-body excitonic spin-exchange and the exciton dissociation processes are investigated. The collision-induced spin-exchange process is found to dominate in the early stage of photoexcitation. The conditions required to obtain a Bose-Einstein condensate with paraexcitons are discussed.

Quantum coherence of1sorthoexcitons inCu2Odriven by resonant two-photon excitation

The intensity of the resonant emission of the $1s$ orthoexcitons in ${\mathrm{Cu}}_{2}\mathrm{O}$ is observed in a weak magnetic field under two-photon resonant excitation at $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ to investigate the quantum interference within the Zeeman sublevels in the Faraday configuration. A specific decrease of $\ensuremath{\sim}15%$ in the emission intensity is observed in the region of $\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. This structure is interpreted as a manifestation of the quantum interference effect between the Zeeman sublevels of the $1s$ orthoexcitons, known as the Hanle effect in atomic spectroscopy. The homogeneous width of the exciton level is estimated to be $5.5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{eV}$. The contribution of the quadrupole exciton polariton is discussed by taking into account the geometry-dependent characteristics of the exciton-polariton on the direction of the magnetic field. The Hanle structure observed is attributed to the interference of the polariton wave packets of the longitudinal excitons, where the phase coherence is transferred via transverse excitons driven by resonant two-photon coupling with the radiation field.

Electric field induced anomalies in frequency shift and collisional perturbations in the 5snd 1D2 Rydberg series of strontium in a heatpipe setup with ionization detection

We have examined the effect of weak DC electric field (2–20 V/cm) complimented by foreign gas collisions on the bound J = 2 even-parity 5snd 1 D 2 Rydberg states of neutral strontium . We use resonant two-photon transverse excitation, employing a narrow bandwidth tunable dye laser and an atomic jet in a heatpipe setup with ionization detection. In this paper we report certain anomalies in the observed spectra covering principal quantum number range n = 27–42 indicating a frequency shift reversal with nearly quadratic dependence on the field strength above an anti-resonance region. Furthermore, we have observed the emergence of highly localized doubly-excited 4d 2 states, including a remarkably broad perturber with large angular momentum , uncovering orbital contraction effect. This non-Rydberg excitation, which intrudes upon the two-photon spectrum with large energy overlap is due to single-photon excitations from the 5s5p 1 P 1 resonance level following molecular dissociation of the Sr 2 dimers suitably governed by binary atomic collisions . Our study which involves laser excitation complimented by electric field and collisions using inexpensive home-made setup opens up the possibilities for a new class of experiments, with considerable simplicity in the choice of excitation schemes for both Rydberg and non-Rydberg transitions, to reach states lying at high energies which cannot otherwise be accessed from the ground state due to parity and selection rules, while allowing one to probe localization properties of atomic wave functions.

Two-Photon Photosensitized Production of Singlet Oxygen: Optical and Optoacoustic Characterization of Absolute Two-Photon Absorption Cross Sections for Standard Sensitizers in Different Solvents

Singlet molecular oxygen, O2(a1Deltag), can be produced upon resonant two-photon excitation of a photosensitizer. In the present study, two molecules that have received recent attention in studies of nonlinear organic materials were characterized for use as standard two-photon sensitizers: 2,5-dicyano-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene, CNPhVB, and 2,5-dibromo-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene, BrPhVB. Absolute two-photon absorption cross sections, delta, were independently determined for these molecules using two techniques that have heretofore not been applied to this problem: an optical technique (time-resolved detection of O2(a1Deltag) phosphorescence) and a nonoptical technique (a time-resolved laser-induced optoacoustic experiment). For experiments performed in toluene, a solvent commonly used for such nonlinear optical studies, appreciable absorption by the solvent itself complicates the measurements. In cyclohexane, however, delta values could be obtained without the interfering effects of solvent absorption. On the basis of these results, we discuss key aspects of the respective techniques used to quantify values of delta. The information reported herein provides some explanation for the lack of consensus that is routinely observed in published values of delta, certainly for experiments performed in aromatic solvents such as toluene and benzene.

Temporal behavior of biexciton luminescence under two-photon excitation in CuCl quantum dots

Temporal behavior of luminescence of biexcitons under two-photon resonant excitation has been measured in CuCl quantum dots. The non-exponential decay profile was observed, and its fast decay component becomes dominant with shortening the decay time for higher pump intensity. This characteristic feature of the biexciton emission is observable only for the two-photon resonant excitation with realization of population inversion in biexciton–exciton system at the initial stage and indicates the possibility of Dicke's superradiance, which is originated from the coherent interaction of diploes among many quantum dots.

Two-photon excitation of low-lying electronic quadrupole states in atomic clusters

A simple scheme of population and detection of low-lying electronic quadrupole modes in free small deformed metal clusters is proposed. The scheme is analyzed in terms of the time-dependent local density approximation calculations. As a test case, the deformed cluster $\mathrm{Na}_{11}{}^{+}$ is considered. Long-living quadrupole oscillations are generated via resonant two-photon (two-dipole) excitation and then detected through the appearance of satellites in the photoelectron spectra generated by a probe pulse. Femtosecond pump and probe pulses with intensities $I=2\ifmmode\times\else\texttimes\fi{}{10}^{10}--2\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ and pulse duration $T=200--500\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ are found to be optimal. The modes of interest are dominated by a single electron-hole pair and so their energies, being combined with the photoelectron data for hole states, allow us to gather full mean-field spectra of valence electrons near the Fermi energy. Besides, the scheme allows us to estimate the lifetime of electron-hole pairs and hence the relaxation time of electronic energy into ionic heat.

Multiphoton processes of CO at 230 nm

High resolution kinetic energy release spectra were obtained for C(+) and O(+) from CO multiphoton ionization followed by dissociation of CO(+). The excitation was through the CO (B (1)Sigma(+)) state via resonant two-photon excitation around 230 nm. A total of 5 and 6 photons are found to contribute to the production of carbon and oxygen cations. DC slice and Megapixel ion imaging techniques were used to acquire high quality images. Major features in both O(+) and C(+) spectra are assigned to the dissociation of some specific vibrational levels of CO(+)(X (2)Sigma(+)). The angular distributions of C(+) and O(+) are very distinct and those of various features of C(+) are also different. A dramatic change of the angular distribution of C(+) from dissociation of CO(+)(X (2)Sigma(+), nu(+) = 1) is attributed to an accidental one-photon resonance between CO(+)(X (2)Sigma(+), nu(+) = 1) and CO(+)(B (2)Sigma(+), nu(+) = 0) and explained well by a theoretical model. Both kinetic energy release and angular distributions were used to reveal the underlying dynamics.

Photoionization of helium atoms irradiated with intense vacuum ultraviolet free-electron laser light. Part II. Theoretical modeling of multi-photon and single-photon processes

We consider the problem of a helium atom under the radiation field of the DESY vacuum ultraviolet (VUV) free electron laser (FEL) (Phase I, $\ensuremath{\hbar}\ensuremath{\cong}13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$). We find by solving numerically the time-dependent Schr\"odinger equation, that there is a large probability for resonant two-photon excitation from the ground state into a low kinetic energy state just above the first He ionization threshold. From this it is possible to go into another quasi-free state higher up, by resonant absorption of an additional photon. There is no double ionization of He. These results are in general agreement with the He photoelectron and time-of-flight (TOF) spectra recorded on March 2002, in the last week of the DESY VUV FEL Phase I operation. A detailed report on the experiments is given in a companion paper.

Effects of spontaneously generated coherence on the group velocity in a V system

We show how the application of an incoherent pumping can produce a variety of effects on the propagation of a weak electromagnetic pulse in a V system with spontaneously generated coherence (SGC). There exists an incoherent pumping rate which makes the group velocity reach the extremum near the region of two-photon resonant excitation. The existence of SGC is just the cause for the occurrence of the extremum, and it may also be regarded as a knob which can be used to manipulate light propagation from subluminal to superluminal.

Resonant two-photon excitation of 1s paraexcitons in cuprous oxide

We have created paraexcitons in Cu 2O via resonant two-photon generation, and examined their population dynamics by means of time-correlated single photon detection. Confining the excitons to a constant volume in a harmonic-potential trap made with inhomogeneous applied stress along the [001] axis, we find that paraexcitons are created directly, and orthoexcitons appear primarily through a up-conversion of the paraexcitons. Also we generate excitons with two colliding pulses, and the luminescence is weaker than that from one beam excitation with same total laser power. These results show that resonant one-beam two-photon generation of paraexcitons is a promising way to pursue Bose–Einstein condensation of paraexcitons.

Study of Orthoexciton-to-Paraexciton Conversion inCu2Oby Excitonic Lyman Spectroscopy

Using time-resolved 1s-2p excitonic Lyman spectroscopy, we study the orthoexciton-to-paraexciton transfer, following the creation of a high density population of ultracold 1s orthoexcitons by resonant two-photon excitation with femtosecond pulses. An observed fast exciton-density dependent conversion rate is attributed to spin exchange between pairs of orthoexcitons. Implication of these results on the feasibility of Bose-Einstein condensation of paraexcitons in Cu(2)O is discussed.

Stable biexcitonic lasing of CuCl quantum dots under two-photon resonant excitation

We have observed ultraviolet lasing of CuCl quantum dots embedded in a NaCl single crystalline matrix. Stable ultraviolet lasing of biexciton (two e–h pairs) luminescence has been achieved below 70 K under the two-photon direct excitation of biexcitons of CuCl quantum dots. Laser action occurs with parallely cleaved surfaces of the NaCl crystalline matrix acting as an optical cavity, which shows a very high efficiency of lasing.