Resonant Laser Excitation Research Articles

Coherent radiation by two-level quantum systems with permanent dipole moments under multiphoton resonant laser excitation

The theory of coherent radiation emission in two-level quantum systems with permanent dipole moments at the multiphoton resonant excitation by moderately strong laser fields is presented. A simple analytic formula for the expectation value of a dipole moment operator is obtained. The spectrum corresponding to harmonic generation and low-frequency radiation on the generalized Rabi frequency is investigated both analytically and numerically. The effect of dynamic Stark shift on a dipole spectrum under direct multiphoton resonant excitation is considered.

Electrically tunable hole tunnelling from a single self-assembled quantum dot embedded in an n-i-Schottky photovoltaic cell

We perform excitation-intensity-dependent measurements of the neutral exciton (X0) photocurrent (PC) peak amplitude from a single InAs/GaAs self-assembled quantum dot (QD) embedded in the intrinsic region of an n-i-Schottky photodiode. Since resonant laser-excitation of the X0 transition cannot occur until the comparatively slow hole tunnels out of the QD, we observe a saturation of the PC peak amplitude towards high excitation-intensities, allowing us to determine the hole tunnelling time by fitting with an appropriate theoretical model. By repeating this measurement for a range of bias voltages, we obtain the hole tunnelling time as a function of vertical electric field, showing that it can be tuned by nearly two orders of magnitude. Finally, we find that the hole tunnelling rate can be described accurately by a theoretical model based on a Wentzel-Kramers-Brillouin approximation to yield precise values for the QD height and hole confinement potential.

Silver nanocluster films as novel SERS substrates for ultrasensitive detection of molecules

Highly reproducible, active and stable SERS substrates were prepared by a nanocluster deposition system. SERS studies were carried out on these substrates with R6G as a test molecule using near Raman resonant and off resonant laser excitation wavelengths. Our results show that Ag nanoclusters of average particle size of (157 ± 119) nm act as very good SERS substrates for the ultra trace detection of R6G with an AEF of 2 × 10 12 corresponding to a detection limit of 10 −16 M. The present substrates could improve the detection limit of R6G by two orders of magnitude as compared to the previous reports.

Laser mass-spectrometry for online diagnosis of reactive plasmas with many species

The purpose of this study is to design a diagnostic system for reactive plasma environment by combining molecular-beam time-of-flight (TOF) mass spectroscopy with laser spectroscopy technique. The combination of TOF mass spectrometers and pulsed lasers is favorable in the diagnosis of intermediate species distribution since they allow the simultaneous but separate recording of the spectra of different species. In the plasma system, the intermediate species in electronic ground state or low lying excited state is pumped to higher energy level with resonant laser excitation, and then, the ionization with a second laser system is possible which can readily be detected by the TOF analyzer. The ionization itself is only used as a detection mechanism for the observation of the excitation of these states. In this manner, the population distribution of intermediate species can be determined with state-selective and mass-selective feature. Also, in this article, a flexible data acquisition and automatic control system based on LABVIEW was designed to integrate all the stand-alone measurement instruments including a TOF spectrometer, a laser system, a high performance oscilloscope, and a digital delay generator into a single personal computer-based control unit. Moreover, a virtual Boxcar integrator with hundreds of channels has been developed to enhance the signal while filtering out the random noises. Finally, the many potentials of this technique in the application of plasma diagnosis will be discussed.

Hydrodynamic nucleation of vortices and solitons in a resonantly excited polariton superfluid

We present a theoretical study of the hydrodynamic properties of a quantum gas of exciton-polaritons in a semiconductor microcavity under a resonant laser excitation. The effect of a spatially extended defect on the superfluid flow is investigated as a function of the flow speed. The processes that are responsible for the nucleation of vortices and solitons in the wake of the defect are characterized, as well as the regimes where the superfluid flow remains unperturbed. Specific features due to the non-equilibrium nature of the polariton fluid are put in evidence.

Nature and Excitation Mechanism of the Emission-dominating Minority Eu-center in GaN Grown by Organometallic Vapor-phase Epitaxy

ABSTRACTIn-situdoped Eu ions in GaN grown by Organometallic Vapor-phase Epitaxy (OMVPE) at different pressures were investigated under different excitation methods and through the use of the following experimental techniques: (1) resonant site-selective laser irradiation (2) electron beam excitation, and (3) a dual excitation using a combination of electron beam and laser irradiation. With these means, we have examined the difference in the excitation pathways that result from resonant laser and electron hole (e-h) pair excitation of Eu ions for two different distinct incorporation sites, which are responsible for most of the luminescence. We have obtained clear evidence that e-h pairs do not have the ability to excite all of the ions and that there is excitation trapping by defects involved in the Eu excitation.

Spectroscopy of the two Lowest Exciton Zero-Phonon Lines in Single CdSe/ZnS Nanocrystals

We study the optical properties of the lowest-energy exciton states in highly photostable individual CdSe/ZnS nanocrystals at low temperatures. We observe two sharp zero-phonon lines which we attribute to the radiative recombination from the two lowest-energy levels of the band-edge exciton fine structure. By using resonant laser excitation on these two lines we measure a spectral broadening of 10 μeV over integration times of 100 ms.

Determination of the first ionization potential of technetium

Using resonance ionization spectroscopy the first-ionization potential of actinium has been determined by analyzing different Rydberg series in two-color resonant laser excitation. Three individual Rydberg series were investigated, converging toward the ionic ground state and toward the first- and second-excited state of the actinium ion, respectively. A combined analysis of the convergence limits leads to a consistent value for the first-ionization potential of Ac of $43\phantom{\rule{0.16em}{0ex}}394.45\phantom{\rule{0.16em}{0ex}}(19)\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$, equivalent to $5.380\phantom{\rule{0.16em}{0ex}}226\phantom{\rule{0.16em}{0ex}}(24)$ eV.

Cesium control and diagnostics in surface plasma negative ion sources

For efficient and reliable negative ion generation it is very important to improve a cesium control and diagnostics. Laser beam attenuation and resonance fluorescence can be used for measurement of cesium distribution and cesium control. Resonant laser excitation and two-photon excitation can be used for improved cesium ionization and cesium trapping in the discharge chamber. Simple and inexpensive diode lasers can be used for cesium diagnostics and control. Cesium migration along the surface is an important mechanism of cesium escaping. It is important to develop a suppression of cesium migration and cesium accumulation on the extraction system.

Confluence of resonant laser excitation and bidirectional quantum-dot nuclear-spin polarization

Resonant laser scattering along with photon correlation measurements established the atom-like character of quantum dots. Here, we show that for a wide range of experimental parameters it is impossible to isolate elementary quantum-dot excitations from a strong influence of nuclear spins; the absorption lineshapes at magnetic fields exceeding 1 T indicate that the nuclear spins get polarized by an amount that ensures locking of the quantum-dot resonance to the incident laser frequency. In stark contrast to earlier experiments, this nuclear-spin polarization is bidirectional, allowing the combined electron–nuclear-spin system to track the changes in laser frequency dynamically on both sides of the resonance. This unexpected feature stems from a competition between two spin-pumping processes that attempt to polarize nuclear spins in opposite directions. We find that the confluence of laser excitation and nuclear-spin polarization suppresses the fluctuations in resonant absorption. A master-equation analysis suggests narrowing of the nuclear-spin distribution, pointing to applications in quantum information processing.

All-optical cw quadruple resonance excitation: A coherently driven five-level molecular system

We report here a continuous-wave, high-resolution, and all-optical quadruple resonance laser excitation experiment. To demonstrate this excitation technique, we have used an inhomogeneously broadened molecular sample (Na-2). We emphasize the enhanced ability afforded by the technique to reach new ranges of internuclear separation as well as access to a large range of molecular electronic states. Using the Autler-Townes split line shape of a rovibronic transition and the E field amplitude of the corresponding coupling laser, we have measured with high precision the absolute value of the molecular electronic transition dipole moment matrix elements, with more flexibility than the double and the triple resonance based excitation schemes used previously. This technique may also be used in a stimulated Raman adiabatic passage configuration for effective transfer of ultracold molecules, formed at long internuclear distance, to the lowest vibrational level of the ground electronic state. (Less)

Competition between zero-phonon and phonon-assisted luminescence in colloidal CdSe quantum dots

We determine the low-temperature optical properties of dark-exciton states in CdSe/CdS nanocrystal quantum dots (NQDs). By using resonant laser excitation we distinguish zero-phonon from phonon-assisted photoluminescence. The NQDs show a decreasing zero-phonon intensity with decreasing temperature, resulting in a redshift of the nonresonant photoluminescence. This redshift is undone by application of a magnetic field. Our results show that dark-exciton luminescence originates from the intricate competition of phonon-assisted and zero-phonon transitions, the latter of which are enhanced by dark--bright-exciton mixing due to unpassivated surface states or a magnetic field.

Acoustic phonon Raman scattering induced by a built-in electric field

We report acoustic phonon resonant Raman scattering experiments in strained piezoelectric [311] ${\text{Ga}}_{0.85}{\text{In}}_{0.15}\text{As}/\text{AlAs}$ superlattices with permanent built-in piezoelectric fields. The acoustic phonon spectra develop upon resonant excitation around the ``forbidden'' $h{h}_{2}\ensuremath{\rightarrow}{e}_{1}$ interband transition into a broad intense structure, which is peaked at the first folded phonons. Standard narrow acoustic phonon doublets are recovered when the power of the resonant laser excitation is increased. Such change in the resonant Raman spectra with increasing power is accompanied by a strong shift and bleaching of a photoluminescence emission related to the $h{h}_{2}\ensuremath{\rightarrow}{e}_{1}$ transition. None of these observations are present in [001] ${\text{Ga}}_{0.85}{\text{In}}_{0.15}\text{As}/\text{AlAs}$ superlattices that lack the built-in fields because of symmetry. We interpret these results as originated in a Raman process resonant with an intermediate transition that becomes allowed due to the built-in fields. We address the possible mechanisms by which the acoustic phonons strongly modulate the dielectric function in these piezoelectric nanostrutures, including an electron-acoustic-phonon interaction involving the phonon modulation of the built-in fields. This mechanism, which is shown to be proportional to the magnitude of the built-in fields, is quenched when the latter are screened by photoexcited carriers.

Theoretical modeling of resonant laser excitation of atoms in a magnetic field

The interaction of near-resonant laser radiation with atoms immersed in a magnetic $\mathbf{B}$ field is calculated using a quantum electrodynamic model. In this model, the magnetic field is assumed to produce a small perturbation such that the degeneracy of the magnetic substates is lifted while maintaining the usual quantum numbers that define the states (the Zeeman effect). The laser radiation is considered to have a narrow bandwidth and to be temporally and spatially coherent. The model produces three general coupled differential equations that describe the state populations and their relative coherences and the optical coherences between levels coupled by the laser radiation. The model can therefore be directly applied to different experiments ranging from atom trapping and cooling experiments through to collision experiments carried out in magnetic and laser fields.

Laser spectroscopy on trapped highly-charged ions using soft x-rays from FLASH

In vast regions of the universe highly charged ions (HCI [1, 2]) are the predominant form of visible matter. Their importance extends to high-temperature terrestrial plasmas, such as those used in fusion research. Yet, accurate prediction of their electronic structure remains a challenge for theory due to the strong electromagnetic field in which the remaining bound electrons dwell. Experimental accuracy has now reached the performance limits of conventional photon spectroscopy in the soft and hard x-ray regions. In this work [3], we report on the resonant laser excitation of the 22S1/2—22P1/2 transition of the Li-like Fe22P1/223+ ion at 48.6 eV, an energy range hitherto unattainable with powerful lasers. The HCI stored in an electron beam ion trap (EBIT [4]) were resonantly excited by ultra-brilliant radiation generated at the Free electron LASer in Hamburg (FLASH [5]). While yielding a relative statistical error of only 2.2·10−5, and extending laser spectroscopy on HCI from the near ultraviolet [6] to the soft X-ray region, this novel experiment demonstrates immediate potential to push the current limits of precision by orders of magnitude. Such experiments allow to verify predictions of quantum electrodynamics (QED) in a strong field environment where perturbation theory [7, 8] fails. Future EBIT experiments at upcoming x-ray free electron lasers (X-FEL) like the Stanford Linear Coherent Light Source (LCLS) or the European X-FEL will pave the way for laser spectroscopy into the hard x-ray region.

Soft X-Ray Laser Spectroscopy on Trapped Highly Charged Ions at FLASH

In a proof-of-principle experiment, we demonstrate high-resolution resonant laser excitation in the soft x-ray region at 48.6 eV of the 2 (2)S(1/2) to 2 (2)P(1/2) transition of Li-like Fe23+ ions trapped in an electron beam ion trap by using ultrabrilliant light from Free Electron Laser in Hamburg (FLASH). High precision spectroscopic studies of highly charged ions at this and upcoming x-ray lasers with an expected accuracy gain up to a factor of a thousand, become possible with our technique, thus potentially yielding fundamental insights, e.g., into basic aspects of QED.

Spectroscopy of laser-excited indium, gallium and indium–gallium systems

The spectroscopy of dense Ga, In and In–Ga vapours is studied via resonant pulsed laser excitation at 403.4 nm (Ga) and 410.3 nm (In). Besides some known atomic Rydberg levels emission, satellite wings on the blue side of the fundamental transitions show up both in the homonuclear and heteronuclear vapours. They are due to the presence of excited homonuclear dimers that form in the collision between excited and ground-state atoms. The formation of heteronuclear dimers is inferred by the time-resolved analysis of some atomic fluorescences.

Laser optogalvanic observations and MQDT analysis of mp5nd J = 3 autoionizing resonances in Ar, Kr and Xe

We report new measurements of the spectra of argon, krypton and xenon in the autoionization region using a two-step resonant laser excitation and optogalvanic detection technique. By selecting (m)p5(m + 1)p′[3/2]2 as an intermediate state (m = 4, 5 and 6 for Ar, Kr and Xe, respectively), we have been able to single out the (m)p5nd[5/2]3 autoionizing resonances in their spectra. The MQDT parameters have been derived from the analysis of the series perturbations among the (m)p5nd[5/2]3, (m)p5nd[7/2]3 and (m)p5nd′[5/2]3 series in the discrete region using the phase shifted formulation of the three-channel quantum defect theory and from the line profile analysis of the autoionizing resonances above the first ionization threshold. The predicted reduced widths for the autoionizing resonances based on the series perturbation analysis show good agreement with those of the experimentally observed profiles. Accurate values of the resonance energies, quantum defects and reduced widths are reported.

Measurement of the 4d-photoionization cross section via two-photon and two-step excitation in sodium

We present a comparison of the photoionization cross sections of the 4d D 3 / 2 , 5 / 2 2 excited levels of sodium by using the two-step resonant laser excitation and by two-photon non-resonant excitation from the ground state. Dye lasers, pumped with a Nd: YAG laser, have been used in conjunction with a thermionic diode ion detector to measure cross sections and the atomic densities as a function of laser energy. By applying the saturation technique, the measured values of the cross sections and atomic densities for the 4d D 5 / 2 2 level by two-photon excitation are 12.2(2.4) Mb and 6.7 × 10 10 cm - 3 , respectively. Where as in the case of two-step excitation, the cross section and number density for the 4d D 3 / 2 2 level via 3p P 1 / 2 2 level and for the 4d D 3 / 2 , 5 / 2 2 levels via 3p P 3 / 2 2 level are determined as 9.6(1.9) Mb, 4.1 × 10 10 cm - 3 and 12.8(2.5) Mb, 3.7 × 10 10 cm - 3 , respectively.

Resonance fluorescence spectroscopy in laser-induced cavitation bubbles

Laser-induced breakdown spectroscopy (LIBS) in liquids using a double-pulse Q-switched Nd:YAG laser system has provided reliable results that give trace detection limits in water. Resonant laser excitation has been added to enhance detection sensitivity. A primary laser pulse (at 532 nm), transmitted via an optical fiber, induces a cavitation bubble and shockwave at a target immersed in a 10 mg l(-1)-100 mg l(-1) indium (In) water suspension. The low-pressure rear of the shockwave induces bubble expansion and a resulting reduction in cavity pressure as it extends away from the target. Shortly before the maximum diameter is expected, a secondary laser pulse (also at 532 nm) is fed into the bubble in order to reduce quenching processes. The plasma field generated is then resonantly excited by a fiber-guided dye laser beam to increase detection selectivity. The resulting resonance fluorescence emission is optically detected and processed by an intensified optical multichannel analyzer system.