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We perform experimental studies of resonantly enhanced sum-frequency mixing (SFM), driven by tunable, spectrally narrowband mid-infrared and fixed-frequency nanosecond laser pulses, aiming at applications in molecular gas detection. The mid-infrared pulses are tuned in the vicinity of two-photon rovibrational transitions in the electronic ground state to provide strong resonance enhancements of the nonlinear susceptibility, while a probe laser at shorter wavelength uses an off-resonant single-photon coupling to excited electronic states. This SFM approach benefits from the advantageous combination of typically small detunings among the mid-infrared, vibrational transitions and the typically large transition dipole moment for couplings of electronic states. Moreover, compared to resonantly enhanced third harmonic generation (THG), a signal wave at much shorter wavelength permits simple and efficient detection. We demonstrate resonantly enhanced SFM via rovibrational states in gaseous hydrogen chloride molecules and compare its features to THG. The SFM spectra offer a large signal-to-noise ratio of 4 orders of magnitude and a detection limit down to a pressure of 0.1 mbar, corresponding to a particle density of 0.35×10^15 per cm^3.

We report on an all-solid-state light source for nanosecond (ns) laser pulses, with broad tunability in the mid-infrared, spectral bandwidth close to the Fourier transform limit, and pulse energy in the mJ regime. To this end, we extend a tunable, continuous wave (cw) singly resonant optical parametric oscillator by an optical parametric pre-amplifier with a periodically poled lithium niobate crystal and a power amplifier stage with two bulk lithium niobate crystals. We demonstrate pulse energies beyond 1 mJ in a tuning range between 3.3 and 3.8 $$\upmu \mathrm {m}$$ μ m center wavelength, with options for even larger output pulse energy and tuning range. The total conversion efficiency in the power amplifier reaches 20%. From measurements of absorption spectroscopy, we determine a very narrow linewidth of 108 MHz (full width at half maximum, FWHM), which is only a factor of 1.4 above the Fourier limit. We demonstrate the applicability and versatility of the laser system for nonlinear spectroscopy by resonantly enhanced third harmonic generation and sum frequency mixing in a gas sample of HCl molecules.

Nonclassical photon sources of high brightness are key components of quantum communication technologies. We here demonstrate the generation of narrowband, nonclassical photon pairs by employing spontaneous four-wave mixing in an optically-dense ensemble of cold atoms within a hollow-core fiber. The brightness of our source approaches the limit of achievable generated spectral brightness at which successive photon pairs start to overlap in time. For a generated spectral brightness per pump power of up to 2 × 109 pairs/(s MHz mW) we observe nonclassical correlations at pump powers below 100 nW and a narrow bandwidth of 2π × 6.5 MHz. In this regime we demonstrate that our source can be used as a heralded single-photon source. By further increasing the brightness we enter the regime where successive photon pairs start to overlap in time and the cross-correlation approaches a limit corresponding to thermal statistics. Our approach of combining the advantages of atomic ensembles and waveguide environments is an important step toward photonic quantum networks of ensemble-based elements.

We studied resonantly enhanced third-harmonic generation, induced by midinfrared, spectrally narrow-band, nanosecond laser pulses tuned in the vicinity of rovibrational molecular resonances. As we drive the frequency conversion process simultaneously close to single-, two-, and three-photon resonances, we get strong resonance enhancements of the third-harmonic yield by several orders of magnitude. The experimental setup requires only a single midinfrared laser beamline; the signal wavelength is easily separable from the much longer driving laser wavelength. We performed extended, systematic studies on the third-harmonic yield, spectral line shape and linewidth with laser intensity, frequency, and pressure. We determined a detection limit of ${10}^{15}$ $\mathrm{molecules}/{\mathrm{cm}}^{3}$, which indicates the potential of the approach for realistic applications in trace gas detection.

We experimentally demonstrate spatial confinement of atomic excitation by adiabatic passage processes in a rare-earth ion-doped Pr3+:Y2SiO5 crystal. In particular, we apply stimulated Raman adiabatic passage (STIRAP) and compare its performance with electromagnetically induced transparency (EIT). Using a Stokes beam with Gaussian and a pump beam with donut shape we localize the atomic population in the zero-intensity center of the latter. Our data confirm that adiabatic passage confines excitation far below the diameter of the driving laser beams, and that this localization rapidly increases with laser intensity. We find, that STIRAP significantly outperforms EIT, as it was predicted by previous theory proposals, i.e., STIRAP reaches small excitation volumes with much lower laser intensity. The experimental data agree very well with numerical simulations. The findings serve as a step towards new applications for STIRAP, to prepare excitation regions or population patterns in space with large resolution.

We present a thorough experimental investigation of the loading process of laser-cooled atoms from a magneto-optical trap into an optical dipole trap located inside a hollow-core photonic bandgap fiber, followed by propagation of the atoms therein. This, e.g., serves to identify limits to the loading efficiency and thus optical depth which is a key parameter for applications in quantum information technology. Although only limited access in 1D is available to probe atoms inside such a fiber, we demonstrate that a detailed spatially resolved characterization of the loading and trapping process along the fiber axis is possible by appropriate modification of probing techniques combined with theoretical analysis. Specifically, we demonstrate the loading of up to $2.1\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms with a transfer efficiency of $2.1%$ during the course of 50 ms and a peak loading rate of $4.7\ifmmode\times\else\texttimes\fi{}{10}^{3}$ atoms ${\mathrm{ms}}^{\ensuremath{-}1}$ resulting in a peak atomic number density on the order of ${10}^{12}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Furthermore, we determine the evolution of the spatial density (profile) and ensemble temperature as it approaches its steady-state value of $T=1400\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$, as well as loss rates, axial velocity and acceleration. The spatial resolution along the fiber axis reaches a few millimeters, which is much smaller than the typical fiber length in experiments. We compare our results to other fiber-based as well as free-space optical dipole traps and discuss the potential for further improvements.

For the efficient, emission-free and low-noise transport of goods and commodities in urban areas, BPW has developed the electric drive "eTransport" for 7.5t light duty trucks. In order to be able to take the specific customer requirements in different application regions into account and to derive load profiles, measurement runs are carried out in the field. If you want to map the loads that actually occur in practice on the entire vehicle or individual vehicle components through measurement campaigns, a good knowledge of the expected loads on the vehicle is very important. Furthermore, it is of particular interest to vehicle manufacturers, such as BPW, to find a reference route near their development centers, on which design changes can be verified quickly and economically under real road conditions.

We present experimental data on adiabatically driven frequency conversion with ultrashort (picosecond) laser pulses towards the vacuum-ultraviolet regime, enhanced by coherent population return and preparation of maximal atomic coherences. We generate the sum frequency of an intense pump pulse and a probe pulse via a two-photon resonance in xenon. When we slightly detune the pump laser from two-photon resonance, the atomic populations are adiabatically driven forth and back from the atomic ground state to an excited state. This coherent population return (CPR) prepares the medium in a transient state of maximal atomic coherence, which enhances frequency mixing with a probe laser. We thoroughly study the variation of CPR-enhanced frequency conversion with experimental parameters and compare CPR with conventional, resonantly enhanced frequency conversion. In particular, we investigate the temporal evolution of the atomic coherence and the effect of inevitable ac Stark shifts. We find that ac Stark shifts induce an asymmetry in the spectral characteristics of CPR, which nevertheless permits enhanced frequency conversion. For the case of resonantly enhanced frequency conversion, we show that the atomic coherences are maintained for several tens of picoseconds after the pump pulse, which permits time-delayed frequency conversion. Moreover, we analyze the pressure dependence of the atomic coherence' lifetime and observe a free induction decay at the two-photon coherence with the generation of a second harmonic field. These findings shall push applications of adiabatic light-matter interactions also to the regime of nonlinear optics, driven by intense laser pulses.

We consider a method of sub-wavelength superlocalization and patterning of atomic matter waves via a two dimensional stimulated Raman adiabatic passage (2D STIRAP) process. An atom initially prepared in its ground level interacts with a doughnut-shaped optical vortex pump beam and a traveling wave Stokes laser beam with a constant (top-hat) intensity profile in space. The beams are sent in a counter-intuitive temporal sequence, in which the Stokes pulse precedes the pump pulse. The atoms interacting with both the traveling wave and the vortex beam are transferred to a final state through the 2D STIRAP, while those located at the core of the vortex beam remain in the initial state, creating a super-narrow nanometer scale atomic spot in the spatial distribution of ground state atoms. By numerical simulations we show that the 2D STIRAP approach outperforms the established method of coherent population trapping, yielding much stronger confinement of atomic excitation. Numerical simulations of the Gross-Pitaevskii equation show that using such a method one can create 2D bright and dark solitonic structures in trapped Bose-Einstein condensates (BECs). The method allows one to circumvent the restriction set by the diffraction limit inherent to conventional methods for formation of localized solitons, with a full control over the position and size of nanometer resolution defects.

International audience; An experimental platform operating at the level of individual quanta and providing strong light-matter coupling is a key requirement for quantum information processing. In our work, we show that hollow-core photonic bandgap fibers filled with laser-cooled atoms might serve as such a platform, despite their typical complicated birefringence properties. To this end, we present a detailed theoretical and experimental study to identify a fiber with suitable properties to achieve operation at the single-photon level. In the fiber, we demonstrate the storage and on-demand retrieval as well as the creation of stationary light pulses, based on electromagnetically induced transparency, for weak coherent light pulses down to the single-photon level with an unconditional noise floor of 0.017(4) photons per pulse. These results clearly demonstrate the prospects of such a fiber-based platform for applications in quantum information networks.

We present strong enhancements of the signal yield in third harmonic generation microscopy by seeding the optically nonlinear sample with some intensity at the third harmonic wavelength, in addition to the driving fundamental beam. By applying a third harmonic bias pulse with a power of less then 0.1% of the fundamental beam, we boost the signal yield by more than a factor of 3000, compared to the conventional third harmonic signal from the microscopy sample. The signal enhancement is most pronounced at low laser intensity and/or weak nonlinear susceptibilities. This makes the concept particularly suitable to improve the signal-to-noise ratio from samples with weak signals, e.g., as typical for applications of nonlinear microscopy. Moreover, we demonstrate improved spatial resolution in beam propagation direction by more than an order of magnitude. This exploits the dependence of our approach for enhanced third harmonic generation from inevitable dispersion in the sample. The improvement is most evident in cases, where the sample geometry allows only weak focusing.

STIRAP (stimulated Raman adiabatic passage) is a powerful laser-based method, usually involving two photons, for efficient and selective transfer of populations between quantum states. A particularly interesting feature is the fact that the coupling between the initial and the final quantum states is via an intermediate state, even though the lifetime of the latter can be much shorter than the interaction time with the laser radiation. Nevertheless, spontaneous emission from the intermediate state is prevented by quantum interference. Maintaining the coherence between the initial and final state throughout the transfer process is crucial. STIRAP was initially developed with applications in chemical dynamics in mind. That is why the original paper of 1990 was published in The Journal of Chemical Physics. However, from about the year 2000, the unique capabilities of STIRAP and its robustness with respect to small variations in some experimental parameters stimulated many researchers to apply the scheme to a variety of other fields of physics. The successes of these efforts are documented in this collection of articles. In Part A the experimental success of STIRAP in manipulating or controlling molecules, photons, ions or even quantum systems in a solid-state environment is documented. After a brief introduction to the basic physics of STIRAP, the central role of the method in the formation of ultracold molecules is discussed, followed by a presentation of how precision experiments (measurement of the upper limit of the electric dipole moment of the electron or detecting the consequences of parity violation in chiral molecules) or chemical dynamics studies at ultralow temperatures benefit from STIRAP. Next comes the STIRAP-based control of photons in cavities followed by a group of three contributions which highlight the potential of the STIRAP concept in classical physics by presenting data on the transfer of waves (photonic, magnonic and phononic) between respective waveguides. The works on ions or ion strings discuss options for applications, e.g. in quantum information. Finally, the success of STIRAP in the controlled manipulation of quantum states in solid-state systems, which are usually hostile towards coherent processes, is presented, dealing with data storage in rare-earth ion doped crystals and in nitrogen vacancy (NV) centers or even in superconducting quantum circuits. The works on ions and those involving solid-state systems emphasize the relevance of the results for quantum information protocols. Part B deals with theoretical work, including further concepts relevant to quantum information or invoking STIRAP for the manipulation of matter waves. The subsequent articles discuss the experiments underway to demonstrate the potential of STIRAP for populating otherwise inaccessible high-lying Rydberg states of molecules, or controlling and cooling the translational motion of particles in a molecular beam or the polarization of angular-momentum states. The series of articles concludes with a more speculative application of STIRAP in nuclear physics, which, if suitable radiation fields become available, could lead to spectacular results., Journal of Physics B: Atomic, Molecular and Optical Physics, 52 (20), ISSN:1361-6455, ISSN:0368-3508, ISSN:0953-4075, ISSN:0022-3700

We investigate the rephasing efficiency of sequences of phased pulses for spin echoes and light storage by electromagnetically induced transparency (EIT). We derive a simple theoretical model and show that the rephasing efficiency is very sensitive to the phases of the imperfect rephasing pulses. The obtained efficiency differs substantially for spin echoes and EIT light storage, which is due to the spatially retarded coherence phases after EIT light storage. Similar behavior is also expected for other light-storage protocols with spatial retardation or for rephasing of collective quantum states with an unknown or undefined phase, e.g., as relevant in single-photon storage. We confirm the predictions of our theoretical model by experiments in a ${\mathrm{Pr}}^{3+}$:${\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ crystal.

We experimentally demonstrate composite stimulated Raman adiabatic passage (CSTIRAP), which combines the concepts of composite pulse sequences and adiabatic passage. The technique is applied for population transfer in a rare-earth doped solid. We compare the performance of CSTIRAP with conventional single and repeated STIRAP, either in the resonant or the highly detuned regime. In the latter case, CSTIRAP improves the peak transfer efficiency and robustness, boosting the transfer efficiency substantially compared to repeated STIRAP. We also propose and demonstrate a universal version of CSTIRAP, which shows improved performance compared to the originally proposed composite version. Our findings pave the way towards new STIRAP applications, which require repeated excitation cycles, e.g., for momentum transfer in atom optics, or dynamical decoupling to invert arbitrary superposition states in quantum memories., 11 pages, 5 figures

We investigate phase-matched harmonic generation of ultrashort (picosecond) laser pulses, tuned in the vicinity of a five-photon resonance in argon, confined in a hollow-core optical waveguide. This combines resonance enhancements and tight spatial confinement to increase the conversion efficiency toward vacuum-ultraviolet radiation pulses. We demonstrate that appropriate choice of the gas pressure maintains optimal phase-matching conditions also in the presence of inevitable dynamic level shifts at high intensities. Moreover, we reveal the considerable contribution of higher-order transversal waveguide modes to the total conversion efficiency and investigate the role of cascading frequency conversion processes. Finally, we study additional signal enhancements by buffer gas admixtures. The experimental data are compared with numerical simulations, taking higher transversal waveguide modes and cascade frequency conversion into account, identifying also the potential of quasi-phase matching by polarization mode beating. Our investigations show that proper choice of experimental parameters enables significant resonance enhancements in the conversion efficiency of harmonic generation.

Heute konzentriert sich der Entwicklungsprozess in der Nutzfahrzeugindustrie nicht nur auf die klassischen Disziplinen wie Entwurfslasten, Festigkeit und Handlingaspekte. Daruber hinaus muss die Kombination verschiedener Themen wie beispielsweise Kraftstoffverbrauch, reale Fahremissionen und variable Verkehrsbedingungen berucksichtigt werden. Im ersten Teil dieses Beitrags werden die Ergebnisse aktueller Untersuchungen zur Nutzung von georeferenzierten Hohendaten in Kombination mit digitalen Karten dargestellt. Der zweite Teil gibt einen Uberblick uber den Entwicklungsprozess der ZF Friedrichshafen AG. Anhand zweier Anwendungsfalle von Getrieben fur Stadtbusse und Nutzfahrzeuge, wird die Integration von georeferenzierten Datenanalysen und statistischen Methoden dargestellt. Der erste Anwendungsfall konzentriert sich auf die derzeit genutzten Auslegungsstrecken der ZF Friedrichshafen AG. Diese werden zunachst unter statistischen Aspekten analysiert, wobei die Zusammensetzung jeder Strecke im Vergleich zu gewunschten Zielmarkten dargestellt wird. Die Untersuchungen werden abgerundet, indem virtuell berechnete Geschwindigkeitsprofile mit gemessenen Profilen verglichen werden. Der zweite Anwendungsfall beschreibt den Transferprozess von bekannten Heimatmarkten zu neuen Kundenmarkten. Der Ansatz folgt der Idee, die Software VMC als Werkzeug zur virtuellen Produktfreigabe fur die Anwendungsbereiche zu nutzen, in denen weniger harte Einsatzbedingungen in Betracht gezogen werden konnen.

We derive a versatile protocol to implement a fast and robust not quantum gate, driven by single-shot-shaped pulses with appropriate time-dependent phase and intensity profiles. The pulses are derived by combining analytic computations and numerical optimizations. We experimentally demonstrate the applicability, efficiency, and robustness of the derived pulses to rephase atomic coherences in a ${\text{Pr}}^{3+}\text{:}{\text{Y}}_{2}{\text{SiO}}_{5}\phantom{\rule{0.222222em}{0ex}}$ crystal.

We introduce universally robust sequences for dynamical decoupling, which simultaneously compensate pulse imperfections and the detrimental effect of a dephasing environment to an arbitrary order, work with any pulse shape, and improve performance for any initial condition. Moreover, the number of pulses in a sequence grows only linearly with the order of error compensation. Our sequences outperform the state-of-the-art robust sequences for dynamical decoupling. Beyond the theoretical proposal, we also present convincing experimental data for dynamical decoupling of atomic coherences in a solid-state optical memory.

We investigate harmonic generation of ultrashort, picosecond pump laser pulses toward the vacuum-ultraviolet spectral regime, enhanced by preparation of dressed states in media of rare gas atoms. A control laser, with appropriately matched frequency and/or peak intensity serves to generate dressed states with large, tunable Autler–Townes splittings in the THz range. The energetic position of the dressed states is laser-controlled to match multiple pump laser photons. This enables resonance enhancement of harmonic generation also for typically fixed-frequency and off-resonant pump laser pulses. Moreover, the tunable Autler–Townes splittings also serve to compensate and even exploit inevitable Stark shifts of atomic resonances, which otherwise reduce the nonlinear response of the medium. We provide convincing experimental data to demonstrate the concept in argon and xenon, yielding enhancements of harmonic generation by 1–2 orders of magnitude.

We report on the first observation of stationary light pulses and narrowband light storage inside a hollow-core photonic crystal fiber. Laser-cooled atoms were first loaded into the fiber core providing strong light-matter coupling. Light pulses were then stored in a collective atomic excitation using a single control laser beam. By applying a second counterpropagating control beam, a light pulse could be brought to a standstill. Our work paves the way towards the creation of strongly-correlated many-body systems with photons and applications in the field of quantum information processing.

We apply time-resolved high-speed particle image velocimetry (PIV) in an optically accessible gasoline engine to determine the effect of the in-cylinder flow field upon combustion dynamics. Our PIV setup involves solid particles as tracer, which enables also measurements at firing top dead center and during the combustion process itself. We analyze the flow field for the entire intake and compression phase, as well as the decay of a prominent large-scale tumble structure in the flow field. The data indicate significant cycle-to-cycle flow field variations, characterized by detection of kinetic energy and tumble center. Measurements in fired engine operation demonstrate the influence of the flow field on combustion dynamics. At stoichiometric operation, we find that variations in the kinetic energy of the flow field are a major cause of cycle-to-cycle variations. From simultaneous imaging of the combustion flame and PIV at lean operation, we find that the velocity distribution in the flow field induces a macroscopic motion of the flame kernel—which significantly effects the combustion process.

We apply laser-induced breakdown spectroscopy (LIBS) to determine local lambda values (i.e. the normalized air-fuel mass ratio) at the ignition location λ ip in a direct-injection single-cylinder optical research engine. The technique enables us to determine variations of λ ip for different fuel injection strategies, as well as correlations between variations in λ ip and the combustion dynamics. In particular we observe, that fluctuations in λ ip are not the major cause of cycle-to-cycle variations in the combustion process. Moreover, our experiments identify insufficient lean λ ip values as a source of misfires in lean combustions. In a combination of LIBS with laser-induced fluorescence (LIF), we obtain additionally information about the two-dimensional λ distribution. These results demonstrate the potential of LIBS to monitor λ values during mixture formation in gasoline engines.

We monitor and characterize near-surface diffusion of miscible, transparent liquids in a microfluidic device by third harmonic microscopy. The technique enables observations even of transparent or index-matched media without perturbation of the sample. In particular, we image concentrations of ethanol diffusing in water and estimate the diffusion coefficient from the third harmonic images. We obtain a diffusion coefficient D = (460 ± 30) μm2/s, which is consistent with theoretical predictions. The investigations clearly demonstrate the potential of harmonic microscopy also under the challenging conditions of transparent fluids.

article i nfo We propose a simple and powerful protocol to map an arbitrary atomic coherence between two quantum states into a population distribution of three metastable states, and later to retrieve the atomic coherence from the population distribution. The protocol applies simple sequences of radiation pulses with arbitrary temporal profile, either as coincident or as consecutive pulses. Mapping of rather short-lived atomic coherences into very long-lived atomic populations permits the prolongation of storage times (e.g. of optical information encoded in atomic coherences) by many orders of magnitude — without the need for complicated techniques to reduce homogeneous broadenings.

Spatial solitons permit optical waveguiding. This holds true for the soliton write beam (i.e. the driving laser beam), as well as for additional probe beams, which may carry optically encoded information. This feature of spatial solitons is of significant interest for applications in optical telecommunication. We pres- ent systematic experimental investigations on single and multiple spatial solitons in the infrared spectral regime (i.e. around optical telecommunication wavelengths), applied as controllable all-optical devices. In particular, we present the implementations of a Y-coupler as an optical signal divider, a switchable Y-coupler as an optical add multiplexer, and a novel design for a 1 � 3 optical beam switch, i.e. applied as a router for infrared signal beams. We report large waveguiding efficiencies up to 40% and transmis- sion rates of 90 Tbit/s in our setups. The presented experimental data are confirmed by numerical simulations.

We report on the experimental implementation of stimulated Raman adiabatic passage (STIRAP), electromagnetically induced transparency (EIT) and the retrieval of light pulses in a Pr 3+ :Y 2 SiO 5 crystal. We apply coherent, adiabatic processes to control population dynamics and atomic coherences in selected hyperfine levels of the praseodymium dopants in the crystal. Our data demonstrate efficient adiabatic population transfer by conventional STIRAP as well as by an alternative transfer process, i.e. b-STIRAP. Moreover, we investigate dark resonances, driven by EIT in a A-type and in a V-type level scheme. We apply EIT to create atomic coherences in Pr3 + :Y 2 SiO 5 . The persistent coherence is monitored by delayed retrieval of light pulses from the medium.

We demonstrate efficient storage and retrieval of light pulses by electromagnetically induced transparency (EIT) in a Pr^{3+}:Y_{2}SiO_{5} crystal. Using a ring-type multipass configuration, we increase the optical depth (OD) of the medium up to a factor of 16 towards OD≈96. Combining the large optical depth with optimized conditions for EIT, we reach a light storage efficiency of (76.3±3.5)%. In addition, we perform extended systematic measurements of the storage efficiency versus optical depth, control Rabi frequency, and probe pulse duration. The data confirm the theoretically expected behavior of an EIT-driven solid-state memory.

We report on rapid adiabatic passage (RAP) in a Pr3+:Y2SiO5 crystal, cooled to cryogenic temperatures. The medium is prepared by optical pumping and spectral hole burning, creating a spectrally isolated two-level system within the inhomogeneous bandwidth of the 3H4 → 1D2 transition of the Pr3+ ions. A chirped laser pulse drives a RAP process in the medium, i.e. inverts the initial population distribution. We study the properties and dynamics of RAP by means of fluorescence detection, absorption spectroscopy and amplified spontaneous emission. Time-resolved absorption measurements serve to monitor the adiabatic population dynamics during the excitation process. In addition, we compare the results with coherent excitation at fixed laser frequency detuned from resonance, i.e. coherent population return (CPR).

We experimentally demonstrate coherent population transfer, driven by stimulated Raman adiabatic passage (STIRAP) between two bound quantum states, coupled via a continuum of states. We present extended numerical and experimental investigations on population transfer from the metastable state 2s 1 S0 to the excited state 4s 1 S0 in metastable helium atoms. While techniques based on incoherent excitation do not permit any population transfer via rapidly decaying continuum states, our data indicate a maximum transfer efficiency of 20% in coherent excitation by STIRAP. We study the transfer efficiency with respect to the relevant experimental parameters. � 2006 Elsevier B.V. All rights reserved.

We demonstrate efficient storage and retrieval of light pulses by electromagnetically induced transparency (EIT) in a Pr^{3+}:Y_{2}SiO_{5} crystal. Using a ring-type multipass configuration, we increase the optical depth (OD) of the medium up to a factor of 16 towards OD≈96. Combining the large optical depth with optimized conditions for EIT, we reach a light storage efficiency of (76.3±3.5)%. In addition, we perform extended systematic measurements of the storage efficiency versus optical depth, control Rabi frequency, and probe pulse duration. The data confirm the theoretically expected behavior of an EIT-driven solid-state memory.

Lassen wir unseren Tagesablauf Revue passieren, so denken wir dabei an alle moglichen schonen und schlechten Erfahrungen, aber nicht ein einziges Mal daran, dass wir mehr als einmal in diesen vierundzwanzig Stunden mit einer menschlichen Schopfung zu tun haben, die fast uberall ihre Photonen im Spiel hat. Die Rede ist vom Laser.

We demonstrate significant enhancement of four-wave mixing in coherently driven mercury isotopes to generate vacuum-ultraviolet radiation at 125 nm. The enhancement is accomplished by preparation of the mercury atoms in a state of maximum coherence, i.e. maximum nonlinear-optical polarization, driven by Stark-chirped rapid adiabatic passage (SCRAP). In this technique, a pump laser at 313 nm excites the two-photon transition between the ground state 6s 21 S0 and the target state 7s 1 S0 in mercury. A strong, offresonant radiation field at 1064 nm generates dynamic Stark shifts. These Stark shifts serve to induce a rapid adiabatic passage process on the two-photon transition. During the process a coherent superposition of the two states is established, which enhances the nonlinear-optical polarization in the medium to the maximum possible value. The maximum coherence permits efficient four-wave mixing of a pump laser and an additional probe laser at 626 nm. The efficiency is further enhanced, as the SCRAP process allows to stimulate the complete set of different mercury isotopes to participate in the frequency conversion process. This enlarges the effective atomic density of the medium. Thus, we observe the generation of vacuum-ultraviolet radiation at 125 nm enhanced by more than one order of magnitude with respect to conventional frequency conversion. Parallel to the frequency conversion process, we monitored the evolution of the population in the medium by laser-induced fluorescence. These data demonstrate efficient coherent population transfer by SCRAP.

The even parity mp1/2 5np′ and mp1/2 5nf′ autoionizing resonances of Ar, Kr, and Xe (m=3,4,5) were investigated experimentally and theoretically by one-photon excitation from lower-lying intermediate levels. In particular, high resolution measurements for the Ar(nf′), Kr(12p′,8f′), and Xe(8p′) resonances are reported; lineshape parameters for these resonances have been derived by a Fano-type analysis, thus yielding reduced resonance widths. The experimental spectra and the resonance parameters are compared with theoretical calculations which are based on the configuration interaction Pauli–Fock approach including core polarization. The measured and calculated lineshapes are in good agreement. In addition, theoretical predictions are presented for other resonances, which have not yet been observed experimentally, and some systematic trends are elucidated.

Using experimental set-ups involving single photon excitation of metastable rare gas atoms Rg(mp 5 (m +1 )s ,J = 2, 0) by a pulsed frequency-doubled dye laser in conjunction with time-of-flight ion detection, we have investigated several even Rg mp 5 1/2 npautoionizing resonances of the heavier rare gas atoms Rg = Ne − Xe (m = 2-5) at experimental bandwidths of 0.06 cm −1 (Ne) and about 0.25 cm −1 (Ar, Kr, Xe). New level energies (quantum defects) and widths for these resonances have been derived by a Fano-type lineshape analysis, thus yielding reduced resonance widths. The experimental spectra and the resonance parameters are compared with theoretical calculations which are based on the configuration interaction Pauli-Fock approach, including core polarization. For Rg = Ne, good agreement between the measured and the calculated results is observed. For the heavier rare gases, significant deviations between some of the existing experimental data and the theoretical results are found, indicating the need for further studies.

We report enhancement of third-harmonic generation in a nonlinear optical medium, prepared in maximum coherence by Stark-chirped rapid adiabatic passage (SCRAP). In this technique a strong off-resonant infrared radiation pulse induces an adiabatic passage process by sweeping the transition frequency of a two-photon transition in Krypton atoms through resonance with a near-resonant pump radiation pulse at 213 nm, appropriately delayed with respect to the infrared radiation. During the adiabatic passage process, a transient two-photon maximum coherence is induced, which beats with the pump laser frequency. Detrimental effects of Doppler broadening and collisional dephasing in the medium are reduced during the SCRAP process. The efficiency of third-harmonic generation, yielding vacuum-ultraviolet radiation at 71 nm is enhanced by more than one order of magnitude with respect to the case of conventional frequency conversion in an unprepared medium.

We compare lineshapes obtained in laser-induced fluorescence and resonantly enhanced multi-photon ionisation, resultingfrom coherent pulsed excitation of a two-photon transition in xenon atoms. We find that excitation profiles probed by ionisation show power broadeningas 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 probingprocess. 2003 Elsevier Science B.V. All rights reserved.

We present and analyze a bichromatic version of the well-known saturation absorption spectroscopy technique used to stabilize lasers to atomic resonances without Doppler broadening. We show that the spectroscopic properties can be transferred between two transitions at very different wavelengths by spectral hole burning in a Doppler-broadened atomic ensemble. Specifically, we demonstrate that this technique can be used to stabilize a laser to the D1 line of Rb87 by obtaining a saturation absorption spectrum with a strong pump laser stabilized to the D2 line. This allows for more freedom in the choice of the probe laser frequency offsets compared to conventional saturation absorption spectroscopy. We compare our experimental results to an analytical model and numerical results based on rate equations.

We present experimental data on quantum interferences in resonantly enhanced frequency up-conversion towards the vacuum-ultraviolet spectral regime. The process is driven in xenon atoms by ultrashort (picosecond) laser pulses. We use two simultaneous frequency conversion pathways via an excited intermediate state, i.e., fifth harmonicgenerationofthefundamentalwavelengthandfour-wavemixingofthefundamentalandtwophotonsof itssecondharmonicwavelength.Bothconversionpathwaysyieldradiationat102nm.Thetwopathwaysinterfere, depending on the relative phase of the fundamental and second harmonic. By appropriate choice of the phase we get constructive interference (resulting in increased conversion efficiency) or destructive interference (resulting in reduced conversion efficiency). The total conversion yield shows very pronounced constructive and destructive quantum interference with a visibility of roughly 90%. A stable and highly accurate phase control setup enables suchstrongquantuminterferencesformorethan260oscillationcycles.Inanextensionoftheexperiment,simultaneouslytofrequencyconversionwealsomonitorlaser-inducedfluorescenceasameasurefortheexcitationprobabilitytotheexcitedintermediatestate.Also,intheexcitationprobabilityweobservestrongquantuminterferences. As an interesting feature, a small phase lag occurs between the quantum interference patterns of frequency conversion and population transfer. This is due to an additional atomic phase acquired during frequency conversion.

Long coherence times are an essential prerequisite for implementations of quantum information technology. This requires techniques to control perturbing processes and hence prolong coherence times in quantum systems. In our work, we present systematic experimental investigations on prolongation of spin coherence times in a rare-earth ion-doped crystal. The approach is based on a combination of established coherence control techniques (i.e., zero first-order Zeeman shifts and simple dynamical decoupling), supported by automatic optimization of experimental control parameters, as well as precise characterization of the optimization loop and the strongly modified complex level structure by spin echoes and high-resolution Raman heterodyne spectroscopy. The spin-echo and Raman heterodyne data clearly prove successful optimization towards proper conditions of zero first-order Zeeman shifts, finally yielding a coherence time of 1 min, i.e., close to the theoretical limit set by the population lifetime in ${\text{Pr}}^{3+}$:${\text{Y}}_{2}$${\text{SiO}}_{5}$.

The spectral width of an atomic absorption line, observed with a steady light source, typically increases as the light intensity increases, an effect known as power broadening. In this paper, we point out classes of pulsed-light observations where power broadening does not always occur. We present analytical and numerical results, supported by experimental data of coherent pulsed excitation probed by photoionization, which show that the extent of power broadening depends crucially upon the nature of excitation and the type of measurement. In particular, we show that a spectral line obtained from measurement performed after pulsed excitation exhibits no power broadening. For pulsed excitation and continuous measurement, the spectral line contains two components: a power-broadened signal collected during the excitation and an unbroadened signal collected after the excitation.

We report the observation of pronounced coherent population trapping and dark resonances in Rydberg states of xenon. A weak two-photon coupling with radiation of P = 250 nm is induced between the 5p 61 S0 ground state of xenon and state 5p 5 6p(1=2)0, leading to (2+1) resonantly enhanced three-photon ionization. The state 5p 5 6p(1=2)0 is strongly coupled by radiation with D ' 600 nm to 5p 5 ns(JC)1 or 5p 5 nd(JC)1 Rydberg states with principal quantum numbers n in the range 18 n 23 and with the rotational quantum number of the ionic core JC =1 = 2o rJC =3 =2. The ionization is monitored through observation of the photoelectrons with an energy resolution E = 150 meV which is sucient to distinguish the ionization processes into the two ionization continua. Pronounced and robust dark resonances are observed in the ionization rate whenever D is tuned to resonance with one of the ns- or nd-Rydberg states. The dark resonances are due to ecient population trapping in the atomic ground state 5p 61 S0 through the suppression of excitation of the intermediate state 5p 5 6p(1=2)0. The resolution is sucient to resolve the hyperne structure of the ns-Rydberg levels for odd xenon isotopes. The hyperne splitting does not vary signicantly with n in the given range. Results from model calculations taking the natural isotope abundance into account are in good agreement with the observed spectral structures. Pronounced dark resonances are also observed when the dressing radiation eld with D is generated from a laser with poor coherence properties. The maximum reduction of the ionization signal clearly exceeds 50%, a value which is expected to be the maximum, when the dip is caused by saturation of the transition rate between the intermediate and the Rydberg state due to incoherent radiation. This work demonstrates the potential of dark resonance spectroscopy of high lying electronic states of rare gas atoms.

We have studied high-order harmonic generation (HHG) from organic molecules irradiated with near-infrared high intensity laser pulses of 70 fs and 240 fs duration. The molecular systems studied were the aromatics benzene and naphthalene and the alkanes cyclopropane and cyclohexane (cyclic) and n-hexane (linear). Harmonic intensities were measured both as a function of laser intensity (in the range 5×1013-5×1015 W cm-2) and as a function of ellipticity of the laser field polarisation. The results were compared with those from the xenon atom. For 70 fs pulses, harmonic generation from the organic systems was similar to that of xenon, revealing an atom-like behaviour for molecules when the laser pulse duration is shorter than the fragmentation timescale of the molecule. We note significant differences between molecules with respect to HHG efficiencies and the suppression of HHG in larger species. We discuss these differences in the context of the molecular properties, electronic structure and behaviour of ionisation and fragmentation that result in enhancement of field ionisation in larger systems. Study of the polarisation ellipticity dependence of HHG shows that the harmonic yield in molecules is less sensitive to the polarisation than for atoms (xenon). This is consistent with the expected behaviour given the larger recollision cross-section presented by the core in the molecular system compared to the atom. Our results suggest that study of HHG from molecules exposed to ultra-short pulses is potentially a powerful tool for understanding the electron dynamics of molecules exposed to an intense field.

The operational characteristics and technical details of a high-intensity discharge source for a pulsed beam with a high flux of helium atoms in their metastable states (exceeding 4 × 10 15 atoms sr −1 s −1 ) are discussed. Owing to the high ionization potential of helium, stable operation of a pulsed discharge in an atomic beam is difficult to achieve. In this work we describe a simple, robust and easy-to-operate design of a reliable high-intensity pulsed source for metastable helium atoms. The principle of operation relies on injection seeding the discharge with electrons emitted from a hot wire, permitting a strong enhancement both in efficiency and instability of the production of metastable atoms.

The interaction of high-intensity short pulse (fs) lasers with atomic or molecular vapors permits the conversion of visible light to intense coherent radiation in the vacuum ultraviolet spectral region. We demonstrate sum frequency mixing of a high-intensity short pulse (fs) laser with a conventional long pulse (ns) laser to extend the accessible wavelength range of harmonic generation. The mixing process does not rely on meeting resonances and can in principle be implemented with any intense short pulse (fs) laser system in any nonlinear medium. The efficiency of the sum frequency mixing process is comparable to the efficiency of harmonic generation of a similar wavelength with the high-intensity short pulse (fs) laser alone. We propose the application of conventional tunable long pulse (ns) lasers to permit the generation of broadly tunable picosecond radiation in the vacuum ultraviolet.