J. Plenge

Field propagation-induced directionality of carrier-envelope phase-controlled photoemission from nanospheres

Near-fields of non-resonantly laser-excited nanostructures enable strong localization of ultrashort light fields and have opened novel routes to fundamentally modify and control electronic strong-field processes. Harnessing spatiotemporally tunable near-fields for the steering of sub-cycle electron dynamics may enable ultrafast optoelectronic devices and unprecedented control in the generation of attosecond electron and photon pulses. Here we utilize unsupported sub-wavelength dielectric nanospheres to generate near-fields with adjustable structure and study the resulting strong-field dynamics via photoelectron imaging. We demonstrate field propagation-induced tunability of the emission direction of fast recollision electrons up to a regime, where nonlinear charge interaction effects become dominant in the acceleration process. Our analysis supports that the timing of the recollision process remains controllable with attosecond resolution by the carrier-envelope phase, indicating the possibility to expand near-field-mediated control far into the realm of high-field phenomena.

Single-shot velocity-map imaging of attosecond light-field control at kilohertz rate

High-speed, single-shot velocity-map imaging (VMI) is combined with carrier-envelope phase (CEP) tagging by a single-shot stereographic above-threshold ionization (ATI) phase-meter. The experimental setup provides a versatile tool for angle-resolved studies of the attosecond control of electrons in atoms, molecules, and nanostructures. Single-shot VMI at kHz repetition rate is realized with a highly sensitive megapixel complementary metal-oxide semiconductor camera omitting the need for additional image intensifiers. The developed camera software allows for efficient background suppression and the storage of up to 1024 events for each image in real time. The approach is demonstrated by measuring the CEP-dependence of the electron emission from ATI of Xe in strong (≈10(13) W/cm(2)) near single-cycle (4 fs) laser fields. Efficient background signal suppression with the system is illustrated for the electron emission from SiO(2) nanospheres.

Carrier–envelope phase-tagged imaging of the controlled electron acceleration from SiO2 nanospheres in intense few-cycle laser fields

Waveform-controlled light fields offer the possibility of manipu- lating ultrafast electronic processes on sub-cycle timescales. The optical light- wave control of the collective electron motion in nanostructured materials is key to the design of electronic devices operating at up to petahertz frequencies. We have studied the directional control of the electron emission from 95nm 10 Authors to whom any correspondence should be addressed.

A pump-probe photoionization mass spectrometer utilizing tunable extreme ultraviolet laser-produced-plasma radiation

An experimental device is reported that utilizes time-correlated nanosecond light pulses in combination with photoionization mass spectrometry. A primary light pulse is generated by a tunable dye laser in the ultraviolet regime, which photolyzes neutral gas targets under collision free conditions. Subsequently, a time-correlated extreme ultraviolet-light pulse comes from a laser-produced plasma that is monochromatized in the 10–25 eV regime. The photolysis products are ionized by one-photon absorption, so that the cations are finally detected by time-of-flight mass spectrometry. The performance of this experimental approach is characterized by investigating the primary photolysis products of chlorine dioxide. Finally, possible applications of this approach are briefly discussed.

Photoionization of the primary photoproducts of A(2∏)-excited ClO

Photoionization of the primary photofragments of chlorine monoxide (ClO) is reported. ClO is photolyzed in the X(2∏)→A(2∏)-regime, yielding Cl(2P) and O(3P,1D). The primary photolysis products, as well as the not photolyzed ClO, are subsequently probed by monochromatic, time-correlated vacuum-ultraviolet radiation from a laser produced plasma source. Autoionization is used for state-specific detection of the atomic photolysis products. The formation of O(3P) is exclusively observed above ≈264 nm. The threshold of O(1D) from A(2∏3/2)-excited ClO is found at 263.71±0.01 nm. The shape of the O(1D) yield near this threshold is discussed in terms of the rotational energy distribution and a rotational barrier of A(2∏3/2)-excited ClO. Direct (nonresonant) one-photon-ionization is used to establish the absolute photoionization cross sections of ClO(X(2∏)), Cl(2P), and O(1D) near 15 eV. Additional experiments on the UV-photolysis of Cl2, yielding Cl(2P), are consistent with the results on ClO. The present work is ...

Photoluminescence properties of cadmium-selenide quantum dots embedded in a liquid-crystal polymer matrix

The photoluminescence properties of cadmium-selenide (CdSe) quantum dots with an average size of ∼3 nm, embedded in a liquid-crystal polymer matrix are studied. It was found that an increase in the quantum-dot concentration results in modification of the intrinsic (exciton) photoluminescence spectrum in the range 500–600 nm and a nonmonotonic change in its intensity. Time-resolved measurements show the biexponential decay of the photoluminescence intensity with various ratios of fast and slow components depending on the quantum-dot concentration. In this case, the characteristic lifetimes of exciton photoluminescence are 5–10 and 35–50 ns for the fast and slow components, respectively, which is much shorter than the times for colloidal CdSe quantum dots of the same size. The observed features of the photoluminescence spectra and kinetics are explained by the effects of light reabsorption, energy transfer from quantum dots to the liquid-crystal polymer matrix, and the effect of the electronic states at the CdSe/(liquid crystal) interface.

Chirped pulse multiphoton ionization of nitrogen: Control of selective rotational excitation in N2+(B Σ2u+)

We report on fluorescence spectra of N(2)(+)(B (2)Sigma(u)(+)) --> N(2)(+)(X (2)Sigma(g)(+)) obtained from multiphoton ionization of molecular nitrogen by 804 nm femtosecond laser pulses. The analysis of the fluorescence spectra reveals that the vibrational levels v = 0 and v = 4 in the B (2)Sigma(u)(+)-state of N(2)(+) are primarily populated. The rotational state distribution of N(2)(+)(B (2)Sigma(u)(+), v = 0) is determined from the rotationally resolved fluorescence spectra. It is demonstrated that the linear chirp of the 804 nm femtosecond laser pulse has a strong influence on the rotational state distribution of the vibrational ground state of the molecular cation N(2)(+)(B (2)Sigma(u)(+), v = 0). Possible mechanisms leading to the experimental results are discussed. The particular population of the vibrational levels as well as the linear chirp dependence of the fluorescence signal gives evidence for the importance of a resonant intermediate state. The N(2) a (1)Pi-state is likely involved in a resonant multiphoton excitation process. This permits to selectively control the rotational population of the cation that is formed via chirped pulse multiphoton ionization.

Coherent control of the ultrafast dissociative ionization dynamics of bromochloroalkanes

We report on the coherent control of the ultrafast ionization and fragmentation dynamics of the bromochloroalkanes C(2)H(4)BrCl and C(3)H(6)BrCl using shaped femtosecond laser pulses. In closed-loop control experiments on bromochloropropane (C(3)H(6)BrCl) the fragment ion yields of CH(2)Cl(+), CH(2)Br(+), and C(3)H(3)(+) are optimized with respect to that of the parent cation C(3)H(6)BrCl(+). The fragment ion yields are recorded in additional experiments in order to reveal the energetics of cation fragmentation, where laser-produced plasma radiation is used as a tunable pulsed nanosecond vacuum ultraviolet radiation source along with photoionization mass spectrometry. The time structure of the optimized femtosecond laser pulses leads to a depletion of the parent ion and an enhancement of the fragment ions, where a characteristic sequence of pulses is required. Specifically, an intense pump pulse is followed by a less intense probe pulse where the delay is 0.5 ps. Similarly optimized pulse shapes are obtained from closed-loop control experiments on bromochloroethane (C(2)H(4)BrCl), where the fragment ion yield of CH(2)Br(+) is optimized with respect to that of C(2)H(4)BrCl(+) as well as the fragment ion ratios C(2)H(2)(+)/CH(2)Br(+) and C(2)H(3)(+)/C(2)H(4)Cl(+). The assignment of the underlying control mechanism is derived from one-color 804 nm pump-probe experiments, where the yields of the parent cation and several fragments show broad dynamic resonances with a maximum at Δt = 0.5 ps. The experimental findings are rationalized in terms of dynamic ionic resonances leading to an enhanced dissociation of the parent cation and some primary fragment ions.

Role of the Polymer Matrix on the Photoluminescence of Embedded CdSe Quantum Dots

The photoluminescence (PL) of CdSe quantum dots (QDs) that form stable nanocomposites with polymer liquid crystals (LCs) as smectic C hydrogen-bonded homopolymers from a family of poly[4-(n-acryloyloxyalkyloxy)benzoic acids] is reported. The matrix that results from the combination of these units with methoxyphenyl benzoate and cholesterol-containing units has a cholesteric structure. The exciton PL band of QDs in the smectic matrix is redshifted with respect to QDs in solution, whereas a blueshift is observed with the cholesteric matrix. The PL lifetimes and quantum yield in cholesteric nanocomposites are higher than those in smectic ones. This is interpreted in terms of a higher order of the smectic matrix in comparison to the cholesteric one. CdSe QDs in the ordered smectic matrix demonstrate a splitting of the exciton PL band and an enhancement of the photoinduced differential transmission. These results reveal the effects of the structure of polymer LC matrices on the optical properties of embedded QDs, which offer new possibilities for photonic applications of QD-LC polymer nanocomposites.

Absolute photoionization cross sections of the primary photofragments of chlorine dioxide and dichlorine monoxide

Photoionization of the primary photofragments of chlorine dioxide (OClO) and dichlorine monoxide (Cl2O) is reported. The nascent photofragments are formed by UV photolysis, they are subsequently photoionized by time-correlated XUV laser radiation and finally detected by time-of-flight mass spectrometry. Primary photolysis of OClO leads to the formation of ClO+O at λ=359.5 nm, whereas ClO+Cl are formed by photolysis of Cl2O at λ=250 nm. The XUV photoionization of the photolysis products relies on single photon ionization. This allows to derive partial photoionization cross sections of the parent cations and their photolysis products from mass spectral intensities by using the absolute photoionization cross sections of the atomic products for calibration. Specifically, we obtain for OClO at E=13.74 eV: σClO=27±5 Mb and σOClO=18.5±3 Mb. Consistent findings are obtained from equivalent experiments on Cl2O. The present results are compared with previous photoionization work on ClO and OClO to demonstrate the reliability of UV-pump/XUV-probe spectroscopy.