2020, Hartmann, Wladick, Varytis, Paris, Gehring, Helge, Walter, Nicolai, Beutel, Fabian, Busch, Kurt, Pernice, Wolfram

Compact, on-chip spectrometers exploiting tailored disorder for broadband light scattering enable high-resolution signal analysis while maintaining a small device footprint. Due to multiple scattering events of light in the disordered medium, the effective path length of the device is significantly enhanced. Here, on-chip spectrometers are realized for visible and near-infrared wavelengths by combining an efficient broadband fiber-to-chip coupling approach with a scattering area in a broadband transparent silicon nitride waveguiding structure. Air holes etched into a structured silicon nitride slab terminated with multiple waveguides enable multipath light scattering in a diffusive regime. Spectral-to-spatial mapping is performed by determining the transmission matrix at the waveguide outputs, which is then used to reconstruct the probe signals. Direct comparison with theoretical analyses shows that such devices can be used for high-resolution spectroscopy from the visible up to the telecom wavelength regime. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2019, Verma D., Tanyag R.M.P., O’Connell S.M.O., Vilesov A.F.

For more than two decades, encapsulation in superfluid helium nanodroplets has served as a reliable technique for probing the structure and dynamics of molecules and clusters at a low temperature of ≈0.37 K. Due to weak interactions between molecules and the host liquid helium, good spectral resolution can usually be achieved, making helium droplets an ideal matrix for spectroscopy in a wide spectral range from infrared to ultraviolet. Furthermore, rotational structure in the spectra of small molecules provides a unique probe for interactions with the superfluid on an atomic scale. This review presents a summary of results and a discussion of recent experimental developments in helium droplet spectroscopy with the emphasis laid on infrared studies. Initially, studies focused on single molecules and have been expanded to larger species, such as metal-molecular clusters, biomolecules, free radicals, ions, and proteins. © 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.

2020, Gazibegović-Busuladžić, A., Busuladžić, M., Čerkić, A., Hasović, E., Becker, W., Milošević, D.B.

We investigate strong-field ionization of linear molecules by a two-color laser field of frequencies rω and sω having coplanar counterrotating or corotating elliptically polarized components (ω is the fundamental laser field frequency and r and s are integers). Using the improved molecular strong-field approximation we analyze direct above-threshold ionization (ATI) and high-order ATI (HATI) spectra. More precisely, reflection and rotational symmetries of these spectra for linear molecules aligned in the laser-field polarization plane are considered. The reflection symmetries for particular molecular orientations, known to be valid for a bicircular field (this is the field with circularly polarized counterrotating components), are valid also for arbitrary component ellipticities. However, specific rotational symmetries that are satisfied for HATI by a bicircular field, are violated for an arbitrary elliptically polarized field with counterrotating components. For the corotating case and the N2 molecule we analyze molecular-orientation-dependent interferences and plateau structures for various ellipticities.

2019, Milošević, D.B.

High-order harmonics generated by a linearly polarized laser field are also linearly polarized. Having in mind that for various application, such as the exploration of magnetic materials, chiral molecules etc., we need circularly polarized high harmonics which serve as coherent soft x-rays, we explore high-order harmonic generation by the so-called bicircular laser field. This field consists of two coplanar counter-rotating circularly polarized fields of different frequencies equal to integer multiples of a fundamental frequency ω. High harmonics generated by such field are circularly polarized with helicity alternating between +1 and −1. Combining a group of such harmonics, instead of obtaining a circularly polarized attosecond pulse train, one obtains a pulse with unusual polarization properties. But, if the harmonics of particular helicity are stronger, i.e., if we have helicity asymmetry in a high-harmonic energy interval, then it is possible to generate an elliptical or even circular pulse train. We theoretically investigated a wide range of bicircular field-component intensities (I1 and I2) and found regions where both the harmonic intensity is high and the helicity asymmetry is large. Particular attention is devoted to the ω−2ω and ω−3ω bicircular fields and atoms having the s and p ground states. In our calculations we use strong-field approximation and quantum-orbit theory. We show that, even in the extreme case of I2 = 8I1, for an ω−3ω bicircular field, high-order harmonic generation is more efficient than in the I2 = I1 case. The obtained results are explained analyzing the relevant electron trajectories and velocities, which follow from the quantum-orbit theory. For the atoms having p ground state the helicity asymmetry parameter is large for a wide range of high-harmonic photon energies, while for the atoms having s ground state the helicity asymmetry parameter can be large only for low harmonics. We confirm this by averaging the obtained results over the intensity distribution in the laser focus.

2020, Grunwald R., Bock M.

Needle beams are highly attractive for applications which take advantage from a spatial and temporal localization of photons. High intensities, high resolution and extended depth of focus lead to fundamental advances in the optical system performance. Ultrashort, fringe-free, self-reconstructing nondiffracting pulses with undistorted temporal transfer are obtained by generating truncated Bessel beams under self-apodization conditions. Nondiffracting Talbot self-imaging of needle beam arrays enables to transfer near field information to the Fraunhofer zone. With addressable arrays of needle beams, reconfigurable time-wavefront sensors are built up. Moreover, spatial light modulators and flexible axicons are used to realize structured, highly localized wavepackets, accelerating beams and nondiffracting images. © 2020, © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.

2019, Trivikram, T.M., Salumbides, E.J., Jungen, Ch., Ubachs, W.

Bound and free quantum resonances of molecular hydrogen exhibiting wave-function density at large internuclear separation, (Formula presented.) 4–5 a.u., are excited via multi-step laser spectroscopy. Highly excited vibrational levels of H (Formula presented.) are prepared via two-photon UV-photolysis of H (Formula presented.) S. Subsequent two-photon Doppler-free precision measurements are performed connecting (Formula presented.) levels with (Formula presented.) outer-well levels. Detection and spectroscopic labelling of the quantum states is assisted by further laser excitation into the auto-ionisation continuum employing a third UV-laser. Level energies of high rotational states ((Formula presented.)) in the outer-well state (Formula presented.) are accurately determined. The three-laser study demonstrates a method for probing resonances in the H (Formula presented.) ionisation continuum with wave-function density at large internuclear separation (Formula presented.) 4–5 a.u., large angular momenta J, and energy range 131,100–133,000 cm-1, a hitherto unexplored territory. © 2019, © 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.

2019, Gross, M., Qian, H.J., Boonpornprasert, P., Chen, Y., Good, J.D., Huck, H., Isaev, I., Koschitzki, C., Krasilnikov, M., Lal, S., Li, X., Lishilin, O., Loisch, G., Melkumyan, D., Mohanty, S.K., Niemczyk, R., Oppelt, A., Shaker, H., Shu, G., Stephan, F., Vashchenko, G., Will, I.

Laser pulse shaping is one of the key elements to generate low emittance electron beams with RF photoinjectors. Ultimately high performance can be achieved with ellipsoidal laser pulses, but 3-dimensional shaping is challenging. High beam quality can also be reached by simple transverse pulse shaping, which has demonstrated improved beam emittance compared to a transversely uniform laser in the 'pancake' photoemission regime. In this contribution we present the truncation of a Gaussian laser at a radius of approximately one sigma in the intermediate (electron bunch length directly after emission about the same as radius) photoemission regime with high acceleration gradients (up to 60 MV/m). This type of electron bunch is used e.g. at the European XFEL and FLASH free electron lasers at DESY, Hamburg site and is being investigated in detail at the Photoinjector Test facility at DESY in Zeuthen (PITZ). Here we present ray-tracing simulations and experimental data of a laser beamline upgrade enabling variable transverse truncation. Initial projected emittance measurements taken with help of this setup are shown, as well as supporting beam dynamics simulations. Additional simulations show the potential for substantial reduction of slice emittance at PITZ. © Published under licence by IOP Publishing Ltd.

2020, Mao F., Hong J., Wang H., Chen Y., Jing C., Yang P., Tomm J.W., Chu J., Yue F.

Broad emission bands due to defects in (In,Ga,Al)N laser diodes operating at 440 nm are investigated using continuous-wave and pulsed currents. In addition to known yellow-green and short-wave infrared bands, defect emissions were observed even in the medium-wave infrared range. A separation from thermal radiation is possible. When using pulsed currents, a super-linearly increasing emission occurs at ∼1150 nm, which could be attributed to amplified spontaneous emission mainly due to the electroluminescence of deep defects in the optically active region. These results may be useful in interpreting the output power bottleneck of GaN-based lasers compared to mature GaAs-based lasers. © 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5143802

2020, Reitsma, G., Hummert, J., Dura, J., Loriot, V., Vrakking, M.J.J., Lépine, F., Kornilov, O.

The efficiency of energy transfer in ultrafast electronic relaxation of molecules depends strongly on the complex interplay between electronic and nuclear motion. In this study we use wavelength-selected XUV pulses to induce relaxation dynamics of highly excited cationic states of naphthalene. Surprisingly, the observed relaxation lifetimes increase with the cationic excitation energy. We propose that this is a manifestation of a quantum mechanical population trapping that leads to delayed relaxation of molecules in the regions with a high density of excited states. © 2019 Published under licence by IOP Publishing Ltd.

2020, Milošević, D.B., Becker, W.

High-order harmonic generation by orthogonally polarized two-color (OTC) laser fields is analysed using strong-field approximation and quantum-orbit theory. Results for the field components frequency ratio of 2:1 and 3:1 are presented and compared. We have shown that, depending on the relative phase between the field components, the shape of the high-harmonic spectrum can be very different from that obtained by a monochromatic linearly polarized laser field. It is also shown that it is possible to generate elliptically polarized high-order harmonics with very high photon energies using OTC laser field with the frequency ratio of 3:1 and a long fundamental wavelength. An effective relative phase control of the harmonic emission is demonstrated. The obtained results are explained using the quantum-orbit theory. © Published under licence by IOP Publishing Ltd.