Dominik Hoff

Modified polymethylmethacrylate as a base for thermostable optical recording media

A possibility to improve the thermal properties of holographic gratings in a photosensitive system based on polymethylmethacrylate (PMMA) and to enhance simultaneously the adhesion of the photopolymer to soda-lime glass is demonstrated. The modified PMMA was prepared by radical copolymerisation of methylmethacrylate (MMA) with acrylic acid (AA). Polymer films deposited from samples of the copolymer of MMA with AA containing 9,10-phenanthrenequinone additives were used as a photosensitive material for the recording of holographic gratings. It is possible to generate gratings that are thermally stable up to 200 masculineC using this modified PMMA. Dynamic thermogravimetry, differential thermal analysis and thermal mechanic analyses were used to determine the dependence of the thermal stability of the modified PMMA on the composition and the structure of its macromolecules.

New thermostable copolymers for holographic storage based on methylmethacrylate with methacrylamide or methacrylic acid

Two new copolymers based on methylmethacrylate (MMA) with methacrylamide (MAA) and methacrylic acid (MA) containing distributed phenanthrenequinone (PQ) molecules are fabricated for the purpose of optical recording by generating holographic diffractive structures under argon laser illumination. Chemical conditions for the formation of holograms are discussed and confirmed by the spectral characteristics of the material. Thermal and adhesive properties were improved with the aim of expanding the range of polymer application and generating of diffractive elements with long term stability and high optical quality.

Accurate determination of absolute carrier-envelope phase dependence using photo-ionization.

The carrier-envelope phase (CEP) dependence of few-cycle above-threshold ionization (ATI) of Xe is calibrated for use as a reference measurement for determining and controlling the absolute CEP in other interactions. This is achieved by referencing the CEP-dependent ATI measurements of Xe to measurements of atomic H, which are in turn referenced to ab initio calculations for atomic H. This allows for the accurate determination of the absolute CEP dependence of Xe ATI, which enables relatively easy determination of the offset between the relative CEP measured and/or controlled by typical devices and the absolute CEP in the interaction.

Sub-1.5-cycle pulses from a single filament

The temporal dynamics of ultrashort laser pulses undergoing filamentary propagation are investigated with a real-time stereographic above-threshold ionization (ATI) phasemeter. The experimental setup is capable of measuring the pulse duration as well as the carrier-envelope phase distribution of pulses originating from a femtosecond filament, which is either truncated in length or fully propagated. Truncation, by means of a semi-infinite gas cell, allows to elucidate the nonlinear evolution and temporal dynamics of ultrashort laser pulses as a function of the propagation length. We observe the formation of few-cycle pulses as well as temporal pulse splitting dynamics during the propagation of the pulse inside the filament. For the first time, we demonstrate the compression of 35 fs pulses down to a duration of sub-4 fs in a single femtosecond filament. This corresponds to sub-1.5 cycles of the electric field.

Tracing the phase of focused broadband laser pulses

In different applications the Gouy phase is used to describe broadband lasers, but new 3D measurements of the spatial dependence of a focused laser pulse show serious deviations from the Gouy phase. Precise knowledge of the behaviour of the phase of light in a focused beam is fundamental to understanding and controlling laser-driven processes. More than a hundred years ago, an axial phase anomaly for focused monochromatic light beams was discovered and is now commonly known as the Gouy phase1,2,3,4. Recent theoretical work has brought into question the validity of applying this monochromatic phase formulation to the broadband pulses becoming ubiquitous today5,6. Based on electron backscattering at sharp nanometre-scale metal tips, a method is available to measure light fields with sub-wavelength spatial resolution and sub-optical-cycle time resolution7,8,9. Here we report such a direct, three-dimensional measurement of the spatial dependence of the optical phase of a focused, 4-fs, near-infrared pulsed laser beam. The observed optical phase deviates substantially from the monochromatic Gouy phase—exhibiting a much more complex spatial dependence, both along the propagation axis and in the radial direction. In our measurements, these significant deviations are the rule and not the exception for focused, broadband laser pulses. Therefore, we expect wide ramifications for all broadband laser–matter interactions, such as in high-harmonic and attosecond pulse generation, femtochemistry10, ophthalmological optical coherence tomography11,12 and light-wave electronics13.

Waveform characterization of few-cycle laser pulses in real-time using above-threshold ionization

Since the time-dependent electric field dictates strong-field laser-matter dynamics, characterization of the waveform is critical for the understanding and control of these interactions. Moreover, the precise determination of these parameters is especially important for applications in the few-cycle regime, e.g. the production of extreme-ultraviolet (XUV) pulses, which serve as the basis for much of the burgeoning field of attosecond science.

Attosecond electronic recollision as field detector

We demonstrate the complete reconstruction of the electric field of visible-infrared pulses with energy as low as a few tens of nanojoules. The technique allows for the reconstruction of the instantaneous electric field vector direction and magnitude, thus giving access to the characterisation of pulses with an arbitrary time-dependent polarisation state. The technique combines extreme ultraviolet interferometry with the generation of isolated attosecond pulses.

Vectorial optical field reconstruction by attosecond spectral interferometry

An electrical pulse is completely defined by its time-dependent amplitude, phase and polarization state. For optical and near-infrared pulses the manipulation and characterization of the last one is fundamental due to its relevance in several scientific and technological fields. Although the complete characterization of optical waveform has been already demonstrated [1, 2], a technique both capable to fully characterize also weak probe pulses, with energy in the 10–100nJ, and, at the same time, free of systematic distortions, would be highly desirable. In this work we report on new theoretical and experimental results to demonstrate a novel approach for the complete characterization of the electric field of an optical pulse. Our method is based on the combination of two elements: the implementation of extreme ultraviolet (XUV) interferometry [3], with time resolution in the attosecond domain, and the demonstration that the motion of an attosecond electronic wave packet, created by an intense laser pulse, allows to sample an unknown electric field along a controllable, fixed direction. Combining these elements, we demonstrate the full reconstruction of electric fields with intensities as low as I∼109 W/cm2 and with a generic time-dependent polarization state, with an all-optical method.

Performance tests of the 5 TW, 1 kHz, passively CEP-stabilized ELI-ALPS SYLOS few-cycle laser system (Conference Presentation)

ELI-ALPS in Hungary, one of the three pillars of the Extreme Light Infrastructure, aims at providing diverse light sources, including energetic attosecond pulses at the highest possible repetition rates. One of the main laser systems for driving plasma and gas-based HHG stages, is a state-of-the-art 1 kHz few-cycle laser called SYLOS. Targeted pulse parameters are an energy of 100 mJ and a duration shorter than two optical cycles (<6 fs), with outstanding energy, phase and pointing stability as well as high spatiotemporal quality. The first phase of the laser system has already set a new standard in kHz laser system engineering and technology. The performance and reliability of the SYLOS laser have been consistently tested over the course of a six-month trial period. During this time the system was running at least 8 hours a day at full power for more than 5 months. The current output parameters are 5 TW peak power, 45 mJ pulse energy with 9 fs duration and 300 mrad CEP stability, while the spectrum spans over 300 nm around 840 nm central wavelength. The layout follows the general scheme NOPCPA architecture with a passively CEP-stabilized front-end. The pulses are negatively chirped for the amplification process and compressed by a combination of large aperture bulk glass blocks and positively chirped mirrors under vacuum conditions at the output. During the trial period, the laser system demonstrated outstanding reliability. Daily startup and shutdown procedures take only a few minutes, and the command-control system enables pulse parameters to be modified instantly. Controlling the delays of individual NOPCPA stages makes it possible to tailor the output spectrum of the pulses and tune the central wavelength between 770 nm and 940 nm. We performed several experimental tests to find out the pulse characteristics. Pulse duration was verified with Wizzler, chirp-scan, autocorrelation methods and a stereo-ATI independently. All of them confirmed the sub-9 fs pulse duration. We recorded the long-term waveform and pointing stabilities of the beam in order to find out the effect of the temperature load on optical elements. Excluding a short initial warm up time, stable signals were observed in general. The in-loop and out-of-loop CEP stability was cross-checked between f-to-2f and stereo-ATI devices. Moreover, the inherent CEP stability of the system without feedback loop was also found to be surprisingly robust thanks to the passive CEP stabilization of the front-end. The polarization contrast was better than 1000:1. The temporal contrast was also measured independently with Sequoia and Tundra cross-correlators, and on the ns scale with a fast photodiode and GHz oscilloscope as well. Results showed that the pulse pedestal generally consists of parametric superfluorescence below the 1E-7 level and about 100 ps long, well in accordance with the pump duration. Delaying the pump pulse allows us to shift the seed pulse to the front and reach a pre-pulse pedestal below 1E-11 at 30 ps before the pulse peak. Detailed findings on all the examined pulse characteristics of the SYLOS laser will be reported in this presentation.

Holographic Volume Absorption Grating in Glass-Like Polymer Recording Material

Contribution of the phase and absorption gratings into the total diffraction efficiency has been analysed in polymer recording materials based on poly(methyl methacrylate) containing phenanthrenequinone.