News Archive

Manuscript on two-color photoemission published in PRL — with a commentary in “Physics”

Ohm Prize ceremony on 28/10/16

Our bachelor student René Barczyk was honored with an Ohm Prize at the graduation ceremony of the Department of Physics for his Bachelor thesis: “Characterisation of Graphene under Ultrastrong and Ultrashort Laser Pulses.” This award honours the best theses of the past year. In his work, René shows that the damage threshold of graphene under laser illumination strongly depends on the pulse duration. This result is of great interest for the investigation of novel strong-field phenomena in graphene.

Congratulations!

Ablation-cooled material removal with ultrafast bursts of pulses

Two bursts of ultrashort pulses in the time domain at fundamental pulse repetition rates of 100 MHz and 500.

Dental tissue material processing results at 500 MHz and 100 MHz ultrashort pulse bursts in comparison to removal results obtained in the traditional processing regime using uniform single pulse repetition rate at e.g. 25kHz. As illustrated, the ablation cooling regime enables a material removal circumventing heat-diffusion into the material that would lead to thermal damaging of the sample. Average power, processing time, single pulse temporal duration and beam size are maintained for each experiment.

For the last decades, laser systems emitting high energy ultrashort pulses represent a material processing tool of high precision and reliability. However, the traditional laser devices are limited in terms of short pulse repetition rates, typically located in the kHz-regime, in which the corresponding time window between two successive pulses allows for heat dissipation deep into the remaining material, potentially allowing for thermal damages. Here, we present a novel material processing technique circumventing these drawbacks by “recycling” the remaining heat that is deposited by the preceding pulse in the processing region before heat diffusion occurs. This implies that a) the already heated up material reduces the energy requirements for the successive pulse, and b) the immediately following ablation of the hot material significantly lowers the average power of the entire sample. This effect is defined as “ablation cooling”, and is used for atmospheric flights of e.g. ballistic warheads since the 1950’s. Applied to femtosecond laser pulse material processing, we have demonstrated an increase in ablation efficiency by up to an order of magnitude compared to the traditional processing procedures. The immediacy of the ablation-sequences is ensured by an extremely high fundamental repetition rate of ultrashort pulses, in which no uniform pulse trains but bursts of pulses are incident on the probe. A kHz-burst repetition rate allows for both optimization of the laser beam repositioning as well as lowering the laser beam average power. This novel material processing technique has been investigated in the research group of Prof. Dr. F.Ö. Ilday at University Bilkent, Ankara, Turkey, in collaboration with the Menlo Systems GmbH in Martinsried, Germany, where our PhD student Heinar Hoogland is working on his thesis. A detailed theoretical and experimental description of this new technique is published in Nature, September 1^st 2016 (Vol. 537, No. 7618; pp. 7-128).

Puplication in nature.

Manuscript on coherent control of nanotip photoemission accepted at Phys. Rev. Lett.:

In this experiment, femtosecond laser pulses of central wavelength 1560 nm (red) and 780 nm (blue) are incident on a tungsten nanotip with a variable delay [left]. The current emitted from the tip can be switched by virtue of the optical phase between the two laser pulses [right]. Sept. 2016

Nanotips are excellent electron emitters making them the prime choice for electron guns in electron microscopes. Therefore, also laser-triggered emission from nanotips has gained much attention. We now demonstrate that the laser-triggered current can be controlled almost perfectly with the phase between two simultaneously incident laser pulses of commensurate frequencies. As can be seen in the figure, modulation depths of up to 94% are reached in the experiment. With the help of density functional theory calculations performed by the group of Prof. Burgdörfer at TU Vienna we investigate the mechanism of control. We find that our results can be explained in terms of two different quantum pathways leading to electron emission, where the phase between the quantum pathways plays the role of a switch. Our research opens the door for further studies of coherence at solids on a fundamental level but also to create electron pulses of a desired spatio-temporal shape.

A preprint of our research can be found on arXiv: https://arxiv.org/abs/1603.01516

ACHIP collaboration meeting at FAU

The ACHIP (Accelerator on a CHip International Program) collaboration held its semiannual conference in Erlangen on September 15-16th, 2016, hosted and organized by our team at FAU. The collaboration members from Stanford University, UCLA, TU Darmstadt, Purdue University, Paul Scherrer Institute, DESY, SLAC, Tech-X and FAU not only presented their recent accomplishments but also discussed future directions of the collaboration, with guidance from an advisory board consisting of Professors Lia Merminga, Chan Joshi and Reinhard Brinkman. The collaboration looks forward towards research aiming to realize the goal of a laser based miniaturized accelerator on a chip.

The participants of the ACHIP collaboration workshop in front of the meeting hall, FAU’s Orangerie. Sept. 2016.

Hiking tour in Franconian Switzerland

Group pictures of the attendees of the hiking tour at the start in Ebermannstadt. Sept. 2016

Light-field-driven currents in graphene

Measurement setup for the generation of light-waveform-dependent currents in graphene (left). Graphene’s characteristic “Dirac-cone” dispersion relation significantly relaxes the requirement to bring electrons into the light-field driven regime, and electrons undergo combined dynamics of band-to-band transitions and acceleration within single bands (right). July 2016.

We show that one can control an electric current flowing in graphene, an atomically thin layer of carbon atoms, by the optical field of light. The current is determined by the optical field waveform oscillating on a timescale of 1 femtosecond (1/1,000,000,000,000,000 second), which is one millionth of a billionth of a second, and orders of magnitudes faster than the fields that drive electrons in common electronics. This finding represents a major step in the rapidly growing interdisciplinary field of strong-field physics in solids by broadening its scope to an entirely new and eminently important material class – metals –.  A door is opened for integrating two important technologies, optics and electronics, to a single platform on the basis of controlling electrons by an optical electric field.

preprint on arXiv https://arxiv.org/abs/1607.04198

Near fields at nanotips captured with attosecond streak camera

Modulation of the photoelectron spectrum from a nanotip as a function of delay between photoemitting XUV pulse and an IR pulse, which excites a near field. May 2016

Nanostructures allow to concentrate electromagnetic fields to scales much smaller than the wavelength of light – this is in stark contrast to free space, where the wavelength forms a natural barrier. This idea is the foundation of the field of nano-optics and we use it extensively in our experiments, for example for photoemission from nanotips and for dielectric laser acceleration. Together with our colleagues of the Max Planck Institute of Quantum Optics and the Ludwig-Maximilians-Universität in Munich we have now succeeded for the first time to determine the strength and temporal shape of a near field with attosecond resolution. To this end we use an infrared laser pulse to excite a near field at a nanotip. An attosecond XUV pulse simultaneously emits photoelectrons from the tip. The photoelectron energy at the detector is dependent on the exact phase and strength of the near field at the time of photoemission. We can therefore reconstruct the near field from the modulation of the highest electron energy as a function of delay between IR and XUV pulse.

Publication in Nature Communications

Press release at MPQ

Les Houches–WE-Heraeus-Workshop on Ultrafast Phenomena at Nanostructures: Attosecond physics meets plasmonics