Georg Herink

Field-driven photoemission from nanostructures quenches the quiver motion

Strong-field physics, an extreme limit of light–matter interaction, is expanding into the realm of surfaces and nanostructures from its origin in atomic and molecular science. The attraction of nanostructures lies in two intimately connected features: local intensity enhancement and sub-wavelength confinement of optical fields. Local intensity enhancement facilitates access to the strong-field regime and has already sparked various applications, whereas spatial localization has the potential to generate strong-field dynamics exclusive to nanostructures. However, the observation of features unattainable in gaseous media is challenged by many-body effects and material damage, which arise under intense illumination of dense systems. Here, we non-destructively access this regime in the solid state by employing single plasmonic nanotips and few-cycle mid-infrared pulses, making use of the wavelength-dependence of the interaction, that is, the ponderomotive energy. We investigate strong-field photoelectron emission and acceleration from single nanostructures over a broad spectral range, and find kinetic energies of hundreds of electronvolts. We observe the transition to a new regime in strong-field dynamics, in which the electrons escape the nanolocalized field within a fraction of an optical half-cycle. The transition into this regime, characterized by a spatial adiabaticity parameter, would require relativistic electrons in the absence of nanostructures. These results establish new degrees of freedom for the manipulation and control of electron dynamics on femtosecond and attosecond timescales, combining optical near-fields and nanoscopic sources.

Field emission at terahertz frequencies: AC-tunneling and ultrafast carrier dynamics

We demonstrate ultrafast terahertz (THz) field emission from a tungsten nanotip enabled by local field enhancement. Characteristic electron spectra which result from acceleration in the THz near-field are found. Employing a dual frequency pump–probe scheme, we temporally resolve different nonlinear photoemission processes induced by coupling near-infrared (NIR) and THz pulses. In the order of increasing THz field strength, we observe THz streaking, THz-induced barrier reduction (Schottky effect) and THz field emission. At intense NIR-excitation, the THz field emission is used as an ultrashort, local probe of hot electron dynamics in the apex. A first application of this scheme indicates a decreased carrier cooling rate in the confined tip geometry. Summarizing the results at various excitation conditions, we present a comprehensive picture of the distinct regimes in ultrafast photoemission in the near- and far-infrared.

Real-time spectral interferometry probes the internal dynamics of femtosecond soliton molecules

Spectral interferometry images the formation, binding, and internal dynamics of optical soliton complexes. Probing the interaction of solitons As a pulse of light propagates through a medium, scattering and dispersion processes usually result in the pulse diffusing. However, under certain circumstances, the dispersion processes can be balanced by nonlinearities to produce localized structures known as solitons or optical bullets. Herink et al. used spectral interferometry to image and track the formation of soliton complexes as they propagated in a laser cavity. Real-time access to the formation processes and complex interaction dynamics could help in modeling other nonlinear systems. Science, this issue p. 50 Solitons, particle-like excitations ubiquitous in many fields of physics, have been shown to exhibit bound states akin to molecules. The formation of such temporal soliton bound states and their internal dynamics have escaped direct experimental observation. By means of an emerging time-stretch technique, we resolve the evolution of femtosecond soliton molecules in the cavity of a few-cycle mode-locked laser. We track two- and three-soliton bound states over hundreds of thousands of consecutive cavity roundtrips, identifying fixed points and periodic and aperiodic molecular orbits. A class of trajectories acquires a path-dependent geometrical phase, implying that its dynamics may be topologically protected. These findings highlight the importance of real-time detection in resolving interactions in complex nonlinear systems, including the dynamics of soliton bound states, breathers, and rogue waves.

Optical field emission from resonant gold nanorods driven by femtosecond mid-infrared pulses

We demonstrate strong-field photoelectron emission from gold nanorods driven by femtosecond mid-infrared optical pulses. The maximum photoelectron yield is reached at the localized surface plasmon resonance, indicating that the photoemission is governed by the resonantly-enhanced optical near-field. The wavelength- and field-dependent photoemission yield allows for a noninvasive determination of local field enhancements, and we obtain intensity enhancement factors close to 1300, in good agreement with finite-difference time domain computations.

Phase space manipulation of free-electron pulses from metal nanotips using combined terahertz near fields and external biasing

This work studies the manipulation of photoelectron emission from metallic nanostructures by intense single-cycle terahertz transients. Specifically, the authors employ streaking spectroscopy to study the kinetic energy of photoelectrons emitted from metal nanotips exposed to tailored static and terahertz electrical fields. Supported by detailed numerical simulations, the measurements provide quantitative information on the temporal and spatial properties of terahertz near fields at metal nanotips. The study illustrates far-reaching control over the trajectories and phase-space density evolution of photoelectron wavepackets acted upon by strong static and dynamic near fields. The results are relevant for applications of nanoscopic photoelectron sources employed in ultrafast electron diffraction and microscopy.

THz near-field streaking spectroscopy at biased metal nanotips

We demonstrate terahertz (THz) near-field streaking at metal nanotips. In this scheme, photoelectrons are generated at the apex of a nanotip (gold, tungsten) by near-infrared (NIR) femtosecond pulses and accelerated in the THz-induced near-field (cf. Fig. 1a). The high local field strengths and the nanometric spatial confinement of the THz near-field allow electrons to leave the THz electric field within a fraction of an optical cycle, and the kinetic energy of the photoelectrons is determined by the electric field in the moment of photoemission. Such a sub-cycle interaction of the photoelectrons with the streaking field is a characteristic feature of near-field streaking at nanostructures [1]. We record streaking spectrograms (see Fig. 1b) by varying the relative delay between the THz and the NIR pulses to map the temporal evolution of the THz near-field onto the kinetic energy of the photoelectrons [2].

Real-time detection of soliton interactions in a few-cycle femtosecond oscillator

Mode-locked lasers are a paramount example of dissipative nonlinear systems that support bound-states of multiple sohtons [1, 2]. However, rapid dynamic interactions between sohton complexes with pico-to femtosecond separation are mostly inaccessible to standard laser characterization techniques, including scanning interferometric autocorrelation, temporally-averaging spectroscopy or single-shot measurements at low repetition rates.

Low-noise passive harmonically mode-locked Yb:CALCO laser oscillator

Harmonic mode-locking of a solid-state bulk laser combines the benefits of long resonators with large pulse energy and high repetition rates. With this technique it is possible to operate a system at different repetition rates without changing the optical setup. The signal to noise ratio of a fundamentally mode-locked oscillator is typically much better than in harmonic operation [1]. In this contribution we present a SESAM based solitary mode-locked diode pumped Yb:CALGO bulk oscillator. This system provides femtosecond pulses at a fundamental repetition rate of 94 MHz. For the second order harmonic mode-locking operation at the repetition rate of 188 MHz and the third harmonic at 282 MHz we have only to increase the pump power. At both operation points we need no active stabilization for a long term low signal to noise operation.

Harmonically mode-locked Yb:CALGO laser oscillator

We present a passively mode-locked Yb:CALGO oscillator with harmonic repetition rate operation up to the third order. It is operated in the solitary regime with a fundamental roundtrip rate of 94 MHz and pulse durations between 200 fs and 600 fs. Harmonic operation was observed being stable for several days. The harmonic mode-locking regions are analyzed depending on intra-cavity dispersion. The transient pulsing dynamics converging to the stable harmonic modes is tracked and a theoretical model describing the pulse moving mechanisms is presented.

Clocking plasmon nanofocusing by THz near-field streaking

We apply terahertz (THz) near-field streaking in a nanofocusing geometry to investigate plasmon polariton propagation on the shaft of a conical nanotip. By evaluating the delay between a streaking spectrogram for plasmon-induced photoemission with a measurement for direct apex excitation, we obtain an average plasmon group velocity, which is in agreement with numerical simulations. Combining plasmon-induced photoemission with THz near-field streaking facilitates extensive control over localized photoelectron sources for time-resolved imaging and diffraction.