Ultrafast Surface Plasmon Probing of Interband and Intraband Hot Electron Excitations.

Upon the interaction of light with metals, nonthermal electrons are generated with intriguing transient behavior. Here, we present femtosecond hot electron probing in a noveloptical pump/plasmon probe scheme. With this, we probed ultrafast interband and intraband dynamics with 15 nm interface selectivity, observing a two-component-decay of hot electron populations. Results are in good agreement with a three-temperature model of the metal; thus, we could attribute the fast (∼100 fs) decay to the thermalization of hot electrons and the slow (picosecond) decay to electron-lattice thermalization. Moreover, we could modulate the transmission of our plasmonic channel with ∼40% depth, hinting at the possibility of ultrafast information processing applications with plasmonic signals.

Carrier-envelope phase on-chip scanner and control of laser beams.

The carrier-envelope phase (CEP) is an important property of few-cycle laser pulses, allowing for light field control of electronic processes during laser-matter interactions. Thus, the measurement and control of CEP is essential for applications of few-cycle lasers. Currently, there is no robust method for measuring the non-trivial spatial CEP distribution of few-cycle laser pulses. Here, we demonstrate a compact on-chip, ambient-air, CEP scanning probe with 0.1 µm3 resolution based on optical driving of CEP-sensitive ultrafast currents in a metal-dielectric heterostructure. We successfully apply the probe to obtain a 3D map of spatial changes of CEP in the vicinity of an oscillator beam focus with pulses as weak as 1 nJ. We also demonstrate CEP control in the focal volume with a spatial light modulator so that arbitrary spatial CEP sculpting could be realized.

Ultrasensitive probing of plasmonic hot electron occupancies.

Non-thermal and thermal carrier populations in plasmonic systems raised significant interest in contemporary fundamental and applied physics. Although the theoretical description predicts not only the energies but also the location of the generated carriers, the experimental justification of these theories is still lacking. Here, we demonstrate experimentally that upon the optical excitation of surface plasmon polaritons, a non-thermal electron population appears in the topmost domain of the plasmonic film directly coupled to the local fields. The applied all-optical method is based on spectroscopic ellipsometric determination of the dielectric function, allowing us to obtain in-depth information on surface plasmon induced changes of the directly related electron occupancies. The ultrahigh sensitivity of our method allows us to capture the signatures of changes induced by electron-electron scattering processes with ultrafast decay times. These experiments shed light on the build-up of plasmonic hot electron population in nanoscale media.

Nonadiabatic Nano-optical Tunneling of Photoelectrons in Plasmonic Near-Fields.

Nonadiabatic nano-optical electron tunneling in the transition region between multiphoton-induced emission and adiabatic tunnel emission is explored in the near-field of plasmonic nanostructures. For Keldysh γ values between ∼1.3 and ∼2.2, measured photoemission spectra show strong-field recollision driven by the nanoscale near-field. At the same time, the photoemission yield shows an intensity scaling with a constant nonlinearity, which is characteristic for multiphoton-induced emission. Our observations in this transition region were well reproduced with the numerical solution of Schrödinger's equation, mimicking the nanoscale geometry of the field. This way, we determined the boundaries and nature of nonadiabatic tunneling photoemission, building on a key advantage of a nanoplasmonic system, namely, that high-field-driven recollision events and their signature in the photoemission spectrum can be observed more efficiently due to significant nanoplasmonic field enhancement factors.

Application of pulsed laser ablation (PLA) for the size reduction of non-steroidal anti-inflammatory drugs (NSAIDs).

We studied the application of pulsed laser ablation (PLA) for particle size reduction in non-steroidal anti-inflammatory drugs (NSAIDs). Grinding of the poorly water-soluble NSAID crystallites can considerably increase their solubility and bioavailability, thereby the necessary doses can be reduced significantly. We used tablets of ibuprofen, niflumic acid and meloxicam as targets. Nanosecond laser pulses were applied at various wavelengths (KrF excimer laser, λ = 248 nm, FWHM = 18 ns and Nd:YAG laser, λ1 = 532 nm/λ2 = 1064 nm, FWHM = 6 ns) and at various fluences. FTIR and Raman spectra showed that the chemical compositions of the drugs had not changed during ablation at 532 nm and 1064 nm laser wavelengths. The size distribution of the ablated products was established using two types of particle size analyzers (SMPS and OPC) having complementary measuring ranges. The mean size of the drug crystallites decreased from the initial 30-80 µm to the submicron to nanometer range. For a better understanding of the ablation mechanism we made several investigations (SEM, Ellipsometry, Fast photography) and some model calculations. We have established that PLA offers a chemical-free and simple method for the size reduction of poorly water-soluble drugs and a possible new way for pharmaceutical drug preformulation for nasal administration.

Plasmon-plasmon coupling probed by ultrafast, strong-field photoemission with <7 Å sensitivity.

The coupling of propagating surface plasmon waves and localized plasmon oscillations in nanostructures is an essential phenomenon determining electromagnetic field enhancement on the nanoscale. Here, we use our recently developed ultrafast photoemission near-field probing technique to investigate the fundamental question of plasmon-plasmon coupling and its effect on large field enhancement factors. By measuring and analyzing plasmon field enhancement values at different nanostructured surfaces, we can separate the contributions from propagating and localized plasmons. When resonance conditions are met, a significant field enhancement factor can be attributed to the generation of localized plasmons on surface nanostructures, acting as dipole sources resonantly driven by the propagating plasmon field. Our plasmon-plasmon coupling results can contribute directly to applications in surface-enhanced Raman scattering (SERS) and the development of plasmonic sensors and nanostructured photocathodes.

Measurement of Nanoplasmonic Field Enhancement with Ultrafast Photoemission.

Probing nanooptical near-fields is a major challenge in plasmonics. Here, we demonstrate an experimental method utilizing ultrafast photoemission from plasmonic nanostructures that is capable of probing the maximum nanoplasmonic field enhancement in any metallic surface environment. Directly measured field enhancement values for various samples are in good agreement with detailed finite-difference time-domain simulations. These results establish ultrafast plasmonic photoelectrons as versatile probes for nanoplasmonic near-fields.

Interactions at the Peptide/Silicon Surfaces: Evidence of Peptide Multilayer Assembly.

Selective deposition of peptides from liquid solutions to n- and p-doped silicon has been demonstrated. The selectivity is governed by peptide/silicon adhesion differences. A noninvasive, fast characterization of the obtained peptide layers is required to promote their application for interfacing silicon-based devices with biological material. In this study we show that spectroscopic ellipsometry-a method increasingly used for the investigation of biointerfaces-can provide essential information about the amount of adsorbed peptide material and the degree of coverage on silicon surfaces. We observed the formation of peptide multilayers for a strongly binding adhesion peptide on p-doped silicon. Application of the patterned layer ellipsometric evaluation method combined with Sellmeier dispersion led to physically consistent results, which describe well the optical properties of peptide layers in the visible spectral range. This evaluation allowed the estimation of surface coverage, which is an important indicator of adsorption quality. The ellipsometric findings were well supported by atomic force microscopy results.