Ultrafast laser-induced phenomena in solids studied by time-resolved interferometry

In this work we have presented the technique for ultrafast time-resolved imaging interferometry and its application to the two di?erent problems of laser-matter interaction: femtosecond laser ablation of absorbing solids and optical breakdown in dielectrics. The presented detailed analysis of the technique including the optical design of the Michelson- and Mach-Zehnder-type imaging interferometers, analysis of the image formation and its relation to the 2D-Fourier-transform algorithm, artifacts in the reconstructed phase and amplitude maps as well as the physical interpretation of phase measurements represent a signi?cant development in the ?eld of time-resolved imaging interferometry. Without such analysis the results of interferometric measurements would be not so valuable and their interpretation not unique. Interferometric measurements at an ablating GaAs-surface allowed us to directly observe several types of transient surface deformations of laser-excited material both below and above the ablation threshold. The results of interferometric measurements support the theoretically predicted inhomogeneous bubble-like internal structure of an ablating layer. The expansion velocity of a hot pressurized laser-molten layer of material is shown to slow down during the ?rst few hundred of picoseconds of expansion, which strongly indicates the build up of tensile stresses in a liquid upon expansion (negative pressure). The observed extremely slow large-amplitude reversible surface deformations could be explained by the frustrated liquid-gas phase transition. This motivates further theoretical investigations of femtosecond laser ablation, which must be focused on the properties of metastable liquids under negative pressure. The variety of new ?ndings deduced from the measurements in GaAs motivate further interferometric studies in di?erent materials and possibly using slightly di?erent experimental con?gurations. As in the case of the universal Newton fringe phenomena we anticipate transient surface deformations to be driven by a material-independent mechanism. Finally, the price/quality ratio of interferometric measurements at ablating surfaces appeared to be very attractive and the chances of fully understanding the basic physical mechanisms of femtosecond laser ablation in the near future are very good. Interferometric measurements in transmission made with the help of imaging Mach-Zehnder-type interferometry aimed to clarify the ionization mechanisms in dielectrics irradiated by single intense femtosecond laser pulses. Signi?cant e?orts have been made to get rid of the propagation e?ects such as self-phase modulation and selffocusing, which only represent additional complications in these types of experi-ments. The 50 fs-time resolution achieved allowed us to follow the extremely fast dynamics of free carriers in sapphire and fused silica just after excitation. We were able to clearly demonstrate that at relatively low intensities below 10 TW/cm2 the dominant ionization mechanism is the 6-photon ionization, which is polarization dependent. The surface breakdown threshold does also slightly depend on laser polarization. The cross-sections of multiphoton ionization have been determined with an accuracy, which is much better than in all previously reported studies. However, at high intensities the spatial averaging in the propagation direction has been shown to be important. The comparison of experimental data with the results of model calculations of 1D-pulse propagation in dielectrics suggests that in fused silica the multiphoton ionization might be the dominant ionization mechanism up to the surface breakdown threshold, whereas for sapphire the ionization mechanism must be di?erent in the pre-breakdown regime. The attempts to compare the experimental data with the predictions of Keldyshs theory of photoionization were not successful. Without redoing Keldyshs calculations we were able to understand the limitations and assumptions behind his model calculations. Whereas Keldyshs general approach is very interesting and elegant from a theoretical point of view, his model calculations could not be applied for the given experimental situation. The most important problem is that high-?eld carrier transport in dielectrics induces extremely fast electron-lattice collisions, which are not included in Keldyshs approach. Finally, the price/quality ratio of interferometric measurements in dielectrics appeared to be rather moderate and the chances of understanding the ionization mechanisms in dielectrics in the pre-breakdown regime in the near future are slim

Topologically Induced Optical Activity in Graphene-Based Meta-Structures

Non-reciprocity and asymmetric transmission in optical and plasmonic systems is a key element for engineering the one-way propagation structures for light manipulation. Here we investigate topological nanostructures covered with graphene-based meta-surfaces, which consist of a periodic pattern of sub-wavelength stripes of graphene winding around the (meta-) tube or (meta-)torus. We establish the relation between the topological and plasmonic properties in these structures, as justified by simple theoretical expressions. Our results demonstrate how to use strong asymmetric and chiral plasmonic responses to tailor the electrodynamic properties in topological meta-structures. Cavity resonances formed by elliptical and hyperbolic plasmons in meta-structures are sensitive to the one-way propagation regime in a finite length (Fabry-Perot-like) meta-tube and display the giant mode splitting in a (Mach-Zehnder-like) meta-torus.Comment: 20 pages, 5 figures + TOC figure, accepted by ACS Photonic

Nonlinear surface magneto-plasmonics in Kretschmann multilayers

The nonlinear magneto-plasmonics aims to utilize plasmonic excitations to control the mechanisms and taylor the efficiencies of the non-linear light frequency conversion at the nanoscale. We investigate the mechanisms of magnetic second harmonic generation in hybrid gold-cobalt-silver multilayer structures, which support propagating surface plasmon polaritons at both fundamental and second harmonic frequencies. Using magneto-optical spectroscopy in Kretschmann geometry, we show that the huge magneto-optical modulation of the second harmonic intensity is dominated by the excitation of surface plasmon polaritons at the second harmonic frequency, as shown by tuning the optical wavelength over the spectral region of strong plasmonic dispersion. Our proof-of-principle experiment highlights bright prospects of nonlinear magneto-plasmonics and contributes to the general understanding of the nonlinear optics of magnetic surfaces and interfaces.Comment: Main Manuscript: 5 pages, 3 figures. Supplementary Information: 10 pages, 7 figure

Magneto‐Plasmonics and Optical Activity in Graphene‐Based Nanowires

Nowadays, graphene plasmonics shows a great number of features unusual for traditional (metal‐based) plasmonics from high localization and large propagation distance of surface plasmon‐polaritons (SPPs) through the existence of both TE‐ and TM‐polarized SPPs to the possibility of controlled SPPs by graphene chemical potential (or, equivalently, by gate voltage or chemical doping). Cylindrical graphene‐based plasmonic structures have some advantages in contrast to planar geometry: absence of edge losses, existence of high‐order azimuthal modes, etc. In this work, we discuss some ways to obtain an optical activity in cylindrical graphene‐based plasmonic structures and its possible applications to SPPs manipulation

Nonlinear acousto-magneto-plasmonics

We review the recent progress in experimental and theoretical research of interactions between the acoustic, magnetic and plasmonic transients in hybrid metal-ferromagnet multilayer structures excited by ultrashort laser pulses. The main focus is on understanding the nonlinear aspects of the acoustic dynamics in materials as well as the peculiarities in the nonlinear optical and magneto-optical response. For example, the nonlinear optical detection is illustrated in details by probing the static magneto-optical second harmonic generation in gold-cobalt-silver trilayer structures in Kretschmann geometry. Furthermore, we show experimentally how the nonlinear reshaping of giant ultrashort acoustic pulses propagating in gold can be quantified by time-resolved plasmonic interferometry and how these ultrashort optical pulses dynamically modulate the optical nonlinearities. The effective medium approximation for the optical properties of hybrid multilayers facilitates the understanding of novel optical detection techniques. In the discussion we highlight recent works on the nonlinear magneto-elastic interactions, and strain-induced effects in semiconductor quantum dots.Comment: 30 pages, 12 figures, to be published as a Topical Review in the Journal of Optic

Anatomy of ultrafast quantitative magneto-acoustics in freestanding nickel thin films

We revisit the quantitative analysis of the ultrafast magneto-acoustic experiment in a freestanding nickel thin film by Kim and Bigot [1] by applying our recently proposed approach of magnetic and acoustic eigenmodes decomposition by Vernik et al. [2]. We show that the application of our modeling to the analysis of time-resolved reflectivity measurements allows for the determination of amplitudes and lifetimes of standing perpendicular acoustic phonon resonances with unprecedented accuracy. The acoustic damping is found to scale as $\propto\omega^2$ for frequencies up to 80~GHz and the peak amplitudes reach $10^{-3}$. The experimentally measured magnetization dynamics for different orientations of an external magnetic field agrees well with numerical solutions of magneto-elastically driven magnon harmonic oscillators. Symmetry-based selection rules for magnon-phonon interactions predicted by our modeling approach allow for the unambiguous discrimination between spatially uniform and non-uniform modes, as confirmed by comparing the resonantly enhanced magneto-elastic dynamics simultaneously measured on opposite sides of the film. Moreover, the separation of time scales for (early) rising and (late) decreasing precession amplitudes provide access to magnetic (Gilbert) and acoustic damping parameters in a single measurement.Comment: 9 pages, 7 figure

Resonant phonon-magnon interactions in free-standing metal-ferromagnet multilayer structures

We analyze resonant magneto-elastic interactions between standing perpendicular spin wave modes (exchange magnons) and longitudinal acoustic phonon modes in free-standing hybrid metal-ferromagnet bilayer and trilayer structures. Whereas the ferromagnetic layer acts as a magnetic cavity, all metal layers control the frequencies and eigenmodes of acoustic vibrations. The here proposed design allows for achieving and tuning the spectral and spatial modes overlap between phonons and magnons that results in their strong resonant interaction. Realistic simulations for gold-nickel multilayers show that sweeping the external magnetic field should allow for observing resonantly enhanced interactions between individual magnon and phonon modes in a broad range of frequencies spanning from tens of GHz up to several hundreds of GHz, which can be finely tuned through the multilayer design. Our results would enable the systematic study and the deep understanding of resonantly enhanced magneto-elastic coupling between individual phonon and magnon modes up to frequencies of great contemporary fundamental and applied interest.Comment: 9 pages, 6 figure

Magnetoplasmonic Interferometers and Applications

Comunicación presentada en el 2nd Early Stage Researchers Workshop in Nanoscience, celebrado en Madrid el 28 y 29 de junio de 2012.Surface plasmons polaritons (SPP) are evanescent waves that propagate along a dielectric-metal interface. They can be confined in subwavelength metal structures, i.e. below the diffraction limit, which leads to many possible applications, including miniaturized optical devices. Within that context, the development of active plasmonics is important to achieve nanophotonic devices with advanced functionalities. This requires a system where the plasmon properties can be manipulated using an external agent. Among the different control agents considered so far, the magnetic field seems a promising candidate, since it is able to modify the dispersion relation of SPP at reasonable magnetic field strengths, and with a high switching speed. This modulation comes from the non-diagonal elements of the dielectric tensor, εij, appearing when the magnetic field is turned on. For noble metals, the ones typically used in plasmonics, these elements are proportional to the applied magnetic field but, unfortunately, very small at field values reasonable for developing applications. On the other hand, ferromagnetic metals have sizeable εij values at small magnetic fields (proportional to their magnetization), but are optically too absorbent. A smart system to develop magnetic field tunable plasmonic devices is the use of multilayers of noble and ferromagnetic metals. That is the framework of the present work, where we analyze the magnetic field induced SPP wavevector modulation (Ak) in Au/Co/Au films as a function of the wavelength and its possible application as a sensor.N