31 research outputs found
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
MagnetoâPlasmonics and Optical Activity in GrapheneâBased Nanowires
Nowadays, graphene plasmonics shows a great number of features unusual for traditional (metalâbased) plasmonics from high localization and large propagation distance of surface plasmonâpolaritons (SPPs) through the existence of both TEâ and TMâpolarized SPPs to the possibility of controlled SPPs by graphene chemical potential (or, equivalently, by gate voltage or chemical doping). Cylindrical grapheneâbased plasmonic structures have some advantages in contrast to planar geometry: absence of edge losses, existence of highâorder azimuthal modes, etc. In this work, we discuss some ways to obtain an optical activity in cylindrical grapheneâbased 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 for frequencies up to 80~GHz and the peak
amplitudes reach . 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
ComunicaciĂłn 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, Δij, 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 Δij 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