37 research outputs found
Generalized 4 4 Matrix Formalism for Light Propagation in Anisotropic Stratified Media: Study of Surface Phonon Polaritons in Polar Dielectric Heterostructures
We present a generalized 4 4 matrix formalism for the description of
light propagation in birefringent stratified media. In contrast to previous
work, our algorithm is capable of treating arbitrarily anisotropic or
isotropic, absorbing or non-absorbing materials and is free of discontinous
solutions. We calculate the reflection and transmission coefficients and derive
equations for the electric field distribution for any number of layers. The
algorithm is easily comprehensible and can be straight forwardly implemented in
a computer program. To demonstrate the capabilities of the approach, we
calculate the reflectivities, electric field distributions, and dispersion
curves for surface phonon polaritons excited in the Otto geometry for selected
model systems, where we observe several distinct phenomena ranging from
critical coupling to mode splitting, and surface phonon polaritons in
hyperbolic media
Femtosecond electrons probing currents and atomic structure in nanomaterials
The investigation of ultrafast electronic and structural dynamics in
low-dimensional systems like nanowires and two-dimensional materials requires
femtosecond probes providing high spatial resolution and strong interaction
with small volume samples. Low-energy electrons exhibit large scattering cross
sections and high sensitivity to electric fields, but their pronounced
dispersion during propagation in vacuum so far prevented their use as
femtosecond probe pulses in time-resolved experiments. Employing a
laser-triggered point-like source of either divergent or collimated electron
wave packets, we developed a hybrid approach for femtosecond point projection
microscopy and femtosecond low-energy electron diffraction. We investigate
ultrafast electric currents in nanowires with sub-100 femtosecond temporal and
few 10 nm spatial resolutions and demonstrate the potential of our approach for
studying structural dynamics in crystalline single-layer materials.Comment: 18 pages, 4 figures, includes 8 pages supplementary informatio
Low-temperature infrared dielectric function of hyperbolic -quartz
We report the infrared dielectric properties of -quartz in the
temperature range from to . Using an
infrared free-electron laser, far-infrared reflectivity spectra of a single
crystal -cut were acquired along both principal axes, under two different
incidence angles, in S- and P-polarization. These experimental data have been
fitted globally for each temperature with a multioscillator model, allowing to
extract frequencies and damping rates of the ordinary and extraordinary,
transverse and longitudinal optic phonon modes, and hence the
temperature-dependent dispersion of the infrared dielectric function. The
results are in line with previous high-temperature studies, allowing for a
parametrized description of all temperature-dependent phonon parameters and the
resulting dielectric function from up to the
--phase transition temperature, . Using
these data, we predict remarkably high quality factors for polaritons in
-quartz's hyperbolic spectral region at low temperatures
Terahertz sum-frequency excitation of a Raman-active phonon
In stimulated Raman scattering, two incident optical waves induce a force
oscillating at the difference of the two light frequencies. This process has
enabled important applications such as the excitation and coherent control of
phonons and magnons by femtosecond laser pulses. Here, we experimentally and
theoretically demonstrate the so far neglected up-conversion counterpart of
this process: THz sum-frequency excitation of a Raman-active phonon mode, which
is tantamount to two-photon absorption by an optical transition between two
adjacent vibrational levels. Coherent control of an optical lattice vibration
of diamond is achieved by an intense terahertz pulse whose spectrum is centered
at half the phonon frequency of 40 THz. Remarkably, the carrier-envelope phase
of the driving pulse is directly imprinted on the lattice vibration. New
prospects in infrared spectroscopy, light storage schemes and lattice
trajectory control in the electronic ground state emerge
Second-harmonic phonon spectroscopy of -quartz
We demonstrate midinfrared second-harmonic generation as a highly sensitive
phonon spectroscopy technique that we exemplify using -quartz (SiO)
as a model system. A midinfrared free-electron laser provides direct access to
optical phonon resonances ranging from to $1400\
\mathrm{cm}^{-1}T_c=846\ \mathrm{K}\alpha\beta$-quartz,
demonstrating the technique's potential for studies of phase transitions
Second Harmonic Generation from Critically Coupled Surface Phonon Polaritons
Mid-infrared nanophotonics can be realized using sub-diffractional light
localization and field enhancement with surface phonon polaritons in polar
dielectric materials. We experimentally demonstrate second harmonic generation
due to the optical field enhancement from critically coupled surface phonon
polaritons at the 6H-SiC-air interface, employing an infrared free-electron
laser for intense, tunable, and narrowband mid-infrared excitation. Critical
coupling to the surface polaritons is achieved using a prism in the Otto
geometry with adjustable width of the air gap, providing full control over the
excitation conditions along the polariton dispersion. The calculated
reflectivity and second harmonic spectra reproduce the full experimental data
set with high accuracy, allowing for a quantification of the optical field
enhancement. We also reveal the mechanism for low out-coupling efficiency of
the second harmonic light in the Otto geometry. Perspectives on surface phonon
polariton-based nonlinear sensing and nonlinear waveguide coupling are
discussed
Second Harmonic Generation from Phononic Epsilon-Near-Zero Berreman Modes in Ultrathin Polar Crystal Films
Immense optical field enhancement was predicted to occur for the Berreman
mode in ultrathin films at frequencies in the vicinity of epsilon near zero
(ENZ). Here, we report the first experimental proof of this prediction in the
mid-infrared by probing the resonantly enhanced second harmonic generation
(SHG) at the longitudinal optic phonon frequency from a deeply
subwavelength-thin aluminum nitride (AlN) film. Employing a transfer matrix
formalism, we show that the field enhancement is completely localized inside
the AlN layer, revealing that the observed SHG signal of the Berreman mode is
solely generated in the AlN film. Our results demonstrate that ENZ Berreman
modes in intrinsically low-loss polar dielectric crystals constitute a
promising platform for nonlinear nanophotonic applications
Layer-Resolved Resonance Intensity of Evanescent Polariton Modes in Anisotropic Multilayers
Phonon polariton modes in layered anisotropic heterostructures are a key
building block for modern nanophotonic technologies. The light-matter
interaction for evanescent excitation of such a multilayer system can be
theoretically described by a transfer matrix formalism. This method allows to
compute the imaginary part of the p-polarized reflection coefficient
Im, which is typically used to analyze the polariton dispersion of
the multilayer structure, but lacks the possibility to access the
layer-resolved polaritonic response. We present an approach to compute the
layer-resolved polariton resonance intensity in aribtrarily anisotropic layered
heterostructures, based on calculating the Poynting vector extracted from a
transfer matrix formalism. Our approach is independent of the experimental
excitation conditions, and fulfills an empirical conservation law. As a test
ground, we study two state-of-the-art nanophotonic multilayer systems, covering
strong coupling and tunable hyperbolic surface phonon polaritons in twisted
\MoO~double layers. Providing a new level of insight into the polaritonic
response, our method holds great potential for understanding, optimizing and
predicting new forms of polariton heterostructures in the future.Comment: 7 pages, 2 figure