Research statement – Dr. Yonatan Sivan
My research involves a variety of subjects in nano-photonics including linear and nonlinear optical
effects in plasmonic and semiconductor nanostructures, graphene, metamaterials and photonic
crystals. The research relies on a combination of analytical, asymptotical and numerical techniques,
but almost always involves collaboration with an experimental group.
Some of the specific projects that occupy us are:
1. The use of metallic nanoparticles for performance improvement in super-resolution
microscopy. This project is based on my recent prediction that metallic nanoparticles can be
used to improve the resolution and signal brightness in Stimulated-emission-depletion
microscopy (Y. Sivan et al., ACS Nano 2012; Y. Sivan, Appl. Phys. Lett. 2012), see Fig. 1. The
project involves analytical work, heavy computations, nanofabrication and optical
characterization (spectroscopy, imaging, lifetime imaging etc.). The work is done in collaboration
with 2 groups at Imperial College London, as well as with the group pf Nobel Laurate in
Chemistry, Prof. S Hell at Gottingen, Germany.
Fig. 1. A schematic illustration of a (NP-) STED nanoscope with excitation beam shape (green), depletion doughnutshaped beam (dark red), hybrid plasmonic-fluorescent emitter (yellow-red metal shell nanoparticle) and overall subdiffraction fluorescence signal (light red). Insets show the transverse cross sections of the field distributions and
signal.
We were recently able to perform proof-of-concept measurements of the technique. Using
metal nano-shells of about 150nm in size, we demonstrated a 4-fold reduction of the
intensity required to obtain super-resolution, see Fig. 2 and [Sonnefraud et al. Nano Lett.
2014]. Further work demonstrated similar performance with gold spheres coated by
fluorescent silica of sizes 30-60nm. More recently, we also repeated these measurements
with 10 by 30 nm metal rods coated with fluorophores attached to actin chains. We are now
in advanced stages of testing the viability of the technique for live cell imaging.
Fig. 2. Resolution improvement using core-shell NPs, as a function of the STED depletion power density used. Blue
disks: bare doped silica cores only. Black dotted circles: core-shell nanoparticles. The red dashed (resp. green dotted)
line is a least square fit of the bare silica (resp. core-shell) data.
2. Ultrashort optical pulse reversal in semiconductor nano-photonic structures. This project is
based on a series of joint publications with Prof. Sir J. Pendry (Sivan and Pendry, Phys. Rev. Lett.
2010; Sivan and Pendry, Opt. Express 2010; Sivan and Pendry, Phys. Rev. A 2010) whereby we
showed how to employ dynamically tuned photonic crystals for extreme manipulations of
ultrashort pulses such as ultrafast switching, time-resolved spectroscopy, time-reversal, storage
and stopping. The current theoretical work on light and charge carrier dynamics in
semiconductor waveguides accompanies the experimental effort done in the University of
Twente, The Netherlands. Specifically, we showed in [Sivan at al., Optics Express 2015] that
transient gratings enable optical switching at femtosecond features, i.e., orders of magnitude
faster than the conventional limits. This is enabled by diffusion of the free carriers along the
periodicity of the grating, so that the grating contract self-erase on sub-picosecond time scales.
Fig. 3. A spatiotemporal contour map of wave evolution in the switchable mirror time-reversal scheme developed by
Sivan and Pendry. The illustration shows the splitting of the incident wave to a forward and a backward (timereversed) wave.
Another direction is to explore additional applications of the technique such as short pulse
generation. This is done by turning on a Bragg grating that partially overlaps a longer signal
pulse. This work is done in collaboration with the group of Amiel Isha’aya at BGU using silica
fibers. We also explore some additional material platforms for these applications such as
plasmonic waveguides or graphene ribbons.
All the above work is accompanied by a careful derivation of a simplified and accurate set of
governing equations by extending the standard coupled-mode theory to pulse propagation and
for time-varying optical properties. This work is currently will be submitted for publication soon.
3. Nonlinear nanofocusing in plasmonic waveguides. In this project we are studying the prospects
of focusing intense beams into nanometric light spots. We study the interplay between linear
focusing provided by the plasmonic environment, nonlinear focusing and losses in the metal.
4. Novel designs of magnetic and negative refractive index metamaterials. This work is based on a
modal analysis of metal wire clusters. The work is done in collaboration with a group at Tel Aviv
University. As part of that, we performed the first ever multipole expansion for wire media
under general oblique incidence conditions, and provided a simple polarization-based
explanation to the seemingly contradictory assigning of magneto-electric coupling to these
structures. These findings appear in [P.Y. Chen, Y. Ben-Yakar and Y. Sivan, Phys. Rev. B,
submitted].
5. Frequency up-conversion using plasmonic waveguides and nano-particles. Here we aim to
exploit various recent ideas from nonlinear optics to realize highly efficient frequency mixing
processes in short plasmonic waveguides. We employ heavy analytical methods such as the
spectral decomposition and transformation optics. We also study these processes in various
nanoparticle complexes.