Research

Plasmonics

The optical response of metal nanoparticles is governed by the collective oscillation of their surface electrons, called plasmons. When light interacts with these plasmon resonances, it creates highly confined electromagnetic fields with volumes much smaller than the diffraction limit of free space light beams. We utilize these fields to enhance and manipulate the light-matter interaction at nanoscale. By using nanofabrication techniques to create metal nanosystems that have a desired geometry which in turn tailors their optical properties such as color, photonic density of states, ultrafast optical response etc. These plasmonic nanosystems can be used to enhance the performance of nanoscale emitters or photodetectors. They can also be used as building blocks for optical metamaterials and metasurfaces: artificial macroscopic objects with engineered optical response.

The image below shows a scanning electron micrograph of a plasmonic system (dipole optical antenna) made of 2 gold nanorods (left), and spectra of metal nanoparticles of different sizes showing the plasmonic resonance (right).

Nonlinear optics at nanoscale

The interaction of light with matter is generally described by the exciting electric field E and its induced polarization P

P = εχ⁽¹⁾E + χ⁽²⁾EE + χ⁽³⁾EEE + . . . (1)

Most of the optical phenomena are described by the first term in this equation when the relationship between the excitation field and the polarization currents is linear. However, for strong excitation fields, this linear relationship breaks down and higher order terms in Eq. 1 come into play. This leads to emergence of new phenomena that can be utilized for frequency conversion, modulation and switching. Thus, the nonlinearity is an additional “knob” that allows to tune the light matter interaction. In nanoscale materials the nonlinear response is not governed only by χ⁽ⁱ⁾ tensor elements but also by the geometry of the system which gives us even further tunabilty. The example below shows signal generation of two different nonlinear processes on a plasmonic dimer system. The same nanoparticle can generate different spatial distribution of the signal for second harmonic generation (second order nonlinear process) and four wave mixing (third order nonlinear process). Scalebar is 100 nm.

Ultrafast Optics

We use nanophotonic materials to study the effect of nanoscale confinement of light and matter on the electronic transitions and ultrafast dynamics. In bulk materials at equilibrium, these processes are relatively well understood. However at dimensions comparable to the mean free path of electrons (~10 nm) and at characteristic timescales of excited carrier relaxation (~10 fs) new processes emerge that have a profound effect on our understanding of electron dynamics.

The example below demonstrates how the ultrafast electron dynamics can be manipulated at the nanoscale. The figures show the ultrafast transient response of plasmonic samples with different degrees of mode confinement. The strong confinement on nanometer scale (right) leads to large-amplitude femtosecond response due to the excitation of hot electrons in metal. In the absence of plasmonic mode confinement (left) the ultrafast dynamics features picosecond response dominated by electron-phonon coupling of thermalized electron gas.