
log in  contact us 



Photon momentum driven lightmatter interactions The fundamental conservation laws dictate total energy and momentum of the system of particles should be conserved through the interaction process. For optical excitation, the photon of energy E=ħw is absorbed by an electron, providing its transition to higher energy state, so the total energy of the system is conserved. The photon also carries momentum p=ħw/c, however, in free space, it is ~10^210^3 times smaller than that of an electron. Thus, optically allowed transitions are depicted as vertical arrows in energy band diagram  free space photon cannot change an electron momentum. Among most recognizable examples, figure on the left shows energymomentum space for pure silicon, with its lowest energetically allowed direct transition from G point at ~3.5 eV (350 nm). Below this energy, the transitions are considered momentumforbidden in Si  it requires an assistance of lattice phonons to change electron momentum and move it horizontally across the reciprocal space of the first Brillouin zone.
However, opposite to free space, the momentum of the confined photon shouldn’t be any longer neglected in lightmatter interaction. The spatial confinement dr leads to an increase of momentum value by ~ħp/dr, and it becomes comparable to that of an electron when photon is trapped at dr ~0.35 nm scales. This means, highly localized optical field can directly change both energy and momentum of an electron, changing the concept of light matter interaction. It makes diagonal transitions on the energy diagram possible.


Nonlinear optics and multiphoton processes in semiconductors Identifying strong and fast nonlinearities (both absorption and refraction) for today's photonic applications is an ongoing effort. Materials and devices are typically sought to achieve increasing nonlinear interactions to enhance or enable new photonic effects for generation, detection and manipulation of light. One example of an optical nonlinearity is simultaneous twophoton absorption that occurs in semiconductors, generating a single electron–hole pair in the process. We explore this effect to enable novel chemical sensing and visualization methods and modalities. Inorganic and organic semiconducting systems were historically considered to be excellent candidates for photonic switching because of their relatively large thirdorder nonlinearities. They have been the subject of extensive studies, both experimental and theoretical in many materialwavelength combinations. However, little attention has been paid to theoretical and experimental predictions for the case where the input wavelengths are vastly different. At the same time, although twophoton processes in a semiconductor are typically weak effects, the rate of absorption increases dramatically when the two photons have very dissimilar wavelengths. A quantummechanical model using Keldysh theory successfully predicts the observed scaling relationship between the bandgap, photon energy, and degenerate twophoton absorption (DTA) rate indicating the coefficient increases from zero at the halfbandgap (ħω = Egap/2) to a maximum value at 2/3 of the bandgap. This theory can further be generalized to include the case of nondegenerate twophoton absorption (NTA). Many had assumed that the nonlinear absorption rate depends primarily on the photon energy sum ħω1+ ħω2, and not on the individual scaling factors ħω1,2/Egap. Surprisingly, the nonlinear absorption function exhibits a remarkable singularity when either ħω1,2/Egap becomes small. What many had considered to be the optimal condition for DTA is actually a saddlepoint: along the degeneracy line (ħω1/Egap= ħω2/Egap), the absorption is minimum, whereas in the orthogonal direction, it reaches a maximum.


Excitons at quantum limit This thrust dives into the mechanisms and origins of longlived optical excitations in semiconducting systems and it explores effects and processes, related to bound states of electronhole pairs, i.e. excitons. Excitons holds extraordinary properties and potential:


Chemical sensitive spectrosopies at low limits Mediation of energy transfer between light and matter by surface plasmon or field enhancement helped overcome many fundamental hurdles in optical spectroscopy and microscopy, most notably, overcome diffraction limits manyfold and probe matter on single molecule level. When motion of chemical bonds within molecules can be observed in real time in the form of vibrational wave packets prepared and interrogated through ultrafast nonlinear spectroscopy, such nonlinear optical measurements are commonly performed on large ensembles of molecules and are limited to the extent that ensemble coherence can be maintained. Surface plasmon mediated signals allow sensitivities required to detect the motion of a single molecule under ambient conditions. In contrast with measurements in ensembles, the vibrational coherence on a single molecule does not undergo pure dephasing. It develops phase fluctuations with characteristic statistics  time evolution of discretely sampled statistical states. From more application perspective, such sensitivity enhancement enables very simple, fast, alloptical concept for live detection of low amounts of molecules that are common to our daily life, hence important from industrial and environmental perspectives (e.g. in situ ammonia monitoring). 

Ultrafast spectroscopy and imaging modalities in THz THz spectral region is of special interest for condensed matter  it is comprised of many molecular signatures, including lowenergy thermal rotations, bending and torsions. When used in pulsed regime and in combination with ultrafast optical exciation, this phasesensitive concept can directly probe the evolution of real and imaginary parts of dielectric function, hence complex optical conductivity. Moreover, many pulse arrnagement enables multidimensional correlation type measurements to probe molecular couplings and its harmonicity, as rich 2DRaman and 2D THzTHzRaman. 



