This week we feature the Nanophotonics Group in the Physics department of the Weizmann Institute of Science (Rehovot. Israel)

The Nanophotonics group was established about 4 years ago at the Physics department of the Weizmann Institute of Science, located in Rehovot, Israel. With 10 researches coordinated by Dr. Dan Oron, the group’s main line of research relates to the nonlinear optical properties of colloidal nanocrystals. In particular, their potential applications in optical microscopy, and particularly in nonlinear and sub-diffraction limited imaging.

Overall, the Nanophotonics group collaborates with several other research groups in Israel, as well as with researchers in China, France, Germany, Switzerland, the United States and is in an interface between optics and materials science.

“We attempt to do everything, from colloidal synthesis, through characterization to optical studies in-house. Our research work involves first understanding the properties of materials and hybrids and alloys. From this understanding we hope to derive some insight and understanding, which will help us design and engineer materials with a particular optical purpose in mind. By then studying these properties experimentally, we can iteratively both learn more on the fundamental science side, and improve our design capabilities. I believe this interface, which requires knowledge and expertise both in physics and in chemistry, is a highly promising arena to work in”, Dr. Oron says.

According to Dr. Oron “Colloidal nanocrystals allow us to tune their resonant energy structure via either size control (as in semiconductor nanocrystals), shape control (as in plasmon-resonant noble metal nanoparticles) and composition.  The spectrum is of utmost importance when considering coherent nonlinear interactions. Semiconductor nanocrystals (“quantum dots”) also provide additional incoherent nonlinearity mechanisms via exciton-exciton interactions. These lead, for example, to intensity-dependent changes in the spectral properties (emission color) and dynamic properties (luminescence lifetime) of quantum dots”.

On one hand, the group is studying the fundamental processes occurring in nanocrystals (quantum dot blinking, low temperature photophysics, carrier cooling mechanism, exciton-exciton interactions, new synthetic protocols for nanocrystals doping etc.). These studies often involve a variety of ultrafast characterization methods, as many of the processes occurring in nanocrystals are extremely rapid. On the other hand, the group attempt to utilize the insights gained in these studies towards various applications.

In particular, these effects can be utilized for fabricating nanoparticles with unique optical properties for potential use in bioimaging applications. For example, “we have recently designed and fabricated hybrid colloidal semiconductor nanocrystals which have extremely high second order nonlinear susceptibility. Despite a diameter of less than 10nm, they exhibit blinking-free, ultra-stable harmonic emission with measured rates exceeding 10⁵ photons/second. Other examples include strong third-harmonic generating metallic nanoparticles and quantum dots exhibiting an extremely large luminescence wavelength shift when strongly excited. We hope to implement some of these in bioimaging scenarios in the near future” Dr. Oron says

In a related study in collaboration with the group of Valentina Emiliani at Universite Rene Descartes (Paris), the group developed an optical method for sculpting light for controlled multiphoton stimulation of neurons inside brain tissue. This method combines digital holography with the nonlinear optical technique of temporal focusing offering unprecedented control over the photo-stimulation process and is currently under intensive study.

Another field in which the group is active is development of novel photovoltaic light harvesting devices. Their main research thrust in this arena is on incorporation of semiconductor nanocrystals inside “standard” dye-sensitized photovoltaic cells for potentially improved performance. In this work, performed in collaboration with the group of Prof. Arie Zaban in Bar-Ilan University (Israel), semiconductor quantum dots are incorporated in the anode of the dye cell, and act as nano-antennas, funneling absorbed light via resonant energy transfer to nearby dye molecules. This introduces a lot of flexibility to the choice of organic dyes, as the requirement for panchromatic absorption is relieved, and also aids in increasing the absorber concentration inside the device, towards making thinner devices with lower losses.

Source:

Dr. Dan Oron,

Nanophotonics group
Department of physics of complex systems
Weizmann institute of science

More information at: www.weizmann.ac.il/complex/DOron/index.html

Relevant publications:

S. Buhbut, S. Itzhakov, E. Tauber, M. Shalom, I. Hod, T. Geiger, Y. Garini, D. Oron, A. Zaban, “Built-in quantum dot antennas in dye-sensitized solar cells”, ACS Nano 4, 1293 (2010). PDF

E. Papagiakoumou, V. DeSars, D. Oron, V. Emiliani, “Patterned two-photon illumination by spatiotemporal shaping of ultrashort pulses”, Opt. Express 16, 22039 (2008). PDF

D. Gachet, A. Avidan, I. Pinkas, D. Oron, “An upper bound to carrier multiplication efficiency in type-II colloidal quantum dots”, Nano Lett. 10, 164 (2010). PDF