Monolayers of transition-metal dichalcogenides (TMDCs) are recently isolated materials combining strong light-matter interactions and high charge mobilities. Many TMDCs possess direct bandgap - a necessary property for optoelectronic and photonic applications. On the other hand, colloidal semiconductor nanocrystals (NQDs) exhibit high emission efficiency, chemical lability and excellent bandgap tunability via size quantization. Joining the two classes of materials in hybrid structures aims to utilize their respective strengths.
Among those, hybrids where two constituents are coupled via non-radiative energy transfer (NRET) present a particular interest. In the NRET process, exciton energy is transferred from NQD donor to TMDC acceptor via near-field, dipole-dipole energy coupling. This process plays an important role in photosynthetic plants and has been recently considered for energy harvesting in NQD/semiconductor architectures. Its efficiency depends on the distance, spectral overlap and dielectric screening properties of the acceptor material and its dimensionality. With the emergence of 2D materials, there is strong motivation, both for fundamental reasons and for the new applications, to study NRET in these novel systems.
We have studied NRET coupling between several types of NQDs and MoS2 monolayers using photoluminescence (PL) and femtosecond transient absorption (TA) spectroscopies. Both methods indicate very efficient NRET into the MoS2 acceptor, with donor PL intensity quenching concurrent with energy influx into acceptor as observed by TA. These effects are facilitated by reduced dielectric screening inherent to strongly polarizable TMDC materials as described by classical electromagnetic model. We envision energy coupling in 0D-2D hybrids enabling applications in photosensing, photovoltaics and light emission.
We present our recent advances toward the development of high-performance solid-state optical limiting devices using reverse saturable absorption (RSA) dyes doped into optical host materials. Femtosecond transient absorption spectroscopy was employed to determine both the spectral regions of strong RSA, and the singlet-triplet excited-state dynamics. The optical limiting in the visible spectrum in both metallo-phthalocyanines and metallo-porphyrins is due to a combination of singlet and triplet RSA. Optical limiting performance was studied for RSA dyes in dual tandem limiters (both in solution and solid-state). Our best results in the solid-state yielded an attenuation of 400X, and a damage threshold of up to several mJ at f/5 focusing. The optical limiting at f/5 is further enhanced, particularly in the solid-state, by self-defocusing thermal nonlinearities.
Parameters of refractive-index profile, the birefringence value, and mode interference pattern of buried planar waveguides fabricated by proton irradiation with doses from 1014 to 1017 cm-2 have been investigated experimentally. It has been shown that the Epsteyn model is a good approximation of waveguide refractive-index profile. The mechanical stress distribution in such waveguide layers has been studied by optical methods. A method for reconstructing the radiation induced defect distribution in the collision region has been proposed. The value of the proportionality factor in the refractive-index increment dependence on dose was determined more exactly on the basis of experimental data. Changes of the waveguide parameters dependent on the temperature of isochronous annealing also have been investigated.