Laser-Induced Chirality of Plasmonic Nanoparticles Embedded in Porous Matrix.

Chiral plasmonic nanostructures have emerged as promising objects for numerous applications in nanophotonics, optoelectronics, biosensing, chemistry, and pharmacy. Here, we propose a novel method to induce strong chirality in achiral ensembles of gold nanoparticles via irradiation with circularly-polarized light of a picosecond Nd:YAG laser. Embedding of gold nanoparticles into a nanoporous silicate matrix leads to the formation of a racemic mixture of metal nanoparticles of different chirality that is enhanced by highly asymmetric dielectric environment of the nanoporous matrix. Then, illumination with intense circularly-polarized light selectively modifies the particles with the chirality defined by the handedness of the laser light, while their "enantiomers" survive the laser action almost unaffected. This novel modification of the spectral hole burning technique leads to the formation of an ensemble of plasmonic metal nanoparticles that demonstrates circular dichroism up to 100 mdeg. An unforeseen peculiarity of the chiral nanostructures obtained in this way is that 2D and 3D nanostructures contribute almost equally to the observed circular dichroism signals. Thus, the circular dichroism is neither even nor odd under reversal of direction of light propagation. These findings will help guide the development of a passive optical modulator and nanoplatform for enhanced chiral sensing and catalysis.

Lattice Rayleigh Anomaly Associated Enhancement of NH and CH Stretching Modes on Gold Metasurfaces for Overtone Detection.

Molecular overtones stretching modes that occupy the near-infrared (NIR) are weak compared to the fundamental vibrations. Here we report on the enhancement of absorption by molecular vibrations overtones via electromagnetic field enhancement of plasmonic nanoparallelepipeds comprising a square lattice. We explore numerically, using finite element method (FEM), gold metasurfaces on a transparent dielectric substrate covered by weakly absorbing analyte supporting N-H and C-H overtone absorption bands around 1.5 µ m and around 1.67 µ m, respectively. We found that the absorption enhancement in N-H overtone transition can be increased up to the factor of 22.5 due to the combination of localized surface plasmon resonance in prolonged nanoparticles and lattice Rayleigh anomaly. Our approach may be extended for sensitive identification of other functional group overtone transitions in the near-infrared spectral range.

Differential extinction of vibrational molecular overtone transitions with gold nanorods and its role in surface enhanced near-IR absorption (SENIRA).

Resonant coupling between plasmonic nanoantennas and molecular vibrational excitations is employed to amplify the weak overtone transitions that reside in the near-infrared. We explore for the first time the differential extinction of forbidden molecular overtone transitions coupled to the localized surface plasmons. We show a non-trivial interplay between the molecular absorption enhancement and suppression of plasmonic absorption in a coupled system. When the resonance conditions are met at 1.5 µm, two orders of magnitude enhancement of differential extinction as compared to the extinction of the same amount of free probe molecules is achieved. Our results pave a road toward a new class of surface enhanced near-infrared absorption-based sensors.

Boosting Terahertz Photoconductive Antenna Performance with Optimised Plasmonic Nanostructures.

Advanced nanophotonics penetrates into other areas of science and technology, ranging from applied physics to biology, which results in many fascinating cross-disciplinary applications. It has been recently demonstrated that suitably engineered light-matter interactions at the nanoscale can overcome the limitations of today's terahertz (THz) photoconductive antennas, making them one step closer to many practical implications. Here, we push forward this concept by comprehensive numerical optimization and experimental investigation of a log-periodic THz photoconductive antenna coupled to a silver nanoantenna array. We shed light on the operation principles of the resulting hybrid THz antenna, providing an approach to boost its performance. By tailoring the size of silver nanoantennas and their arrangement, we obtain an enhancement of optical-to-THz conversion efficiency 2-fold larger compared with previously reported results for similar structures, and the strongest enhancement is around 1 THz, a frequency range barely achievable by other compact THz sources. We also propose a cost-effective fabrication procedure to realize such hybrid THz antennas with optimized plasmonic nanostructures via thermal dewetting process, which does not require any post processing and makes the proposed solution very attractive for applications.