Tip-enhanced Stokes and anti-Stokes Raman scattering in defect-enriched carbon films

Anti-Stokes Raman scattering is one of the mechanisms that lie behind an optical refrigeration due to release of photons with greater energy than of incoming photons. To achieve a cooling regime the enhancement of anti-Stokes scattering is necessary, since spontaneous Stokes scattering dominates over anti-Stokes scattering under normal conditions. Here, we investigate the opportunity of enhancement of spontaneous anti-Stokes Raman scattering in defect-enriched carbon film by means of localized plasmon resonances. In our simulations, incoherence of Raman scattering results in excess of anti-Stokes intensity over Stokes one. However, when the field is localized within the phonon coherence volume (coherent regime), the anti-Stokes intensity is lower compared to Stokes one. The provided analysis shows that plasmon-enhanced anti-Stokes Raman scattering can be achieved in highly-defective carbon films. The results are beneficial for Raman-based temperature measurements on the nanoscale.

Plasmon Coupling - The Root Cause of Raman Anomaly and Laser Cooling in Nanocrystal Ge

Laser cooling of matter through anti-Stokes photoluminescence, where the emitted frequency of light exceeds that of the impinging laser by virtue of absorption of thermal vibrational energy, has been successfully realized in condensed media, and in particular with rare earth doped systems achieving sub-100K solid state optical refrigeration. Studies suggest that laser cooling in semiconductors has the potential of achieving temperatures down to ~10K and that its direct integration can usher unique high-performance nanostructured semiconductor devices. While laser cooling of nanostructured II-VI semiconductors has been reported recently, laser cooling of indirect bandgap semiconductors such as group IV silicon and germanium remains a major challenge. Here we report on the anomalous observation of dominant anti-Stokes photoluminescence in germanium nanocrystals principally associated with plasmon coupling. Specifically, we attribute this Raman anomaly to the confluence of ultra-high purity nanocrystal germanium, generation of high density of electron-hole plasma, the inherent degeneracy of longitudinal and transverse optical phonons in non-polar indirect bandgap semiconductors, and commensurate spatial confinement effects. At high laser intensities, plasmon-assisted laser cooling with lattice temperature as low as ~50K is inferred.

Opto-refrigerative tweezers

Optical tweezers offer revolutionary opportunities for both fundamental and applied research in materials science, biology, and medical engineering. However, the requirement of a strongly focused and high-intensity laser beam results in potential photon-induced and thermal damages to target objects, including nanoparticles, cells, and biomolecules. Here, we report a new type of light-based tweezers, termed opto-refrigerative tweezers, which exploit solid-state optical refrigeration and thermophoresis to trap particles and molecules at the laser-generated cold region. While laser refrigeration can avoid photothermal heating, the use of a weakly focused laser beam can further reduce the photodamages to the target object. This novel and noninvasive optical tweezing technique will bring new possibilities in the optical control of nanomaterials and biomolecules for essential applications in nanotechnology, photonics, and life science.

Giant excitonic upconverted emission of two-dimensional semiconductor in doubly resonant plasmonic nanocavity

Phonon-assisted upconverted emission lies at the heart of energy harvesting, bioimaging, optical cryptography and optical refrigeration. It has been demonstrated that the emerging two-dimensional (2D) semiconductors can provide a great platform for efficient phonon-assisted upconversion due to the enhanced optical transition strength and phonon-exciton interaction of 2D excitons. However, the research on the further enhancement of excitonic upconverted emission in 2D semiconductors is almost blank. Here we report the enhanced multiphoton upconverted emission of 2D excitons in doubly resonant plasmonic nanocavity. Owing to the enhanced light collection, enhanced excitation rate and quantum efficiency enhancement arising from Purcell effect, the upconverted emission amplification of > 1000 folds and the decrease of 2 ~ 3 orders of magnitude for saturated excitation energy density are achieved. These findings pave the way to the development of excitonic upconversion lasing, nanoscopic thermometry and sensing, and open up the possibility of optical refrigeration in future 2D electronic or excitonic devices.