Optical detection of infectious SARS-CoV-2 virions by counting spikes.

Existing methods for the mass detection of viruses are limited to the registration of small amounts of a viral genome or specific protein markers. In spite of high sensitivity, the applied methods cannot distinguish between virulent viral particles and non-infectious viral particle debris. We report an approach to solve this long-standing challenge using the SARS-CoV-2 virus as an example. We show that wide-field optical microscopy with the state-of-the-art mesoscopic fluorescent labels, formed by a core-shell plasmonic nanoparticle with fluorescent dye molecules in the core-shell that are strongly coupled to the plasmonic nanoparticle, not only rapidly, i.e. in less than 20 minutes after sampling, detects SARS-CoV-2 virions directly in a patient sample without a pre-concentration step, but can also distinguish between infectious and non-infectious virus strains by counting the spikes on the lipid envelope of individual viral particles.

Ultra-bright and narrow-band emission from Ag atomic sized nanoclusters in a self-assembled plasmonic resonator.

We have proposed, implemented and investigated a novel, efficient quantum emitter based on an atomic-sized Ag nanocluster in a plasmonic resonator. The quantum emitter enables the realization of: (1) ultra-bright fluorescence, (2) narrow-band emission down to 4 nm, (3) ultra-short fluorescence lifetime. The fluorescence cross-section of a quantum emitter is on the order of σ ∼ 10-14 cm2, which is comparable to the largest fluorescence cross-sections of dye molecules and quantum dots, and enables a light source with a record high intensity known only for plasmon nanolasers. The results presented suggest a unique method for fabricating nanoprobes with high brightness and wavelength-tunable spectrally narrow fluorescence, which is needed for multiplex diagnostics and detection of substances at extremely low concentrations.

Resonant Concentration-Driven Control of Dye Molecule Photodegradation via Strong Optical Coupling to Plasmonic Nanoparticles.

Photobleaching is one of the basic chemical processes that occur naturally in organic molecules. In this work, we investigate the quantum dynamics of Cy 7.5 dye molecules optically coupled to Au nanorod particles and experimentally demonstrate the decrease of the photobleaching rate in this hybrid system. We discover the effect of a resonance-like behavior not observed before for any type of emitter─the photobleaching rate of the dye molecules reaches a minimum for a suitable number of molecules coupled to the nanoparticle. The manifestation of the effect is the consequence of shifts in the energy levels in the hybrid system caused by the change in the number of molecules coupled to a nanoparticle. The energy shifts are the prerequisite for the effective depopulation of the triplet level, which is responsible for the photodegradation mechanism. The discovered effect paves the way for increasing the efficiency of optoelectronic and photovoltaic devices.

Ultrafast, Ultrasensitive Detection and Imaging of Single Cardiac Troponin-T Molecules.

The fluorescence-based methods of single-molecule optical detection have opened up unprecedented possibilities for imaging, monitoring, and sensing at a single-molecule level. However, single-molecule detection methods are very slow, making them practically inapplicable. In this paper, we show how to overcome this key limitation using the expanded laser spot, laser excitation in a nonfluorescent spectral window of biomolecules, and more binding fluorescent molecules on a biomolecule that increases the detection volume and the number of collected photons. We demonstrate advantages of the developed approach unreachable by any other technique using detection of single cardiac troponin-T molecules: (i) 1000-fold faster than by known approaches, (ii) real-time imaging of single troponin-T molecules dissolved in human blood serum, (iii) measurement of troponin-T concentration with a clinically important sensitivity of about 1 pg/mL. The developed approach can be used for ultrafast, ultrasensitive detection, monitoring, and real-time imaging of other biomolecules as well as of larger objects including pathogenic viruses and bacteria.

Split Hole Resonator: A Nanoscale UV Light Source.

Because of strong light absorption by metals, it is believed that plasmonic nanostructures cannot be used for generating intensive radiation harmonics in the ultraviolet (UV) spectral range. This work presents results of investigation of nonlinear optical interaction with a single gold nanostructure, the split-hole resonator (SHR) under the state-of-the-art experimentally realized conditions. To realize interaction with all spectral components of a 6 fs laser pulse several multipole plasmon resonances were simultaneously excited in the SHR nanostructure. To the best of our knowledge, this paper reports for the first time a strong nonlinear optical interaction at the frequencies of these resonances that leads to (i) the second harmonic generation, (ii) the third harmonic generation (THG), and (iii) the light generation at mixed frequencies. The THG near field amplitude reaches 0.6% of the fundamental frequency field amplitude, which enables the creation of UV radiation sources with a record high intensity. The UV THG may find many important applications including biomedical ones (such as cancer therapy).

Giant enhancement of two photon induced luminescence in metal nanostructure.

We experimentally demonstrate a drastic increase in the rate of radiative process of a nanoscale physical system with implementation of the three physical effects: (1) the size effect, (2) plasmon resonance and (3) the optical Tamm state. As an example of a nanoscale physical system, we choose a single nanohole in Au film when the nanohole is embedded in a photonic crystal of a specific type that maintains an optical Tamm state and as a radiative process - a nonlinear photoluminescence. The efficiency of the nonlinear photoluminescence is increased by more than 10(7) times in compare to a bulk material.

Subwavelength light localization based on optical nonlinearity and light polarization.

We propose and experimentally realize subwavelength light localization based on the optical nonlinearity of a single nonlinear element in nanoplasmonics-a split hole resonator (SHR). The SHR is composed of two basic elements of nanoplasmonics, a nanohole, and a nanorod. A peak field intensity occurs at the single spot of the SHR nanostructure. We demonstrate the use of the SHR as a highly efficient nonlinear optical element for (i) the construction of a polarization-ultrasensitive nanoelement and, as a practical application, (ii) the building up of an all-optical display.

Giant optical nonlinearity of a single plasmonic nanostructure.

We realize giant optical nonlinearity of a single plasmonic nanostructure which we call a split hole resonator (SHR). The SHR is the marriage of two basic elements of nanoplasmonics, a nanohole and a nanorod. A peak field intensity in the SHR occurs at the single tip of the nanorod inside the nanohole. The peak field is much stronger than those of the nanorod and nanohole, because the SHR field involves contributions from the following two field-enhancement mechanisms: (1) the excitation of surface plasmon resonances and (2) the lightning-rod effect. Here, we demonstrate the use of the SHR as a highly efficient nonlinear optical element for: (i) the generation of the third harmonic from a single SHR; (ii) the excitation of intense multiphoton luminescence from a single SHR.

Single nanohole and photoluminescence: nanolocalized and wavelength tunable light source.

We are first to demonstrate a broadband, nanometer-scale, and background-free light source that is based on photoluminescence of a single nanohole in an Au film. We show that a nanohole with a diameter of as small as 20 nm in a 200-nm thick Au film can be used for this purpose. Further development of the localized source that involves the use of a photon-crystal microcavity with a Q-factor of 100 makes it possible to create a 30-fold enhanced, narrowband tunable light source and with a narrow directivity of the radiation.