Filipp V. Ignatovich

Surface plasmon interference excited by tightly focused laser beams.

We show that interfering surface plasmon polaritons can be excited with a focused laser beam at normal incidence to a plane metal film. No protrusions or holes are needed in this excitation scheme. Depending on the axial position of the focus, the intensity distribution on the metal surface is either dominated by interferences between counterpropagating plasmons or by a two-lobe pattern characteristic of localized surface plasmon excitation. Our experiments can be accurately explained by use of the angular spectrum representation and provide a simple means for locally exciting standing surface plasmon polaritons.

Nano-optofluidic Detection of Single Viruses and Nanoparticles

The reliable detection, sizing, and sorting of viruses and nanoparticles is important for biosensing, environmental monitoring, and quality control. Here we introduce an optical detection scheme for the real-time and label-free detection and recognition of single viruses and larger proteins. The method makes use of nanofluidic channels in combination with optical interferometry. Elastically scattered light from single viruses traversing a stationary laser focus is detected with a differential heterodyne interferometer and the resulting signal allows single viruses to be characterized individually. Heterodyne detection eliminates phase variations due to different particle trajectories, thus improving the recognition accuracy as compared to standard optical interferometry. We demonstrate the practicality of our approach by resolving nanoparticles of various sizes, and detecting and recognizing different species of human viruses from a mixture. The detection system can be readily integrated into larger nanofluidic architectures for practical applications.

Real-time optical detection of single human and bacterial viruses based on dark-field interferometry

The rapid and sensitive detection and characterization of human viruses and bacteriophage is extremely important in a variety of fields, such as medical diagnostics, immunology and vaccine research, and environmental contamination and quality control. We introduce an optical detection scheme for real-time and label-free detection of human viruses and bacteriophage as small as ~24 nm in radius. Combining the advantages of heterodyne interferometry and dark-field microscopy, this label-free method enables us to detect and characterize various biological nanoparticles with unsurpassed sensitivity and selectivity. We demonstrate the high sensitivity and precision of the method by analyzing a mixture containing HIV virus and bacteriophage. The method also resolves the distribution of small nano-impurities (~20-30 nm) in clinically relevant virus samples.

Optical Detection of Single Nanoparticles and Viruses

We have developed two different optical techniques for the detection of nanoscale particles. One of the methods is based on measuring the optical gradient force exerted on a nanoparticle as it passes through a confined optical field, and the other method uses a background-free interferometric scheme to detect the scattered field amplitude from a laser-irradiated particle. In both cases, the measured signal depends on the third power of the particle size (R3) as opposed to the R6 dependence inherent to traditional scattering-based detection methods. The weaker size dependence in our schemes leads to a better signal-to-noise ratio (SNR) for small particles. Similar to mass spectrometry, the first detection method influences the trajectory of a particle as it passes through a tightly focused laser beam. On the other hand, the second detection method combines an interferometer with a split detector that yields no signal in the absence of a particle. For both systems, we demonstrate real-time (1 ms) detection of single nanoparticles in a microfluidic system and discuss the limits of each detection approach

Experimental study of nanoparticle detection by optical gradient forces

We experimentally investigate a particle detection scheme that makes use of optical gradient forces. The path of nanoscale particles carried in a microfluidic channel is perturbed by a strongly focused laser beam. The perturbation is interferometrically recorded with a quadrant detector and the temporal asymmetry of the detector signal yields information about the force exerted on the particles by the laser focus. Large particles experience strong forces, thus rendering a highly asymmetric signal whereas small particles pass the laser focus almost unperturbed thus rendering a symmetric signal. We analyze the influence of experimental parameters such as laser power and fluid speed on the precision of the method and discuss the influence of Brownian motion.

Detection of nanoparticles using optical gradient forces

Abstract We present a detection scheme for nanoscale particles based on the gradient force and torque near a tightly focused laser beam. The focus affects the path of nanoparticles passing by and a quadrant detector records the particle trajectory. A feedback system continuously adjusts the laser power and thereby prevents the particles from being trapped. Particle size and shape can be assessed by evaluating the time-trace of the quadrant detector signal.

The Kruger problem and neutron spin games

We present a compact solution to the one-dimensional problem of neutron scattering from two mutually perpendicular magnetic fields: permanent and rotating ones. Both fields are confined inside a layer of matter or a layer of a free space. The applications of this solution to the interpretation of some experiments with polarized neutrons are considered.

Nanofluidic preconcentration and detection of nanoparticles

The fast detection and characterization of nanoparticles, such as viruses or environmental pollutants, are important in fields ranging from biosensing to quality control. However, most existing techniques have practical throughput limitations, which significantly limit their applicability to low analyte concentrations. Here, we present an integrated nanofluidic scheme for preconcentration and subsequent detection of nanoparticle samples within a continuous flow-through system. Using a Brownian ratchet mechanism, we increase the nanoparticle concentration ∼27-fold. Single nanoparticles are subsequently detected and characterized by optical heterodyne interferometry. A wide range of potential applications can be foreseen, including real-time analysis of clinically relevant virus samples and contamination control of processing fluids used in the semiconductor industry.

Precision interferometric measurements of refractive index of polymers in air and liquid

We have developed a procedure for precise measurement of the group refractive index for materials in air and liquid environments, using a low coherence interferometer. For example, in manufacturing of soft contact lenses, the lenses are always kept hydrated in a saline solution. Knowing accurate refractive index of the lens is important to metrology and quality control purposes. The small refractive index difference between the liquid and the lens makes such tasks especially challenging. The developed procedure allows us to obtain measurement repeatability for group refractive index less than 1 x 10-3 for materials with thicknesses on the order of 100 microns, when measured in liquid. The measurement repeatability further improves for measurements in air, or for thicker materials.

Development of a calibration standard for spherical aberration

There are no calibration standards currently available for metrology equipment used to measure spherical aberration. We have selected a set of plano-convex lenses that can be used as spherical aberration calibration standards. The key parameters of the lenses were measured using a nodal optical bench and a low coherence interferometer. Spherical aberrations of the lenses were measured using a commercially available aberrometer the CrystalWave™, based on a Shack-Hartmann wavefront sensor. The lenses were then modeled in optical modeling software, where the spherical curvatures of the lenses were adjusted to match the key parameters. The measured spherical aberrations were then compared to the values provided by the modeling software.