Yuliya Kuznetsova

Imaging interferometric microscopy–approaching the linear systems limits of optical resolution

The linear systems optical resolution limit is a dense grating pattern at a lambda/2 pitch or a critical dimension (resolution) of lambda/4. However, conventional microscopy provides a (Rayleigh) resolution of only ~ 0.6lambda/NA, approaching lambda/1.67 as NA ?lambda1. A synthetic aperture approach to reaching the lambda/4 linear-systems limit, extending previous developments in imaginginterferometric microscopy, is presented. Resolution of non-periodic 180-nm features using 633-nm illumination (lambda/3.52) and of a 170-nm grating (lambda/3.72) is demonstrated. These results are achieved with a 0.4-NA optical system and retain the working distance, field-of-view, and depth-of-field advantages of low-NA systems while approaching ultimate resolution limits.

Fabrication of enclosed nanochannels using silica nanoparticles

We report a simple and inexpensive approach to the fabrication of enclosed nanoscale channels composed of silica nanoparticles on planar Si surfaces using interferometric lithography to define the long-range pattern in a photoresist film followed by spin-coating self-assembly of colloidal silica nanoparticles and high-temperature calcination to remove the photoresist leaving open nanochannels. Channel structures with channel width and height ranging from 100 nm to over 1 m were formed over large areas with different particle sizes and channel profiles. The dimensional scale of these ordered arrays of enclosed channels can be easily controlled through the parameters in the photoresist patterning and the spin-coating steps. Complex, multilayer structures have been generated using this approach as well. This process opens a route to fabricating ordered enclosed nanochannels with potential uses in photonics, molecular/biological sensors, biological separations and catalysis.

Structured illumination for the extension of imaging interferometric microscopy

Structured illumination applied to imaging interferometric microscopy (IIM) allows extension of the resolution limit of low numerical aperture objective lenses to ultimate linear systems limits (<approximately lambda/4 in air) without requiring a reference beam around the objective lens. Instead, the reference beam is provided by an illumination beam just at the edge of the optical system numerical aperture resulting in a shift of the recorded spatial frequencies (equivalent to an intermediate frequency). The restoration procedure is discussed. This technique is adaptable readily to existing microscopes, since extensive access to the imaging system pupil plane is not required.

Optical resolution below λ/4 using synthetic aperture microscopy and evanescent-wave illumination

Evanescent-wave illumination is applied to synthetic-aperture microscopy on a transparent solid substrate to extend the resolution limit to λ/2(n+1) (where n is the substrate refractive index) independent of the lens NA. Using a 633 nm source and a 0.4 NA lens, a resolution to 150 nm (λ/4.2) is demonstrated on a glass (n=1.5) substrate. Further extension to ~74-nm resolution (λ/8.6) is projected with a higher index substrate (n=3.3).

Ridged atomic mirrors and atomic nanoscope

Recent results on the reflection of waves from ridged mirrors are discussed. Numerical calculations, analytical estimates and the direct measurements of coefficient of specular reflection of atomic waves from solid-state mirrors are combined. The reflectivity is approximated as an elementary function of period L of ridges, their width l, wavenumber K, grazing angle θ and effective depth w of the van der Waals potential. In a special case L = l, the fit reproduces the reflectivity of flat surfaces. Our approximation allows us to optimize the L at given l and estimate the maximum performance of a ridged mirror. Such a mirror is suggested as a focusing element for the nano-scale imaging system.

Solid-immersion imaging interferometric nanoscopy to the limits of available frequency space

Imaging interferometric nanoscopy (IIN) is a synthetic aperture approach offering the potential of optical resolution to the linear-system limit of optics (~λ/4n). The immersion advantages of IIN can be realized if the object is in close proximity to a solid-immersion medium with illumination and collection through the substrate and coupling this radiation to air by a grating on the medium surface opposite the object. The spatial resolution as a function of the medium thickness and refractive index as well as the field-of-view of the objective optical system is derived and applied to simulations.

High-Resolution THz Spectroscopy to Measure Strong THz Absorption Signatures of si-RNA in Solution

Molecular spectroscopy of aqueous solutions has been more challenging in the THz region than at near-infrared wavelengths for several reasons, such as the strong absorption by liquid water. We have measured high-absorbance ( ≈ 0.8) and high-Q ( ≈ 90) resonant signatures around 1.0 THz from linear small-interfering double-stranded RNA molecules suspended in buffer solution in nanofluidic channels. The measurement instrument is a coherent photomixing transceiver. A physical model is developed to explain the measurements based on high-order flexural resonances of the entire double-strand with polar coupling to the THz radiation via the ionized phosphate groups. The predicted resonant frequencies are within 2% of experiment for the three double-strands studied (15, 19, and 23 base pairs), supporting the conclusion that each si-RNA molecule has a unique THz resonant frequency.

Concentration Methods for High-Resolution THz Spectroscopy of Nucleic-Acid Biomolecules and Crystals

Biomolecules can exhibit low-lying vibrational modes in the THz region which are detectable in transmission given a strong molecular dipole moment and optical depth, and a spectrometer of adequate sensitivity. The nucleic acids are particularly interesting because of applications such as label-free gene assay, bio-agent detection, etc. However for nucleic acids, sample preparation and THz coupling are of paramount importance because of the strong absorption by liquid water and the small concentration of molecules present in physiological solutions. Concentration methods become necessary to make the THz vibrational modes detectable, either by concentrating the nucleic-acid sample itself in a small volume but large area, or by concentrating the THz radiation down to the volume of the sample. This paper summarizes one type of the first method: nanofluidic channel arrays for biological nucleic acids; and two types of the second method: (1) a circular-waveguide pinhole, and (2) a circular-waveguide, conical-horn coupling structure, both for DNA crystals. The first method has been demonstrated on a very short artificial nucleic acid [small-interfering (si) RNA (17-to-25 bp)] and a much longer, biological molecule [Lambda-phage DNA (48.5 kbp)]. The second method has been demonstrated on small (~100 micron) single crystals of DNA grown by the sitting-drop method.

THz signatures of DNA in nanochannels under electrophoretic control

During the past several years we have utilized our nanofluidic-chip technology and high-resolution frequency-domain THz spectroscopy to detect absorption signatures in biomolecules and bioparticles of various types, especially the nucleic acids. Some of the signatures have been surprisingly narrow (<; 20 GHz FWHM), leading to the hypothesis that the nanofluidic chips can enhance certain vibrational resonances because of their concentrating and linearizing effects. In this work, we take the technology one step further by utilizing electrophoretic control of the absorbing biomolecules. A demonstration is provided of the variation in THz transmission through aqueous Lambda DNA at fixed frequency at one of its strongest sub-THz solutions. The THz transmission is found to be highly correlated to the electrophoretic current in the nanochannels, and to decrease with time. This is consistent with an increasing DNA concentration, or increasing oscillator strength, by the electrophoretic effect.

Imaging interferometric microscopy for enhanced resolution

Using the principle of imaging interferometry we resolve structures with a relatively low NA microscope objection which could not be resolved in the conventional illumination setup. We show experimental results for the cases of 700- and 4000-nm period gratings. We compare these results with theoretical simulations and estimate the maximum resolution potential. Also we evaluate further advantages of our approach, such as field of view and working distance.