Darshan B. Desai

Hemispherical digital optical condensers with no lenses, mirrors, or moving parts

We present a simple method for obtaining direct non-scanning images in the far-field with subwavelength resolution. Our approach relies on the use of a digital optical condenser comprised of an array of light emitting diodes uniformly distributed inside of a hollow hemisphere. We demonstrate experimental observation of minimum feature sizes of the order of λ/6 with the proposed technique. Although our experiments were performed at visible frequencies, we anticipate that the proposed approach to subwavelength resolution can be extended to the ultraviolet and infrared spectral regions.

Ultra-thin condensers for optical subwavelength resolution microscopy

We present optical subwavelength resolution images of periodic patterned nanostructures using ultra-thin condensers (UTCs) illuminated by evanescent waves. We demonstrate bright and dark field microscopy using UTCs based on two types of surface wave illumination: surface plasmon polaritons and evanescent waves related to total internal reflection. We provide a discussion about the potential of UTCs for deep subwavelength resolution microscopy, and we discuss the similarities and differences between proposed UTCs, traditional bulky optical condensers, and several demonstrated superlenses.

Super-resolution imaging of photonic crystals using the dual-space microscopy technique.

We present an experimental implementation of the recently proposed dual-space microscopy (DSM), an optical microscopy technique based on simultaneous observation of an object in the position and momentum spaces, using computer-controlled hemispherical digital condensers. We demonstrate that DSM is capable of resolving structures below the Rayleigh resolution limit.

Versatile optical microscopy using a reconfigurable hemispherical digital condenser

We present a computer-controlled hemispherical digital condenser and demonstrate that a single device can be used to implement a variety of both well established and novel optical microscopy techniques. We verified the condenser capabilities by imaging fabricated periodic patterned structures and biological samples.

Optical condensers formed in wet-mounting setup

We reveal that a practical and simple sample arrangement for optical microscopy that is commonly used in biomedical imaging laboratories is a microscope condenser. This unnoticed but high quality microscope condenser is formed when the object under observation immersed in a liquid is sandwiched between two glass coverslips, and is observed using an oil-immersion objective lens. We demonstrate that the advantages in image resolution and contrast provided by this imaging arrangement come from the resulting microscope condenser. We also demonstrate that this overlooked condenser can be reconfigured as a variable numerical aperture microscope condenser by depositing a drop of low boiling point liquid on top of it. We present and discuss several experiments suggesting that the condenser-like rings observed in the Fourier plane images are formed when the incoming light bounces off, or gets scattered by the inner edge of the top aperture of the metal cage of the oil-immersion objective lens toward the top surface of the sample arrangement, and is either reflected, or totally-internally reflected back at a highly inclined angle toward the object under observation.

Simulation study of dual-space microscopy

We explore the convergence of the dual-space microscopy (DSM) phase-recovery algorithm. DSM is an optical microscopy technique based on simultaneous observation of an object in the position and momentum spaces. We present one-dimensional (1D) simulations of this technique, demonstrating that the DSM technique is capable to resolve periodic and nonperiodic structures with a resolution well below the Rayleigh resolution limit. Using a simple and faster 1D version of the full 2D DSM algorithm, we simulated the DSM technique for thousands of different samples. Our results demonstrate that the DSM algorithm always converges rapidly to the correct optical disturbance.

Comprehensive study of unexpected microscope condensers formed in sample arrangements commonly used in optical microscopy.

We show that various setups for optical microscopy which are commonly used in biomedical laboratories behave like efficient microscope condensers that are responsible for observed subwavelength resolution. We present a series of experiments and simulations that reveal how inclined illumination from such unexpected condensers occurs when the sample is perpendicularly illuminated by a microscopes built-in white-light source. In addition, we demonstrate an inexpensive add-on optical module that serves as an efficient and lightweight microscope condenser. Using such add-on optical module in combination with a low-numerical-aperture objective lens and Fourier plane imaging microscopy technique, we demonstrate detection of photonic crystals with a period nearly eight times smaller than the Rayleigh resolution limit.

Chapter 5 – Plasmonic and Nonplasmonic Characterization of Nanomaterials

Optical characterization has been one of the primary tools for researchers since time immemorial. However, most of the conventional techniques have either been diffraction-limited or require mechanical scanning and/or intensive numerical postprocessing that prohibits real-time observation. This chapter explores the contribution of different surface-wave imaging modalities for noninterferometric far-field imaging and characterization of nanoscale objects with truly high lateral resolution at visible frequencies using plasmonic and nonplasmonic evanescent surface waves excited using fluorescent molecules.