Fundaments of optical far-field subwavelength resolution based on illumination with surface waves

Spin-Direction-Spin Coupling of Quasiguided Modes in Plasmonic Crystals

We report an unusual spin-direction-spin coupling phenomenon of light using the leaky quasiguided modes of a waveguided plasmonic crystal. This is demonstrated as simultaneous input spin-dependent directional guiding of waves (spin-direction coupling) and wave-vector-dependent spin acquisition (direction-spin coupling) of the scattered light. These effects, manifested as the forward and the inverse spin Hall effect of light in the far field, and other accompanying spin-orbit interaction effects are observed and analyzed using a momentum (k) domain polarization Mueller matrix. Resonance-enabled enhancement of these effects is also demonstrated by utilizing the spectral Fano resonance of the hybridized modes. The fundamental origin and the unconventional manifestation of the spin-direction-spin coupling phenomenon from a relatively simple system, ability to probe and interpret the resulting spin-orbit phenomena in the far field through momentum-domain polarization analysis, and their regulated control in plasmonic-photonic crystals open up exciting avenues in spin-orbit-photonic research.

Vectorial physical-optics modeling of Fourier microscopy systems in nanooptics

Fourier microscopy, which makes direct observation of the angular distribution possible, is widely used in the nanooptics community. The theory of such systems is typically based on ideal lenses. However, the real lenses in the typical complex lens systems have an impact on the image quality in the experiment. Therefore, it is desirable to have a model of the entire system, which is capable of predicting such phenomena, in order to conduct a preliminary detailed analysis of the setup before building it in the lab. In this work, we perform a vectorial physical-optics simulation of Fourier microscopy systems, which considers the real lenses; it also includes the nanostructure (e.g., photonic crystal). The systems are used to image the emission diagram of a single molecule as well as to analyze the angular-spectral property of a photonic crystal. We analyze various effects of the entire systems, e.g., Fresnel effects of the real lens surfaces, diffraction, polarization, chromatic aberration, and the effects of misalignment. We find that the above-mentioned effects have an influence on the final results, which should be taken into account when performing similar real-life experiments.

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 microscope's 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.

Metal slab superlens-negative refractive index versus inclined illumination: discussion

I describe experiments using a combination of an optical microscope and a plasmonic ultrathin condenser (UTC). The UTC's structure is similar to the original proposal of a silver slab superlens. I show that the observed improvement in image resolution is determined by the well-known equation describing the resolution obtainable when a condenser is included in the microscope setup. I argue that the described experiments indicate that the so-called metal slab superlens is better described as a novel ultrathin microscope condenser, and the observed improvement in resolution is due to the illumination of the object under observation with inclined light produced by the plasmonic UTC. Implications of this opinion for the development of an optical nanoscope based on plasmonic UTCs are presented.

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.

Experimental solution for scattered imaging of the interference of plasmonic and photonic mode waves launched by metal nano-slits

Using an L-shaped metal nanoslit to generate waves of the pure photonic and plasmonic modes simultaneously, we perform an experimental solution for the scattered imaging of the interference of the two waves. From the fringe data of interference, the amplitudes and the wavevector components of the two waves are obtained. The initial phases of the two waves are obtained from the phase map reconstructed with the interference of the scattered image and the reference wave in the interferometer. The difference in the wavevector components gives rise to an additional phase delay. We introduce the scattering theory under Kirchhoff's approximation to metal slit regime and explain the wavevector difference reasonably. The solution of the quantities is a comprehensive reflection of excitation, scattering and interference of the two waves. By decomposing the polarized incident field with respect to the slit element, the scattered image produced by slit of arbitrary shape can be solved with the nanoscale Huygens-Fresnel principle. This is demonstrated by the experimental intensity pattern and phase map produced by a ring-slit and its consistency with the calculated results.

Extracting surface wave-coupled emission with subsurface dielectric gratings

Conventional surface plasmons (SPs) or Bloch surface waves (BSWs) have a wave vector exceeding that of light in vacuum, and, therefore, the surface plasmon-coupled emission (SPCE) or Bloch surface wave-coupled emission (BSWCE) cannot escape from the corresponding structures. With the aid of a high-refractive-index prism or an oil-immersion objective, the SPCE or BSWCE can be coupled into free space. But the large volumes of the prism and objective are certainly unfavorable for miniaturization of the optical systems or inconvenient for applications such as the optical displays. Here we experimentally demonstrate a new method to extract the SPCE or BSWCE with a subsurface dielectric grating. The experimental results verify that the chip-like substrate with two decorated sides can bring out the directional fluorescence emission in free space. The emitting direction and emitting patterns can be tuned by the period size and dimensionality of the gratings. Our work provides a new strategy to realize free-space directional fluorescence emission at a very low cost and compact configuration, which has potential applications in fluorescence-based sensing, imaging, light-emitting diodes, optical displays, and other near-field optical devices.

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.

Tamm plasmon- and surface plasmon-coupled emission from hybrid plasmonic-photonic structures

Photonic and plasmon-coupled emissions present new opportunities for control on light emission from fluorophores, and have many applications in the physical and biological sciences. The mechanism of and the influencing factors for the coupling between the fluorescent molecules and plasmon and/or photonic modes are active areas of research. In this paper, we describe a hybrid photonic-plasmonic structure that simultaneously contains two plasmon modes: surface plasmons (SPs) and Tamm plasmons (TPs), both of which can modulate fluorescence emission. Experimental results show that both SP-coupled emission (SPCE) and TP-coupled emission (TPCE) can be observed simultaneously with this hybrid structure. Due to the different resonant angles of the TP and SP modes, the TPCE and SPCE can be beamed in different directions and can be separated easily. Back focal plane images of the fluorescence emission show that the relative intensities of the SPCE and TPCE can be changed if the probes are at different locations inside the hybrid structure, which reveals the probe location-dependent different coupling strengths of the fluorescent molecules with SPs and TPs. The different coupling strengths are ascribed to the electric field distribution of the two modes in the structure. Here, we present an understanding of these factors influencing mode coupling with probes, which is vital for structure design for suitable applications in sensing and diagnostics.