Erik J. Sánchez

Near-field optical imaging using metal tips illuminated by higher-order Hermite-Gaussian beams

Abstract We propose a new scheme for high-resolution near-field optical imaging. The method relies on the highly enhanced fields at sharp metal tips under laser illumination. These fields are laterally confined to the tip size and can be used to locally excite the sample surface. Illumination along the tip axis with a higher-order beam mode (Hermite–Gaussian (1,0) mode) and detection of nonlinear responses (two-photon fluorescence, generation of second/third harmonics) ensure sufficient background discrimination. We outline the theory of laser beams beyond the paraxial approximation and investigate the electromagnetic fields for the proposed scheme.

Fluorescence quenching in tip-enhanced nonlinear optical microscopy

We describe the theoretical treatment of fluorescence quenching in tip-enhanced nonlinear optical microscopy (TENOM). Finite difference time domain simulations demonstrate that while sharp pyramidal probes yield fluorescence signal enhancement that decays monotonically as a function of probe-fluorophore distance, more commonly used conical probes cause more complex image contrast. Fluorescence quenching can thus explain the halo-type images that are sometimes observed in TENOM. Formation of a dielectric spacer layer on the TENOM probe should alleviate the complications associated with quenching.

A simple approach to neutral atom microscopy

Scanning surfaces using a beam of noncharged atoms or molecules allows for especially nondestructive and low-energy surface imaging, with the potential to obtain new information about surfaces that cannot be easily obtained otherwise. We have developed a new approach, operating with the sample at a close working distance from an aperture, the need for optics to focus the beam is obviated. Compared to more complex approaches, the theoretical performance has no other disadvantage than the short working distance. Resolution of 1.5 μm has been achieved, and submicron resolution appears to be practical. Construction of the microscope and results are presented, including first images done in reflection mode, theory for optimization of the design and avenues for future improvement.

Increased resolution in neutral atom microscopy

The neutral atom microscope uses a beam of thermal noncharged atoms or molecules to probe an atomic surface with very low interaction energies (<70 meV). Continued optimization of the ‘pinhole’ neutral atom microscope has improved resolution to 0.35 μm. Recent images are presented demonstrating resolution and the contrast mechanisms identified so far. The future potential for sub‐100 nm resolution is discussed.

Apertureless near-field/far-field CW two-photon microscope for biological and material imaging and spectroscopic applications.

We present the development of a versatile spectroscopic imaging tool to allow for imaging with single-molecule sensitivity and high spatial resolution. The microscope allows for near-field and subdiffraction-limited far-field imaging by integrating a shear-force microscope on top of a custom inverted microscope design. The instrument has the ability to image in ambient conditions with optical resolutions on the order of tens of nanometers in the near field. A single low-cost computer controls the microscope with a field programmable gate array data acquisition card. High spatial resolution imaging is achieved with an inexpensive CW multiphoton excitation source, using an apertureless probe and simplified optical pathways. The high-resolution, combined with high collection efficiency and single-molecule sensitive optical capabilities of the microscope, are demonstrated with a low-cost CW laser source as well as a mode-locked laser source.

Field programmable gate array based reconfigurable scanning probe/optical microscope

The increasing popularity of nanometrology and nanospectroscopy has pushed researchers to develop complex new analytical systems. This paper describes the development of a platform on which to build a microscopy tool that will allow for flexibility of customization to suit research needs. The novelty of the described system lies in its versatility of capabilities. So far, one version of this microscope has allowed for successful near-field and far-field fluorescence imaging with single molecule detection sensitivity. This system is easily adapted for reflection, polarization (Kerr magneto-optical (MO)), Raman, super-resolution techniques, and other novel scanning probe imaging and spectroscopic designs. While collecting a variety of forms of optical images, the system can simultaneously monitor topographic information of a sample with an integrated tuning fork based shear force system. The instrument has the ability to image at room temperature and atmospheric pressure or under liquid. The core of the design is a field programmable gate array (FPGA) data acquisition card and a single, low cost computer to control the microscope with analog control circuitry using off-the-shelf available components. A detailed description of electronics, mechanical requirements, and software algorithms as well as examples of some different forms of the microscope developed so far are discussed.

Fabrication of a versatile substrate for finding samples on the nanometer scale

With increasing interest in nanometer scale studies, a common research issue is the need to use different analytical systems with a universal substrate to relocate objects on the nanometer scale. Our paper addresses this need. Using the delicate milling capability of a focused ion beam (FIB) system, a region of interest (ROI) on a sample is labelled via a milled reference grid. FIB technology allows for milling and deposition of material at the sub 20‐nm level, in a similar user environment as a standard scanning electron microscope (SEM). Presently commercially available transmission electron microscope (TEM) grids have spacings on the order 100 μm on average; this technique can extend this dimension down to the submicrometre level. With a grid on the order of a few micrometres optical, FIBs, TEMs, scanning electron microscopes (SEMs), and atomic force microscopes (AFM) are able to image the ROI, without special chemical processes or conductive coatings required. To demonstrate, Au nanoparticles of ∼ 25 nm in size were placed on a commercial Formvar®‐ and carbon‐coated TEM grid and later milled with a grid pattern. Demonstration of this technique is also extended to bulk glass substrates for the purpose of sample location. This process is explained and demonstrated using all of the aforementioned analytical techniques.

Experimental validation of phase using Nomarski microscopy with an extended Fried algorithm

Reconstruction of an image (or shape or wavefront) from measurements of the derivatives of the image in two orthogonal directions is a common problem. We demonstrate how a particular reconstructor, commonly referred to as the Fried algorithm, can be used with megapixel derivative images to recover the original image. Large datasets are handled by breaking the derivative images into smaller tiles, applying the Fried algorithm and stitching the tiles back together. The performance of the algorithm is demonstrated using differential interference contrast microscopy on a known test object.

Simplified neutral atom microscopy

For some decades scientists have been interested the possibility of imaging surfaces using a focused beam of neutrally charged atoms or molecules. Scattering of neutral atoms provides a low-energy, surface-sensitive probe, and imaging with this could result in new insights. However, it has been difficult to form a sharply focused and sufficiently intense beam of neutral particles. The first image from such a microscope was published in 2008. We have succeeded with a new approach, using pinhole optics and a mechanically scanned sample. Resolution has reached 3 µm. Recent images and design theory are presented.

Effect of particle clustering on decay rates of admolecules at the interface of a composite material substrate

Abstract The decay rates of admolecules at the surface of a composite material are investigated theoretically in the limit of low volume fraction for the metallic particles in the composite. The optical properties of the substrate are described by both the Maxwell-Garnett and the fractalcluster models, simulating situations where the metal particles distribute themselves randomly, and where they coalesce to form localized clusters within the host material, respectively. It is found that in the case of particles coalescing to form clusters, a very small fraction of particles could lead to very large surface-induced damping for the admolecule at low emission frequencies, together with a red-shifted resonance peak in the decay rate spectrum. Hence, studies on the damping of vibrational states of admolecules at such composite material surfaces could lead to information concerning the distribution of particles in the substrate.