R. Eckert

Near-field fluorescence imaging with 32 nm resolution based on microfabricated cantilevered probes

High-resolution near-field optical imaging with microfabricated probes is demonstrated. The probes are made from solid quartz tips fabricated at the end of silicon cantilevers and covered with a 60-nm-thick aluminum film. Transmission electron micrographs indicate a continuous aluminum layer at the tip apex. A specially designed instrument combines the advantages of near-field optical and beam-deflection force microscopy. Near-field optical data of latex bead projection patterns in transmission and of single fluorophores have been obtained in constant-height imaging mode. An artifact-free optical resolution of 31.7±3.6 nm has been deduced from full width at half maximum values of single molecule images.

Observation of enhanced transmission for s-polarized light through a subwavelength slit

Enhanced optical transmission (EOT) through a single aperture is usually achieved by exciting surface plasmon polaritons with periodic grooves. Surface plasmon polaritons are only excited by p-polarized incident light, i.e. with the electric field perpendicular to the direction of the grooves. The present study experimentally investigates EOT for s-polarized light. A subwavelength slit surrounded on each side by periodic grooves has been fabricated in a gold film and covered by a thin dielectric layer. The excitation of s-polarized dielectric waveguide modes inside the dielectric film strongly increases the s-polarized transmission. A 25 fold increase is measured as compared to the case without the dielectric film. Transmission measurements are compared with a coupled mode method and show good qualitative agreement. Adding a waveguide can improve light transmission through subwavelength apertures, as both s and p-polarization can be efficiently transmitted.

Enhanced transmission from a single subwavelength slit aperture surrounded by grooves on a standard detector

An enhanced transmission is detected through a single slit of subwavelength width surrounded by grooves in a gold layer that is added as a postprocess to a standard complementary metal oxide semiconductor (CMOS) fabricated detector. The enhanced transmission results from constructive interference of surface waves, which interact with the incident light. The measured enhanced transmission shows strong qualitative agreement with that predicted by the modal expansion method. With the decreasing dimensions available in standard CMOS process, such nanostructures in metals could be used to replace current optical systems or to improve performance by increasing the signal to noise ratio and/or allowing polarization selection.

Fabrication and characterization of a silicon cantilever probe with an integrated quartz-glass (fused-silica) tip for scanning near-field optical microscopy

A cantilever-based probe is introduced for use in scanning near-field optical microscopy (SNOM) combined with scanning atomic-force microscopy (AFM). The probes consist of silicon cantilevers with integrated 25-mum-high fused-silica tips. The probes are batch fabricated by microfabrication technology. Transmission electron microscopy reveals that the transparent quartz tips are completely covered with an opaque aluminum layer before the SNOM measurement. Static and dynamic AFM imaging was performed. SNOM imaging in transmission mode of single fluorescent molecules shows an optical resolution better than 32 nm.

Characterization and fabrication of fully metal-coated scanning near-field optical microscopy SiO2 tips

The fabrication of silicon cantilever‐based scanning near‐field optical microscope probes with fully aluminium‐coated quartz tips was optimized to increase production yield. Different cantilever designs for dynamic‐ and contact‐mode force feedback were implemented. Light transmission through the tips was investigated experimentally in terms of the metal coating and the tip cone‐angle. We found that transmittance varies with the skin depth of the metal coating and is inverse to the cone angle, meaning that slender tips showed higher transmission. Near‐field optical images of individual fluorescing molecules showed a resolution < 100 nm. Scanning electron microscopy images of tips before and after scanning near‐field optical microscope imaging, and transmission electron microscopy analysis of tips before and after illumination, together with measurements performed with a miniaturized thermocouple showed no evidence of mechanical defect or orifice formation by thermal effects.

A beta-scintillator for surface measurements of radiotracer kinetics in the intact rodent cortex.

Beta+-sensitive probes are useful tools for the measurement of radiotracer kinetics in small animals. They allow the cost-effective development of new PET tracers and offer the possibility to investigate a variety of cerebral processes. The studys main aim was the in vivo evaluation of a probe system for cerebral surface acquisitions. The detector system is a 0.2-mm thick scintillating disk of 3-mm diameter, positioned close to the cerebral surface. The study consists of 4 subparts: (1) simulation of the detection volume, (2) direct comparison with the classic intracortical beta probe regarding its capability to acquire kinetic data, (3) test of the ability to detect local tracer accumulations during infraorbital nerve (ION) electrostimulation and (4) demonstration of the feasibility to measure tracer kinetics in awake animals. Kinetic data acquired with 18F-fluorodeoxyglucose and 15O-H2O were fitted with standard compartment models. The surface probe measurements were in good agreement with those obtained using the intracortical scintillator. ION electrostimulation induced a marked increase in tracer accumulation adequately detected by the surface probe. In the head-fixed animal, a marked change in FDG kinetics was detected between the awake and anesthetized state. The novel surface probe system proved to be a valuable instrument for in vivo radiotracer studies of the cerebral cortex. Its main advantage is the absence of any tissue damage. In addition, serial acquisitions of tracer kinetics in the awake animal turned out to be feasible.

Polarization properties of fully metal coated scanning near-field optical microscopy probes

We have presented a design of silicon cantilever based scanning near-field optical microscopy (SNOM) probes with aluminum coated quartz tips which have been batch fabricated using micromachining technology. The 12 /spl mu/m high probe tips are made of SiO/sub 2/ and fabricated at the end of silicon cantilevers. A hole in the cantilever, located underneath the tip base, permits light insertion and collection through the cantilever. Transmission electron microscopy reveals a 60 nm thick polycrystalline aluminum layer that covers the tip entirely. Still, far-field measurements show the typical polarization properties of conventional SNOM aperture probes. In order to study the mechanism of light transmission through such a tip, theoretical modelling and several experiments were performed.

Nanostructured metallic surfaces for enhanced transmission and polarization filtering in CMOS fabricated photodetectors

The miniaturization of photodetectors often comes at the expense of a smaller photosensitive area. This can reduce the signal and thus limit the image quality. One way to overcome this limitation is to reduce the photosensitive area but with no reduction of signal i.e. harvest the light. Here we investigate, theoretically and experimentally, light harvesting with nanostructured metals. Nanostructured metals can also give additional functionality such as polarization filtering which is also investigated. After defining the figure of merits used when characterizing light harvesting and polarization filtering structures, we detail the fabrication and measurement process. Structures were made on glass substrate, as a post process step on CMOS fabricated detectors and directly in the CMOS fabrication of the detectors. The optical characterization results are presented and compared with theory. Finally, we discuss the challenges and advantages of integrating metallic nanostructures within the CMOS process.

Enhanced transmission of s-polarized light through a metal slit

Enhanced optical transmission (EOT) through subwavelength apertures is usually obtained for p-polarized light. The present study experimentally investigates EOT for s-polarized light. A subwavelength slit surrounded on each side by periodic grooves has been fabricated in a gold film and covered by a thin dielectric layer. The excitation of s-polarized dielectric waveguide modes inside the dielectric film strongly increases the s-polarized transmission. Transmission measurements are compared with a coupled mode model and show good qualitative agreement. Adding a waveguide can improve light transmission through subwavelength apertures, as both s and p-polarization can be efficiently transmitted.

Theoretical and experimental characterization of fully metal coated scanning near-field optical microscopy SiO/sub 2/ tips

In spite of the considerable progress achieved in fabricating scanning near-field optical microscopy (SNOM) probes, the process to prepare smooth tip surfaces with aperture sizes under 100 nm is still expensive and time consuming. Recently, silicon cantilever-based SNOM probes with aluminum-coated quartz (fused silica) tips have been batch fabricated using micro machining technology. The tip apex is fully covered with a metal layer in the range of 60 nm thickness. It is the objective of this work to theoretically and experimentally elucidate the mechanism responsible for the light transmission through these novel probes.