Bennett B. Goldberg
Professor of Physics and Electrical and
Computer Engineering
Boston University
590 Commonwealth Ave.
Boston, MA 02215
(617) 353-5789/1712
goldberg@bu.edu
Three Dimensional Nanoscale Imaging and Spectroscopy in Hard and Soft Materials
The grand challenges in nanoscale measurements in the optical regime fall into several different
categories. Here I discuss several, specifically proximity probe techniques, far-field subsurface imaging,
and imaging in soft materials and biological systems.
Surface proximity nanoscale optical imaging status: In the last 5 years, nanoscale measurements in the
optical regime have vastly improved for several classes of materials and systems. Scanned probe
microscopy with tip-enhanced, near-field scanning optical or near-field apertureless techniques have
become today’s standard and are reducing optical resolution down to 10’s of nanometers for hard
materials. [See, for example, Novotny, Kielmann, Saito and others]. Single carbon nanotubes have been
resolved spectroscopically with tip-enhanced Raman, and sophisticated optical tips have been used to
resolve the distribution of the optical matrix element in single quantum dots. These approaches lend
themselves to strongly resonant (to reduce background), fairly sparse systems located within a few
nanometers of the surface of the material, as they are all proximity probe techniques. In this regime of
Surface proximity nanoscale optical imaging and spectroscopy challenges:
1. Develop approaches for 3-D tomography, allowing one to examine subsurface structures, and
being to understand how buried systems can be imaged.
2. Develop tip-enhanced and aperture and apertueless probes that are reproducible, easily fabricated
and robust. The field of AFM exploded following the introduction of mass produced silicon and
silicon nitride cantilevers. A similar effort toward metal nanorods imbedded in tips, solid
immersion lenses and cantilevers needs to be built. This is largely a technological limitation.
Subsurface far-field optical imaging and spectroscopy status: In the field of subsurface imaging,
where proximity probes can no longer access any relevant information, the large advances in the past few
years have been in the development of solid immersion lens techniques. Our group [Goldberg and Unlu]
and others [see Karrai, Novotny, Grober] has pioneered the use of solid immersion microscopy, and in the
area of silicon SILs for subsurface imaging of ICs, currently have the highest resolution images in the
world in the optical regime. Solid immersion microscopy is now being introduced to take silicon
processing requirements to the next node and beyond. SILs will become more and more prevalent as we
deploy them on cantilevers, figure out ways to use them on many materials and extend the wavelength
range in the uv and infra-red. Thermal imaging is readily accomplished with SILs, and we have
demonstrated lambda/5 resolution for black body radiation.
Subsurface far-field optical imaging and spectroscopy challenges:
1. Development of modeling the process of solid immersion imaging in heterogeneous systems. We
know how to calculate optical response in a homogeneous system, but when the lens and
substrate material differ, and when the object under interest is close to other heterogeneous
materials, we do not yet know how the system will respnd
2. Development of lenses and lens systems for solid immersion microscopy. The technical challenge
is to build SILs and NAILs where the effective surface contact area is small, so that surface
roughness is not an issue in imaging. Yet the lens must have very high index, low absorption and
broad transmission from the uv to IR. These should be placed on easily controlled mechanical
systems.
Far-field imaging and spectroscopy in biological systems and soft materials status: The last decade
has seen the steady growth of techniques and approaches that have continued to improve biological
imaging, especially in the area of fluorescent techniques. The advances have fallen into two broad
categories; first single molecule imaging, and second, fluorescent techniques for biological systems with
sparse, yet large numbers of fluorphores. Advances have seen the development of two-photon techniques
by Webb and others, stimulated depletion by Hell and co-workers, and an interesting array of
interferometric techniques using patterned excitation, and interferometric excitation and detection. Our
group [Goldberg, Unlu, and Swan] have developed spectral self-interference microscopy that has
demonstrated 10 nanometer resolution in one dimension and which we believe will someday provide true
3D biological imaging at 20nm or so.
Far-field imaging and spectroscopy in biological systems and soft materials chalenges:
1. Probes: Molecular and nanocrystal probes that are brighter, and live forever
2. Detectors: There is a critical need for hyperspectral arrays. These are CCDs or similar that have
spectroscopic capability at each pixel. Having 1000 points of wavelength information at each
pixel would go a long way toward developing all the new interferometric techniques.
3. Modeling: We need new tools to understand all the influences of a heterogeneous environment on
interferometric biological imaging.