Eric Betzig

Near-Field Optics: Microscopy, Spectroscopy, and Surface Modification Beyond the Diffraction Limit

The near-field optical interaction between a sharp probe and a sample of interest can be exploited to image, spectroscopically probe, or modify surfaces at a resolution (down to ∼12 nm) inaccessible by traditional far-field techniques. Many of the attractive features of conventional optics are retained, including noninvasiveness, reliability, and low cost. In addition, most optical contrast mechanisms can be extended to the near-field regime, resulting in a technique of considerable versatility. This versatility is demonstrated by several examples, such as the imaging of nanometric-scale features in mammalian tissue sections and the creation of ultrasmall, magneto-optic domains having implications for highdensity data storage. Although the technique may find uses in many diverse fields, two of the most exciting possibilities are localized optical spectroscopy of semiconductors and the fluorescence imaging of living cells.

Breaking the Diffraction Barrier: Optical Microscopy on a Nanometric Scale

In near-field scanning optical microscopy, a light source or detector with dimensions less than the wavelength (λ) is placed in close proximity (λ/50) to a sample to generate images with resolution better than the diffraction limit. A near-field probe has been developed that yields a resolution of ∼12 nm (∼λ/43) and signals ∼104- to 106-fold larger than those reported previously. In addition, image contrast is demonstrated to be highly polarization dependent. With these probes, near-field microscopy appears poised to fulfill its promise by combining the power of optical characterization methods with nanometric spatial resolution.

Single Molecules Observed by Near-Field Scanning Optical Microscopy

Individual carbocyanine dye molecules in a sub-monolayer spread have been imaged with near-field scanning optical microscopy. Molecules can be repeatedly detected and spatially localized (to ∼λ/50 where λ is the wavelength of light) with a sensitivity of at least 0.005 molecules/(Hz)1/2 and the orientation of each molecular dipole can be determined. This information is exploited to map the electric field distribution in the near-field aperture with molecular spatial resolution.

Combined shear force and near‐field scanning optical microscopy

A distance regulation method has been developed to enhance the reliability, versatility, and ease of use of near‐field scanning optical microscopy (NSOM). The method relies on the detection of shear forces between the end of a near‐field probe and the sample of interest. The system can be used solely for distance regulation in NSOM, for simultaneous shear force and near‐field imaging, or for shear force microscopy alone. In the latter case, uncoated optical fiber probes are found to yield images with consistently high resolution.

Near‐field magneto‐optics and high density data storage

Near‐field scanning optical microscopy (NSOM) has been used to image and record domains in thin‐film magneto‐optic (MO) materials. In the imaging mode, resolution of 30–50 nm has been consistently obtained, whereas in the recording mode, domains down to ∼60 nm have been written reproducibly. Data densities of ∼45 Gbits/in.2 have been achieved, well in excess of current magnetic or MO technologies. A brief analysis of speed and other issues indicates that the technique may represent a viable alternative to these and other methods for anticipated high density data storage needs.

Near-Field Spectroscopy of the Quantum Constituents of a Luminescent System

Luminescent centers with sharp (<0.07 millielectron volt), spectrally distinct emission lines were imaged in a GaAs/AIGaAs quantum well by means of low-temperature near-field scanning optical microscopy. Temperature, magnetic field, and linewidth measurements establish that these centers arise from excitons laterally localized at interface fluctuations. For sufficiently narrow wells, virtually all emission originates from such centers. Near-field microscopy/spectroscopy provides a means to access energies and homogeneous line widths for the individual eigenstates of these centers, and thus opens a rich area of physics involving quantum resolved systems.

Nanoscale complexity of phospholipid monolayers investigated by near-field scanning optical microscopy

Near-field scanning optical microscopy of phospholipid monolayers doped with fluorescent lipid analogs reveals previously undescribed features in various phases, including a concentration gradient at the liquid-expanded/liquid-condensed domain boundary and weblike structures in the solid-condensed phase. Presumably, the web structures are grain boundaries between crystalline solid lipid. These structures are strongly modulated by the addition of low concentrations of cholesterol and ganglioside GM1 in the monolayer.

Polarization contrast in near-field scanning optical microscopy

Recent advances in probe design have led to enhanced resolution (currently as significant as ~ 12 nm) in optical microscopes based on near-field imaging. We demonstrate that the polarization of emitted and detected light in such microscopes can be manipulated sensitively to generate contrast. We show that the contrast on certain patterns is consistent with a simple interpretation of the requisite boundary conditions, whereas in other cases a more complicated interaction between the probe and the sample is involved. Finally application of the technique to near-filed magneto-optic imaging is demonstrated.

Optical spectroscopy of a GaAs/AlGaAs quantum wire structure using near‐field scanning optical microscopy

We report the first spectroscopic study using a low temperature near‐field scanning optical microscope. We have studied an array of GaAs/AlGaAs cleaved edge overgrowth quantum wires. The three luminescence peaks originate from different structures in the sample: The (001)‐oriented multiple quantum wells, the (110)‐oriented single quantum well, and the quantum wires. The linewidth of the quantum wire emission is related to roughness in the (110)‐oriented single quantum well. Quenching of the multiple quantum wells and single quantum well emission near the quantum wires is attributed to diffusion of photoexcited carriers into the wires.

Design and implementation of a low temperature near‐field scanning optical microscope

The design and implementation of a low temperature (T≥1.5 K), near‐field scanning optical microscope are described herein. This microscope, which is based on the recently developed tapered fiber probe, is optimized for luminescence imaging and spectroscopy of mesoscopic semiconductor systems.