C. L. Jahncke

Raman imaging with near‐field scanning optical microscopy

Raman spectroscopy in conjunction with near‐field scanning optical microscopy is used to image Rb‐doped KTiOPO4 within a spectral feature with high spatial resolution. We present Raman spectra as well as the first Raman images obtained in the near field. Differences between near‐field and far‐field Raman measurements are discovered and discussed.

Nano-Raman Spectroscopy and Imaging with a Near-Field Scanning Optical Microscope

Raman spectroscopy was performed using a near-field scanning optical microscope. This avoids the diffraction limit inherent in conventional optical microscopy techniques involving far-field optical components, and allows volumes significantly smaller than the cube of the wavelength to be investigated. The small sample volume coupled with the light-starved nature of the Raman effect itself makes such nano-Raman studies difficult. A near-field Raman microscope is described and results showing near-field effects in an investigation of Rb-doped KTP are presented. An image taken within a Raman feature demonstrates that nano-Raman imaging is indeed possible if the near-field instrument has considerable long-term stability, and that several unique aspects of the near-field data recommend this approach.

A versatile stable scanning proximal probe microscope

We present a novel scanning proximal probe microscope design utilizing a piezoelectric driven coarse positioning mechanism in x, y, and z, while maintaining relatively small lateral dimensions. The instrument is suitable for insertion into a Dewar. The primary purpose of this work is to develop a stable yet versatile instrument in order to meet the signal averaging limitations imposed by low signal level measurements. We have implemented a near field scanning optical microscope with this system, whose key features include simultaneous detection of reflected and transmitted signals, unique “center of mass” tip oscillator for shear force feedback, and overall microscope stability.

Choosing a preamplifier for tuning fork signal detection in scanning force microscopy

In scanning probe microscopy, it is critical to maintain small probe sample separations for high resolution imaging. Quartz crystal tuning forks are typically used for detecting shear forces in near-field scanning optical microscopy and normal forces in other atomic force-related microscopies. In this article we compare several tuning fork based detection schemes to determine which solution gives the best signal to noise ratio. The high impedance and low signals produced by the tuning fork necessitate care in selection of an appropriate preamplifier. We find that a carefully guarded voltage preamplifier sensing a mechanically driven tuning fork performs the best, but an electrically driven fork with a current preamplifier offers simpler construction with only 25% lower signal to noise ratio on average.

Fundamental differences between micro‐ and nano‐Raman spectroscopy

Electric field polarization orientations and gradients close to near‐field scanning optical microscope (NSOM) probes render nano‐Raman fundamentally different from micro‐Raman spectroscopy. With x‐polarized light incident through an NSOM aperture, transmitted light has x, y and z components allowing nano‐Raman investigators to probe a variety of polarization configurations. In addition, the strong field gradients in the near‐field of a NSOM probe lead to a breakdown of the assumption of micro‐Raman spectroscopy that the field is constant over molecular dimensions. Thus, for nano‐Raman spectroscopy with an NSOM, selection rules allow for the detection of active modes with intensity dependent on the field gradient. These modes can have similar activity as infra‐red absorption modes. The mechanism can also explain the origin and intensity of some Raman modes observed in surface enhanced Raman spectroscopy.

Time as a contrast mechanism in near-field imaging

Abstract Any modulation of a detected signal can be used as a contrast mechanism in imaging applications. Insofar as the time dependence of optical properties of imaged structures can be used to elucidate material properties, such time dependences can provide a modulation which can then be used as a contrast mechanism in imaging. We introduce a system in which time can be used as a contrast mechanism in imaging. We introduce a system in which time can be used as a contrast mechanism to study material nanostructures. Single-crystal silicon wafers are imaged in the infrared using a HeNe laser while the wafer is simultaneously pulsed with visible radiation. By studying the time dependence of the infrared transmittance, defect distribution on the nanometer scale can be imaged, and sample nanostructure can be studied.

Excellent thermal stability of superconducting properties in YBa2Cu3O7 films irradiated by pulsed excimer lasers

We have investigated the microstructure and superconducting properties of YBa2Cu3O7 films irradiated by nanosecond pulsed excimer lasers. The superconducting YBa2Cu3O7 thin films were deposited in situ on (100) LaAlO3 substrates using the pulsed laser evaporation technique. The virgin unirradiated YBa2Cu3O7 films exhibited excellent superconducting properties with Tco (temperature for zero resistance) of 90 K and critical current densities Jc (at 77 K and zero magnetic fields) of approximately 5.0 × 106 A/cm2. The films were further irradiated by a pulsed excimer XeCl laser (λ=308 nm, τ=45×10−9 s) with energy density varying from 30 to 300 mJ/cm2. Excellent thermal stability was observed for YBa2Cu3O7 films on LaAlO3 substrate for laser irradiation at all energy densities between 30 and 300 mJ/cm2, with no deterioration in the Tco values (90±0.3 K). An improvement in the Jc of the films was observed at low‐energy density irradiation, however, for energy densities above the melt threshold the Jc values dec...

Stabilizing wide bandwidth, tuning fork detected force feedback with nonlinear interactions

Near-field scanning optical microscope force feedback can be destabilized by the anisotropy in response times engendered by nonlinear tip sample interactions. This nonlinear interaction, the tapping of the tip on layers adsorbed on the sample, is important when the intrinsic damping of the system is low. We present strong evidence of tapping on adlayers rather than the sample surface at operational distances, and numerically solve a model to find the dynamics of tip motion. These results illuminate the origins of feedback problems when using tuning fork detection of oscillation amplitude, and show an optimal technique that uses the rapid response of the tip-adlayer nonlinearity to circumvent the slow damping response and enable wide bandwidth, stable distance regulation for these systems.

Dynamics of the tip–sample interaction in near-field scanning optical microscopy and the implications for shear force as an accurate distance measure

Near-field scanning optical microscopy uses shear-force feedback as the primary method to control the probe–sample distance. We describe the nonlinear interaction between the tip and sample with a simple truncated driven harmonic oscillator model. The model accurately describes the measured dynamics of this system. Insights are gained into the mechanism behind this interaction, and we give strong evidence that the probe taps on sample surface adlayers in normal operation, but will tap the underlying sample surface when the oscillation is nearly quenched.

The effects of probe boundary conditions and propagation on nano-Raman spectroscopy

Raman spectra obtained in the near‐field, with collection of the Raman‐shifted light in reflection, show selective enhancement of vibrational modes. We show that the boundary conditions for an electric field near a metal surface affect propagation of the reflected signal and lead to this selection. The enhancement of certain Raman forbidden vibrations is explained by the presence of an electric field gradient near the metal‐apertured fibre probe.