Fred F. Froehlich

Minimum detectable displacement in near‐field scanning optical microscopy

The probe‐to‐sample separation in near‐field scanning optical microscopes can be regulated by a noncontact atomic shear force sensing scheme that allows simultaneous acquisition of optical and shear force images. We have measured the minimum detectable displacement that can be achieved with a scheme based on diffracting a focused laser beam from the vibrating probe. The minimum detectable displacement determines the smallest resolvable change in force acting on the probe. The measured shot‐noise‐limited value is 2.8×10−3 Arms/√Hz, and the practical sensitivity is limited by thermal vibration noise to 7×10−3 Arms/√Hz. These values compare well with those calculated theoretically.

Linear behavior of a near-field optical scanning system

A hybrid finite-difference time-domain and angular-spectrum propagation modeling technique is used to study the imaging properties of a near-field optical scanning system with dielectric samples. The model is used to calculate system transfer functions based on scanning sinusoidal gratings of various spatial periods or on scanning a straight edge and then taking a derivative and a Fourier transform. Results from these two methods are in good agreement. A square-wave grating is simulated by linear addition of component sine-wave grating images that are weighted by the transfer function. The image generated by this method agrees well with an image generated by direct use of the hybrid model. In the region of parameter space investigated with the model, the near-field optical scanning system exhibits nearly linear behavior. The region of linear operation depends on the index of the sample and on the probe-to-sample spacing.

Near-field optical detection of asperities in dielectric surfaces

A finite-difference time-domain code is used to model near-zone electromagnetic probe fields of subwavelength dimensions and the interactions of these fields with a dielectric sample. The magnitude and the phase of the electric and the magnetic fields are determined in the region in which the energy leaving the probe interacts with the sample. An angular-spectrum code is then used to propagate the electric field into the far zone, in which signal detection takes place. TE and TM polarizations in a two-dimensional waveguide are modeled. We examine the effects of scanning the probe over a surface asperity in a dielectric sample. Two different far-zone detection schemes (total-energy detection and differential detection with a split-cell detector) are studied. When the probe scans a well, total-energy detection by TE polarization yields the closest estimate of the well’s actual width, whereas differential detection by TM polarization yields the sharpest profile of the well’s edges. Differential detection is shown to be less sensitive to variations in the probe-to-sample separations during a scan and has minimal distortions with both TE and TM polarizations.

Detection of probe dither motion in near-field scanning optical microscopy

The probe-to-sample separation in near-field scanning optical microscopes can be regulated by a noncontact shear-force sensing technique. The technique requires the measurement of a minute dither motion applied to the probe. We have characterized an optical detection method for measuring this motion to determine the optimum detection configuration in terms of sensitivity and stability. A scalar diffraction model of the detection method is developed for calculating sensitivity, and experimental results are found to be in good agreement with the theoretical predictions. We find that maximum sensitivity and stability cannot be achieved simultaneously, and it may be desirable in practice to trade sensitivity for enhanced stability.

Mechanical resonance behavior of near-field optical microscope probes

The mechanical resonance behavior of near-field optical microscope probes is examined with a simple experiment on a flat pyrex sample. While our tapered-fiber probe is locked on the second resonance for servo control, the vibration characteristics around the first resonance are investigated. We find that the overwhelming cause of decreased vibration amplitude as the tip approaches the sample is an increase in damping presumably due to a fluidlike layer on the sample. A small additional effect is also observed that could be due to force derivatives.

Numerical analysis of a two-dimensional near-field probe

Abstract A hybrid finite-difference-time-domain and angular spectrum technique is used to model near-field optical probe fields and the interaction of the fields with a pyrex substrate. Included in this analysis is a study of the effect of various nanometer sized asperities in the substrate. The hybrid technique provides a straightforward and highly accurate method to simulate the complex electromagnetic problems associated with probe fields.

Heating mechanisms in a near-field optical system

A finite-difference-time-domain and two finite-difference-thermal models are used to study various heating mechanisms in a near-field optical system. It is shown that the dominant mechanism of sample heating occurs from optical power that is transferred from the probe to a metallic thin-film sample. The optical power is absorbed in the sample and converted to heat. The effects of thermal radiation from the probe s coating and thermal conduction between the probe and the sample are found to be negligible. In a two-dimensional waveguide with TE polarization, most of the optical power is transferred directly from the aperture to the sample. In a two-dimensional waveguide with TM polarization, there is significant optical power transfer between the probe s aluminum coating and the sample. The power transfer results in a wider thermal distribution with TM polarization than with TE polarization. Using computed temperature distributions in a Co -Pt film, we predict the relative size of thermally written marks in a three-dimensional geometry. The predicted mark size shows a 30 % asymmetry that is due to polarization effects.

High-resolution optical lithography with a near-field scanning subwavelength aperture

We report on a novel method for generating sub-micron lithographic patterns in photoresist through the use of a scanned sub-wavelength optical aperture. The aperture consists of the tip of a single-mode optical fiber that is drawn down to a diameter of 80 nm and coated with aluminum. The fiber tip is manipulated with a modified scanning tunneling microscope (STM) that brings the tip into proximity of a photoresist-coated substrate. The resolution is primarily a function of the aperture diameter and tip-to-sample separation. A linewidth of 200 nm has been achieved in preliminary experiments.

Differential spot-size focus servo

We describe performance of a differential spot-size (wax-wane) focus servo. Cross talk from the tracks are analyzed in the single detector and differential focus circuits. Magnitude of the cross talk is reduced by a factor of three in the differential circuit. A false FES signal is present when the spot crosses sector marks at na angle.

Micro-optic lens for data storage

We describe a new type of microlens for data storage applications that has improved off-axis performance. The lens consists of a micro-Fresnel pattern on a curved substrate. The radius of the substrate is equal to the focal length of the lens. If the pattern and substrate are thin, the combination satisfies the Abbe sine condition. Therefore, the lens is free of coma. We analyze a 0.5 numerical aperture, 0.50 mm focal length lens in detail. A 0.16 numerical aperture lens was fabricated holographically, and results are presented.