Mark R. Freeman

Picosecond resolution in scanning tunneling microscopy

A method has been developed for performing fast time-resolved experiments with a scanning tunneling microscope. The method uses the intrinsic nonlinearity in the microscopes current versus voltage characteristics to resolve optically generated transient signals on picosecond time scales. The ability to combine the spatial resolution of tunneling microscopy with the time resolution of ultrafast optics yields a powerful tool for the investigation of dynamic phenomena on the atomic scale.

Periodic magnetic microstructures by glancing angle deposition

An advanced deposition technique known as glancing angle deposition (GLAD) [K. Robbie, J. C. Sit, and M. J. Brett, J. Vac. Sci. Technol. B 16, 1115 (1998); K. Robbie and M. J. Brett, U.S. Patent No. 5,866,204 (filed 1999)] has been used to fabricate periodic arrays of magnetic pillars and randomly seeded magnetic helices, posts, and chevrons. Because of the nature of initial film nucleation, the GLAD process normally distributes posts randomly on the substrate surface. We can grow periodic arrays of GLAD microstructures by suppressing the randomness inherent within the initial nucleation stage of film growth. Shadowing sites were fabricated by pre-patterning a thin titanium layer on silicon substrates into a square array using electron beam lithography. These sites shadow regions of the substrate from incident flux during film deposition and act as preferred nucleation sites for film growth. Using this process, we have fabricated periodic arrays of cobalt posts with a regular elemental period of 600 nm an...

Ultrafast magneto‐optic sampling of picosecond current pulses

We report the demonstration of a magneto‐optic sampling technique that allows for the detection of picosecond electrical pulses propagating down a microstrip transmission line. A unique feature of the method is that it directly measures the current in ultrashort electrical pulses. The technique is based on time‐resolved detection of the magneto‐optic Faraday effect in a Bi‐substituted yttrium‐iron‐garnet (Bi‐YIG) film. The detailed magnetic response is found to be in excellent agreement with the Bloch‐Bloembergen equations. The bandwidth of the sampling system has been demonstrated to scale with the ferromagnetic resonance frequency up to 82.3 GHz, with a potential for increase towards 1 THz.

Picosecond time‐resolved magnetization dynamics of thin‐film heads

The application of picosecond magneto‐optic sampling techniques to studies of the magnetization dynamics of thin‐film recording heads is described. Time‐resolved magnetization measurements are performed using picosecond stroboscopic scanning Kerr microscopy. A Faraday probe is used to monitor the coil excitation current with the same optical setup, yielding a direct measure of the magnetic propagation delay through the head. This delay is found to have a marked spatial dependence across the face of the pole tips. As an aid to visualization of the spatial dependence, scanned images of the polar Kerr rotation at fixed time delays are acquired.

Metal-wire terahertz time-domain spectroscopy

Terahertz (THz) pulses propagating on a metal-wire waveguide are used to perform terahertz time-domain spectroscopy of lactose powder dispersed on top of the wire. The THz pulses are generated by a photoconductive switch at one end of the metal wire and are detected at the other end by electro-optic sampling in a ZnTe crystal that can be moved parallel to the axis of the metal wire. A large enhancement in the peak amplitude of the THz signal is observed by contacting the metal wire to one of the electrodes of the photoconductive switch. The propagation characteristics of the THz pulse along the metal wire and near its end are studied. Potential applications of metal-wire terahertz time-domain spectroscopy are discussed.

High-resolution pattern generation using the epoxy novolak SU-8 2000 resist by electron beam lithography

We report the fabrication of high-resolution sub 50 nm patterns by electron beam lithography using the epoxy novolak SU-8 2000 resist formulation. The minimum linewidth achieved is on the order of 30 nm and corresponds to a threefold reduction in minimum linewidth over previous reports describing similar resist chemistries. Our results also show that it is possible to fabricate dense linear grating elements without proximity correction. The dry etch resistance of native SU-8 2000 was found to be nearly twice that of poly(methylmethacrylate), making it ideal for applications that require pattern transfer. These studies are intended to explore the feasibility of SU-8 2000 as an electron beam resist for pattern generation on length scales below 50 nm.

Time-resolved scanning Kerr microscopy of ferromagnetic structures (invited)

Time-resolved microscopy enables valuable new measurements of the dynamics of resonance and relaxation in a range of magnetic systems. An overview of the scope of applications to ferromagnetic microstructures is presented. These include observations of ferromagnetic resonance and spatially nonuniform modes of oscillation, studies of magnetization reversal, and characterizations of the speed of magnetic recording devices.

Picosecond pulsed magnetic fields for studies of ultrafast magnetic phenomena

Ultrafast optical techniques were used to develop novel time-resolved probes of magnetic dynamics. Picosecond-scale magnetic field pulses were launched from a photoconductive switch and applied to small samples which were then probed by time-delayed optical pulses, yielded temporal information in a conventional stroboscopic manner. Characteristics of the magnetic field pulses are described and illustrated through measurements of rapid relaxation effects in europium sulfide films at low temperatures. >

Microwave frequency sensor for detection of biological cells in microfluidic channels

We present details of an apparatus for capacitive detection of biomaterials in microfluidic channels operating at microwave frequencies where dielectric effects due to interfacial polarization are minimal. A circuit model is presented, which can be used to adapt this detection system for use in other microfluidic applications and to identify ones where it would not be suitable. The detection system is based on a microwave coupled transmission line resonator integrated into an interferometer. At 1.5 GHz the system is capable of detecting changes in capacitance of 650 zF with a 50 Hz bandwidth. This system is well suited to the detection of biomaterials in a variety of suspending fluids, including phosphate-buffered saline. Applications involving both model particles (polystyrene microspheres) and living cells-bakers yeast (Saccharomyces cerevisiae) and Chinese hamster ovary cells-are presented.

Nanoscale torsional optomechanics

Optomechanical transduction is demonstrated for nanoscale torsional resonators evanescently coupled to optical microdisk whispering gallery mode resonators. The on-chip, integrated devices are measured using a fully fiber-based system, including a tapered and dimpled optical fiber probe. With a thermomechanically calibrated optomechanical noise floor down to 7 fm/Hz, these devices open the door for a wide range of physical measurements involving extremely small torques, as little as 4×10−20N·m.