Robert D. Grober

Piezoelectric tip‐sample distance control for near field optical microscopes

An aluminum coated tapered optical fiber is rigidly attached to one of the prongs of a high Q piezoelectric tuning fork. The fork is mechanically dithered at its resonance frequency (33 kHz) so that the tip amplitude does not exceed 0.4 nm. A corresponding piezoelectric signal is measured on electrodes appropriately placed on the prongs. As the tip approaches within 20 nm above the sample surface a 0.1 nN drag force acting on the tip causes the signal to reduce. This signal is used to position the optical fiber tip to about 0 to 25 nm above the sample. Shear forces resulting from the tip‐sample interaction can be quantitatively deduced.

Optical antenna: Towards a unity efficiency near-field optical probe

We demonstrate that an antenna can be used to realize a near-field optical probe that combines spatial resolution well below the diffraction limit with transmission efficiency approaching unity. The probe consists of a planar bow-tie antenna with an open-circuited gap at its apex. We present proof-of-principle measurements using microwave radiation and discuss scaling the antenna to the visible optical spectrum.

Origin of the magnetoelectric coupling effect in Pb(Zr0.2Ti0.8)O{3}/La{0.8}Sr{0.2}MnO{3} Multiferroic heterostructures.

The electronic valence state of Mn in Pb(Zr0.2Ti0.8)O{3}/La{0.8}Sr{0.2}MnO{3} multiferroic heterostructures is probed by near edge x-ray absorption spectroscopy as a function of the ferroelectric polarization. We observe a temperature independent shift in the absorption edge of Mn associated with a change in valency induced by charge carrier modulation in the La0.8Sr0.2MnO3, demonstrating the electronic origin of the magnetoelectric effect. Spectroscopic, magnetic, and electric characterization shows that the large magnetoelectric response originates from a modified interfacial spin configuration, opening a new pathway to the electronic control of spin in complex oxide materials.

Playing with Polarity

We review the influence of GaN crystal polarity on various properties of epitaxial films and electronic devices. GaN films grown on sapphire by MOCVD or HVPE usually exhibit Ga-face polarity. N-face polarity is obtained either on the backside of such layers after removal from the substrate, or by turning the crystal polarity in MBE growth via a thin AlN buffer layer. In addition to rather obvious differences in their structural and morphological features, Ga- and N-face samples differ also in their electronic properties. Thus, different Schottky barrier heights are observed for both polarities, the position and detailed properties of spontaneously formed two-dimensional electron gases vary with polarity, and the adsorption of gases and ions also show an influence of the two different surfaces. A particular interesting possibility is the growth of lateral polarity heterostructures with predetermined macroscopic domains of different polarity separated by inversion domain boundaries. These structures make use of the crystal polarity as a new degree of freedom for the investigation of electronic properties of III-nitrides and for novel devices.

Fundamental limits to force detection using quartz tuning forks

This paper explores the fundamental limits of the use of quartz tuning forks as force detectors in scanned probe microscopy. It is demonstrated that at room temperature, pressure, and atmosphere these force sensors have a noise floor of 0.62 pN/Hz and exhibit a root mean square Brownian motion of only 0.32 pm. When operated as a shear force sensor both dissipative and reactive forces are detected on approach to the sample. These forces are sufficient to reduce the amplitude of motion of the probe nearly to zero without physically contacting the surface. It is also demonstrated that conventional proportional-integral feedback control yields closed loop responses at least 40 times faster than their open loop response.

Realization of numerical aperture 2.0 using a gallium phosphide solid immersion lens

We report a study of a gallium phosphide, hemispherical, solid immersion lens through the imaging of 40-nm-diam fluorescent dye balls. A spatial resolution as small as 139 nm has been achieved at a wavelength of 560 nm, which is equivalent to a diffraction-limited system of numerical aperture 2.0. This resolution is a 33% improvement over conventional oil immersion objectives and previously reported solid immersion lenses, which typically have a numerical aperture around 1.5.

Piezo-electric tuning fork tip—sample distance control for near field optical microscopes

Abstract In a recent paper [Karrai and Grober, Appl. Phys. Lett. 66 (1995) 1842], a new technique was developed in order to control the distance separation between a tapered metal-coated optical fiber tip and the surface of a sample. This new technique is based on a piezo-electric tuning fork used as a shear-force detector. The fiber tip, which is attached along one of the arms of the tuning fork, acts as a shear-force pick-up. We present in this article the idealized model analysis that leads to the design parameters of a tuning fork optimized for near-field scanning optical microscopy.

Temperature dependence of the magnetoelectric effect in Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 multiferroic heterostructures

The magnetoelectric response of Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 (PZT/LSMO) artificial multiferroic heterostructures as a function of temperature, electric, and magnetic field, shows that the largest magnetoelectric coupling is attained at temperatures near the magnetic critical point of LSMO, at ∼180 K (−13.5 Oe cm kV−1). The magnetoelectric coupling displays a strong temperature dependence, changing sign at 150 K and saturating to positive values below ∼100 K (+6 Oe cm kV−1). The magnetoelectric curve switches hysteretically between two states in response to the ferroelectric switching. The peak in the magnetoelectric response coincides with the observation of on/off switching of magnetism in LSMO near the critical region, where the sensitivity to electric field is largest, making it a promising approach for device applications.

Spatially resolved photoluminescence of inversion domain boundaries in GaN-based lateral polarity heterostructures

Intentionally grown GaN inversion domain boundaries (IDBs) of lateral polarity heterostructures have been spectroscopically imaged at low temperature using high spatial resolution photoluminescence. It is shown that the IDBs are not only optically active, but are more than an order of magnitude brighter than the GaN bulk material. Our findings are in agreement with calculations predicting that IDBs should not adversely affect near-band-gap photoluminescence due to the absence of midgap electronic states. Typical linewidths are on the order of 10–20 meV, however, features less than 0.6 meV are observed. The boundary emission is found to be neither spectrally nor spatially uniform. Also, a strong polarization dependence of the IDB photoluminescence is measured and determined to be oriented parallel to the boundary between GaN of N- or Ga-face polarity.

Near‐field optical spectroscopy of single quantum wires

Low temperature near‐field scanning optical microscopy is used for spectroscopic studies of single, nanometer dimension, cleaved edge overgrown quantum wires. A direct experimental comparison between a two dimensional system and a single genuinely one dimensional quantum wire system, inaccessible to conventional far field optical spectroscopy, is enabled by the enhanced spatial resolution. We show that the photoluminescence of a single quantum wire is easily distinguished from that of the surrounding quantum well. Emission from localized centers is shown to dominate the photoluminescence from both wires and wells at low temperatures. A factor of 3 absorption enhancement for these wires compared to the wells is concluded from the photoluminescence excitation data.