Abstract

Quartz tuning forks, driven to electromechanical resonance by piezo elements, are widely used as distance sensing elements in scanning near-field optical microscopy (SNOM) [1]. Distance controls, based on tuning forks without piezo elements, have been successfully employed in atomic force microscopes [2]. We extended this technique to the case of low temperature SNOM. In our experimental set-up, the fibre is fixed at two points. The first point is glued to a rigid support two centimeters far from the tip, while the very end of the tip is attached to the fork itself (see fig. 1). When the system is driven at its resonance frequency, by an alternating voltage applied to the tuning fork, the electrical impedance is reduced to a minimum. When the tip and the surface are located a few hundred nanometers apart, the oscillatory parameters (resonance frequency, phase difference between voltage and current, width of the resonance) change depending on the magnitude of the shear forces acting on the tip. A feedback control of the tip-surface distance is performed by directly monitoring the amplitude and the phase of the electrical current in the fork. This is achieved by standard current lock-in detection technique. The main advantage is the compactness of the system, requiring neither piezo elements nor voltage preamplifiers. Preamplifiers, normally employed in systems where tuning forks are mechanically driven by piezo elements, are usually connected next to the fork and this causes serious problems when the whole system is at temperature of a few Kelvin. By varying the position of the contacts on the fibre, the resonance frequency can be tuned between 30 kHz and 32.5 kHz, and the Q-factors can be changed between 100 and 2000, allowing fast response or high sensitivity, respectively The system remains stable over several hours at a base temperature of 8K, allowing long acquisition times. These are needed when the intensity of light emission is low, as is the case in low excitation photoluminescence spectroscopy. Using uncoated fibre tips, which maximise light collection, we reach a spatial resolution of 200 nm and we obtain photoluminescence emission maps at very low excitation densities Fine structures in semiconducting heterostructures can be therefore observed. In the case of quantum wires, we show that the photoluminescence emission comes from elongated boxes (fig 2) and for the first time we have observed the spatial distribution and coherence length of ID excitons in the homogeneous regime.

© 2000 IEEE