Resolution improvement of pulsed laser experiments with a micro-optomechanical focussing tip

摘要

The increased use of micromechanical devices demands the miniaturization of corresponding testing- and evaluation methods. The well known scanning probe methods (SPM) have very high lateral resolution. Pulsed laser acoustic experiments on the other hand have the advantage of very high temporal resolution, whereas the lateral resolution is limited by the fact that the minimal spot size to which a laser pulse can be optically focussed amounts to several wavelengths of light. The mechanical wavelength of acoustic waves excited by a ultra short laser pulse amounts to 10 to 20 nm. In contrast the spot size of the laser pulse is three orders of magnitude larger. The presented approach to improve the lateral resolution is the combination of the conventional scanning probe methods with pulsed laser acoustic methods. The introduction of a micro-opto-mechanical focusing tip in which the mechanical waves are focussed leads to a new potential time resolved scanning probe technique. An elastodynamic finite difference method has been developed to investigate the ultrasonic wave propagation in the tip numerically. The mechanical wave propagation for a conical tip geometry is discussed. The numerically calculated results are verified by experiments with structures having macroscopic dimensions. Scaling effects and restrictions due to the pulsed laser experiment are considered in order to translate the results into the microscopic scale.