Resonance micromechanical mass sensor with holographic interferometer

Abstract

Micromechanical resonators are widely used as inertial balances to detect small quantities of adsorbed mass through shifts in oscillation frequency [1]. In such applications, an interferometer is considered as a promising instrument for detecting the oscillations which could have ultra-small amplitude. However, due to its high sensitivity, an interferometer is open to an influence of external impacts such temperature drift, industrial noise, etc. Moreover classical homodyne interferometer is very sensitive to a quality of probe beam. Alternatively the interferometer can be implemented with use of dynamic holograms recorded in a photorefractive crystal [2]. Due to adaptive properties of dynamic holograms, such an interferometer can stably operate in industrial environment with light waves having complex wave fronts.In this work we present a prototype of the mass measurement system with pico-gram resolution based on micromechanical cantilever as a sensitive element and adaptive interferometry technique with using dynamic hologram recorded in photorefractive crystal for signal decoding.In the measurement system, laser light pulses at wavelength 532 nm with energy 0.5 mJ and duration 7 ns are used to excite a micro-mechanical cantilever with dimensions 200×40×5 m 3 . The cantilever out-of-plane mechanical vibrations were detected and their resonant frequency was measured by means of adaptive interferometer based on vectorial wave mixing in orthogonal geometry in CdTe crystal operated at cw laser light at 1,06 m [3].Organic nano-objects (with dimensions from 40 to 600 nm) were preliminary adsorbed at the end of cantilever by sputter coating technique. Masses of them were subject to a measurement. Every laser shot resulted in removing part of adsorbed microscopic objects (indicated by the arrows on Fig. 1). Figure 2 shows the evolution of the cantilever resonant frequency with laser pulse shots. As seen the frequency has increased by 751 Hz which corresponds to a decrease in the mass absorbed at the cantilever by 410×10 -12 g. The system has resolution of mass measurement was 6×10 -12 g.Adaptive properties of the interferometer based on using dynamic holograms allow one to drastically reduce size of cantilever (and, as sequence, to enhance a mass detection sensitivity) down to sub-wavelength scale without significant worsening of SNR. It is experimentally shown possibility of using cantilevers with dimensions 15×0,3×0,3 m 3 which opens prospects of easy detection of 10 -17 g masses. Fig.1. Cantilever with absorbed nano-objects before (a) and after (b) impact of laser pulses. Arrows show adsorbed nano-objects Fig.2. Micro-cantilever resonance frequency evolution with laser pulse shotsThe results obtained allow one to conclude that adaptive interferometer technique based on using dynamic photorefractive holograms can be a promising tool for detection ultra-small objects and measurement there masses. The research is supported by Ministry of Science and Education of Russian Federation.