A quadrature optical detection technique, based on polarized balanced-homodyne interferometry, has been developed for specific application to apertureless near-field scanning optical microscopy (ANSOM). With such technique, multiplicative background interference, inficiating quantitative optical imaging in standard homodyne-based ANSOM, can be suppressed. Periodic modulation of interferometric optical phase, typically employed in heterodyne-based ANSOMs even to such purpose, is not needed in the present configuration. Homodyne detection also facilitates detection of harmonic components of the ANSOM optical signal at the probe/sample distance modulation frequency, necessary for near-field discrimination and suppression of artifacts. Furthermore, since amplitude signal is not affected by phase fluctuations generated in the optical path of the interferometer, an optical fiber could be included in one interferometer arm, to couple the ANSOM head to the detection system, obtaining improved versatility of the instrument. A demonstration of the interferometer performance is given by a test confocal optical scan of a mirror surface. This technique, as applied to near-field microscopy, is anticipated to provide absolute values of optical contrast not depending on background interference and topography artifacts.
REFERENCES
We notice that the presence of a polarization component of the signal beam parallel to the reference one would give raise to some multiplicative BG interference, therefore a linear polarizer (P2) is included in the signal arm (Fig. 1). Such solution here is preferable to the one used in Ref. 17 employing a PBSC in place of BS1 and P2.
Incidentally, in balanced-homodyning , , , . The difference with multiple-pass homodyning is that the four measurements are simultaneous and no mirror motion is necessary.
