Berlin 2012 – scientific programme

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O: Fachverband Oberflächenphysik

O 64: Plasmonics and nanooptics III

O 64.9: Talk

Thursday, March 29, 2012, 12:30–12:45, MA 005

Phase-sensitive near-field mapping — •Stefan Griesing, Andreas Englisch, and Uwe Hartmann — Institute of Experimental Physics, Universität des Saarlandes, P.O. Box 15 11 50, D-66041 Saarbrücken

The combination of a standard SNOM set-up with an interferometer give an access to the phase information of the optical near-field. Fiber-based aperture SNOMs connected to a pseudoheterodyne set-up are sensitive to low-frequency phase variations which are caused by environmental effects. In order to stabilize the reference phase, we introduced an active phase stabilization system in our setup. The detected near-field signal is demodulated by a lock-in amplifier. The real part of the second harmonic of the fiberstretchers drive frequency is detected and used as input signal for a PID feedback loop. The stretcher is then driven by a signal composed of the modulation part and the feedback part. In the case of a slow PID loop which solely follows the environmentally caused phase fluctuations, amplitude and phase information of the sample are obtained by demodulation of the first harmonic. The most interesting application is the investigation of highly dispersive media. In this case, a second wavelength is injected in the system. The PID feedback loop is fed with the real part of the second harmonic of one wavelength as described above. If a short time constant of the feedback-loop is chosen it becomes fast and compensates completely for phase variations of the input wavelength. Solely the difference phase of both wavelengths, caused by the dispersion of the sample, remains in the interference signal and can be measured.