Monday, 20 June 2011 09:54

NSOM Phase Mapping of Nano Photonic Waveguides

We highlight here the work of the group of Prof. Uriel Levy, Hebrew University Israel, showing a simple method for phase mapping characterization of nano photonic structures.  The group showed that by exploiting the intrinsic oscillations of an apertured NSOM probe phase mapping could be achieved.

Typical NSOM measurements reveal the intensity of the light but give no information regarding the complex nature of the measured electromagnetic field.  While other methods allow such measurements they have the drawback of very complicated setups like Hetrodyne and Homodyne setups with complex background removal algorithms.  The proposed method is compact, cost effective, align-free and does not require external modulators. The system can be integrated to an existing NSOM setup in relative ease and has high potential as a characterization tool of various nanophotonic structures.  In this paper, several maps were shown providing significant information about the phase and amplitude of the electromagnetic field within the waveguide.

The figure shows a superimposed Phase NSOM signal on AFM topography.

In the image above we see a phase map of a Si serpentine s-shaped waveguide (blue and red colors). The measurements were obtained with the Nanonics MultiView4000 MultiProbe SPM System integrated with a fiber interferometric setup.   Light is emitted from a laser into a beam splitter sending the majority of light into the silicon waveguide. The NSOM probe was brought into proximity with the waveguide and kept in contact with its surface with intermittent contact. The light was collected by the NSOM tip, combined with the reference beam using a beam splitter and detected by a Photo Detector.

The NSOM probe was modulated at a frequency around 40 KHz, which is the characteristic eigen-frequency of the tuning fork of the tip. Such a relatively low frequency component can be detected and demodulated with relative ease with a standard detector and Lock-In Amplifier.  The demodulated signal is the phase NSOM signal giving the phase map.

The open architecture of the MultiView 4000 combined with the ability to scan the tip are critical elements to the setup.  The ability to keep the sample stationary to the input light while scanning the tip over the structure is a key feature. In this manner the electromagnetic field inside the waveguide is unaffected during the measurement.  This paper is one important example of the key role that NSOM and Nanonics are playing in the field of nano-photonic structure characterization.

For further reading-

Near field phase mapping exploiting intrinsic oscillations of aperture NSOM probe

Liron Stern, Boris Desiatov, Ilya Goykhman, Gilad M. Lerman, and Uriel Levy,

OSA, Optics Express, Vol. 19, Issue 13, pp. 12014-12020 (2011)

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