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Research - Photonic Crystal Optical Modulators

Voltage and Laser-Tuned Optical Modulators

We are researching and engineering optical limiting filters to block laser light transmission for protection of eyes and sensors.  The optical limiting effect is possible through a resonant reflection phenomenon of a photonic crystal slab.  Highly efficient optical limiting filters are designed with reflections over a narrow range of the target wavelength. The devices are made on flexible plastic substrates and over large areas of several cm2 using a nanoreplica molding process.  This is crucial for protection of large area sensors and for mechanical integration onto non planar surfaces.  The photonic crystal slab requires only a single deposition step of a thin film which is a major advantage over conventional distributed Bragg reflectors.  The device architecture also facilitates an easy and simple way to incorporate a dynamic or agile behavior into the optical limiting filter.

We are able to electro optically tune the reflection wavelength of the optical limiting filter by controlling the molecular orientation of liquid crystals within the filter using applied electric fields.  This reorientation will result in a change of the refractive index of a layer in the photonic crystal slab, and the consequent change in the optical response is manifested as a shift of the reflection wavelength.  Using either DC or AC applied electric fields, reflection wavelength tuning is possible over a 10 nm range.

Unique azobenzene liquid crystal molecules are also employed to enable all optical tuning of the reflection wavelength.  These molecules are able to change their physical molecular structure and exist in a stable and meta-stable state.  Induced changes in the molecular states are possible through the absorption of light at appropriate wavelengths.  The associated refractive index change is responsible for the all optical tuning of the reflection wavelength.  We have demonstrated tuning over a 25 nm range using such devices. 

This project is funded through the Army Natick Soldier Systems Center in Natick, Massachusetts.

Photograph of electro-optic tunable optical limiting filter.

Transmission spectra of an optical limiting filter.  All optical tuning of reflection wavelength is achieved using pulsed Nd:YaG laser.

References

1. "Optically tunable guided-mode resonance filter," D. Dobbs and B.T. Cunningham, Applied Optics, Vol. 45, Issue 28, pp. 7286-7293, 2006.

2. "A voltage-tuned resonant reflectance optical filter for visible wavelengths fabricated by nanoreplica molding," F. Yang, G. Yen, and B.T. Cunningham, Applied Physics Letters, Vol. 90, p. 261109-26111

3. Fuchyi Yang, Gary Yen, and Brian T. Cunningham. Voltage tuned resonant reflectance optical filter for visible wavelengths fabricated by nanoreplica molding, IEEE LEOS Conference, October 2007.

4. "Optically tuned resonant optical reflectance filter," F. Yang, G. Yen, G. Rasigade, J. Soares, and B.T. Cunningham,  Applied Physics Letters, Vol. 92, No. 9, p. 091115, DOI: 10.1063/1.289071, 2008.

5. Fuchyi Yang, Gary Yen, and Brian T. Cunningham. Photonic Crystal Optical Limiting Filter, CLEOS/QELS, May 2008.