By building a photonic crystal reflectance filter that incorporates material with tunable refractive index by the application of electric fields or by illumination with light from a laser, we can fabricate devices that can modulate the transmission of light at wavelengths from the ultraviolet through the infrared part of the electromagnetic spectrum.  The period of the photonic crystal is designed to acquire a reflectance peak around a desired center wavelength.  In addition, both 1D and 2D photonic crystals can be fabricated to design polarization dependent and independent devices, respectively.  Devices with these capabilities are potentially useful for optical signal modulation in the telecommunication field, video display, and protection of sensors and eyes from lasers.

We have demonstrated 1D photonic crystals that incorporate liquid crystals (LC), such as those used in LCD displays, that can selectively reflect a single wavelength around 650nm within a narrow tuning range by reorientation of the LC molecules using applied electric fields through optically transparent electrodes.  The reorientation of the LC produces a change in its refractive index, which can tune the reflectance peak 4nm with 60V of applied voltage.

We have also demonstrated 1D and 2D photonic crystals that incorporate azobenzene molecules that undergo a rapid transformation when illuminated by a laser to quickly move a reflected wavelength band around from one wavelength to another.  Devices were fabricated for both 650nm and 850nm reflection peaks.  The refractive index change of the azobenzene molecules can achieve tuning of the reflectance peak up to 60nm.  Using azobenzene LC with a fast response time, we achieved tuning speeds on the range of milliseconds for CW laser pumping.

 

References

1. “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. 261109-261111, 2007.
2. “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.
3. “Optically Tunable Photonic Crystal Reflectance Filter,” D. Dobbs and B.T. Cunningham, Applied Optics, Vol. 45, Issue 28, pp. 7286-7293, 2006.
4. “Integrated 2D photonic crystal stack filter fabricated using nanoreplica molding,” F. Yang, G. Yen, and B.T. Cunningham, Optics Express, Vol. 18, No. 11, p. 11846-11858, 2010.
5. “Enhanced quantum dot optical downconversion using asymmetric 2D photonic crystals,” F. Yang and B.T. Cunningham, Optics Express, Vol. 19, No. 5, p. 3908-3918, 2011.