Boston University

Optical Characterization and Nanophotonics Laboratory








Polarization Sensing with Resonant Cavity Enhanced Photodetectors

Alumni: Bora Onat, Gökhan Ulu


We created a method for sensing the linear polarization of light using Resonant Cavity Enhanced (RCE) photodetectors. The RCE detectors are constructed by integrating a thin absorption region into an asymmetric Fabry-Perot cavity. The top reflector is formed by the semiconductor air interface, while the bottom mirror is a Distributed Bragg Reflector (DBR). Quantum efficiency of these RCE devices can be controlled by tuning the cavity length by recessing the top surface of the detector. For off-normal incidence of light, the reflectivity of the semiconductor-air interface can be significantly different for TE (s) and TM (p) polarizations. A pair of monolithically integrated RCE photodetectors with cavity lengths tuned for resonance and anti-resonance provide a large contrast in response to TE and TM polarizations. An alternative polarization sensor can be formed by vertically integrating a conventional and a RCE photodetector. We have shown that a large contrast in the TE/TM responsitivities of the Vertical Cavity Polarization Detectors (VCPD) can be achieved, thus combining detection and polarization sensing in a single mesa semiconductor device. These devices alleviate the problems associated with the bulkiness and critical alignment constraints of the conventional sensors based on polarizing filters or splitters, and have potential for fabrication of monolithic smart pixels and imaging arrays.

Application Examples

Magneto-Optic Hard drives

In magneto-optical (M-O) drives, the content of the stored data is coded as a change in the polarization of light. The conventional M-O reading head configuration employ polarizing beam splitters and dedicated detectors for the two polarization components.

The use of bulk discrete optical components require individual alignment in three spatial and three angular coordinates with narrow tolerances resulting in increasing cost and limited range of applications. Another main consideration of this implementations is the problem of having heavy and bulky optical elements which limit access time of the data reading head. It is, therefore, strongly desirable to integrate polarization selectivity and detection in a single semiconductor device structure.

Utilizing the Vertical Cavity Polarization Detector (VCPD) for this application, the new M-O reading head configuration would provide great enhancement to the performance and cost.

Imaging Arrays

In imaging, the polarized light sensitivity is expected to underlie a visual quality similar to color vision that might permit the detection of objects that are otherwise blended in the background.

The scene above was taken by Cronin et. al., by mounting a polarization filter in front of a CCD camera. Although the sky and sea were actually blue in color, the highly polarized light reflected off the sea produces a strong contrast at the horizon line.


M. S. Ünlü, and B. Onat, "Polarization sensitive photodetectors and detector arrays ," United States Patent Issued, No. 5,767,507, 16 June 1998

B. Onat, G. Ulu, and M. S. Ünlü, "Compact polarization sensors with vertically integrated photodetectors," Proceedings of SPIE: Optoelectronic Integrated Circuits II, Vol. 3290, 28-30 January 1998, pp. 52-56

G. Robinson, "Detector Registers Polarized Light," EETimes, 10 March 1997

B. Onat, and M. S. Ünlü, "Vertical cavity polarization detectors," Proceedings of IEEE Lasers and Electro-Optics Society 1996 Annual Meeting, Vol. 2, 18-21 November 1996, pp. 236-237

B. Onat, and M. S. Ünlü, "Polarization Sensing with Resonant Cavity Enhanced Photodetectors," IEEE Journal of Selected Topics in Quantum Electronics, Vol. 2, No. 1, April 1996, pp. 135-140

M. S. Ünlü, and H. P. Zengingönül, "Polarization Sensing with Resonant Cavity Enhanced Photodetectors," Electronics Letters, Vol. 32, No. 6, 14 March 1996, pp. 591 - 593

M. S. Ünlü, B. Onat, H. P. Zengingönül, R. H. Henderson, and E. Towe, "Polarization Senisitive Photodetector Arrays," Proceedings of the International Semiconductor Device Research Symposium (ISDRS), December 1995

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