1.  Top DBR mirror replaced with CTF and chiral STF bilayers  The CTF (QWP) introduces a pi/2 retardance to compensate the polarization mismatch between the two reflectors.  Top DBR mirror replaced with CTF and chiral STF bilayers  The CTF (QWP) introduces a pi/2 retardance to compensate the polarization mismatch between the two reflectors. Towards Circularly-Polarized Light Emission from Vertical-Cavity Surface-Emitting Lasers Fan Zhang, Jian Xu and Akhlesh Lakhtakia* Department of Engineering Science and Mechanics, Penn State University, State College, PA, 16802 *akhlesh@psu.edu, Tel: (814)863-4319, Fax: (814)865-9974 Compact circularly polarized (CP) light sources have recently attracted wide attention for direct chip-level integration due to potential applications in the fields of optical information processing and data storage, optical communication, quantum computing, and bio/chemical detection. So, it is highly desirable to have on-chip CP light emitters with precise controls over CP handedness and wavelength. The authors report the development of a class of chiral-mirror-based vertical-cavity surface-emitting lasers (VCSELs). The advances in sculptured thin film (STF) technology will eventually lead to the development of a new family of CP photonic devices that are efficient, compact, and fully integrable into optical/optoelectronic chips for a wide range of applications of CP light. Introduction vv Vacuum pump Source Vacuum chamber Vapor Substrate Quartz crystal monitor CTF chiral STF Schematic of the basic system for PVD of STFs Schematics of depositions of CTFs and chiral STFs  Oblique angle deposition  A tilted and rotating/fixed substrate corresponds to chiral STF/CTF deposition.  Atomic self-shadowing (Low energy adatom diffusion). STF deposition RCP Right-handed chiral STF mirror RCP Right-handed chiral STF mirror RCP Conventional mirror LCP RCPLCP Conventional mirror An example of well-developed circular Bragg regime Difference between chiral STF mirror and conventional mirror  Circular Bragg phenomenon (CBP)  A well-developed CBP displays high selective reflection of CP light and is confined to a defined spectral regime.  Microcavity built with chiral mirrors  Chiral STF mirror: CP states preserved by reflection.  Conventional mirror: CP states NOT preserved, due to  shift. Chiral-mirror microcavity Schematic of the device  Device structure CP emission from QDs LEDs  Chiral mirrors: structurally left-handed STFs made of TiO 2 with the circular Bragg regime centered at 610 nm  Device characterization LCP and RCP emission spectra of the NQDs confined in the chiral-STF-based microcavity Measured reflectance spectrum of the microcavity device for incident LCP light  Spectrally: narrower FWHM; higher peak intensity; large discriminable difference between CP handedness.  LCP emission peak in good agreement with the position of spectral hole.  Large discriminable difference between CP handedness is persistent under different pumping light power.  Spatially: narrower emission angle (strongly directed normal to the surface). Polarization control in external cavity diode laser  System setup 1-Laser diode; 2-collimating lens; 3-Soleil Babinet Compensator; 4-Left-handed Chiral STF mirror  LD: one facet is coated for enhanced reflectivity; the other is antireflection-coated.  The fast axis of the intra-cavity QWP was aligned at 45°with respect to the polarization of the TE mode in the LD.  Chiral mirrors: left-handed STFs made of TiO 2 with the circular Bragg regime centered at 660nm.  System lasing behavior Light output energy as a function of driving current (Inset: spectrum of the LCP laser emission) Polar plot of the normalized analyzer transmission vs the angle between the optical axes of the analyzer and the Fresnel-rhomb retarder CP ratio=112 CP ratio=32  I th = 46 mA  LCP lasing output  Side-mode suppression ratio is 26 dB Bottom DBR mirror n-contact layer Active layer (MQWs) p-contact layer Top chiral STF m /2 cavity CTF (QWP) CP emission from VCSELs  Device design  Device characterization Reflectance spectra of the CTF and RH chiral STF bilayers (Inset: cross-section SEM image of the CTF and STF bilayers) Light output as a function of the pumping light energy (Inset: spectrum of the RCP lasing emission) References A. Lakhtakia and R. Messier. Sculptured Thin Films: Nanoengineered Morphology and Optics, SPIE Press (2005). F. Zhang, J. Xu, A. Lakhtakia, S. M. Pursel, M. W. Horn, and A. Wang, Appl. Phys. Lett. 91, 023102 (2007). F. Zhang, J. Xu, A. Lakhtakia, T. Zhu, S.M. Pursel, and M.W. Horn, Appl. Phys. Lett. 92, 111109 (2008). F. Zhang, Ph.D. Dissertation, Pennsylvania State University (2008). Acknowledgement The authors thank Sean M. Pursel and Dr. Mark W. Horn for providing help on initial STF depositions.