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University of California, Berkeley  
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High-Contrast Subwavelength Grating
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High-Contrast Gratings - thin, sub-wavelength gratings bounce light better

Students: Chris Chase, James Ferrara, Vadim Karagodsky, Fanglu Lu, Yi Rao, Tianbo Sun, Weijian Yang, Li Zhu

Highly reflective mirrors are one of the key fundamental optical elements in optics. Regular metallic mirrors aren't reflective enough at optical wavelengths due to absorption, so most designs rely on distributed Bragg reflectors (DBRs), which consist of thick stacks of different materials. Our group has developed an alternative that is two orders of magnitude thinner that can potentially replace all DBRs: the High-Index-Contrast Grating (HCG) [1]. They are easy to integrate onto a vertical cavity surface-emitting laser (VCSEL) and silicon photoncs.

ABC News report on High-Contrast Subwavelength Grating Mirrors fabricated at UC Berkeley

We describe the HCG as a single layer of regular, alternating stripes of semiconductor and air (or silica) with subwavelength periodicity. HCGs work because different sets of mathematical functions (modes) govern light's behavior outside of and inside the HCG, and as with other optical interfaces, the HCG's front and back surfaces translate between the two [2]. The HCG's thickness is such that all of the incoming light's modes destructively interfere at its back plane, in such a way that only evanescent modes of free space on the other side are excited. As a result, the light has no place to go but back, as a near-total reflection. Functionally, HCGs differ from DBRs in that they are broadband, reflecting over a range of wavelengths, and are sensitive to the incident light's polarization. We report on several devices we build using HCGs: a tunable-frequency VCSEL, a high-Q optical resonator, a shallow-angle reflector, a lens and a hollow-core waveguide that uses an HCG to redirect light rather than reflect it. Based on this unique structure, we report the fastest wavelength-tunable VCSEL [3, 4], long wavelength VCSELs [5], multiwavelength VCSELs [6], a high-Q optical resonator [7], a planar high focusing power reflector/lens [8] and a hollow-core waveguide [9].


We have demonstrated a HCG-based VCSEL that operates at 1550 nm [5], which is a standard wavelength for optical networks. For this relatively long wavelength, DBR reflectors need to be especially thick, which makes them difficult to manufacture. Our design, in contrast, uses an HCG that can be grown in a single epitaxy step and a low cost, proton-implantation current aperture to output >1 mW of power continuously at room temperature. This promises simpler to manufacture, more efficient, and lower-cost VCSELs for optical communications applications.


HCG Laser LIV and Spectrum

Planar Lens

We proposed using a HCG as an optical lens and focusing reflective element [8]. Regular lenses are usually made of glass and are thick and bulky. By using an HCG we can create a lens or focusing reflector using one extremely thin (~1 Ám), single layer of semiconductor, making the lens orders of magnitude thinner and lighter. The lenses can have numerical apertures as high as 0.96 with losses less than 0.2 dB. Also, the lens can be made out of silicon using standard semiconductor processing equipment, so it can be manufactured at a very low cost and integrated with other optical devices. This type of lens opens the door to a radically different lens in the applications such as CCDs, solar cells, microscopes, telescopes, and lasers, potentially making the whole systems smaller, lighter weight, and lower cost.

HCG Lens

Hollow core waveguide

Hollow core waveguides based on HCG have also been proposed. Chip-scale long optical delay with low intrinsic loss is useful for optical signal processors, RF filtering, optical buffers, and optical sensing. HCG-HW [9] eliminates the core material and thereby minimizes high losses due to material absorption and scattering. Field intensity inside the HCG is small, and this further decreases the loss compared with the traditional DBR. With the optimization of different HCG parameters, a 15 μm core HCG slab HW with propagation loss as low as 0.006 dB/m is designed. Lateral confinement can be achieved by choosing different HCG period as well as air gap for the core and cladding region. These two HCG designs provide different reflection phases, and thus their effective indices (as a slab waveguide) are different.

HCG Hollow Core Waveguide

Selected Publications:

  1. C. Mateus, M. Huang, Y. Deng, A. Neureuther, and C. Chang-Hasnain, “Ultrabroadband Mirror Using Low-Index Cladded Subwavelength Grating,” IEEE Photonics Technology Letters, vol. 16, no. 2, pp. 518-520, 2004.
  2. V. Karagodsky, F. G. Sedgwick, and C. J. Chang-Hasnain, “Theoretical analysis of subwavelength high contrast grating reflectors,” Optics Express, vol. 18, no. 16, pp. 16973-16988, 2010.
  3. C. Chase, Y. Zhou, and C. J. Chang-Hasnain, “Size effect of high contrast gratings in VCSELs,” Opt. Express, vol. 17, pp. 24002–24007, 2009.
  4. M. C. Huang, Y. Zhou, and C. J. Chang-Hasnain, “Nano electro-mechanical optoelectronic tunable VCSEL,” Optics Express, vol. 15, no. 3, pp. 1222–1227, 2007.
  5. C. Chase, Y. Rao, W. Hofmann, and C. J. Chang-Hasnain, “1550 nm high contrast grating VCSEL,” Optics Express, vol. 18, no. 15, p. 15461, 2010.
  6. V. Karagodsky, B. Pesala, C. Chase, W. Hofmann, F. Koyama, and C. J. Chang-Hasnain, “Monolithically integrated multi-wavelength VCSEL arrays using high-contrast gratings,” Opt. Express, vol. 18, pp. 694–699, 2010.
  7. Y. Zhou, M. Moewe, J. Kern, M. C. Huang, and C. J. Chang-Hasnain, “Surface-normal emission of a high-Q resonator using a subwavelength high-contrast grating,” Optics Express, vol. 16, no. 22, pp. 17282–17287, 2008.
  8. F. Lu, F. G. Sedgwick, V. Karagodsky, C. Chase, and C. J. Chang-Hasnain, “Planar high-numerical-aperture low-loss focusing reflectors and lenses using subwavelength high contrast gratings,” Optics Express, vol. 18, no. 12, pp. 12606-12614, Aug. 2010.
  9. Y. Zhou, V. Karagodsky, B. Pesala, F. G. Sedgwick, and C. J. Chang-Hasnain, “A novel ultra-low loss hollow-core waveguide using subwavelength high-contrast gratings,” Optics Express, vol. 17, no. 3, pp. 1508–1517, 2009.





Last Updated December 2010