Chris Chase, James Ferrara, Vadim Karagodsky, Fanglu Lu, Yi
Rao, Tianbo Sun, Weijian Yang, Li Zhu
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
distributed Bragg reflectors (DBRs), which consist of thick stacks of
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) .
They are easy to integrate onto a vertical
cavity surface-emitting laser (VCSEL) and silicon photoncs.
News report on High-Contrast
Subwavelength Grating Mirrors fabricated at UC Berkeley
describe the HCG as a single layer of regular,
alternating stripes of semiconductor and air (or silica) with
HCGs work because different sets of mathematical functions (modes)
light's behavior outside of and inside the HCG, and as with other
interfaces, the HCG's front and back surfaces translate between the
The HCG's thickness is such
that all of the incoming light's modes destructively interfere at its
plane, in such a way that only evanescent modes of free space on the
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
broadband, reflecting over a range of wavelengths, and are sensitive to
incident light's polarization. We report on several devices we build
HCGs: a tunable-frequency VCSEL, a high-Q optical resonator, a
reflector, a lens and a hollow-core waveguide that uses an
redirect light rather than reflect it. Based on this unique structure, we report the fastest
long wavelength VCSELs
a high-Q optical resonator
a planar high focusing power
and a hollow-core waveguide
have demonstrated a HCG-based VCSEL that operates at 1550
which is a standard
wavelength for optical networks. For this relatively long wavelength,
reflectors need to be especially thick, which makes them difficult to
manufacture. Our design, in contrast, uses an HCG that can be grown in
epitaxy step and a low cost, proton-implantation current aperture to
mW of power continuously at room temperature. This promises simpler to
manufacture, more efficient, and lower-cost VCSELs for optical
proposed using a HCG as an optical lens and focusing
. 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
single layer of semiconductor, making the lens orders of magnitude
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
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
smaller, lighter weight, and lower cost.
core waveguides based on HCG have also been proposed.
Chip-scale long optical delay with low intrinsic loss is useful for
signal processors, RF filtering, optical buffers, and optical sensing.
 eliminates the core material and thereby minimizes high losses due
material absorption and scattering. Field intensity inside the HCG is
and this further decreases the loss compared with the traditional DBR.
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
period as well as air gap for the core and cladding region. These two
designs provide different reflection phases, and thus their effective
(as a slab waveguide) are different.
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.
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.
Chase, Y. Zhou, and C. J. Chang-Hasnain, “Size effect of high contrast
gratings in VCSELs,” Opt. Express, vol. 17, pp. 24002–24007, 2009.
Huang, Y. Zhou, and C. J. Chang-Hasnain, “Nano electro-mechanical
optoelectronic tunable VCSEL,” Optics Express, vol. 15, no. 3, pp.
Chase, Y. Rao, W. Hofmann, and C. J. Chang-Hasnain, “1550 nm high
contrast grating VCSEL,” Optics Express, vol. 18, no. 15, p. 15461,
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.
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.
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.
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.