November 15, 2004 / Vol. 29, No. 22 / OPTICS LETTERS 2641
Low-threshold two-dimensional annular Bragg lasers
Jacob Scheuer
Department of Applied Physics, Mail Stop 128-95, California Institute of Technology, Pasadena, California 91125
William M. J. Green and Guy DeRose
Department of Electrical Engineering, Mail Stop 128-95, California Institute of Technology, Pasadena, California 91125
Amnon Yariv
Departments of Applied Physics and Electrical Engineering, Mail Stop 128-95, California Institute of Technology,
Pasadena, California 91125
Received July 29, 2004
Lasing at telecommunication wavelengths from annular resonators employing radial Bragg ref lectors is demon-
strated at room temperature under pulsed optical pumping. Submilliwatt pump threshold levels are observed
for resonators with 0.5 –1.5-wavelength-wide defects of radii
7
8 mm. The quality factors of the resonator
modal fields are estimated to be of the order of a few thousand. The electromagnetic field is shown to be
guided by the defect. Good agreement is found between the measured and the calculated spectra. © 2004
Optical Society of America
OCIS codes: 140.4780, 140.5960, 250.3140, 230.1480.
Ring resonators are versatile elements with various ap-
plications ranging from telecommunication and sens-
ing to basic scientific research.
1–5
During the past few
years, considerable effort has been focused on improv-
ing the quality factors (Q) of resonators and reducing
their modal volume (see, e.g., Refs. 5 and 6 and refer-
ences therein).
Recently we proposed a novel type of annular reso-
nator based on radial Bragg reflection.
7,8
These de-
vices, annular Bragg resonators (ABRs), offer smaller
dimensions than those of conventional resonators that
employ total internal ref lection while retaining a high
Q. In addition, such structures exhibit superior sen-
sitivity compared with conventional resonators for bio-
logical and chemical sensing applications.
9
This class
of resonators is closely related to the family of circular-
grating distributed Bragg ref lector resonators, which
also exhibit lasing patterns with low angular propaga-
tion coeff icients.
10,11
In this Letter we report on the observation of
photoluminescence and lasing from ABRs realized in
semiconductor material (see Fig. 1). The structure
consisted of a circumferentially guiding defect sur-
rounded by radial Bragg ref lectors. Because of the
circular geometry, the optimal layer widths required
to confine the light in the defect are not constant
but rather monotonically decreasing with the radial
distance. The widths of the layers are determined by
the zeros and extrema of the Bessel function of order
m, where m is the angular propagation coefficient
of the mode for which the device was designed. For
simplicity we label the separation between a zero and
the nearest extremum and between successive zeros
the quarter-wavelength and the half-wavelength, re-
spectively. Although these distances are not constant
across the device, their role in the construction of
the distributed reflector is similar to that of their
Cartesian counterparts.
12
The ref lector layers could
also be of higher Bragg order (i.e.,
3兾4l, 5兾4l, etc.,
in the same notation). Although such an approach
requires more Bragg layers to confine the light in
the defect (compared with a ref lector based on l兾4
layers), it facilitates the fabrication of the devices.
The defect width of an ABR is a multiple integer
of half-wavelength, meaning that its interfaces are
located at zeros of the field.
ABRs of several geometries and Bragg ref lector or-
ders were fabricated within a 250-nm-thick membrane
of InGaAsP with six 7.5-nm quantum wells positioned
at the center. After the ABR patterns were etched
into the active material, the original InP substrate
was removed and the membrane was transferred to
a sapphire plate by use of an ultraviolet-curable opti-
cal adhesive to improve the vertical confinement of the
electromagnetic field in the device.
13
Fig. 1. Scanning electron microscope images of the fabri-
cated ABRs: A, l兾2-wide high-index defect. The defect
is the slightly narrower sixth ring from the center, marked
by the arrow; B, 3l兾2-wide high-index defect; C, 3l兾2-wide
air defect; D, close up of the ref lector layers.
0146-9592/04/222641-03$15.00/0 © 2004 Optical Society of America
2642 OPTICS LETTERS / Vol. 29, No. 22 / November 15, 2004
To simplify the design and the modeling of the de-
vices we employed the effective index approximation
in the vertical dimension. The effective index of
the membrane was found to be approximately 2.8 for
the
H
z
polarization and 2.09 for the E
z
polarization.
Since the H
z
polarization is more confined than the E
z
polarization and the optical gain of the compressively
strained quantum-well structure used favors the H
z
polarization,
14
we optimized the radial structure to
this polarization.
8
To facilitate the fabrication of the devices we
adopted a mixed Bragg order approach.
8
The high-
index Bragg layers (n
eff
苷 2.8) were 3兾4l wide (second
order), and the low-index layers (n
eff
苷 1.0 for air gaps
and n
eff
苷 1.54 for adhesive-filled gaps) were l兾4
wide (first order). In addition to the relaxed fabrica-
tion tolerances, such a layer structure improves the
vertical confinement (larger material filling factor)
and induces efficient vertical emission. Although
the latter decreases the overall Q factor of the cavity,
it also permits simple observation of the intensity
pattern that evolves in the device.
15
Figure 1 depicts
scanning electron microscope micrographs of some of
the fabricated devices.
The near-field intensity pattern and the emitted
spectrum of the ABRs were examined at room tem-
perature under pulsed optical pumping. The pump
beam was focused through the transparent sapphire
substrate on the backside of the sample. Half of the
pump beam intensity was split by a 3-dB beam splitter
and was focused on a detector to obtain the pump
power. The vertical emission from the front side of
the sample was either focused on an IR camera to
obtain the near-field intensity pattern or coupled into
a multimode f iber to obtain the spectrum.
The spectrum emitted from the pumped, unpat-
terned, quantum-well layer structure consisted of
a wide peak centered at 1559 nm. When an ABR
was pumped, the emission characteristics changed
significantly. Although the specific details varied
from device to device, the overall behavior was similar.
Once a certain pump intensity threshold was ex-
ceeded, clear and narrow emission lines appeared in
the spectrum (see Fig. 2). As the pump intensity
was increased, the intensity of the emission lines in-
creased while broadening toward shorter wavelengths.
Increasing the pump power further resulted in the
appearance of additional emission lines.
Figure 2 depicts the emitted spectra from an ABR
for various pumping levels. The specific device con-
sisted of a half-wavelength-wide defect surrounded by
five and ten ref lection grating periods in the inner and
outer sides, respectively. The inset of Fig. 2 shows a
L
L curve of the same device, indicating a threshold
at P
th
苷 1.0 mW. Other devices exhibited even lower
threshold levels, the lowest being 0.6 mW.
At threshold, emission lines spaced approximately
14 nm apart appeared in the spectrum. At twice
the threshold level, two additional emission lines
appeared at l 苷 1.608 mm and l 苷 1.623 mm. At
P
pump
苷 2.4 mW three additional emission lines
appeared at l 苷 1.593 mm, l 苷 1.612 mm, and
l 苷 1.626 mm. Increasing the pump even further
resulted only in variation of relative intensities of the
emitted modes.
To understand the spectral characteristics, we used
a finite-difference time domain simulation tool to
model the device.
16
Figure 3 shows a comparison be-
tween the A, measured and the B, calculated spectra.
Good agreement was found between the measured
and the calculated spectra not only for the resonance
Fig. 2. Evolution of the emitted spectrum from the ABR
shown in Fig. 1A as a function of pump intensity. Inset,
L
L curve of the device.
Fig. 3. A, measured and B, calculated spectral responses
of the ABR shown in Fig. 1A. The D modes are confined
in the defect; the I modes are confined in one of the rings of
the internal ref lector; the M modes are not confined to a
single ring. FDTD, finite-difference time domain.
November 15, 2004 / Vol. 29, No. 22 / OPTICS LETTERS 2643
Fig. 4. IR image of the vertical emission from an ABR.
The lasing pattern corresponds to mode M
2
.
wavelengths but also for the relative amplitudes. It
should be noted that the measured spectrum is to some
extent compressed compared with the calculated one.
This is because the finite-difference time-domain
model does not account for the material dispersion
of the membrane. The f ield profiles of the various
modes can be classified into three distinct categories
(see Fig. 3A). The modes belonging to class D are
confined within the defect with different angular
propagation coefficients. The modes belonging to
class I are confined in the ring closest to the defect
from the internal side. These modes are actually
guided by total internal ref lection and are supported
by the structure because of the use of second-order
high-index layers. The rest of the modes, labeled M,
are not localized in a specific layer but are distributed
over several grating periods, peaking both in the
defect and in one of the rings of the internal ref lector.
Unlike the I class, the M family modes are conf ined
by Bragg ref lection. The angular propagation factors
of M
1
and M
2
are 41 and 31, respectively, correspond-
ing to effective indices of 1.48 and 1.12, defined by
n
eff
苷 ml兾2pr
eff
, where l and r
eff
are, respectively,
the resonance wavelength and the effective radius of
the mode. These effective indices are lower than the
refractive index of the adhesive filling the gaps, which
confirms that the radial confinement mechanism is
indeed Bragg ref lection.
Figure 4 shows the intensity pattern emitted from
the device at a pump level of 1.6 mW. The emitted
pattern consisted of two concentric and relatively
wide rings of light. The radius of the outer ring
corresponds to the radius of the defect, indicating that
the lasing pattern belongs to the M family.
In summary, we have demonstrated annular Bragg
lasers in a thin membrane of InGaAsP active semi-
conductor material. Lasing was achieved at room
temperature under pulsed optical pumping condi-
tions. Submilliwatt threshold levels were observed
for ABRs with 7
8-mm defect radii pumped by a
30-mm-diameter spot. Imaging the vertical IR emis-
sion from the devices indicated localization of the field
within the defect.
The authors thank Axel Scherer and Oskar Painter
for access to fabrication facilities and George Paloczi
and Reginald Lee for fruitful discussions. This work
was supported by the National Science Foundation,
the Defense Advanced Research Projects Agency,
and the U.S. Air Force Office of Scientific Research.
J. Scheuer’s e-mail address is koby@caltech.edu.
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