High-quality-factor (Q~10^4-10^5) AlGaAs microdisks for atomic and semiconductor quantum dot cavity QED experiments are demonstrated. Cavities embedded with InAs/InGaAs dots-in-a-well exhibit room-temperature pulsed lasing with thresholds <17uW. Continuous wave operation at room-temperature is also observed

Barclay, Paul E.

Krishna, Sanjay

Painter, Oskar

Srinivasan, Kartik

Stintz, Andreas

Caltech Authors - Main

QME5
2005 Quantum Electronics and Laser Science Conference (QELS)
High-quality-factor AlGaAs optical microcavities for atomic Cs
and semiconductor quantum dot cavity QED experiments
Kartik Srinivasan, Paul E. Barclay, and Oskar Painter
Thomas J. Watson, Sr: Laboratory ofApplied Physics, California Institute ofTechnology, Pasadena, CA 91125, USA.
phone: (626) 395-6269,,fax: (626) 795-7258. e-mail: kartik@caltech.edu
Andreas Stintz and Sanjay Krishna
Centerfor High Technology Materials, University ofNew Mexico, Albuquerque, NM 87106, USA.
Abstract: High-quality-factor (Q - 104- 105) AlGaAs microdisks for atomic and semiconductor quan-
tum dot cavity QED experiments are demonstrated. Cavities embedded with InAs/IlnGaAs dots-in-a-well
exhibit room-temperature pulsed lasing with thresholds < 17uW. Continuous wave operation at room-
temperature is also observed.
© 2005 Optical Society of America
OCIS codes: (230.5750) Resonators; (270.0270) Quantum Optics: (140.5960) Semiconductor Lasers
Within the past year, demonstrations of high quality factor (Q), small mode volume (Veff) silicon photonic crystal
microcavities[ 1, 2] (Q - 4x 104,Veff _ (X/n)3) and microdisks[3] (Q - 5x 105,Vef 5(X/n)3) have indicated the po-
tential of semiconductor microcavities for chip-based quantum optics experiments, such as the recent demonstrations
of vacuum Rabi splitting in a microcavity-quantum dot system[4, 5]. Experiments integrating such microcavities with
chip-level magnetic nmicrotraps to deliver cooled Cs atoms to the cavity near-field for experiments within the strong
coupling domain of cavity quantum electrodynamics (cQED) have been proposed[6]. However, the Cs D2 transition
commonly used in cQED experiments is at a wavelength X -852 nm, where silicon is highly absorbing. The bandgap
of AlGaAs, on the other hand, can be tailored (as a function of Al content) so that it is transparent in this wavelength
region, and is thus a more suitable choice for such experiments. Furthermore, AlGaAs has the advantage over silicon
(and other materials such as silica, where ultra-high-Q, larger Veffn microtoroid cavities have been demonstrated[7])
in that quantum-confined structures such as quantum wells and dots can be incorporated during its epitaxial growth,
allowing for purely semiconductor cQED experiments[4, 5].
Here, we report on recent progress in creating high-Q AlGaAs microdisks in the - 850 nm (Q - 3.4x 104) and
1550 nm (Q - 1.5 x 105) wavelength bands, which compare favorably with the highest Q factors measured for AlGaAs
microcavities[8, 9, 4, 5]. The utility of such cavities has been demonstrated in their integration with quantum dots-in-
a-well (DWELL) epitaxies[ 10, 11] to create optically pumped, low threshold, room temperature quantum dot lasers at
X - 1.2 Mm. Laser thresholds of < 17 pW ofpeak external pump power have been achieved in disks containing a single
DWELL layer, representing over an order of magnitude decrease in comparison to recent experimental results[12].
The loss in these cavities is low enough that the lasers operate at thresholds approaching the transparency levels of the
DWELL material, previously only seen in broad-area stripe lasers with mm-scale cavity lengths[ I 1].
A-XI edestal
Fig. 1. Scanning electron microscope (SEM) images of microdisk cavities after the (a) SiN, etch, and (b)-(c) AlGaAs etch
and undercut.
Initial passive Q measurements are performed on d = 6pm diameter inicrodisk cavities fabricated in a 180 nm thick
A]0.3Gao.7As layer supported by a 1.5 ,um thick AIO.8Gao.2As pedestal (Fig. 1). The cavities are created through: (i)
electron beam lithography and subsequent reflow of the resist, (ii) SF6/C4F8 inductively-coupled plasma reactive ion
etching (ICP-RIE) of a deposited SixNY mask layer (Fig. I (a)), (iii) Ar-Cl2 ICP-RIE etching of the AIo03Gao.7As layer,
and (iv) wet chemical etching of the underlying Al 0o8Ga0.2As layer to form the supporting pedestal (Fig. I (b)-(c)). The
Si5N, etch step is particularly important, as any roughness in this mask layer is transferred into the underlying layers.
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Srinivasan, et al., High quality factor AlGaAs optical microcavities... QELS/2005Page2
This etch has thus been calibrated to produce as smooth a sidewall surface as possible (Fig. 1(a)), without particular
concern for the verticality of the etched sidewall. The subsequent Ar-Cl 2 etch is highly selective so that the angled
mask does not result in erosion of the AlGaAs sidewalls.
The Q factors of the cavity modes are directly probed using an optical-fiber-based evanescent coupling approach[2,
3]. In Fig. 2(a), we show the resonance of a microdisk mode in the 850 nm wavelength band. The linewidth of this
resonance (y), determined by a Lorentzian fit to the data, corresponds to Q - 3.4x 104, and is a minimum estimate
for the Q, as loading of the cavity due to the presence of the taper causes y to broaden past its cold-cavity value.
Similarly, in Fig. 2(b), we show a wavelength scan for one of the disks in the 1550 nm band. In this scan, we see a
number of resonances with Qs as high as 1.5x 105. We believe that the lower Q factors in the 850 nm band might be
due to incomplete removal of a GaAs cap layer, which will be much more strongly absorbing at lower wavelengths.
Further improvements in this step, along with etched sidewalls whose roughness is comparable to those in the high-Q
Si microdisks[3] indicate that Q> 105 at 850 nm should be achievable.
(a)°.°2 * (b)
0
E 0-975 4 t]i
r X K *, 0.998
Q03AX104 y5
* *0 Q-'13Q-1.5X1,
.* _ Q--3 0.996 --1 -5x1 8 iQ~109
,x,50.996 Q'1.5x105
O. 0.994853.14 853.18 853.22 853.26 1587.22 1587.26 1587.30 1587.34
wavelength (nm) wavelength (nm)
Fig. 2. (a) Normalized taper transmission for a 6 pm diameter microdisk in the 850 nm range. (b) Normalized taper transmis-
sion for a 6 pm diameter microdisk in the 1550 nm range.
The InAs/InGaAs DWELL system has proven to be a technologically important system due to its near-IR emission
wavelengths which approach the zero dispersion region of optical fibers as well as their superior performance in very
low threshold laser devices[l 1]. The epitaxy used here consists of one or three layers of InAs quantum dots embedded
in (1-3) In015GaO.85As quantum wells, which are in turn sandwiched between GaAs/AI03Gao.7As layers to create
a total waveguide layer that is 255 nm thick. This layer is grown on top of a 1.5 pm thick Al o.8GaO.2As sacrificial
layer, and the disks are fabricated using the process described earlier. Cavities (d=6 pm) are optically pumped at room
temperature using a pulsed 830 nm semiconductor laser, and the emitted laser light at 1.2 pm is collected by a
microscope objective and resolved by an optical spectrum analyzer (OSA). For both the 1- and 3-DWELL devices,
lasing action is observed for a few (-2-5) modes in a given microdisk, and the linewidth of the resonant modes (taken
at sub-threshold pump powers) are as narrow as the resolution limit of the OSA (inset of Fig. 3(a)). Fig. 3(a) shows
a typical light-in-light-out (L-L) curve for a 3-DWELL device pumped with a 300 ns period and 10 ns pulse width;
the estimated threshold value from this curve is
-22,uW. Similarly, Fig. 3(b) shows an L-L curve for a 1-DWELL
device. This material's ground state saturated modal gain is estimated to be 3.6 cm-' [I1], indicating that a minimum
cavity Q 4x 104 is required for this single layer of dots to provide enough gain compensation just to achieve lasing.
Lasing from the quantum dot ground states is indeed observed, and furthermore, the threshold of 16.4,uW (Fig. 3(b))
is significantly lower than the 750 uW threshold value recently reported for similarly sized microdisk quantum dot
lasers[12]. The thresholds we report are the peak pump power incident on the sample surface; the absorbed pump
power is conservatively estimated to be roughly 16% of this value. The equivalent threshold current density, useful for
comparing their performance to previously demonstrated broad area stripe lasers[11], can be estimated by knowing
the pump spot size (i- 16 um2) and assuming an internal quantum efficiency 1. From this calculation, we arrive
at estimated threshold current densities of -'15 and 11 A/cm2 for the 3- and 1-DWELL devices, respectively. In
comparison, the estimated transparency current densities in previous work are 20.1 and 10.1 A/cm 2, respectively[I 1].
The proximity of the demonstrated laser thresholds to these transparency values is due to the high cavity Q factors that
have largely reduced non-intrinsic losses from the system. Finally, we note that room temperature, continuous wave
(CW) lasing was observed for both 3- and 1-DWELL lasers (Fig. 3(c)), albeit at somewhat higher thresholds.
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Srinivasan, et al., High quality factor AlGaAs optical microcavities... QELS/2005 Page 3
(a) Y cb)/ ' (C)
lxs >. aXo.lo.10nmt .-...
aO 1983 11983 1199.3f1 10 15 20 25 30 35,il .a. 11983 Wavelength (nm) ' PeakEt PumpPowerp-(J ,W
ai 3DWELL disk X 1 DWELL disk
i)of10ns pulse width ,.Jf 10 ns pulse width I DWELL disk
o-..*4 3300 ns pulse period 300 ns pulse period CW Lasing
V , Pthreshold"22-1 jiW Pthreshold"16A4 .iW *; Pthreshold"5535 tW
20 60 100 140 20 60 100 140 20 60 100 140
Peak External Pump Power (gW) Peak External Pump Power (giW) Peak External Pump Power (jiW)
Fig. 3. L-L curves for: (a) 3DWELL inicrodisk laser The inset shows the sub-threshold spectrum of cavity mode with a
resolution limited linewidth. (b) IDWELL microdisk laser. The inset shows a zoomed-in near-threshold L-L curve obtained
by using an attenuator to achieve higher resolution when incrementing the pump beam power. (c) IDWELL microdisk laser
under CW pumping conditions.
In addition to the aforementioned results, a number of extensions of this work that are currently in progress will
be discussed. These include improvements in the Q of 6 pm microdisks at X 850 nm and investigations on smaller
diameter disks and on the high-Q, ultrasmall Veff photonic crystal cavities previously demonstrated in silicon[2]. Fur-
thermore, the advantage of using the optical-fiber-probing technique in experiments will be highlighted. In particular.
the fiber taper can serve not only as a probe to measure Q factors, but also as an input coupler that allows for light to be
efficiently transferred from an optical fiber into the wavelength scale mode volume of the cavity, where it can interact
with embedded quantum dots or introduced Cs atoms, and as a subsequent output coupler. This technique can greatly
improve upon the coupling efficiency to wavelength-scale microcavities such as disks and photonic crystals[ 13], which
typically do not have a radiation pattern that can efficiently be collected by free-space optics or a cleaved fiber[5]. This
is of particular importance in quantum optics experiments where the light intensity required for single-photon-based
experiments can be quite low, and is integral to recently proposed cQED experiments with cooled Cs atoms[6]. Pre-
liminary applications of this method towards fiber-based pumping and collection schemes for the microdisk DWELL
lasers described above will be examined. Finally, initial low temperature (T< 10 K) measurements of the DWELL
epitaxy and DWELL-containing microcavities will be discussed.
References
1. Y. Akahane, T. Asano, B.-S. Song, and S. Noda, "High-Q photonic nanocavity in a two-dimensional photonic crystal," Nature 425, 944-947
(2003).
2. K. Srinivasan, P. E. Barclay, M. Borselli, and 0. Painter, "Optical-fiber-based measurement of an ultrasmall volume, high-Q photonic crystal
microcavity,' Phys. Rev. B 70,081206(R) (2004).
3. M. Borselli, K. Srinivasan, P. Barclay, and 0. Painter, "Rayleigh scattering, mode coupling, and optical loss in silicon microdisks,' Appi.
Phys. Lett. 85, 3693-3695 (2004).
4. J. Reithmaier, G. Sek, A. Loffer, C. Hoffman, S. Kuhn, S. Reitzenstein, L. Keldysh, V. Kulakovskii. T Reinecke, and A. Forchel, "Strong
coupling in a single quantum dot-semiconductor microcavity system," Nature 432, 197-200 (2004).
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7. D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, "Ultra-high-Q toroid microcavity on a chip," Nature 421, 925-928 (2003).
8. B. Gayral, J. M. Gerard, A. Lemahtre, C. Dupuis, L. Manin, and J. L. Pelouard, "High-Q wet-etched GaAs microdisks containing InAs
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11. G. Liu, A. Stintz, H. Li. T. Newell, A. Gray, P. Varangis, K. Malloy, and L. Lester, "The Influence of Quantum-Well Composition on the
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High-quality-factor AlGaAs optical microcavities for atomic Cs

and semiconductor quantum dot cavity QED experiments

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High-quality-factor AlGaAs optical microcavities for atomic Cs and semiconductor quantum dot cavity QED experiments

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