IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 2, FEBRUARY 2004
359
Mode Characteristics of Semiconductor Equilateral
Triangle Microcavities With Side Length of 5–20 m
Qiao-Yin Lu, Xiao-Hong Chen, Wei-Hua Guo, Li-Juan Yu, Yong-Zhen Huang, Senior Member, IEEE, Jian Wang,
and Yi Luo
Abstract—Semiconductor equilateral triangle microresonators
(ETRs) with side length of 5, 10, and 20 m are fabricated by the
two-step inductively coupled plasma (ICP) etching technique. The
mode properties of fabricated InGaAsP ETRs are investigated experimentally by photoluminescence (PL) with the pumping source
of a 980-nm semiconductor laser and distinct peaks are observed
in the measured PL spectra. The wavelength spacings of the distinct peaks agree very well with the theoretical longitudinal mode
intervals of the fundamental transverse modes in the ETRs, which
verifies that the distinct peaks are corresponding to the enhancement of resonant modes. The mode quality factors are calculated
from the width of the resonant peaks of the PL spectra, which are
about 100 for the ETR with side length of 20 m.
Index Terms—GaInAsP–InP, microresonator, photoluminescence (PL), quantum wells.
I. INTRODUCTION
O
PTICAL microresonators have the potential applications
in the fabrication of microlasers and optical add–drop filters. Optical microdisk or microring lasers as typical semiconductor microcavity lasers have achieved great success [1]–[6].
Recently, we have numerically shown that the equilateral triangle microresonator (ETR) is a good choice for realizing semiconductor microlasers with directional emission [7]–[9]. In this
letter, we fabricated GaInAsP–InP ETRs by the inductively coupled plasma (ICP) etching technique, and studied the mode characteristics for the fabricated ETRs. Resonant peaks are observed
in the photoluminescence (PL) spectrum with the wavelength
spacings in good agreement with the analytical mode wavelength intervals. In addition, the mode quality factors are calculated from the width of the resonant peaks and the cavity losses
are estimated accordingly.
II. EXPERIMENTAL RESULTS
A laser wafer with the active region composed of six quantum
wells was grown by metal-organic chemical vapor deposition
(MOCVD) technique. The thickness of the InGaAsP quaternary (Q) quantum wells and the 1.2- m-Q barrier layers are
Manuscript received February 25, 2003; revised August 26, 2003. This work
was supported by the National Nature Science Foundation of China under Grant
60225011, and Major State Basic Research Program under Grant G2000036606.
Q.-Y. Lu, X.-H. Chen, W.-H. Guo, L.-J. Yu, and Y.-Z. Huang are with
State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China (e-mail:
[email protected]).
J. Wang and Y. Luo are with State Key Laboratory on Integrated Optoelectronics, Department of Electronical Engineering, Tsinghua University, Beijing
100084, China.
Digital Object Identifier 10.1109/LPT.2003.821063
Fig. 1. SEM pictures of the GaInAsP-InP ETR microcavities fabricated by
two-step ICP etching. (a) Oblique side view of an ETR with side length a
5 m. (b) Oblique side view of arrayed ETRs with side length a 20 m.
=
=
11 nm. To enhance the emission of the TM mode 0.3% tensile strain is introduced into the quantum wells. The active region is sandwiched by 80-nm-thick 1.1- m-Q confining layers,
and the upper layers consist of 1.7- m-thick InP cladding layer
and a p -InGaAs ohmic contact layer. An 800-nm SiN layer
was deposited by plasma-enhanced chemical vapor deposition
(PECVD) on the laser wafer as the mask for etching InGaAsP
and InP. After forming the triangle patterns on the photoresist
by the standard photolithography, we first transferred the patterns onto the SiN layer with the reaction gas of SF and then
etched InP–GaInAsP with the CH –Cl –Ar gas mixtures by a
Plasmalab100 ICP system. The etching depth is about 6 m
that is larger than the range of the lateral field distribution. The
scanning electron microscope (SEM) pictures of oblique side
view of fabricated ETRs are shown in Fig. 1, Fig. 1(a) for an
ETR with side length of 5 m and Fig. 1(b) for arrayed ETRs
with side length of 20 m. The etched pillars are nearly vertical and the surface at the bottom is smooth, however, the vertices of the ETRs are round, which is mainly distorted in the
1041-1135/04$20.00 © 2004 IEEE
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 2, FEBRUARY 2004
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140
a = 5 µm
40
10 µm
120
20 µm
100
Quality Factor
Intensity (a.u.)
50
30
20
(a)
a=5µm
10µm
80
20µm
60
40
10
0
1300
20
1350
1400
1450
1500
1550
0
1300
1600
1350
1400
1450
Fig. 2. PL spectra for GaInAsP/InP ETRs with side length of 5, 10, and 20 m.
1550
1600
120
(b)
a = 20µm
λ=1523 nm
110
Quality Factor
photolithography process. The ETR vertices are very difficult to
keep during the photolithography and the ICP etching process
especially for deep etching. The polymers formed in the etching
process with CH gas mixture help to protect the sidewalls and
result in smoother sidewalls in the lower layer than the upper
layer as shown in Fig. 1(a).
The fabricated ETRs were optically pumped by a 980-nm
semiconductor laser at room temperature (RT) under continuous
wave (CW) condition, with the pump beam focused onto the
ETR through a tipped single-mode fiber. The output spectrum
was collected by another tipped single-mode fiber and measured by an optical spectrum analyzer (OSA). The measured
5, 10, and
PL spectra for the ETRs with the side length
20 m are shown in Fig. 2, distinct peaks are observed in the
PL spectra and the peak wavelengths are not influenced by the
position and angle of the detection fiber relative to the ETR.
By Lorentzian fitting of the measured PL spectra, we can obtain the wavelength and width of each peak. In the next section, we will show that these distinct peaks are corresponding to
the resonant peaks of different longitudinal modes of the fundamental transverse mode in the ETR and the peak wavelength is
the mode wavelength. We can calculate the mode quality factor
as the ratio of the peak wavelength to the width and estimate the
cavity loss. In Fig. 3(a), the quality factors are plotted as functions of the mode wavelength at
5, 10, and 20 m. The
calculated mode quality factors are about 35 and 60 at
5
and 10 m, respectively, as the wavelength increases from 1300
to 1550 nm. The results indicate that the cavity loss is much
larger than the band-to-band transition absorption of the active
region at
10 m, so its quality factor varies a little from
1400 to 1580 nm. For the ETR with the side length of 20 m,
the quality factor increases from 101 to 142 as the wavelength
increases from 1505 to 1580 nm, which corresponds to the mode
loss decreasing from 1.8 10 to 1.2 10 cm . The mode
loss difference of 6 10 cm can be considered mainly from
the difference of the band-to-band transition absorption between
1505 and 1580 nm for the 20- m ETR. The increase of the mode
quality factor in the short-wavelength side may be caused by the
decrease of the cavity loss. Fig. 3(b) represents the mode quality
factors versus pumping intensity for the modes with wavelength
1488, 1505, and 1523 nm at
20 m. With the increase
of the pumping intensity the mode quality factor for
1505
nm increases from 86 to 106 and the corresponding mode loss
1500
Wavelength (nm)
Wavelength (nm)
100
1505 nm
1488 nm
90
80
0
4
8
12
16
20
24
28
Pump Intensity (a.u.)
Fig. 3. (a) Mode quality factors versus mode wavelengths for the ETRs with
side length of 5, 10, and 20 m. (b) Mode quality factors versus pumping
intensity for different longitudinal modes in an ETR with side length of 20 m.
decreases from 2.1 10 to 1.7 10 cm . The cavity loss
is too large for lasing in the ETRs. The imperfect vertices and
rough sidewalls greatly reduce the mode quality factors of confined modes in the ETRs.
III. COMPARISON WITH THEORETICAL RESULTS
In this section, the mode wavelength intervals obtained from
the PL spectra are compared with the theoretical results. The
mode wavelength of the confined modes in the ETR can be
written as [9]
(1)
where the phase shift for the TM mode is
(2)
is the side length of the ETR, is the effective index of the
guided modes, and and are transverse and longitudinal mode
numbers. The transverse and longitudinal propagation constant
and and the decay constant
satisfy
(3)
(4)
(5)
LU et al.: MODE CHARACTERISTICS OF SEMICONDUCTOR EQUILATERAL TRIANGLE MICROCAVITIES
80
IV. CONCLUSION
Experiment
Mode Spacing (nm)
70
In conclusion, we fabricated semiconductor equilateral triangle microcavities by the two-step ICP etching process. The
PL spectra of the ETRs were measured at room temperature
with the pump source of a 980-nm semiconductor laser. Distinct peaks are observed in the PL spectra for the ETRs with
side length of 5, 10, and 20 m. Comparing with the theoretical
results, we find that the distinct peaks in the PL spectra are resonant peaks of different longitudinal modes of the fundamental
transverse modes. In addition, mode quality factors and cavity
losses are estimated from the width of the resonant peaks.
Theory
60
a = 5 µm
50
40
10 µm
30
20 µm
20
10
1300
1400
361
1500
1600
Mode Wavelength (nm)
Fig. 4. Mode intervals versus mode wavelengths obtained from the PL spectra
and the theoretical formulae for the ETRs with side length of 5, 10, and 20 m.
where
is the wavenumber in vacuum. Since the
mode wavelength covers a wide wavelength range, the wavelength dependence of the mode refractive index should be accounted in calculating the mode wavelength. Assuming the effective index varying linearly with photon energy, we obtain the
0.8 eV from
mode effective index as
the group index obtained from the mode wavelength interval of
a ridge-waveguide Fabry–Pérot laser fabricated from the same
laser wafer. Based on (1)–(5) and the mode refractive index, we
calculate the mode wavelength and then obtain the mode wavelength interval as the function of mode wavelength. In Fig. 4, the
measured mode wavelength intervals versus mode wavelengths
are compared with the theoretical mode intervals for the ETRs
with the side length of 5, 10, and 20 m. The experimental mode
intervals agree very well with the theoretical results of the longitudinal mode intervals of the fundamental transverse modes.
This verifies that the distinct peaks of the PL spectra are associated with the resonant fundamental transverse modes in the
ETRs.
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