Download Carrier plasma shift in GaInAsP photonic crystal point defect cavity

Carrier plasma shift in GaInAsP photonic
crystal point defect cavity
observed at l ¼ 1.536 mm with a full width at half maximum of
3.8 nm and the corresponding Q factor of 410.
T. Baba, M. Shiga, K. Inoshita and F. Koyama
input light
A 1.43-mm-GaInAsP photonic crystal slab with a point defect cavity
and line defect waveguides has been fabricated. A 1.5 mmwavelength light is inserted into the waveguide and the cavity resonant
spectrum observed. A 5.6 nm blue shift in the resonant peak arising
from the carrier plasma effect by photopumping is also observed.
These results will be applicable to all-optical switches and tunable
devices.
cavity
output spot
air
PC slab
output spot
10 µm
substrate
Fig. 2 Near field pattern of light at l ¼ 1.504 mm, propagating in
waveguide
Top view (left) and side view of output end (right)
Introduction: In recent years, applications of semiconductor photonic
crystal (PC) slabs have been actively explored, e.g. point defect lasers
[1], line defect waveguides [2], and resonant filters composed of point
and line defects [3, 4]. However, there have been no reports of
experiments on light control devices in PC slabs. Even apart from
PC slabs, opal PCs infiltrated with liquid-crystals [5] are limited
examples that show the possibility of such devices. A tunable PC with
the carrier plasma effect has been discussed theoretically [6].
Normally, the response time of this effect is no shorter than nanoseconds. However, in a point defect cavity in a PC slab, the carrier
relaxation is accelerated by the Purcell effect [7], so a response time
of the order of picoseconds is possible. In this Letter, we report the
fabrication of a filter structure of a point defect cavity and line defect
waveguides in a GaInAsP PC slab, and the evaluation of
the transmission characteristics with the carrier plasma effect by
photopumping.
Fabrication: An epiwafer was prepared with a 0.28 mm-thick 1.43 mm
GaInAsP layer on InP substrate. The PC had a triangular lattice
structure of airholes with a lattice constant a of 0.40 mm and an airhole
diameter of 0.17–0.28 mm. The fabrication process flowed as (i) Ti=Ni
evaporation, (ii) electron beam lithography, (iii) pattern transfer by CF4
inductively coupled plasma (ICP) and Ar electron cyclotron resonance
plasma etchings, (iv) hole opening by Cl2=Xe-ICP etching, (v) formation of input=output ends by cleavage, and (vi) formation of the
airbridge PC slab by HCl wet etching. A fabricated device is shown
in Fig. 1. The point defect cavity of four missing airholes is sandwiched by PC photonic bandgap mirrors with four airholes and singleline defect waveguides. The total length of the device depended on
cleaved positions and was typically 100 mm.
Fig. 1 Top-view of fabricated device
Measurements: Tunable laser light was controlled by transverseelectric polarisation and focused on the input end of the waveguide.
The near field pattern (NFP) of light propagation from the top and
lateral side of the sample was observed using vidicon cameras. The
NFP for a resonant wavelength l ¼ 1.504 mm is shown in Fig. 2. Only
around this wavelength did we observe scattered light at the cavity
and a light spot at the output end. Then we shadowed the sample
except for the output end using an aperture and detected the light
output using an optical power meter. The spectrum of the light output
observed by changing the source wavelength is shown in Fig. 3a.
(Here, the sample is different from that for Fig. 2.) A resonant peak is
Fig. 3 Spectra of light output (sample is different from that for Fig. 2)
a Effective pump power of 850 mW
b 13 mW
c No excitation
Next, CW laser light of 0.98 mm wavelength was irradiated in to
the cavity from the top using a lensed fibre with a spot diameter of
10 mm. Considering the slab thickness, an absorption coefficient of
20 000 cm1 for irradiated light and a surface reflectivity of 30%,
the absorbed power was estimated to be 30% of the irradiated power.
Spectra near l ¼ 1.536 mm under such light irradiation are shown in
Fig. 3b and c. The resonant peak blue-shifted by <1 nm for an absorbed
power of 13 mW and 5.6 nm for 850 mW.
Considering the surface recombination at airholes [7] and assuming a
carrier lifetime of 4 ns, a pump power of 850 mW corresponds to a
carrier density of 5.7 1017 cm3. It is expected to provide an index
change Dn of 3.9 103 due to the carrier plasma effect. For this Dn,
we calculated the resonant wavelength shift in the cavity to be 2.0 nm
by the finite-difference time-domain method. Thus the measured value
is 2.8 times larger than the theoretical value. One reason considered,
which can explain this difference, is the resonant absorption of
irradiated light in the PC slab. According to a three-dimensional
photonic band calculation of the PC slab [8], the G point of the
second band locates at a normalised frequency a=l 0.41, which
corresponds to l 0.98 mm for a ¼ 0.4 mm. Therefore, the guided
resonance effect [9] can strongly enhance the absorption of irradiated
light. If it realises the perfect absorption, the wavelength shift will be
6.6 nm, which is much closer to the measured value.
Conclusions: We have fabricated a filter structure of a point defect
cavity and line defect waveguides in a GaInAsP PC slab and observed a
resonant transmission spectrum. In addition, we have confirmed the
carrier plasma shift of the resonant peak by photopumping, which can
be explained by the resonant absorption effect. This result will be
ELECTRONICS LETTERS 16th October 2003 Vol. 39 No. 21
applicable to fast optical switches and tunable devices based on
PC slabs.
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Acknowledgments: This work was supported by Nano-Photonic and
Electron Devices Technology Project, and 21st Century COE program
of the Ministry of Education, Culture, Sports, Science and Technology, and by CREST project of Japan Science and Technology
Corporation.
# IEE 2003
Electronics Letters Online No: 20030930
DOI: 10.1049/el:20030930
4 July 2003
T. Baba, M. Shiga and K. Inoshita (Department of Electrical and
Computer Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogayaku, Yokohama, 240-8501, Japan)
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E-mail: [email protected]
F. Koyama (P&I Laboratory, Tokyo Institute of Technology, 4259
Nagatsuta, Midoriku, Yokohama, 226-8503, Japan)
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