Appl. Phys. A 73, 125–127 (2001) / Digital Object Identifier (DOI) 10.1007/s003390100890
Applied Physics A
Materials
Science & Processing
Rapid communication
Strong enhancement of spontaneous emission in
amorphous-silicon-nitride photonic crystal based coupled-microcavity
structures
M. Bayindir
∗
, S. Tanriseven, A. Aydinli, E. Ozbay
Department of Physics, Bilkent University, Bilkent, 06533 Ankara, Turkey
Received: 8 March 2001/Accepted: 17 March 2001/Published online: 23 May 2001 – Springer-Verlag 2001
Abstract. We investigated photoluminescence (PL) from
one-dimensional photonic band gap structures. The pho-
tonic crystals, a Fabry–Perot (FP) resonator and a coupled-
microcavity (CMC) structure, were fabricated by using al-
ternating hydrogenated amorphous-silicon-nitride and hydro-
genated amorphous-silicon-oxide layers. It was observed that
these structures strongly modify the PL spectra from optically
active amorphous-silicon-nitride thin films. Narrow-band and
wide-band PL spectra were achieved in the FP microcavity
and the CMC structure, respectively. The angle dependence
of PL peak of the FP resonator was also investigated. We also
observed that the spontaneous emission increased drastically
at the coupled-cavity band edge of the CMC structure due to
extremely low group velocity and long photon lifetime. The
measurements agree well with the transfer-matrix method re-
sults and the prediction of the tight-binding approximation.
PACS: 42.70.Qs; 78.55.-m; 42.60.Da;78.66.Jg
Ability to control spontaneous emission is expected to have
practical importance in certain commercial applications.
Thus, in the past decade, photonic band gap materials were
proposed for alteration (inhibition and enhancement) of the
spontaneous emission from atoms [1–14].
Recently, we reported a new type of propagation mech-
anism in which photons move along the localized coupled-
cavity modes [15, 16]. Moreover, it was observed that the
group velocity tends to zero and photon lifetime increases
drastically at the coupled-cavity band edges [17]. In this pa-
per, we experimentally demonstrate the modification of spon-
taneous emission from the hydrogenated amorphous-silicon-
nitride active layers in a Fabry–Perot (FP) resonator and
a coupled-microcavity (CMC) structure.
Since the density of electromagnetic modes (ω) is modi-
fied by the surrounding environments, the spontaneous emis-
sion from atoms can be controlled by placing the atoms inside
∗
Corresponding author.
(Fax: +90-312/266-4579, E-mail: bayindir@fen.bilkent.edu.tr)
cavities. The spontaneous emission rate is directly propor-
tional to the photon density of modes via Fermi’s golden rule:
Γ
s
∝ (ω) ∝ 1/v
g
[6]. Thus, it is expected that spontaneous
emission from a CMC structure can be enhanced by a low
group velocity.
Our structures were composed of alternating hydro-
genated amorphous-silicon-nitride (Si
3
N
4
) and hydrogenated
amorphous-silicon-oxide (SiO
2
) multilayers [18]. The SiO
2
and Si
3
N
4
layers were deposited on glass and silicon sub-
strates by plasma-enhanced chemical vapour deposition
(PECVD) at 250
◦
C. Nitrogen (N
2
) balanced 2% silane
(SiH
4
), pure ammonia (NH
3
) and nitrous oxide (N
2
O) were
used as the silicon, nitride and oxide sources, respectively.
The refractive indices and thicknesses of layers were n
SiO
2
=
1.46, n
Si
3
N
4
= 1.98, d
SiO
2
= 124.8 nm, and d
Si
3
N
4
= 92.0nm.
The λ/2(d
cavity
= 184 nm) cavities were deposited with an in-
tercavity distance Λ = 4.5 pairs. The structure of the sample
and experimental setup are shown in Fig. 1.
Spontaneous Emission
Laser
Ar
+
N
34
Si
Spectrom
eter
θ
Λ
SiO
2
Fig. 1. Schematic of a coupled-microcavity structure and the experimental
setup for measuring the photoluminescence spectra
126
The room temperature photoluminescence (PL) measure-
ments were performed using a 1 m double monochromator,
equipped with a cooled GaAs photomultiplier tube and stan-
dard photon counting electronics, at θ = 0
◦
with respect to
the surface normal and with a spectral resolution of 2 nm. An
Ar
+
laser operating at 488 nm with 120 mW output power
was focused with a 15 cm focal-length cylindrical lens on the
sample. The transmission spectrum was taken by an Ocean
Optics S2000 fiber spectrometer.
First, we fabricated a FP microcavity which consisted of
16 λ/4-thick Si
3
N
4
/SiO
2
pairs and a λ/2-thick Si
3
N
4
cav-
ity layer (see the inset in Fig. 2a). The measured transmission
characteristics are displayed in Fig. 2a. We also plot the PL
spectra of the FP microcavity (solid line) and a single Si
3
N
4
layer (dotted line) in Fig. 2b. In Fig. 2b, the PL spectrum of
the Si
3
N
4
layer was multiplied by a factor of five. As shown
in Fig. 2b, the PL spectrum was strongly modified in the pres-
ence of the FP structure. We achieved a narrow-band PL peak
at wavelength λ = 722 nm. Recently, similar observations
have been reported by other scientists [13, 14].
We also measured the PL spectra at different collecting
angles θ (see the inset in Fig. 3); they are plotted in Fig. 3.
We observed that the resonance wavelength was shifted to-
wards lower wavelengths (blue-shift), and the peak intensity
decreased significantly as we increased θ.
Next, we fabricated a CMC structure (see Fig. 1 for
the schematics of this structure) having 36 Si
3
N
4
/SiO
2
pairs and four Si
3
N
4
cavity layers. Figure 4a shows the
measured (solid line) and calculated (dotted line; using
the transfer matrix method, TMM [19]) transmission char-
acteristics of the CMC sample with four cavities. Nearly
100% transmission was achieved throughout the CMC
band. We observed that (a) spontaneous emission was en-
hanced at the photonic band edge [6], (b) a strong enhance-
ment of spontaneous emission was achieved for a wide
0.01
0.10
1.00
Transmittance
500 600 700 800 900
Wavelength (nm)
0.0
1.0
PL Intensity (arb. units)
Microcavity
Bulk Si
3
N
4
x5
Fig. 2. a Measured transmission spectrum of a hydrogenated amorphous-
silicon-nitride Fabry–Perot (FP) microcavity. Inset: Schematics of the FP
microcavity structure. b Measured photoluminescence from the hydro-
genated amorphous-silicon-nitride thin film (dotted line) and FP micro-
cavity (solid line). The photoluminescence spectrum was significantly
modified
500 600 700 800 900
Wavelength (nm)
0
0.5
1
PL Intensity (arb. units)
θ=0
o
θ=10
o
θ=15
o
Si
Ar
+
Laser
θ
)
Spectrometer
Microcavity
Fig. 3. Measured photoluminescence intensity as a function of wavelength
for various collecting angles, θ. Inset: Schematics of experimental setup for
measuring the photoluminescence spectrum
0.01
0.10
1.00
Transmittance
Experiment
TMM
500 600 700 800 900
Wavelength (nm)
0.0
1.0
2.0
PL Intensity (arb. units)
Fig. 4. a Measured (solid line) and calculated (dotted line) transmission
through the SiO
2
/Si
3
N
4
coupled-microcavity (CMC) structure. Nearly
100% transmission was achieved throughout the cavity band extending from
690 to 770 nm. b Measured photoluminescence from the CMC structure.
The photoluminescence spectrum was modified, and enhanced significantly
at the cavity band edge
range of wavelengths (cavity band) extending from 690
to 770 nm, and (c) the spontaneous emission was sig-
nificantly enhanced at the coupled-cavity band edge. It
is important to note that the spontaneous emission dis-
played an oscillatory behavior near the edge of photonic
band gap.
In conclusion, we investigated photoluminescence from
hydrogenated amorphous-silicon-nitride Fabry–Perot micro-
cavity and coupled-microcavity structures. We observed that
the spontaneous emission spectra can be altered (inhibited
or enhanced) using these structures. It was also observed
that a strong enhancement of spontaneous emission can be
achieved throughout the coupled-cavity band. These results
open up a variety of possibilities in optoelectronic applica-
127
tions, such as coupled-cavity broadband high brightness light-
emitting devices.
Acknowledgements. This work was supported by NATO Grant No.
SfP971970, National Science Foundation Grant No. INT-9820646, Turkish
Department of Defense Grant No. KOBRA-001, and Thales JP8.04.
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