Multi-wavelength surface emitting quantum cascade laser based on equivalent phase
shift
J. C. Zhang, F. Q. Liu, D. Y. Yao, L. J. Wang, F. L. Yan, J. Q. Liu, and Z. G. Wang
Citation: Journal of Applied Physics 115, 033106 (2014); doi: 10.1063/1.4862649
View online: http://dx.doi.org/10.1063/1.4862649
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JOURNAL OF APPLIED PHYSICS 115, 033106 (2014)
Multi-wavelength surface emitting quantum cascade laser based
on equivalent phase shift
J. C. Zhang,a) F. Q. Liu,b) D. Y. Yao, L. J. Wang, F. L. Yan, J. Q. Liu, and Z. G. Wang
Institute of Semiconductors, Key Laboratory of Semiconductor Materials Science, Chinese Academy of
Sciences, Beijing, 100083, People’s Republic of China and Beijing Key Laboratory of Low Dimensional
Semiconductor Materials and Devices, Beijing 100083, China
(Received 9 October 2013; accepted 7 January 2014; published online 16 January 2014)
A novel surface emitting distributed feedback quantum cascade laser emitting around k 4.6 lm is
demonstrated by employing an equivalent phase shift (EPS) of quarter-wave (k/4). The EPS is
fabricated through extending one sampling period by 50% in the center of a sampled Bragg
grating. Single-lobed far-field radiation pattern with a low divergence angle of about 0.6 16.8
is obtained. Selective single-mode lasing with a mean side mode suppression ratio above 20 dB and
wavelength coverage range of 72 nm is achieved simultaneously on a single wafer only by
C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4862649]
changing the sampling period. V
I. INTRODUCTION
Since their invention in 1994, quantum cascade lasers
(QCLs) have undergone a rapid development and are now
well established as reliable semiconductor light sources
in the mid-infrared, far-infrared, and terahertz spectral
region.1,2 For chemical sensing and pollution monitoring,
however, there is a need for widely tunable single-mode surface emitting lasers. Since light generated in laser core is
transverse magnetic (TM) polarized, QCLs are not instinctively suitable for vertical cavity but in-plane configuration.
Second-order distributed feedback (DFB) gratings,3–5
photonic crystal (PhC) lattices,6,7 and ring cavity grating8,9
utilizing their first-order Fourier diffraction are capable
of surface emission with spectra and spatial purifications.
Among these designs, the fabrication of PhC patterns and
ring cavity grating experience a technologically challenging
in the device preparation. Therefore, the easy-to-fabricate
second-order DFB grating is preferable to realize QCLs with
both single mode and surface emission. An elegant and, up
to this point, unexplored method to obtain surface emission
is the fabrication of DFB QCLs utilizing a sampled second
order Bragg grating. This has the additional advantage that
the multi-wavelength surface emitting QCLs can be fabricated on a single wafer through changing the sampling
period. Only conventional holographic exposure combined
with the optical photolithography technology is needed,
leading to improved flexibility, repeatability, and costeffectiveness. In addition, the sampled Bragg grating with an
equivalent phase shift (EPS) of quarter-wave (k/4) should be
proposed in order to stabilize the lasing frequency and
achieve single-lobed far-field radiation pattern. Though this
k/4-EPS technology has been demonstrated for the edge
emitting devices at wavelengths around 1.5 lm,10 it was first
used for the surface emitting devices.
In our experiment, we designed and fabricated a
sampled Bragg grating with k/4-EPS in the transmission
a)
Electronic mail: [email protected].
Electronic mail: [email protected]
b)
0021-8979/2014/115(3)/033106/4/$30.00
spectrum with Fourier order of þ1st (positive first order) in a
surface emitting DFB QCL. Selective single-mode lasing at
different wavelengths with a mean side mode suppression
ratio (SMSR) above 20 dB is achieved simultaneously on a
single wafer by changing the sampling period. A wide wavelength coverage range of 72 nm is obtained in pulsed mode
at room temperature. Furthermore, single-lobed far-field
radiation pattern with a low divergence angle of about
0.6 16.8 is obtained.
II. DEVICE DESIGN AND FABRICATION
The EPS structure is demonstrated to realize various
physically realizable functions by a specially designed
sampled Bragg grating. The principle of the EPS structure
can be described briefly as follows:11
2pz
þ c:c;
(1)
DnðzÞ ¼ sðzÞ exp j
K
X
2mpz
sðz Þ ¼
;
(2)
Fm exp j
Z
m
where K is the period of the Bragg grating, s(z) is the sampling modulation as a periodic function, Z is the sampling
period, and Fm is the Fourier coefficient of the mth order
channel of the sampled Bragg grating. When a sampling
period increase DZ is applied to s(z) at z0, the index modulation of the mth order channel can be expressed as follows:
8
>
2pz
2mpz
>
> Fm exp j
þj
z z0
<
K
Z
(3)
Dnm ðzÞ ¼
>
2pz
2mpz
>
>
þj
jh
z > z0 ;
: Fm exp j
K
Z
where the phase h is obtained as
h ¼ 2mp
DZ
:
Z
(4)
When m 6¼ 0 and DZ is changed smoothly, an equivalent
chirp can be achieved. Especially, if one sampling period at
115, 033106-1
C 2014 AIP Publishing LLC
V
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033106-2
Zhang et al.
J. Appl. Phys. 115, 033106 (2014)
the center of the sampled Bragg grating is extended by 50%,
k/4-EPS will be obtained in the reflection bands with odd
Fourier orders. The formed reflection band at odd Fourier
orders is similar to the one created by the uniform grating
with a p phase shift structure.
The subtle grating design is to utilize the þ1st order
reflection of the sampled Bragg grating with k/4-EPS for
high performance surface emitting operation, which possesses three distinct features: (i) robust single-mode emissions at high current level, (ii) realize wide wavelength
tunability, and (iii) achieve single-lobed far-field radiation
pattern. For the surface emitting devices lasing at the þ1st
order wavelength, the sampling period Z can be precisely
designed and determined as12
Z¼
k0 ¼ neff K ;
(5)
k0 2
;
2neff ðkþ1 k0 Þ
(6)
where k0 is the Bragg wavelength of the base grating, kþ1 is
the þ1st order wavelength, and neff is the effective index of
the DFB mode. Thus, the lasing wavelength of surface emitting devices at the þ1st mode can be easily controlled by the
manipulation of the sampling periods Z. As is similar to
edge emitting DFB QCLs, the Bragg wavelength of surface
emitting devices is also set at the edge of the gain curve,
while the þ1st order wavelength is set around the peak of the
gain curve.13 The base second order Bragg grating period,
sampling period, and duty cycle (the ratio of the grating area
to the sampling period) are 1.38 lm, 13 lm, and 50%,
respectively. The neff of the DFB mode is about 3.16, and
then the resulting k0, kþ1 are 4.331 lm and 4.592 lm, respectively. Fig. 1 shows the calculated transmission spectrum of
the sampled second order Bragg grating with k/4-EPS based
on transfer matrix simulation.
The QCL wafer was grown on an n-doped (Si,
3 1017 cm3) InP substrate by solid-source molecular beam
epitaxy (MBE) based on a two-phonon resonance design, as
is similar to that in Ref. 14. The active core and waveguide
layers are identical to that in Ref. 13. The sampled grating
with k/4-EPS is then formed on highly doped InP cladding
layer by conventional holographic exposure combined with
FIG. 1. The calculated transmission spectrum of the sampled second order
grating with k/4-EPS.
the usual photolithography. The base second order Bragg
grating with a period of K ¼ 1.38 lm (duty cycle of 35%)
was fabricated using holographic lithography technique and
transferred by wet chemical etching to the depth of about
525 nm. The sampling periods (changing from 7 lm to
22 lm) were formed by the usual photolithography. Fig. 2
shows the scheme and scanning electron microscope (SEM)
images of the sampling grating with k/4-EPS. Then the wafer
was etched into double-channel waveguide laser with an
average core width of 11 lm using a photolithography and
nonselective wet chemical etching. The ridge structures were
passivated with 450 nm-thick SiO2 layers deposited by
chemical vapor deposition. The devices were then fabricated
with e-beam evaporated Ti/Au layer, followed by a
4 lm-thick electroplated gold layer. After thinned down to
about 120 lm, the Ge/Au/Ni/Au metal was deposited as substrate contact layer followed by metal lift-off leaving 150 lm
wide windows to allow emission of the first-order diffracted
output beams. The processed wafer was cleaved to a 2-mm
long laser bars and a high-reflectivity coating consisting of
Al2O3/Ti/Au/Ti/Al2O3 (200/10/100/10/120 nm) was deposited on both of the facet.
III. DEVICE CHARACTERIZATION
The lasers were mounted epilayer side down on copper
heat sinks with indium solder. The lasers were then wire
bonded, and mounted on a holder containing a thermistor
combined with a thermoelectric cooler (TEC) to monitor and
adjust the submount temperature. The emitted optical power
from the substrate of the laser was measured with a calibrated thermopile detector placed directly in front of the
laser. All measurements are taken under pulsed operation at
a duty cycle of 1% with 1 ls pulses.
Fig. 3 depicts the surface emission spectra of a DFB
QCL with sampling period Z ¼ 11 lm operated in pulsed
mode at different heat sink temperatures between 293 K and
333 K. The measurements were performed using a Fourier
transform infrared (FTIR) spectrometer with 0.125 cm1
resolution in rapid scan mode. It can be seen clearly that the
lasing wavelength locates accurately in the center of the þ1st
order Bragg modes, which demonstrates the effectiveness of
the design. Since the center Bragg wavelength is designed in
the edge of gain curve, the lasing at þ1st order Bragg mode
is easy to occur. Single-mode emission with SMSR about
20 dB in all tested temperature range is observed, which is
limited by our measurement setup. The peak emission tunes
FIG. 2. (a) Microscope (b) SEM images of the sampled grating with k/4EPS.
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033106-3
Zhang et al.
J. Appl. Phys. 115, 033106 (2014)
FIG. 3. Surface emission spectra of a DFB QCL with sampling period
Z ¼ 11 lm operated in pulsed mode at currents of 1.05 Ith for different heat
sink temperatures of 293–333 K. The inset of upper panel shows lasing spectra at different currents from 0.55 A to 0.85 A with a step of 0.1 A at 293 K.
The inset of lower panel shows light power-current characteristics of surface
emitting DFB QCL measured at 293 K in pulsed mode.
linearly with temperature from 4.576 lm to 4.594 lm as the
temperature changes from 293 K to 333 K. This corresponds
to a wavelength tuning coefficient Dk/DT ¼ 0.45 nm/K,
which is of the same order as those of complex-coupled
sampled grating edge emitting DFB QCLs operated in pulsed
mode.13 The inset of upper panel shows the lasing spectra of
the same device at different currents with a step of 0.1 A at
293 K. Single-longitudinal-mode operation to relatively high
current is expected. In fact, a certain amount of gain/loss
coupling is always present in a second order DFB laser. This
loss coupling should prevent the device from oscillation in
both stop-band modes. The inset of lower panel shows light
output-current characteristics of surface emitting DFB QCL
measured at 293 K in pulsed mode. The typical threshold
current density of the device is 2.59 kA/cm2 and the peak
output power is 60 mW.
Also we calculated the coupling coefficient of sampled
grating surface emitting DFB QCL based on finite-element
method. A 40 nm Ti layer was taken into consideration. For
the complex-coupled DFB QCL, the coupling coefficient can
be approximatively given by the equation15
j¼
p
Da
Dneff þ i ;
kB
2
order DFB QCLs can lift the degeneracy of the two gap
modes. This explains why we do not observe any instability
between the two gap modes or spectral sensitivity.
Radiation patterns for k/4-EPS grating devices were
measured at 293 K in pulsed mode with a mercury cadmium
telluride (MCT) detector at a distance of 30 cm from the
laser chip with a slit covered the detector window to limit its
lateral size. Measurement results from the Z ¼ 11 lm device
are shown in Fig. 4. The other k/4-EPS grating devices also
had similar radiation patterns. The single-lobed pattern with
a small beam divergence of about 0.6 full width at half
maximum (FWHM) in the direction along the waveguide is
observed, which is associated with the 2 mm wide longitudinal aperture. As is similar to p phase shift of uniform grating,
the selection of the mode is independent considerably on
manufacturing nonuniformities and residual facet reflections.
The implementation of defect at the central of cavity has
yielded single-lobed far-field behavior. On the other hand,
the beam is relatively divergent (16.8 ) in the perpendicular
direction due to a waveguide width that is of the order of the
wavelength.
The wavelength tunability of the device is also examined. Selective single-mode lasing with a different lasing
wavelength is achieved on a single wafer. In a different area
of this wafer, the sampling period is changed from Z ¼ 7 lm
to Z ¼ 22 lm, leading to other þ1st order Bragg wavelengths.
As is shown in Fig. 5, a wide wavelength coverage range of
72 nm (from 4.533 lm to 4.605 lm) is obtained at a heat sink
temperature of 293 K by changing sampling period. Robust
single-mode emissions with a mean SMSR above 20 dB are
maintained for all the wavelengths. The lasing wavelengths
of all the ten surface emitting lasers coincide with the
designed þ1st order modes of the corresponding sampled
second order Bragg gratings. The method of using þ1st order
mode as the lasing wavelength, combined with the formation
way of the sampled grating will make it easy to fabricate the
multi-wavelength lasers by changing the sampling period
on a single wafer instead of using the expensive and
(7)
where Dneff is the amplitude of the periodic modulation of
the real part of the effective index. The corresponding modulation of the absorption coefficient has an amplitude Da. In
the calculation, the sampled grating coupling coefficient is
simply given by the product of the coupling coefficient of
the uniform grating times the duty cycle of the superstructure.16 The calculated values for both Dneff and Da is
3.29 104 and 2.59 cm1, so the index-coupling coefficient
ji and gain-coupling coefficient jg can be estimated to be
2.26 cm1 and 1.29 cm1 in the complex-coupled waveguide, respectively. It can be seen that the certain amount
modulation of absorption coefficient present in the second
FIG. 4. Far-field radiation pattern measured from the Z ¼ 11 lm k/4-EPS
grating device at currents of 1.5 Ith. The low-divergence profile was obtained
in the direction along the waveguide and the wide-divergence profile was
obtained in the direction perpendicular the waveguide.
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033106-4
Zhang et al.
J. Appl. Phys. 115, 033106 (2014)
operation with a different lasing wavelength is achieved on a
single wafer only by changing sampling period. A wide
wavelength coverage range of 72 nm is obtained in pulsed
mode at room temperature. These devices can be integrated
into a surface emitting DFB array, which will be fabricated
easier and more cost-effective.
ACKNOWLEDGMENTS
This work was supported by the National Basic
Research Program of China (Grant Nos. 2013CB632801,
2013CB632803), National Natural Science Foundation of
China (Grant No. 61306058, 61274094), National Scientific
Instrument Developing Project of China (2011YQ1300180204). We acknowledge the contributions of Liang Ping and
Hu Ying in the device fabrication.
FIG. 5. Surface emission spectra of DFB QCLs with different sampling periods ranging from Z ¼ 7 lm to Z ¼ 22 lm operated in pulsed mode at a heat
sink temperature of 293 K.
1
TABLE I. Detailed values of the designed Z, k, and output power.
Laser
1
2
3
4
5
6
7
8
9
10
Sampling period Z (lm)
Wavelength k (lm)
Power (mW)
7
8
9
10
11
12
13
14
17
22
4.601
4.597
4.589
4.582
4.576
4.571
4.564
4.557
4.549
4.535
65
56
58
44
60
44
53
62
74
50
time-consuming electron beam lithography. The detailed
relation between sampling periods, lasing wavelengths and
peak output power is shown on Table I, respectively. The
output power of the ten lasers varied between 44- and
74-mW, respectively.
IV. CONCLUSIONS
In conclusion, we have demonstrated the first tunable
surface emitting DFB QCLs based on the k/4-EPS technology. The implementation of a central defect has yielded
single-lobed far-field radiation patterns. Single-mode
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