Broadband Emission in Quantum-Dash Semiconductor Laser

11
dash variation from different dash stacks. The light-current (L-I) curve of the short cavity
Qdash laser (L = 300µm) yields a J
th
and slope efficiency of 2.3 kA/cm
2
and 0.46 W/A,
respectively, as depicted in Fig. 7(a). Measuring the temperature dependent J
th
over a range
of 10-50 ºC, reveals the temperature characteristic (T
o
) of 41.3 K. On the other hand, the long
cavity Qdash laser (L = 1000µm) yields J
th
= 1.18 kA/cm
2
, slope efficiency of 0.215 W/A, and
T
o
of 46.7 K over the same temperature range, as shown in Fig. 8(a). Fig. 6. The lasing spectra show the changes of multi-state emission, from ground state (GS),
first excited state (ES 1) and second excited state (ES 2) of the 50 x 500 μm
2
broad area Qdash
intermixed laser, under different current injection of 1.1 x I
th

different temperatures. Up to ~340 mW total output power (from both facets) has been
measured at J = 4.0 x J
th
at 20ºC. (b) The progressive change of lasing spectra above
threshold condition.
lasing modes (Hadass et al., 2004), as can be seen in the lasing spectra of Fig. 7(b). In
addition, the observation of kink in the L-I curve for device tested at low temperature might
also be a result of mode competition in the gain-guided, broad area cavity devices. The
calculated Fabry-Perot mode spacing of ~1.1 nm is well resolved in the measurement across
the lasing wavelength span at low injection before a quasi-supercontinuum lasing is
achieved, where the spectral ripple is less than 1 dB.
Subsequent injections contribute to the stimulated emission from longer wavelength or
lower order subband energies while suppressing higher order subbands as shown in Fig.
7(b). This Qdash laser behavior is fundamentally different from the experimental
observation from Qdot lasers with short cavity length, where the gain of lower subband is
too small to compensate for the total loss, and lasing proceeds via the higher order subbands
(Markus et al., 2003; Markus et al., 2006). In short-cavity Qdash laser, the initial lasing peak
at shorter wavelength (~1525 nm) is dominantly emitted from different groups of smaller
size Qdash ensembles instead of higher order subbands of Qdash. Hence, the significant
difference of ~11 meV as compared to the dominant lasing peak of ~1546 nm at high
injection will contribute to photon reabsorption by larger size Qdash ensembles and seize
the lasing actions at shorter wavelength. Regardless, a smooth L-I curve at the injection
above 3 x J
th
due to the only dominant lasing modes at long wavelength demonstrates the
high modal gain of the Qdash active core (Lelarge et al., 2007). These observations indicate
that carriers are easily overflows to higher order subbands (Tan, et al., 2009) because of the
large cavity loss and the small optical gain (Shoji et al., 1997) at moderate injection. At high
injection, carrier emission time becomes shorter, when equilibrium carrier distribution is
reached and lasing from multiple Qdash ensembles is seized (Jiang & Singh, 1999).

th
, simultaneous two-state laser emission, which is
attributed to two groups of Qdash ensembles as mentioned previously, is noticed from short
cavity Qdash lasers. On the other hand, a broad linewidth laser emission from a single Fig. 9. The presence of different lasing Qdash ensembles with cavity length at the injection
of J = 2.25 x J
th
. The inset shows the progressive red-shift of lasing peak emission with cavity
length at the injection of J = 1.1 x J
th
.

Fig. 10. The effect of cavity dependent on quasi-supercontinuum broadband emission from
intermixed Qdash laser at an injection of J = 4 x J
th
.
Advances in Optical and Photonic Devices

14
dominant wavelength is shown in longer cavity Qdash lasers of 850 µm and 1000 µm, as
depicted in Fig. 9. As a result, a quasi-supercontinuum broad laser emission could be
achieved at high injection, as shown in Fig. 7. An ultrabroad quasi-supercontinuum lasing
coverage from Qdash devices with L = 500µm (Tan et al., 2008) results from emission in
different order of energy subbands and groups of ensemble, which will be discussed in the
following section.
The broad lasing spectra from devices with different L suggest there is collective lasing from
Qdashes with different geometries. However, the broad laser spectra of Qdash lasers
obtained at room temperature are different from that of Qdot lasers which shows similar

, there is only ground state lasing E
0
with the
wavelength coverage of ~10 nm [Fig. 11(b)]. The broad E
0
lasing spectrum suggests the
collective lasing from Qdashes with different geometries. At J > 1.5×J
th
, the bi-state lasing is
noted. The simultaneous lasing from both E
0
and E
1
is attributed to the relatively slow carrier
relaxation rate and population saturation in the ground state in low-dimensional quantum
heterostructures (Zhukov et al., 1999). The transition from mono-state to bi-state lasing is
marked with a slight kink in the L-I characteristics. The bi-state lasing spectrum is
progressively broadened with increasing carrier injection up to a wavelength coverage of 54
nm at J = 4.5×J
th
. The corresponding side-mode suppression ratio is over 25 dB and a ripple
measured from the wavelength peak fluctuation within 10 nm span is less than 3 dB.
Bangap-tuned broad area lasers with optimum cavity length (L = 500 μm) that gives largest
quasi-supercontinuum coverage of lasing emission, as presented in Fig. 10, are fabricated.
The L-I curve of the Qdash laser yields an improved J
th
and slope efficiency of 2.1 kA/cm
2

and 0.423 W/A, which is depicted in Fig. 12(a), as compared to that of as-grown laser with

~1 W per device at room temperature before any sign of thermal roll-over. This shows that
injection above 6 kA/cm
2
provides enough carriers for population inversion in all the
available or possible radiative recombination energy states and thus the energy-state-
hopping is absent. Fig. 12. (a) L-I characteristics of the 50 x 500 μm
2
broad area Qdash laser at different
temperatures. Up to ~1 W total output power has been measured at J = 5.5 x J
th
at 20ºC
before showing sign of thermal roll-off. (b) The lasing spectra above threshold condition that
are acquired by an optical spectrum analyzer with wavelength resolution of 0.05 nm.
Measuring the temperature dependence J
th
over a range of 10-60 ºC reveals the improved T
o

of 56.5 K as compared to the as-grown laser of 43.6 K (
b
Djie et al., 2007). This result is
Advances in Optical and Photonic Devices

16
comparable to the T
o
range (50-70 K) of the equivalent QW structure. In Fig. 12(b), only a

th
. Fig. 14. (a) Spaced and quantized energy states from ideal Qdot samples. (b) Large
broadening of each individual quantized energy state contributes to laser action across the
resonantly activated large energy distribution. (c) Variation in each individual quantized
energy state owing to inhomogeneous noninteracting quantum confined nanostructures in
addition to self broadening effect demonstrate a broad and continuous emission spectrum.
Broadband Emission in Quantum-Dash Semiconductor Laser

17
different geometries occur before a quasi-supercontinuum broad lasing bandwidth with a
ripple of wavelength peak fluctuation that is less than 1 dB is achieved. This idea can be
illustrated clearly in Fig. 14, when a peculiarly broad and continuous spectrum is
demonstrated from a conventional quantum confined heterostructures utilizing only
interband optical transitions. The effect of variation in each individual quantized energy
state owing to large ensembles of noninteracting nanostructures with different sizes and
compositions, in addition to self inhomogeneity broadening within each Qdot/Qdash
ensemble, will contribute to active recombination and thus quasi-supercontinuum emission.
5. Conclusion
In conclusion, the unprecedented broadband laser emission at room temperature up to 76
nm wavelength coverage has been demonstrated using the naturally occurring size
dispersion in self-assembled Qdash structure. The unique DOS of quasi-zero dimensional
behavior from Qdash with wide spread in dash length, that gives different quantization
effect in the longitudinal direction and band-filling effect, are shown as an important role in
broadened lasing spectrum as injection level increases. After an intermediate degree of
postgrowth interdiffusion technique, laser emission from multiple groups of Qdash
ensembles in addition to multiple orders of subband energy levels within a single Qdash
ensemble has been experimentally demonstrated. The suppression of laser emission in short

Djie, H. S.; Ooi, B. S.; Fang, X. –M.; Wu, Y.; Fastenau, J. M.; Liu, W. K. & Hopkinson, M.
(2007). Room-temperature broadband emission of an InGaAs/GaAs quantum dots
laser. Opt. Lett., Vol. 32, No. 1, (January 2007) 44-46
b
Djie, H. S.; Tan, C. L. ; Ooi, B. S.; Hwang, J. C. M.; Fang, X. –M.; Wu, Y.; Fastenau, J. M.; Liu,
W. K.; Dang, G. T. & Chang, W. H. (2007). Ultrabroad stimulated emission from
quantum-dash laser. Appl. Phys. Lett., Vol. 91, No. 111116, (September 2007) 111116
1-3
Djie, H. S.; Wang, Y.; Ding, Y. H.; Wang, D. –N.; Hwang, J. C. M.; Fang, X. –M.; Wu, Y.;
Fastenau, J. M.; Liu, A. W. K.; Dang, G. T.; Chang, W. H. & Ooi, B. S. (2008).
Quantum dash intermixing. IEEE J. Sel. Top. Quantum Electron., Vol. 14, No. 4,
(July/August 2008) 1239-1249
Garbuzov, D.; Kudryashov, I. & Dubinskii, M. (2005). 110 W (0.9 J) pulsed power from
resonantly diode-laser-pumped 1.6-μm Er:YAG laser. Appl. Phys. Lett., Vol. 87, No.
121101, (September 2005) 121101 1-3
Gmachl, C.; Sivco, D. L.; Colombelli, R.; Capasso, F. & Cho, A. Y. (2002). Ultra-broadband
semiconductor laser. Nature, Vol. 415, No. 6874, (February 2002) 883-887
Gontijo, I.; Krauss, T.; Marsh, J. H. & De La Rue, R. M. (1994). Postgrowth control of
GaAs/AlGaAs quantum well shapes by impurity-free vacancy diffusion. IEEE J.
Quantum Electron., Vol. 30, No. 5, (May 1994) 1189-1195
Hadass, D.; Alizon, R.; Dery, H.; Mikhelashvili, V.; Eisenstein, G.; Schwertberger, R.; Somers,
A.; Reithmaier, J. P.; Forchel, A.; Calligaro, M.; Bansropun, S. & Krakowski, M.
(2004). Spectrally resolved dynamics of inhomogeneously broadened gain in
InAs/InP 1550 nm quantum-dash lasers. Appl. Phys. Lett., Vol. 85, No. 23,
(December 2004) 5505-5507
Harris, L.; Mowbray, D. J.; Skolnick, M. S.; Hopkinson, M. & Hill, G. (1998). Emission spectra
and mode structure of InAs/GaAs self-organized quantum dot lasers. Appl. Phys.
Lett., Vol. 73, No. 7, (August 1998) 969-971
Jiang, H. & Singh, J. (1999). Nonequilibrium distribution in quantum dots lasers and
influence on laser spectral output. J. Appl. Phys., Vol. 85, No. 10, (May 1999) 7438-

Ooi, B. S.; Mcllvaney, K.; Street, M. W.; Helmy, A. S.; Ayling, S. G.; Bryce, A. C.; Marsh, J. H.
& Roberts, J. S. (1997). Selective quantum-well intermixing in GaAs-AlGaAs
structures using impurity-free vacancy diffusion. IEEE J. Quantum Electron., Vol. 33,
No. 10, (Oct 1997) 1784-1793
Ooi, B. S.; Djie, H. S.; Wang, Y.; Tan, C. L.; Hwang, J. C. M.; Fang, X. –M.; Fastenau, J. M.;
Liu, A. W. K.; Dang, G. T. & Chang W. H. (2008). Quantum dashes on InP substrate
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Popescu, D. P. & Malloy, K. J. (2006). Anisotropy of carrier transport in the active region of
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Sek, G.; Poloczek, P.; Podemski, P.; Kudrawiec, R.; Misiewicz, J.; Somers, A.; Hein, S.;
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Shoji, H.; Nakata, Y.; Mukai, K.; Sugiyama, Y.; Sugawara, M.; Yokoyama, N. & Ishikawa, H.
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type based upon Lord Rayleigh's ‘concave’ whispering gallery mode (WGM) for the
optoelectronic large-scale integration circuits (McCall et al., 1992). The above lasers were
however two dimensional (2D) WGM which is troubled with the well-known WGM light
spread problem. For the remedy of this problem, asymmetric WGM lasers of stadium type
(Nockel & Stone, 1997) were then introduced to control the spreading light beam. Quite
recently, a novel micro-cavity of limaçon shape has shown the capability of highly
directional light emission with a divergence angle of around 40-50 degrees, which is a big
improvement to the light spreading problem.(Wiersig & Hentschel, 2008)
On the other hand, when we employ a new micro-cavity of vertically reflecting distributed
Bragg reflector (DBR) structures added below and above quantum well (QW) planes, say a
few active 80Å (Al) GaAs QWs, a 3D toroidal cavity is formed giving rise to helix standing
waves in 3D whispering cave modes (WCMs) as shown Fig. 1 (Ahn et al., 1999). The
photonic quantum ring (PQR) laser of WCMs is thus born without any intentionally
fabricated ring pattern structures, which will be elaborated later. The PQR’s resonant light is
radiating in 3D but in a surface-normal dominant fashion, avoiding the 2D WGM’s in-plane
light spread problem. Bessel (J
m
)
field profile
Helical wave

Fig. 1. Planar 2D Bessel function WGMs vs. toroidal 3D knot WCM (Park et al., 2002). The
3D WCM is a toroid with a circular helix symmetry not reducible to the simple 2D rotational
symmetry
Advances in Optical and Photonic Devices

22

i
eI
π
φητ
×
+ (1)
N
1D
is the 1D transparency carrier density, τ the carrier lifetime, η the quantum efficiency,
and I
i
stands for internal loss (Ahn et al., 1999; Kwon et al., 2006). The PQR formulae are
now compared with the actual data in Fig. 3, which show quite an impressive agreement
except some random deviations due to device imperfections. For smaller diameters (
φ
) the
active volume decreases below 0.1
μ
m
3
, and with the cavity Q factor over 15,000. The
corresponding spontaneous emission coefficient
β
will become appreciable enough for
threshold-less lasing without a sharp turn-on threshold, which often occurs in the PQR
light-current analyses. As listed in Fig. 3, the wide-spread data suggest a fuzzy ring trend
growing as the device shrinks due to the growing leaky implantation boundary around the
implant-isolated holes, and the hole PQR threshold data are actually approaching the curve
B, whose formula is derived for the mesa by assuming that the Rayleigh region is now
nothing but a piece of annular quantum well plane of random recombinant carriers instead:

0.06
0.09
0.12
0.15
0.18
0.21
0.24
15
μ
m
12
μ
m
10
μ
m
9
μ
m
845 846 847 848 849 850
FWHM = 0.055 nm
I = 800
μ
A
D = 10
μ
m

Wavelength (nm)
FWHM,

that the Rayleigh region of quantum well planes is deeply buried beneath a few micron
thick AlAs/GaAs Bragg reflectors not accessible for direct observation. However, recent
experiments and modeling work on dynamic interactions between carriers and transient
field in a quantum well plane is a close case in point (Gehrig & Hess, 2004). It thus appears
that the transient quantum wire-like features considered here seem to persist within the
relevant time scale through thermal fluctuations. For an ensemble of carriers randomly
distributed in the regional quantum well plane of concentration 10
12
cm
-2
for instance, tens-
of-nm scale local field-driven drifts of given carriers to a neighboring imminent PQR site
should generate the proposed PQR ordering for an imminent recombination event of
annihilating electron-hole pairs. For example, one can imagine a transient formation of the
two separate Rayleigh rings instantly via light field-induced migration of random carriers
within the W
Rayleigh
region as schematically shown for curve A in Fig. 3. We expect the
standing waves in the Rayleigh region to give rise to a weak potential barrier for such a
dynamic electron-hole pair process, perhaps an opposite case of extremely shallow quantum
well excitons at room temperature where even the shallow barriers tend to assure at least
one bound state according to square well quantum mechanics.
3. Spatio-temporal dynamic simulation of PQR standing waves and carriers
Although it is limited to 2D cases, recent spatiotemporal dynamic simulation work in a
straight waveguide case (see Fig.5) faithfully reveals such a tangled but otherwise quantum-
wire-like ordering of recombinant carriers undergoing some picosecond-long exciton process,
consistent with the photonic quantum corral effect due to a strong carrier-photon coupling.
The images of several standing light-wave-like carrier distribution patterns within a 1 micron
wide quantum well stripe emerge, as a function of time from-5-to-8 psec after about 5 psec
chaotic regime as indicated along the horizontal time axis of 10 psec full range, shown in Fig. 6

For single mode lasers we have made non-conventional PQRs of hyperboloid drum shape
like Figs. 8 (a) and (b) (Kim et al., 2003) having a submicron active diameter with
φ
= 0.9 μm,
where as its top region of a few micron diameter serves as metallic contact area for electro
pumping. Figs. 8 (c) and (d) show the threshold data with a 0.46 Å linewidth exhibit the
smallest threshold of about 300 nA, (Yoon et al., 2007) observed so far among the injection
lasers of quantum well, wire, or dot type to the best of our, although the external quantum
efficiency observed right after the threshold is poor suffering from the soft lasing turn-on
behavior here.
Advances in Optical and Photonic Devices

26
845 846 847 848 849 850
m
15
m
13
m
11
m
9
m
7
m
5
m
3
m
1

uniformly injecting current on the device surface. The used epitaxial wafer structure of a p-
i(MQW: multi quantum well)-n diode was grown on an n-type GaAs (001) substrate by
metal-organic vapor-phase epitaxy. The structure consists of two distributed Bragg reflector
(DBR) mirrors surrounding the i-region of a one-λ cavity active region (269.4 nm thick)
including three GaAs/Al
0.3
Ga
0.7
As quantum well structures, tuned to yield a resonance
wavelength of 850 nm. The p- and n- type DBR mirrors consist of alternating 419.8 Å
Al
0.15
Ga
0.85
As and 488.2 Å Al
0.95
Ga
0.05
As layers, 21.5 periods and 38 periods respectively.
Figures 9(a) and (b) show scanning electron microscopy (SEM) images for top view and
cross section of 1M PQR hole laser array, respectively, whose SEM pictures exhibit a bit
rough cross section as compared with single device side walls in Figs. 9(c) and (d).
Photonic Quantum Ring Laser of Whispering Cave Mode

27

Fig. 9. (a) Top and (b) cross section SEM images of 1M PQR hole array (c) SEM micrographs
of mesa and hole type PQR structures.
Figure 10(a) shows the CCD images of the illuminant 1M PQR hole array near the
transparent current, 0.08 A (80 nA/cell) and near the threshold current, 0.7 A (700 nA/cell).

scatters, and low modulation frequencies less than MHz ranges. Although the LED
performances are improving, lasers can be the alternative answer with the usual GHz range
modulation capability. In particular, the PQR laser is an attractive candidate for next
generation display, based upon the special PQR characteristics as explained in the preceding
sections like extremely low threshold currents, thermally stable spectra, and high-density
chip capabilities. The PQR of WCMs can have both concave and convex modes, which are
the fundamental properties exploited for fabricating high power flower type PQR lasers as
elaborated in the end for display applications.
The high power PQR laser properties will now be presented to compare with conventional
LEDs, in terms of properties such as power-saving features, color purity, luminous
efficiency, and beam shape properties:
The spectral data for a conventional LED has a linewidth of about 25 nm which may be
reduced further down to several nm in the case of resonant cavity LEDs, while the linewidth
of the PQR is usually around or below 0.1 nm, as illustrated in Fig. 12, the spectra for a PQR
of
φ
= 7 μm. Namely, if the linewidths of the PQR and LED are about 0.1 and 25 nm,
respectively, the electric power consumption of the PQR is about 1/250 of the LED power
consumption. It means that the low threshold current and sharp discrete mode PQRs offer
high brightness as LED with much less amount of electric current because the sum of each
sharp peak can replace the broad peak of LED spectrum. The PQR’s color purity is about 1
which means high color rendering ability.
Fig. 13(a) shows the emission image of the 16x16 mesa type red PQR laser array. A single
red PQR emission reveals two different regions at a given injection current (I=24uA/cell).
The PQR lasing occurs in the periphery of the active disk called the Rayleigh band and the
Photonic Quantum Ring Laser of Whispering Cave Mode

29
LED emission occurs in the middle part of the disk. Luminous efficiency of the 16x16 red
PQR array is 7.20lm/w at the 670nm wavelength, which, if translated to 620nm with the

hard to achieve CW at room temperature.
On the other hand, we are making the blue PQR lasers which is CW operated at room
temperature lasing in 3D but emitting dominantly in surface normal direction. Our blue
PQR lasers with wavelengths between 420 and 470nm are fabricated using a GaN wafer
with sapphire substrates removed via laser lift-off (LLO) procedures (Fig. 14).
The multi mode lasing spectra from the blue PQR as shown in Fig. 15 and this tentative
result was reported in the reference (Kim et al., 2006). Fig. 14. Blue PQR array with the edge region affected by spontaneous background emission
(in red circle). Fig. 15. Multimode spectra from a blue PQR (in red circle in Fig. 14) CW at room
temperature with I = 60uA/cell to 1.63mA/cell.
7. PQR laser beam propagation characteristics
For 3D beam profile studies, we have used a home-built 2D/3D single photon scanning
system for measuring the PQR beam profile and polarization with a resolution of
0.5μm/step.