Jpn. J. Appl. Phys. Vol. 40 (2001) pp. L 875–L 877
Part 2, No. 8B, 15 August 2001c2001
The Japan Society of Applied Physics

Low-Threshold Continuous-Wave Lasing in Photopumped GaInAsP Microdisk Lasers
Masayuki F UJITA, Kousou T ESHIMA and Toshihiko BABA
Faculty of Electrical and Computer Engineering, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan(Received February 19, 2001; accepted for publication June 29, 2001)

We demonstrated continuous-wave lasing at room temperature in a photopumped GaInAsP microdisk laser fabricated by Cl2 /Xe
inductively coupled plasma etching. The minimum threshold pump power was as low as 30 µW. This value is 0.7 times thelowest threshold in the current injection device due to the uniform carrier distribution by photopumping. Higher thermal
resistance and odd-order azimuthal mode lasing as a result of a narrower pedestal and no upper post structure were observed.
KEYWORDS: GaInAsP, microcavity, microdisk, photopumped, semiconductor laser

flow rate. It was reduced to 3 sccm to suppress the appearanceof crystallographic planes caused by the excess chemical reaction. The disk shape with the InP pedestal, as shown in
the inset in Fig. 1, was formed by the selective wet etchingof the InP lower cladding. In the measurement, the device
was pumped by the 0.98-µm laser light. The emitted light
was detected by a multimode fiber and analyzed by an optical
spectrum analyzer. The effective pump power was estimatedby the formula Po · (a 2 /w2 ) · (1 − R) · (1 − e−αd ), where Po
is the irradiated power, a the disk radius, w the focused spotradius (∼ 10 µm in this experiment), R the reflectivity at thedisk surface (∼ 0.3), α the average optical absorption coefficient of the disk (∼ 2 × 104 cm−1 ),15) and d the disk thickness(∼ 250 nm).
Figure 1 shows the laser mode peak intensity versus effective pump power characteristics at RT under the cw condition. The lowest threshold pump power Pth is 30 µW forthe 3.0-µm-diameter device. The threshold power density is400 W/cm2 . Using the relation P/h̄ω = I /e0 with the angular frequency ω, the current I and the electron charge e0 , thethreshold power density can be converted to a current density of 520 A/cm2 . This value is 0.7 times the lowest threshold current density for the injection device,5) and is almost
the same as that evaluated for the injection device excluding

Laser Mode Peak Intensity [arb. units]

The microdisk laser has a sub-µm3 order small cavity witha high Q.1, 2) Therefore, ultralow-threshold lasing by the volume effect or zero-threshold lasing by a large spontaneousemission factor C, the coupling efficiency of spontaneousemission energy to a lasing mode, close to 13, 4) are expected.
Thus, a threshold current of 40 µA5) and a C factor of 0.16)
were achieved in current injection devices at room temperature (RT) under a continuous-wave (cw) condition. On theother hand, photopumped devices have also been studied bymany research groups.1, 7–13) They are attractive because of
their simple structure, easy fabrication and evaluation, andthe potential of a large-scale integration for all optical processings. However, RT cw lasing has not been achieved without special processes, e.g., wafer bonding and selective oxidation.9, 13) According to discussions in refs. 9 and 13, thisis due to the large thermal resistance in normal microdisks
exposed to the air. However, quantitative discussion on thismatter is very limited. The thermal resistance was estimatedto be 0.5 − 1.5 × 105 K/W for the injection device,14) but has
never been evaluated for photopumped devices. In addition, aserious problem of the above-mentioned processes is that they
degrade the active material and increase the threshold to themilliwatt order. Under such conditions, the threshold characteristic and the large C factor have not been fully investigatedyet. In this study, we realized ultralow-threshold RT cw lasing
in a photopumped device fabricated by the standard process
we developed for the injection device. For these devices, wecarefully investigated the lasing characteristic and the thermal resistance, and found some unique properties different
from those of the injection device. This paper describes theseresults, and finally discusses the key issues for their ultimate
performance.
In this experiment, we prepared an metal-organic vaporphase-epitaxy grown 1.55-µm GaInAsP-InP wafer with a 1%
compressively strained quantum-well (QW) active layer. The
thickness of each of the five quaternary (Q) QWs was 5 nm,
and that of 1.20-µm Q barrier layers was 12 nm. They were
sandwiched by 1.20-µm Q, 1.15-µm Q and 1.10-µm Q
cover layers, each of which was 30 nm thick. Cover layers
were expected to suppress the carrier dissipation at disk surfaces by the surface recombination. After drawing circular
dot patterns by the electron-beam lithography, vertical mesas
were formed by Cl2 /Xe inductively coupled plasma etching.
The key issues of this process are complete roundness, andsmooth and vertical side walls. The etching condition wasalmost the same as that described in ref. 5 except for the Cl20295 K, CW

2 = 3.0 m
3.1
3.0

3.31m

10 20 30 40 50 60 70
Effective Pump Power P [ W]80Fig. 1. Laser mode peak intensity versus effective pump power. Circles,
diamonds and squares denote disk diameter 2a = 3.0, 3.1 and 3.3 µm,respectively. Open and closed symbols represent a smaller pedestal device
with a disk wing of ∼ 1 µm width exposed to air and a larger pedestal
device with a disk wing of ∼ 0.5 µm width, respectively.

L 875

Jpn. J. Appl. Phys. Vol. 40 (2001) Pt. 2, No. 8B

the excess current caused by the nonuniform carrier distribution.5) The full width at half maximum (FWHM) of the
background spontaneous emission spectrum at 1.1 × Pth is
80 nm. This is comparable to or smaller than that observed
for the injection device recording the lowest threshold current
density. Considering the fact that FWHMs are generally determined by the pump level in semiconductors, the observed
FWHM indicates that the estimated threshold for the photopumped device is comparable to or lower than that of the injection device. This result is consistent with the above discussion. Figure 1 also suggests that a smaller disk diameterand a smaller pedestal result in a lower threshold due to thevolume effect and the reduction in light scattering loss, respectively. Regarding the scattering loss, the difference in Q
factor was observed in resonant spectra at the pump level forthe transparent carrier density; Q = 3000 for the 3.0-µmdiameter device with the smaller pedestal, while Q = 2000
for the 3.3-µm-diameter device with the larger pedestal.
Figure 2 shows lasing spectra of the 3.0-µm-diameter device. The lasing wavelength is 1610 nm. The mode peak intensity at 2 × Pth is 30 dB higher than the background spontaneous emission level. The linewidth at this pump level is0.3 nm, which is the resolution limit of the measurement. InFig. 2, the blueshift at the lower pump level due to the carrier effect and the redshift at the higher pump level due to thethermal effect are observed, as in the injection device.14) At
the higher pump level, the observed redshift λ/ P is 20–
50 nm/mW, where λ is the wavelength shift and P is theincrease of the pump power. The thermal resistance is givenby the expression ( λ/ T )−1 · ( λ/ P), where ( λ/ T )is the temperature dependence of the wavelength shift (typically 0.1 nm/K for GaInAsP). This estimation seems to be reasonable, since the temperature increase of the InP substrate isless than 0.1 times that of the disk, even considering the freecarrier absorption of the pump light in the substrate. FromFig. 2, it is estimated to be 2–5 × 105 K/W. This is higher
than 0.5–1.5 × 105 K/W for injection devices having almost
the same disk diameter. The reason considered is that the heatdiffusion in photopumped devices with a very small pedestal
is more difficult than in injection devices having a relatively
large pedestal, an upper post and a contact layer.
Figure 3 shows the C factor estimated by the careful curve

10 dB0.3 nm

Intensity

P = 60 W

3926141600

1605
1610
1615
Wavelength [nm]1620

Fig. 2. Lasing spectra against various pump powers. 2a = 3.0 µm. Resolution limit of measurement is 0.3 nm.

M. F UJITA et al.

100
Spontaneous Emission Factor C

L 876

2 =3 m
10-1

10-2

Fsp

Mode

m- sp

10-3-100-50
050
100
Detuning m- sp [nm]150

Fig. 3. Dependence of C factor on mode detuning against spontaneousemission peak. Open and closed symbols denote photopumped and current
injection devices, respectively. Triangles, circles, diamonds and squares
denote 2a = 2.7, 3.0, 3.1 and 3.3 µm, respectively.

fitting described in ref. 6. In this experiment, spectrally integrated laser mode power versus effective pump power characteristics were obtained from far below the threshold to above
threshold. Theoretical values were calculated from rate equations without the carrier diffusion effect.2) Here, we assumed
the logarithmic gain G = G 0 ln(N /N0 ) with G 0 = 150 cm−1and the transparent carrier density N0 = 1.8 × 1018 cm−3 , theradiative recombination coefficient B = 2 × 10−10 cm3 /s, theAuger recombination coefficient C A = 5 × 10−29 cm3 /s andthe modal index of 2.65. The curve fitting was performed forsome devices of nearly 3 µm in diameter. The small fluctuation of the diameter changed the modal wavelength and theC factor. The largest value is 0.03 for the photopumped device. However, Fig. 3 clearly shows that the experimental C
factor agrees with the theoretical curve obtained by the simple expression with the observed spontaneous emission spectrum,14) and is increased by the fine mode tuning to the spontaneous emission peak. All the lasing modes locate at longer
wavelengths than the spontaneous emission peak, since thegain peak generally redshifts against the spontaneous emission peak in semiconductors. Therefore, the maximum theoretical value of 0.2 can be obtained by making the modal
wavelength shorter than the gain peak.
Figure 4 shows the lasing wavelengths measured for various disk diameters. In injection devices, lasing always occurs by even-order azimuthal modes so that the scattering
loss at the edge of posts having the square crosssection isminimized.16) The finite-difference time-domain simulationof light indicates that the scattering loss at the post edge forodd-order azimuthal modes is larger than 10 cm−1 , while thatfor even-order azimuthal modes ≤2 cm−1 . However, the post
width cannot be reduced freely, since it disturbs the smooth
carrier diffusion. On the other hand, in uniformly photopumped devices with a sufficiently small pedestal, the scattering loss can be less than 1 cm−1 for any modes. In fact, the11th mode lasing is observed, as shown in Fig. 4.
In summary, we realized RT cw lasing in GaInAsP microdisk lasers by photopumping with a threshold power of30 µW. This low value is due to the uniform excitation of thedisk as well as the small disk diameter of ∼ 3 µm. The spontaneous emission factor was evaluated to be of 10−2 order. ThisJpn. J. Appl. Phys. Vol. 40 (2001) Pt. 2, No. 8B

M = 6 7 8 9 10 11 12 13 143 4 5 6151216

1.613171.5

1.4141815191L 877

RFTF 97P00103 and Research Fellowship #05045.

7 8 9 10 11

Modal Wavelength m [ m]1.7M. F UJITA et al.

23Disk Diameter 2a [ m]4Fig. 4. Lasing wavelength versus disk diameter. Symbols and lines indicate experiment and theory, respectively. Open and closed symbols denotephotopumped and current injection devices, respectively.

measurement indicated that a larger value more than 0.2 will
be possible by fine mode tuning to the spontaneous emission
peak. As expected, the thermal resistance was as high as the105 –106 K/W order. Thus, a low refractive index cover layer,
which maintains the strong optical confinement and accelerates the heat diffusion, will be necessary for the large-scale
integration of this type of device.17) In addition, we observed
the relatively weak selection of the lasing mode in this device.
For stable single-mode operation, we will discuss a modified
structure elsewhere.18)
This work was partly supported by The Grant-in-Aid#13305009 by the Ministry of Education, Culture, Sports,
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