Observation of laser oscillation without population inversion in InGaAsP microdisk lasers

Pergamon

Solid State Communications, Vol. 91, No. 9, pp. 699-701, 1994 Elsevier Science Ltd Printed in Great Britain 0038-1098(94)$7.00+.00 0038-1098(94)00436-6

OBSERVATION OF LASER OSCILLATION WITHOUT POPULATION INVERSION IN InGaAsP MICRODISK LASERS

Bei Zhang, Ruo-Peng Wang, Xiao-Min Ding, Lun Dai, Shu-Min Wang

Physics Department and Mesoscopic Physics Labs, Peking University, Beijing 100871, P.g.China

(Received 5 June 1994 by Z.ZGan) We report the results on the microfabrication of optical pumped InGaAsP single quantum well microdisk lasers and the measurement of their emitting spectra. The laser oscillation without population inversion was observed at the liquid nitrogen temperature. Keywords: A. quantum wells, A. semiconductors, B. epitaxy, D. optical properties, E. luminescence Recently, the studies on semiconductor microcavity

output power in the lasing mode is much higher than that in

lasers have become a topic of increasing interest for both

other optical modes. By reason of the high spontaneous

theoretieians

emission coefficient, this lasing condition may be achieved

and

experimenters.

In

a

microcavity

even without population inversion in a microcavity laser 1.

semiconductor laser, due to the extremely small volume of the high-Q resonator, only a few optical modes are covered by the gain spectrum of the work material in the active

Several

kinds of high-Q microcavity have been

region of the lasers, and the spontaneous emission

introduced,

coefficient of the lasing mode may be much higher than that

demonstrated by McCall and coworkers ~3 are of particular

in a conventional laser. In a conventional laser, the laser

interest 4. As the optical mode structure of the microdisk

a m o n g them

the

microdisk

resonator

oscillation takes place when the optical loss, which includes

resonator is relative simple, so more precise analyses on the

the radiation loss and the internal loss, of the lasing mode is

photon emitting process are possible. Comparing with the

compensated by the optical gain, and is accompanied by a

Fabry-Perot mode microeavities, the microdisk resonator is

sharp change in the quantum efficient. The situation for a

easier to be realized experimentally, since the complexity of

microcavity laser is, however, somewhat different. Because

multilayer distributed Bragg reflector, which is necessary for

of the

the

obtaining of high quality factor in Fabry-Perot micro-

contribution of the spontaneous emission in the lasing mode

resonators, being avoided. The high quality factor is

optical output is no longer negligible, and

the emitting

achieved by the so called "whispering gallery (WG) mode"

quantum efficiency of a microcavity laser increases more

in the microdisk resonators. The optical wave in the WG

smoothly with the increasing of the pumping power. For this

mode can be thought as a narrow photon flux concentrated

reason, this kind of lasers are regarded as "thresholdless

near the disk edge, propagating along and reflected

lasers". This character of the microcavity lasers makes the

continually at large incident angle by the disk edge. The

criterion for laser oscillation being not so straight. In this

optical field of a WG mode is characterized by the factor of

letter, we talk about laser oscillation when the optical

exp(iM0), where M is the order of the WG mode. For high

high

spontaneous

emission

coefficient,

699

700

OBSERVATION OF LASER OSCILLATION

Vol. 91, No. 9

order WG mode, only a extremely small part of the optica!

lithography. The electron beam lithography was performed

flux may pass through the disk edge, so the high-Q factor is

in a modified NEC S-530 scanning electron microscope

achieved. The order of the WG mode is a increasing

system controlled by a personal computer. The selective

function of the microdisk's diameter, so the larger the

chemical etching processing in KKI 5 and HCI etchant was

diameter, the higher the quality factor, but the smaller

used. In Fig. 1 we present the scanning electron microscope

difference between adjoin modes at same time. To achieve

(SEM) image of the typical LPE InGaAsP microdisk lasers.

high spontaneous emission coefficient in a microdisk

The diameter of these microdisks is about 7 /am, and the

resonator made of InGaAsP lattice matched to InP, a

thickness is of 300 - 400 rim. The microdisks are covered by

diameter of several microns is most suitable. Due to the

a 90 nm thick SiO2 mask, and supported by a InP pedestal

symmetry of the microdisk resonator, the WG modes are

about 2 lam on a side and 2.0/am in high. Apparently, this

two-fold degenerated, in this case the upper limit of the

beautiful microdisk looks like a tiny thumbtack standing on

spontaneous emission coefficient is 0.5. In this letter, we

the InP substrate.

present the results on the microfabrication of InGaAsP/InP The

microdisk lasers and the observation of laser oscillations without population inversion in these microlasers.

Photoluminescence

(PL)

experiments were

carried out on these LPE InGaAsP microdisk array together

with

unprocessed

wafers.

During

the

PL

The microdisk lasers were fabricated from a single

experiments, the samples were cooled by liquid nitrogen and

quantum well (SQW)wafer grown by means of liquid phase

optically pumped by a 488.0 nm Ar laser beam. PL spectra

epitaxy (LPE) technique. An 1.5 -2.0 /am undoped InP

were measured by HRD 1 spectrometer using liquid nitrogen

buffer layer and a SQW structure was successively grown

cooled Ge detector. In Fig. 2, the curve (A) and (B) are the

on a (100) InP substrate. The SQW structure consisted of a

488.0 nm Ar laser , t ~ SQW lnGaAsP/InP excitation / ~

20-40 nm central InGaAsP layer with bandgap wavelength ;~ about 1.50/am bounded by two InGaAsP layers with ~,s

\ without resonator

//

at 1.25 /am and 1.03 /am on both sides sequentially. The total thickness of this SQW structure was in the range of 200-400 nm A 90 nm thick SiO2 layer was deposited on the

e.

top of the LPE wafer by chemical vapor phase deposition (CVD). The microdisks' patterns were produced by the

microdisks

conventional photolithography and the electron beam

-

~

I

I

I

1.2

1.3

1.4

1.5

1.6

Wavelength (/am) Fig.2 The photoluminescence (PL) spectra of the single quantum well InGaAsP/InP structure at 77K. Fig. 1 Scanning electron microscope image of a array of InGaA.sP microdisk lasers.

(A) PL spectrum of the unprocessed LPE wafer. (B) PL spectrum of microdisk lasers.

Vol. 91, No. 9

OBSERVATION OF LASER OSCILLATION

701

spectra of SQW InGaAsP microdisk array and the same, but

mode is much narrower than the natural spectral line width

unprocessed, epitaxial wafer, respectively. The full width

of the resonator that is determined by the resonator's quality

half-maximum value for the peak at 1.408 measured from

factor. The situation may be different in a micro-resonator,

curve (A) in Fig.2 is 80 nm, this is the case of the

where the laser oscillation is also possible

unprocessed wafer without the resonator effect, in case of

population inversion. The microdisk laser reported in this

the microdisks, the PL spectrum exhibits a sharp peak at

letter is one of the microlasers having this property. The

1.434 ~m, with A~, of 2 nm, as shown by the curve ('I3) in

quality factor of the microdisk at transparency carrier

Fig.2. In this case, the estimated effective quality factor of

density calculated by the effective refraction index method is

the WG mode was about 700. The emission intensity of the

above 104, corresponding a natural spectral line width AZ.

microdisks is about 60 times higher than that of the

smaller than 0.1 nm. The measured spectral line width is

unprocessed wafer. Approximately, the pumping power per

much greater than this value, this demonstrates that the

each microdisk was definitely less than 1 I,tw.

working material in the microdisk is still in a passive state,

without

that means no population inversion happened when the Generally, the spectral line width broadening is related

iasing operation took place in the microdisk lasers under

to the optical loss in the optical resonator by the relation:

optical pumping. The further studies on the optical modes

Ak/3.= a/k, where c~ is the optical loss and k the modules of

and the threshold behavior of the microdisk lasers are going

the wave vector. The optical loss in a resonator is mainly

to be reported in separate papers.

due to the internal absorption and the radiation loss. In a conventional laser, the contribution of the spontaneous

Aclmowledgments - - The authers would like to thank Z.J.

emission in the lasing mode is negligible, and the laser

Yang for providing the SQW wafers, they are also grateful

oscillation requests optical amplification, that means the

to Prof.L.Z.Zhang, B.R.Zhang and Prof. C.G.Zhang, J.Xiu,

population inversion. In this case the effective internal

H.Z.Zhang for their help in earring out the PL and SEM

absorption is negative, so the spectral line width of a lasing

measurements.

REFERENCES 1.

Y.Yamamoto, S.Machida, G.Bjork, Opt. Quantum

4.

(1993).

Electron., 24, S215 (1992). 2.

S.L.McCalI,

A.F.J.Levi,

R.ESlusher,

S.J.Pearton,

AppL Phys. Lett., 60, 289 (1992). 3.

A.F.J.Levi, R.E.Slusher,

S.L.McCalI, T.Tanbun-EK,

D.L.Coblentz, S.J.Pearton, Electron. Left., 28,1010 (1992).

Y.Yamamoto, R.E.Slusher, Physics Today, June, 66

5.

T.Kambayash, C.Kitahara, K.Iga, dpn. £ AppL Phys., 19, 79 (1980).