ThA6 9:45am-I 0:OOam

Far- and near-field observation of whispering gallery modes in 1.7-2.9 p m microdisk lasers

D. J. Shin, H.Y. Ryy S. H. Kim, H. G. Park, andY. H. Lee

Department of Physics, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea Phone : +82-42-869-2576, Fax : +82-42-869-2510, E-mail : s-djshin@nad.kaist.ac.kr

The whispering gallery modes(WGMs) in microdisk lasers have attracted great attention due to not only their

potential applications to photonic integrated circuits but also fundamental interests in the interaction of

electromagnetic waves and microcavities. For circular microdisks realized on semiconductor gain materials,

there have been series of experiments on the emission properties of WGMs in far- [l] and near-field [2] regimes.

However, in-plane emission of the WGMs and practical problem of pumping geomtry of microdisk lasers have

restricted all the experiments to access only part of the emission properties. In this paper, we present novel

measurements on the lasing modes in microdisk lasers, which provide conprehensive features on out-coupling

of WGMs in far- and near-field regimes. The far-field emission patterns are analyzed in polarization-resolved

twodimensional(2-D) angular distribution measurements in the full half space. The near-field emission patterns

are analyzed in spectrally resolved near-field scanning optical microscopy(NS0M).

Our microdisk lasers are built on a thin InGaAsP slab waveguide structure as shown in Fig. 1. The high-

index(n=3.4) active layer containing a compressively strained multiple quantum well(QW) is sandwiched by air

and a low-index(n=1.44) S i Q layer. The microdisks are then patterned on the wafer by electron beam

lithography and chemicaLassisted ion-beam etching(CA1BE). The experimental setups for the far- and near-field

measurements are shown in Fig. 2. In order to freely characterize the laser emission into the front side of the

laser structure, an external pumping laser beam is illuminated on the back side of the substrate. This back-side

pumping is allowed by transparency of the GaAs substrate at the 980-nm pumping laser wavelength. The

polarization-resolved 2-D emission patterns are obtained by scanning the detector over the surface of a sphere

centered at the microdisk laser. The spectrally resolved near-field images are obtained by scanning an uncoated

tapered fiber over the disk plane with shear-force feedback.

Fig. 3 presents far-field emission patterns of the microdisk laser whose diameter and thickness are 1.7 pm and

326 nm The polarization-resolved images show that the antinodes of the standing wave WGM manifest

themselves as TEpolarized directional emissions evenly distributed in the azimuthal directions. The number of

directional emissions experimentally identifies the azimuthal mode number of the WGM. Near-field images

obtained from the microdisk laser whose diameter and thickness are 2.2 pm and 230 nm are shown in fig. 4. The

antinodes of the WGMs are clearly observed in the spectrally resolved near-field images with super-resolution

beyond the diffraction limit. We observe that the standing wave WGM is spatially locked even under

perturbation of the probe scanning in close proximity to the microdisk

References [I] T. D. Lee, P. H. Cheng, J. S. Pan, R. S. Tsai, Y. Lai, and K. Tai, Appl. Phy. Lett. 72,2223 (1998).

[2] T. Baba, H. Yamada, and A. Sakai, Appl. Phys. Lett. 77,1584 (2000).

0-7803-71 05-4/01/$10.0002001 IEEE 66 1

q m epmy GaAs sub,

Fig. 2. Experimental setups for far-field(a)

and near-field(b) measurements. In (a), the

detector scans from 0" to 90" and from 0'

to 360' in 8 and @directions, respectively.

Fig. 1. SEM image(a) and slab

structure(b) of a fabricated

microdisk lase1

Fig. 3. Far-field emission patterns of the microdisk laser. (a) SEM image of the disk. (b) Total intensity. (c) I$

polarization. (d) 8 polarization. The data over the curved surface of a sphere (e, I$) are represented on a flat

surface (x, y) by a mapping, ~ = I T ( ~ + Y ' ) ~ . ~ and +Arg(x+yi). The image (c) and (d) are rendered in the same

vertical scale.

Fig. 4. Near-field images of the microdisk laser. (a) SEM image of the disk. 6) Below threshold, PL. (c)

Above threshold, lasing mode and PL, (d) Above threshold, spectrally resolved lasing mode. (e) Topography

of (d).

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