A two-dimensional photonic crystal defined hexagonal disk laser which relies on Bragg reflection rather than the total internal reflection as in traditional microdisk lasers is described. The devices are fabricated using a selective etch to form free standing membranes suspended in air. Room temperature lasing at 1650nm for a 150nm thick, ~15μm wide cavity fabricated in InP/GaAsP is demonstrated with pulsed optical pumping

Kitzke, B.

Lee, R. K.

Painter, O. J.

Scherer, A.

Yariv, A.

English

Caltech Authors - Main

laser to longer wavelengths resulted in CW rather than mode- 
locked operation. 
In conclusion, we have demonstrated the first passively mode- 
locked diodepumped bulk Er3+:Y3+:glass laser. We obtained 
shorter pulses and significantly higher pulse energies than previ- 
ously obtained with actively modelocked bulk Er3+:Y3+:glass lasers 
[41. 
Acknowledgment: This research was supported by the Swiss Prior- 
ity Program in Optics II. 
0 IEE 1999 
Electronics Letters Online No: 19990379 
DOI: 10.1049/el:I9990379 
G.J. Spiihler, L. Gallmann, R. Fluck, G. Zhang and U. Keller 
(Ultrafast Laser Physics, Institute of Quantum Electronics, Swiss 
Federal Institute of Technology, ETH Honggerberg - HPT, CH-8093 
Zurich, Switzerland) 
E-mail: spuehler@iqe.phys.ethz.ch 
14 January 1999 
L.R. Brovelli and C. Harder (Uniphase Laser Enterprise, Binzstrasse 
17, CH-8045 Zurich, Switzerland) 
P. Laporta (I.N.F.M., Politecnico di Milano, Dipartimento di Fisica, 
Piazza Leonardo da Vinci 32, 20133 Milano, Italy) 
References 
5 
6 
7 
8 
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bulk erbium-ytterbium glass laser at 1.5 microns’, IEEE Photonics 
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KELLER, U,: ‘Q-switching stability limits of continuous-wave passive 
mode locking’, J. Opt. Soc. Amer. B, 1999, 16, pp. 46-56 
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slow and fast saturable absorbers - What’s the difference?’, IEEE J. 
Sel. Topics Quantum Electron., 1998, 4, pp. 159-168 
Photonic bandgap disk laser 
R.K. Lee, O.J. Painter, B. Kitzke, A. Scherer and 
A. Yariv 
A two-dimensional photonic crystal defmed hexagonal disk laser 
which relies on Brag reflection rather than the total internal 
reflection as in traditional microdisk lasers is described. The 
devices are fabricated using a selective etch to form free standing 
membranes suspended in air. Room temperature lasing at 1650nm 
for a 150nm thick, - 1 5 ~  wide cavity fabricated in InPhGaAsP 
is demonstrated with pulsed optical pumping. 
ELECTRONICS LETTERS 1st April I999 Vol. 35 
Since their introduction in 1992, [l] microdisk lasers have 
attracted sigmfkant attention as high-Q light sources with very 
strong optical confinement. Their lasing modes approximate whis- 
pering-gallery modes which depend on total internal reflection at 
the curved boundary of the semiconductor disk. In this Letter, we 
describe a demonstration of lasing action in a hexagonally shaped 
disk with a similar whispering-gallery-like mode based on Bragg 
reflection from a photonic bandgap crystal instead of the large 
dielectric discontinuity between the semiconductor layer and a sur- 
rounding low index medium. [2] The confimement of whispering- 
gallery modes depends strongly on the curvature of the disk 
boundary. The use of a photonic bandgap allows the lateral con- 
fmement and the device size to be decoupled. 
Fig. 1 Scanning electron micrograph (top view) of typical hexagonal 
membrane cavity bounded by 2 0  photonic bandgap crystal 
We have designed and fabricated two-dimensional photonic 
crystals in a thin semiconductor membrane bounded above and 
below by air. An electron micrograph of a typical device is shown 
in Fig. 1. The optical cavity consists of a I f2  thick dielectric slab 
waveguide suspended in air. The strong confinement provided by 
the index contrast between the semiconductor membrane and air 
helps to reduce the scattering out of the plane of the waveguide by 
the photonic crystal. The membrane is patterned with a hexagonal 
two-dimensional array of air holes which is used to define the 
boundaries of a hexagonal cavity. The ratio of the hole size to the 
lattice space (r/a) was -0.35. The calculated band structure for this 
photonic crystal, taking into account the finite extent of the phot- 
onic crystal in the third dimension, exhibits a TE bandgap from a 
normalised frequency of a/h = 0.3476 to a/h = 0.4654 for modes 
guided within the semiconductor slab. Details of this calculation 
have been previously described [3 - 51. The cavities were designed 
with the photonic bandgap centred around the peak emission 
wavelength at a normalised frequency of aJh = 0.406. In this Let- 
ter, cavities -10-15p across were examined. At the emission 
wavelength of 1 . 5 5 p ,  this structure forms a multimode cavity. 
The membrane consisted of six lattice matched InGaAs quan- 
tum wells with InGaAsP barriers grown by metal organic vapour 
phase epitaxy (OMVPE) on an InP substrate. The total waveguide 
thickness was 150nm. The InGaAsP system was chosen for its rel- 
atively low surface recombination velocity since the photonic crys- 
tal has a relatively large surface-to-volume ratio. The photonic 
crystal structure was patterned using electron beam lithography. 
This pattern was transferred from the electron beam resist into a 
multilayer mask and then etched into the semiconductor mem- 
brane using Ar/Cl, chemically assisted ion beam etching (CAIBE). 
A dilute HCI solution was then used to selectively etch away the 
InP material beneath the membrane while leaving the InGaAsP 
waveguide layer. This results in a free standing, air suspended 
membrane. 
The laser cavities were optically pumped normal to the mem- 
brane using an InGaAs laser emitting at 980nm focused to a spot 
size of - 1 5 p .  The pump laser was pulsed with l0ns pulses at a 
duty cycle of 0.34.5%. The low duty cycle was used in order to 
minimise heating of the membrane which lacks a good thermal 
conduction path for heat dissipation. Luminescence from the 
quantum wells at h = 1 . 5 5 ~  was collected normal to the mem- 
brane, in the same direction as the pump. Hexagonal photonic 
crystal disks similar to the device in Fig. 1 showed a lasing spec- 
trum at room temperature as shown in Fig. 3. The lasing spectrum 
No. 7 569 
was centred at h = 1645nm with a linewidth measured to be less 
than - 2A (limited by the spectrometer resolution). The lasing 
wavelength was significantly red-shifted from the designed quan- 
tum well emission peak of h = 1550nm because of heating of the 
active layer by the pump laser which shifted the gain peak towards 
longer wavelengths. These lasers were fabricated with a lattice 
spacing of a = 640nm. For a lasing wavelength of 1645nm, this 
corresponds to a normalised frequency of 0.389, within the phot- 
onic bandgap, near the bandgap centre frequency of 0.406. Fig. 3 
also shows a number of side modes, spaced by -30nm with full- 
width at half-maximum (FWHM) linewidths of 20nm, corre- 
sponding to a cavity quality factor Q = 80. 
Fig. 2 Calculated field distribution for whispering gallery-like mode in 
this cavity 
In this image, field intensity is represented by shading; triangular 
array of circles represent air holes etched into semiconductor 
waveguide 
-75 I 2.5- 
E 
-80 
I 
P 
0 
c 
5 -85 
._ - 
U 
m 
- 
- 
8 -90 
-95 
1400 1450 1500 1550 1600 1650 
1, nm m 
Fig. 3 Spectrum from lasing hexagonal PBG disk slightly above thresh- 
old with spectral resolution of 5nm 
Lasing line is clearly visible at 1650nm; width of line as well as its rec- 
tangular profile are due to wide spectral resolution 
Inset: light intensity against pump power demonstrating a clear 
threshold 
0 point where spectrum was measured 
The light output against pump power response for the laser is 
shown in the inset of Fig. 3. The lasing threshold occurred at a 
peak pumping power of 66mW. The relatively high threshold 
required is due largely to the mismatch between the pumping pro- 
file and the lasing mode as well as the relatively low Q of this non- 
optimised structure. For a whispering gallery-like mode as shown 
in Fig. 2, the majority of the field intensity lies around the perime- 
ter of the cavity. However, optically pumping the device with a 
Gaussian beam concentrates the peak pumping power at the 
centre of the cavity. This poor pump to mode overlap was experi- 
mentally verified. Lasing occurred with a pump spot size of 1 5 p  
however, when the pump spot was focused to smaller sizes (2- 
l o p ) ,  it was not possible to reach lasing threshold even with 
peak pump powers up to - 150mW. The lasing mode in Fig. 3 
does not correspond to one of the prominent equispaced (AX = 
30nm) modal peaks but occurs between two such resonances. 
Below threshold, only these regularly spaced side modes are visible 
with very little emission occurring at the lasing wavelength. This 
indicates that these side modes have a strong overlap with the 
pump profile, giving strong spontaneous emission, but low Q so 
that the lasing threshold is not reached. The lasing mode is a 
much higher Q mode, which lowers the lasing threshold but has a 
relatively poor overlap with the pump. These regularly spaced 
modes have been previously observed [6]. 
Finite difference time domain calculations have shown that the 
relatively large hexagonal cavities exhibit an extremely complex 
modal structure including whispering gallery-like modes (Fig. 2), 
as well as more plane-wave like modes reflecting between oppos- 
ing faces of the hexagon. However, because the photonic crystal 
has a very wide bandgap (and is thus highly reflective over a wide 
range), these are all relatively high Q modes. This suggests the 
possibility of using pumping geometry to select the lasing mode 
(and therefore also the lasing wavelength) due to the differences in 
spatial field distribution between the modes. By adjusting the 
pump beam alignment with the laser cavity, we have been able to 
observe an abrupt shift of the lasing wavelength to h = 16601x11 
corresponding to one of the previously mentioned regularly spaced 
modes. 
In summary, we have demonstrated planar hexagonal disk 
lasers based on a two-dimensional photonic band gap structure in 
the InP material system. Lasing occurred at room temperature 
under pulsed pumping conditions in spite of the poor thermal con- 
duction path for heat removal. Different lasing modes in the disk 
were selected by adjusting the pump alignment. A relatively high 
lasing threshold was observed due to the poor overlap between the 
pump and the lasing mode and the relatively high membrane tem- 
perature. Heat dissipation could be improved by including a sup- 
porting post similar to that found in microdisk lasers. 
Acknowledgment: The authors thank Ortel Corporation for sup- 
plying the material grown for this Letter. This work was sup- 
ported by the Army Research Office (ARO), the National Science 
Foundation (NSF), the Office of Naval Research (ONR) and the 
Defense Advanced Research Projects Agency (DARFA). R.K. Lee 
and O.J. Painter are both supported by the National Science and 
Engineering Research Council of Canada (NSERC). 
0 IEE 1999 
Electronics Letters Online No: 19990415 
D 01: IO. 1049/el: 199941 5 
R.K. Lee, O.J. Painter, B. Kitzke, A. Scherer and A. Yariv (Applied 
Physics and Electrical Engineering, California Institute of Technology, 
1200 E. California Blvd., Pasadena, CA 91125, USA) 
26 February 1999 
References 
1 MCCALL, s.L., LEVI, A.F.J., SLUSHER, RE, PEARTON, s.J., and 
LOGAN, R.A.: ‘Whispering-gallery mode microdisk lasers’, Appl. 
Phys. Lett., 1992, 60, (3), pp. 289-291 
2 YEH, P., and YARIV, A.: ‘Bragg reflection waveguides’, Opt. 
Commun., 1976, 19, (3), pp. 427430 
3 FAN, s.H., VILLENEUVE, P.R., JOANNOPOULOS. J.D., and SCHUBERT, E.F.: 
‘High extraction efficiency of spontaneous emission from slabs of 
photonic crystals’, Phys. Rev. Lett., 1997, 78, (17), pp. 32943297 
4 PAINTER, o.J., VUCKOVIC, J., and SCHERER, A.: ‘Defect modes of a 
two-dimensional photonic crystal in an optically thin dielectric 
slab, J. Opt. Soc. Am. B, 1999, 16, (2), pp. 275-285 
5 RUSSELL, P. ST. J., ATKIN, DM. ,  and BIRKS, T.A.: ‘Bound modes of 
two-dimensional photonic crystal waveguides’ (Kluwer Academic 
Publishers, The Netherlands, 1996), pp.203-218 
6 LEE, R.K., PAINTER, o.J., D’URSO, B., SCHERER, A., and YARIV, A.: 
‘Measurement of spontaneous emission from a two-dimensional 
photonic band gap defined microcavity at near-infrared 
wavelengths’, Appl. Phys. Lett., 1999, 74, (1 I ) ,  pp. 
570 ELECTRONICS LETTERS 1st April 1999 Vol. 35 No. 7 


Photonic bandgap disk laser

Bound modes of two-dimensional photonic crystal waveguides’

Bragg reflection waveguides’,

Defect modes of a two-dimensional photonic crystal in an optically thin dielectric slab,

ELECTRONICS LETTERS 1st April 1999 Vol. 35 No. 7

FAN, s.H., VILLENEUVE, P.R., JOANNOPOULOS. J.D., and SCHUBERT, E.F.: ‘High extraction efficiency of spontaneous emission from slabs of photonic crystals’, Phys. Rev. Lett., 1997, 78, (17), pp. 32943297

High extraction efficiency of spontaneous emission from slabs of photonic crystals’,

LEE, R.K., PAINTER, o.J., D’URSO, B., SCHERER, A., and YARIV, A.: ‘Measurement of spontaneous emission from a two-dimensional photonic band gap defined microcavity at near-infrared wavelengths’, Appl. Phys. Lett., 1999, 74, (1 I ) , pp.

MCCALL, s.L., LEVI, A.F.J., SLUSHER, RE, PEARTON, s.J., and LOGAN, R.A.: ‘Whispering-gallery mode microdisk lasers’, Appl. Phys. Lett., 1992, 60, (3), pp. 289-291

Measurement of spontaneous emission from a two-dimensional photonic band gap defined microcavity at near-infrared wavelengths’,

PAINTER, o.J., VUCKOVIC, J., and SCHERER, A.: ‘Defect modes of a two-dimensional photonic crystal in an optically thin dielectric slab, J. Opt. Soc. Am. B, 1999, 16, (2), pp. 275-285

RUSSELL, P. ST. J., ATKIN, DM. , and BIRKS, T.A.: ‘Bound modes of two-dimensional photonic crystal waveguides’ (Kluwer Academic Publishers, The Netherlands, 1996), pp.203-218

Whispering-gallery mode microdisk lasers’,

YEH, P., and YARIV, A.: ‘Bragg reflection waveguides’, Opt. Commun., 1976, 19, (3), pp. 427430

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Photonic bandgap disk laser

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