Abstract

An experimental study of the lasing characteristics of photonic crystal lasers based on the conjugated polymer 2-methoxy-5-(2′- ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) is reported in this letter. One and two dimensional (1D, 2D) photonic crystal structures were patterned on a glass substrate through interferometric lithography on photoresist layers. A 1.5 µm layer of polymethylglutarimide (PMGI) was deposited to prevent photoxidation of the polymer. Lasing action was observed under optically pumped conditions. Instabilities associated with pumping geometries were demonstrated in the case of 2D photonic crystal laser. As a result, the laser spectrum and threshold gain were found to be strongly dependent on the excitation geometry. The broad spectrum of the amplified spontaneous emission (ASE) allows laser tunability by engineering the effective refractive index of the devices or by controlling the periodicity of the photonic crystal.

© 2005 Optical Society of America

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References

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Adv. Mater

J. Gao, G. Yu, and A. J. Heeger, �??Polymer p-i-n Junction Photovoltaic Cells,�?? Adv. Mater 10, 692-695 (1998)
[CrossRef]

M. D. McGehee, A.J. Heeger, �??Semiconducting (Conjugated) Polymers as Materials for Solid-State Lasers,�?? Adv. Mater, 22, 1655-1668 (2000)
[CrossRef]

Appl. Phys. Lett.

T. Sakanoue, E. Fujiwara, R. Yamada, and H. Tada, �??Visible light emission from polymer-based field effect transistors,�?? Appl. Phys. Lett. 84, 3037-3039 (2004)
[CrossRef]

H. Kogelnik and C. V. Shank, �??Stimulated emission in a periodic structure,�?? Appl. Phys. Lett. 18 152 (1971)
[CrossRef]

M. D. McGehee, M. A. Diaz-Garcia, F. Hide, R. Gupta, E. K. Miller, �??Semiconducting polymer distributed feedback lasers,�?? Appl. Phys. Lett. 72, 1536-1538 (1998)
[CrossRef]

S. Riechel, C. Kallinger, U. Lemmer, J. Feldman, A. Gombert, V. Wittwer, U. Sherf, �??A nearly limited surface emitting conjugated polymer laser utilizing a two-dimensional photonic band structure,�?? Appl. Phys. Lett. 77, 2310-2312 (1998)
[CrossRef]

J. Quantum Electron.

M. Toda, �??Proposed cross grating single-mode DFB laser,�?? J. Quantum Electron. 28 1653 -1662 (1992)
[CrossRef]

W. Streifer, D. R. Scifres and R. D. Burham, �??Couple wave analysis of DFB and DBR Lasers,�?? J. Quantum Electron. 13 134-141 (1977)
[CrossRef]

Macromol. Symp.

Y. Shi, J. Liu and Y. Yang, �??Device performance and polymer morphology in polymer light emitting diodes : morphology dependent emission spectra,�?? Macromol. Symp. 154, 187-197 (2000)
[CrossRef]

Nature

N.M. Lawandy,R.M. Balachandran, A.S.L. Gomez and E. Sauvain, �??Laser action in strongly scattering Media,�?? Nature, 368 436-438 (1994)
[CrossRef]

Phys. Rev. B

G. Turnbull, P. Andrews, W. Barnes, I. Samuel, �??Relationship between photonic band structure and emission characteristics of a polymer distributed feedback laser,�?? Phys. Rev. B 64 125122-1 (2001)
[CrossRef]

Synth. Met.

J. Gao F. Hide, and H. Wang, �??Efficient photodetectors and photovoltaic cells from composites of fullerenes and conjugated polymers: Photoinduced electron transfer,�?? Synth. Met. 84, 979-980 (1997)
[CrossRef]

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Figures (7)

Fig. 1.
Fig. 1.

Normalized absorption (dashed) and photoluminescence (continuous) spectra of thin films MEH-PPV. The films were cast by spin coating from toluene solution with a concentration of 5 g/dm3. A Nd:YAG laser operating at 532 nm, 2.5 ns pulse length with a repetition rate of 10 Hz was used as the excitation source for the photoluminescence

Fig. 2.
Fig. 2.

Interferometric lithography setup. Grating periods from 150 nm to 2 µm can be fabricated by selecting the appropriate angle θ. An angle of 17.75 degrees was selected to obtain a grating period of 400 nm. The laser source is an argon laser operating at 244 nm with an intracavity second harmonic crystal.

Fig. 3.
Fig. 3.

Atomic force microscope (AFM) image of the 1D photonic crystal structure. The period of the corrugation was about 400 nm with an average depth of 70 nm.

Fig. 4.
Fig. 4.

(a). Theoretical calculation of the fundamental mode of the polymer waveguide structure. The refractive index profile of the device shows excellent overlapping between the field distribution and the grating region, thus ensuring low threshold gain for the laser. Fig. 4(b). Amplified spontaneous emission (ASE) spectrum (dashed) obtained by photopumping an unpatterned film of MEH-PPV coated with PMGI. The solid line represents lasing action from the photonic crystal laser. The spectrum was recorded at 12 inches from the device with a FWHM around 0.65 nm, which is limited by the resolution of the spectrometer.

Fig. 5.
Fig. 5.

Atomic force microscope (AFM) image of the 2D photonic crystal structure. The period of the corrugation was about 400 nm in both directions with an average depth of 80 nm.

Fig. 6.
Fig. 6.

(a) Laser action from a 2D-PCL as pumped with circular spot. This pumping configuration presents better spectral characteristics compare with the elliptical geometry. Fig. 6(b). Laser spectrum from a 2D-PCL as pumped with an elliptical spot. Notice the diffraction slope around the lasing wavelength.

Fig. 7.
Fig. 7.

(a) Second order diffraction mechanism in 2D-PCL. In this configuration the grating vector couples the forward and backward waves in two steps. Surface emission is achieved through crossing the origin of the k-space coordinates after the first diffraction step. The device behaves as a two independent resonators and thus performance is dependent on pumping geometry. Fig. 7(b). Fourth order process in 2D-PCL. This configuration is more stable since both gratings must work simultaneously to couple the forward and backward waves, thus only one resonator is defined.

Equations (2)

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Λ = λ 2 sin θ .
Λ = n λ B 2 n eff .

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