2005 Conference on Lasers and Electro-Optics Europe

Dual-wavelength semiconducting-polymer DFB laser

G.E. TownDepartment ofElectronics (E6A), Macquarie University, /SW 2109, Australia

Tel: +612 9850 9063, Fax: +612 9850 9128, Email: gtown.ics.mq.edu.au

A. Vasdekis, F. Bain, G.A. Turnbull, and I.D.W. SamuelOrganic Semiconductor Centre & Ultrafast Photonics Collaboration, School ofPhysics and Astronomy,

University ofSt Andrewvs, St Andrewvs, Fife KYJ6 9SS, United Kingdom

Dual and multiwavelength optical output from a single laser cavity is known to be possible in homogeneouslybroadened gain media only if gain cross-saturation between the lasing modes can be overcome. Dual andmultiwavelength outputs from semiconducting polymer lasers previously have been observed, e.g. [1], however in thelasing wavelengths were orthogonal modes of the cavity and hence were not able to be defined independently.

In this work we show for the first time that two lasing wavelengths may be set independently in a semiconducting-polymer laser by using a doubly-periodic (i.e. Moire) DFB grating. As in other similar single-wavelength DFB lasers[2], the gain medium was a thin film of semiconducting polymer (MEH-PPV), and a second-order grating was used tosimultaneously provide feedback and outcoupling at the signal wavelengths, in this case around 635nm. Unlike previousreports, the 2nd-order Moire grating defining the lasing wavelengths was written holographically in an SU-8 film on asilica substrate.

The doubly-periodic grating was formed in a 250 nm thick film of SU-8 on a silica substrate by double holographicexposure using a He:Cd laser and a simple interferometer in which the period of the interference pattern could bechanged between exposures. Before applying the MEH-PPV film the SU-8 grating was measured using an atomic forcemicroscope (AFM); the grating topography was observed to be sinusoidal, with peak-peak depth of approximately 60nm. Once the MEH-PPV film had been applied, the structure was placed in a vacuum chamber and was evacuated toapproximately 10-3 mbar. The structure was then pumped at 532 nm from normal to the MEH-PPV film using a Q-switched microchip laser, focussed to an approximately 1.4 mm diameter spot on the film. The pump intensity wasvaried using neutral density (ND) filters.

Figure 1 shows the output spectrum of the laser at a pumping intensity of 48.15 VJ/cm2 (with ND=0), measured using afibre-coupled CCD array (optical spectrum analyser) with 0.7 nm resolution. Two lasing peaks were evident at Xi =

627.3 nm, and 2 = 644.0 nm. Within experimental error, the spacing between the lasing wavelengths was consistentwith the spacing between the two components in the DFB grating (2.16 % = AA/Aav - AAXav = 2.62 %).

~~~~~~~. iT f I>vI. iZo9.i2

Fig 1. Optical spectrum of the planar DFB laser.

The result is interesting because semiconducting polymers are usually regarded as having a homogeneously broadenedgain-spectrum, and so it is surprising that two spatially non-orthogonal modes could be made to lase simultaneously atnearby wavelengths. Further studies are required, e.g. to test for temporal variation of the two modes, before firmconclusions can be made about the line-broadening in MEH-PPV in the wavelength range observed.

[1] S.V. Frolov, M. Shkunov, and Z.V. Vardeny, "Ring microlasers from conducting polymers," Phys. Rev. B 56,R4363-66 (1997).[2] G. A. Tumbull, P. Andrew, M. J. Jory, W. L. Barnes, and 1. D. W. Samuel, "Relationship between photonic bandstructure and emission characteristics of a polymer distributed feedback laser," Phys. Rev. B 64, 125 122 (2001 ).

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