Elsevier

Optics Communications

Volume 281, Issue 11, 1 June 2008, Pages 3159-3162
Optics Communications

Vertically emitting, dye-doped polymer laser in the green (λ  536 nm) with a second order distributed feedback grating fabricated by replica molding

https://doi.org/10.1016/j.optcom.2008.02.020Get rights and content

Abstract

Lasing in the green from a distributed feedback (DFB) structure, based upon a second order grating fabricated by replica molding in a dye-doped UV curable polymer, has been demonstrated. For a Bragg grating having a periodicity and depth of 360 ± 2 nm and 78 ± 5 nm, respectively, a coumarin 540-polymer laser operates at 535.6 nm, which is in agreement with calculations of the photonic band diagram for the structure. The fabricated laser exhibits a linewidth of 0.15 nm, a threshold pump fluence of ∼0.7 mJ cm−2 at 355 nm, and a slope efficiency of ∼14%. Incorporation of the dye gain medium into a one- (or two-) dimensional photonic crystal and fabrication of the grating by replica molding at room temperature provides an inexpensive approach to fabricating polymer-based DFB lasers on flexible substrates of large area.

Introduction

Since the demonstrations (in 1971) of lasing in which optical feedback was realized by a corrugated, periodic refractive index structure [1], [2], organic dyes have offered a versatile gain medium for the evaluation of integrated resonator designs, waveguide configurations and microlaser fabrication techniques [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Suitable for incorporation into polymers, sol–gel silica [6], and other hosts such as zirconia [7], dyes have conveniently provided the requisite gain for the development of optical resonators based on distributed feedback (DFB) [6], [7], [8], [9], [10], [11], [12], distributed Bragg reflectors (DBRs) [4] or microdisks [3]. DFB structures offer periodic modulation of either the gain [6], [7], [8], [9] or refractive index [10], [11], [12] of the medium and are generally preferred for solid state dye lasers, principally because end-on or out-of-plane emission, broad tunability, and narrow linewidth [8] are available. A drawback of conventional refractive index-modulated DFB dye laser designs, however, is the requirement for electron beam lithography [10] or interference photolithographic techniques [11], [12] to define the one- or two-dimensional surface grating.

Recently introduced embossing and imprinting techniques have simplified grating fabrication with inexpensive processes amenable to high throughput manufacturing and yet capable of precisely reproducing submicron features in a polymer substrate. Although embossing methods have reproduced submicron periodic grating patterns, the elevated pressures and temperatures required result in the accumulation of defects in the embossing tool and render the process incompatible with plastic film substrates (such as polyester and polycarbonate). It should also be noted that Ichikawa et al. [13] recently described a DFB laser fabricated by thermally-cured replica molding but the linewidth was measured to be 0.6 nm.

Ultraviolet (UV)-cured nanoreplica molding is, in contrast, a room temperature, low pressure process for the transfer of surface structure from a silicon wafer onto a flexible plastic substrate. Features with depths and widths as small as 10 nm, and one- and two-dimensional gratings with periods as low as 230 nm, have been reported [14]. Photonic crystal optical filters and biosensors fabricated by nanoreplica molding are currently manufactured by a large area, continuous roll-to-roll process [15].

This Letter reports the demonstration and characterization of lasing in the green (λ  536 nm) from a polymer-based DFB laser in which the dye gain medium is incorporated into a high-index, second order DFB grating. Fabricated by nanoreplica molding and optically pumped at 355 nm and an arbitrary angle of incidence, this integrated laser exhibits a threshold pump fluence of ∼0.7 mJ cm−2 for coumarin 540 dye, a slope efficiency of ∼14%, and a lifetime (to half power) of ∼800 shots.

Section snippets

Device structure and fabrication

A cross-sectional diagram (not to scale) of the DFB laser structure adopted for the present experiments is shown in Fig. 1. A polymer cladding layer was first applied to one surface of a polyethylene-terephthalate (PET) substrate by replica molding. A liquid, ultraviolet (UV)-curable polymer (Luvantix WR-354) with a refractive index of 1.39 was squeezed between the PET substrate and a planar, 10 cm (4 in.) dia. Si wafer having no surface structure. In order to ensure the thickness uniformity of

Grating characteristics

In addition to the integration of the gain medium into the DFB structure, a key aspect of the laser design of Fig. 1 is the incorporation of a second order grating. Unlike first order DFB structures, second order gratings support both edge and surface (out-of-plane) emission modes. Specifically, the second order DFB grating couples laser radiation into a vertically-emitted mode by first order diffraction [16]. Furthermore, maximum reflectivity for this one-dimensional photonic crystal occurs at

Experimental results and discussion

Optical pumping experiments were conducted by exciting the coumarin 540, DFB structure with ≲10 ns FWHM pulses from a frequency-tripled, Q-switched Nd:YAG laser. Pump fluence at the sample was varied with neutral density filters and emission emanating from the grating was monitored along the axis orthogonal to the surface with a spectrometer (Ocean Optics HR4000) having a resolution (in first order) of 0.12 nm FWHM. Spectra representative of those recorded for pump fluences of 0.5 mJ cm−2 and 1.5 mJ 

Conclusions

In summary, lasing in the green from a polymer-based DFB laser, having dye incorporated into a second order grating, has been demonstrated and characterized. Exhibiting a threshold pump fluence of ∼0.7 mJ cm−2 and fabricated by a room temperature replica-molding process, this DFB polymer-dye structure is inexpensive and manufacturable on plastic substrates of large surface area. These devices offer the opportunity to pursue coupled cavity configurations and phased arrays in a compact planar

Acknowledgements

This work was supported by SRU Biosystems and the National Science Foundation under Grant No. 0427657. Any opinions, findings, and conclusions or recommendations expressed in this article are those of the authors and do not necessarily reflect the views of the National Science Foundation. The technical assistance of C.J. Wagner and B.J. Ricconi of the Laboratory for Optical Physics and Engineering is gratefully acknowledged.

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