**Fabrication of Circular Grating Distributed Feedback Dye Laser by Nanoimprint Lithography**

Yan Chen1, Zhenyu Li1, Zhaoyu Zhang1 and Axel Scherer2 *1Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences 2California Institutes of Technology 1China 2USA* 

### **1. Introduction**

196 Recent Advances in Nanofabrication Techniques and Applications

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Within recent years the development of polymer dye lasers has progressed to higher levels of performance and functionality. The most attractive advantages of polymer dye lasers include low-cost processing, wide choice of emission wavelengths, and easy fabrication on flexible substrates. Several waveguide dye lasers have been studied with emission wavelengths ranging from ultraviolet to near infrared (Y. Oki et al., 2002). By simply changing the fluorophore doped in the polymer, these lasers can be used as the tunable sources for various applications, such as spectroscopy (Y. Oki et al., 2002) and fluorescence excitation source (C. Vannahme, 2011). Furthermore, microlaser array with multiwavelength emissions can be achieved (Y. Huang et al., 2010) for more applications such as compact displays and multiwavelength biosensors.

Currently the integration of miniaturized active light sources such as lasers into microfluidic systems becomes an attractive approach for biological and chemical processes (D. Psaltis et al., 2006). A majority of microfluidic systems are based on external light sources. However, the coupling of optical signals in and out of the devices, typically by optical fibers, remains one of the major challenges in integrated optics. By making on-chip light sources, we can eliminate the optics alignment, which greatly reduces the complexity of the system (E. Verpoorte, 2003). For applications in biochemical analysis in microfluidic systems, a surface emitting laser would appear to be more useful than other lasers because of its stacked substrate structure. Therefore, we choose a circular grating structure as the laser resonator design to produce low-threshold surface emitting lasing. The laser operating characteristics can be significantly improved by the two-dimensional nature of the resonator structure, and they are suitable to serve as low-threshold, surface-emitting coherent light source in microfluidic networks.

The 1-D distributed feedback (DFB) structure is a widely employed resonator geometry, and has been previously demonstrated for polymer lasers (Y. Oki et al., 2002). However, operating characteristics can be significantly improved within 2-D structures. Here, we choose a circular grating distributed feedback structure to obtain low threshold operation, a well-defined output beam, and vertical emission perpendicular to the device plane.

Fabrication of Circular Grating Distributed Feedback Dye Laser by Nanoimprint Lithography 199

Fig. 1. Schematic diagram of a nanoimprinted circular grating dye laser chip

Laser (Light Amplification by Stimulated Emission of Radiation) is composed of a gain medium and a resonant optical cavity (A. E. Siegman, 1986; F.P.Schafer, 1990). The gain medium amplifies the beam by stimulated emission, and the resonant cavity provides the feedback necessary for the lasing operation. For our solid-state dye lasers, we choose the circular grating resonator as laser cavities for their unique two-dimensional nature and

The circular grating structure proposed (T. Erdogan, 1990) and demonstrated (T. Erdogan, 1992) by Erdogan provides a natural 2-D extension of the basic DFB structure. It allows feedback to be applied in all in-plane directions, and the second-order grating couples the emitted radiation perpendicularly out of the surface of the sample. Figure 2 shows a general design of a circular grating distributed feedback structure. A theoretical analysis of circular grating lasers is described in detail elsewhere (C. M. Wu et al., 1991; T. Erdogan et al., 1992; P. L. Greene et al., 2001; G. F. Barlow et al.,2004; G. A. Turnbull et al.,2005) predicting that only the radial propagating components define the modes in the circularly symmetric grating. The distributed feedback scheme indicates that the gain material is directly implemented in the grating structure. The circular grating DFB structure satisfies the second-order Bragg

of the waveguide mode, and is the grating period, with an inner cavity providing a

The second-order grating is used to obtain surface emission, because it not only couples counter-propagating radial waves (via second-order Bragg reflection), but also induces coupling of radially propagating waves into the direction normal to the grating surface (via first-order Bragg reflection). The corrugations in the grating structure provide both distributed feedback and output coupling of the guided optical mode via second-order and

*Bragg* is the emission wavelength, *neff* is the effective index

**2. Laser cavity design** 

enhanced lasing performance.

condition,

first-order Bragg scattering.

*Bragg neff* , where

quarter- or half-wavelength shift similar to the classical DFB case.

Although surface emitting circular grating lasers using semiconducting polymers have been previously demonstrated (Bauer et al., 2001; Turnbull et al., 2005), their lasers were fabricated by depositing the organic gain material onto prepatterned dielectric substrates, limiting the depth and the accuracy of the shape of the grating.

For better geometric control, we choose nanoimprint lithography (S. Y. Chou et al., 1996) as a direct patterning method. Nanoimprint lithography is the technique that can effectively produce nano pattern with line width below 100nm. In general, a hard mold is used to transfer patterns with high fidelity into target polymers, and this technique has become an attractive approach to define nanofabricated optical resonator structures. Conjugated polymer lasers fabricated by hot embossing have been studied (J. R. Lawrence et al., 2002), and 1-D DFB lasers based on organic oligomers using a room temperature nanoimprint method were reported (D. Pisignano et al., 2003, 2004).

The basic idea of nanoimprint lithography is to press a mold with nanostructures on its surface into a thin layer of resist on a substrate, followed by the removal of the mold. Nanoimprint is a low cost nanopatterning technology based on the mechanical deformation of a resist, and it is a high-throughput alternative to traditional serial nanolithography technologies.

The imprint step creates a thickness contrast and duplicates the nanostructures in the resist film. During the imprint process, the resist is heated to a temperature above its glass transition temperature. At this temperature, the resist, which is thermoplastic, becomes a viscous liquid and can be deformed into the shape of the mold. Therefore, this method allows the nanostructure on the mold to be faithfully transferred to the polymer substrate.

The well developed nanoimprint technology provides a convenient way of mass production and large-scale fabrication of low-cost dye laser arrays with a wide wavelength output range. It is also straightforward to build on-chip dye lasers with waveguides to replace the optical fibers necessary for the integrated optics. The miniaturized dye lasers can serve as surface emitting coherent light sources, which are very important in various biochemical applications, such as laser-induced fluorescence and spectroscopy.

In this chapter, we report the fabrication of a circular grating distributed feedback laser on dye-doped poly(methylmethacrylate) (PMMA) films (Y. Chen et al., 2007). The schematic diagram of a nanoimprinted circular grating dye laser chip is illustrated in Figure 1. The laser was fabricated on a glass substrate using a low-cost and manufacturable nanoimprint method. In this solid-state dye laser device, the laser dye is doped in the polymer forming the laser resonator, which can produce high-intensity and narrow-linewidth lasing with a well-defined output beam. With certain selected grating period, surface emission lasing with single frequency at 618 nm and a linewidth of 0.18 nm was measured from the polymer dye laser exhibiting a threshold value of 1.31 μJ/mm2. The laser operation characteristics of the circular grating resonator are improved through the high accuracy and aspect ratio nanoimprint pattern transfer. Moreover, the mold can be re-used repeatedly, providing a convenient way of mass production and large-scale fabrication of low-cost polymer dye laser arrays. The on-chip dye lasers allow the integration of coherent light sources with other microfluidic and optical functionalities, and provide possibilities for building more complete "lab-on-achip" systems.

Fig. 1. Schematic diagram of a nanoimprinted circular grating dye laser chip
