**5. Conclusion**

206 Recent Advances in Nanofabrication Techniques and Applications

Threshold Fluence: 1.31 J/mm2

30 40 50 60 70 80 90 100

Absorbed pump Energy (nJ)

Figure 10 (a) represents the far-field image of the emission pattern recorded by a CCD camera, and Figure 10 (b) shows the far-field radiation patterns of the laser passing through a linear polarizer with different orientation angles. The laser is expected to be azimuthally polarized (R. H. Jordan et al., 1997), as illustrated in the polarization patterns. The azimuthal polarization also results in a zero electrical field (a dark spot) at the center of the laser (T. Erdogan et al., 1992). The Polarization studies of circularly symmetric beams verified theoretical predictions that these beams are azimuthally polarized. In the lasing process, many spatial modes can be excited with their mode thresholds very close to each other (T. Erdogan et al., 1992). The fundamental mode is normally the favored one, because higher

We observe decreases in the laser emission with increasing exposure time. This result is consistent with previous studies on polymer DFB structures (G. Heliotis et al, 2004). The lifetime of polymer dye laser can last over 106 shots of pump laser pulse, and if the characterization of the laser device is carried out under vacuum to inhibit photo-oxidation, the lifetime can be further extended (P. Del Carro et al., 2006). Because of the low cost of materials and fabrication, replication molded devices are disposable and may not require a long lifetime. With the mass production capability, nanoimprinted solid-state dye lasers are

The integration of solid-state dye laser with microfluidic platform is important. Because of its stacked substrate structure, the alignment of surface emitting dye lasers with microfluidic channels would be straight forward. Since optofluidic dye lasers also have great advantages in microfluidics integration, many on-chip liquid dye lasers with distributed feedback structure have been demonstrated (Z. Y. Li et al., 2006; M. Gersborg-Hansen and A. Kristensen, 2006; S. Balslev and A. Kristensen, 2005) by soft lithography (Y. N. Xia and G. M. Whitesides, 1998).

Fig. 9. The output laser power vs. the absorbed pump energy curve. The threshold pump

0

order modes do not overlap well with the gain region.

suitable for disposable light sources for integration in microsystems.

fluence is 1.31 μJ/mm2

5

10

15

Output Laser Power (W)

20

25

30

35

In summary, we have described the fabrication of a surface emitting polymer dye laser with circular grating distributed feedback structure using nanoimprint lithography. We have

Fabrication of Circular Grating Distributed Feedback Dye Laser by Nanoimprint Lithography 209

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achieved excitation thresholds as low as 1.31 μJ/mm2 and FWHM linewidths of 0.18 nm. The technique described here enables the fabrication of low cost, high quality and mass producible laser arrays, which may be deployed as compact and inexpensive coherent light sources for lab-on-a-chip applications such as sensing and spectroscopy.

Future work will be focused on improving the laser cavity quality factor (Q) values with better electromagnetic design, optimizing the dye concentration, and fabricating smoother surfaces. One of the future research directions is to use novel soft lithography technique to develop optofluidic dye lasers based on circular grating geometry, in order to realize dye solution circulating and wavelength tuning. Another direction is to use conductive polymer as the gain medium to enable electrically pumped laser scheme. The ultimate goal is to reduce the lasing threshold to enable the use of LEDs or laser diodes as integrated and inexpensive pump sources for on-chip polymer lasers, and integrate the laser chip into microfluidics for further development of complete "lab-on-a-chip" systems.

### **6. Acknowledgment**

This work is supported by the Caltech DARPA Center for Optofluidic Integration under Contract No. HR0011-04-1-0032 and by the Boeing Corporation.

#### **7. References**


achieved excitation thresholds as low as 1.31 μJ/mm2 and FWHM linewidths of 0.18 nm. The technique described here enables the fabrication of low cost, high quality and mass producible laser arrays, which may be deployed as compact and inexpensive coherent light

Future work will be focused on improving the laser cavity quality factor (Q) values with better electromagnetic design, optimizing the dye concentration, and fabricating smoother surfaces. One of the future research directions is to use novel soft lithography technique to develop optofluidic dye lasers based on circular grating geometry, in order to realize dye solution circulating and wavelength tuning. Another direction is to use conductive polymer as the gain medium to enable electrically pumped laser scheme. The ultimate goal is to reduce the lasing threshold to enable the use of LEDs or laser diodes as integrated and inexpensive pump sources for on-chip polymer lasers, and integrate the laser chip into

This work is supported by the Caltech DARPA Center for Optofluidic Integration under

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sources for lab-on-a-chip applications such as sensing and spectroscopy.

microfluidics for further development of complete "lab-on-a-chip" systems.

Contract No. HR0011-04-1-0032 and by the Boeing Corporation.

**6. Acknowledgment** 

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**7. References** 


**11** 

*Japan* 

Masaki Yanagisawa

*Sumitomo Electric Industries, Ltd.* 

**Application of Nanoimprint Lithography to** 

The recent growth of information-communication facilities such as the Internet, mobile telecommunication and video-on-demand services has led to explosion of worldwide communication traffic, which increase demand for faster and denser communication infrastructures including optical communication networks. Distributed feedback laser diodes (DFB LDs) have been widely used as optical sources in networks because of their high selectivity and stability of wavelength. Although they had limitedly been used in longhaul and high-speed network at one time, they are now increasingly required in metro and end-user fields because of the increasing traffic. Thus, necessity for inexpensive DFB LDs

The characteristics of a DFB LD with uniform (constant period) gratings depend on the grating phase at the cleaved facet (Matsuoka et al., 1984). The variation of characteristics with the facet phase is a serious issue in view of productivity and usability of the LDs. One of the most effective ways of reducing the facet phase effect is to adopt phase-shifted gratings instead of uniform gratings (Kaden et al., 1992). The uniformities of the LD characteristics such as mode-stability and output power are improved by adopting phaseshifted gratings, thus the yield of LDs increase and their production cost is effectively

In general, there are various fabrication methods for diffraction gratings of DFB LDs, for example, interference exposure, electron beam lithography (EBL), and optical projection exposure. Interference exposure cannot feasibly be used for fabricating phase-shifted gratings, because it exclusively generates exposure patterns with a uniform bright-and-dark period. Although EBL has sufficient resolution to be used for phase-shifted gratings, exceedingly expensive apparatus is necessary for volume production with sufficient throughput. For the optical projection method, a forefront stepper having sufficient resolution for gratings is also expensive, and the cost is too high for fabricating DFB LDs, of which production volume is relatively small compared to that of such semiconductor

Nanoimprint lithography (NIL) has been studied by many organizations since the middle of the 1990s. Chou et al. indicated that sub-10-nm features could be formed by imprint, which started the era of NIL technology (Chou et al., 1995). A novel method of NIL using a UVcurable resin was introduced by Haisma et al (Haisma et al., 1996), and Bailey et al.

**1. Introduction** 

increases rapidly.

reduced.

devices as LSIs.

**Distributed Feedback Laser Diodes** 

