Towards Hydrogel Based Organic Distributed Feedback Laser Utilizing Supramolecular Host–Guest Complex

Abstract

Organic dye lasers base their operation on \pi-conjugated molecules or polymers as an active material. One major advantage of these lasers is their wavelength tunability that arises from broad emission spectra of dye molecules and variety of available dyes to be used. Solid-state dye lasers are under constant development to overcome inconveniences related to more traditional liquid dye lasers, such as large size and exposure to organic solvents. Polymers have proved to be attractive host material option for compact solid-state dye lasers as they offer easy processability and low-cost fabrication. Moreover, additional tuning via external stimuli can be achieved by responsive soft materials, such as hydrogels. Hydrogels are crosslinked polymers that can reversibly swell/deswell by absorbing water in response to external stimuli, such as temperature or light. The premise underlying this work is that by combining swelling of hydrogels with distributed feedback (DFB) geometry of laser resonator realized through surface relief gratings (SRG), the geometry of the resonator could be controlled externally. This could open various possibilities for tunable laser sources sensitive to different stimuli.

Organic dye molecules unfortunately possess some properties that limit their use in lasing. They tend to aggregate especially in aqueous solutions, which can lead to fluorescence quenching. Another problem arises from photobleaching due to optical pumping: Because of excess energy received by the molecules, they can undergo photochemical reactions and permanently lose the ability to fluoresce. An elegant way to prevent these problems is through supramolecular host–guest complexes, where host molecule encapsulates the dye molecule, preventing aggregation and restricting rotational and vibrational freedom.

In this work, a known concept of supramolecular host–guest complex between cucurbit[7]uril (CB7) and rhodamine B (RhB) dye is taken from aqueous solution to solid-state and possibility of the developed material to be used as DFB laser is studied. A poly(N-isopropylacrylamide)-based (PNIPAm) hydrogel with covalently bonded RhB was fabricated and the effect of host–guest complex on fluorescence and photostability was characterized. In addition, material gain was studied and SRG were inscribed on hydrogel film via photo- and soft lithography by fabricating master SRG, replicating it on PDMS film and subsequently on hydrogel.

Incorporation of the dye in PNIPAm hydrogel films proved to be successful and post-polymerization rinsing of the hydrogel led to dye leakage of 10%. In addition, films had good optical quality and uniformity. The host–guest complexation led to significant improvements in fluorescence and photostability of the system. Fluorescence emission intensity showed as high as 6-fold increase in swollen hydrogel once CB7 was introduced to the system. Similarly, the photobleaching half-life was increased by factor of 9. Hydrogel material exhibited gain between 595 nm–660 nm with peaks around 605 nm and 615 nm in swollen and dry state, respectively. Replication of SRG on hydrogel film was partially successful. Grating period remained constant during the process but final grating height on hydrogel was reduced to only 10% of the initial value before the replication. Therefore, further optimization is required in SRG replication process.

In conclusion, the supramolecular host–guest complex was observed to notably improve fluorescence and photostability of the hydrogel system. Additionally, material showed optical gain and SRG required for DFB laser was replicated on hydrogel film. Based on these results, a groundwork for hydrogel-based DFB laser with enhanced performance through host–guest complex structure is provided. Main parts left for the future studies prior to lasing experiments are the optimization of SRG replication process and the waveguide structure of the device.