Sensors and microfluidic devices in PCF

The aim here is to explore the use of PCF as a vehicle for new kinds of optical sensors and devices, offering strong interactions between light and gaseous/fluidic samples over path-lengths that are much longer than in conventional gas/liquid cells [Cubillas (2013)]. There are many opportunities for miniaturization and flexibility compared to table-mounted optical systems, with the potential for continuous high sensitivity monitoring in remote or hazardous environments. 

Solid glass cores as sensors

Solid-core PCFs can also be used as sensors, provided the evanescent field of the guided mode extends far enough into the sample, and the so-called Mercedes design offers the combination of strong field penetration and large (~30 µm) fluidic channels for the sample solution to pass through. It consists of a solid silica core held in place by three silica nanomembranes. The interaction of the guided light with the sample can be adjusted by appropriate core design, and provided it is strong enough, the system will provide high sensitivity over long interaction lengths with very small sample volumes. Quantitative absorption measurements can be made on chemical samples introduced into the cladding holes, with sample volumes 1000 times smaller than in a conventional cuvette [Euser (2008)]

(a) Scanning electron micrographs of two Mercedes fibres. (b) Measured field intensity profiles at 700 nm and 1000 nm wavelength. (c) Figure 4: The measured broadband absorption spectrum of NiCl in a solid core PCF agrees quantitatively with data measured in a standard cuvette.

Controlled photochemistry in photonic crystal fibre microreactors

The development of new photoactivated anticancer complexes is of major importance for the effective treatment of cancer. Such complexes are inert, nontoxic and are activated only locally in cancer cells by light. In collaboration with co-workers from the University of Warwick in the UK, we have developed a new method that allows rapid testing and optimization of photochemical compounds [Chen (2010); Chen (2010a); Unterkofler (2012)]. It makes use of the unique waveguiding properties of liquid-filled hollow-core photonic crystal fibres to bring together for the first time the fields of microfluidics, chemistry, and optics. The hollow core is loaded with chemicals and acts as a miniature photochemical reactor. Both the chemicals and the excitation light are confined to the core. Consequently, the entire sample volume interacts strongly with the light, resulting in much shorter reaction times than in conventional cuvettes. Importantly, the required sample volume is reduced by four orders of magnitude. The reaction is monitored in real-time by in-fibre broadband absorption spectroscopy, offering insight in the dynamics of the chemistry. As a proof-of-principle study, the photochemical conversion of Vitamin B12a to vitamin B12b upon excitation with blue laser light was investigated. In a continuous-flow configuration, the method enables a rapid optimization of the excitation wavelength and light dosage of photoactivated anticancer complexes.

(a) Scanning electron micrograph of a kagomé-style hollow-core photonic crystal microreactor. (b) In-fibre measurement of the temporal evolution of the absorption spectrum of Vitamin B12 upon excitation with blue laser light.

Photoswitching in PCF

PCF nanoreactors can also be used to study fast, reversible photoswitching processes in real-time [Chen (2010a)]. The figure shows photoisomerization of an azobenzene derivative: upon irradiation with blue light, the thermally stable trans molecules are converted to the cis form at a rate that is proportional to the irradiance. The cis molecules are subsequently converted back to the trans form by both photochemical and (slow) thermal processes. After a certain exposure time, a photostationary state (PSS) is reached. In typical photoswitching experiments, high laser intensities are required to obtain a large fraction of cis molecules in the PSS. The strong confinement of both light and sample in hollow core PCF results in a high intensity throughout the entire sample volume. Consequently, the laser power required is reduced by at least five orders of magnitude compared to that in a standard cuvette.

Top: Reversible isomerization between the trans and the cis geometric isomers of azobenzene. Left: temporal evolution of molar absorptivity at 455 nm measured in 39 cm kagome HC-PCF infiltrated with 0.75 µM of DO1 for the trans to cis photoisomerization induced by irradiation at 488 nm at 3 µW. Right: cis to trans thermal isomerization.

Photochemistry in soft-glass HC-PCF

To date, the use of liquid-filled HC-PCF in photochemical experiments have only been performed in fused silica HC-PCFs, which limits the choice of solvent to liquids with refractive index less than that of silica (nG = 1.45 at 600 nm), if the benefits of single-mode guidance are to be preserved. This is because the guidance mechanism switches to total internal reflection (TIR) for nL > nG, resulting in increasingly multi-mode operation as nL rises and making accurate spectroscopic measurements very difficult. We have overcome this limitation by employing single-ring HC-PCF made from a lead-silicate glass (Schott SF6) with index nG = 1.80 at 600 nm. The fibre was fabricated using an improved "stack-and-draw" technique. In a proof-of-principle experiment, this fibre was used to demonstrate photochemical conversion of an organometallic complex in toluene [Cubillas (2017)].

Scanning electron micrographs (SEMs) of (a) the soft-glass single-ring HC-PCF and (b) a close-up of a portion of the surrounding glass membranes. The fibre has a core diameter of ~25 ?m and average core wall thickness of 600 nm. (c) Left: Normalized transmission in air-filled (solid blue) and toluene-filled (dashed black) HC-PCF. The loss spectrum for an air-filled fibre (dashed red) is calculated analytically, assuming a capillary waveguide with the same core diameter and wall thickness. Right: Measured near-field mode profiles in the air-filled (upper) and toluene-filled (bottom) single-ring HC-PCF. (d) The set-up used for the photochemical experiments. A 35-cm-long soft-glass single-ring HC-PCF is used. Two custom-built liquid cells were used for filling toluene solvent into the fibre. Laser light is coupled to the fibre using an (under-filled) 4x 0.1 NA objective, and the output is imaged onto a CCD camera and a detector. (e) (symbols) Measured absorbance decay of a sample with an initial concentration C=10 ?M of (HBPz´3)Rh(CO)2 in toluene, in a 35-cm-long soft-glass single-ring ARR HC-PCF. The colours correspond to different input powers. The grey data points correspond to a reference measurement in a 1-cm-long cuvette with C =100 ?M.