Cavity-enhanced optical methods for online microfluidic analysis

Cavity-based absorption spectroscopy techniques

“…Techniques and applications of CRDS have also been well covered by two earlier books (Busch and Busch, 1999;Van Zee and Looney, 2002) and by many informative review articles, notably those by Scherer et al (1997), Paul and Saykally (1997), Wheeler et al (1998), Berden et al (2000Berden et al ( , 2002, Wang et al (2000), Wagner et al (2002), McIlroy and Jeffries (2002), Brown (2003), Ball and Jones (2003), Vallance (2005), Mazurenka et al (2005), Kachanov (2005, 2006), Tarsa and Lehmann (2008), van Rushworth et al (2012). The CRDS literature is extensive and rapidly growing, so we focus on specialised aspects of particular interest.…”

“…CEAS, also known as integrated cavity output spectroscopy (ICOS), is a useful complementary alternative to CRDS (Berden et al , 2000(Berden et al , , 2002Curl and Tittel, 2002;Brown, 2003;Mazurenka et al , 2005;Paldus and Kachanov, 2005;Ball and Jones, 2009;Fiddler et al , 2009;van der Sneppen et al , 2009a;van Helden et al , 2009;Orr and He, 2011;Schnippering et al , 2011;Rushworth et al , 2012). Whatever its name, the CEAS/ICOS technique entails measurement of either the peak amplitude or the time-integrated signal amplitude of light transmitted by a high-fi nesse cavity (rather than the accompanying ringdown decay, as in CRDS).…”

“…Finally, in the context of cavity-based absorption spectroscopy in the liquid phase, we note that surface tension enables liquids to form droplets with intrinsic optical cavity properties (such as whispering-gallery modes, cavity-ringdown effects and interesting dynamical phenomena) and leading to 'cavity enhanced droplet spectroscopy' (Nockel and Chang, 2002;Symes et al , 2004). In recent work Rushworth et al , 2012) have used broadband CEAS/ICOS to record real-time absorption spectra of μ L-sized aqueous-phase droplets in a microfl uidic chip assembly.…”

Guided-mode-resonance-enhanced measurement of thin-film absorption

“…Sophisticated techniques, such as ellipsometry, interferometry, and photoacoustics, have been applied to measure absorption in thin films [3][4][5]. Alternatively, cavity-enhanced techniques effectively increase the optical path length by placing the sample within an optical cavity, where excitation light undergoes multiple passes through the sample [6][7][8]. Nanophotonic devices that support optical cavity modes have also been utilized to amplify the interaction of the excitation light and analyte [9][10][11][12].…”

Real-time optical pH measurement in a standard microfluidic cell culture system

“…The main difficulty in implementing absorption measurements in microfluidic systems, however, is related to the short light path through the absorbing medium. Several approaches have been suggested to increase the light path to improve detection accuracy, through modification of the microfluidic chip design such as the integration of optical fibers, complex mirror designs, and/or integrated optical cavities [14], again adding significantly to fabrication complexity and potentially interfering with the design flexibility of the microfluidic system.…”