Femtosecond laser structuring and selective etching of microchannels in lithium niobate for micro-optofluidics and lab-on-chip applications

In this work, lithium niobate (LiNbO_3, LN) was explored as a platform for microfluidic and micro-optofluidic applications. Using femtosecond laser-assisted selective etching (FLASE) combined with post-etching annealing, optically smooth monolithic microfluidic channels buried within the LN volume were achieved. This approach enables more compact and mechanically stable devices, in contrast to conventional methods that seal planar structures on a substrate with a cover piece. Although LN is not widely used for microfluidic or optofluidic platforms, its unique acousto-optical, electro-optical, and nonlinear optical properties, along with its ability to support surface acoustic waves (SAWs), can be harnessed to develop multifunctional, miniaturized systems, such as lab-on-chip devices, for the manipulation and characterization of analytes. For the first time, selective etching of LN by FLASE was systematically investigated using 40 % hydrofluoric acid (HF). Consistent with observations in yttrium aluminum garnet (YAG) [1], it was found that the dependence of the etch depth d on duration t is not linear but follows the expression d^2 = 2D_γt^γ, where D_γ and γ are the diffusion coefficient and the anomaly parameter, respectively. Here, γ > 1 corresponds to an anomalous diffusion regime, in contrast to normal diffusion where γ = 1. The fastest diffusion coefficient obtained was D_γ = 24 398 μm^{2}/h^{1.5}, which could allow the selective etching of a 1 cm long, 25 μm × 8 μm channel from both sides in 3 days, comparable to what is achieved in glass [2] and seven times faster than in YAG [1]. Microfluidic channels with aspect ratios of 1000 (length-width) and 100 (width-height) were achieved. Various monolithic microfluidic elements — including curved channels, T-, Y-, and cross-junctions — were successfully demonstrated, proving that FLASE is a versatile tool for fabricating monolithic microfluidic elements in LN. Furthermore, thermal annealing — already established for surface smoothening in glass [3, 4] and polymers [5] — was applied here to reduce the post-etch roughness of the produced microchannels to as low as 2 nm, which has previously not been achieved in selectively etched microchannels. The influence of annealing time and temperature on microchannels’ final roughness and shape rounding due to surface tension was investigated. The evolution of surface roughness during annealing was found to follow a functional dependence on initial roughness, annealing temperature, and duration. With this dependence, the final roughness of a selectively etched microchannel (with initial roughness < 60 nm) after a certain annealing time could be estimated. As a first application, a micro-optofluidic refractive index (RI) sensor was fabricated in LN comprising a fs-laser written waveguide intersecting an annealed smooth channel wall in a monolithic configuration. The sensor was characterized using a broadband light source, and the Fabry-Pérot (FP) interference in the microchannel was recorded in reflection. A strong contrast of 24 dB was observed at 1550 nm in the FP interference spectrum when the microchannel of the sensor was air-filled. The refractive index of sucrose mixtures with RI steps of 10^{-3} was measured with a sensitivity of 1215 nm/RIU and an accuracy of 8.5 × 10^{-5} in the vicinity of 1554 nm. Repeated measurements showed that under stabilized temperature conditions, the sensor can achieve reliable RI measurement with a precision equivalent to 3.7 × 10^{-6} RIU. 1. K. Hasse, D. Kip, and C. Kränkel. “Influence of diluted acid mixtures on selective etching of MHz- and kHz-fs-laser inscribed structures in YAG,” Opt. Mater. Express 11(5), 1546–1554 (2021). 2. S. Kiyama et al. “Examination of etching agent and etching mechanism on femotosecond laser microfabrication of channels inside vitreous silica substrates,” J. Phys. Chem. C. 113(27), 11560–11566 (2009). 3. J. Drs, T. Kishi, and Y. Bellouard. “Laser-assisted morphing of complex three dimensional objects,” Opt. Express 23(13), 17355–17366 (2015). 4. A. Krishnan and F. Fang. “Review on mechanism and process of surface polishing using lasers,” Front. Mech. Eng. 14(3), 299–319 (2019). 5. B. Jung, C. K. Ober, and M. O. Thompson. “Controlled roughness reduction of patterned resist polymers using laser-induced sub-millisecond heating,” J. Mater. Chem. C 2, 9115–9121 (2014).