Abstract
A micro-interferometer based on surface third-harmonic generation (THG) at two-photon-polymerized SU-8 cuboids for real-time monitoring of the refractive index changes of target fluids, which can be easily integrated into microfluidic photonic systems, is demonstrated. The third-harmonic (TH) interferogram is selectively generated only from the target volume by a simple vertical pumping, thereby eliminating the needs for complicated coupling and alignments. The dependence of the generated TH to the input pump polarization state is thoroughly investigated. The THG efficiency by linearly polarized excitation is found to be 2.6 × 10-7, which is the most efficient at the SU-8-air interface and independent of the input polarization direction. The THG efficiency from the SU-8-air interface is 12.17 times higher than that from the glass-air interface and 4.93 times higher than that from the SU-8-glass interface. Real-time monitoring of argon gas pressure is demonstrated using the micro- interferometer. The surface TH from two-photon-polymerized 3D structures offers novel design flexibility to the nonlinear optical light sources for microfluidic and microelectronic devices.
Highlights
Real-time on-chip measurement of optical refractive indices with high precision is of great interest in lab-on-a-chip devices and micro-electro-mechanical systems (MEMS) for ultrasensitive detection of fluid flow in bio-sensing [1,2,3], chemical interaction monitoring [4,5], and environmental monitoring [6]
We demonstrate a two-photon-polymerized micro-sized third-harmonic optical interferometer for the real-time, on-chip monitoring of microfluidic refractive indices
We have thoroughly characterized TH excited at different interfaces of twophoton-polymerized SU-8 structures with different incident powers, input polarization states, and excitation depths
Summary
Real-time on-chip measurement of optical refractive indices with high precision is of great interest in lab-on-a-chip devices and micro-electro-mechanical systems (MEMS) for ultrasensitive detection of fluid flow in bio-sensing [1,2,3], chemical interaction monitoring [4,5], and environmental monitoring [6]. Optical micro- and nano- structures are manufactured through mask-based photolithography [23], mask-less electron-beam lithography [23], and focused ion beam processes [20]. Because TPP is based on a nonlinear optical two-photon process, the printing resolution can overcome the traditional optical diffraction limit; various novel micro- structures have been demonstrated in the past decade [24]. Lab-on-a-chip devices require compact coherent light sources for high-precision refractive index (RI) measurements within a small volume near to target samples. For this purpose, two micro-sized parallel cuboids were additively manufactured via TPP using 100-fs near-infrared femtosecond laser pulses; these structures worked as the nonlinear optical media generating coherent third-harmonic photons with high spectral purity. The real-time RI measurement was demonstrated up to 25 frames-per-second, which is limited by the camera frame rate, which can be improved to hundreds of kHz by introducing a higher speed camera
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