Ultra-sensitive mid-infrared evanescent field sensors combining thin-film strip waveguides with quantum cascade lasers

Abstract

We demonstrate ultra-sensitive chemical sensing in the mid-infrared spectral regime with a combination of quantum cascade lasers (QCLs) with GaAs/Al(0.2)Ga(0.8)As strip waveguides fabricated via metal-organic vapor-phase epitaxy (MOVPE) and reactive ion etching (RIE) using evanescent field absorption spectroscopy. These strip waveguides have been designed with a width of 200 μm, thereby facilitating 2-D confinement and mode-matched propagation of mid-infrared radiation emitted from a distributed feedback (DFB) QCL at a wavelength of 10.3 μm. Acetic anhydride was detected with a limit of detection (LOD) of 18 pL (19.4 ng) deposited at the waveguide surface by overlapping of the vibrational absorption of the methyl group with the emission frequency of the QCL. The obtained results indicate a remarkable enhancement in sensitivity by three orders of magnitude compared to previously reported multimode planar silver halide waveguides. Further reduction of the waveguide strip width to 50 μm resulted in an additional sensitivity enhancement yielding a calculated LOD of 0.05 pL for the exemplary analyte acetic anhydride, which is among the most sensitive evanescent field absorption measurements with a miniaturized mid-infrared sensor system reported to date.

Cocaine detection by a mid-infrared waveguide integrated with a microfluidic chip

“…6, in which the waveguide output light intensity was measured when we switched different concentrations of cocaine in real time. The flow rate was 50 ml min 21 . Between each concentration, we added pure PCE solvent at the same flow rate for 5-7 min.…”

“…Cocaine detection is a pilot example of this technology. The detection limit of this device still needs optimization when it is compared with other optical methods which have detection limits in the range of several ng ml 21 to several mg ml 21 , 18 but this device has the advantage of the ability to obtain spectral information if a tunable QCL is used. The compact size of the QCL and the waveguides allows the design of portable systems.…”

“…This is tested by using a long (y20 cm) and a short (y6 cm) tube for measuring the switch from pure PCE to a cocaine concentration of 100 mg ml 21 . Fig.…”

“…20 Compared to a slab waveguide, a single mode strip waveguide has a higher fraction of energy in the evanescent field, and hence increases the sensitivity. 21 The material for the microfluidic system is a UV-curable adhesive NOA81 (Norland Products Inc., Cranbury, NI, USA). Compared to PDMS (polydimethylsiloxane), the most widely used polymeric material for microfluidic devices, this low-cost, commercially available adhesive has better chemical resistance, is impermeable to air and water vapor, and is less prone to swelling upon contact with infrared-transparent organic solvents.…”

Monolithically integrated mid-infrared sensor using narrow mode operation and temperature feedback

“…They are widely used in spectroscopic applications and are often integrated in sensing devices. [17][18][19] In this letter, we demonstrate a monolithically integrated lab-on-a-chip using a DFB QCL to improve the selectivity and sensitivity. Weakly coupled DFB lasers with a FWHM of 2 cm À1 are fabricated with the aim to fulfill two important tasks.…”