Diamonds Are a Spectroscopist’s Best Friend: Thin-Film Diamond Mid-Infrared Waveguides for Advanced Chemical Sensors/Biosensors

Abstract

The first combination of mid-infrared (MIR) tunable quantum cascade lasers (tQCLs) with thin-film diamond strip waveguides (DSWGs) suitable for advanced chemical sensing/biosensing is demonstrated. The sensing system is composed of thin diamond films grown on surface-passivated Si wafers via chemical vapor deposition (CVD) and microstructured using inductively coupled plasma (ICP) etching, serving as photonic waveguides for radiation emitted by a broadly tunable quantum cascade laser (tQCL) in the spectral regime of 5.78-6.35 μm (1570-1730 cm(-1)). The characterization of the free-standing diamond waveguides reveals excellent transmission properties across a broad MIR band. As a proof of concept, the detection of acetone in D2O via evanescent field absorption is demonstrated achieving a limit of detection (LOD) as low as 200 pL, which indicates a significant sensitivity improvement compared to conventional MIR slab/strip waveguides reported to date. Providing characteristic absorption features within the tuning range of the tQCL, studies using anisaldehyde as an analyte further corroborate the potential of tQCL-DSWG-based chemical sensors/biosensors.

Mid‐infrared thin‐film diamond waveguides combined with tunable quantum cascade lasers for analyzing the secondary structure of proteins

“…The main features observed in the FTIR spectra are also observed in the spectra obtained via the QCL‐DSWG system. As observed herein, slight differences in the shape of the featured bands have also been documented in literature , i.e., ascribed to residual H 2 O absorption affecting the QCL measurements .…”

“…Diamond waveguides were fabricated by thin‐film deposition with subsequent optical photolithography and inductively coupled plasma etching (ICP) to obtain laterally defined thin‐film diamond waveguides, as previously described . Firstly, microwave plasma‐assisted chemical vapor deposition (CVD) was used to grow a 14‐μm thick diamond film on top of a 200‐nm Si 3 N 4 layer and a 2‐μm SiO 2 cladding layer deposited onto a Si wafer substrate (from Diamond Materials GmbH) with a thickness of 1 mm.…”

“…Also in the MIR, diamond has excellent optical properties such as low self‐absorption and scattering, and transparency over a wide spectral range, which – combined with its high refractive index and its hardness and chemical resistance – render it an ideal material for chem/bio sensing applications in the MIR regime. The combination of tunable quantum cascade lasers (tQCLs) with thin‐film diamond strip waveguides (DSWGs) was recently demonstrated for chemical sensing, thereby revealing a significant sensitivity improvement compared to conventional MIR waveguides .…”

Portable Infrared Laser Spectroscopy for On-site Mycotoxin Analysis

“…Quantitative evanescent field absorption (A) utilizing thin-film waveguides may accordingly be described via a pseudo-Lambert-Beer relationship A = (εcl)r, where ε is the molar absorptivity, c is the concentration of the analyte, l is the equivalent optical path length, and r is the fraction of radiation power residing outside the waveguide core (i.e., within the evanescent field). Consequently, any intensity enhancement of the evanescent field above the waveguide surface directly increases the obtainable signal-to-noise ratio (SNR), and thus, the overall sensitivity of absorption measurements using such thin-film waveguides2428. Consequently, the high spectral density provided by QCLs in combination with the sensitive thin-film waveguides appear ideal for the spectroscopic investigation of minor components and minute spectral changes in complex matrices29.…”

Advances in Mid-Infrared Spectroscopy for Chemical Analysis

“…However, novel detection principles for MIR radiation such as quantum cascade detectors (QCDs) (12,(30)(31)(32) and advanced thin-film (33)(34)(35)(36) and hollow waveguide (HWG) technologies (37)(38)(39) have also contributed to the evolution of conventional MIR spectroscopy into miniaturized chem/bio sensing and assay platforms, as recently reviewed (40,41).…”