Advances in Mid-Infrared Imaging: Single-Pixel Microscopy Modernized with Quantum Lasers

Artistic impression of QCL microscope © Oranuch - stock.adobe.com

Chronicling Mid-Infrared Imaging with Quantum Lasers

In a significant advancement for infrared (IR) microscopy, researchers from the Research Center for Non-Destructive Testing GmbH (RECENDT) and Johannes Kepler University Linz have demonstrated a mid-infrared hyperspectral microscope utilizing single-pixel imaging (SPI) technology and a quantum cascade laser (QCL). Their work, published in Optics Express, addresses long-standing limitations IR microscopy, paving the way for faster, more affordable, and high-resolution chemical imaging (1,2).

IR microscopy is essential for analyzing chemically and spatially diverse samples. The technique allows scientists to identify molecular composition non-destructively, making it invaluable across various scientific disciplines. Historically, Fourier-transform infrared (FT-IR) spectrometers equipped with thermal IR sources have dominated the field. However, these conventional methods often suffer from drawbacks such as slow data acquisition, limited spectral brightness, and high costs due to complex focal plane array detectors (FPAs).

The team, led by Augustin Zuljevic, Alexander Ebner, and colleagues, tackled these challenges by integrating a QCL—a high-brightness laser source tunable between 8.3 and 11.1 µm—into a single-pixel imaging system. Unlike traditional FPAs, SPI uses a digital micromirror device (DMD) to modulate light spatially and record intensity with a single detector, significantly reducing costs while enhancing performance (1,2).

Technical Innovations and Findings

A key challenge in designing the microscope was managing diffraction effects caused by the DMD's microstructure. The team conducted a detailed analysis of diffraction patterns, comparing the zeroth and first diffraction orders. They found that while the zeroth order lacked sufficient contrast to distinguish "on" and "off" states of the micromirrors, the first diffraction order provided optimal modulation contrast—a critical factor for SPI accuracy (1).

The system’s spatial resolution was rigorously tested using a reflective resolution target, demonstrating performance beyond 24.8 µm at a 10.1 µm wavelength. This level of detail represents a significant improvement over many conventional IR systems (1).

To validate the microscope's spectral capabilities, the researchers analyzed an 8 µm-thick polypropylene (PP) foil and compared the results with reference FT-IR spectra. The QCL-based system accurately identified characteristic absorption features of polypropylene, confirming its efficacy for chemical detection. This validation highlights the system's potential for practical applications in fields ranging from materials science to biomedical diagnostics (1).

Overcoming Limitations and Future Directions

Despite its successes, the system faces certain limitations. The reliance on the first diffraction order introduces wavelength-dependent diffraction angles, complicating the optical setup and narrowing the spectral bandwidth. Additionally, while QCL illumination is intense, using the first diffraction order reduces the signal-to-noise ratio (S/N) since much of the diffraction intensity is concentrated in the zeroth order (1).

The researchers suggest that future improvements could involve custom optical components to address bandwidth limitations or enhancements to the zeroth order’s diffraction contrast. Such advancements could further boost the S/N and simplify the system’s design. Another promising direction is exploring structured illumination techniques, which could mitigate heat damage in sensitive samples, expanding the system's applicability (1).

This pioneering work by Zuljevic, Ebner, and their colleagues demonstrates the transformative potential of combining single-pixel imaging with QCLs for mid-infrared microscopy. By overcoming key challenges associated with traditional FT-IR systems, this new approach offers a faster, more affordable, and highly precise tool for chemical analysis. As the technology continues to advance, it holds promise for diversifying research and industrial applications in microscopic imaging.

References

(1) Zuljevic, A.; Ebner, A.; Gattinger, P.; Zorin, I.; Rankl, C.; Hingerl, K.; Brandstetter, M. Mid-Infrared Hyperspectral Single-Pixel Microscopy With a Quantum Cascade Laser. Optics Express 2024, 32 (20), 35184–35193. DOI: 10.1364/OE.535296

(2) Weida, M.J. and Yee, B., 2011, February. Quantum Cascade Laser-Based Replacement for FT-IR Microscopy. In Imaging, Manipulation, and Analysis of Biomolecules, Cells, and Tissues, IX, SPIE 2011, 7902, 280–286. DOI:10.1117/12.873954