Peter Tidemand-Lichtenberg

This paper investigates silver gallium selenide (AGSe) as a promising alternative to silver gallium sulfide (AGS) for upconversion detection beyond 10 µm. AGSe offers a wide transparency range extending to ∼20 µm, making it a candidate for upconversion detection in the challenging 10 to 20 µm range. Beyond 12 µm, AGS is not an option due to high absorption. An experimental setup is implemented for crystal comparison in the 10 to 12 µm range, based on a tunable quantum cascade laser (QCL). Phase matching constraints in AGSe required an 1880 nm mixing laser, whereas AGS was conveniently implemented with a 1064 nm mixing laser. Detection of the upconverted signal is consequently based on InGaAs (for AGSe) and Si detectors (for AGS), respectively. Results show that AGSe offers higher upconversion efficiency and superior phase matching properties, making it a promising crystal for long-wavelength infrared (IR) detection.

In this paper, a novel approach based on frequency upconversion in ultra-thin nonlinear crystals is investigated for use in high-resolution infrared (IR) microscopy in the 5–12 µm range, an important domain for biomedical research. Traditional IR imaging encounters spatial resolution constraints due to diffraction, which are addressed via upconversion imaging using ultra-thin crystals. The present work combines a tunable IR quantum cascade laser and a short wavelength mixing laser to circumvent the classical resolution limit dictated by the Rayleigh criterion. A detailed numerical model for small signal upconversion imaging at μm-scale resolution shows good agreement with experimental data. The presented approach opens new avenues for IR applications for label-free biomedical diagnostics and spectral imaging.

Since the inception of the second-order nonlinear frequency conversion in 1961, enhancing the inherent low conversion efficiency has been a primary objective. This goal has been successfully accomplished through the utilization of cm-long nonlinear crystals characterized by high quality and nonlinearity, coupled with versatile phase-matching strategies and high-power mixing lasers. However, the reliance on lengthy nonlinear crystals and the necessity for precise phase-matching introduce stringent tolerances on acceptance angles and spectral bandwidths for the interacting fields, thereby constraining its widespread applicability in scientific and industrial domains. This challenge is addressed by combining a broadly tunable ∼5 mW quantum cascade laser operating in the 9.5–12.5 µm range with upconversion detection in ∼100 µm long AGS crystals. Using a tightly focused continuous wave Nd:YVO₄ laser with 20 mW output power and spatial filtering of the upconverted beam lead to a SNR of 55 for 50 µs averaging time sufficient for many applications.

In this Letter, a novel approach for unidirectional operation of a 1064 nm solid-state ring laser is demonstrated based on difference frequency mixing. Unidirectional operation is achieved exploiting the directional parametric gain from a single-pass diode laser, facilitated through a periodically poled LiNbO₃ crystal. In addition to achieving unidirectional operation, the nonlinear process further enables the generation of single-frequency mid-infrared light. Using a single-pass tapered diode laser, tunable in the range from 780 to 815 nm, the generated mid-infrared signal covers the 2.9 to 3.5 µm range while optimizing the phase-match condition of the difference frequency generation process.

In this Letter, an efficient tunable upconversion module is demonstrated and characterized. The module combines high conversion efficiency and low noise with broad continuous tuning, covering the spectroscopically important range from 1.9 to 5.5 µm. A fully computer-controlled, compact, portable system is presented and characterized in terms of efficiency, spectral coverage, and bandwidth, using simple globar illumination. The upconverted signal is in the 700–900 nm range, ideal for Si-based detection systems. The output from the upconversion module is fiber coupled, enabling flexible connection to commercial NIR detectors or NIR spectrometers. In order to cover the spectral range of interest using periodically poled LiNbO₃ as the nonlinear material, poling periods ranging from 15 to 23.5 µm are needed. The full spectral coverage is reached using a stack of four fanned poled crystals, enabling maximal upconversion efficiency of any spectral signature of interest in the 1.9 to 5.5 µm range.

We investigate synchronous upconversion of a pulsed, tunable quantum cascade laser (QCL) in the important 5.4–10.2 µm range, with a 30 kHz, Q-switched, 1064 nm laser. The possibility to accurately control the repetition rate and pulse duration of the QCL results in a good temporal overlap with the Q-switched laser, leading to an upconversion quantum efficiency of 16% in a 10 mm-long AgGaS₂ crystal. We investigate the noise properties of the upconversion process in terms of pulse-to-pulse energy stability and timing jitter. For QCL pulses in the 30–70 ns range the upconverted pulse-to-pulse stability is approximately 1.75%. The demonstrated combination of broad tunability and high signal to noise in the system is well-suited for mid-IR spectral analysis of highly absorbing samples.

We report on mid-infrared optical coherence tomography (OCT) at 4 µm based on collinear sum-frequency upconversion and promote the A-scan scan rate to 3 kHz. We demonstrate the increased imaging speed for two spectral realizations, one providing an axial resolution of 8.6 µm, and one providing a record axial resolution of 5.8 µm. Image performance is evaluated by sub-surface micro-mapping of a plastic glove and real-time monitoring of C O 2 in parallel with OCT imaging.

Near- and mid-infrared optical coherence tomography (OCT) is evaluated as a non-destructive and non-contact reflection imaging modality for inspection of industrial and marine coatings. Near-infrared OCT was used to obtain high-resolution images (~6/2 µm lateral/axial) of hidden subsurface cracks and defects in a resin base coating, which had been exposed to high pressure and high temperature to study coating degradation in hostile environments. Mid-infrared OCT was employed for high-resolution (~15/8.5 µm lateral/axial) subsurface inspection of highly scattering marine coatings, demonstrating monitoring of wet film thickness and particle dispersion during curing of a 210 µm layer of antifouling coating, and detection of substrate corrosion through 369 µm of high-gloss alkyd enamel. Combining high-resolution and fast, non-invasive scanning, OCT is therefore considered a promising tool for studying coating performance and for industrial inspection.

Mid‐infrared (MIR) spectroscopy, which has important applications in medicine, environmental monitoring, materials, and food science, is widely performed using Fourier transform infrared (FTIR) spectrometers as gold standard. Despite decades of development, FTIR systems are vulnerable to vibration and have limited temporal resolution due to reliance on mechanically scanned mirrors and MIR direct detectors that have a slower response than their near‐infrared counterparts. Using cryogenically cooled detectors, state‐of‐the‐art FTIR systems have reached a signal‐to‐noise ratio (SNR) of ≈6000 at 1 s integration time (at 4 cm⁻¹ spectral resolution). Here, a novel MIR upconversion spectrometer (MIRUS) is presented with a record‐high SNR > 10 000 at 1 s (6 cm⁻¹ resolution), outperforming FTIR systems by circumventing the need for sophisticated cooling and any moving part, thus enabling operation in harsh environments. It has a spectral coverage of 6–12 µm—the broadest for an upconversion spectrometer to date. The MIRUS uses broadband intracavity upconversion to convert the signal spanning the MIR fingerprint region to the near‐infrared where sensitive Si‐detector based spectrometers operate with rates easily reaching kilo spectra per second. Applications of MIRUS for gas sensing, plastic identification, and rapid photopolymerization monitoring are demonstrated.