Conference Agenda

This study presents the development of a novel closed-loop fiber-optic Raman probe system, designed to enhance and simplify current open-loop methods for fiber-based Raman spectroscopy. One aspect of this system is the integration of a computer-controlled liquid lens within the optical setup, enabling rapid autofocus capabilities. The design process involved evaluating various optical configurations against criteria, such as focal quality, working distance, and the numerical aperture (NA) for the signal collection. The chosen configuration achieved an image NA of 0.46, a working distance range of 7.6~10.3 mm, and an RMS radius of 49.5 μm. A unique three-channel probe was developed for the system. It includes a channel for the laser source to focus on the sample, and two additional channels for signal collection and analysis of which one is used for a photodiode detector and the other to perform Raman spectroscopy. The photodiode detector plays a crucial role in the closed-loop system by providing feedback for the autofocus mechanism. A new autofocus algorithm was designed to dynamically maintain the focal point on the sample, operating effectively within 50 ms, allowing handheld applications. The probe's performance was tested using polystyrene and polycarbonate sample. Results from these experiments, particularly when compared to control experiments with a fixed focal length, demonstrated the probe's ability to acquire accurate Raman spectra at various sample distances and speed. The study confirms that the 50 ms response time of the autofocus system is suitable for handheld operation, marking a significant advancement in fiber-optic Raman spectroscopy techniques.

Mid-infrared (mid-IR) spectroscopy is a powerful analytical technique, offering label-free, molecule-specific detection based on the fundamental vibrational transitions of chemical bonds. It is particularly well-suited for applications in the life sciences, including real-time monitoring of biochemical processes, breath analysis for medical diagnostics, and detection of biomarkers in complex biological matrices. A key technological enabler for such applications is the development of compact, stable, and tunable laser sources operating in the mid-IR range. Quantum Cascade Lasers (QCLs) have emerged as the leading technology in this domain due to their inherent spectral flexibility, high output power, and potential for integration into portable sensing systems.

In this contribution, we present the design, fabrication, and characterization of a series of quantum cascade lasers covering an exceptionally broad emission wavelength range from 3.8 μm to 14 μm. These devices were realized using bandstructure engineering tailored for different wavelength regimes, relying on InGaAs/AlInAs active regions grown on InP substrates using molecular beam epitaxy (MBE) and metal-organic vapor phase epitaxy (MOVPE). Waveguide designs were optimized to ensure good modal confinement, efficient heat dissipation, and robust single-facet emission across the spectral range. The lasers demonstrate continuous-wave (CW) operation with typical output powers in the range of tens to hundreds of milliwatts, depending on the emission wavelength and cavity configuration.

For spectroscopy applications requiring high spectral resolution and mode selectivity, we have developed single-mode QCLs based on distributed feedback (DFB) and coupled-cavity configurations. DFB QCLs with integrated gratings were fabricated for targeted wavelengths such as 5.2 μm and 11 μm, corresponding to strong absorption features of biologically relevant molecules including nitric oxide (NO), carbon monoxide (CO), and various volatile organic compounds (VOCs). The DFB structures exhibit robust single-mode emission with side mode suppression ratios (SMSRs) exceeding 30 dB and wavelength tuning capabilities via temperature and current control.

In addition to DFB devices, we have implemented coupled-cavity QCLs utilizing optical feedback from passive resonators and Vernier effect-based tuning mechanisms. These configurations provide enhanced spectral selectivity and allow for broader tuning ranges compared to conventional DFB lasers, enabling the interrogation of multiple analytes or overlapping spectral features in complex biological samples.

All devices were comprehensively characterized using spectroscopic and electrical techniques, including high-resolution Fourier-transform infrared (FTIR) spectroscopy, beam profiling, and power-current-voltage (P-I-V) measurements. Long-term stability and reproducibility were also assessed to ensure suitability for integration into field-deployable spectroscopic instruments.

The presented results demonstrate the maturity and versatility of QCL technology as a foundation for mid-IR spectroscopic tools tailored for life science applications. The combination of broad wavelength coverage, single-mode operation, and high output power opens the door to advanced sensing platforms for healthcare, environmental monitoring, and biochemical diagnostics.

The growing demand for monitoring specific molecules in environmental, health, and security applications has created a need for inexpensive and power-efficient light sources. In particular, the mid-infrared (MIR) wavelength ranges from 3 µm to 9 μm is of high interest for gas-sensing. Many trace gases ubiquitous to industrial sites have their strongest absorption bands in this region, e.g. carbon dioxide, nitric oxide, water, and various important hydrocarbons. They show absorption strengths that are several orders of magnitude higher than those in other spectral areas.

Aiming at providing reliable broadband and cost-effective alternatives to standard optical gas analysis, nanoplus has developed light-emitting diodes (LEDs) in the MIR. The novel substrate-side-emitting devices rely on the innovative nanoplus technology for distributed feedback (DFB) interband cascade lasers (ICL) and are available at customized wavelengths between 2800 nm and 6500 nm. They display higher wall-plug efficiencies and maximum output powers than previous MIR LEDs, operating in continuous-wave (cw) at room temperature.

nanoplus has specialized in designing DFB lasers for high-precision gas sensing in industry and research. Based on a ridge waveguide structure, which is independent of the material system, nanoplus designs CW DFB lasers at any wavelength between 760 nm and 14 µm. The nanoplus flagship product is a DFB ICL with target wavelengths from 2800 nm to 6500 nm and an extremely narrow linewidth of below 3 MHz. At top-rated wavelengths, the laser shows output powers above 15 mW and is hence perfectly suitable for highly sensitive gas detection. In this talk we will present various applications which utilize DFB ICLs or MIR LEDs and give a general overview of this technology including latest results from interband cascade photodiodes (PD) and LED-arrays in the MIR or up to 10 µm and some of our R&D projects.

The development of novel energy materials, such as organic photovoltaics, next-generation batteries, and hydrogen storage media, is pivotal for enabling sustainable energy solutions. These materials often exhibit unique nanoscale features and highly localized functional properties due to their increased surface-to-volume ratios, requiring advanced characterization techniques to fully understand and optimize their performance. Nanoscale infrared microscopy offers exceptional spatial resolution combined with broadband infrared spectroscopy capabilities, making it a powerful tool for the comprehensive investigation of these materials. In applications ranging from solid-state and metal-air batteries to ion-exchange membranes, nanoscale infrared methods enable detailed chemical mapping and correlation of morphology with electrochemical functionality. Similarly, in organic and perovskite-based photovoltaics, nanoscale infrared techniques reveal critical insights into local structure–property relationships, morphology, and degradation mechanisms—ultimately aiding in the enhancement of power conversion efficiency and long-term stability. By bridging optical, electrical, and mechanical modalities in one technique, nanoscale infrared microscopy is transforming how researchers design and optimize next-generation energy materials for both academic research and industrial applications. This presentation highlights recent applications enabled by infrared nanoscale technology.

Spectrally resolved sensing gains increasing interest for deriving analytical information from a sample in contact-free and fast manner. Regarding spatially resolved information, this has evolved towards hyper spectral imaging (for low resolution imaging). Alternatively, miniaturizing and parallelizing spectrally resolving systems remains a challenge, especially in the context of mobile applications. In a recent work, we have introduced an approach based on planar micro-optics technologies. It is based on stretched grating spectrometers that are arranged in a two-dimensional array configuration [1]. The array configuration features the option to increase the system’s number of channels without increasing the complexity of the optical system.

This approach has been advanced towards spectral resolution in the 3 nm range for approximately 30 simultaneous measurements in the visible spectral range (400..800 nm), and has been combined with a fiber based light delivery for distributed sensing applications. Therefore, a dedicated optics design has been developed, which combines (i) an effective medium grating to reach polarization independent diffraction efficiency across the full spectral range, (ii) an array of identical micro-optical channels with an aperture in the 1.5 mm range to reach the required spectral resolution, and (iii) sufficient lateral imaging capability of each channel to enable laterally resolved detection along the slit. The latter enables to read out three fibers per spectrometer channel. The number of channels depends on the image sensor used: For the 9 mm diagonal sensor Sony IMX178 we manage to arrange 13 channels, whilst each channel analyzes the spectra from 3 fibers.

To set up this kind of system, refractive elements have been prepared by wafer-level polymer-on-glass replication, including the preparation of achromatic lenslets. The transmission grating fabricated by electron lithography has been arranged inside a micro-optical prism-grating-prism configuration. The optical elements have been arranged by passive integration on chip scale; fiber array and image sensor have been mounted under active alignment to ensure light throughput as well as sharpness of the image. In total, the optical system features a length of 18 mm only, resulting in volume of less than 8 ml.

The presentation will motivate and illustrate optical system design, the components being used as well as initial characterization results.

^[1] N. Danz, B. Höfer et al., Optics Express 27 (2019) 5719

Bridging the gap between chemical specificity and spatial resolution, the optical photothermal mid-infrared microscope (OPTIR) provides a new path for materials and life science research. The first commercial OPTIR instrument has been developed to collect infrared spectra and acquire images in point-based scan mode with dwell times down to a 1 ms per pixel. OPTIR images have sub-micron lateral resolution and are less impacted by anomalous scattering phenomena than direct quantum cascade laser (QCL)-based or Fourier transform infrared (FTIR) spectroscopic imaging [1,2]. Recently wide-field OPTIR techniques were developed for capturing images of large areas (20 – 100 µm in diameter) at submicron spatial resolution in a few hundreds of milliseconds [2]. An innovative implementation will be showcased, motivated by these advancements to enhance the field of view to larger than 200 µm with a high-power free-electron laser (FEL). The wide-field setup was first implemented in a time-gate manner with QCL excitation as pump source (50 KHz repetition rate), LED as a probe source (50 KHz) and 24 MP CMOS camera (1 - 400 Hz frame rate) as a detector using a waveform generator as the trigger source. The CMOS camera was used to capture images while the infrared beam was on (hot) and off (cold). The difference between hot and cold image generates photothermal contrast. Using a 40×, 0.6 NA objective in a field of about 40×40 µm², the system demonstrated a spatial resolution of less than 1.6 µm. Moreover, IR spectra can be reconstructed by a series of images at 5 cm^-1 wavenumber intervals. Additionally, the QCL was replaced with the FEL, and hyperspectral images were obtained from mouse brain tissue and THP-1 cells. The FEL-based setup imaged an area over 19 times larger than QCL based. The FEL-based wide-field detection with a high-speed CMOS would facilitate widefield image acquisition with improved time resolution. This advancement could significantly benefit various fields, including neuroscience, by enabling researchers to explore the dynamics of chemical composition in functional brain tissues from large areas and to map neurotransmitters and other important biomolecules.

Acknowledgement: This work is funded within the Leibniz Center for Photonics in Infectious Research (LPI) by BMBF.

[1] C. Krafft, R. Salzer, S. Seitz, C. Ern, M. Schieker. Differentiation of individual human mesenchymal stem cells probed by FTIR microscopic imaging. Cytometry A, 2005, 64A, 53-61

[2] Teng, Xinyan, et al. "Mid-infrared Photothermal Imaging: Instrument and Life Science Applications." Analytical Chemistry 96.20 (2024): 7895-7906..