The terahertz (THz) portion of the electromagnetic frequency spectrum—approximately ranging from 0.3 THz to 3 THz—is recently gaining significant attention. Numerous applications, such as security screening and high data-rate wireless links to name two relevant examples, can benefit the non-ionizing character of THz radiation with respect to X-rays and the increased resolution and bandwidth with respect to microwave techniques. It is essential to accurately characterize the THz properties of materials with appropriate techniques capable of dealing with dielectric, metal-dielectric, and conductive samples and provide results over a relatively wide range of frequencies with sufficient frequency resolution. In this respect, THz time-domain spectroscopy (THz-TDS) is a valuable tool, although the commonly used transmission mode fails to be effective when used on highly reflective samples, such as thin conducting films or metasurfaces, which are both components that are widely used in THz applications. On the other hand, the less explored THz-TDS in reflection mode has recently been proven to be a very effective technique for accurately characterizing such kinds of components, as well as most of the dielectric and conductive materials used in THz applications. For this purpose, in this abstract, we will show the measurement protocols that we recently developed for obtaining an accurate electromagnetic characterization of materials and components at THz. Specific care will be paid to the dispersive models employed to simultaneously fit the THz-TDS measurements and comply with electromagnetic theory. We will show a variety of experimental validations over commercial dielectric materials (e.g., Rogers, ciclo-olephins, and dry resists), novel 2-D materials (e.g., graphene, graphene oxide, and transition metal dichalcogenides), thin conducting films (e.g., aluminum zinc oxide, indium tin oxide, and titanium), and metasurfaces (e.g., patch arrays, strip gratings, and fishnets). Finally, we will briefly comment on how the latter components can profitably be used to realize THz radiation devices.

In this study, the all-optical C-band switching capability of the nonlinear characteristics of Europium Oxide-Polyvinyl Alcohol (Eu₂O₃-PVA)-based saturable absorber (SA) thin film is experimentally shown. Eu2O3-PVA-based thin film saturable absorber was fabricated using a simple method and its all-optical switching performance was obtained by installing an all-fibre-based experimental testbed. In the experiments, a continuous wave (CW) signal at 1555 nm with an optical power of 20.41 mW generated by a tuneable laser source was converted into a variable frequency square-wave-modulated switching signal using a mechanical shutter/modulator. This modulated switching signal was then applied to the Eu2O3-PVA-based SA thin film with a CW signal at 1545 nm with an optical power of 3.47 mW provided by another tuneable laser source to switch the absorption characteristics of the SA between high-loss and low-loss cases. It has been shown that the 1545 nm CW signal was optically ON--OFF switched due to nonlinear absorption loss variations in SA generated by the saturating switching light at 1555 nm. In the experiments, all-optical switching in Eu2O3-PVA-based saturable absorber thin film succeeded for switching frequencies up to 4 KHz. The rise and fall times of the modulated light measured at 302 Hz were 500 µs and 350 µs on average, respectively.

In the field of photonics and biophotonics, the need for higher resolution for microscopy and nanoscopy is of prime interest for many applications like the semiconductors industry (carrier density) or biological studies and diagnostics. After a detailed description of the free space approach, using tips just like those used for AFM or original ones that we proposed a few years ago, we will focus on the guided wave approach and particularly on a solution for microscopy-on-chip with an integrated photonic source that generates terahertz waves. Both approaches with free space or guided waves are presented with an original topology in each case. For free space infrared and terahertz waves, the shape of the tip plays a key role for an antenna-like coupling with the sample. For the guided mode approach, the electromagnetic confinement around a single conductor, which acts as a plasmonic circuit, is exploited to efficiently bring the sensing wave to the sample. Concerning biological studies, we will also see how the proposed approaches are compatible with liquid media, opening many areas of biophotonic application. The key point is the on-chip-integration of the active photonic device, the UTC-PD (Uni-traveling-carrier Photodiode), that can generate millimetre and terahertz waves with an ultrabroad behaviour that is useful for hyperspectral imaging.

The modulation of the optical response of structures fabricated in reactive mesogens (RMs) has proven to be a versatile and innovative strategy in creating polymeric devices with the tailored optical properties typical of liquid crystals. [1-3]

Among the fabricating techniques, two-photon lithography (TPL) stands out as one of the most suitable technologies for producing complex 3D objects with high sub-wavelength resolution [4] and, at the same time, it allows us to modify the RMs' internal architecture.

Here, we show how the nematic phase is perturbed when TPL is performed along an angle that is not parallel or perpendicular to the director vector, resulting in the reordering of RMs. This leads to a reorientation of the optical axis and birefringence, which can be finely controlled in terms of the writing parameters (i.e., laser power, scan speed, periodicity).

Similar studies performed on cholesteric RMs aim to demonstrate the possibility of fabricating devices with customizable photonic band gaps across the entire visible spectrum, simply by tuning the energy dose delivered during manufacturing [5]. A further modulation of the optical device response can be achieved through temperature control. [6]

These studies are exploited to fabricate innovative security devices that cannot be replaced with other technologies or materials.

References:

[1] H. Zeng et al. Adv. Mater., 2014, 26, 15, 2319.

[2] A. M. Flatae et al., Light Sci., 2015, Appl. 4, 4, e282.

[3] R. Wei et al. Macromol. Rapid Commun., 2013, 34, 4, 330.

[4] S. Engelhardt, Direct Laser Writing. In Laser Technology in Biomimetics; V. Schmidt, M. R. Belegratis, Eds.; Springer-Verlag, Berlin/Heidelberg 2013.

[5] T. Ritacco et al, Adv. Optical Mater., 2021, 2101526.

[6] T. Ritacco et al, Liquid Crystals., 2024, 1,9.

Infrared small target tracking on complex backgrounds is a challenging research hotspot, which plays an important role in target warning, ground monitoring and flight guidance. In an infrared background image, the target occupies few pixels and has low contrast and no specific shape; the target edge is not obvious, lacks texture and is point-like; and there are often difficulties in the tracking process such as target occlusion, background clutter and rapid movement, which increase the difficulty of tracking infrared weak and small targets. In order to improve the tracking performance of traditional correlation filters for small targets, we design a target-tracking algorithm based on the structure tensor, which is a symmetric, semi-positive definite matrix that can represent a certain regularity in the neighbourhood of a point in the space, and the structural tensor feature extraction method is used as an epistemic model of the tracking algorithm, which can adequately describe the characteristics of the small targets that exhibit sudden changes in grey scale and are not correlated with the neighbourhood. Compared with origional correlation filters, it has a great advantage in extracting the structural information of the small target. Experiments show that, compared with other tracking algorithms, our proposed tracking algorithm effectively avoids the influence of false alarms in infrared target tracking under complex environments and has a significant advantage in terms of its accuracy index and high computational efficiency.

Purpose: Stem cell therapy has the potential to treat currently incurable diseases, such as geographic atrophy in macular degeneration. However, tracking stem cells after transplantation is a major challenge. This study demonstrates an advanced, non-invasive, high-resolution, multimodality platform technology for the longitudinal visualization of damaged retinal pigment epithelium (RPE) using photoacoustic microscopy (PAM), optical coherence tomography (OCT), and fluorescence microscopy (FM) in living rabbits.

Methods: Millisecond laser photocoagulation was applied to 12 New Zealand rabbits to create RPE damage. On day 4 post-laser treatment, each eye received a subretinal injection of 30 µL (3.3×10⁶ cells/ µL) human-induced pluripotent stem cells differentiated to RPE (hiPSC-RPE) cells labeled with ultrapure chain-like gold nanoparticle (CGNP) clusters conjugated with indocyanine green. The CGNP clusters have a red-shifted optical absorption in the near-infrared window, and their diameter of 7-8 nm after disassembly enables renal excretion. hiPSC-RPE cells were followed up to 8 months after transplantation by color fundus photography, PAM, OCT, and FM.

Results: PAM images at 650 nm showed the distribution of the hiPSC-RPE cells, demonstrating that the cells rapidly localized to laser burns within 1 week and remained at the laser burn sites with signal for a duration of 8 months. PAM images at 578 nm visualized the microvasculature. Cells were observed using FM up to 28 days post-injection with a significant reduction in fluorescence signal by 1 month. The co-registration of PAM and OCT images validated the location of hiPSC-RPE cells to RPE injury sites. Histological and immunofluorescence images confirmed the imaging results. TEM and confocal images demonstrated CGNP clusters within hiPSC-RPE cells without affecting the cell’s morphology, pigmentation, and RPE differentiation or function.

Conclusions: This research presents an innovative platform technology for the longitudinal imaging of cell-based therapies in living animals for a duration of 8 months using PAM, OCT, and FM imaging.

In the Fourier Single-Pixel Imaging (F-SPI) framework, Fourier basis patterns serve as orthogonal bases. An LED light source is reflected onto a Digital Micromirror Device (DMD) to modulate the light field, which is subsequently focused onto the sample's focal plane using an optical setup. A single-pixel detector captures the light intensity, and the object image is reconstructed via the four-step phase shift algorithm. An L-norm-based compressed reconstruction algorithm is introduced to achieve high-quality F-SPI at reduced spectrum sampling rates. Both simulation and experimental results indicate that the NESTA algorithm outperforms the D-AMP algorithm in terms of imaging efficiency, while ensuring superior imaging quality. The NESTA algorithm is adopted as the compressed sensing framework (CSF). These findings suggest that the proposed method can significantly enhance the sparse sensing module of F-SPI, facilitating rapid and high-quality image reconstruction. The Fourier regularization reconstruction mode and sub-pixel shift artifact removal algorithm are introduced to further reduce the imaging time consumption and suppress the imaging noise artifacts to a certain extent. Building upon the experimental validation of imaging efficiency in the Fourier Single-Pixel Imaging Compressed Sensing System (SPI-CSS), we discuss the Pixel Single-Pixel Imaging (P-SPI) scheme, which utilizes Zernike polynomials as orthogonal bases. In this approach, the modulated light field conforms to the Zernike polynomial patterns. The imaging process involves repeating the steps of Fourier Single-Pixel Imaging to record light intensity, followed by iteratively summing the product of Zernike polynomials and Zernike moments to reconstruct the target image. Experimental results demonstrate that P-SPI can effectively reconstruct images even at low sampling rates. Furthermore, compared to F-SPI, P-SPI exhibits a superior robustness to background noise within the SPI-CSS framework. This advantage extends the applicability of P-SPI to more complex environments, highlighting its potential for broader use in challenging imaging scenarios.

The quantitative phase imaging technique based on the Kramers–Kronig relation is an innovative computational imaging method that does not require iteration and only needs four low-resolution images to achieve a large spatial bandwidth–time product, optimizing imaging performance. Based on this, the method employs polarization-multiplexed LED illumination, with three LED components each covered by 0°/45°/135° polarization filters, providing numerically aperture-matched illumination for the sample. Using Malus's law, the three polarized light fields recorded by the polarization camera are decoupled, allowing us to obtain intensity images of LED illumination from three different angles in a single measurement. The intensity images for each polarization channel are separated and processed, and the spectral subregions of different channels are rotated and shifted to reconstruct the spectrum. Compared to traditional Fourier ptychography and rapid quantitative phase imaging techniques based on the spatial-domain Kramers–Kronig relation, this technology uses single-frame capture to successfully deduce the quantitative phase distribution of samples from intensity information, significantly increasing the frame rate of acquisition and reducing the overall acquisition time. It is highly suitable for dynamic imaging, such as live cell observation, and minimizing the impact of motion artifacts. Because it can provide higher resolution and a broader range of applications, this technology is expected to become an important tool in the fields of biomedical and surface inspection.

Imaging through scattering media is an important research topic in many fields, but it remains challenging. Imaging techniques based on optical memory effects have been extensively studied, but most methods require operations in dark environments. The extra interfering light submerges the speckle information in the spatial noise, resulting in a lower reconstruction quality of the image. In this paper, a principal component analysis method based on singular value decomposition is proposed for imaging under strong interference light. By selecting an appropriate singular value decomposition mode, the environment noise and effective speckle information are fitted to reconstruct the target object. The feasibility and reconstruction effect of this method are verified by experiments. The effective information extracted by the method proposed in this paper is very similar to that obtained under dark room conditions and strong interference light conditions. The selection of singular values is also discussed. This method can still extract speckle information about the unexposed part of the image even if some information is lost due to exposure. Our method can realize the separation of effective speckle information and ambient noise without prior knowledge, breaking through the limitation of the dark room environment of scattering imaging, and because our method can directly separate the target object information from the original speckle, it may be combined with other scattering imaging techniques.

In contrast to the array detector, the single-pixel detector (SD) has a range of advantages, including heightened quantum efficiency, attenuated dark noise, and expedited response time. These advantages engender the extensive utility of single-pixel imaging (SPI) within numerous imaging fields. Traditional single-pixel imaging (SPI) encounters challenges such as a high sampling redundancy and poor imaging quality, constraining its widespread application. Despite a range of orthogonal modulation modes having been employed in structured illumination to enhance imaging performance, some encoding issues still persist in information sampling, impeding the further progression of SPI. We propose an SPI method based on orthogonal Hermite–Gaussian (HG) moments, achieving improved imaging reconstruction through differential modulation of HG basis patterns and linear weighting of the acquired intensity. Moreover, we incorporate compressed sensing algorithms within the framework of HG-SI, integrating moment-based sampling strategies to optimize imaging capability under sparse measurements.

Both simulations and experiments confirm the superior imaging quality and computation efficiency of our proposed Hermite–Gaussian single-pixel imaging (HG-SI), especially at low measurement levels. Our research underscores the effectiveness of HG modulation in SPI reconstruction, enabling high-quality outcomes via compressed sampling. Our method entails no additional setups or constraints compared to other SPI modes, making it applicable to a wide range of SPI scenarios. This advancement propels the investigation of optical field modulation modes within SPI and holds promise in offering a universal solution for weak-intensity and non-visible light microscopy.