An emerging frontier in condensed matter physics involves novel electromagnetic responses, such as the anomalous Hall effect (AHE), in ferromagnetic Weyl semimetals (FM-WSMs). Candidate FM-WSMs have been limited to materials that preserve inversion symmetry and generate Weyl crossings by breaking the time-reversal symmetry. These materials share three common features: a centrosymmetric lattice, a collinear FM ordering, and a large AHE observed when the field is parallel to the magnetic easy axis. Here we present CeAlSi as a FM-WSM in which the Weyl nodes are stabilized by breaking the inversion symmetry, but their positions are tuned by breaking the time-reversal symmetry. Unlike the other FM-WSMs, CeAlSi has a noncentrosymmetric lattice, a noncollinear FM ordering, and a novel AHE that is anisotropic between the easy and hard magnetic axes. It also exhibits large FM domains that are promising for exploring both device applications and the interplay between the Weyl nodes and FM domain walls.

Despite the fundamental nature of the edge state in topological physics, direct measurement of electronic and optical properties of the Fermi arcs of topological semimetals has posed a significant experimental challenge, as their response is often overwhelmed by the metallic bulk. However, laser-driven currents carried by surface and bulk states can propagate in different directions in nonsymmorphic crystals, allowing for the two components to be easily separated. Motivated by a recent theoretical prediction G. Chang et al., Phys. Rev. Lett. 124, 166404 (2020), we have measured the linear and circular photogalvanic effect currents deriving from the Fermi arcs of the nonsymmorphic, chiral Weyl semimetal RhSi over the 0.45–1.1 eV incident photon energy range. Our data are in good agreement with the predicted spectral shape of the circular photogalvanic effect as a function of photon energy, although the direction of the surface photocurrent departed from the theoretical expectation over the energy range studied. Surface currents arising from the linear photogalvanic effect were observed as well, with the unexpected result that only two of the six allowed tensor element were required to describe the measurements, suggesting an approximate emergent mirror symmetry inconsistent with the space group of the crystal.

Recent experiments in the topological Weyl semimetal TaAs have observed record-breaking second-harmonic generation (SHG), a nonlinear optical response at $2\omega$ generated by an incoming light source at $\omega$. However, whether SHG is enhanced in topological semimetals in general is a challenging open question because their band structure entangles the contributions arising from trivial bands and topological band crossings. In this work, we circumvent this problem by studying RhSi, a chiral topological semimetal with a simple band structure with topological multifold fermions close to the Fermi energy. We measure SHG in a wide frequency window, $\omega \in [0.27,1.5]\mathrm{eV}$ and, using first-principles calculations, we establish that, due to their linear dispersion, the contribution of multifold fermions to SHG is subdominant as compared with other regions in the Brillouin zone. Our calculations suggest that parts of the bands where the dispersion is relatively flat contribute significantly to SHG. As a whole, our results suggest avenues to enhance SHG responses.

The direct detection of singlet-state oxygen (¹O₂) constitutes the holy grail dosimetric method for type-II photodynamic therapy (PDT), a goal that can be quantified using multispectral singlet oxygen near-infrared luminescence dosimetry (MSOLD). The optical properties of tissues, specifically their scattering and absorption coefficients, play a crucial role in determining how the treatment and luminescence light are attenuated. Variations in these properties can significantly impact the spatial distribution of the treatment light and hence the generation of singlet oxygen and the detection of singlet oxygen luminescence signals. In this study, we investigated the impact of varying optical properties on the detection of ¹O₂ luminescence signals during Photofrin-mediated PDT in tissue-mimicking phantoms. For comparison, we also conducted Monte Carlo (MC) simulations under the same conditions. The experimental and simulations are substantially equivalent. This study advances the understanding of MSOLD during PDT.

SignificanceDirect detection of singlet-state oxygen (O12) is a critical objective in Type II photodynamic therapy (PDT) due to its pivotal role in mediating therapeutic effects. Although multispectral singlet oxygen dosimetry (MSOLD) has demonstrated the capability to detect O12 luminescence both in vitro and in vivo, there remains no standardized method for accurately quantifying reactive singlet oxygen, [O2]rx1, based on these measured signals. By contrast, the singlet oxygen explicit dosimetry (SOED) model offers a robust framework for calculating [O2]rx1. Demonstrating that O12 luminescence obtained through MSOLD can reliably quantify [O2]rx1, as achieved by the SOED model, is essential for advancing the accuracy and applicability of PDT dosimetry.AimWe aim to evaluate the accuracy and reliability of MSOLD in quantifying O12 concentrations from measured O12 luminescence in benzoporphyrin derivative (BPD)-mediated PDT. The performance of MSOLD is assessed by comparing its results with those derived from the SOED model.ApproachA continuous-wave 690 nm laser was used to excite a nanoparticle formulation of BPD, tradename Visudyne® in methanol at varying concentrations (2 to 6 mg/L). The singlet oxygen luminescence was measured using an InGaAs spectrometer and analyzed using a singular value decomposition algorithm. Near-infrared singlet oxygen emission at ∼1270 nm was extracted as O12 luminescence. Real-time singlet oxygen spectra were collected over 900 s using a 1.5 mm diameter fiber optic. Ground-state oxygen concentration was measured with a commercial oxygen probe, photosensitizer concentration was determined with a custom-made contact probe, and photon fluence rate was assessed with an isotropic detector. [O2]rx1 was then calculated based on the SOED model.ResultsThe extracted singlet oxygen (O12) luminescence exhibited clear concentration-dependent trends, with higher BPD concentrations producing stronger O12 luminescence. Over time, the O12 luminescence decayed due to photosensitizer bleaching. In addition, a strong linear correlation was observed between the O12 luminescence measured via MSOLD and the reactive oxygen species (ROS) concentrations calculated using the SOED model. We also investigated the impact of tissue optical properties on singlet oxygen luminescence detection and developed correction factors to account for their variations.ConclusionsWe demonstrate that singlet oxygen (O12) detected through MSOLD can reliably quantify ROS concentrations in BPD-mediated PDT with accuracy comparable to the SOED model, which requires separate measurements of light fluence rate, photosensitizer concentration, and oxygen availability, followed by modeling to estimate the amount of reactive singlet oxygen. By contrast, MSOLD can also be a more cost-effective, simpler, and faster alternative to SOED as it directly measures the singlet oxygen luminescence to quantify reactive singlet oxygen. Under appropriate correction for tissue optical properties, MSOLD presents a promising, robust, and direct dosimetry solution for clinical PDT applications.

Cherenkov photons generated during External Beam Radiation Therapy (EBRT) provide real-time optical feedback on the radiation field distribution. While Cherenkov emission correlates with radiation dose, patient-specific anatomical variations, particularly breast geometry, introduce inconsistencies in the Cherenkov emission-to-dose ratio. This study investigates the influence of breast contour, specifically curvature, on the accuracy of Cherenkov-based dosimetry. To address these variations, patient-specific Cherenkov imaging data is corrected using Lambertian geometric correction and compared with the planned radiation dose to derive a correction function as a function of breast curvature. Additionally, scintillator-based Cherenkov imaging data is analyzed and compared with in vivo dosimetry to assess the feasibility of scintillators as real-time dosimeters for EBRT.