Riichiro Saito

Fluorescent quantum defects enable single-wall carbon nanotubes (SWCNTs) to emit brightly in the short-wave infrared (SWIR), but how these sites evolve during oxidative degradation, and whether their emission survives, remains unclear. Here we show that hypochlorous acid (HOCl) drives defect-specific transformation in chirality-pure (6,5) SWCNTs under the conditions examined. In accessible surfactant environments, HOCl converts a parent ether defect (O-(6,5)) emitting at ~1124 nm into a distinct oxidized emissive state at ~1163 nm before further lattice damage, whereas aryl sp ³ defects (ArNO ₂ -(6,5)) and pristine nanotubes (Pris-(6,5)) predominantly lose emission. Coating and pH control the formation of this intermediate by regulating oxidant access and reactive-chlorine speciation. Time-resolved photoluminescent spectroscopy and single-particle hyperspectral imaging resolve the distinct two emissive states, and density-functional-theory supports a vicinal diol-like sp ³ (OH) ₂ -(6,5) motif as the most plausible oxidized structure among the candidates examined. Its D-band intensity per defect resembles that of conventional sp ³ defects, indicating a comparable nanotube defect geometry. Consequently, O-(6,5) oxidizes more rapidly than ArNO ₂ -(6,5) yet retains a larger fraction of total defect emission. These findings establish oxidation-driven conversion between emissive quantum defects and identify defect configuration and surface coating as design parameters for degradable, oxidation-responsive SWIR probes with sustained fluorescence.

We present temperature-dependent electrical transport and Raman spectroscopy measurements of 1T-SnSe₂ single crystals. Room-temperature in-plane optical absorption spectra through spectroscopic ellipsometry analysis reveal indirect and direct band gaps of 1.19 and 1.52 eV, respectively. The transport data exhibit a broad conductivity maximum near 100 K and can be described by a thermally activated semimetallic conduction channel over 28-300 K, coexisting with a two-dimensional variable-range hopping conductivity. Temperature-dependent Raman spectra resolve the first-order E_g and A_1g phonon modes at 109 and 183 cm^-1. These phonon modes are fitted using Breit-Wigner-Fano (BWF) and Voigt line shapes, respectively. Notably, the Fano asymmetry parameter and the Voigt broadening exhibit anomalies near 100 K that quantitatively track the transport signature, whereas the peak positions and spectral widths evolve monotonically from 10 to 300 K. First-principles calculations of the electronic band structure and phonon dispersion corroborate the extracted band gap energies and spectral assignments. These findings establish a consistent picture for the electronic band structure, charge transport, and lattice dynamics in 1T-SnSe₂.

Helicity-resolved Raman spectra are measured for silicon (100), (110), and (111) surfaces in a back-scattering geometry using a 532 nm laser and compared with linearly-polarized Raman spectra. The polar plots of the helicity-resolved Raman intensity for the (110) and ( 111) surfaces as a function of the rotating angle of the polarizer for the scattered light are quantitatively described by rotating the Raman tensor from the (100) surface. Notably, the polar patterns of the T 2g phonon mode at 520 cm -1 for the (110) surface exhibit a tilted angular dependence in the helicity-changing Raman spectra. For the (100) surface, a weak Raman peak is observed in the helicity-conserved spectra that can not be captured by the standard T 2g Raman tensor, indicating a resonance-induced renormalization of the Raman tensor. This effect is reproduced by introducing a small diagonal component to the Raman tensor, which is supported by the first-principles resonant Raman calculations. The calculations further show that the relative value of the diagonal to off-diagonal component increases with increasing laser excitation energy. Our findings establish helicity-resolved Raman spectroscopy as a non-destructive method for probing symmetry-dependent Raman selection rules and identifying surface crystallographic orientation in silicon. This approach provides direct information on the Raman tensor and is applicable to other semiconductor materials.

We calculate polarized Raman intensity of diamond and zinc‐blende crystal structure for arbitrary surfaces in a backscattering geometry using both linearly and circularly polarized light. Rotated Raman tensors are applied to the phonon mode of diamond and the (LO/TO) phonon modes of zinc blende to evaluate how the Raman intensity evolves as the surface orientation is varied from (100) toward (110) and (111). Polar plots versus sample rotation angle or analyzer angle show distinct behaviors for parallel (VV) and cross (VH) configurations under linear polarization, as well as helicity‐dependent configuration. Helicity‐dependent Raman spectroscopy reveals surface‐dependent tilts of the polar pattern. We derive the analytical expressions for the Raman intensities and for the angular positions of maxima and minima as explicit functions of . These surface orientation‐sensitive signatures enable identification of unknown , including sample miscut, and allow predictive modeling of polar‐plot evolution, improving the precision of polarized Raman surface metrology.

In this review, we summarize recent progress in the first-principles calculation of resonant Raman spectra in solids. We begin by describing methods for computing Raman spectra using tight-binding and plane-wave approaches. We then review our recent software packages, QER aman and QR ² -code, and compare their capabilities with other existing tools such as electron–phonon Wannier interpolation over k- and q -points. Finally, we discuss future perspectives for computational approaches to Raman spectroscopy.

Symmetry breaking in van der Waals materials enables the realization of quantum states and advanced device functionalities. Janus transition-metal dichalcogenides (TMDs) exhibit distinctive nonlinear optical properties due to their broken out-of-plane mirror symmetry. However, the dynamic control of second harmonic generation (SHG) anisotropy and resonance behavior via optical excitation remains elusive. In this work, we investigate the SHG response of Janus MoSSe/MoS₂ heterostructures with 2H and 3R stackings. We can tune the SHG response by varying the incident photon wavelength from 800 to 1000 nm, which shows a resonance-dependent enhancement in intensity and a deviation from 6-fold symmetry, indicating wavelength-dependent anisotropy. The ratio between maximum and minimum intensity in the armchair directions, associated with the SHG anisotropy, reaches a value of 1.73 at the excitation wavelength of 1000 nm. Group theory analysis and first-principles calculations reveal that the observed anisotropy arises from optically induced strain. Our findings highlight the role of symmetry breaking and optical resonance contributing to the optomechanical tuning of SHG anisotropy, offering opportunities for developing Janus TMD-based photonic devices for frequency conversion, light generation, and optical switching.