Search Results (397)

In this work, the structural, electronic, optical, and vibrational properties of gallium nitride (Ga_xN_x) nanostructures were systematically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). A series of nanoparticles was constructed starting from a cubic-like Ga₄N₄ building unit, leading to one-dimensional (1D), two-dimensional (2D), three-dimensional (3D), and hexagonal configurations. Geometry optimizations and vibrational frequency calculations were performed at the B3LYP/def2-TZVP level, while optical properties were investigated using TD-DFT with the CAM-B3LYP functional. Only dynamically stable structures without imaginary vibrational frequencies were considered for spectroscopic analysis. The results reveal a strong dependence of the optical and vibrational behavior on nanoparticle size and geometry. Larger and lower-symmetry systems exhibit broader and red-shifted UV–Vis absorption spectra together with richer IR vibrational features. In contrast, elongated low-dimensional configurations such as Ga₁₂N₁₂–1D and Ga₁₆N₁₆–1D/2D were found to be dynamically unstable. The investigated nanostructures also show a clear tendency toward structural reorganization from cubic-like motifs to compact hexagonal arrangements related to the wurtzite phase of bulk GaN. Benchmark analysis demonstrates that CAM-B3LYP provides reliable excitation energies at moderate computational cost. Overall, the obtained results highlight the strong coupling between structure and optoelectronic properties in Ga_xN_x nanostructures and indicate their potential for nanoscale optoelectronic and photonic applications. Full article

The linear and third-order nonlinear optical (NLO) properties of a water-soluble perylenediimide derivative, N,N′-di(2-(trimethylammonium iodide) ethylene) perylenediimide (TAIPDI), doped with semiconductor nanoparticles (NPs), were systematically investigated under femtosecond laser excitation. ZnO and TiO₂ NPs were synthesized using a pulsed laser ablation technique. Nanocomposite systems were prepared by incorporating different concentrations of ZnO and TiO₂ NPs into the TAIPDI dye solution. The optical properties were characterized using UV–visible absorption spectroscopy together with open- and closed-aperture Z-scan measurements at 800 nm. Linear absorption measurements revealed concentration-dependent modifications in the optical band gap, indicating electronic interaction between the dye molecules and the semiconductor NPs. Open-aperture Z-scan results demonstrated strong nonlinear absorption (NLA) behavior dominated by two-photon absorption and excited-state absorption processes. Closed-aperture measurements showed a negative nonlinear refractive (NLR) index, corresponding to self-defocusing behavior. Both the NLA coefficient and the NLR index increased with increasing NP concentration, resulting in a significant enhancement of the third-order nonlinear susceptibility of the nanocomposite systems. In addition, optical limiting measurements revealed a pronounced reduction in the limiting threshold with increasing nanoparticle concentration, demonstrating improved laser attenuation capability. These findings indicate that ZnO@TAIPDI and TiO₂@TAIPDI nanocomposites are promising candidates for applications in optical limiting, all-optical switching, and advanced photonic devices. Full article

Accurate nanoscale displacement readout is essential for optical inertial sensors, precision positioning, and micro-vibration characterization. In this work, we develop a free-space optical heterodyne interferometric readout system for low-frequency nanoscale displacement sensing and establish an SNR-guided adaptive demodulation framework. Two complementary demodulation strategies are integrated: Bessel-function-based frequency-domain sideband extraction for small-amplitude low-SNR motion and IQ quadrature phase tracking for larger-amplitude displacement. The experimentally demonstrated framework maps the applicability regimes of the two methods and enables wavelength-referenced displacement readout over a range from sub-nanometer narrowband detection to 250 nm under the present experimental conditions. The implemented system achieves a repeated-measurement repeatability of 0.40 nm under a 10 Hz excitation condition, and spectral SNR analysis is consistent with time-domain statistical evaluation. Finally, the readout system is applied to a quartz pendulum inertial structure, demonstrating its potential for photonic displacement sensing and optical inertial sensor characterization. Full article

The photoreceptor bacteriorhodopsin (HsBR) from Halobacterium salinarum is a model system for studying ultrafast photoinduced reactions in proteins. Recent time-resolved serial femtosecond crystallography (TR-SFX) experiments require high pump energies, raising concerns about nonlinear excitation and multi-photon effects. Here, we systematically investigate the influence of excitation energy, pulse duration and the sign of the chirp on the initial HsBR photo-reaction using femtosecond Vis-pump IR-probe spectroscopy in the retinal C=C stretching region. An acousto-optic programmable dispersive filter enabled independent control of pulse energy and chirp. Within the tested range, the retinal dynamics were independent of pulse duration and chirp, indicating that fluence alone does not fully describe excitation conditions. Increasing excitation energy leads to nonlinear saturation of the retinal signals and the appearance of an additional band near 1550 ${\mathrm{cm}}^{-1}$. However, this band rises linearly with the excitation energy. Hence, the additional band is not directly caused by non-resonant multi-photon absorption. Spectral decomposition reveals two components: a low-energy contribution consistent with the known retinal isomerization dynamics and a high-energy contribution attributed to a small population of photo-damaged HsBR likely formed via a resonant two-photon process. These findings clarify the role of excitation conditions in ultrafast HsBR spectroscopy and suggest that spectral changes at high pump energies mainly arise from damaged species upon resonant two-photon excitation. Full article

KTaO₃ (KTO) is a quantum paraelectric perovskite oxide which belongs to the cubic space group Pm$\overline{3}$m in a large temperature range. Polar optical modes with a T_1u symmetry of KTO are infrared-active and Raman-inactive according to the centrosymmetric exclusion principle of the selection rule. In general, the soft modes responsible for ferroelectric instability are infrared-active and Raman-inactive in the paraelectric phase. Therefore, there are still not enough studies on Raman-inactive soft modes and related phonon polaritons. In the present study, Raman-inactive polar modes and related polaritons of KTO crystals are studied by Terahertz Time-Domain spectroscopy (THz-TDS) and FTIR. The real and imaginary parts of a dielectric constant along the [100] axis are uniquely determined by transmission and reflection THz-TDS without any fitting in the low-frequency range between 6 and 225 cm⁻¹, which covers the two lowest-frequency polar modes. The reflectivity is determined by reflection FTIR in the range between 50 and 1200 cm⁻¹, and the complex dielectric constant is also estimated by the fitting in the range between 6 and 1200 cm⁻¹. The phonon–polariton dispersion relations of the real and imaginary parts of the polariton wavevector are also studied in the range between 6 and 1200 cm⁻¹. The crossover from photon-like to phonon-like polaritons and related polariton decay are observed, while no anomaly related to polariton scattering and coupling to other elementary excitations is observed in the polariton dispersion. Full article

We present a high-precision theoretical study of attosecond transient absorption spectroscopy (ATAS) of atomic hydrogen by numerically solving the time-dependent Schrödinger Equation (TDSE). A broadband extreme ultraviolet (XUV) attosecond pulse creates a wave packet of singly-excited bound states, which is subsequently probed by a time-delayed synthesized optical attosecond pulse (SOAP) with varying bandwidths and durations. When the SOAP has a narrow bandwidth (1.3–1.5 eV) and a long duration (~17 fs), the absorption spectrum exhibits conventional features, namely AC Stark shifts, half-cycle modulations (1.48 fs), and light-induced intermediate states, consistent with previous ATAS studies. In contrast, when the SOAP has a broad bandwidth (0.5–5.5 eV) and an attosecond duration (400 as), the dynamics are completely different. The spectrum reveals transverse wavelike modulations along the absorption lines and, remarkably, quantum beats with distinct frequencies, which are different from previous reports in hydrogen ATAS. To interpret these observations, we employ a dipole-control model. The model quantitatively reproduces the dominant modulation frequencies, identifying resonant couplings via two-photon processes (TPPs, 1.89 eV, period 2.18 fs) and three-photon processes (THPPs, 10.2 eV and 12.1 eV), as well as higher-order couplings. The validity of the $\delta$-like pulse approximation is quantitatively assessed. The model remains accurate for pulse durations shorter than 700 as (bandwidth broader than 3.5 eV) but fails for longer pulses (exceeding 4 fs), where energy level splittings emerge. Our results demonstrate that the dipole-control model provides a reliable and intuitive framework for interpreting complex multiphoton interactions in ATAS, and highlight the unique capability of broadband SOAP probes to resolve attosecond-scale quantum beats inaccessible with conventional few-cycle infrared pulses. Full article

m-Aminostyrene (MAS) is a key molecular scaffold with an electron-donating amino group and a conjugating vinyl group, exhibiting significant potential in photonic materials and biological applications due to its rotamerism and photoinduced behavior. Despite its importance, a comprehensive, rotamer-resolved investigation of its vibronic and cationic spectroscopic properties is lacking. Here, we report a high-resolution study on the cis and trans rotamers of jet-cooled MAS using two-color resonant enhanced multi-photon ionization (2C-REMPI), UV-UV hole-burning (HB), and mass-analyzed threshold ionization (MATI) spectroscopies, combined with density functional theory (DFT) calculations. The HB technique unambiguously resolves the vibronic spectra of each rotamer, overcoming the limitations of previous one-color REMPI studies. The excitation energies (S₁ ← S₀) are determined to be 30,416 cm⁻¹ (cis) and 30,932 cm⁻¹ (trans). The MATI spectra yield precise adiabatic ionization energies (AIEs) of 61,569 cm⁻¹ (cis) and 61,274 cm⁻¹ (trans). A comprehensive assignment of vibrational modes in both the S₁ and D₀ states is provided, revealing distinct mode activities and frequency shifts between the two rotamers. A propensity for Δν = 0 upon ionization is observed, indicating high geometrical similarity between the S₁ and D₀ states. This work provides a crucial spectroscopic blueprint for understanding the electronic and vibrational structure of MAS rotamers, with implications for the design of functionalized styrene-based molecular systems. Full article

This work presents an FPGA-based edge-event timing front-end for time-resolved sensing and event-driven measurement scenarios. The proposed design is intended as a detector-independent timing subsystem whose architectural choices are motivated by constraints that are common in single-photon avalanche diode (SPAD)-based and other asynchronous time-resolved sensing workflows, including event trustworthiness, dead-time sensitivity, and constrained downstream readout. Rather than treating the implementation as an isolated interpolation macro, this work evaluates it as an experimentally observable timing subsystem that combines carry-chain-based fine interpolation, coarse–fine timestamp formation, explicit event-quality assessment, dead-time-aware handling, and lightweight host-visible export. The experimental validation is organized around two complementary modes. An internal ILA-based mode is used to verify coherent front-end behavior under MHz-range short-pulse excitation, while a UART-based campaign identifies practical host-visible operating regions through baseline, repeatability, pulse-width, safe-versus-aggressive, and intermediate frequency-sweep experiments. The results identify a safe export-compatible operating point, a more exploratory high-rate regime, and an experimentally interpretable transition between them that, while not strictly monotonic in all metrics, does not exhibit catastrophic degradation across the explored frequency range. Taken together, the measurements indicate that the proposed architecture is best understood not as a best-case standalone time-to-digital (TDC) benchmark but as an experimentally characterized timing front-end whose practical behavior can be interpreted across complementary internal and export-visible operating regimes. Full article

Rare-earth-doped microsphere cavities have attracted significant interest for applications in miniaturized photonic devices due to their unique optical properties. In this work, Yb³⁺/Er³⁺ co-doped microsphere cavities were fabricated via a melting method, which enables uniform interior doping at high and tunable rare-earth concentrations through a simpler and more cost-effective process compared with existing coating and fiber-etching approaches. Whispering gallery modes (WGMs) enhanced upconversion luminescence, which was observed using tapered fiber coupling, producing a vivid green fluorescence ring near the equatorial region of the microsphere. The luminescence characteristics of the microsphere cavity were investigated by measuring the fluorescence spectra under varying excitation powers. The results indicated that the fluorescence emission follows a two-photon absorption process, consistent with the upconversion emission mechanism of Er³⁺. A finite difference time domain (FDTD) model was employed to simulate the optical field distribution within the microsphere cavity. At a microsphere diameter of 90 μm and a coupling gap of 0 μm, both the 980 nm pump light and the emitted light were effectively confined near the equatorial region of the microsphere, forming WGM confinement patterns. These findings are expected to advance the application of rare-earth-doped microsphere cavities in fields such as biosensing, bioimaging, optical communications, and upconversion microlasers. Full article

Nanoplatforms combining multiple imaging contrast modalities are gaining interest across life sciences and beyond. Here, we disclose a proof-of-concept series of harmonic nanoparticles (HNPs) conjugated with a variety of lanthanide (Ln) complexes, enabling tunable imaging properties. Building on our previous approach for the conjugation of Gd(III) complexes at the surface of HNPs through copper-catalyzed click chemistry, we first establish a copper-free alternative by benchmarking the signals of the resulting conjugates in magnetic resonance imaging phantoms. We then extend this system to Eu, Tb and Yb conjugates and investigate their photophysical properties, successfully detecting long-lived Ln emissions spanning the visible and near-infrared spectrum. Interestingly, the Ln ion can be efficiently removed and exchanged, allowing reuse of the same HNP with a new optical signature. Most notably, we demonstrate that the Eu luminescence can be indirectly activated via second-harmonic generation from the HNP core upon femtosecond-pulsed irradiation in parallel to direct two-photon excitation. This nonlinear activation scheme paves the way for the preparation of mixtures with multidimensional optical signatures using a single excitation source. Altogether this work provides a versatile framework to further explore HNP-Ln conjugates as multimodal imaging probes. Full article

The development of efficient and accurate methods for analyzing electromagnetic scattering by cylindrical objects has theoretical and practical relevance due to its importance in photonics, optoelectronics, antennas, and remote sensing applications. Modal methods are a category of semi-analytical solvers used in modelling electromagnetic scattering problems. Modal methods have several advantages compared to fully numerical methods, and they are very useful for problems with translational symmetry. In this paper, the interaction of electromagnetic waves with a cylinder with an impedance surface situated between two homogeneous half-spaces with different electromagnetic properties is studied. A forward model of the addressed scattering problem is presented. A theoretical formulation of the problem is deduced within a flexible and comprehensive framework that can be extended to solve other related cylindrical configurations. Furthermore, the modal solver developed within the proposed framework can be further developed to consider other forms of source excitations. Moreover, the impedance-dominant scenario has been investigated, and a convergent scheme has been derived for the special case within this regime. Full article

Three ethynyl-extended benzanthrone derivatives with benzonitrile (Dye A), thiophene (Dye B), and methyl propiolate (Dye C) as substituents were synthesized and investigated to illustrate structure–property relationships governing their nonlinear optical (NLO) behavior. The third-order nonlinear absorption and refractive index of three dyes were studied using open- and closed-aperture z-scan measurements under 532 nm continuous-wave laser excitation. All dyes exhibited reverse saturable absorption dominated by two-photon absorption, with Dye A showing the highest nonlinear absorption coefficient (βeff = 2.3 × 10⁻⁵ cm/W) and two-photon response, attributed to its extended conjugation and smaller HOMO−LUMO gap (6.45 eV). Closed-aperture Z-scans revealed strong nonlinear refraction (n₂), with the thiophene-substituted Dye B displaying the largest n₂ (14.8 × 10⁻⁹ cm²/W) and third-order susceptibility (χ³ = 3.1 × 10⁻⁶ esu). The evaluated optical switching figures of merit met the requirements for all-optical switching and optical limiting. DFT and TDDFT calculations demonstrated that donor substitution and conjugation length govern electronic structure, charge transfer character, and global reactivity descriptors. Enhanced electronic softness and hyperpolarizability in Dye B further support its superior refractive nonlinearity. These results establish clear structure–property correlations and highlight donor engineering as an effective strategy for developing organic nonlinear optical and photonic materials. Full article

We present a checkerboard metasurface integrating interleaved Helmholtz resonator arrays with distinct geometrical parameters, enabling decoupled dual-band coherent perfect absorption (CPA) in both in-phase and anti-phase excitation conditions. Full-wave simulations confirm that the proposed structure achieves absorption rates exceeding 99% at 2.904, 3.024, 3.788 and 3.856 THz, corresponding to two pairs of resonant modes enabled by the asymmetric transmission characteristics. Notably, by actively manipulating the relative phase difference between the two excitation modes, the absorption frequencies associated with each CPA channel can be independently and continuously tuned. Benefiting from the planar checkerboard configuration, which combines compact geometry, suppressed mutual coupling, and balanced energy distribution, the metasurface achieves stable and independent dual-band absorption characteristics. The proposed design provides a promising pathway for the development of terahertz coherent absorbers with enhanced frequency stability and spectral flexibility of dual-mode operations, offering strong potential for practical photonic and electromagnetic applications. Full article

We present a temperature-dependent photoluminescence (PL) study of solution-grown CsPbBr₃ bulk crystal and thin film, using one-photon and two-photon excitations. Twin planes are observed in X-ray diffraction spectra in crystal. In analyzing PL peak position and spectral widths as function of temperature, we find that the electron–phonon interaction is generally stronger in CsPbBr₃ crystals than in films. Moreover, with one photon excitation, emissions from excitons and trapped excitons are observed in CsPbBr₃ crystal. Under two-photon excitation, only the emissions from trapped excitons are observed in bulk crystal. Our work demonstrates that two-photon excitation PL is more sensitive to the trapped excitons inside CsPbBr₃, implicating an optical method to probe the inside quality of the crystal. Full article

We propose a silicon carbide (3C-SiC) periodic grating structure based on quasi-bound states in the continuum (q-BICs), which is used to significantly enhance the second-order optical nonlinear effect, including second-harmonic generation (SHG) and sum-frequency generation (SFG). By introducing a four-segment sub-wavelength grating on the SiC thin film and tailor the dimension, the structure successfully excites two q-BIC modes with ultra-high Q factor (resonant wavelengths at 1713.2 nm and 1804.6 nm respectively), realizing enhanced localization and nonlinear interaction of the strong light field. The simulation results show that under oblique incidence, the structure significantly enhances SFG efficiency and exhibits strong robustness to variations in key structural parameters. In addition, the study also reveals the coexistence of forward and backward SHG, and resonant wavelength tuning can be achieved by adjusting the structure dimension. This work not only provides a new path to enhance the nonlinear conversion efficiency of SiC thin films and solve the problem of difficult phase matching, but also lays the theoretical and technical foundation for the development of compact, efficient and integrated SiC-based nonlinear photonic devices. Full article