Spectral Redshift Research Articles

Role of nuclear dynamics in high harmonic generation redshift of H2 +

The detailed spectral and temporal structures of high-order harmonic generation (HHG) have provided us with a wealth of information about the highly nonlinear response of atoms, molecules, and electronic structures to intense low-frequency laser fields. In this study, we aim to investigate the underlying physics behind the molecular high harmonic spectra redshift beyond the Born–Oppenheimer approximation for and its isotopes with numerical simulations of the time-dependent Schrödinger equation. One widely recognized phenomenon is the occurrence of a spectral redshift at the trailing edge of a laser pulse. However, our findings reveal that even when driven by a sinusoidal laser pulse without a falling edge, the HHG redshift appears and highlights that the observed spectral redshift is not solely due to the declining intensity of the laser pulse. We show that it is a direct consequence of the decrease in ionization energy during the transition from recombination time to ionization time within wave packets at short internuclear distances. Interestingly, our results also demonstrate how a sinusoidal laser pulse without a rising part can disrupt the standard picture of redshift in literatures in the past. This finding suggests that there are additional factors influencing the redshift mechanism beyond just the trailing edge of the laser pulse. These phenomena are elucidated through the analysis of the dynamics of the nuclear wave packet. Our study provides valuable insights into the complex dynamics of HHG processes and sheds light on how different laser pulse characteristics can influence spectral redshifts and harmonic generation in molecular systems.

Ce/Mn/Cr: (Re,Y)3Al5O12 Phosphor Ceramics (Re = Gd, Tb and Lu) for White LED Lighting with Significant Spectral Redshift and Improved Color-Rendering Index.

In order to attain phosphor ceramics with a high Color-Rendering Index (CRI), samples with the composition of Y0.997-xRexCe0.003)3(Al0.9748 Mn2+0.024Cr3+0.0012)5O12(Rex = 0, Gd0.333, Gd0.666, Gd0.997, Tb0.333, Tb0.666, Tb0.997 and Lu0.997 were prepared by solid-state reaction and vacuum sintering, and exhibited potential for high-quality, solid-state lighting. Doping with Cr3+ and Mn2+ effectively enhanced the red component of Ce3+ spectra through the intense energy transfer from Ce3+ ions to Mn2+/Cr3+ ions. The crystal field splitting of [GdO8] and [TbO8] was more extensive than that of [YO8], causing a massive redshift in the Ce3+ emission peaks from 542 to 561 and 595 nm, while [LuO8] had an opposite effect and caused a blueshift with a peak position at 512 nm. White LED devices incorporating Ce/Mn/Cr: (Gd0.333Y0.664)3Al5O12 phosphor ceramic exhibited a high CRI of 83.97, highlighting the potential for enhancing the red-light component of white LED lighting.

Spectral shift in terahertz emission by ultrafast laser-induced demagnetization

We demonstrate a spectral shift in the peak frequency of terahertz (THz) emission by ultrafast laser-induced demagnetization in the Fe20Ni80 (Py) film. Experimentally, red spectral shifts of the THz emission are achieved by increasing the laser fluence or decreasing the Curie temperature of the Py film. Based on the microscopic three-temperature model, it is found that such redshifts are attributed by the slowing down of laser induced demagnetization processes in the Py film. The results not only enrich our understanding of THz emission introduced by ultrafast laser-induced demagnetization but also provide a practical way to detect the timescale of laser-induced ultrafast demagnetization dynamics.

Plasmonic Coupled Modes in a Metal-Dielectric Periodic Nanostructure.

In this study we investigate the optical properties of a 2D-gap surface plasmon metasurface composed of gold nanoblocks (nanoantennas) arranged in a metal-dielectric configuration. This novel structure demonstrates the capability of generating simultaneous multi-plasmonic resonances and offers tunability within the near-infrared domain. Through finite difference time domain (FDTD) simulations, we analyze the metasurface's reflectance spectra for various lattice periods and identify two distinct dips with near-zero reflectance, indicative of resonant modes. Notably, the broader dip at 1150 nm exhibits consistent behavior across all lattice periodicities, attributed to a Fano-type hybridization mechanism originating from the overlap between localized surface plasmons (LSPs) of metallic nanoblocks and surface plasmon polaritons (SPPs) of the underlying metal layer. Additionally, we investigate the influence of dielectric gap thickness on the gap surface plasmon resonance and observe a blue shift for smaller gaps and a spectral red shift for gaps larger than 100 nm. The dispersion analysis of resonance wavelengths reveals an anticrossing region, indicating the hybridization of localized and propagating modes at wavelengths around 1080 nm with similar periodicities. The simplicity and tunability of our metasurface design hold promise for compact optical platforms based on reflection mode operation. Potential applications include multi-channel biosensors, second-harmonic generation, and multi-wavelength surface-enhanced spectroscopy.

GaAs/GaInNAs core-multishell nanowires with a triple quantum-well structure emitting in the telecommunication range

Semiconducting nanowires (NWs) fabricated from III–V materials have gained significant attention for their application in advanced optoelectronic devices. Here, the growth of GaAs/GaInNAs/GaAs core-multishell NWs with a triple quantum-well structure, having about 2% N and 20% In, is reported. The NWs are grown via selective area plasma-assisted molecular beam epitaxy on patterned Si(111) substrates with SiO2 mask holes. The nucleation and growth of the GaAs nanowires' core are carried out by Ga-induced vapor–liquid–solid growth at the open holes. Finely controlled, vertically aligned, regular core-multishell NWs with uniform wire length and diameter are obtained with a 96% yield and targeted nitrogen concentrations of 0%, 2%, and 3%. The GaInNAs NWs exhibit a spectral red shift relative to the GaAs NWs' peak. Their emission wavelength increases with the N content reaching up to 1.26 μm, which makes them a promising tool in telecommunication light sources.

Two-Color iSCAT Imaging of Ag Nanoparticles Resolves Size and Ambient Refractive Index Changes.

The ability to discern noble metal nanoparticles (NPs) with different sizes and in ambient media with different refractive indices has important applications in imaging and sensing. Here a two-color (405 nm, 445 nm) interferometric scattering (iSCAT) detection scheme is applied to characterize the wavelength-dependent iSCAT contrast of Ag NPs with nominal diameters of 10, 20, 40, and 60 nm and to distinguish between NPs of different sizes. The iSCAT contrast also depends on the ambient refractive index and the relative iSCAT contrast on both channels revealed a spectral red-shift for 40 and 60 nm Ag NPs when the ambient refractive index was increased from n = 1.3892 to n = 1.4328. With the selected wavelength channels, the spectral resolution of the two-color imaging strategy was, however, insufficient to resolve spectral shifts induced by refractive index changes for 10 and 20 nm Ag NPs.

In vivo characterization of the human glioblastoma infiltrative edge with label-free intraoperative fluorescence lifetime imaging.

Challenges in identifying a glioblastoma's infiltrative edge during neurosurgical procedures result in rapid recurrence. A label-free fluorescence lifetime imaging (FLIm) device was used to evaluate glioblastoma's infiltrative edge in vivo in 15 patients (89 samples). FLIm data were analyzed according to tumor cell density, infiltrating tissue type (gray and white matter), and diagnosis history (new or recurrent). Infiltrations in white matter from new glioblastomas showed decreasing lifetimes and a spectral red shift with increasing tumor cell density. Areas of high versus low tumor cell density were separated through a linear discriminant analysis with a ROC-AUC=0.74. Current results support the feasibility of intraoperative FLIm for real-time in vivo brain measurements and encourage refinement to predict glioblastoma infiltrative edge, underscoring the ability of FLIm to optimize neurosurgical outcomes.

Strain-Induced Modification of Photoluminescence in Quasi-2D Perovskite Thin Films

Quasi-2D perovskites are recognized as a promising material for lightweight photovoltaic applications. For these applications, it is critical to understand how mechanical stresses affect the perovskite’s optoelectronic properties. Here, we discuss how applied stress impacts the photoluminescence spectrum and lifetime of quasi-2D perovskite thin films and how these effects depend on interlayer cation choice (butylammonium or phenethylammonium) and film processing (hot-casting versus postdeposition annealing). We find that compressive stress induces an enhancement and spectral redshift (>10 meV per percent strain) in the emission from bulk-like grains in the film. Moreover, the charge carrier recombination lifetime is greatly enhanced (up to a factor of 14 with only 1% strain). We show that the phenethylammonium-based perovskites are more robust against strain-induced modifications compared to those containing the butylammonium cation. These results highlight how strongly strain can modify the optoelectronic properties of quasi-2D perovskite thin films and how cation choice and film processing can have important practical implications for developing lightweight photovoltaics.

Manipulation of Organic Afterglow in Fluoranthene-Containing Dopant-Matrix Systems: From Conventional Room-Temperature Phosphorescence to Efficient Red TADF-Type Organic Afterglow.

It remains challenging to fabricate highly-efficient and long-lived organic afterglow materials, especially in the case of red afterglow systems. Here we develop advanced charge transfer (CT) technology to boost afterglow efficiency and lifetimes in fluoranthene-containing dopant-matrix systems. First, organic CT molecules possess singlet-triplet splitting energy (ΔEST ) of around 0.5 eV, much smaller than localized excitation systems. Second, upon doping into suitable organic matrices, dipole-dipole interactions between 1 CT states and organic matrices reduce 1 CT levels with less effect on 3 CT levels, and thus further narrow ΔEST and enhance intersystem crossing. Third, the rigid planar structure of fluoranthene groups and the rigid microenvironment provided by organic matrices can suppress phosphorescence quenching. Forth, the multiple donor design enables spectral red-shifts to red region and switches on TADF mechanism to improve afterglow efficiency to 13.1 % and maintain afterglow lifetime of 0.1 s. Such high-performance afterglow materials have been rarely explored in reported studies.

Simultaneous Luminescence Color Tuning and Spectral Broadening of Cr3+ Near-Infrared Emission for Night Vision Application

Near-infrared (NIR) light serves a key role in many potential applications such as food analysis, biomedical devices, night vision, and so on. As an important component of NIR phosphor-converted light-emitting diodes (pc-LEDs), the adjustment of emission peak position and spectral width of Cr3+-doped phosphors is still considered to be a crucial challenge. Herein, the two emissions at 740 nm from [Cr1O6] and 830 nm from [Cr2O4] were realized to broaden the emission band in the Al2.26Ge0.74O4.87:Cr3+ (AGO:Cr3+) phosphor. Moreover, with increasing Cr3+-doped concentration, the crystal field strength decreases and energy transfer from [Cr1O6] to [Cr2O4] is enhanced. The emission peak originating from [Cr2O4] gradually becomes dominant, resulting in a spectral red shift and broadening. The optimized AGO:0.7% Cr3+ phosphor shows remarkable thermal stability, of which the integrated intensity at 373 K keeps 86% of the initial intensity at 298 K. Under 405 nm excitation, the internal quantum efficiency could reach 70% and the external quantum efficiency was measured to be 23%. The application of night vision can be implemented based on the AGO:Cr3+-fabricated NIR pc-LED. This work achieves simultaneous modulation of luminescence color and spectral width by controlling the Cr3+-doped concentration, which provides a feasible strategy for NIR emission adjustment.

One-pot synthesis of multicolor carbon quantum dots: One as pH sensor, one with ultra-narrow emission as fluorescent sensor for uric acid

In this work, tricolor carbon quantum dots (CQDs) including blue-, green-, and orange-CQDs were prepared in one-pot. Impressively, the full width at half maximum of blue-CQDs narrowed greatly to 8 nm, serving as the narrowest bandwidth emissive CQDs so far, to our knowledge, which even compared to colloidal semiconductor quantum dots. Based on the blue-CQDs exhibited highly selective and sensitive ‘on-off’ response towards Fe3+ and the outstanding antioxidant properties of uric acid, we constructed ‘on-off-on’ switching sensor to achieve quantitative measurement of uric acid. The green-CQDs served as simple pH sensor qualitatively judging acid rain based on ultrasensitive spectral redshift property at pH < 5.6. The sensors based on CQDs withstood actual sample matrix effects, pH and temperature change, and long-term storage. The ultra-narrow bandwidth emissive CQDs and fluorescent sensors based on distinctive optical properties of CQDs are meaningful and attractive research directions.

Metal-Halide Perovskite Nanocrystal Superlattice: Self-Assembly and Optical Fingerprints.

Self-assembly of nanocrystals into superlattices is a fascinating process that not only changes geometric morphology, but also creates unique properties that considerably enrich the material toolbox for new applications. Numerous studies have driven the blossoming of superlattices from various aspects. These include precise control of size and morphology, enhancement of properties, exploitation of functions, andintegration of the material into miniature devices. The effective synthesis of metal-halide perovskite nanocrystals has advanced research on self-assembly of building blocks into micrometer-sized superlattices. More importantly, these materials exhibit abundant optical features,including highly coherent superfluorescence, amplified spontaneous laser emission, and adjustable spectral redshift, facilitating basic research and state-of-the-art applications. This review summarizes recent advances in the field of metal-halide perovskite superlattices. It begins with basic packing models and introduces various stacking configurations of superlattices. The potential of multiple capping ligands is also discussed and their crucial role in superlattice growth is highlighted, followed by detailed reviews of synthesis and characterization methods. How these optical features can be distinguished and present contemporary applications is then considered. This review concludes with a list of unanswered questions and an outlook on their potential use in quantum computing and quantum communications to stimulate further research in this area.

Molecular Mechanism of Spectral Tuning by Chloride Binding in Monkey Green Sensitive Visual Pigment

The visual pigments of the cones perceive red, green, and blue colors. The monkey green (MG) pigment possesses a unique Cl- binding site; however, its relationship to the spectral tuning in green pigments remains elusive. Recently, FTIR spectroscopy revealed the characteristic structural modifications of the retinal binding site by Cl- binding. Herein, we report the computational structural modeling of MG pigments and quantum-chemical simulation to investigate its spectral redshift and physicochemical relevance when Cl- is present. Our protein structures reflect the previously suggested structural changes. AlphaFold2 failed to predict these structural changes. Excited-state calculations successfully reproduced the experimental red-shifted absorption energies, corroborating our protein structures. Electrostatic energy decomposition revealed that the redshift results from the His197 protonation state and conformations of Glu129, Ser202, and Ala308; however, Cl- itself contributes to the blueshift. Site-directed mutagenesis supported our analysis. These modeled structures may provide a valuable foundation for studying cone pigments.

Diagnosis of ultrafast surface dynamics of thin foil targets irradiated by intense laser pulses.

The temporal modulation of an electron bunch train accelerated from a foil target irradiated by an intense laser pulse is studied by measuring the coherent transition radiation (CTR) from the rear surface of a target. We experimentally obtained CTR spectra from a 1 µm thick foil target irradiated at a maximum intensity of 6.5 × 1019 W/cm2. Spectral redshifts of the emitted radiation corresponding to increases in laser intensity were observed. These measurements were compared with the theoretical calculation of CTR spectra considering ultrafast surface dynamics, such as plasma surface oscillation and relativistically induced transparency. Plasma surface oscillations induce a spectral redshift, while relativistic transparency causes a spectral blueshift. Both effects are required to find reasonable agreement with the experiment over the entire range of laser intensities.

Optofluidic R6G microbubble DBR laser: A miniaturized device for highly sensitive lab-on-a-chip biosensing

In this work, we propose a miniaturized optofluidic bio-sensor based on organic Rhodamine 6G optofluidic microbubble distributed Bragg reflector (R6G OMB DBR) laser and investigate its performance using a 3-D finite difference time domain (FDTD) simulation method. For this aim, lasing characteristics are precisely calculated and then exploited to scrutinize the capability of the proposed bio-laser in bio-sensing. The designed laser is pumped with a 532 nm Nd: YAG laser and operates at a wavelength of 561.1 nm. The threshold pump energy density and quality (Q)-factor of this single-mode microlaser are 3.6 μJ/mm2 and 4.28 × 104, respectively. The results declare that the presence of Polio virus-like-particles (VLPs) leads to a spectral redshift in the laser output wavelength while decreasing its output intensity. The obtained sensitivity and intensity sensitivity of the proposed bio-sensor are 1377 nm/RIU and 0.98 × 104%/RIU, respectively. Therefore, the proposed miniaturized device shows proper lasing characteristics as well as high abilities in real-time lab-on-a-chip (LOC) bio-sensing applications.

All-optical observation on activity-dependent nanoscale dynamics of myelinated axons.

In the mammalian brain, rapid conduction of neural information is supported by the myelin, the functional efficacy of which shows steep dependence on its nanoscale cytoarchitecture. Although previous in vitro studies have suggested that neural activity accompanies nanometer-scale cellular deformations, whether neural activity can dynamically remodel the myelinated axon has remained unexplored due to the technical challenge in observing its nanostructural dynamics in living tissues. We aim to observe activity-dependent nanostructural dynamics of myelinated axons in a living brain tissue. We introduced a novel all-optical approach combining a nanoscale dynamic readout based on spectral interferometry and optogenetic control of neural excitation in an acute brain slice preparation. In response to optogenetically evoked neuronal burst firing, the myelinated axons exhibited progressive and reversible spectral redshifts, corresponding to the transient swelling at a subnanometer scale. We further revealed that the activity-dependent nanostructural dynamics was localized to the paranode. Our all-optical studies substantiate that myelinated axon exhibits activity-dependent nanoscale swelling, which potentially serves to dynamically tune the transmission speed of neural information.

Mid-Infrared Photothermal-Fluorescence In Situ Hybridization for Functional Analysis and Genetic Identification of Single Cells.

Simultaneous identification and metabolic analysis of microbes with single-cell resolution and high throughput are necessary to answer the question of "who eats what, when, and where" in complex microbial communities. Here, we present a mid-infrared photothermal-fluorescence in situ hybridization (MIP-FISH) platform that enables direct bridging of genotype and phenotype. Through multiple improvements of MIP imaging, the sensitive detection of isotopically labeled compounds incorporated into proteins of individual bacterial cells became possible, while simultaneous detection of FISH labeling with rRNA-targeted probes enabled the identification of the analyzed cells. In proof-of-concept experiments, we showed that the clear spectral red shift in the protein amide I region due to incorporation of 13C atoms originating from 13C-labeled glucose can be exploited by MIP-FISH to discriminate and identify 13C-labeled bacterial cells within a complex human gut microbiome sample. The presented methods open new opportunities for single-cell structure-function analyses for microbiology.

Phonon-assisted upconversion in twisted two-dimensional semiconductors

Phonon-assisted photon upconversion (UPC) is an anti-Stokes process in which incident photons achieve higher energy emission by absorbing phonons. This letter studies phonon-assisted UPC in twisted 2D semiconductors, in which an inverted contrast between UPC and conventional photoluminescence (PL) of WSe2 twisted bilayer is emergent. A 4-fold UPC enhancement is achieved in 5.5° twisted bilayer while PL weakens by half. Reduced interlayer exciton conversion efficiency driven by lattice relaxation, along with enhanced pump efficiency resulting from spectral redshift, lead to the rotation-angle-dependent UPC enhancement. The counterintuitive phenomenon provides a novel insight into a unique way that twisted angle affects UPC and light-matter interactions in 2D semiconductors. Furthermore, the UPC enhancement platform with various superimposable means offers an effective method for lighting bilayers and expanding the application prospect of 2D stacked van der Waals devices.

Design strategy of infrared 4-hydroxybenzylidene-imidazolinone-type chromophores based on intramolecular charge transfer: A theoretical perspective

Partial genetically encoded 4-hydroxybenzylidene-imidazolinone (HBI)-type chromophores are new promising fluorescent probes, which are suitable for imaging and detection of living cells. However, the lack of infrared chromophores hinders the development seriously. Here more than 30 HBI-type chromophores with regular structure modifications were employed and typical spectral redshift change laws and mechanisms were investigated by quantum methods. Results show that both one-photon spectrum (OPS, absorption/emission) and two-photon absorption (TPA) can achieve large redshift via either extending conjugated lengths of frag-3 or enlarging conjugated areas of frag-1 of HBI skeleton. Spectral redshifts of all chromophores are highly related to intramolecular charge transfer (ICT), but neutral ones are closely related to the total ICT or electron-accepting-numbers of frag-3, and the high correlative factor of anions is the aromaticity of frag-2 bridge. The frag-2 bridge with high aromaticity can open a reverse charge transfer channel in anion relative to neutral, obtaining significant redshift. Based on analysis, a new 6-hydroxyl-naphthalene-imidazolinone (HNI) series, which have larger conjugated area in frag-1, are predicted. The OPS and TPA of anionic HNI ones acquire about 76–96 nm and 119–146 nm red-shift relative to traditional HBI series respectively as a whole. The longest emission of anionic HNI-4 realizes more 244 nm redshift relative to HBI-1. Our work clarifies worthy spectral regularities and redshift mechanisms of HBI-type chromophores and provides valuable design strategy for infrared chromophores synthesis in experiment.

Maximum Correntropy Based Spectral Redshift Estimation for Spectral Redshift Navigation

This paper presents a maximum correntropy method of redshift estimation for spectral redshift navigation (SRN). It converts the problem of spectral redshift estimation to a problem of maximum correntropy. A wavelet transform technique is established to denoise spectral signals and fit spectral continuous portions for spectral line extraction and further construction of a candidate redshift set. Subsequently, the joint correntropy is established by using the correntropy based on the redshift predicted from the SRN system equations to correct the correntropy based on the candidate redshift set. According to the maximum correntropy criterion, the spectral redshift estimation is acquired as the candidate redshift with the maximum joint correntropy. Simulations and comparative analysis demonstrate that in addition to the possessed real-time performance, the proposed method can also improve the accuracy of redshift estimation, leading to the improved accuracy for spectral redshift navigation.