Fluorescence Excitation Research Articles

A red-emitting phosphor Na3AlF6:Mn4+: Green synthesis, optical characteristics, thermal stability and application in high-performance warm WLED

Mn4+-doped fluoride red phosphors have been widely used in warm WLEDs, but the preparation of such phosphor requires the use of a large amount of HF as a fluorine source and an acidic environment. How to reduce the excessive use of highly toxic HF in the preparation process and achieve green synthesis has become a hot research topic. Therefore, a simple green synthesis strategy is proposed in this work to synthesize a series of red-emitting phosphors Na3AlF6:Mn4+. The highly toxic HF solution is replaced by using a low toxicity reaction system (HCl/HNO3 + NH4F), which reduces the direct use of HF. X-ray powder diffraction, energy-dispersive X-ray spectrometer and scanning electron microscope are employed to determine the crystal structure, composition and morphology of all samples. Optical properties are characterized using excitation spectra, emission spectra and fluorescence lifetime curves. The calculation results indicate that the red-light has low correlated color temperature and the excellent color purity, and Na3AlF6 host can provide a strong crystal field environment for Mn4+. In addition, different aluminum sources and Mn4+ doping concentrations are used to explore the optimal preparation condition. The mechanisms of concentration quenching and thermal quenching have also been systematically explored. The stabilities of red-light emission intensity and color are investigated under the high temperature. The integral PL intensity at 423 K is 47 % of the initial value at 298 K. The activation energy, the chromaticity shift and the chromaticity coordinate variation are also systematically calculated. More importantly, a high-performance warm WLED with low correlated color temperature (CCT = 3296 K) and high color rendering index (Ra = 94.7) is achieved by using Na3AlF6:Mn4+ as a red-emitting material. Not only that, the warm WLED exhibits strong output stability under high driving current. The discovery of this work provides a comprehensive understanding for the rational design of high-performance Mn4+-activated fluoride red phosphors via a simple green synthesis strategy.

Interferometric excitation fluorescence lifetime imaging microscopy

Fluorescence lifetime imaging microscopy (FLIM) is a well-established technique with numerous imaging applications. Yet, one of the limitations of FLIM is that it only provides information about the emitting state. Here, we present an extension of FLIM by interferometric measurement of fluorescence excitation spectra. Interferometric Excitation Fluorescence Lifetime Imaging Microscopy (ixFLIM) reports on the correlation of the excitation spectra and emission lifetime, providing the correlation between the ground-state absorption and excited-state emission. As such, it extends the applicability of FLIM and removes some of its limitations. We introduce ixFLIM on progressively more complex systems, directly compare it to standard FLIM, and apply it to quantitative resonance energy transfer imaging from a single measurement.

Digital quantitation of bridging fibrosis and septa reveals changes in natural history and treatment not seen with conventional histology.

Metabolic dysfunction-associated steatohepatitis (MASH) with bridging fibrosis is a critical stage in the evolution of fatty liver disease. Second harmonic generation/two-photon excitation fluorescence (SHG/TPEF) microscopy with artificial intelligence (AI) provides sensitive and reproducible quantitation of liver fibrosis. This methodology was applied to gain an in-depth understanding of intra-stage fibrosis changes and septa analyses in a homogenous, well-characterised group with MASH F3 fibrosis. Paired liver biopsies (baseline [BL] and end of treatment [EOT]) of 57 patients (placebo, n = 17 and tropifexor n = 40), with F3 fibrosis stage at BL according to the clinical research network (CRN) scoring, were included. Unstained sections were examined using SHG/TPEF microscopy with AI. Changes in liver fibrosis overall and in five areas of liver lobules were quantitatively assessed by qFibrosis. Progressive, regressive septa, and 12 septa parameters were quantitatively analysed. qFibrosis demonstrated fibrosis progression or regression in 14/17 (82%) patients receiving placebo, while the CRN scoring categorised 11/17 (65%) as 'no change'. Radar maps with qFibrosis readouts visualised quantitative fibrosis dynamics in different areas of liver lobules even in cases categorised as 'No Change'. Measurement of septa parameters objectively differentiated regressive and progressive septa (p < .001). Quantitative changes in individual septa parameters (BL to EOT) were observed both in the 'no change' and the 'regression' subgroups, as defined by the CRN scoring. SHG/TPEF microscopy with AI provides greater granularity and precision in assessing fibrosis dynamics in patients with bridging fibrosis, thus advancing knowledge development of fibrosis evolution in natural history and in clinical trials.

Enhancing Two-Photon Excited Fluorescence of Metal-Organic Framework Single Crystals through Modulation of Inorganic Nodes.

Regulation of the two-photon excited fluorescence (TPEF) emission intensity and wavelength of metal-organic framework (MOF) crystals with similar constitutions presents a significant challenge. In this study, two MOFs, Zn-BTPPA and Cd3-BTPPA, were constructed using tetrakis(1,1'-biphenyl-4-carboxylic acid)-1,4-benzenediamine (H4BTPPA) as the organic ligand and mononuclear Zn and trinuclear Cd3 inorganic nodes, respectively. The incorporation of H4BTPPA within the MOF structures enables effective TPEF emission in both Zn-BTPPA and Cd3-BTPPA. The TPEF results show that Zn-BTPPA and Cd3-BTPPA exhibited strong emissions at 523 and 463 nm, respectively, when excited with a 780 nm laser. Moreover, Zn-BTPPA and Cd3-BTPPA exhibited much higher two-photon absorption cross sections, approximately 4.9 and 5.2 times higher than that of the reported dinuclear MOF, Cd2-BTPPA, with a similar composition, respectively. With different inorganic nodes, the stacking of chromophores, π···π interactions, and ligand geometry were found to correlate with the enhanced TPEF in Cd3-BTPPA and the blue-shifted TPEF in Zn-BTPPA. This work serves as an inspiration for designing efficient TPEF MOF materials based on the structure-property relationship.

Photobleaching analysis of fluorescent proteins in two-photon microscopy at high repetition rates

Severe photobleaching in two-photon excitation fluorescence microscopy (TPM) has limited its potential for long-term and quantitative imaging of live cells and tissues. One solution is to excite fluorophores with a high-repetition rate which reduces the peak intensity of irradiance. However, there is a lack of knowledge about the general utility of this strategy for fluorescent proteins. Here, using simple photobleaching assay, we studied the photobleaching of EGFP and tdTomato at 80 MHz and 640 MHz repetition rates. Our analysis shows that the impact of high repetition rate excitation on reducing photobleaching in TPM is highly dependent on the excitation wavelength and fluorophores. Our results are useful for selecting optimal parameters to minimize photobleaching and photodamage in TPM.

Multi-modal, Label-free, Optical Mapping of Cellular Metabolic Function and Oxidative Stress in 3D Engineered Brain Tissue Models.

Brain metabolism is essential for the function of organisms. While established imaging methods provide valuable insights into brain metabolic function, they lack the resolution to capture important metabolic interactions and heterogeneity at the cellular level. Label-free, two-photon excited fluorescence imaging addresses this issue by enabling dynamic metabolic assessments at the single-cell level without manipulations. In this study, we demonstrate the impact of spectral imaging on the development of rigorous intensity and lifetime label-free imaging protocols to assess dynamically over time metabolic function in 3D engineered brain tissue models comprising human induced neural stem cells, astrocytes, and microglia. Specifically, we rely on multi-wavelength spectral imaging to identify the excitation/emission profiles of key cellular fluorophores within human brain cells, including NAD(P)H, LipDH, FAD, and lipofuscin. These enable development of methods to mitigate lipofuscin's overlap with NAD(P)H and flavin autofluorescence to extract reliable optical metabolic function metrics from images acquired at two excitation wavelengths over two emission bands. We present fluorescence intensity and lifetime metrics reporting on redox state, mitochondrial fragmentation, and NAD(P)H binding status in neuronal monoculture and triculture systems, to highlight the functional impact of metabolic interactions between different cell types. Our findings reveal significant metabolic differences between neurons and glial cells, shedding light on metabolic pathway utilization, including the glutathione pathway, OXPHOS, glycolysis, and fatty acid oxidation. Collectively, our studies establish a label-free, non-destructive approach to assess the metabolic function and interactions among different brain cell types relying on endogenous fluorescence and illustrate the complementary nature of information that is gained by combining intensity and lifetime-based images. Such methods can improve understanding of physiological brain function and dysfunction that occurs at the onset of cancers, traumatic injuries and neurodegenerative diseases.

Imaging Transgenic Nude Mice Expressing Spectrally-distinct Fluorescent Reporters Emitting From Blue to Far Red Light With One Instrument With Single-nanometer-tuning of Laser-excitation Fluorescence.

Transgenic nude mice expressing green fluorescent protein (GFP), red fluorescent protein (RFP), or cyan fluorescent protein (CFP) were previously developed by our laboratory, AntiCancer Inc. In the present study, we demonstrate imaging of the GFP, RFP, or CFP nude mice with single-nanometer-tuning laser fluorescence excitation with a single instrument. Female transgenic C57/B6 nude GFP, RFP, and CFP mice aged six weeks were used. The images were obtained using the UVP Biospectrum Advanced system (Analytik Jena US LLC) with excitation at 480 nm and peak emission at 513 nm for GFP; 520 nm and 605 nm, respectively, for RFP; and 405 nm and 480 nm, respectively, for CFP. For each color transgenic fluorescent mouse, images without background could be obtained individually with the UVP Biospectrum Advanced system. Using a single instrument, brilliant and well-defined images of GFP, RFP, and CFP mice were obtained with single-nanometer-tuning laser fluorescence excitation. This imaging system will be used in future studies to analyze cancer cells in the colored mice that are spectrally distinct in order to determine how stromal cells and cancer interact in the tumor microenvironment.

Crystal field optimization, luminescence enhancement and thermal stability of novel K2NaAl0.4Ga0.6F6:Mn4+ solid solution red phosphor for high-performance warm WLED

Mn4+-doped fluoride phosphors have important application value in the background light and WLED lighting fields. Therefore, the synthesis of efficient and stable Mn4+-doped red phosphor is of great application value. In this study, a series of red-emitting K2NaAl1-yGayF6:xMn4+ (y = 0, 0.2, 0.4, 0.6, 0.8 and 1) (x = 0.01, 0.03, 0.05, 0.07 and 0.09) solid solution phosphors are successfully synthesized by simple coprecipitation method. The synthesis strategy of simple crystal structure optimization is proposed to realize simultaneously the crystal field optimization, luminescence enhancement and thermal stability of red phosphor. The calculation and theoretical analysis of crystal field strength are used to prove that solid solution matrix K2NaAl0.4Ga0.6F6 is the most ideal matrix material. X-ray powder diffraction, energy-dispersive X-ray spectrometer and scanning electron microscope are employed to determine the crystal structure, composition and morphology of all samples. Optical properties are characterized using excitation spectra, emission spectra and fluorescence lifetime curves. Moreover, the CIE coordinates of red-light show that the phosphor has low correlated color temperature and excellent color purity. Intriguingly, the as-prepared K2NaAl0.4Ga0.6F6:Mn4+ phosphor possesses admirable thermal quenching behavior and color stability at high temperature. At a temperature of 423 K, the photoluminescence intensity can be maintained at 52 % of the original intensity (298 K). The activation energy, the chromaticity shift and chromaticity coordinate variation are also systematically calculated. More importantly, A high-performance warm WLED with low correlated color temperature (CCT = 3982 K) and high color rendering index (Ra = 93.7) is achieved by using K2NaAl0.4Ga0.6F6:Mn4+ as a red-emitting material. These results demonstrate that K2NaAl0.4Ga0.6F6:Mn4+ red phosphor provides a novel choice for the warm WLED required for indoor lighting.

Blue light excitable fluorescent green security ink for anticounterfeit application

AbstractFluorescent security printing inks on flexible substrates is an arena that demands consistent developments to prevent the ever increasing menace of document/product counterfeiting. Visible light is a much more accessible and safer excitation source than the commonly used ultraviolet (UV) light. In this context, a simple Schiff base 4-pyridyl-benzylidene 2,4-difluoro aniline (PBDFA) is synthesized as a colorant with significant solid-state fluorescence for preparing security ink formulation. A huge challenge lies in preparing a security ink that does not fluoresce under UV light but produces a green fluorescence when irradiated with a blue light source. Such prints would be hard to forge as compared to the existing UV luminescent security inks. The screen prints obtained on a UV dull paper substrate using the solvent-based PBDFA ink revealed good blue light excitable green fluorescence, photostability, and colorimetric, densitometric, and rub resistance characteristics.

Land Use Cover and Flow Condition Affect the Spatial Distribution Characteristics of Fluorescent Dissolved Organic Matter in the Yongding River

Dissolved organic matter (DOM) is involved in many biogeochemical processes and plays an important role in aquatic ecosystems. This study integrated three-dimensional fluorescence excitation–emission matrix (EEM), fluorescence regional integration (FRI), and parallel factor analysis (PARAFAC) to better understand the distribution and component characteristics of DOM in the Yongding River and explore the response of DOM to natural and anthropogenic activities. The results showed that the relative abundance of fulvic-like materials of DOM in the river was the highest, with an average of 68.64%. PARAFAC identified three fluorescent components, namely, C1 (microbial humic-like components), C2 (terrestrial humic-like components), and C3 (protein-like components), and their changes with flow confirmed that the riverine DOM was generally influenced by microbial sources and terrestrial inputs. The upper reaches showed strong autochthonous characteristics and a high humification degree of DOM due to a fast flow rate, while the middle reaches showed biological or aquatic bacterial origin due to a moderate flow rate. The lower reaches of the river showed characteristics of biological and bacterial origin, most strongly influenced by human activities. The findings can help provide a basis for identifying DOM characteristics in the Yongding River basin and understanding the geochemical cycle of DOM at a regional scale.

Room-Temperature Solution Fluorescence Excitation Correlation Spectroscopy.

Single-molecule fluorescence spectroscopy is a powerful tool for investigating the physical properties of individual molecules, yet elucidating the fast fluctuation dynamics of freely diffusing single molecules in solution at room temperature, where a variety of chemical and biological processes occur, remains challenging. In this study, we report on fluorescence excitation correlation spectroscopy of room-temperature solutions, which enables the study of spontaneous fluctuation of the excitation spectrum with microsecond time resolution. By employing Fourier transform spectroscopy with broadband femtosecond pulses and time-correlated single-photon counting, we achieved fluorescence excitation spectroscopy of a room-temperature solution at the single-molecule level. Building upon this single-molecule measurement, we obtained an excitation wavelength-resolved fluorescence autocorrelation function in the microsecond to millisecond range, demonstrating the potential of this method to elucidate fast, spontaneous, time-dependent changes of excitation spectra in statistically equilibrated systems. With further development, this method will allow the study of spectral exchange associated with transitions between sub-ensembles of solution-phase molecules with unprecedented time resolution.

Variational calculus approach to Zernike polynomials with application to FCS

Zernike polynomials are a sequence of orthogonal polynomials that play a crucial role in optics and, in particular, modeling microscopy systems. Introduced by Frits Zernike in 1934, they are particularly useful in expressing wavefront aberrations and, thus, imperfections of imaging systems. However, their origin and properties are rarely discussed and proven. Here, we present a novel approach to Zernike polynomials using variational calculus and apply them to describe aberrations in fluorescence microscopy. In particular, we model the impact of various optical aberrations on the performance of one-photon and two-photon excitation fluorescence microscopy.

Efficient multiphoton microscopy with picosecond laser pulses.

Multiphoton microscopes employ femtosecond lasers as light sources because the high peak power of the ultrashort pulse allows for multiphoton excitation of fluorescence in the examined sample. However, such short pulses are susceptible to broadening in a microscope's highly dispersive optical elements and require careful dispersion management, otherwise decreasing excitation efficiency. Here, we have developed a 10 nJ Yb:fiber picosecond laser with an integrated pulse picker unit and evaluated its performance in multiphoton microscopy. Our results show that performance comparable to femtosecond pulses can be obtained with picosecond pulses only by reducing the pulse repetition rate and that such pulses are significantly less prone to the effect of chromatic dispersion. These findings proved that the temporal pulse compression is not always efficient, and it can be omitted by using a smaller and easier-to-use all-fiber setup.

Outcome prediction in metabolic dysfunction-associated steatotic liver disease using stain-free digital pathological assessment.

Computational quantification reduces observer-related variability in histological assessment of metabolic dysfunction-associated steatotic liver disease (MASLD). We undertook stain-free imaging using the SteatoSITE resource to generate tools directly predictive of clinical outcomes. Unstained liver biopsy sections (n = 452) were imaged using second-harmonic generation/two-photon excitation fluorescence (TPEF) microscopy, and all-cause mortality and hepatic decompensation indices constructed. The mortality index had greater predictive power for all-cause mortality (index >.14 vs. </=.14, HR 4.49, p = .003) than the non-alcoholic steatohepatitis-Clinical Research Network (NASH-CRN) (hazard ratio (HR) 3.41, 95% confidence intervals (CI) 1.43-8.15, p = .003) and qFibrosis stage (HR 3.07, 95% CI 1.30-7.26, p = .007). The decompensation index had greater predictive power for decompensation events (index >.31 vs. </=.31, HR 5.96, p < .001) than the NASH-CRN (HR 3.65, 95% CI 1.81-7.35, p < .001) or qFibrosis stage (HR 3.59, 95% CI 1.79-7.20, p < .001). These tools directly predict hard endpoints in MASLD, without relying on ordinal fibrosis scores as a surrogate, and demonstrate predictive value at least equivalent to traditional or computational ordinal fibrosis scores.

Columnar excitation fluorescence microscope for accurate evaluation of quantum properties of color centers in bulk materials

Color centers in wide band-gap semiconductors, which have superior quantum properties even at room temperature and atmospheric pressure, have been actively applied to quantum sensing devices. Characterizing the quantum properties of the color centers in the semiconductor materials and ensuring that these properties are uniform over a wide area are key issues for developing quantum sensing devices based on color centers. In this article, we have developed an optics design protocol optimized for evaluating the quantum properties of color centers and have used this design approach to develop a new microscopy system called columnar excitation fluorescence microscope (CEFM). The essence of this system is to maximize the amount of fluorescence detection of polarized color centers, which is achieved by large-volume and uniform laser excitation along the sample thickness with sufficient laser power density. This laser excitation technique prevents undesirable transitions to undesirable charge states and undesirable light, such as unpolarized color center fluorescence, while significantly increasing the color center fluorescence. This feature enables fast measurements with a high signal-to-noise ratio, making it possible to evaluate the spatial distribution of quantum properties across an entire mm-size sample without using a darkroom, which is difficult with typical confocal microscope systems.

Room temperature luminescence of 1,N2-etheno-2-aminopurine in poly (vinyl alcohol) films.

We studied spectral properties of 1,N2-etheno-2-aminopurine after immobilization in poly (vinyl alcohol) films. The absorption spectrum of 1,N2-ε2APu consists of two peaks centered at 300 and 370 nm, and the fluorescence spectrum has maximum at about 460 nm. The fluorescence quantum efficiency is 62%. The fluorescence anisotropy reaches a value of 0.3 at longer wavelengths, while it is low at shorter wavelengths (corresponding to the second single excited state). The 1,N2-ε2APu has a relatively long fluorescence lifetime of about 16 ns and a noticeable room temperature phosphorescence with a lifetime of about 220 ms. A broad phosphorescence emission band (425-675 nm) is centered at about 530 nm and markedly overlaps with fluorescence at shorter wavelengths. Surprisingly, the phosphorescence excitation spectrum of 1,N2-ε2APu-doped poly (vinyl alcohol) film differs from the absorption and fluorescence excitation spectra. The strongest room temperature phosphorescence excitation is about 335 nm. At longer excitation wavelengths, above 450 nm, where fluorescence cannot be excited, a triplet excitation is still possible. The 1,N2-ε2APu phosphorescence anisotropy spectra confirm direct triplet state excitation. The ability to excite molecules at long wavelengths can find applications in the study of biological molecules that are unstable when excited at high energies.

Photoisomerization of heptamethine cyanine (Cy7) dyes: A theoretical study

In this study, density functional theory (DFT) combined with time-dependent (TD) DFT calculations were employed to investigate the photoisomerization reaction kinetics of two near infrared (NIR) heptamethine cyanine (Cy7-NH3 and Cy7-SO3) dyes in the ground singlet state and the first excited singlet state. We found that the photoisomerization of the ground state all-trans Cy7 molecules results in at least one mono-cis and one all-cis species that demonstrate redshifted emission, in agreement with recently published transient state excitation modulation spectroscopy and fluorescence correlation spectroscopy measurements. The transition states were estimated for a whole photoisomerization pathway for both the ground singlet and first excited singlet state potential energy surfaces. We have found that all-cis isomers of the studied Cy7 dyes can be achieved through a sequential two-step photoisomerization within the excited singlet state potential energy surface, along the double CC bond adjacent to edge group (leading to mono-cis isomer 1) and along the double CC bond adjacent to the central-chain group (leading to mono-cis isomer 2). Computations show that all-trans→ mono-cis isomer 1→all-cis kinetics is limited by the first trans→ mono-cis isomer 1 stage, while the all-trans→ mono-cis isomer 2→all-cis pathway is limited by the second mono-cis isomer 2→all-cis stage. Accounting for the fact that mono-cis isomer 2 demonstrates red-shifted emission compared to the all-trans form and that this mono-cis isomer 2 is reachable through the energetically favorable all-trans→ mono-cis isomer 2 stage, we concluded that the experimentally observed red-shifted emission by Cy7-NH3 and Cy7-SO3 should be assigned to the formation of mono-cis isomer 2 species. If the all-cis isomer is populated through the double-step photoisomerization it can also be considered as a source of red-shifted emission. However, as follows from our simulations, the all-cis isomer is kinetically intricate to achieve compared to the mono-cis isomer 2.

Hyperspectral imaging of 4T1 mammary carcinomas grown in dorsal skinfold window chambers: spectral renormalization and fluorescence modeling.

Hyperspectral imaging (HSI) of murine tumor models grown in dorsal skinfold window chambers (DSWCs) offers invaluable insight into the tumor microenvironment. However, light loss in a glass coverslip is often overlooked, and particular tissue characteristics are improperly modeled, leading to errors in tissue properties extracted from hyperspectral images. We highlight the significance of spectral renormalization in HSI of DSWC models and demonstrate the benefit of incorporating enhanced green fluorescent protein (EGFP) excitation and emission in the skin tissue model for tumors expressing genes to produce EGFP. We employed an HSI system for intravital imaging of mice with 4T1 mammary carcinoma in a DSWC over 14 days. We performed spectral renormalization of hyperspectral images based on the measured reflectance spectra of glass coverslips and utilized an inverse adding-doubling (IAD) algorithm with a two-layer murine skin model, to extract tissue parameters, such as total hemoglobin concentration and tissue oxygenation ( ). The model was upgraded to consider EGFP fluorescence excitation and emission. Moreover, we conducted additional experiments involving tissue phantoms, human forearm skin imaging, and numerical simulations. Hyperspectral image renormalization and the addition of EGFP fluorescence in the murine skin model reduced the mean absolute percentage errors (MAPEs) of fitted and measured spectra by up to 10% in tissue phantoms, 0.55% to 1.5% in the human forearm experiment and numerical simulations, and up to 0.7% in 4T1 tumors. Similarly, the MAPEs for tissue parameters extracted by IAD were reduced by up to 3% in human forearms and numerical simulations. For some parameters, statistically significant differences ( ) were observed in 4T1 tumors. Ultimately, we have shown that fluorescence emission could be helpful for 4T1 tumor segmentation. The results contribute to improving intravital monitoring of DWSC models using HSI and pave the way for more accurate and precise quantitative imaging.

Multidimensional Quality Characteristics of Sichuan South-Road Dark Tea and Its Chemical Prediction

The distinctive quality of Sichuan south-road dark tea (SSDT) is gradually disappearing with processing innovation. Here, near-infrared (NIR) spectroscopy (NIRS) and spectrofluorometric techniques were utilized to determine the spectral characteristics of dried SSDT and its brew, respectively. Combined with chemical analysis, the multidimensional quality characteristics of SSDT will be presented. Finally, the NIR spectral fingerprint of dried SSDT was observed, with Kangzhuan (KZ) and Jinjian (JJ) showing a very similar NIR spectrum. The SiPLS models effectively predicted the levels of theabrownin, caffeine, and epigallocatechin gallate, based on the NIR spectrum, with root-mean-square errors of calibration of 0.15, 0.12, and 0.02 for each chemical compound, root-mean-square errors of prediction of 0.20, 0.09, and 0.03, and both corrected and predicted correlation coefficients greater than 0.90. Meanwhile, the fluorescence characteristics of the SSDT brew were identified based on the parallel factor analysis for the fluorescence excitation–emission matrix (EEM). The KZ and JJ brews could be classified with 100% accuracy using extreme-gradient-boosting discriminant analysis. The integration of NIRS and fluorometric EEM seems to be a powerful technique for characterizing SSDTs, and the results can greatly benefit the production and quality control of SSDTs.

Broadband rapid-scanning phase-modulated Fourier transform electronic spectroscopy

We present a phase-modulated approach for ultrabroadband Fourier transform electronic spectroscopy. To overcome the bandwidth limitations and spatial chirp introduced by acousto-optic modulators (AOMs), pulses from a 1 µm laser are modulated using AOMs prior to continuum generation. This phase modulation is transferred to the continuum generated in a yttrium aluminum garnet crystal. Separately generated phase-modulated continua in two arms of a Mach-Zehnder interferometer interfere with the difference of their modulation frequencies, enabling physical under-sampling of the signal and the suppression of low-frequency noise. By interferometrically tracking the relative time delay of the continua, we perform continuous, rapid-scanning Fourier transform electronic spectroscopy with a high signal-to-noise ratio and spectral resolution. As proof of principle, we measure the linear absorption and fluorescence excitation spectra of a laser dye and various biological samples.