Broadband Absorption Spectroscopy Research Articles

Harnessing tin phthalocyanine for ultrafast mode-locking in an erbium-doped fiber laser

We present the first demonstration, to the best of our knowledge, of a mode-locked erbium-doped fiber laser (EDFL) employing tin phthalocyanines (SnPc) as a saturable absorber (SA). The investigated SA exhibits a notable modulation depth of 18.6%, fabricated via a simple, cost-effective method, wherein the SnPc compound is embedded into a polyvinyl alcohol (PVA) film. Integration of this thin film into the EDFL cavity facilitates the generation of output pulses with a repetition rate of 0.96 MHz and a pulse duration of 2.68 ps. The frequency spectrum showcases a signal-to-noise ratio of up to 73.2 dB, affirming the stability of the mode-locking operation. Operating at a wavelength of 1531.7 nm, this picosecond pulse train holds significant promise for diverse applications, including high-precision optical metrology, frequency-comb generation, and broadband absorption spectroscopy.

Hollow-Core Fiber-Based Broadband Absorption Spectroscopy With Dual-Wavelength Measurements to Remove the Effect of Multigas Interference

Hollow-Core Fiber-Based Broadband Absorption Spectroscopy With Dual-Wavelength Measurements to Remove the Effect of Multigas Interference

Temporal evolution of absorption spectra for diisopropyl methylphosphonate at high temperatures in a shock tube

Absorption spectra and cross-sections of Diisopropyl Methylphosphonate (DIMP) are measured in argon at temperatures of 600–1250 K in a shock tube using rapid scanning, broadband absorption spectroscopy. Measurements are obtained at approximately 2.3 kHz with each complete scan of the 925–1350 wavenumber band taking 90 microseconds. This technique enables spectra behind incident and reflected shocks to be obtained in a single test. In addition, multiple spectra are obtained behind the reflected shock during the test time. A significant reduction in absorption cross section with temperature is observed, and the spectra taken during the test time show no decomposition or significant changes, suggesting that vibrational equilibrium is achieved.

Band Gap Renormalization at Different Symmetry Points in Perovskites.

Using ultrafast broad-band transient absorption (TA) spectroscopy of photoexcited MAPbBr3 thin films with probe continua in the visible and the mid- to deep-ultraviolet (UV) ranges, we capture the ultrafast renormalization at the fundamental gap at the R symmetry point of the Brillouin zone (BZ) and a higher energy gap at the M symmetry point. Advanced global lifetime analysis and lifetime density distribution analysis are applied to extract quantitative information. Our work confirms the similarity of the response at both high-symmetry points, which indicates a band edge renormalization that rises within the instrument response function (IRF, ∼250 fs) and decays in ca. 400-600 fs, undergoing an energy red shift of 90-150 meV. The reported time scale corresponds to the decay of free carriers into neutral excitons. The ability to monitor different high-symmetry points in photoexcited perovskites opens exciting prospects for the characterization of a large class of materials and for photonic applications.

Quantitative 2D reconstruction of temperature and OH concentration in axisymmetric flames based on ultraviolet broadband absorption tomography

Absorption tomography plays a crucial role in quantitatively imaging temperature and absolute concentration in combustion. However, there is scarce research on the tomography of OH radicals, which are significant for combustion diagnostics. This study presented a new tomography method to image OH radicals, termed ultraviolet broadband absorption tomography (UV-BAT). Three strategies were proposed to solve the nonlinear problem in UV-BAT tomography, namely all-spectra fitting, weak-absorption approximation, and multi-temperature approximation. The proposed UV-BAT method was applied for OH imaging in methane/air axisymmetric laminar Bunsen flames with different equivalence ratios (ϕ = 0.9, 1.0, and 1.1). The reconstructed OH distributions showed distinctive structures of Bunsen flames including premixed and diffusion flame areas. The accuracy of the UV-BAT method was validated by comparing tomography results with computational fluid dynamics (CFD) simulations using the USC-Mech II mechanism, demonstrating good agreement in two-dimensional (2D) temperature and OH concentration distributions. This novel method offers new opportunities for tomographic reconstruction of free radicals such as OH and temperature in small-scale flames.Novelty and significanceThis study demonstrated a novel tomography imaging method using ultraviolet broadband absorption spectroscopy to reconstruct temperature and radical concentration in combustion. This method possesses high sensitivity to free radicals and employs compact and inexpensive equipment, making it one of the most viable methods for realizing free radical absorption tomography. The 2D imaging of OH radicals using this method based on axisymmetric absorption tomography was accomplished for the first time. In conclusion, this study enriched the diagnostic methods of flame radicals based on absorption spectroscopy and provided new ideas for small-scale temperature tomography.

Time-resolving state-specific molecular dissociation with XUV broadband absorption spectroscopy.

The electronic and nuclear dynamics inside molecules are essential for chemical reactions, where different pathways typically unfold on ultrafast timescales. Extreme ultraviolet (XUV) light pulses generated by free-electron lasers (FELs) allow atomic-site and electronic-state selectivity, triggering specific molecular dynamics while providing femtosecond resolution. Yet, time-resolved experiments are either blind to neutral fragments or limited by the spectral bandwidth of FEL pulses. Here, we combine a broadband XUV probe pulse from high-order harmonic generation with an FEL pump pulse to observe dissociation pathways leading to fragments in different quantum states. We temporally resolve the dissociation of a specific O2+ state into two competing channels by measuring the resonances of ionic and neutral fragments. This scheme can be applied to investigate convoluted dynamics in larger molecules relevant to diverse science fields.

Study on temperature effects of broadband absorption spectroscopy based on Doppler broadening combined with a multilinear shape superposition

In this study, we determine the relationship between the broadband absorption spectroscopy and temperature using Doppler broadening combined with a multilinear shape superposition. First, a method for temperature effects on broadband absorption spectroscopy is proposed, utilizing the principle of a multiple Gaussian line shape superposition. A theoretical expression of the temperature effect on broadband spectroscopy is then presented for the first time to the best of our knowledge, and the variation law of broadband absorption spectroscopy with temperature is explained. Furthermore, the effectiveness of the expression is demonstrated by comparing experimental and theoretical data of the SO2 broadband absorption spectroscopy. The results demonstrate that the correlation coefficient (r) between the experimental and theoretical spectra of SO2 within the temperature range of 298.15–923.15 K is greater than 0.93. The method and expression presented in this Letter can effectively explain the data of previous research.

High-speed broadband absorption spectroscopy enabled by cascaded frequency shifting loops

Frequency shifting loops, consisting of a fiber optic ring cavity, a frequency modulator, and an amplifier to compensate for loss, enable high-speed frequency scanning with precise and easily controlled frequency steps. This platform is particularly attractive for applications in spectroscopy and optical ranging. However, amplified spontaneous emission noise accumulates due to the repeated amplification of light circulating in the cavity, limiting the frequency scanning range of existing frequency shifting loops (FSLs). Here, we introduce a cascaded approach which addresses this basic limitation. By cascading multiple FSLs in series with different frequency shifts we are able to dramatically increase the accessible scanning range. We present modeling showing the potential for this approach to enable scanning over ranges up to 1 THz—a tenfold increase compared with the state-of-the-art. Experimentally, we constructed a pair of cascaded FSLs capable of scanning a 200 GHz range with 100 MHz steps in 10 ms and used this platform to perform absorption spectroscopy measurements of an H13C14N cell. By increasing the operating bandwidth of FSLs, the cascaded approach introduced in this work could enable new applications requiring precise and high-speed frequency scanning.

Full C-band wavelength-tunable, 250 MHz repetition rate mode-locked polarization-maintaining fiber laser

We demonstrate a full C-band wavelength-tunable mode-locked fiber laser with a repetition rate of 250 MHz, representing the highest repetition rate for C-band tunable mode-locked lasers thus far to the best of our knowledge. The polarization-maintaining fiber-based Fabry–Perot cavity enables a fundamental repetition rate of 250 MHz with a semiconductor saturable absorber mirror as a mode-locker. We observed a stable and single soliton mode-locking state with wide tunability of the center wavelength from 1505 to 1561 nm by adjusting the incident angle of a bandpass filter inside the cavity. The wavelength-tunable high-repetition-rate mode-locked laser covering the full C-band is expected to be a compelling source for many frequency-comb-based applications, including high-precision optical metrology, broadband absorption spectroscopy, and broadband optical frequency synthesizers.

Boosting the terahertz absorption spectroscopy based on the stretchable metasurface.

Terahertz (THz) absorption spectroscopy is widely used for molecular label-free fingerprinting detection, but it is not capable of efficiently analyzing trace-amount sample materials. For improving the efficiency of terahertz absorptance spectroscopy detection, we propose a sensing strategy by treating the geometry sweeping spoof surface plasmon polariton (SSP) of the stretchable metasurface. For the first time, the geometry sweeping can be realized by dynamically stretching the polydimethylsiloxane (PDMS) flexible substrate, leading to the resonant frequency variation of the unit cell. This design provides a significant absorption enhancement factor about 270 times for a 0.1 μm lactose film in a broad terahertz band, enabling the unambiguous identification of different trace-amount samples. The designed method exhibits a novel solution for the enhancement of broad-band terahertz absorption spectroscopy and great application potential in the field of detecting trace-amount samples.

Spatially resolved broadband absorption spectroscopy measurements of temperature and multiple species (NH, OH, NO, and NH[formula omitted]) in atmospheric-pressure premixed ammonia/methane/air flames

The temperature and multi-species concentration distributions, including NH, OH, NO, and NH3, are of great importance to the research on ammonia/methane combustion mechanisms. However, there is scarce research simultaneously measuring these parameters in atmospheric-pressure ammonia/methane flames. In this work, we proposed a measurement method based on the ultraviolet broadband absorption spectroscopy (BAS) to realize the simultaneous measurement of temperature and multiple species concentrations for the first time (absolute concentrations for OH, NH, and NO, and relative concentrations for NH3). Due to the strong absorption of the electronic transitions in the ultraviolet range, the proposed method offers low detection limits for concentration measurements. Taking NH radicals as an example, the detection limit is as low as 31 ppb at 1700 K. High sensitivity for temperature measurements is also achieved due to the multi-line spectra of OH broadband absorption. Moreover, a fitting strategy was proposed to separate the NO and NH3 spectra near 225 nm, enabling a simultaneous determination of both species. We established an optical system based on the proposed BAS method with a spatial resolution of approximately 0.1 mm to perform spatially resolved measurements of temperature and multiple species on atmospheric ammonia/methane/air flames. The ammonia/methane flames were investigated at different equivalence ratios (0.9, 1.0, and 1.1) and different ammonia mole fractions in fuels (10%, 30%, and 50%). As investigated, NH and NH3 concentrations increased linearly with increasing ammonia mole fractions, while OH and NO concentrations decreased in similar trends with increasing equivalence ratios. The experimentally measured temperature and multi-species concentrations were compared with computational fluid dynamics (CFD) simulations using the mechanism of the CRECK modeling group. Good agreements were achieved in absolute values, spatial profiles, and variations with equivalence ratios and ammonia mole fractions.

Ultrafast Symmetry-Breaking Charge Separation in a Perylene Bisimide Dimer Enabled by Vibronic Coupling and Breakdown of Adiabaticity.

Perylene bisimides (PBIs) have received great attention in their applicability to optoelectronics. Especially, symmetry-breaking charge separation (SB-CS) in PBIs has been investigated to mimic the efficient light capturing and charge generation in natural light-harvesting systems. However, unlike ultrafast CS dynamics in donor-acceptor heterojunction materials, ultrafast SB-CS in a stacked homodimer has still been challenging due to excimer formation in the absence of rigidifying surroundings such as a special pair in the natural systems. Herein, we present the detailed mechanism of ultrafast photoinduced SB-CS occurring in a 1,7-bis(N-pyrrolidinyl) PBI dimer within a cyclophane. Through narrow-band and broad-band transient absorption spectroscopy, we demonstrate that ultrafast SB-CS in the dimer is enabled by the combination of (1) vibrationally coherent charge-transfer resonance-enhanced excimer formation and (2) breakdown of adiabaticity (formation of SB-CS diabats) in the excimer state via structural and solvent fluctuation. Quantum chemical calculations also underpin that the participation of strong electron-donating substituents in overall vibrational modes plays a crucial role in triggering the ultrafast SB-CS. Therefore, our work provides an alternative route to facilitate ultrafast SB-CS in PBIs and thereby establishes a novel strategy for the design of optoelectronic materials.

Gas-phase CH-Overtone band spectra of methyl acetate and ethyl acetate via incoherent broad-band cavity-enhanced absorption spectroscopy

Gas-phase overtone bands of methyl acetate and ethyl acetate were detected for up to v = 6–0 by incoherent broad-band cavity-enhanced absorption spectroscopy. The signal-to-noise ratios of v = 4–0 were achieved to be 40–50 by the 100 s integration time, suggesting an application possibility of this spectroscopic technique for environmental monitoring. Profiles of the observed overtone bands were analyzed in detail with the aid of theoretical calculations and their remarkable peaks were assigned to the progressions starting from the symmetric CH-stretching bands. The harmonic frequencies, the anharmonicities, and the dissociation energies were derived.

Experimental study of nonlinear phenomenon of NO ultraviolet broadband absorption spectroscopy

Ultraviolet broadband absorption spectroscopy (UV-BAS) has been widely used to measure the concentration of gas pollutant, such as NO. However, the nonlinear dependence of the absorbance on the optical thickness (<i>XL</i>) caused by the broadening effect of instrument function is observed. In this paper, the nonlinear behavior of NO absorbance is investigated both theoretically and experimentally, and a database using a polynomial to describe the nonlinearity is established to present a simple method of measuring NO concentration. First, the nonlinear relationship between absorbance and <i>XL</i> is deduced. Second, the nonlinearity of an isolated spectral line is simulated, and the dependence of nonlinear behavior on instrument width is investigated. Third, the nonlinerities of peak absorbance in <i>γ</i> (0, 0) band with different instrumental widths are calculated, the nonlinear expression is given in a polynomial form, and the corresponding coefficient database is established. In addition, the nonlinearities in different vibration bands with the same instrumental width are compared with each other. Finally, two spectrometers are used to measure NO absorption spectra in different instrumental widths in order to validate the above-mentioned results of theoretical analysis. The relative error between the measured peak absorbance and theoretical calculation is less than 4%, and that between experimental results and the interpolation polynomial results is less than 8%. The experimental results demonstrate the accuracy of theoretical calculation and the reliability of database.

Thickness dependent optical properties of sputtered Bi2Se3 films on mica

The Bi2Se3 thin films of different thicknesses (100, 200 and 400 nm) were deposited on mica substrates by magnetron sputtering technique and their structural and optical properties have been studied. The X-ray diffraction analysis revealed the crystalline nature of Bi2Se3 films with preferred orientation along the c-axis. The presence of three well-pronounced Raman modes i.e. A11g, E2g, and A21g confirms the formation of pure Bi2Se3 compound on mica substrates. The field emission scanning electron microscopy and elemental analysis measurements showed the growth of Bi2Se3 film with varying grain sizes with thickness while maintaining the stoichiometric ratio of 2:3 of Bi and Se. Surface roughness was analysed using atomic force microscopy and it revealed that the roughness increases from 2.16 to 28.3 nm when the film thickness increases from 100 to 400 nm. The optical bandgap analysed with broadband absorption spectroscopy showed a decrease in bandgap from 1.3 to 1.08 eV with increasing film thickness. The sputtered Bi2Se3 films on mica possess good crystalline and structural quality with tunable bandgap that can be used for futuristic optoelectronic applications.

Broadband absorption spectroscopy of heterogeneous biological tissue.

Absorption spectra (∼600 to 1064 nm) of six tissues in three healthy volunteers were measured by combining dual-slope continuous-wave broadband spectroscopy with self-calibrated frequency-domain measurements of scattering at two wavelengths (690 and 830 nm). The spectral fit with a linear combination of oxy- and deoxyhemoglobin, water, and lipids extinction spectra is improved by a wavelength-independent absorption background. The need to introduce this background is assigned to the inhomogeneous distribution of absorbers in tissue. By using a two-layer model, the relationship between recovered concentrations and their two-layer values was investigated, and the implications for non-invasive tissue spectroscopy are discussed.

Triplet energy migration pathways from PbS quantum dots to surface-anchored polyacenes controlled by charge transfer.

Sensitization of molecular triplets using PbS quantum dots (QDs), followed by efficient triplet fusion, has been developed as a novel route to near-infrared-to-visible photon upconversion. Fundamentally, however, the mechanisms of triplet energy transfer (TET) from PbS QDs to surface-anchored polyacence acceptors remain highly debated. Here we study and side-by-side compare the kinetic pathways of TET from photoexcited PbS QDs to surface-anchored tetracene and pentacene derivatives using broad-band transient absorption spectroscopy spanning multiple decades of timescales. We find that the TET pathways are dictated by charge-transfer energetics at the QD/molecule interface. Charge transfer from QDs to tetracene was strongly endothermic, and hence spectroscopy showed one-step transformation from QD excited states to tetracene triplets in 302 ns. In contrast, hole transfer from QDs to pentacene was thermodynamically favoured and was confirmed by the formation of pentacene cation radicals in 13 ps, which subsequently evolved into pentacene triplets through a 101 ns electron transfer process. These results not only are consistent with a recently-established framework of charge-transfer-mediated TET, but also provide a route to manipulate triplet sensitization using lead-salt QDs for efficient upconversion of near-infrared photons.

Characterization of an RF-driven argon plasma at atmospheric pressure using broadband absorption and optical emission spectroscopy

Atmospheric pressure plasmas in argon are of particular interest due to the production of highly excited and reactive species enabling numerous plasma-aided applications. In this contribution, we report on absolute optical emission and absorption spectroscopy of a radio frequency (RF) driven capacitively coupled argon glow discharge operated in a parallel-plate configuration. This enabled the study of all key parameters including electron density and temperature, gas temperature, and absolute densities of atoms in highly electronically excited states. Space and time-averaged electron density and temperature were determined from the measurement of the absolute intensity of the electron-atom bremsstrahlung in the visible range. Considering the non-Maxwellian electron energy distribution function, an electron temperature (Te) of 2.1 eV and an electron density (ne) of 1.1×1019m−3 were obtained. The time-averaged and spatially resolved absolute densities of atoms in the metastable (1s5 and 1s3) and resonant (1s4 and 1s2) states of argon in the pure Ar and Ar/He mixture were obtained by broadband absorption spectroscopy. The 1s5 metastable atoms had the largest density near the sheath region with a maximum value of 8×1017m−3, while all other 1s states had densities of at most 2×1017m−3. The dominant production and loss mechanisms of these atoms were discussed, in particular, the role of radiation trapping. We conclude with comparison of the plasma properties of the argon RF glow discharges with the more common He equivalent and highlight their differences.

Temperature and OH concentration measurements by ultraviolet broadband absorption of OH(X) in laminar methane/air premixed flames

We report temperature and hydroxyl (OH) concentration measurements in methane/air premixed flat flames based on ultraviolet (UV) broadband absorption spectroscopy (BAS) of the OH electronic transitions (X2Π-A2Σ+). The temperature and OH mole fraction distributions were obtained at different heights above the burner for various equivalence ratios (ϕ = 0.6, 0.7, 0.8, 0.93, and 1.06). The results agreed well with computational fluid dynamics (CFD) simulations using the AramcoMech 1.3 skeletal mechanism, which verifies the BAS measurement accuracy. For stable flames (ϕ = 0.6, 0.7, and 0.8), the root-mean-square differences above the flat flames were all less than 16 K for the temperature and 150 ppm for the OH mole fraction. The BAS results also agreed with tunable diode laser absorption spectroscopy (TDLAS) measurements with a root-mean-square temperature difference of 31.5 K. Experimental and theoretical analysis demonstrates that the UV BAS OH detection limit is more than two orders of magnitude lower than that for infrared (IR) TDLAS, and the OH BAS temperature sensitivity is much higher than that for most line pairs in TDLAS measurements. These advantages make the UV BAS method an accurate method for combustion diagnostics, with more accurate temperature and OH concentration than IR TDLAS.

Ablation-Dominated Arcs in CO2 Atmosphere—Part II: Molecule Emission and Absorption

Molecule radiation can be used as a tool to study colder regions in switching arc plasmas like arc fringes in contact to walls and ranges around current zero (CZ). This is demonstrated in the present study for the first time for the case of ablation-dominated high-current arcs as key elements of self-blast circuit breakers. The arc in a model circuit breaker (MCB) in CO2 with and an arc in a long nozzle under ambient conditions with peak currents between 5 and 10 kA were studied by emission and absorption spectroscopy in the visible spectral range. The nozzle material was polytetrafluoroethylene (PTFE) in both cases. Imaging spectroscopy was carried out either with high-speed cameras or with intensified CCD cameras. A pulsed high-intensity Xe lamp was applied as a background radiator for the broad-band absorption spectroscopy. Emission of Swan bands from carbon dimers was observed at the edge of nozzles only or across the whole nozzle radius with highest intensity in the arc center, depending on current and nozzle geometry. Furthermore, absorption of C2 Swan bands and CuF bands were found with the arc plasma serving as background radiator. After CZ, only CuF was detected in absorption experiments.