Broadband Femtosecond Pulses Research Articles

TPF stitching imaging of rubber tree leaves

This paper presents a two-photon fluorescence (TPF) microscopy platform based on a femtosecond oscillator. The system images rubber tree leaf samples by exciting and detecting TPF signals generated by the tissue using broadband femtosecond pulses. The imaging system utilizes a detection window of 565–615 nm to capture TPF signals from flavin adenine dinucleotide (FAD) in the rubber tree leaf tissue. Additionally, multiple TPF images were acquired through sample movement and Z-axis scanning, followed by image stitching to achieve large-area comprehensive visualization of the rubber tree leaf samples. This study provides detailed observations and analyses of various biological structures within the rubber tree leaf samples, contributing significantly to the understanding of plant growth, physiological functions, and environmental adaptability.

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.

Two-photon imaging of rabbit mammary tissue with 3D reconstruction

A broadband two-photon fluorescence (TPF) imaging system, based on a femtosecond oscillator, has been constructed. This system enables TPF imaging of rabbit mammary gland tissue samples by detecting TPF signals generated from the tissue upon excitation by broadband femtosecond pulses. Specifically, TPF imaging utilizes a detection window of 565–615 nm to capture TPF signals from the fluorescent adenine dinucleotide (FAD) present in lactating cells within breast tissue. Moreover, the 3D reconstruction algorithm has been optimized to visualize the 3D structure of the rabbit mammary gland acinar cavity.The optimized three-dimensional reconstruction algorithm can reduce ambiguous signal regions, making the reconstructed three-dimensional structure more reliable and accurate. This is of significant importance for studying the microscopic structure and cell distribution of breast tissue, contributing to a deeper understanding of the tissue's physiological status.

Efficient femtosecond optical parametric generation in group-velocity-matched MgO:PPLN at 10 MHz.

We report on efficient single-pass optical parametric generation (OPG) of broadband femtosecond pulses in the mid-infrared at 10 MHz by exploiting group-velocity-matched interaction in a 42-mm-long MgO:PPLN crystal. Using a microchip-started femtosecond amplified Mamyshev oscillator at 1064 nm as the pump, the OPG source can provide tunable femtosecond pulses across 1516-1566 nm in the signal and 3318-3568 nm in the idler, with slope efficiencies of ∼93% and ∼41%, respectively. For 650 mW of average input pump power, signal powers of up to 283 mW at 1524 nm are generated, with more than 200 mW over the entire tuning range. Idler average powers of up to 104 mW at 3450 nm, with more than 80 mW across the full range, are also obtained. For input pump pulses of ∼182 fs, the generated signal pulses have a duration of ∼460 fs at 1516 nm. The idler pulses have a typical bandwidth of ≥100 nm over the entire tuning range, and as wide as 181 nm at 3457 nm. The OPG source exhibits excellent passive power stability, better than 0.5% rms in the signal and 0.6% rms in the idler, over 1 h, both in Gaussian TEM00 spatial profile with M2 < 1.5.

High-Speed Stimulated Raman Scattering Microscopy Using Inertia-Free AOD Scanning

High-throughput stimulated Raman scattering (SRS) microscopy is highly desired for large tissue imaging with chemical specificity. However, the mapping speed remains as the major short board of conventional SRS, primarily owing to the mechanical inertia existing in galvanometers or other laser scanning alternatives. Here, we developed inertia-free acousto-optic deflector (AOD)-based high-speed large-field stimulated Raman scattering microscopy, in which both the speed and integration time are ensured by immune of the mechanical response time. To avoid laser beam distortion induced by the intrinsic spatial dispersion of AODs, two spectral compression systems are implemented to compress the broad-band femtosecond pulse to picosecond laser. We achieved an SRS imaging of a 12 × 8 mm2 mouse brain slice in only 8 min at an image resolution of approximately 1 μm and 32 slices from a whole brain in 12 h. The AOD-based inertia-free SRS mapping can be much faster after further upgrading and allow broad-spectrum applications of chemical imaging in the future.

Transient Stimulated Raman Excited Fluorescence Spectroscopy

The pursuit of better sensitivity has always been one of the central themes in Raman spectroscopy. Recently, all-far-field single-molecule Raman spectroscopy has been demonstrated by a novel hybrid spectroscopy that couples Raman scattering with fluorescence emission. However, such frequency-domain spectroscopy lacks efficient hyperspectral excitation methods and encounters intrinsic strong fluorescence backgrounds from electronic transitions, hindering its applications in advanced Raman spectroscopy and microscopy. Here we report the ultrafast time-domain spectroscopy counterpart named transient stimulated Raman excited fluorescence (T-SREF): excited by two successive broadband femtosecond pulse pairs (i.e., the pump and Stokes pulses) with time-delay scanning, strong vibrational wave packet interference is revealed on the time-domain fluorescence trace, resulting in background-free spectra of the corresponding Raman modes after the Fourier transform. T-SREF achieves background-free Raman spectra of electronic-coupled vibrational modes with sensitivity up to the level of a few molecules, which paves the way for supermultiplexed fluorescence detection and molecular dynamics sensing.

Attosecond stable dispersion-free delay line for easy ultrafast metrology

We demonstrate a dispersion-free wavefront splitting attosecond resolved interferometric delay line for easy ultrafast metrology of broadband femtosecond pulses. Using a pair of knife-edge prisms, we symmetrically split and later recombine the two wavefronts with a few tens of attosecond resolution and stability and employ a single-pixel analysis of interference fringes with good contrast using a phone camera without any iris or nonlinear detector. Our all-reflective delay line is theoretically analyzed and experimentally validated by measuring 1st and 2nd order autocorrelations and the SHG-FROG trace of a NIR femtosecond pulse. Our setup is compact, offers attosecond stability with flexibility for independent beam-shaping of the two arms. Furthermore, we suggest that our compact and in-line setup can be employed for attosecond resolved pump-probe experiments of matter with few-cycle pulses.

Uncovering surface plasmon optical resonances in nanohole arrays through interferometric photoemission electron microscopy

The role of surface plasmon polaritons (SPPs) in nanohole array optical extinction spectra is explored using a time-resolved technique capable of isolating the air/metal interfacial SPP contribution to the typical Fano profile in optical transmission curves. A pair of interferometrically locked broad-band femtosecond pulses is used to launch SPPs from lithographically patterned plasmonic nanohole arrays. SPPs launched in the co- and counter-propagating directions are probed using a third probe pulse in a photoemission electron microscope. Using this approach, we record interferometric SPP–SPP linear autocorrelations that selectively report on the resonances of SPPs launched from arrays of varying pitches and hole diameters. Aside from advancing an approach to selective SPP spectroscopy, we illustrate that resonant coupling in the counter-propagating direction may be exploited to control the spatial, temporal, and spectral characteristics of SPPs. For the counter-propagating direction, we show that tuning the array pitch near the fundamental plasmon resonance generates color-tuned (∼770–820 nm), narrow bandwidth SPPs, and the bandwidth may be controlled by changing the ratio of pitch to hole diameter. The SPP resonances we recover through Fourier transforms of the interferometric autocorrelations shed light on the classical problem of Fano interference in nanohole array extinction spectra.

Bright and Dark Exciton Coherent Coupling and Hybridization Enabled by External Magnetic Fields.

Magnetic field- and polarization-dependent measurements on bright and dark excitons in monolayer WSe2 combined with time-dependent density functional theory calculations reveal intriguing phenomena. Magnetic fields up to 25 T parallel to the WSe2 plane lead to a partial brightening of the energetically lower lying exciton, leading to an increase of the dephasing time. Using a broadband femtosecond pulse excitation, the bright and partially allowed excitonic state can be excited simultaneously, resulting in coherent quantum beating between these states. The magnetic fields perpendicular to the WSe2 plane energetically shift the bright and dark excitons relative to each other, resulting in the hybridization of the states at the K and K' valleys. Our experimental results are well captured by time-dependent density functional theory calculations. These observations show that magnetic fields can be used to control the coherent dephasing and coupling of the optical excitations in atomically thin semiconductors.

Morphology-Controllable Ultrafast Fiber Lasers Based on Intracavity Manipulation of Transverse Modes

The performances of mode-locked fiber lasers rest with the design and configuration of the cavity, and their temporal morphologies are rarely linked with the modulation of transverse modes. Here, we demonstrate a morphology-controllable ultrafast fiber laser based on the intracavity manipulation of transverse modes, capable of producing dual-color pulses, narrowband picosecond pulses, and broadband femtosecond pulses. Unlike traditional multicolor pulses with unequal group velocities, the dual-color pulse operates in a synchronous state, displaying a multipeak structure where the modulation period relies on the group-delay difference of two modes in the fiber. Simulation results fully reproduce the experimental observations and show that the laser follows the minimum-loss principle with pulse morphologies depending on the mode-interference-induced filtering effect. This work connects transverse-mode modulation with pulse morphology, providing a stable and cost-effective laser source for terahertz-wave generation and nonlinear spectroscopy.

Intensity distortion in dual-axis galvo mirror scanning TPF imaging system

Two-photon fluorescence (TPF) imaging system with dual-axis galvo mirror as scanning unit is built. Based on the optical path of imaging system built, mechanism of how intensity distortion generating in dual-axis galvo mirror scanning TPF imaging system is theoretically analyzed. In experiment, TPF imaging is realized with Rhodamine B sample through detecting TPF signal generated from Rhodamine B molecule at excitation of broadband femtosecond pulses. The imaging result verifies the existence of intensity distortion with distribution agreeing well with the theoretical analyzation. To eliminate the intensity distortion in TPF imaging result, a mathematical method is proposed in which a calibration matrix is calculated, As the calibration matrix is applied to the distorted imaging result of Rhodamine B sample, intensity distortion is relieved effectively and precise imaging result is achieved.

Generation of narrowband pulses from chirped broadband pulse frequency mixing.

We extend an approach based upon sum-frequency generation of oppositely chirped pulses to narrow the bandwidths of broadband femtosecond pulses. We efficiently generate near-transform-limited pulses with durations of several picoseconds, while reducing the pulse bandwidth by a factor of 120, which is more than twice the reduction reported in previous literature. Such extreme bandwidth narrowing of a broadband pulse enhances the effects of dispersion nonlinearities. Precise chirp control enables us to characterize the efficacy of frequency mixing broadband pulses with nonlinear temporal chirps. We demonstrate the use of these narrowband pulses as probes in coherent anti-Stokes Raman spectroscopy.

Broad-band femtosecond pulses, λ3 diffraction and managing of electron mirrors

The experimental and theoretical investigations of attosecond pulses diffraction describe one unexpected parabolic deformation of their intensity profile at few diffraction lengths. This deformation was called by some authors λ3 diffraction. In our previous works it was shown that this phenomenon can be solved analytically in the frame of linear non-paraxial evolution equation of the amplitude of the electrical field. In addition, by solving numerically this equation for phase-modulated femtosecond pulses (20–30 fs) we demonstrated that broad-band pulses also diffract in λ3 regime. Shortly, Fresnel's law does not work for broad-band laser pulses and their diffraction is similar to attosecond ones. On the other hand, by using diffraction grating or other optical dispersion elements the spectrum of the femtosecond pulse can be not only extended, but also the sign of the chirp parameter can be changed. In the present work, it is studied more precisely the influence of the sign and value of the chirp parameter on the linear evolution of phase-modulated femtosecond pulses.

Yb:CALYO-based femtosecond chirped pulse regenerative amplifier for temporally resolved pump-probe spectroscopy

Diode-pumped femtosecond chirped pulse regenerative amplifiers based on Yb3+-materials are of practical importance for wide range of scientific, industrial and biomedical applications. The aim of this work was to study the amplification of broadband chirped femtosecond pulses in regenerative amplifier based on Yb3+:CaYAlO4crystal.Such systems use femtosecond mode-locked lasers as seed pulse sources and amplify nJ-seed pulses to sub-mJ energy range. Most chirped pulse regenerative amplifier systems described in the literature use seed lasers with typical pulse spectral width at the level of 10–15 nm full width at half maximum (FWHM) that limit the seed pulse duration of about 90 fs and amplified pulse duration at the level of 200 fs due to strong influence of gain narrowing effect on the amplified pulse parameters. Yb3+-doped crystals with wide and smooth gain bandwidth as an active medium of chirped femtosecond pulse regenerative amplification systems allow to reduce negative contribution of gain narrowing effect and lead to shortening of amplified pulses. In this research we study the chirped pulse regenerative amplification of broad-band femtosecond pulses (60 nm spectral width FWHM) in the Yb3+:CaYAlO -based chirped pulse regenerative amplifier. Substantial reduction of the amplified pulse duration down to 120 fs (19.4 nm spectral width FWHM) with average power of 3 W at 200 kHz pulse repetition frequency was demonstrated without any gain narrowing compensation technique.The results of experimental investigation of broad-band seeded Yb3+:CaYAlO -based chirped pulse regenerative amplifier are reported for the first time to our knowledge. 120 fs-pulses (19.4 nm FWHM) with average output power of 3 W were demonstrated without any gain narrowing compensation technique. Despite the significant reduction of amplified pulse duration the task of improvement group velocity dispersion balance (including high orders of group velocity dispersion) remains relevant.

High-Resolution Rb Two-Photon Transition Spectroscopy by a Femtosecond Frequency Comb via Pulses Control**Supported by the National Basic Research Program of China under Grant No 2012CB921603, the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China under Grant No IRT13076, and the National Natural Science Foundation of China under Grant Nos 61378049, 10934004, 11404198,

We experimentally observe the high resolution direct frequency comb spectroscopy using counter-propagating broadband femtosecond pulses on two-photon transitions in room-temperature 87Rb atoms. The Doppler broadened background is effectively eliminated with the pulse shaping method and the spectrum modulation technique. The combination of the pulse shaping method and the spectra modulation technique provides a potential approach to reduce background of at least 99%.

Discrete dispersion scanning as a simple method for broadband femtosecond pulse characterization.

A simple and easy to implement technique for femtosecond pulse characterization is proposed and experimentally verified. It is based on the introduction of a known amount of dispersion (by controlling the number of passes through dispersive material) and subsequent recording of the spectral positions of second harmonic peaks obtained in a non-linear crystal. Such dependence allows for direct retrieval of the pulse spectral phase. The presented pulse characterization method is beneficial especially for broadband pulses, where the second harmonic spectrum exceeds the detection bandwidth of a single spectrometer.

Electronic resonances in broadband stimulated Raman spectroscopy.

Spontaneous Raman spectroscopy is a formidable tool to probe molecular vibrations. Under electronic resonance conditions, the cross section can be selectively enhanced enabling structural sensitivity to specific chromophores and reaction centers. The addition of an ultrashort, broadband femtosecond pulse to the excitation field allows for coherent stimulation of diverse molecular vibrations. Within such a scheme, vibrational spectra are engraved onto a highly directional field, and can be heterodyne detected overwhelming fluorescence and other incoherent signals. At variance with spontaneous resonance Raman, however, interpreting the spectral information is not straightforward, due to the manifold of field interactions concurring to the third order nonlinear response. Taking as an example vibrational spectra of heme proteins excited in the Soret band, we introduce a general approach to extract the stimulated Raman excitation profiles from complex spectral lineshapes. Specifically, by a quantum treatment of the matter through density matrix description of the third order nonlinear polarization, we identify the contributions which generate the Raman bands, by taking into account for the cross section of each process.

Fraunhofer type diffraction of phase-modulated broad-band femtosecond pulses

Attosecond optical pulses with one-two cycles under the envelope diffract in nonparaxial regime on several diffraction lengths. Their intensity profile takes a form similar to Fraunhofer distribution. An analytical theory was developed, where was pointed, that such type of diffraction depends on the spectral width of the optical pulse. In this paper is shown that even for a broad-band phase-modulated femtosecond pulses the diffraction is also of Fraunhofer type.

Generation of high-power femtosecond supercontinua in the near-IR spectral range using broadband parametric frequency conversion in LBO and DCDA crystals pumped at

The pump wavelength of parametric amplifiers based on CLBO, DCDA and LBO crystals and pumped by the second harmonic of a femtosecond Cr : forsterite laser is close to optimal for broadband amplification because of the proximity of group velocities of interacting pulses. Injection of a broadband continuum into the range of the signal-wave gain in LBO and DCDA parametric amplifiers, pumped at , leads to generation of broadband femtosecond pulses with a spectrum ranging from 1050 to and peak powers up to .

Spectrally modulated stimulated Raman scattering imaging with an angle-to-wavelength pulse shaper

The stimulated Raman scattering signal is often accompanied by unwanted background arising from other pump-probe modalities. We demonstrate an approach to overcome this challenge based on spectral domain modulation, enabled by a compact, cost-effective angle-to-wavelength pulse shaper. The pulse shaper switches between two spectrally narrow windows, which are cut out of a broadband femtosecond pulse and selected for on- and off- Raman resonance excitation, at 2.1 MHz frequency for detection of stimulated Raman scattering signal. Such spectral modulation reduced the unwanted pump-probe signals by up to 20 times and enabled stimulated Raman scattering imaging of molecules in a pigmented environment.