White-light Continuum Pulses Research Articles

Full-Spectrum CARS Microscopy of Cells and Tissues with Ultrashort White-Light Continuum Pulses.

Coherent anti-Stokes Raman scattering (CARS) microscopy is an emerging nonlinear vibrational imaging technique that delivers label-free chemical maps of cells and tissues. In narrowband CARS, two spatiotemporally superimposed picosecond pulses, pump and Stokes, illuminate the sample to interrogate a single vibrational mode. Broadband CARS (BCARS) combines narrowband pump pulses with broadband Stokes pulses to record broad vibrational spectra. Despite recent technological advancements, BCARS microscopes still struggle to image biological samples over the entire Raman-active region (400-3100 cm-1). Here, we demonstrate a robust BCARS platform that answers this need. Our system is based on a femtosecond ytterbium laser at a 1035 nm wavelength and a 2 MHz repetition rate, which delivers high-energy pulses used to produce broadband Stokes pulses by white-light continuum generation in a bulk YAG crystal. Combining such pulses, pre-compressed to sub-20 fs duration, with narrowband pump pulses, we generate a CARS signal with a high (<9 cm-1) spectral resolution in the whole Raman-active window, exploiting both the two-color and three-color excitation mechanisms. Aided by an innovative post-processing pipeline, our microscope allows us to perform high-speed (≈1 ms pixel dwell time) imaging over a large field of view, identifying the main chemical compounds in cancer cells and discriminating tumorous from healthy regions in liver slices of mouse models, paving the way for applications in histopathological settings.

Dispersive white light continuum single Z-scan for rapid determination of degenerate two-photon absorption spectra

We present an experimental technique to determine the degenerate two-photon absorption (2PA) spectra by performing a single Z-scan using a high-spectral-irradiance white light continuum (WLC) generated by a hollow core fiber. The hollow fiber was filled with Argon (Ar) gas at a pressure of 0.6 bar and was pumped with 500 mJ, 30 fs, and 800 nm pulses. The broadband WLC pulses with 350 nm bandwidth in the range of 600–950 nm were compressed to sub-8 fs pulses. To characterize and interpret the data obtained from this method, the spectral, temporal and spatial characteristics of the WLC were first analyzed. The WLC emerging from the compressor was dispersed using a prism pair and then focused into the sample by a cylindrical lens. Since different spectral components are spatially separated, any part of the sample in the beam cross section is irradiated with almost single wavelength pulses leading to only a degenerate 2PA process. The nonlinear transmittance was then measured by a charge-coupled-device (CCD) line camera as a function of the sample position while the sample was moved along the beam direction by a motorized translation stage. In this way the Z-scans at different wavelengths in the WLC spectral range can be measured and thus the wavelength-resolved degenerate 2PA spectra can be obtained by performing a single scan using dispersive WLC. This method was verified on a well-characterized dye Rhodamine B and yield a reasonable agreement with the data found in the literature. We used this method to determine the 2PA spectra of some two-photon initiators (2PIs) developed for two-photon polymerization (2PP) based 3D micro-structuring.

40-µJ passively CEP-stable seed source for ytterbium-based high-energy optical waveform synthesizers.

We demonstrate experimentally for the first time a ~40-µJ two-octave-wide passively carrier-envelope phase (CEP)-stable parametric front-end for seeding an ytterbium (Yb)-pump-based, few-optical-cycle, high-energy optical parametric waveform synthesizer. The system includes a CEP-stable white-light continuum and two-channel optical parametric chirped pulse amplifiers (OPCPAs) in the near- and mid-infrared spectral regions spanning altogether a two-octave-wide spectrum driven by a regenerative amplifier. The output pulses are compressed and fully characterized to demonstrate the well-behaved spectral phase of this seed source.

Phase retrieval and compression of low-power white-light pulses

We characterize and compress sub-nJ visible white-light continuum (WLC) pulses generated by self-phase modulation in yttrium aluminium garnet. The spectral phase is retrieved by spectrally resolving the transient reflectivity from an optically excited transition metal oxide. This measured phase is compensated by applying the appropriate distortion to a deformable mirror. By comparing the response of two different materials, we show that the white-light pulses can be compressed to approximately 10 fs duration.

Effect of protonation on the singlet–singlet excited-state absorption of meso-tetrakis( p-sulphonatophenyl) porphyrin

This work reports on the excited singlet-state absorption cross-section spectra of both biprotonated and nonprotonated states of meso-tetrakis( p-sulphonatophenyl) porphyrin obtained by means of the Z-scan technique with white-light continuum pulses. Although our main purpose was to determine these photo-physical parameters in a wide spectral range, we also analyzed the role of accumulative effects arising as consequence of the pulse chirp caused by the group velocity dispersion. This effect leads to apparent changes in the measured excited-state absorption spectra, which can be corrected during the fitting procedure to yield meaningful absorption cross-section values.

Ultrafast Dynamics of Polarons in Conductive Polyaniline: Comparison of Primary and Secondary Doped Forms

Time- and wavelength-resolved pump-probe measurements are performed on the conductive, primary and secondary doped, forms of polyaniline in solution to investigate the relaxation dynamics of photoexcited polarons. Contrasting dynamics observed in the two forms allow investigation of electronic (and structural) relaxation of this material. Pump pulse at 800 nm photoexcites an electron from the valence to polaron band, and subsequent relaxation dynamics are probed by a white light continuum pulse. Three distinct features are observed in the primary doped sample: (1) absorption near time zero in the 900-1025 nm probe wavelength region; (2) delayed absorption from 850 to 1025 nm; (3) pronounced oscillations with frequencies of 165 and 210 cm(-1). The first two features are associated with intraband absorptions to higher-lying states in the polaron band from the initial excited and conformationally changed intermediate states. The oscillations reflect torsional motions associated with photoexcitation and relaxation. In the secondary doped material, only bleaching and stimulated emission are observed throughout the whole spectral region; neither transient absorption signals nor oscillatory dynamics are observed. Kinetic modeling is performed to establish the mechanism of relaxation. We propose that the excited polaron relaxes nonradiatively to the ground state through an intermediate state(s) with a twisted geometry. Our measurements and analysis allow us to describe the structure of the polaron bands for primary and secondary doped polyaniline; they are in the small and large polaron limits, respectively, and are consistent with a bandgap for the primary doped form and without a bandgap (i.e., metallic) for the secondary doped material.

Nonlinear absorption spectrum of ytterbium bis-phthalocyanine solution measured by white-light continuum Z-scan technique

In this work we have measured the nonlinear absorption spectrum of ytterbium bis-phthalocyanine (YbPc 2) in chloroform solution employing the Z-scan technique with white-light continuum (WLC) pulses. Two distinct excited state absorption behaviors were observed: a saturable absorption (SA) at the Q-band region and a reverse saturable absorption (RSA) around 530 nm. A three-energy-level diagram was used to explain the experimental results, leading to the excited state absorption cross-section determination from 500 up to 675 nm.

High dynamic range characterization of ultrabroadband white-light continuum pulses

We demonstrate an improved method for characterizing ultrashort pulses. Our apparatus is based on spectral phase interferometry for direct electric-field reconstruction (SPIDER) and particularly suited for measurement of compressed white-light continuum pulses with their strong spectral amplitude variations. Phase-sensitive noise rejection allows for a significant increase of the dynamic range of the SPIDER trace. We show that characteristic SPIDER artifacts can be avoided and demonstrate the method by characterizing 4.3-fs pulses from a two-stage hollow-fiber compressor.

White-light continuum Z-scan technique for nonlinear materials characterization

We present a technique for rapid characterization of degenerate nonlinear absorption and refraction spectra using a femtosecond white-light continuum (WLC) pulse to perform Z-scans. The spectral components of the WLC source are temporally and spatially dispersed to minimize nondegenerate two-photon absorption (2PA) processes. We demonstrate the validity of the method by measuring the 2PA spectrum of a well-characterized semiconductor, ZnSe.

Generation and applications of phase-locked white-light continuum pulses

We report on an experiment demonstrating the high degree of mutual phase coherence between two white-light continuum pulses independently generated by phase-locked ultrashort laser pulses. We show that these secondary pulses are locked in phase and exhibit surprisingly clear and stable Young's interference fringes when overlapped on a screen in the far field. We discuss the use of such phase-locked continuum pulses in the realization of a new kind of ultrabroad frequency comb for absolute frequency measurements throughout the visible spectrum.

Application of a spatial light modulator for programmable optical pulse compression to the sub-6-fs regime

We present a grating-pair-formed pulse shaper with a spatial light modulator (SLM) for programmable chirp compensation by applying voltages on the SLM. When linear and nonlinear chirp are optimally compensated for, corresponding to the quadratic, cubic, and quartic phases of -400 fs/sup 2/, 1100 fs/sup 3/, and 2000 fs/sup 4/ at the center wavelength 780 nm on the SLM, a stronger chirped white light continuum (WLC) pulse with a spectral bandwidth of 570-970 nm is compressed to the sub-6-fs regime.

Phase-locked white-light continuum pulses: toward a universal optical frequency-comb synthesizer

We demonstrate that two white-light continuum pulses that are independently generated by phase-locked ultrashort laser pulses are locked in phase and show surprisingly clear and stable Young interference fringes. The experiment shows that the two generated continua emit essentially in phase and that random phase jitter can remain negligible. This result is not only of interest for studies of nonlinear field-matter interactions but also suggests that such white-light continuum pulses can be used to realize a broad frequency comb for absolute frequency measurements from the IR to the UV.

Femtosecond noncollinear and collinear parametric generation and amplification in BBO crystal

We perform comprehensive theoretical and experimental analysis of ultrafast noncollinear and collinear parametric generation and amplification in type-I phase-matched BBO crystal, pumped with 390 nm wavelength, 150-fs-duration pulses from a frequency-doubled regenerative-amplified Ti:sapphire laser. We investigate conditions for group-velocity-matched generation and amplification, seeded with white-light continuum pulses by considering mean propagation velocity of pump and parametric pulses. We show that under certain conditions, effective matching can be achieved for noncollinear geometry with a 390-nm pump. We report detailed energy, spectral, and temporal measurement of the parametric processes in BBO and show that our effective matching conditions agree with experimentally observed angular and spectral properties of the parametric gain. The maximum tuning range for noncollinear process was from 460 nm to 2.56 μm, with the highest conversion efficiency of up to 20%. For collinear geometry, no effective matching of average group velocity was found. Because of this, the maximum conversion efficiency with UV pump in single-pass OPA is limited to 1.5%.

Propagation of Femtosecond White-Light Continuum Pulse in Polymer Latex Powder Investigated by Optical Kerr Gate and Time-Resolved Diffuse Reflectance Spectroscopy

Propagation of a femtosecond white-light continuum pulse (350 fs) in poly (methyl methacrylate) (PMMA) latex powders with particle diameters of 1, 5, 20, 60, and 100 µm was investigated by means of an optical Kerr gate and time-resolved diffuse reflectance spectroscopy, which is a pump-probe technique for multiple light scattering materials. The temporal profile of a diffuse reflected light pulse from the powder and the temporal response of transient absorption of the powder doped with an organic molecule were measured. The Kubelka-Munk model was extended to address the propagation of a short light pulse, and its numerical simulations reproduced both experimental results quite well. It is suggested that in a dense scattering medium such as powder, an effect that light is confined or proceeds in a twisted manner is of importance. Fitting the experimental data with the diffusion theory was unsuccessful, which is also discussed.

Second-harmonic generation frequency-resolved optical gating in the single-cycle regime

The problem of measuring broad-band femtosecond pulses by the technique of second-harmonic generation frequency-resolved optical gating (SHG FROG) is addressed. We derive the full equation for the FROG signal, which is valid even for single-optical-cycle pulses. The effect of the phase mismatch in the second-harmonic crystal, the implications of the beam geometry, and the frequency-dependent variation of the nonlinearity are discussed in detail. Our numerical simulations show that, under carefully chosen experimental conditions and with a proper spectral correction of the data, the traditional FROG inversion routines work well even in the single-cycle regime. The developed description of the SHG FROG signal was applied to measure the white-light continuum pulses in the spectral region of 500-1100 nm. The obtained spectral phase of these pulses served as a target function for the pulse compressor design. The pulses produced by compression around 800 nm were also characterized by SHG FROG. The resulting pulse duration measures 4.5 fs which corresponds to /spl sim/2.5 optical cycles.

Automatic time delay optimization between the pump and seed pulses of a broadly tunable femtosecond optical parametric amplifier

We propose a scheme for optimizing the time delay between the pump and seed pulses of an optical parametric amplifier (OPA) over a large spectral range. The efficiency of this method is demonstrated for a femtosecond BBO parametric amplifier seeded with a white-light continuum pulse. The error signal used for intensity stabilization results from a modulation of the temporal delay between the pump and the continuum pulses and phase-sensitive detection of the amplified signal. It allows us to lock the delay to the position that maximizes the OPA gain.

Convenient Method of Measuring the Chirp Structure of Femtosecond White-Light Continuum Pulses

We have developed a convenient method of measuring the temporal chirp structure of white-light continuum pulses used in femtosecond time-resolved absorption spectroscopy. This method, based on the nonresonant optical Kerr effect, is superior in many respects to the existing one that uses the sum or difference frequency generation by a nonlinear crystal.

Generation of tunable near-infrared amplified femtosecond laser pulses and time-correlated white-light continuum

We report the generation of kilohertz-rate, amplified, sub-100-fsec-duration laser pulses tunable from 785 to 825 nm. A novel two-stage amplification scheme is used to produce amplified, tunable optical pulses, with microjoule energies. Diffraction-limited focusing of the amplified laser pulses in ethylene glycol to peak intensities greater than 1013 W/cm2 results in the efficient generation of white-light continuum pulses with a bandwidth extending from 0.4 to 1.0 μm. This tunable laser system is well suited for sensitive femtosecond spectroscopic measurements in the near infrared, and its tunability may easily be extended to cover the wavelength range 600–900 nm.

Picosecond time-resolved absorption in the mid-infrared by seeded optical parametric generation

An apparatus to probe picosecond photoinduced absorption in semiconductors between 2 and 5 microm with picosecond resolution is described. The IR source is based on difference frequency mixing an amplified picosecond dye laser pulse with a synchronized white light continuum pulse. The choice of liquid for continuum generation is discussed, and applications to transient absorption in germanium and trans-polyacetylene are presented.

Femtosecond optical pulses

Recent advances in generation, amplification, compression, and frequency broadening of femtosecond optical pulses are reviewed. We describe use of colliding pulse mode locking to generate pulses of 65 fs duration and pulse compression to reduce those pulse durations to 30 fs. Amplification of femtosecond pulses to gigawatt powers and frequency broadening to obtain white light continuum pulses while retaining femtosecond pulse durations are also examined.