Collinear opto-optical loss modulation for carrier-envelope offset stabilization of a fiber frequency comb.

Opto-optical loss modulation (OOM) for stabilization of the carrier-envelope offset (CEO) frequency of a femtosecond all-fiber laser is performed using a collinear geometry. Amplitude-modulated 1064 nm light is fiber coupled into an end-pumped semiconductor saturable absorber mirror (SESAM)-mode-locked all-polarization-maintaining erbium fiber femtosecond laser, where it optically modulates the loss of the SESAM resulting in modulation of the CEO frequency. A noise rejection bandwidth of 150 kHz is achieved when OOM and optical gain modulation are combined in a hybrid analog/digital loop. Collinear OOM provides a simple, all-fiber, high-bandwidth method for improving the CEO frequency stability of SESAM mode-locked fiber lasers.

Mid-infrared frequency comb with 6.7 W average power based on difference frequency generation.

We report on the development of a high-power mid-infrared frequency comb with 100 MHz repetition rate and 100 fs pulse duration. Difference frequency generation is realized between two branches derived from an Er:fiber comb, amplified separately in Yb:fiber and Er:fiber amplifiers. Average powers of 6.7 W and 14.9 W are generated in the 2.9 µm idler and 1.6 µm signal, respectively. With high average power, excellent beam quality, and passive carrier-envelope phase stabilization, this light source is a promising platform for generating broadband frequency combs in the far infrared, visible, and deep ultraviolet.

Mid-infrared frequency combs at 10 GHz.

We demonstrate mid-infrared (MIR) frequency combs at 10 GHz repetition rate via intra-pulse difference-frequency generation (DFG) in quasi-phase-matched nonlinear media. Few-cycle pump pulses (≲15fs, 100 pJ) from a near-infrared electro-optic frequency comb are provided via nonlinear soliton-like compression in photonic-chip silicon-nitride waveguides. Subsequent intra-pulse DFG in periodically poled lithium niobate waveguides yields MIR frequency combs in the 3.1-4.8 µm region, while orientation-patterned gallium phosphide provides coverage across 7-11 µm. Cascaded second-order nonlinearities simultaneously provide access to the carrier-envelope-offset frequency of the pump source via in-line f-2f nonlinear interferometry. The high-repetition rate MIR frequency combs introduced here can be used for condensed phase spectroscopy and applications such as laser heterodyne radiometry.

Mid-Infrared Frequency Comb Generation and Spectroscopy with Few-Cycle Pulses and χ

The mid-infrared atmospheric window of 3-5.5 µm holds valuable information regarding molecular composition and function for fundamental and applied spectroscopy. Using a robust, mode-locked fiber-laser source of <11 fs pulses in the near infrared, we explore quadratic (χ^{(2)}) nonlinear optical processes leading to frequency comb generation across this entire mid-infrared atmospheric window. With experiments and modeling, we demonstrate intrapulse difference frequency generation that yields few-cycle mid-infrared pulses in a single pass through periodically poled lithium niobate. Harmonic and cascaded χ^{(2)} nonlinearities further provide direct access to the carrier-envelope offset frequency of the near infrared driving pulse train. The high frequency stability of the mid-infrared frequency comb is exploited for spectroscopy of acetone and carbonyl sulfide with simultaneous bandwidths exceeding 11 THz and with spectral resolution as high as 0.003 cm^{-1}. The combination of low noise and broad spectral coverage enables detection of trace gases with concentrations in the part-per-billion range.

Tunable mid-infrared generation via wide-band four-wave mixing in silicon nitride waveguides.

We demonstrate wide-band frequency down-conversion to the mid-infrared (MIR) using four-wave mixing (FWM) of near-infrared (NIR) femtosecond-duration pulses from an Er:fiber laser, corresponding to 100 THz spectral translation. Photonic-chip-based silicon nitride waveguides provide the FWM medium. Engineered dispersion in the nanophotonic geometry and the wide transparency range of silicon nitride enable large-detuning FWM phase-matching and results in tunable MIR from 2.6 to 3.6 µm on a single chip with 100-pJ-scale pump-pulse energies. Additionally, we observe up to 25 dB broadband parametric gain for NIR pulses when the FWM process is operated in a frequency up-conversion configuration. Our results demonstrate how integrated photonic circuits pumped with fiber lasers could realize multiple nonlinear optical phenomena on the same chip and lead to engineered synthesis of broadband, tunable, and coherent light across the NIR and MIR wavelength bands.

Mid-infrared frequency comb generation via cascaded quadratic nonlinearities in quasi-phase-matched waveguides.

We experimentally demonstrate a simple configuration for mid-infrared (MIR) frequency comb generation in quasi-phase-matched lithium niobate waveguides using the cascaded-χ(2) nonlinearity. With nanojoule-scale pulses from an Er:fiber laser, we observe octave-spanning supercontinuum in the near-infrared with dispersive wave generation in the 2.5-3 µm region and intrapulse difference frequency generation in the 4-5 µm region. By engineering the quasi-phase-matched grating profiles, tunable, narrowband MIR and broadband MIR spectra are both observed in this geometry. Finally, we perform numerical modeling using a nonlinear envelope equation, which shows good quantitative agreement with the experiment-and can be used to inform waveguide designs to tailor the MIR frequency combs. Our results identify a path to a simple single-branch approach to mid-infrared frequency comb generation in a compact platform using commercial Er:fiber technology.

Selectivity of Electronic Coherence and Attosecond Ionization Delays in Strong-Field Double Ionization.

Experiments are presented on real-time probing of coherent electron dynamics in xenon initiated by strong-field double ionization. Attosecond transient absorption measurements allow for characterization of electronic coherences as well as relative ionization timings in multiple electronic states of Xe^{+} and Xe^{2+}. A high degree of coherence g=0.4 is observed between ^{3}P_{2}^{0}-^{3}P_{0}^{0} of Xe^{2+}, whereas for other possible pairs of states the coherences are below the detection limits of the experiments. A comparison of the experimental results with numerical simulations based on an uncorrelated electron-emission model shows that the coherences produced by strong-field double ionization are more selective than predicted. Surprisingly short ionization time delays, 0.85 fs, 0.64 fs, and 0.75 fs relative to Xe^{+} formation, are also measured for the ^{3}P_{2}, ^{3}P_{0}, and ^{3}P_{1} states of Xe^{2+}, respectively. Both the unpredicted selectivity in the formation of coherence and the subfemtosecond time delays of specific states provide new insight into correlated electron dynamics in strong-field double ionization.

Generating high-contrast, near single-cycle waveforms with third-order dispersion compensation.

Femtosecond laser pulses lasting only a few optical periods hold the potential for probing and manipulating the electronic degrees of freedom within matter. However, the generation of high-contrast, few-cycle pulses in the high power limit still remains nontrivial. In this Letter, we present the application of ammonium dihydrogen phosphate (ADP) as an optical medium for compensating for the higher-order dispersion of a carrier-envelope stable few-cycle waveform centered at 735 nm. The ADP crystal is capable of removing the residual third-order dispersion present in the spectral phase of an input pulse, resulting in near-transform-limited 2.9 fs pulses lasting only 1.2 optical cycles in duration. By utilizing these high-contrast, few-cycle pulses for high-harmonic generation, we are able to produce nanojoule-scale, isolated attosecond pulses.

Coherent electron hole dynamics near a conical intersection.

The coherent evolution of an electron hole in a photoionized molecule represents an unexplored facet of charge transfer phenomena occurring in complex systems. Using ultrafast extreme ultraviolet spectroscopy, we investigate the real-time dynamics of an electron hole wave packet created near a conical intersection in CO_{2}. We resolve the oscillation of the electron hole density between σ and π character, driven by the coupled bending and asymmetric stretch vibrations of the molecule. We also quantify the mixing between electron hole configurations and find that the wave packet coherence diminishes with time due to thermal dephasing.

Ultrafast dynamics of neutral superexcited oxygen: a direct measurement of the competition between autoionization and predissociation.

Using ultrafast extreme ultraviolet pulses, we performed a direct measurement of the relaxation dynamics of neutral superexcited states corresponding to the nlσ(g)(c(4)Σ(u)(-)) Rydberg series of O(2). An extreme ultraviolet attosecond pulse train was used to create a temporally localized Rydberg wave packet and the ensuing electronic and nuclear dynamics were probed using a time delayed femtosecond near-infrared pulse. We investigated the competing predissociation and autoionization mechanisms in superexcited oxygen molecules and found that autoionization is dominant for the low n Rydberg states. We measured an autoionization lifetime of 92±6 fs and 180±10 fs for the (5s,4d)σ(g) and (6s,5d)σ(g) Rydberg state groups, respectively. We also determine that the disputed neutral dissociation lifetime for the ν=0 vibrational level of the Rydberg series is 1100±100 fs.

Attosecond-resolved evolution of a laser-dressed helium atom: interfering excitation paths and quantum phases.

Using high-order harmonic attosecond pulse trains, we investigate the photoionization dynamics and transient electronic structure of a helium atom in the presence of moderately strong (∼10(12) W cm(-2)) femtosecond laser pulses. We observe quantum interferences between photoexcitation paths from the ground state to different laser-dressed Floquet state components. As the intensity ramps on femtosecond time scales, we observe switching between ionization channels mediated by different atomic resonances. Using precision measurements of ion yields and photoelectron distributions, the quantum phase difference between interfering paths is extracted for each ionization channel and compared with simulations. Our results elucidate photoionization mechanisms in strong fields and open the doors for photoabsorption or photoionization control schemes.

Health-Related Quality of Life in Adrenocortical Carcinoma: Development of the Disease-Specific Questionnaire ACC-QOL and Results from the PROFILES Registry.

We aimed to develop a disease-specific adrenocortical carcinoma (ACC) health-related quality of life (HRQoL) questionnaire (ACC-QOL) and assess HRQoL in a population-based cohort of patients with ACC. Development was in line with European Organization for Research and Treatment of Cancer (EORTC) guidelines, though not an EORTC product. In phase I and II, we identified 90 potential HRQoL issues using literature and focus groups, which were reduced to 39 by healthcare professionals. Pilot testing resulted in 28 questions, to be used alongside the EORTC QLQ-C30. In Phase III, 100 patients with ACC were asked to complete the questionnaires twice in the PROFILES registry (3-month interval, respondents: first 67, second 51). Confirmatory factor analysis demonstrated the structural validity of 26 questions with their scale structure (mitotane side-effects, hypercortisolism/hydrocortisone effects, emotional effects). Internal consistency and reliability were good (Cronbach's alpha 0.897, Interclass correlation coefficient 0.860). Responsiveness analysis showed good discriminative ability (AUC 0.788). Patients diagnosed more than 5 years ago reported a good HRQoL compared with the Dutch reference population, but experienced residual fatigue and emotional problems. Patients who underwent recent treatment reported a lower HRQoL and problems in several domains. In conclusion, we developed an ACC-specific HRQoL questionnaire with good psychometric properties.

Ultrafast strong-field dissociation of vinyl bromide: An attosecond transient absorption spectroscopy and non-adiabatic molecular dynamics study.

Attosecond extreme ultraviolet (XUV) and soft x-ray sources provide powerful new tools for studying ultrafast molecular dynamics with atomic, state, and charge specificity. In this report, we employ attosecond transient absorption spectroscopy (ATAS) to follow strong-field-initiated dynamics in vinyl bromide. Probing the Br M edge allows one to assess the competing processes in neutral and ionized molecular species. Using ab initio non-adiabatic molecular dynamics, we simulate the neutral and cationic dynamics resulting from the interaction of the molecule with the strong field. Based on the dynamics results, the corresponding time-dependent XUV transient absorption spectra are calculated by applying high-level multi-reference methods. The state-resolved analysis obtained through the simulated dynamics and related spectral contributions enables a detailed and quantitative comparison with the experimental data. The main outcome of the interaction with the strong field is unambiguously the population of the first three cationic states, D 1, D 2, and D 3. The first two show exclusively vibrational dynamics while the D 3 state is characterized by an ultrafast dissociation of the molecule via C-Br bond rupture within 100 fs in 50% of the analyzed trajectories. The combination of the three simulated ionic transient absorption spectra is in excellent agreement with the experimental results. This work establishes ATAS in combination with high-level multi-reference simulations as a spectroscopic technique capable of resolving coupled non-adiabatic electronic-nuclear dynamics in photoexcited molecules with sub-femtosecond resolution.

Infrared electric field sampled frequency comb spectroscopy.

Probing matter with light in the mid-infrared provides unique insight into molecular composition, structure, and function with high sensitivity. However, laser spectroscopy in this spectral region lacks the broadband or tunable light sources and efficient detectors available in the visible or near-infrared. We overcome these challenges with an approach that unites a compact source of phase-stable, single-cycle, mid-infrared pulses with room temperature electric field-resolved detection at video rates. The ultrashort pulses correspond to laser frequency combs that span 3 to 27 µm (370 to 3333 cm-1), and are measured with dynamic range of >106 and spectral resolution as high as 0.003 cm-1. We highlight the brightness and coherence of our apparatus with gas-, liquid-, and solid-phase spectroscopy that extends over spectral bandwidths comparable to thermal or infrared synchrotron sources. This unique combination enables powerful avenues for rapid detection of biological, chemical, and physical properties of matter with molecular specificity.

Disentangling conical intersection and coherent molecular dynamics in methyl bromide with attosecond transient absorption spectroscopy.

Attosecond probing of core-level electronic transitions provides a sensitive tool for studying valence molecular dynamics with atomic, state, and charge specificity. In this report, we employ attosecond transient absorption spectroscopy to follow the valence dynamics of strong-field initiated processes in methyl bromide. By probing the 3d core-to-valence transition, we resolve the strong field excitation and ensuing fragmentation of the neutral σ* excited states of methyl bromide. The results provide a clear signature of the non-adiabatic passage of the excited state wavepacket through a conical intersection. We additionally observe competing, strong field initiated processes arising in both the ground state and ionized molecule corresponding to vibrational and spin-orbit motion, respectively. The demonstrated ability to resolve simultaneous dynamics with few-femtosecond resolution presents a clear path forward in the implementation of attosecond XUV spectroscopy as a general tool for probing competing and complex molecular phenomena with unmatched temporal resolution.

Universality in eight-arm star polystyrene and methylcyclohexane mixtures near the critical point.

Measurements of the coexistence curve and turbidity were made on different molecular mass samples of the branched polymer-solvent system eight-arm star polystyrene in methylcyclohexane near its critical point. We confirmed that these systems belong in the Ising universality class. The location of the critical temperature and composition as well as the correlation length, susceptibility, and coexistence curve amplitudes were found to depend on molecular mass and the degree of branching. The coexistence curve diameter had an asymmetry that followed a "complete scaling" approach. All the coexistence curve data could be scaled onto a common curve with one adjustable parameter. We found the coexistence curve amplitude to be about 12% larger for branched than linear polystyrenes of the same molecular mass in either solvent cyclohexane or methylcyclohexane. The two-scale-factor universality ratio R was found to be independent of molecular mass or degree of branching.