Interferometric Frequency-resolved Optical Gating Research Articles

Asymmetric double-pulse interferometric FROG for visible-wavelength time-domain spectroscopy.

To extend the detection range of time-domain spectroscopy into the challenging visible frequencies, we propose an interferometry-type frequency-resolved optical gating (FROG). Our numerical simulation shows that, when operating in a double-pulse scheme, a unique phase-locking mechanism can be activated and preserves both zero- and first-order phases (φ0, φ1)-indispensable for phase-sensitive spectroscopic study-that are otherwise inaccessible to standard FROG measurement. Followed by time-domain signal reconstruction and analysis protocol, we show that time-domain spectroscopy with sub-cycle temporal resolution is enabled and well suits the need of a ultrafast-compatible and ambiguity-free method for complex dielectric function measurement at visible wavelengths.

Asymmetric interferometric frequency resolved optical gating for complete unambiguous ultrashort pulse characterisation

We propose a modification of the second-harmonic generation frequency resolved optical gating method (SHG-FROG) utilizing asymmetric spectral interferometry (aiFrog) for complete unambiguous reconstruction of ultrashort optical pulse intensity and phase. The large amount of information encoded in the aiFrog trace allows a straightforward calculation of the fundamental pulse spectrum and a direct reconstruction of the pulse envelope and phase. A simple and robust iterative algorithm with fast convergence is developed for pulse reconstruction from the aiFrog trace, taking advantage of large data redundancy and noise averaging. Due to the asymmetry of the setup, our method is free from the known challenges of a standard SHG-FROG (direction-of-time ambiguity, almost identical FROG traces for well separated pulses with different phases and for asymmetric well separated pulses). The method is highly sensitive to the weak satellite pulse of the main pulse, which can be used to track pre- or post-pulses even without full processing of the measured aiFrog trace.

Single-scan, dual-functional interferometer for fast spatio-temporal characterization of few-cycle pulses.

Accurate and fast characterization of spatio-temporal information of high-intensity, ultrashort pulses is crucial in the field of strong-field laser science and technology. While conventional self-referenced interferometers were widely used to retrieve the spatial profile of the relative spectral phase of pulses, additional measurements of temporal and spectral information at a particular position of the laser beam, however, were necessary to remove the indeterminacy, which increases the system complexity. Here we report an advanced, dual-functional interferometer that is able to reconstruct the complete spatio-temporal information of ultrashort pulses with a single scan of the interferometer arm. The setup integrates an interferometric frequency-resolved optical gating (FROG) with a radial shearing Michelson interferometer. Through scanning one arm of the interferometer, both the cross-correlated FROG trace at the central part of the laser beam and the delay-dependent interferograms of the entire laser profile are simultaneously obtained, allowing a fast three-dimensional reconstruction of few-cycle laser pulses.

Common pulse retrieval algorithm: a fast and universal method to retrieve ultrashort pulses

We present a common pulse retrieval algorithm (COPRA) that can be used for a broad category of ultrashort laser pulse measurement schemes including frequency-resolved optical gating (FROG), interferometric FROG, dispersion scan, time domain ptychography, and pulse shaper assisted techniques such as multiphoton intrapulse interference phase scan (MIIPS). We demonstrate its properties in comprehensive numerical tests and show that it is fast, reliable and accurate in the presence of Gaussian noise. For FROG it outperforms retrieval algorithms based on generalized projections and ptychography. Furthermore, we discuss the pulse retrieval problem as a nonlinear least-squares problem and demonstrate the importance of obtaining a least-squares solution for noisy data. These results improve and extend the possibilities of numerical pulse retrieval. COPRA is faster and provides more accurate results in comparison to existing retrieval algorithms. Furthermore, it enables full pulse retrieval from measurements for which no retrieval algorithm was known before, e.g., MIIPS measurements.

Field enhancement of multiphoton induced luminescence processes in ZnO nanorods

The near-ultraviolet photoluminescence of ZnO nanorods induced by multiphoton absorption of unamplified Ti:sapphire pulses is investigated. Power dependence measurements have been conducted with an adaptation of the ultrashort pulse characterization method of interferometric frequency-resolved optical gating. These measurements enable the separation of second harmonic and photoluminescence bands due to their distinct coherence properties. A detailed analysis yields fractional power dependence exponents in the range of 3–4, indicating the presence of multiple nonlinear processes. The range in measured exponents is attributed to differences in local field enhancement, which is supported by independent photoluminescence and structural measurements. Simulations based on Keldysh theory suggest contributions by three- and four-photon absorption as well as avalanche ionization in agreement with experimental findings.

Third-harmonic interferometric frequency-resolved optical gating

The ability to generate third-harmonic light with a broad phase-matching bandwidth at any dielectric interface offers a promising tool for the characterization of ultrafast laser pulses. Third-harmonic, interferometric frequency-resolved optical gating (TH-iFROG) exploits this nonlinearity. The TH-iFROG traces contain an abundance of information, but their complicated analytical form poses a problem for standard FROG algorithms. Here we solve the pulse retrieval problem with an evolutionary algorithm known as differential evolution. The algorithm processes a TH-iFROG trace, extracting three novel types of FROG traces that are subsequently used for pulse retrieval. The analytical forms of these FROG traces are also presented. Built-in error correction mechanisms and the large amount of redundant data make the pulse retrieval robust against measurement noise and systematic errors. The technique is demonstrated via the characterization of unamplified few-cycle pulses using a simple fused silica window as the nonlinear medium. Comparison to a reference measurement made with a commercial pulse characterization device shows excellent agreement between the measured complex electric fields.

Pulse retrieval algorithm for interferometric frequency-resolved optical gating based on differential evolution

A novel algorithm for the ultrashort laser pulse characterization method of interferometric frequency-resolved optical gating (iFROG) is presented. Based on a genetic method, namely, differential evolution, the algorithm can exploit all available information of an iFROG measurement to retrieve the complex electric field of a pulse. The retrieval is subjected to a series of numerical tests to prove the robustness of the algorithm against experimental artifacts and noise. These tests show that the integrated error-correction mechanisms of the iFROG method can be successfully used to remove the effect from timing errors and spectrally varying efficiency in the detection. Moreover, the accuracy and noise resilience of the new algorithm are shown to outperform retrieval based on the generalized projections algorithm, which is widely used as the standard method in FROG retrieval. The differential evolution algorithm is further validated with experimental data, measured with unamplified three-cycle pulses from a mode-locked Ti:sapphire laser. Additionally introducing group delay dispersion in the beam path, the retrieval results show excellent agreement with independent measurements with a commercial pulse measurement device based on spectral phase interferometry for direct electric-field retrieval. Further experimental tests with strongly attenuated pulses indicate resilience of differential-evolution-based retrieval against massive measurement noise.

Interferometric time-domain ptychography for ultrafast pulse characterization.

A novel pulse characterization method is presented, favorably combining interferometric frequency-resolved optical gating (FROG) and time-domain ptychography. This new variant is named ptychographic-interferometric frequency-resolved optical gating (πFROG). The measurement device is simple, bearing similarity to standard second-harmonic FROG, yet with a collinear beam geometry and an added bandpass filter in one of the correlator arms. The collinear beam geometry allows tight focusing and circumvents possible geometrical distortion effects of noncollinear methods, making πFROG especially suitable for the characterization of unamplified few-cycle pulses. Moreover, the direction-of-time ambiguity afflicting most second-order FROG variants is eliminated. Possible group delay dispersion of pulses leads to a characteristic tilt in the πFROG traces, allowing the detection of uncompensated dispersion without a retrieval. Using nanojoule, three-cycle pulses at 800nm, the πFROG method is tested, and the results are compared with spectral phase interferometry for direct electric field reconstruction measurements. Measured pulse durations agree within a fraction of a femtosecond. As a further test, the πFROG measurements are repeated with added group delay dispersion, and found to accurately reproduce the dispersion computed with Sellmeier equations.

ZnO nanorods for efficient third harmonic UV generation

ZnO nanorods grown by both high temperature vapour phase transport and low temperature chemical bath deposition are very promising sources for UV third harmonic generation. Material grown by both methods show comparable efficiencies, in both cases an order of magnitude higher than surface third harmonic generation at the quartz-air interface of a bare quartz substrate. This result is in stark contrast to the linear optical properties of ZnO nanorods grown by these two methods, which show vastly different PL efficiencies. The third harmonic generated signal is analysed using intensity dependent measurements and interferometric frequency resolved optical gating, allowing extraction of the laser pulse parameters. The comparable levels of efficiency of ZnO grown by these very different methods as sources for third harmonic UV generation provides a broad suite of possible growth methods to suit various substrates, coverage and scalability requirements. Potential application areas range from interferometric frequency resolved optical gating characterization of few cycle fs pulses to single cell UV irradiation for biophysical studies.

First Measurement of the Non-instantaneous Response Time of a χ(3)Nonlinear Optical Effect

The third harmonic of a few-cycle pulse, generated at different dielectric surfaces, is investigated using interferometric frequency-resolved optical gating. We present direct experimental evidence for a non-instantaneous nonlinear response in a TiO 2 thin film whereas surface third-harmonic generation in a SiO 2 sample does not show any indication for non-instanteneity. To the best of our knowledge, this constitutes the first report of a non-instantaneous nonlinear optical response of a dielectric optical material.

Highly efficient THG in TiO_2 nanolayers for third-order pulse characterization

Third harmonic generation (THG) of femtosecond laser pulses in sputtered nanocrystalline TiO2 thin films is investigated. Using layers of graded thickness, the dependence of THG on the film parameters is studied. The maximum THG signal is observed at a thickness of 180 nm. The corresponding conversion efficiency is 26 times larger compared to THG at the air-glass interface. For a demonstration of the capabilities of such a highly nonlinear material for pulse characterization, third-order autocorrelation and interferometric frequency-resolved optical gating (IFROG) traces are recorded with unamplified nanojoule pulses directly from a broadband femtosecond laser oscillator.

Few-Femtosecond Plasmon Dephasing of a Single Metallic Nanostructure from Optical Response Function Reconstruction by Interferometric Frequency Resolved Optical Gating

The precise characterization of the ultrafast optical response of metals and metallic nanostructures has remained an experimental challenge. We probe the few-femtosecond electronic dephasing of a local surface plasmon polariton excitation using symmetry-selective second-harmonic (SH) Rayleigh scattering of a nanoscopic conical gold tip as an individual plasmonic nanostructure. The full reconstruction of the optical response function of the plasmon excitation with phase and amplitude without any model assumptions is demonstrated from the analysis of the two-dimensional spectrogram obtained by simultaneous time- and frequency-domain SH measurements, using interferometric frequency resolved optical gating. The measured dephasing time of T(2) = 18 +/- 5 fs indicates the plasmon damping is dominated by nonradiative decay, consistent with a Drude-Sommerfeld dielectric response for gold. Even for the nominally homogeneous localized plasmon response, deviations are observed from the ideal harmonic oscillator phase behavior, which may reflect the underlying inhomogeneous electronic response with its different scattering channels. The presented technique is generally applicable for the reconstruction of the plasmon dynamics of complex nanostructures: information that cannot be obtained by conventional dark-field scattering.

Advanced methods for the characterization of few-cycle light pulses: a comparison

We discuss and compare four methods for measuring the width and pulse profile of ultrashort pulses. For our comparison, we use stable sub-7 fs pulses from a Ti:sapphire oscillator. Interferometric autocorrelation, spectral phase interferometry for direct electric-field reconstruction (SPIDER), a spatially-encoded variant of SPIDER, and interferometric frequency-resolved optical gating (IFROG) are utilized for characterizing pulses from the oscillator. The methods are found to agree within 5% as far as determination of the pulse width is concerned. However, differences are observed in the satellite structure reconstructed by either method. The current state of the art of measuring ultrashort pulses with these methods is reviewed and current limitations, in particular for characterizing complex pulse shapes, are discussed.

Structures of interferometric frequency-resolved optical gating

We report on a novel method for the characterization of complex ultrashort pulses, in both amplitude and phase. This method is the straightforward extension of interferometric autocorrelation (IAC) toward frequency-resolved optical gating (FROG) and therefore displays all advantages of a collinear method for measuring few-cycle pulses. The interferometric FROG (IFROG) trace contains two independent time-frequency distributions, namely the standard second-harmonic (SH)-FROG trace and a novel non-semidefinite FROG trace, which requires new retrieval algorithms. We discuss one suitable retrieval concept that is based on an adaption of generalized projections and demonstrate it with different types of short laser pulses. The IFROG trace contains additional information that can be used for intrinsic calibration of the FROG traces. The strong redundancy of data increases the robustness of the method. IFROG is a pulse characterization method that is particularly suited for the complex sub-10-fs pulse shapes as they can arise during supercontinuum compression and gives detailed insight into the satellite structures of partly compressed continua.

Interferometric frequency-resolved optical gating

We demonstrate a novel variant of frequency-resolved optical gating (FROG) that is based on spectrally resolving a collinear interferometric autocorrelation rather than a noncollinear one. From the interferometric FROG trace, one can extract two terms, the standard SHG-FROG trace and a new phase-sensitive modulational component, which both allow for independent retrieval of the pulse shape. We compare the results of both methods and a separate SPIDER measurement using 6.5-fs pulses from a white-light continuum. We find that the novel modulational component allows for robust retrieval of pulse shapes in the few-cycle regime. Together with the added cross-checks, our method significantly enhances choices for pulse characterization in this regime.

Measurement of ultrafast ionization dynamics of gases by multipulse interferometric frequency-resolved optical gating.

Ultrafast ionization dynamics of femtosecond laser-irradiated noble and simple diatomic gases were studied using a novel two-color time-domain technique which eliminated significant complications seen in past experiments. Ultrafast depletion of the probing laser pulse was observed strictly coincident with the ionization front and attributed to a previously unobserved nonlinear frequency mixing via the transverse plasma current [F. Brunel, J. Opt. Soc. Am. B 7, 52 (1990)]. Good quantitative agreement of the measured single-atom ionization rates with Ammosov-Delone-Krainov rates was found, except for O (2) which showed a 200x smaller rate.

Coherent ultrafast MI-FROG spectroscopy of optical field ionization in molecular H/sub 2/, N/sub 2/, and O/sub 2/

Quantitative phase-sensitive measurements of ultrafast optical-field ionization rates in molecular H/sub 2/, N/sub 2/, and O/sub 2/ are obtained using a temporally gated frequency-domain interferometric pulse measurement technique: multipulse interferometric frequency-resolved optical gating (MI-FROG). By measuring the pump-induced frequency change on a weak copropagating probe pulse, the optical field ionization dynamics can be completely time-resolved with sub-pulsewidth time resolution. A one-dimensional nonrelativistic electromagnetic fluid code model is used to compute the ionization dynamics and optical field propagation through the plasma. Using the Ammosov-Delone-Krainov (ADK) tunnel ionization rate model originally developed for atoms, the relatively simple model proposed here has been shown to compare favorably with the MI-FROG measured ionization rates in noble gases in the intermediate intensity regime (10/sup 14/ W/cm/sup 2/) (Siders et al, Phys. Rev. Lett.). We attempt to unify our studies in noble gases and molecules by performing experiments on N/sub 2/ and O/sub 2/, which have nearly identical ionization potentials to Ar and Xe, respectively. For the molecules studied here, we show that an ADK-like description of molecular ionization rates calculated from the model agree with the experimentally measured rates using the MI-FROG technique for H/sub 2/ and N/sub 2/. In the case of O/sub 2/, however, the experimentally measured ionization rate is approximately two orders of magnitude lower than that expected from the standard ADK formula. This is in agreement with the previously observed suppressed O/sub 2/ ionization rate in ion mass spectroscopy studies (Guo, 2000). We attribute the suppressed ionization rate in O/sub 2/ to a multielectron screening effect and show that a modified version of the ADK formula, taking into account the electron screening as proposed by Guo, well approximates the MI-FROG O/sub 2/ ionization rate data.

Multipulse interferometric frequency-resolved optical gating

We review multipulse interferometric frequency-resolved optical gating (MI-FROG) as a technique, uniquely suited for pump-probe coherent spectroscopy using amplified visible and near-infrared short-pulse systems and/or emissive targets, for time-resolving ultrafast phase shifts and intensity changes. Application of polarization-gate MI-FROG to the study of ultrafast ionization in gases is presented.

Multipulse interferometric frequency-resolved optical gating: real-time phase-sensitive imaging of ultrafast dynamics

We demonstrate a powerful new tool for real-time single-shot imaging of ultrafast phase shifts based on multipulse interferometric frequency-resolved optical gating that can directly measure and display ultrafast-time-scale phase shifts without computation. In addition, this technique can, with the application of interferogram analysis and iterative phase-retrievial techniques, recover the intensity and phase of three pulses in a single shot and exhibits a linear sensitivity to the pulse field in the wings.