Interferometric Autocorrelation Research Articles

Observation of both second-harmonic and multiphoton-absorption-induced luminescence in ZnO

The confusion over second-harmonic generation (SHG) and multiphoton-absorption (MPA)-induced luminescence in ZnO is observed under excitation with femtosecond laser between 800 and 1000 nm at room temperature. Excitation power dependence measurements show SHG is a two-photon process, and MPA is a three- or four-photon process. These are confirmed with interferometric pulse autocorrelation experiments. Excitation power density is a main regulatory factor for this confusion.

Single-pulse coherent anti-Stokes Raman scattering microscopy employing an octave spanning pulse

We demonstrate two complementary types of microscopy using an identical setup for single-pulse coherent anti-Stokes Raman scattering (CARS) imaging, which employs an ultrabroadband laser pulse with a spectral bandwidth of 4800 cm(-1) and enables the suppression of nonresonant CARS signals. One is a novel type of microscopy that uses spectral phase modulation for the selective excitation of a single Raman mode. The selective excitation is achieved by the modulated pulse focusing its difference-frequency spectrum into a narrow spectral region. Another type is Fourier-transform CARS (FT-CARS) microspectroscopy based on the measurement of the CARS spectrum obtained from the Fourier-transform of the interferometric autocorrelation (IAC) signal. Vibrational spectral imaging of chemical and biological samples is demonstrated using the two types of microscopy.

Ex vivo characterization of sub-10-fs pulses

An all-mirror dispersion-compensation setup is used to correct for quadratic and cubic phase distortions induced within a custom nonlinear optical microscope. Mouse tail tendon is used to characterize sub-10-fs pulses by interferometric autocorrelation. This is an ideal method for characterizing dispersion from the optical system, immersion medium, and wet biological sample. The generation of very short autocorrelations demonstrates the ability to compensate for phase distortions within the imaging system and efficient second-harmonic upconversion of the ultrashort pulse spectrum within collagen. Compensated autocorrelation traces are presented for biologically relevant objective lenses.

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.

Generation and complete characterization of intense mid-infrared ultrashort pulses

Intense mid-infrared pulses tunable between 5 and 14 µm with pulse energies of several microjoules were generated by difference-frequency mixing (DFM) in a GaSe crystal. Longer wavelengths (up to 18 µm) were achieved by DFM in a CdSe crystal. The infrared pulses were then characterized using various techniques: The spectrum was measured using a Fourier-transform spectrometer, which was then modified to determine the interferometric second-order autocorrelation. The electric field spectral phase was measured using the same setup, thus leading to a full characterization of the mid-infrared pulses. The spectral phase was measured using the time-domain homodyne optical technique for spectral phase interferometry for direct electric field reconstruction, where spectral interferometry was replaced with time-domain interferometry. The measured pulse duration was 100 fs, nearly transform limited.

Ultrafast dynamics and carrier-envelope phase sensitivity of multiphoton photoemission from metal surfaces

Subcycle dynamics of multiphoton-induced photoelectron emission from metal surfaces is analysed using a simple phenomenological model to assess optimum conditions for direct carrier-envelope phase measurement. To gain further insight femtosecond time-resolved measurements were carried out on a polycrystalline gold surface with ultrashort laser pulses to explain the recently found, unexpectedly low carrier-envelope phase dependence of the photoemission process in this particular case. In the higher-order interferometric autocorrelation distribution additional short side wings appeared suggesting that ultrafast dynamics of hot electrons reduce the carrier-envelope phase dependence of the photoemission electron yield produced by few-cycle laser pulses. Other metals can be investigated with this simple and fast method to pave the way towards the construction of a solid-state-based, direct carrier-envelope phase detector.

Carrier-envelope phase control by a composite plate

We demonstrate a new concept to vary the carrier-envelope phase of a mode-locked laser by a composite plate while keeping all other pulse parameters practically unaltered. The effect is verified externally in an interferometric autocorrelator, as well as inside the cavity of an octave-spanning femtosecond oscillator. The carrier-envelope frequency can be shifted by half the repetition rate with negligible impact on pulse spectrum and energy.

Pulse retrieval from interferometric autocorrelation measurement by use of the population-split genetic algorithm

The population-split genetic algorithm (PSGA) was successfully applied to retrieve femtosecond optical fields from interferometric autocorrelation traces. PSGA strikes a balance between diversity and the size of population in the genetic algorithm. As a result, PSGA is less likely prematurely converging to sub-optimal solutions. Theoretical and experimental studies indicate that the PSGA can yield more accurate results in shorter time compared with conventional genetic algorithm and the iterative method. compared with conventional genetic algorithm and iterative method.

Unambiguous chirp characterization using modified-spectrum auto-interferometric correlation and pulse spectrum

Modified-spectrum auto-interferometric correlation (MOSAIC), derived from a conventional second order interferometric autocorrelation trace, is a sensitive and visual chirp diagnostic method for ultrashort laser pulses. We construct several pairs of example pulse shapes that have nearly identical MOSAIC traces and demonstrate that chirp ambiguity can result when the field amplitude or spectrum are not known, thus making MOSAIC a qualitative tool for chirped pulses. However, when the pulse spectrum is known, a unique chirp reconstruction is possible. With the help of a new reconstruction technique, we experimentally demonstrate complete pulse characterization using MOSAIC envelopes and the pulse spectrum.

Development of Near-Infrared 35 fs Laser Microscope and Its Application to the Detection of Three- and Four-Photon Fluorescence of Organic Microcrystals

Femtosecond near-infrared laser microscope was developed with a home-built cavity-dumped chromium:forsterite laser as a light source centered at 1.26 microm. Optimization of the pulse duration achieved 35 fs fwhm at the sample position of the microscope after passing through a 100x objective. This system was applied to the detection of multiphoton fluorescence of some organic microcrystals. Excitation intensity dependence and the interferometric autocorrelation detection of the fluorescence clearly demonstrated that simultaneous three- and four-photon absorption processes are responsible for the production of the excited state for perylene and anthracene microcrystals, respectively. The spatial resolution along the optical axis and its dependence on the order of the multiphoton process were also discussed.

Efficient multiphoton-absorption-induced luminescence in single-crystalline ZnO at room temperature

At room temperature, multiphoton absorption- (MPA-) induced photoluminescence in ZnO strongly driven by a femtosecond (fs) near-infrared laser is studied. Two-photon absorption and three-photon absorption are proved to be responsible for the intense luminescence, when the wavelength of the fs excitation laser is above and below the half-bandgap of ZnO, respectively. Strong MPA absorption in ZnO is unambiguously evidenced by the interferometric autocorrelation measurements of the luminescence signal.

The effect of errors and detector noise on sensitive detection of chirp and pulse asymmetry of ultrashort laser pulses from interferometric autocorrelation signals

We have studied the effect of various types of systematic and non-systematic errors on unbalanced spectrally modified interferometric autocorrelation signals for detection of pulse chirp and asymmetry of ultrashort laser pulses. The effect of systematic errors arising due to limited number of data points per fringe, scan rate etc, and non-systematic errors due to phase noise, additive noise, multiplicative noise and quantization of interferometric autocorrelation (IAC) signals is illustrated using a linearly chirped asymmetric laser pulse. It is seen that the spectrally modified IAC signals based on difference of normalized envelope functions corresponding to different frequencies are not much sensitive to various noises, permitting their use for sensitive detection of pulse chirp and asymmetry. The analysis is supported by experimentally recorded IAC signals under different conditions.

Measurement of electric field by interferometric spectral trace observation

We present a new methodology that obtains, in an analytical way, the complex electric field of ultrashort pulses. This methodology is based only on Fourier analysis of the frequency components of spectrally resolved interferometric collinear autocorrelations. We present an experimental demonstration of this technique and the results are compared with the conventional second-harmonic generation frequency-resolved optical gating technique.

Two-photon QWIPs for quadratic detection of weak mid-infrared laser pulses

We report on QWIPs optimized for nonlinear detection using a bandstructure design with three energetically equidistant subbands. The double resonant character of the two-photon transition from the ground state into the second excited state results in quadratic dependence of the photocurrent on the incident power down to excitation densities as low as 0.1 W/cm 2. We have used these two-photon QWIPs as quadratic detectors in second-order interferometric autocorrelation measurements allowing for the separation of coherent and incoherent contributions to the photocurrent. We have studied the carrier dynamics and the influence of the QW doping on both phase and energy relaxation times. The measurements also demonstrate that these devices provide an ultra-sensitive tool with sub-ps response time for the characterization of ultrashort (∼ps) mid-infrared pulses in the pJ-regime such as those generated by modelocked quantum cascade lasers.

Second- and third-order interferometric autocorrelations based on harmonic generations from metal surfaces

A metal-based second-order autocorrelation technique has previously been shown to be advantageous compared to conventional nonlinear crystal-based technique. In this paper, we have fully characterized the dependence of the metal-based autocorrelation signal on incident laser beam angle and light polarization. The results provide us a more complete knowledge to optimize this autocorrelation technique. Furthermore, we have extended this technique to both second-and third-order interferometric autocorrelation measurements that are more suitable for sub-20-fs ultrashort pulse measurements. These results demonstrate the potential of this autocorrelation technique for temporally characterizing ultrafast light sources with a super-broad spectrum, such as those produced from photonic-crystal fibers.

Calibration of interferometric autocorrelation trace for femtosecond pulse in frequency domain and the real-time chirp diagnostic for Ti:sapphire oscillator

The time delay of interferometric autocorrelation trace of femtosecond pulse was calibrated in frequency domain. By virtue of the calibrated interferometric aut ocorrelation trace, root-mean-square width and chirp parameter of the femtosecon d pulse to be measured were calculated. Based on these parameters, the Ti:sapphi re oscillator was optimized to generate 115fs pulse in full width at half ma ximum, which was very close to the width of the transform-limited pulse. The mea surement involved only an integration and a fast Fourier transform. Consequently , real-time chirp diagnostics for Ti:sapphire oscillator was feasible.

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.

Resonant interferometric autocorrelator for estimation of laser pulse duration

We present a simple design for a Mach–Zehnder interferometric autocorrelator that is aimed at real-time laser pulse diagnostics. The device is based around a symmetric and balanced mechanical oscillator that confers a simple and compact design, while still allowing the measurement of a broad range of pulse durations with high speed and high resolution. The device requires a very low drive level and possesses a very weak mechanical coupling to the environment. The apparatus provides two outputs: one output is proportional to the instantaneous velocity of the interferometer arms, which can be integrated to determine the arm position. This gives the means to calibrate the arm position accurately and thereby account for any nonuniformity of the motion. The second output, from a two-photon photodetector, is used to determine the pulse duration. The present device can measure pulses from the femtosecond through to the picosecond domain.

Use of commercial grade light emitting diode in auto-correlation measurements of femtosecond and picosecond laser pulses at 1054 nm

Commercial grade AlGaAs based light emitting diodes (LEDs) have been characterized at 1054 nm for their non-linear photoconductivity and effect of aging and used as a two-photon detector to record auto-correlation signals of ultra-short laser pulses. The lifetime of these LEDs is found to be ∼6×1011 and ∼2×1011 laser pulses at an average incident power level of 20 and 60 mW of 200 fs laser pulses, respectively. These LEDs have been used for real time recording of interferometric auto-correlation (IAC) to determine pulse duration and sensitive detection of the frequency chirp of 200 fs laser pulses. A twofold enhancement in sensitivity of new modified signal, derived from the envelope functions of IAC signal, has been achieved. The suitability of these LEDs to record intensity auto-correlation in multi-picosecond regime is also demonstrated using 30ps laser pulses from an Nd:YLF oscillator.