Pulse Shaping Apparatus Research Articles

Spatio-temporal ultrafast pulse shaping at the femtosecond-nanometer scale.

Optical pulse shaping is a fundamental tool for coherent control of the light-matter interaction. While such control enables the measurement of ultrafast temporal dynamics, simultaneous spatiotemporal control is required for studying non-local ultrafast charge dynamics at the nanoscale. However, obtaining accurate spatial control at a sub-wavelength resolution with conventional optical elements poses significant difficulty. Here, we use the spatiotemporal coupling naturally arising in a spatial light modulator based pulse shaping apparatus to achieve accurate control with femto-nano spatiotemporal resolution. We experimentally demonstrate spatial steering at the sub-micron scale of second harmonic generation from nanostructures. In addition, we apply an absolute-value spectral phase to achieve controlled double pulses for nanoscale excitation. We introduce a novel, to the best of our knowledge, scheme for accurate tunable spatiotemporal pump-probe experiments. This method offers rich insight into materials with ultrafast transport phenomena at the femtosecond-nanometer regimes.

Spectral envelope control for a flat frequency comb spectrum

Laser frequency combs have properties that make them promising spectrograph calibration light sources. One drawback for this application is the high dynamic range in the supercontinuum spectra of some frequency combs. We aim to flatten the spectrum of a Ti:sapphire laser frequency comb to improve the calibration performance for a Fourier transform spectrograph. For this, we develop a compact Fourier transform optical pulse shaping setup, which enables control of the spectral envelope via dispersion of the light onto a spatial light modulator (SLM). We demonstrate that this setup allows us to flatten the comb spectrum in the wavelength range 780 to 980 nm. For 90% of the wavelength range, the dynamic range is below 3 dB. In the flattened spectra 3% more comb lines can be detected. The line center fit precision, however, is reduced due to noise caused by the SLM.

Polarization envelope helicity dependent photovoltage in GaAs/Al0.3Ga0.7As modulation-doped quantum well.

In this study, we demonstrate the switching of the direction of the photocurrent in an n-type GaAs/Al0.3Ga0.7As modulation-doped quantum well using a polarization pulse-shaping apparatus containing a 4f setup. The right- and left-polarization-twisting pulses with a polarization rotation frequency in the THz-regime are incident on a modulation-doped quantum well. The results show that the sign of the photovoltage is dependent on the direction of rotation of the polarization-twisting pulses, which can be explained by the circular photogalvanic effect combined with the production of a classical edge photocurrent from the acceleration of free electrons in the vicinity of the sample edge by the incident optical electric field. The wide range over which the polarization-rotation frequency may be tuned makes this method a powerful tool to investigate the response of an extensive variety of materials in the THz-regime.

Modifications of filament spectra by shaped octave-spanning laser pulses

In this paper we examine the spectral changes in a white light laser filament due to different pulse shapes generated by a pulse shaping setup. We particularly explore how the properties of the filament spectra can be controlled by parametrically tailored white light pulses. The experiments are carried out in a gas cell with up to 9 bar of argon. Plasma generation and self-phase modulation strongly affect the pulse in the spectral and temporal domain. By exploiting these effects we show that the pulse spectrum can be modified in a desired way by either using second order parametric chirp functions to shift the filament spectrum to higher or lower wavelengths, or by optimizing pulse shapes with a genetic algorithm to generate more complex filament spectra. This paper is one of the first examples of the application of complex, parametrically shaped white light pulses.

White-Light Optimal Control of Photoinduced Processes

The photofragmentation of Cu3 and Al4 clusters during charge reversal with white-light pulses is optimized by using an evolutionary algorithm and a pulse shaping setup. By controlling the spectral phase over the full spectral range of the supercontinuum, tailored pulses are generated with the aim of adapting the energetic and temporal structure of the pulse to the characteristic dynamics and electronic manifold of the investigated systems. In this context, maximizing the ratio of the cationic yield of a selected fragment (Cu2+ and Al+) and the parent ion is chosen as an optimization target. Significant enhancement factors are achieved, demonstrating a high selectivity in populating specific points on the potential energy surface, which facilitates the targeted photofragmentation of the investigated systems. The optimal pulse shapes indicate that both vibronic as well as electronic wave packets are probed. Additional laser-induced dissociation experiments suggest that fragmentation of the Cu3 clusters occu...

Influence of nonlinear effects on the three-photon excitation of L-Tryptophan in water using phase-shaped pulses

We show that nonlinear effects like self-phase modulation in a sample have to be considered for phase control of three-photon excitations. Furthermore, we demonstrate the control of three-photon excitation of L-Tryptophan in water using a pulse-shaping setup. Simulations of the propagation of the laser pulses in the cuvette exhibit good agreement with the experimental fluorescence scans at different laser intensities and show large discrepancies when neglecting nonlinear effects prior to the three-photon process. This can lead to improvements in selective excitation of amino acids by a near-infrared femtosecond laser source.

Compression of ultrashort laser pulses via gated multiphoton intrapulse interference phase scans

Delivering femtosecond laser light in the focal plane of a high numerical aperture microscope objective is still a challenge, despite significant developments in the generation of ultrashort pulses. One of the most popular techniques, used to correct phase distortions resulting from propagation through transparent media, is the multiphoton intrapulse interference phase scan (MIIPS). The accuracy of MIIPS however is limited when higher order phase distortions are present. Here we introduce an improvement, called Gated-MIIPS, which avoids shortcomings of MIIPS, reduces the influence of higher order phase terms, and can be used to more efficiently compress broad band laser pulses even with a simple 4f pulse shaper setup. In this work we present analytical formulas for MIIPS and Gated-MIIPS valid for chirped Gaussian pulses; we show an approximated analytic expression for Gated-MIIPS valid for arbitrary pulse shapes; finally we demonstrate the increased accuracy of Gated-MIIPS via experiment and numerical simulation.

Femtosecond pulse shaping using the geometric phase

We demonstrate a femtosecond pulse shaper that utilizes polarization gratings to manipulate the geometric phase of an optical pulse. This unique approach enables circular polarization-dependent shaping of femtosecond pulses. As a result, it is possible to create coherent pulse pairs with orthogonal polarizations in a 4f pulse shaper setup, something until now that, to our knowledge, was only achieved via much more complex configurations. This approach could be used to greatly simplify and enhance the functionality of multidimensional spectroscopy and coherent control experiments, in which multiple coherent pulses are used to manipulate quantum states in materials of interest.

2D IR Spectroscopy using Four-Wave Mixing, Pulse Shaping, and IR Upconversion: A Quantitative Comparison

Recent technological advances have led to major changes in the apparatuses used to collect 2D IR spectra. Pulse shaping offers several advantages including rapid data collection, inherent phase stability, and phase-cycling capabilities. Visible array detection via upconversion allows the use of visible detectors that are cheaper, faster, more sensitive, and less noisy than IR detectors. However, despite these advantages, many researchers are reluctant to implement these technologies. Here we present a quantitative study of the S/N of 2D IR spectra collected with a traditional four-wave mixing (FWM) apparatus, with a pulse shaping apparatus, and with visible detection via upconversion to address the question of whether weak chromophores at low concentrations are still accessible with such an apparatus. We find that the enhanced averaging capability of the pulse shaping apparatus enables the detection of small signals that would be challenging to measure even with the traditional FWM apparatus, and we demonstrate this ability on a sample of cyanylated dihydrofolate reductase.

Two-Color Two-Dimensional Electronic Spectroscopy Using Dual Acousto-Optic Pulse Shapers for Complete Amplitude, Phase, and Polarization Control of Femtosecond Laser Pulses

We demonstrate a dual pulse-shaper setup capable of independent polarization, phase, and amplitude control over each pulse. By using active phase stabilization, we achieve a phase stability of ~λ/314 between the two pulse shapers, making the dual-shaper setup suitable for both two-quantum and one-quantum measurements. The setup is compact and easily switchable between pump-probe and collinear geometries. We further illustrate the functionality of the dual-shaper setup by performing two-color 2D visible spectroscopy on colloidal CdSe quantum dots in pump-probe geometry.

Influence of finite spatial resolution on single- and double-pass femtosecond pulse shapers

We investigate the influence of the finite spatial resolution of a typical 4f pulse-shaping setup in a single- and double-pass configuration on the shaped waveforms. Specifically, we calculate and measure the space-frequency distribution at the focal plane of a lens following the shaping setup and show that steep amplitude or phase jumps, as they appear with pixelated spatial light modulators, are affected by a finite spatial resolution.

A fiber Mach–Zehnder interferometer for the unique phase retrieval of ultrafast pulses with a 1 THz gap

Unique phase retrieval of ultrashort pulses with well-separated frequency components was demonstrated using a cross-correlation technique with a conditioning filter. Here, this filter was designed as an unbalanced fiber Mach–Zehnder interferometer. A pulse-shaping apparatus was used to generate pulses with a 1 THz gap. A fast phase-retrieval algorithm was then employed to reconstruct these pulses. Ambiguities of relative-phase or other characteristics were not found under any circumstances.

Full control over the electric field using four liquid crystal arrays

We present a femtosecond pulse shaper setup that is capable of shaping the phase, amplitude, and polarization simultaneously and independently. The modulator utilizes four liquid crystal arrays, a pair of half-wave plates, and a polarizer to gain full control of the electrical field. This is done in a common-path common-optic scheme without using interferometry. The functionality of the setup is demonstrated by using systematic parameter scans and example pulses.

Space-time coupling in femtosecond pulse shaping and its effects on coherent control

We present a Fourier optical analysis of a typical femtosecond pulse shaping apparatus and derive analytic expressions for the space-time dependence of the emerging waveform after the pulse shaper and in the focal volume of an additional focusing element. For both geometries the results are verified experimentally. Hereafter, we analyze the influence of space-time coupling on nonlinear processes, specifically second harmonic generation, resonant interaction with an atomic three-level system, and resonant excitation of a diatomic molecule.

Femtosecond pulse shaping in two dimensions: Towards higher complexity optical waveforms

We demonstrate a new Fourier pulse shaping apparatus capable of achieving simultaneous high resolution and broad bandwidth operation by dispersing frequency components in a two dimensional geometry through simultaneous use of a high resolution and a broad bandwidth spectral disperser. We show experimental results which demonstrate significant improvements in achievable waveform complexity (number of controllable temporal/spectral features). We also demonstrate experiments of line-by-line pulse shaping with optical frequency combs. In this regime our configuration would allow significant enhancement of the number of controllable spectral lines which may further enhance recently demonstrated massively parallel approaches to spectroscopic sensing using frequency combs.

Interferometric generation of parametrically shaped polarization pulses

We demonstrate the capabilities of the recently introduced interferometric parallel pulse shaper setup and present a method for fully tailoring the three-dimensional electrical field of femtosecond laser pulses. The possibility of producing parametric polarization pulses with arbitrary orientations and ellipticities in time is demonstrated with a selection of example pulses.

Shaping a femtosecond pulse with a programmable thermo-optically driven phase modulator

A femtosecond pulse shaping apparatus based on a thermo-optically driven spatial phase modulator is presented. The modulator cell is illuminated by a standard projector and is easily controllable with a computer. In principle the setup allows for two dimensional pulse shaping, however, all initial demonstrations reported here, such as feedback controlled dispersion compensation or pulse train generation, are performed in a one dimensional phase-only shaping geometry.

Selective spectral filtering of molecular modes of β‐carotene in solution using optimal control in four‐wave‐mixing spectroscopy

AbstractFemtosecond laser pulses are spectrally broad and therefore coherently excite several molecular modes. While the temporal resolution is high, usually no mode‐selective excitation is possible. In this work, we demonstrate that, by phase shaping one of the two pulses, which excite the vibrational modes of β‐carotene within a coherent anti‐Stokes Raman scattering (CARS) process, specific modes can be enhanced or suppressed. Using the pulse shaper setup in a feedback‐controlled closed‐loop optimizes this mode selection. Here, the ratio of signal intensities observed in the CARS spectrum serves as a feedback function for an evolutionary algorithm responsible for the optimization. The optimized phase structure of the femtosecond pulse is characterized using the frequency resolved optical gating (FROG) technique to get an insight into the optimization process. Furthermore, it is shown that the temporal resolution after optimization is still high enough to investigate the ultrafast molecular dynamics. The suppression or relative enhancement of vibrational modes persists over the entire coherence time. Copyright © 2006 John Wiley & Sons, Ltd.

Control of Li2 wave packet dynamics by modification of the quantum mechanical amplitude of a single state

Sequences of pulses with different spectra are used to control rotational wave packet dynamics in Li(2) by exploiting quantum interference phenomena. Wave packet superpositions are excited in a two-step resonant Raman process by two different pulses. Interferences between individual states shared by both wave packets can be used to enhance or destroy specific components of a superposition by varying the time delay between the pulses and/or the relative phase within the pulses. Elimination of selected quantum beats is achieved by greater than 94% for each case. A simple, yet effective, method for generating different color phase-locked pairs of laser pulses in a liquid-crystal pulse shaper setup without the need for interferometric stabilization schemes is described. The ability to manipulate single states of a superposition is an important advancement for intuitive control schemes and provides a potential new approach for initialization schemes in the field of quantum information.

Application of adaptive feedback loop for ultra-violet femtosecond pulse shaper control

We apply an adaptive feedback loop to control a ultra-violet (UV) femtosecond pulse shaping apparatus. The adaptive feedback control is implemented by a continuous parameter genetic algorithm. We use the adaptive shaper to compensate for the pulse chirp. The genetic algorithm produces a pulse with a width of 115 fs, identical to that of the transformlimited pulse. We then apply the adaptive shaper to the Stokes pulse in a femtosecond coherent anti-Stokes Raman scattering (CARS) experiment on dipicolinic acid solution. The algorithm maximizes the first CARS beat signal at the probe pulse delay of 650 fs. We confirm that a transformlimited Stokes pulse achieves the best detection sensitivity.