Interferometric Autocorrelation Measurements Research Articles

Temporal Dynamics of Collective Resonances in Periodic Metasurfaces.

Temporal dynamics of confined optical fields can provide valuable insights into light-matter interactions in complex optical systems, going beyond their frequency-domain description. Here, we present a new experimental approach based on interferometric autocorrelation (IAC) that reveals the dynamics of optical near-fields enhanced by collective resonances in periodic metasurfaces. We focus on probing the resonances known as waveguide-plasmon polaritons, which are supported by plasmonic nanoparticle arrays coupled to a slab waveguide. To probe the resonant near-field enhancement, our IAC measurements make use of enhanced two-photon excited luminescence (TPEL) from semiconductor quantum dots deposited on the nanoparticle arrays. Thanks to the incoherent character of TPEL, the measurements are only sensitive to the fundamental optical fields and therefore can reveal clear signatures of their coherent temporal dynamics. In particular, we show that the excitation of a high-Q collective resonance gives rise to interference fringes at time delays as large as 500 fs, much greater than the incident pulse duration (150 fs). Based on these signatures, the basic characteristics of the resonances can be determined, including their Q factors, which are found to exceed 200. Furthermore, the measurements also reveal temporal beating between two different resonances, providing information on their frequencies and their relative contribution to the field enhancement. Finally, we present an approach to enhance the visibility of the resonances hidden in the IAC curves by converting them into spectrograms, which greatly facilitates the analysis and interpretation of the results. Our findings open up new perspectives on time-resolved studies of collective resonances in metasurfaces and other multiresonant systems.

Integration of liquid crystal optical delay and mechanical stage optical delay for measurement of ultrafast autocorrelations and terahertz pulses

To improve the temporal resolution in an optical delay system that uses a conventional mechanical delay stage, we integrate an in-line liquid crystal (LC) wave retarder. Previous implementations of LC optical delay methods are limited due to the small temporal window provided. Using a conventional mechanical delay stage system in series with the LC wave retarder, the temporal window is lengthened. Additionally, the limitation on temporal resolution resulting from the minimum optical path alteration (resolution of 400 nm) of the conventionally used mechanical delay stage is reduced via the in-line wave retarder (resolution of 50 nm). Interferometric autocorrelation measurements are conducted at multiple laser emission frequencies (349, 357, 375, 394, and 405 THz) using the in-line LC and conventional mechanical delay stage systems. The in-line LC system is compared to the conventional mechanical delay stage system to determine the improvements in temporal resolution relating to maximum resolvable frequency. This work demonstrates that the integration of the in-line LC system can extend the maximum resolvable frequency from 375 to 3000 THz. The in-line LC system is also applied for measurement of terahertz pulses.

Spectral phase reconstruction of femtosecond laser pulse from interferometric autocorrelation and evolutionary algorithm

We report on the complete temporal characterization of femtosecond laser pulses from second-order interferometric autocorrelation and laser spectrum measurements. The method exploits a newly developed autocorrelator based on a two photon-absorption signal produced directly within a camera sensor so as to provide a single-shot interferometric autocorrelation of great reliability and robustness. Interferometric autocorrelation trace and laser spectrum are exploited for a spectral phase retrieval via an evolutionary algorithm. The quality of the reconstruction for highly modulated spectral phases imprinted by a pulse shaper confirms the reliability of the method. The autocorrelator is compact, robust and easy to use. Possible improvements are discussed.

Controlling Quantum Cascade Laser Optical Frequency Combs through Microwave Injection

AbstractIn this work, control over the precise state emitted by quantum cascade laser frequency combs through strong radio‐frequency current modulation close to their repetition frequency is demonstrated. In particular, broadening of the spectrum from about 20 cm−1 to an average bandwidth of 60 cm−1 can be achieved throughout most of the current dynamical range while preserving the coherence, as measured by shifted wave interference Fourier transform spectroscopy (SWIFTS). The required modulation frequency to achieve this broadening is red‐shifted compared to the free‐running beatnote frequency at increasing modulation powers starting from 25 dBm, whereas the range where it occurs narrows. Outside of this maximum‐bandwidth range, the spectral bandwidth of the laser output is gradually reduced and the new center frequency is red‐ or blue‐shifted, directly dependent on the detuning of the modulation frequency. By switching between two modulation frequencies detuned symmetrically with respect to the free‐running beatnote, two multiplexed spectral regions with negligible overlap from the same device at rates of at least 20 kHz can be generated. In the time‐domain, with both SWIFTS and interferometric autocorrelation (IAC) measurements a transition from quasi‐continuous output to long‐pulsed ( ps) output is shown by ramping up the injection power to 35 dBm.

Space- and time-resolved second harmonic spectroscopy of coupled plasmonic nanocavities

Abstract Localized surface plasmon resonances of individual sub-wavelength cavities milled in metallic films can couple to each other to form a collective behavior. This coupling leads to a delocalization of the plasmon field at the film surface and drastically alters both the linear and nonlinear optical properties of the sample. In periodic arrays of nanocavities, the coupling results in the formation of propagating surface plasmon polaritons (SPP), eigenmodes extending across the array. When artificially introducing dislocations, defects and imperfections, multiple scattering of these SPP modes can lead to hot-spot formation, intense and spatially confined fluctuations of the local plasmonic field within the array. Here, we study the underlying coupling effects by probing plasmonic modes in well-defined individual triangular dimer cavities and in arrays of triangular cavities with and without artificial defects. Nonlinear confocal spectro-microscopy is employed to map the second harmonic (SH) radiation from these systems. Pronounced spatial localization of the SPP field and significant enhancements of the SH intensity in certain, randomly distributed hot spots by more than an order of magnitude are observed from the triangular arrays as compared to a bare silver film by introducing a finite degree of disorder into the array structure. Hot-spot formation and the resulting enhancement of the nonlinear efficiency are correlated with an increase in the lifetime of the localized SPP modes. By using interferometric SH autocorrelation measurements, we reveal lifetimes of hot-spot resonances in disordered arrays that are much longer than the few-femtosecond lifetimes of the localized surface plasmon resonances of individual nanocavity dimers. This suggests that hot spot lifetime engineering provides a path for manipulating the linear and nonlinear optical properties of nanosystems by jointly exploiting coherent couplings and tailored disorder.

Measuring two-photon microscopy ultrafast laser pulse duration at the sample plane using time-correlated single-photon counting

.Two-photon microscopy (2PM) has revolutionized biomedical imaging by allowing thin optical sectioning in relatively thick biological specimens. Because dispersive microscope components in 2PM, such as objective lens, can alter temporal laser pulse width (typically being broader at the sample plane), for accurate measurements of two-photon absorption properties, it is important to characterize pulse duration at the sample plane. We present a simple modification to a two-photon microscope light path that allows for second-harmonic-generation-based interferometric autocorrelation measurements to characterize ultrafast laser pulse duration at the sample plane using time-correlated single-photon counting (TCSPC). We show that TCSPC can be used as a simple and versatile method to estimate the zero time delay step value between two adjacent ultrafast laser pulses for these measurements. To demonstrate the utility of this modification, we measured the Coherent Chameleon-Ultra II Ti:sapphire laser pulse width at the sample plane using a air, air, or water-immersion objective lens. At 950-nm two-photon excitation, the measured pulse width was , , and (, ), respectively.

Sub-femtosecond electron transport in a nanoscale gap

The strong fields associated with few-cycle pulses can drive highly nonlinear phenomena, allowing the direct control of electrons in condensed matter systems. In this context, by employing near-infrared single-cycle pulse pairs, we measure interferometric autocorrelations of the ultrafast currents induced by optical field emission at the nanogap of a single plasmonic nanocircuit. The dynamics of this ultrafast electron nanotransport depends on the precise temporal field profile of the optical driving pulse. Current autocorrelations are acquired with sub-femtosecond temporal resolution as a function of both pulse delay and absolute carrier-envelope phase. Quantitative modelling of the experiments enables us to monitor the spatiotemporal evolution of the electron density and currents induced in the system and to elucidate the physics underlying the electron transfer driven by strong optical fields in plasmonic gaps. Specifically, we clarify the interplay between the carrier-envelope phase of the driving pulse, plasmonic resonance and quiver motion. Single-cycle interferometric autocorrelation measurements of electrons tunnelling across the gap of a plasmonic bowtie antenna and quantitative models provide insight into the physical interactions that drive the electron transfer.

Interferometric autocorrelation measurements of supercontinuum based on two-photon absorption

We report on interferometric autocorrelation measurements of broadband supercontinuum light in the anomalous dispersion regime using two-photon absorption in a GaP photodetector. The method is simple, low-cost, and provides a direct measure of the second-order coherence properties, including quantitative information on the coherence time and average duration of the supercontinuum pulses as well as on the presence of temporally coherent sub-structures. We report measurements in regimes where the supercontinuum is coherent and incoherent. In the former case, the interferometric measurements are similar to what is observed for mode-locked laser pulses while in the latter case the interferometric measurements and coherence properties are shown to have characteristics similar to that of a stationary chaotic light source.

Adiabatic Nanofocusing on Ultrasmooth Single-Crystalline Gold Tapers Creates a 10-nm-Sized Light Source with Few-Cycle Time Resolution

We demonstrate adiabatic nanofocusing of few-cycle light pulses using ultrasharp and ultrasmooth single-crystalline gold tapers. We show that the grating-induced launching of spectrally broad-band surface plasmon polariton wavepackets onto the shaft of such a taper generates isolated, point-like light spots with 10 fs duration and 10 nm diameter spatial extent at its very apex. This nanofocusing is so efficient that nanolocalized electric fields inducing strong optical nonlinearities at the tip end are reached with conventional high repetition rate laser oscillators. We use here the resulting second harmonic to fully characterize the time structure of the localized electric field in frequency-resolved interferometric autocorrelation measurements. Our results strongly suggest that these nanometer-sized ultrafast light spots will enable new experiments probing the dynamics of optical excitations of individual metallic, semiconducting, and magnetic nanostructures.

Nonlinear Fourier-transform spectroscopy ofD2using high-order harmonic radiation

High-order harmonic radiation is available to investigate ultrafast nonlinear optical phenomena, such as two-photon absorption process, in the vacuum and extreme ultraviolet wavelength region. We have determined the sequential two-photon dissociative ionization pathways of D2 with the simultaneous irradiation of multiple order harmonics of a Ti:sapphire laser in the visible-vacuum ultraviolet region with the aid of both an interferometric autocorrelation measurement and a Fourier-transform spectroscopy. By analyzing the interferometric autocorrelation signals appearing in the measured momentum distribution of D + , we were able to decompose the measured momentum distribution into three momentum distribution images representing the three distinct dissociation pathways, which are sequentially two-photon excitation processes of D2. \({\mathrm{D}}_{2}^{+}\) is prepared in the electronic ground state of \({\mathrm{D}}_{2}^{+}\), and then excited to the first excited state, leading to the dissociation into D + and D.

Spectral phase retrieval from interferometric autocorrelation by a combination of graduated optimization and genetic algorithms

We describe a method for retrieving spectral phase information from second harmonic interferometric autocorrelation measurements supplemented by the use of the observed spectral intensity. By applying a combination of graduated optimization and genetic algorithms, accurate phase retrieval of laser pulses as short as a few optical cycles was obtained from the measured autocorrelation and spectral intensity. The effectiveness of the combined algorithms is demonstrated on a set of significantly different femtosecond pulse shapes.

Development of a nonlinear nanoprobe for interferometric autocorrelation based characterization of ultrashort optical pulses

Near-field scanning can achieve nanoscale resolution while ultrashort pulse diagnostic tools can characterize femtosecond pulses. Yet currently it is still challenging to nonperturbatively characterize the near field of an ultrashort optical pulse with nanofemtoscale spatiotemporal resolution. To address this challenge, we propose to develop a nonlinear nanoprobe composed of a silica fiber taper, a nanowire, and nonlinear fluorescent spheres. Using such a nanoprobe, we also report proof-of-principle characterization of femtosecond optical pulse through interferometric autocorrelation measurement.

Sub-two-cycle pulses by soliton self-compression in highly nonlinear photonic crystal fibers

We demonstrate the direct generation of sub-two-cycle pulses by soliton self-compression of femtosecond pulses from a Ti:sapphire laser at 85 MHz using a 4.85-mm-long highly nonlinear photonic crystal fiber. Sub-nanojoule, 41 fs input pulses were compressed down to 4.6 fs without additional phase compensation schemes. To our knowledge, these are the shortest pulses obtained by soliton-effect compression of a laser oscillator. Efficient, near-dispersionless collimation of the fiber output was achieved with a simple lens and an octave-spanning double-chirped mirror pair. The full electric field of the compressed pulses was retrieved with a genetic algorithm applied to spectral and interferometric autocorrelation measurements, and the results are well described by numerical simulations.

Two-photon autocorrelation measurements on a single InGaN/GaN quantumdot

We report on the use of interferometric autocorrelation measurements to investigate thenon-linear absorption processes evident in single InGaN/GaN quantum dots. The nearquadratic excitation intensity dependence of the photoluminescence signal inconjunction with the asymmetric collinear autocorrelation trace unambiguouslyconfirms the process as being one involving two photons via an intermediate virtualstate. These results highlight the inherently non-linear optical properties of thesestructures.

Two-photon imaging using adaptive phase compensated ultrashort laser pulses

An adaptive pulse shaper controlled by multiphoton intrapulse interference phase scanning (MIIPS) was used, together with a prism-pair, to measure and cancel high-order phase distortions introduced by a high-numerical-aperture objective and other dispersive elements of a two-photon laser-scanning microscope. The delivery of broad-bandwidth (approximately 100 nm), sub-12-fs pulses was confirmed by interferometric autocorrelation measurements at the focal plane. A comparison of two-photon imaging with transform-limited and second-order-dispersion compensated laser pulses of the same energy showed a 6-to-11-fold improvement in the two-photon excitation fluorescence signal when applied to cells and tissue, and up to a 19-fold improvement in the second harmonic generation signal from a rat tendon specimen.

Few-cycle laser pulses tested with double ionization of H2

Different compression schemes for the production of sub-10 fs laser pulses are tested using double ionization of H2. Compression schemes differ in the laser beam guiding modes which are necessary in order to get a spatially-homogeneous spectral broadening through self-phase modulation in argon. Guiding may take place in a hollow fibre waveguide or may be self-induced in a filament. We tested four set-ups: a single hollow fibre, two hollow fibres, a hollow fibre then a filament, and two filaments. The shortest pulses around 8 fs were recorded with the set-ups involving one or two filaments. Double ionization of H2 and the associated proton spectra give results in good agreement with the spectral and interferometric autocorrelation measurements. The two-filament set-up gives the shortest pulses. In addition, the H2 experiment reveals that the spectral phase induced with two filaments cannot be fully compensated with second-order chirped mirrors as for the other tested set-ups.

Autocorrelation measurements of free-electron laser radiation using a two-photon QWIP

The two-photon QWIP comprises three equidistant subbands, namely two bound states localized in the quantum well and an extended state in the continuum. This device is very promising for quadratic autocorrelation measurements of pulsed mid-infrared lasers due to its resonantly enhanced optical nonlinearity and sub-ps time resolution. We report on interferometric autocorrelation measurements of ps optical pulses from a free-electron laser (FEL). The intense FEL radiation further allows us to study the saturation properties of two-photon QWIPs at liquid nitrogen temperature and their detection properties at 300K. The device is well suited for standard diagnostics of the FEL pulse shape via interferometric autocorrelation.

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.

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.

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.