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Stem cells have received much attention recently for their potential utility in regenerative medicine. The identification of their differentiated progeny often requires complex staining procedures, and is challenging for intermediary stages which are a priori unknown. In this work, the ability of label-free quantitative coherent anti-Stokes Raman scattering (CARS) micro-spectroscopy to identify populations of intermediate cell states during the differentiation of murine embryonic stem cells into adipocytes is assessed. Cells were imaged at different days of differentiation by hyperspectral CARS, and images were analysed with an unsupervised factorization algorithm providing Raman-like spectra and spatially resolved maps of chemical components. Chemical decomposition combined with a statistical analysis of their spatial distributions provided a set of parameters that were used for classification analysis. The first 2 principal components of these parameters indicated 3 main groups, attributed to undifferentiated cells, cells differentiated into committed white pre-adipocytes, and differentiating cells exhibiting a distinct protein globular structure with adjacent lipid droplets. An unsupervised classification methodology was developed, separating undifferentiated cell from cells in other stages, using a novel method to estimate the optimal number of clusters. The proposed unsupervised classification pipeline of hyperspectral CARS data offers a promising new tool for automated cell sorting in lineage analysis.

We have investigated the ability of dual-frequency Coherent Antistokes Raman Scattering (D-CARS) micro-spectroscopy, based on femtosecond pulses (100 fs or 5 fs) spectrally focussed by glass dispersion, to distinguish the chemical composition of micron-sized lipid droplets consisting of different triglycerides types (poly-unsaturated glyceryl trilinolenate, mono-unsaturated glyceryl trioleate and saturated glyceryl tricaprylate and glyceryl tristearate) in a rapid and label-free way. A systematic comparison of Raman spectra with CARS and D-CARS spectra was used to identify D-CARS spectral signatures which distinguish the disordered poly-unsaturated lipids from the more ordered saturated ones both in the CH-stretch vibration region and in the fingerprint region, without the need for lengthy CARS multiplex acquisition and analysis. D-CARS images of the lipid droplets at few selected wavenumbers clearly resolved the lipid composition differences, and exemplify the potential of this technique for label-free chemically selective rapid imaging of cytosolic lipid droplets in living cells.

Nanoparticles have attracted enormous attention for biomedical applications as optical labels, drug-delivery vehicles and contrast agents in vivo. In the quest for superior photostability and biocompatibility, nanodiamonds are considered one of the best choices due to their unique structural, chemical, mechanical and optical properties. So far, mainly fluorescent nanodiamonds have been utilized for cell imaging. However, their use is limited by the efficiency and costs in reliably producing fluorescent defect centres with stable optical properties. Here, we show that single non-fluorescing nanodiamonds exhibit strong coherent anti-Stokes Raman scattering (CARS) at the sp(3) vibrational resonance of diamond. Using correlative light and electron microscopy, the relationship between CARS signal strength and nanodiamond size is quantified. The calibrated CARS signal in turn enables the analysis of the number and size of nanodiamonds internalized in living cells in situ, which opens the exciting prospect of following complex cellular trafficking pathways quantitatively.

There is increasing evidence showing that cytosolic lipid droplets, present in all eukaryotic cells, play a key role in many cellular functions. Yet their composition at the individual droplet level and how it evolves over time in living cells is essentially unknown due to the lack of suitable quantitative nondestructive measurement techniques. In this work, we demonstrate the ability of label-free hyperspectral coherent anti-Stokes Raman scattering (CARS) microscopy, together with a quantitative image analysis algorithm developed by us, to quantify the lipid type and content in vol/vol concentration units of individual lipid droplets in living human adipose-derived stem cells during differentiation over 9 days in media supplemented with different fatty acids. Specifically, we investigated the addition of the polyunsaturated linoleic and alpha-linolenic fatty acids into the normal differentiation medium (mostly containing monounsaturated fatty acids). We observe a heterogeneous uptake which is droplet-size dependent, time dependent, and lipid dependent. Cells grown in linoleic-acid-supplemented medium show the largest distribution of lipid content across different droplets at all times during differentiation. When analyzing the average lipid content, we find that adding linoleic or alpha-linolenic fatty acids at day 0 results in uptake of the new lipid components with an exponential time constant of 22 ± 2 h. Conversely, switching lipids at day 3 results in an exponential time constant of 60 ± 5 h. These are unprecedented findings, exemplifying that the quantitative imaging method demonstrated here could open a radically new way of studying and understanding cytosolic lipid droplets in living cells.

We demonstrate a technique for simultaneous detection of coherent anti-Stokes Raman scattering (CARS) at four vibrational frequencies, using simple passive optical elements and without spectrally resolved detection. The technique is based on pump and Stokes femtosecond pulses selectively exciting vibrational resonances through spectral focusing. By replicating the pump and Stokes pair into four pairs, each traveling through appropriate glass elements, we simultaneously excite four different vibrational frequencies. The resulting CARS is a periodic train of intensities detected by a single photomultiplier and frequency analyzed to retrieve its Fourier coefficients. We demonstrate detection of methanol and ethanol mixtures in water and quantitative determination of their concentration owing to the improved chemical selectivity of this quadruplex CARS scheme.

Coherent Raman Scattering (CRS) has emerged in the last decade as a powerful multiphoton microscopy technique offering chemically specific label-free imaging in real time with high three-dimensional spatial resolution. Many technical realizations of CRS microscopy have been proposed to remove, suppress, or account for the non-resonant background in the nonlinear susceptibility which complicates spectral analysis and reduces image contrast. Here, we demonstrate coherent anti-Stokes Raman scattering microscopy using a dual-polarization balanced heterodyne detection in epi-geometry (eH-CARS), providing background-free chemically specific image contrast for nanoparticles and interfaces, shot-noise limited detection, and phase sensitivity. We show the sensitivity and selectivity of eH-CARS in comparison with forward CARS and stimulated Raman scattering on polystyrene beads in agarose gel. As an important biologically relevant application, we demonstrate eH-CARS imaging of individual lipid bilayers with high contrast and topographic sensitivity.

The surface conformation and accessibility of oligonucleotides within arrays are two key parameters that affect the utility of immobilized nucleic acids in sensor technologies. In this work, a novel combination of analytical techniques was used to compare two methods for DNA immobilization on glass. The aim of the study was to identify a method that generated a high surface density of hybridization-accessible oligonucleotides in a true planar monolayer. The first method based on direct coupling of silanized DNA to the glass surface showed a high immobilization density of 0.013 molecules/nm2 but low surface accessibility, as shown by the hybridization measurements (or =15%). The second method, based on the biotin-streptavidin interaction, generated a high immobilization density (0.02 molecules/nm2) and high surface accessibility (90%). Atomic force microscopy and X-ray photoelectron spectroscopy indicated that both methods achieved uniform surfaces. Using the biotin-streptavidin system, the intermolecular distance between the hybridized molecules could be tightly controlled by titrating biotinylated complementary and noncomplementary oligonucleotides.

Coherent anti-Stokes Raman scattering (CARS) microscopy using linearly chirped femtosecond laser pulses and spectrally and time-integrated detection is discussed both theoretically and experimentally. By matching the linear chirp of Stokes and pump pulses, the spectral resolution is reduced to the Fourier limit of the chirped pulse duration (spectral focusing). We discuss the effect of the time-ordering of Pump and Stokes pulses on the signal strength and selectivity, and give analytical expressions for the relevant quantities. We consider linear chirp obtained by glass elements of known group-velocity dispersion, and analyze the limit of the spectral resolution by higher order glass dispersion. Experiments corroborating the theory are presented, using 100 fs laser pulses, SF57 glass elements, and polystyrene beads as test samples.

We demonstrate a novel imaging modality based on the detection of resonant four-wave mixing emitted by colloidal quantum dots. This third-order signal is excited and detected at a visible wavelength in resonance with an electronic transition, and results in a significant improvement of the spatial resolution with respect to a twophoton fluorescence microscope.\ud \ud In particular, we have investigated the dependence of the transient reduced absorption of the sample induced by the illumination (photobleaching) on the exposure time and excitation intensity, and its influence to the fourwave mixing signal.

Using a pump-probe differential transmission experiment in heterodyne detection, we measured the refractive index dynamics at the ground-state excitonic transition in electrically pumped InAs/GaAs quantum-dot amplifiers emitting near 1.3 µm at room temperature. We compare three samples differing only in the level of p-doping, and interpret the measured index changes taking into account the gain dynamics in these devices. We find that in absorption, the excess hole density due to p-doping accelerates the recovery and reduces the refractive index change, since filling of the hole states by p-doping shifts the induced changes in the hole population toward high energy states. Conversely, in gain, the reduced electron reservoir in the excited states in p-doped devices results in slower gain recovery dynamics and in larger refractive index changes compared to undoped devices operating at the same modal gain. The linewidth enhancement factor inferred from these measurements shows that p-doping is effective in reducing this parameter mainly due to the larger differential gain in p-doped devices in the gain regime. © 2009 IEEE.

Using a differential transmission pump-probe experiment in heterodyne detection, the ultrafast gain and refractive-index dynamics of the ground-state transition in InAs/GaAs quantum- dot amplifiers emitting near 1.3 mum at working condition, that is at room temperature and under electrical injection were measured. An ultrafast gain recovery on a subpicosecond time scale is observed at high electrical injection indicating fast carrier relaxation into the dot ground state, which is appealing for high-speed applications with these devices. Comparing p-doped and undoped devices of otherwise identical structure and operating at the same gain, a faster absorption recovery but a slower gain dynamics in p-doped amplifiers was observed. This finding should help in elucidating the role of p-doping in the design of QD-based devices with high-speed performances.

We report a wide-field imaging method to rapidly and quantitatively measure the optical extinction cross-section σ(ext) (also polarisation resolved) of a large number of individual gold nanoparticles, for statistically-relevant single particle analysis. We demonstrate a sensitivity of 5 nm(2) in σ(ext), enabling detection of single 5 nm gold nanoparticles with total acquisition times in the 1 min range. Moreover, we have developed an analytical model of the polarisation resolved σ(ext), which enabled us to extract geometrical particle aspect ratios from the measured σ(ext). Using this method, we have characterized a large number of nominally-spherical gold nanoparticles in the 10-100 nm size range. Furthermore, the method provided measurements of in-house fabricated nanoparticle conjugates, allowing distinction of individual dimers from single particles and larger aggregates. The same particle conjugates were investigated correlatively by phase-resolved transient resonant four-wave mixing micro-spectroscopy. A direct comparison of the phase-resolved response between single gold nanoparticles and dimers highlighted the promise of the four-wave mixing technique for sensing applications with dimers as plasmon rulers.

Mammalian oocytes contain lipid droplets that are a store of fatty acids, whose metabolism plays a substantial role in pre-implantation development. Fluorescent staining has previously been used to image lipid droplets in mammalian oocytes and embryos, but this method is not quantitative and often incompatible with live cell imaging and subsequent development. Here we have applied chemically specific, label-free coherent anti-Stokes Raman scattering (CARS) microscopy to mouse oocytes and pre-implantation embryos. We show that CARS imaging can quantify the size, number and spatial distribution of lipid droplets in living mouse oocytes and embryos up to the blastocyst stage. Notably, it can be used in a way that does not compromise oocyte maturation or embryo development. We have also correlated CARS with two-photon fluorescence microscopy simultaneously acquired using fluorescent lipid probes on fixed samples, and found only a partial degree of correlation, depending on the lipid probe, clearly exemplifying the limitation of lipid labelling. In addition, we show that differences in the chemical composition of lipid droplets in living oocytes matured in media supplemented with different saturated and unsaturated fatty acids can be detected using CARS hyperspectral imaging. These results demonstrate that CARS microscopy provides a novel non-invasive method of quantifying lipid content, type and spatial distribution with sub-micron resolution in living mammalian oocytes and embryos., Summary: CARS microscopy provides a novel and non-invasive method of quantifying lipid content, type and spatial distribution in living mouse oocytes and pre-implantation embryos.

We measured the intrinsic ground-state exciton dephasing and population dynamics in colloidal quasi-two-dimensional (2D) CdSe nanoplatelets at low temperature (5–50 K) using transient resonant four-wave mixing in heterodyne detection. Our results indicate that below 20 K the exciton dephasing is lifetime limited, with the exciton population lifetime being as fast as 1 ps. This is consistent with an exciton lifetime given by a fast radiative decay due to the large in-plane coherence area of the exciton center-of-mass motion in these quasi-2D systems compared to spherical nanocrystals. The radiative rate in such 2D platelet systems can be controlled by the platelet area over orders of magnitude.

We measure the dephasing time of the first optically-active excited-state excitonic transition in strongly confined InGaAs quantum dots using transient four-wave mixing. The optically-driven excited-state polarization shows a bi-exponential decay with a significant fraction of the probed excited states exhibiting a very long dephasing time, in the nanosecond range at 10 K. The full time-dependent four-wave mixing polarization is microscopically calculated by taking into account both virtual and real acoustic phonon-assisted transitions. The bi-exponential decay is qualitatively explained by a model of two-bright excitonic excited states non-degenerate in the absence of in-plane cylindrical symmetry. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we review the subject of dephasing processes and population dynamics in self-assembled InGaAs/GaAs quantum dot materials and devices. Our aim is to give a comprehensive overview of our experimental results and an up-to-date discussion in comparison with present theories and other experiments reported in the literature. Additionally, we present new experimental results on InGaAs quantum dots emitting near 1.3 µm wavelength. Concerning the understanding of the fundamental physical processes, we will address the issues of carrier–phonon interaction and carrier–carrier Coulomb interaction and compare dephasing times with population dynamics to highlight the role of pure dephasing processes. Furthermore, we will point out the impact of the measured dynamics specifically at room temperature and under electrical injection in active waveguide structures, for the application of InGaAs/GaAs quantum dots in optoelectronic devices.

We report on four-wave-mixing experiments in self-assembled InAs/GaAs quantum dots and compare with a theory that accounts for the pure-dephasing induced by acoustic phonons. The measurements demonstrate an unusual non-monotonous temperature dependence of the initial decoherence time in full agreement with previous theoretical predictions made on the basis of a model that concentrates on the lowest electronic states. For samples exhibiting strong electronic confinement this model even yields a good quantitative agreement with the experiments while for less confined samples there are some systematic deviations although many qualitative features are still well reproduced by the model.

Self-assembled InAs/GaAs quantum dot molecules have been studied spectroscopically in order to obtain a comprehensive picture of the exciton states confined in them. Photoluminescence studies show a characteristic splitting of the quantum dot shells, which clearly hints at a tunnel coupling of the dots. This coupling was confirmed by exciton fine structure studies, where distinct anticrossings are observed in its magnetic field dispersion. These avoided crossings would not occur if the two dots were decoupled. Four-wave mixing studies show a strong shortening of the exciton dephasing in the molecules as compared to quantum dots which appears to be problematic for application of excitons as the genuine bits in quantum information processing. Finally, we demonstrate that the molecule coupling can be controlled by applying an electric field along the molecule axis.

In this paper, the present status of the experimental study of the optical properties of biexcitons in semiconductor microcavities is reviewed. In particular, a detailed investigation of a polariton–biexciton transition in a high-quality single quantum well GaAs/AlGaAs microcavity is reported. The binding energy and the coherent dynamics of the biexciton state are measured at low temperature using differential reflectivity and transient four-wave mixing spectroscopy, respectively. It is found that the biexciton binding energy is not strongly influenced by the exciton–photon coupling in the microcavity, even if the vacuum Rabi splitting exceeds the biexciton binding energy. However, the presence of a longitudinal built-in electric field that results in a Stark effect slightly reducing the binding energy compared to the value measured on a reference bare quantum well is experimentally pointed out and compared with calculations. Additionally, the polarization decay of the transition from the crystal ground state to the biexciton is measured and is shown to be larger by approximately a factor of two compared to the value measured on the reference quantum well.

We present measurements of the dephasing time of biexcitonic transitions in self-organized InGaAs quantum dots in the temperature range from 10 K to 70 K. We use an heterodyne four-wave mixing technique that allows direct measurement of the dephasing time in an inhomogeneously broadened quantum-dot ensemble. By applying an electrical injection, we progressively increase the number of carriers in the quantum dots and investigate the passage from a four-wave mixing response dominated by the one-exciton transition to a response dominated by biexcitonic and multiexcitonic transitions. The temperature dependence of the homogeneous broadening of the biexciton-to-exciton transition is extrapolated to zero injection current and its physical origin in terms of radiative recombination and phonon interactions is discussed.

We present the experimental observation and the theoretical modelling of self-induced transparency signatures such as nonlinear transmission, pulse retardation and reshaping for subpicosecond pulse propagation in a 2 mm-long InGaAs quantum-dot ridge waveguide at 10 K. The measurements were obtained using a cross-correlation frequency resolved optical gating technique which allows us to retrieve the field amplitude of the propagating pulses.

We present measurements of the dephasing time and of optical Rabi oscillations in the excitonic ground-state transition of an InGaAs quantum dot ensemble at low temperature, using an heterodyne pump-probe technique. Dephasing times of several hundred picoseconds are measured, enabling coherent light-matter interaction with picosecond optical pulses. Optical Rabi oscillations which are damped versus pulse area and change period when changing pulse duration are observed. Comparison with calculations indicates that the observed damping is not intrinsic to a single dot. Dephasing processes and the biexciton resonance change the amplitude and the period of the oscillations, respectively, while the damping versus pulse area is due to a distribution of transition dipole moments in the ensemble.

We present an extensive experimental study of the exciton relaxation and dephasing in InGaAs quantum dots (QDs) in the temperature range from 10 K to 295 K. The QDs are embedded in the active region of an electrically pumped semiconductor optical amplifier. Ultrafast four-wave mixing and differential transmission spectroscopy on the dot ground-state transition are performed with a sensitive heterodyne detection technique. The importance of the population relaxation dynamics to the dephasing is determined as a function of injection current and temperature. Above 150 K dephasing processes much faster than the population relaxation are present, due to both carrier-phonon scattering and Coulomb interaction with the injected carriers. Only at low temperatures (

The dephasing time of biexcitons in a semiconductor quantum well microcavity is measured at low temperature using transient four-wave mixing spectroscopy. The homogeneous linewidth corresponding to the dephasing of the transition from the crystal ground state to the biexciton is found to be approximately twice the one measured in a reference quantum well without Bragg reflectors.

Time-resolved four-wave mixing in strongly confined and electrically pumped InGaAs quantum dots is measured at different temperatures. Without electrical injection, the dephasing time of the dot ground state transition is lifetime-limited and of several hundred picoseconds at low temperature. Under electrical injection, the sum of the contributions from empty and populated dots with a relative phase shift of π is evidenced near transparency.

In this work, we demonstrate the applicability of coherent anti-Stokes Raman scattering (CARS) micro-spectroscopy for quantitative chemical imaging of saturated and unsaturated lipids in human stem-cell derived adipocytes. We compare dual-frequency/differential CARS (D-CARS), which enables rapid imaging and simple data analysis, with broadband hyperspectral CARS microscopy analyzed using an unsupervised phase-retrieval and factorization method recently developed by us for quantitative chemical image analysis. Measurements were taken in the vibrational fingerprint region (1200–2000/cm) and in the CH stretch region (2600–3300/cm) using a home-built CARS set-up which enables hyperspectral imaging with 10/cm resolution via spectral focussing from a single broadband 5 fs Ti:Sa laser source. Through a ratiometric analysis, both D-CARS and phase-retrieved hyperspectral CARS determine the concentration of unsaturated lipids with comparable accuracy in the fingerprint region, while in the CH stretch region D-CARS provides only a qualitative contrast owing to its non-linear behavior. When analyzing hyperspectral CARS images using the blind factorization into susceptibilities and concentrations of chemical components recently demonstrated by us, we are able to determine vol:vol concentrations of different lipid components and spatially resolve inhomogeneities in lipid composition with superior accuracy compared to state-of-the art ratiometric methods.

We demonstrate a method to increase the acquisition speed in coherent anti-Stokes Raman scattering (CARS) hyperspectral imaging while retaining the relevant spectral information. The method first determines the important spectral components of a sample from a hyperspectral image over a small number of spatial points but a large number of spectral points covering the accessible spectral range and sampling the instrument spectral resolution at the Nyquist limit. From these components we determine a small set of frequencies needed to retrieve the weights of the components with minimum error for a given measurement noise. Hyperspectral images with a large number of spatial points, for example covering a large spatial region, are then measured at this small set of frequencies, and a reconstruction algorithm is applied to generate the full spectral range and resolution. The resulting spectra are suited to retrieve from the CARS intensity the CARS susceptibility which is linear in the concentration, and apply unsupervised quantitative analysis methods such as FSC3 [1]. We demonstrate the method on CARS hyperspectral images of human osteosarcoma U2OS cell, with a reduction in the acquisition time by a factor of 25. This method is suited also for other coherent vibrational microscopy techniques such as stimulated Raman scattering, and in general for hyperspectral imaging techniques with sequential spectral acquisition.

The ultrafast gain dynamics in an electrically pumped InAs/InGaAs/GaAs quantum-dot amplifier are measured at room temperature with femtosecond resolution, and compared with results on an InGaAsP bulk amplifier. The role of spectral hole burning and carrier heating in the recovery of the gain compression is investigated. Reduced carrier heating for both gain and refractive index dynamics of the quantum-dot device is found, which is a promising prerequisite for high-speed applications.

We present microscopic modeling and experimental measurements of femtosecond-pulse interactions in a semiconductor optical amplifier. Two novel nonlinear propagation effects are demonstrated: pulse break-up in the gain regime and pulse compression in the transparency regime. These propagation phenomena highlight the microscopic origin and important role of adiabatic following in semiconductor optical amplifiers. Fundamental light-matter interactions are discussed in detail and possible applications are highlighted.

The temperature dependent linewidths of the polariton resonances in a GaAs/AlGaAs single quantum well microcavity are measured. Due to the dominant homogeneous broadening of the investigated resonances, a direct linewidth analysis of the reflectivity spectra allows us to investigate the role of scattering mechanisms in the lower polariton branch compared to the middle and upper polaritons. We find that the lower polariton linewidth is smaller than the middle and upper polariton linewidths at all investigated temperatures from 11 to 100 K, in agreement with expectations in literature.

The temperature dependent linewidths of homogeneously broadened GaAs/AlxGa1 - xAs microcavity polaritons are investigated. The linewidths of the lower, middle, and upper polariton resonances are measured directly from reflection spectra at normal incidence (k(parallel to) = 0). The Linewidth of the lower polariton is found to be smaller than the linewidths of the middle and upper polaritons at all investigated temperatures ranging from 11 to 100 K, The results clearly show the reduction of dynamic scattering processes in the lower polariton compared to the middle and upper polaritons, in agreement with theoretical predictions in literature. A non trivial temperature dependence of the linewidth is found and its physical origin is discussed.

The ultrafast gain and index dynamics in a set of InAs-InGaAs-GaAs quantum-dot (QD) amplifiers are measured at room temperature with femtosecond resolution. The role of spectral hole-burning (SHB) and carrier heating (CH) in the recovery of gain compression is investigated in detail. An ultrafast recovery of the spectral hole within /spl sim/100 fs is measured, comparable to bulk and quantum-well amplifiers, which is contradicting a carrier relaxation bottleneck in electrically pumped QD devices. The CH dynamics in the QD is quantitatively compared with results on an InGaAsP bulk amplifier. Reduced CH for both gain and refractive index dynamics of the QD devices is found, which is a promising prerequisite for high-speed applications. This reduction is attributed to reduced free-carrier absorption-induced heating caused by the small carrier density necessary to provide amplification in these low-dimensional systems.

Summary form only given. Semiconductor quantum dots (QDs) are receiving increasing attention for fundamental studies on zero-dimensional confinement and for device applications. Quantum-dot lasers are expected to show superior performances, like high material gain, low and temperature-independent threshold current and chirp-free operation, due to the delta-like density of states (DOS). We have measured the dephasing time at room temperature of InAs QDs embedded in a waveguide to estimate the lower limit for the energy-broadening of the DOS given by the homogeneous linewidth. The sample consists of 3 stacked layers of InAs-InGaAs-GaAs quantum dots.

Biexciton binding energies and biexciton dephasing in In0.18Ga0.82As/GaAs single quantum wells have been measured by time-integrated and spectrally resolved four-wave mixing. The biexciton binding energy increases from 1.5 to 2.6 meV for well widths increasing from 1 to 4 nm. The ratio between exciton and biexciton binding energy changes from 0.23 to 0.3 with increasing inhomogeneous broadening, corresponding to increasing well width. From the temperature dependence of the exciton and biexciton four-wave mixing signal decay, we have deduced the acoustic-phonon scattering of the exciton-biexciton transition. It is found to be comparable to that of the exciton transition, indicating that the deformation potential interactions for the exciton and the exciton-biexciton transitions are comparable.

The temperature and density dependencies of the exciton dephasing time in In0.18Ga0.82As/GaAs single quantum wells with different thicknesses have been measured by degenerate four-wave mixing. The exciton-phonon scattering contribution to the dephasing is isolated by extrapolating the dephasing rate to zero-exciton density. From the temperature dependence of this rate we have deduced the linewidth broadening coefficients for acoustic and optical phonons. We find acoustic-phonon coefficients that increase from 1.6 to 3 μeV/K when increasing the well width from 1 to 4 nm. This is in quantitative agreement with theoretical predictions when the spatial extension of the exciton wave function, strongly penetrating into the GaAs barrier in thin InxGa1-xAs quantum wells, is taken into account. The optical-phonon coefficient does not show a systematic dependence on well thickness, and is comparable with the value for bulk GaAs.

In this work we present an experimental technique for investigating ultrafast carrier dynamics in semiconductor optical amplifiers at room temperature. These dynamics, influenced by carrier heating, spectral hole-burning and two-photon absorption, are very important for device applications in information processing at high data transfer rates. The technique is based on single pulse propagation through the device, with pulse duration tunable from 150fs to 11ps, and the characterization of the pulse at the output of the device by measuring the total pulse energy, the real time profile and the frequency spectrum. Results on a InGaAsP bulk amplifier are shown as an example.

The coherent response of excitons in semiconductor nanostructures measured in four-wave mixing (FWM) depends strongly on the inhomogeneous broadening of the exciton transition. We investigate GaAs/Al0.3Ga0.7As single quantum wells (SQW) and AlxGa1-xAs mixed crystals. Additional to the usual phase-space filling nonlinearity, excitation-induced dephasing (EID) and biexciton formation (BIF) are important. EID leads to a strong dependence of the signal on the angle between the linear input polarizations. We find that EID persists in inhomogeneous systems, showing that the mutual density-dependent dephasing rate difference between two subsystems within the inhomogeneous distribution is strongly dependent on their energy difference. BIF is strongly affecting the cross-linear polarized FWM response. The signal for positive delay is dominated by the transitions from the one-exciton state X to the two-exciton states. Here, the third-order polarization at X-XX is not at the same resonance as the first-order polarization at 0-X. Consequently, the rephasing of the microscopic third-order polarization to the macroscopic FWM photon echo is blurred by the non-perfect correlation of X and XX energies, leading to a fast and non-exponential signal decay in delay time. For inhomogeneous broadenings larger than the biexciton binding energy, we find an enhanced biexciton binding energy, and a quenching of the oscillator strength of the unbound biexciton.

Exciton formation and relaxation in GaAs bulk epilayers have been studied by means of time-resolved photoluminescence techniques. It is found that the time evolution of the free exciton luminescence, nonresonantly excited at low temperature and low intensity, is extremely slow, with a rise time of the order of 1 ns and a decay time of several ns. Simulations based on Monte Carlo solution of the set of coupled Boltzmann-like equations for free carriers and excitons show a nice agreement with the experimental data, and suggest a dominant role played by acoustic phonons in the exciton relaxation.

The energy relaxation and the capture of free carriers in InxGa1—xAs/GaAs quantum wells have been investigated by means of continuous-wave and time-resolved photoluminescence for excitation energies tuned over a wide interval above the GaAs band-gap. The strong interaction between free electrons and longitudinal-optical phonons gives rise to a 42 meV-period modulation of the efficiency of carrier collection into the well and of the corresponding collection time. The energy position of the electronic level driving the capture into the well has been deduced from the data and does not coincide with any calculated energy level of the well structure. The capture time turns out to be at the limit of our time resolution (≈ 20ps), while the collection time oscillates with an amplitude of about 80 ps, because of the emission of acoustic phonons.

The polarization dependence of the ultrafast nonlinear response of localized excitons in GaAs/AlGaAs quantum wells has been investigated by spectrally resolved, transient four-wave mixing. The role of the excitation-induced dephasing in the delay dependence of the signal is discussed in detail. In particular, we show experimentally that the excitation-induced dephasing gives rise to a photon echo in inhomogeneous systems. We develop a simple analytical model for the delay dependence of the signal, which enables us to deduce the dephasing time from the four-wave mixing decay.

Photoluminescence excitation (PLE) measurements have been performed in several GaAs-based structures by monitoring the electron-carbon acceptor and the exciton bound to carbon recombinations. In both cases we show that the separate capture of free carriers makes the main contribution to the electron-carbon transition and to the formation of the bound exciton. A dip is indeed observed in the PLE spectra at the energy of the free exciton. The contribution of the bound exciton relaxation to the two-hole transition is pointed out.

The photoluminescence of GaAs / AlAs source multiple quantum wells structures under optical ps excitation is investigated for carrier densities in the range 1018−4 × 1019 cm−3 with frequency and time-resolved spectroscopic techniques. The measurements give a direct evidence of the occurrence in the sample of carrier heating. This energy up-conversion gives rise to photoluminescence from the states near the Fermi level whose intensity and time evolution depend on the carrier density in a strongly non-linear way. The observed behaviour can be explained introducing in the carrier dynamics an up-conversion mechanism due to Auger-like processes.

We show that the insertion of extremely narrow AlAs layers in double-barrier GaAs/AlAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum wells results in a variety of electronic configurations, thus providing a powerful tool for tailoring the electronic transitions in GaAs heterostructures. In particular, the transition from type-I to type-II recombination is shown to occur in correspondence with variations by a single monolayer in the thickness of the AlAs and/or GaAs layers. Drastic changes in the recombination lifetimes are correspondingly observed; at the same time, the photoluminescence efficiency is found to be almost independent of the type-I--type-II character of the transition.

The ultrafast gain recovery dynamics of the first excited state (ES) is studied in an electrically pumped InGaAs quantum-dot amplifier at room temperature and compared with the ground-state (GS) gain dynamics. Pump-probe differential transmission experiments are performed in heterodyne detection and the gain dynamics are investigated as a function of injection current. An ultrafast (

In the last decade, coherent anti-Stokes Raman scattering (CARS) microscopy has emerged as a powerful multiphoton imaging technique offering label-free chemical sensitivity and high three-dimensional resolution. However, its widespread application in the life sciences has been hampered by the use of costly pulsed lasers, the existence of a nonresonant background requiring involved technical solutions for its efficient suppression, and the limited acquisition speed of multiplex techniques addressing several vibrational resonances, if improved chemical specificity is needed. We have recently reported a differential CARS technique (D-CARS), which simultaneously measures two vibrational frequencies, enhancing the chemical selectivity and sensitivity without introducing costly hardware, while maintaining fast acquisition. In this study, we demonstrate a compact, fully automated, cost-effective module, which integrates on hardware and software level with a commercial multiphoton microscope based on a single 100 fs Ti:Sapphire oscillator and enables D-CARS microscopy in a user-friendly format for applications in the life sciences.

We report a simple, rapid, and quantitative wide-field technique to measure the optical extinction $\sigma_{\rm ext}$ and scattering $\sigma_{\rm sca}$ cross-section of single nanoparticles using wide-field microscopy enabling simultaneous acquisition of hundreds of nanoparticles for statistical analysis. As a proof of principle, we measured nominally spherical gold nanoparticles of 40\,nm and 100\,nm diameter and found mean values and standard deviations of $\sigma_{\rm ext}$ and $\sigma_{\rm sca}$ consistent with previous literature. Switching from unpolarized to linearly polarized excitation, we measured $\sigma_{\rm ext}$ as a function of the polarization direction, and used it to characterize the asphericity of the nanoparticles. The method can be implemented cost-effectively on any conventional wide-field microscope and is applicable to any nanoparticles.

Using a phase-sensitive degenerate four-wave mixing technique in heterodyne detection we measured the ultrafast changes of the complex polarizability in single gold nanoparticles at the surface plasmon resonance. Two components in the nanoparticle nonlinear response are distinguished, depending on the linear polarization direction of pump and probe pulses and particle geometry. One component is quantitatively modeled as the variation of the complex dielectric constant induced by the initial increase in the electron gas temperature following the absorption of the pump pulse, and the subsequent electron thermalization with the lattice and the surrounding. The dependence of the measured four-wave mixing on the probe field polarization direction is well reproduced by including a deviation from spherical symmetry in the nanoparticle dielectric constant. The second component manifests as a significant increase in the measured nonlinearity when the probe field is co-polarized with the pump field at pump–probe time overlap. We attribute this component to the nonlinear response from coherent surface plasmons before dephasing into single electron excitations.