As they have gained maturity, colloidal nanocrystals (NCs) have also expand the spectral range over of which they could be used for photonic and optoelectronic applications. In particular, the infrared use of NCs has become of utmost interest to develop cost-effective alternatives to current technologies. It is then critical not to let the material dictate the light-matter interaction, which is why the coupling of NCs to photonic cavities has been proposed. For infrared NCs, this approach has first been devoted to the control of absorption with in mind the increase of the signal magnitude for detectors. A Lot of efforts have been focused on the use of metallic metasurfaces. However, these generate significant optical losses and yield low quality factor. Here, this study rather focus on the coupling of infrared NCs to a dielectric mirror cavity. HgTe/CdS core-shell NCs are used and integrated into a cavity made of aperiodic dielectric mirrors. The effect of the substrate is systematically study on spectral linewidth, carrier dynamic, and emission directivity. The cavity is shown to narrow the PL by a factor 10, while focusing the emission over a 12 degrees angle. Monitoring the power dependence of the emission, this study shows that the cavity leads to 250 K increase in the effective electronic temperature. Schematic of narrow band gap HgTe nanocrystals within a dielectric mirror cavity. Inside the cavity, the photoluminescence spectrum is drastically narrowed, the effective electronic temperature of the carriers is strongly increased, while the emission is also made more directive. image

Ultrafast acoustic imaging experiments are a powerful tool to investigate, at the nanometer scale, cell mechanical properties such as stiffness, viscosity, and adhesion, properties that play some roles in the life and death of cells. However, due to cell complex structures, the ultrafast acoustic signal analysis is quite intricate and depends on multiple parameters. Complex data analysis with poorly known parameters can be handled by a data clustering method as already shown in particle physics and biology. In this work, ultrafast acoustic data analysis is tackled by a spectral clustering method followed by a hierarchical agglomerating method. Coupled to conventional microscopy performed on the very same cell, the clustered data can be assigned to inner-cell features such as the nucleus, the cytoplasm, and the cytoskeleton. The signal dependency on the cell thickness and stiffness is highlighted. Moreover, thanks to the improvement of the signal-to-noise ratio, the nature of the adhesion is also assessed through observation and characterization of a polymerlike layer as thin as a few nanometers.

Intraband absorption in doped nanocrystals offers an interesting alternative to narrow-band-gap materials to explore mid-infrared optoelectronic device designs. However, the performance of such a device clearly lags behind the ones relying on intrinsic materials. Livache et al. proposed a dye-sensitized approach to overcome the limitations observed from intraband materials (high dark current, slow response, low activation energy), where an intraband absorber is coupled with an undoped material, which takes care of the charge conduction. Here, we unveil the coupling between both materials using mid-infrared transient reflectivity (TR) measurement. We show that the hybrid material displays a unique feature in the TR signal that we attribute to a charge transfer for which the dynamics matches the hopping time. Then, we developed a strategy to enhance the photodetection performances of the hybrid material by coupling the intraband absorption to a light resonator for the first time. The latter is used to enhance the absorption by a factor of 4 and enables an increase in the operating temperature by 80 K compared to the reference device. The obtained device matches the performance of the best devices relying on intraband absorption.

Because of their role of information transmitter between the spinal cord and the muscle fibers, motor neurons are subject to physical stimulation and mechanical property modifications. We report on motoneuron elasticity investigated by time-resolved pump and probe spectroscopy. A dual picosecond geometry simultaneously probing the acoustic impedance mismatch at the cell-titanium transducer interface and acoustic wave propagation inside the motoneuron is presented. Such noncontact and nondestructive microscopy, correlated to standard atomic force microscopy or a fluorescent labels approach, has been carried out on a single cell to address some physical properties such as bulk modulus of elasticity, dynamical longitudinal viscosity, and adhesion.

Visible nanocrystal-based light-emitting diodes (LEDs) are about to become commercially available. However, their infrared counterparts suffer from two key limitations. First, III–V semiconductor technologies are strong competitors. Second, their potential for operation beyond 1.7 µm remains unexplored. The range from 1.5 to 4 µm corresponds to a technological gap in which the efficiency of interband quantum-well-based devices vanishes and quantum cascade lasers are not efficient enough. Powerful infrared LEDs in this range are needed for applications such as active imaging, organic molecule sensing and airfield lighting. Here we report the design of a HgTe nanocrystal-based LED with luminescence between 2 and 2.3 µm. With an external quantum efficiency of 0.3% and radiance up to 3 W Sr−1 m−2, these HgTe LEDs already present a competitive performance for emission above 2 µm. Near-infrared emission at around 2 µm is observed from HgTe nanocrystals. LEDs based on this material platform could prove to be a useful low-cost, convenient light source for applications in gas sensing and other tasks.

Perturbation of tissue rigidity is associated with different types of pathology. However, it is sometimes difficult to conclude if this change in stiffness of cells or tissue is the effect or the source of the pathologies. This is why the characterization of the mechanical properties of cells is essential to understand their behavior during mitosis, apoptosis, adhesion, mobility and disease development. However, the complexity of the inner cell composition and the intricate meshwork formed by molecular mediators of the transmembrane cell-substrate interactions requires non-invasive techniques to probe and quantify local mechanical properties of cells, including modulus of elasticity, viscoelastic properties, adhesion, and forces created at the single-cell scale. Several recent reviews describe tools used to study cell mechanics and to apply forces on them. Acoustic waves generated by lasers provide a very adequate tool for probing the mechanical properties of biological cells or tissues in a non-contact, non-invasive configuration. We report on detection of Brillouin (BO) signature used for mapping elastic properties in single neuron-like cells. To our knowledge, this is the first time this technique is used on this type of cells, in which the regeneration processes are closely related to cell elasticity. The elasticity contrasts thus revealed between the nucleus and the cytoskeleton in the reticulate cells also obviously exist in living cells for which the study is more subtle given the very specific conditions necessary for their maintenance . The frequencies and the lifetimes of the BOs are mapped across the cell using Pearson correlation method. The influence of hydration, i.e. a more biologically relevant conditions, on the cell elasticity is investigated. Finally a correlative microscopy coupling elastic mapping, atomic force microscopy and fluorescence will be presented in motoneuron single cells.

We report on elasticity measurements in neuronlike cells using picosecond acoustics pump and probe spectroscopy. The stimulated Brillouin oscillations were mapped in PC12 cells to reveal their internal elastic structure. Thanks to a Pearson correlation coefficient mapping, different areas could be distinguished. The nucleus material shows a bulk modulus equal to 12.9 GPa in the case of a dry cell. Attenuation of the Brillouin signature gives access to dynamical longitudinal viscosity equal to 10.6 mPa ⋅ s, one order of magnitude higher than that of water. The modulus considerably drops to 2.6 GPa in the most physiologically relevant case of a hydrated cell.

Solitons are self-preserving traveling waves of great interest in nonlinear physics, offering many interesting application such as high-bandwidth optical fiber communication. Solitons can also appear in ultrafast acoustics, and several observations suggest acoustic solitons as short as 200 fs. Here, the authors designed an experimental setup to observe and characterize acoustic solitons traveling through a GaAs(001) substrate. The experimental signal is explained well with the help of the Korteweg--de Vries equation, demonstrating the particlelike nature and the unique properties of the solitons. Moreover, the temporal distribution of the solitons is also analyzed with the help of the inverse scattering method. Such investigations provide a new tool to probe transient properties of highly excited matter through the study of the solitons due to the acoustic pulse emitted after laser excitation.

The transient acoustic pulse emitted by a single InAs quantum dots layer was measured depending on the pump wavelength. By tuning the pump wavelength through the transition energies of the GaAs barrier, the wetting layer and the quantum dots themselves, the acoustic phonon emission is shown to be strongly correlated to the electronic structure of the quantum dots layer system. The contributions of the wetting layer and the quantum dots to the acoustic signal were clearly identified and quantified by comparing different regions of the same sample containing or not quantum dots.

Acoustic attenuation rates in vitreous silica in the 20–400 GHz frequency range have been measured using a multiple-pulse optical technique for generation of tunable multicycle acoustic waves that are detected interferometrically after traversal of the sample. The results connect the frequency ranges of several measurement methods, yielding a consistent description of the acoustic behavior.

We present a laser ultrasonics experiment with imaging capabilities performed in transmission through semiconductor substrates. Such experiment provides a new kind of bulk wavefront imaging using an ultrafast optical generation and detection scheme for gigahertz waves. The data may be viewed as two-dimensional slices of the three-dimensional acoustic wavefront revealing the basic anisotropy of the solid with 0.3 ps time resolution and 3 μm space resolution and allowing the determination of the whole set of elastic constants for various experimental conditions. As examples, measurements are carried out in a Si(001), GaAs(001), and GaAs(111) sample at 20 and 300 K.

In standard ultrafast acoustic experiments very short coherent acoustic pulses are generated by the absorption of a femtosecond laser pulse in a thin metallic transducer deposited on the sample. Subsequently the acoustic pulses and the heat generated in the transducer cross the sample and are partially transmitted in the underlaying substrate. At low temperature, heating of the metallic transducer gives rise to the emission of incoherent phonons wave packets which propagate ballistically over large distances in the substrate. We report on a series of experiments which demonstrate the acoustic rectification [1-3] of these wave packets as they propagate through large GaAs or Si substrates. [1] B. Perrin, J. de Phys. C8 (1979) 216. [2] S. M. Avanesyan, V. E. Gusev, Solid State. Commun. 54 (1985) 1065. [3] B. Perrin, E. Péronne, L. Belliard, Ultrasonics, 44 (2006) 1277.

Picosecond ultrasonic experiments are widely used as pump-probe technique using longitudinal acoustic pulses and measuring time dependence of the signal. We present an experiment which combines an interferometer with the control of the pump-probe spatial overlap. Such set-up can perform a new kind of bulk phonon imaging based on ultrafast optical generation and detection scheme. When ultrafast acoustic longitudinal wave propagate in crystals they generate coherent transverse acoustic wave by diffraction. Transverse waves are known to give rise to phonon focusing patterns. The imaging of such patterns in time and space allowed us to partially reconstruct the wave front surface of bulk acoustic wave in crystals. The data may be viewed as 2D slices of the 3D acoustic wavefront revealing the basic anisotropy of the solid with unprecedented micronic lateral and in-depth resolution. Measurements are carried out in a 356 μm-thick GaAs sample as an example, but many different experimental conditions are compatible with the present set-up. This technique provides a new way to perform phonon energy and phonon polarisation dependent experiment with high frequency phonons.

As the size of the structure decrease toward the nanoscale, their acoustic properties get closer to the THz range. Traditional techniques based on piezoelectric transducer or the Brillouin diffusion can hardly reach the adequate frequency range. However, if we tightly focus short optical pulses, we can expect to generate high frequencies due to nonlinear effect during the propagation of the acoustic pulses. Moreover, such small source could be used to image buried structures. We have developed an pump-probe experiment based on a Ti:Sa oscillator using microscope objectives and 2 axis displacement stage to study nonlinear propagation and diffraction of acoustic wave in 3 dimension. We have studied 356-μm thick GaAs substrate and have demonstrated the generation of acoustic longitudinal wavelength as short as 10 nm. Due to the acoustic dispersion during the propagation, acoustic solitons are observed for high excitation power. Thanks to the use of interferometric detector, the acoustic pulses are characterized in great details. Such experimental development can help us to study the properties of transmission and reflection of buried nanosystem such as single quantum well or phonon nanocavities.

Acoustic solitons have been recently observed in different systems (Si, Sapphire, MgO, alpha-quartz). Such acoustic waves could lead to sub-picosecond acoustic pulses. In this paper, we report on the formation of acoustic solitons in a GaAs crystalline slab. A short picosecond acoustic pulse is generated by absorption of a femtosecond laser pulse in an aluminum thin film deposited on one side of the slab. This strain pulse travels through the sample up to the opposite side where it is detected by a time delayed laser pulse reflected by an aluminum transducer. We use interferometric detection to measure independently the real and imaginary parts of the relative change in optical reflectivity induced by the acoustic pulse. We find that, at low temperature and with a laser pump pulse energy of 10 nJ, an acoustic soliton clearly separates from the acoustic pulse in GaAs slab. The soliton shape is compared with numerical simulations for different excitation conditions. From the very unique properties of solitons, we infer a soliton pulse duration of about 2.3 ps which corresponds to a spatial extent of only 12 nm.

The rotational revival structure of asymmetric top molecules, following irradiation by an intense picosecond laser pulse, is explored theoretically and experimentally. Numerically we solve nonperturbatively for the rotational dynamics of a general asymmetric top subject to a linearly polarized intense pulse, and analyze the dependence of the dynamical alignment on the field and system parameters. Experimentally we use time-resolved photofragment imaging to measure the alignment of two molecules with different asymmetry, iodobenzene, and iodopentafluorobenzene. Our numerical results explain the experimental observations and generalize them to other molecules. The rotational revival structure of asymmetric tops differs qualitatively from the intensively studied linear top case. Potentially it provides valuable structural information about molecules.

We have investigated optical properties of high-density InAs self-assembled quantum dots (QDs) in an ${\mathrm{In}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}\mathrm{As}$ matrix, lattice matched to an InP (001) substrate. The weak lattice mismatch $(\ensuremath{\sim}3%)$ results in a 90% coverage of the ${\mathrm{In}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}\mathrm{As}$ surface with InAs QDs. By means of interband and intraband spectroscopies crossed with atomic force microscopy (AFM) measurements, we have determined that the InAs QDs optical properties depend on the deposited amount of InAs. Photoinduced absorption spectroscopy has been used to investigate midinfrared intraband absorptions. For three monolayers (ML) InAs deposit thickness, just above two-dimensional (2D)/3D growth mode transition (2.5 ML), the islands form as isolated elliptical dots elongated along the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction and exhibit intraband resonances polarized either along the [110] or the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction. For thicker deposition $(>3\mathrm{ML}),$ InAs islands form chains of elliptical dots along the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction where the quantum confinement is lost, resulting in a quantum-wire-like behavior. In this paper, we also report on photoluminescence and photocurrent spectroscopies, in order to get insight into the ${\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}\mathrm{As}$ island band structure. These experimental results are in good agreement with that of a multiband $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ model.