Emmanuel Abraham

Investigation on reconstruction methods applied to 3D terahertz computed tomography.

3D terahertz computed tomography has been performed using a monochromatic millimeter wave imaging system coupled with an infrared temperature sensor. Three different reconstruction methods (standard back-projection algorithm and two iterative analysis) have been compared in order to reconstruct large size 3D objects. The quality (intensity, contrast and geometric preservation) of reconstructed cross-sectional images has been discussed together with the optimization of the number of projections. Final demonstration to real-life 3D objects has been processed to illustrate the potential of the reconstruction methods for applied terahertz tomography.

Wide-field optical coherence tomography: imaging of biological tissues

We describe a two-dimensional optical coherence tomography technique with which we were able to obtain multiple longitudinal slices of a biological sample directly in a single Z scan. The system is based on a femtosecond Cr4+:forsterite laser and an infrared camera for wide-field imaging of the sample with a depth resolution of 5 microm. With this imaging apparatus we were able to investigate human skin and mouse ear samples and to observe the different constitutive tissues.

Photophysics of 4-dimethylamino 4′-cyanostilbene and model compounds: dual excited states revealed by sub-picosecond transient absorption and Kerr ellipsometry

4-Dimethylamino 4′-cyanostilbene (DCS) and two selectively bridged compounds are investigated using sub-picosecond time-resolved absorption and Kerr ellipsometry experiments. The latter technique makes it possible to work at low excitation energy and low concentrations, thereby avoiding intensity and concentration effects which exist in DCS derivatives. Using this technique, in a non-polar solvent (cyclohexane), only the presence of a single excited state is observed for all studied compounds. In polar solvents, the bridged derivative where twisting of the anilino moiety is prevented but double bond twisting is allowed also reveals the presence of only one excited state. On the other hand, in polar solvents, for the DCS compound and the related bridged derivative where the anilino moiety is still able to twist, a precursor-successor relationship is clearly observed between two different excited states. These results can be understood within a four excited states model derived from a previously suggested diagram: the DE state (delocalized excited state), the ICT state (internal charge transfer, highly polar, nearly planar configuration, formed quasi-instantaneously from the DE state by electronic reorganization), the CRICT state (conformational relaxed ICT state, highly polar and fluorescent, involves conformational geometric changes such as twisting of the anilino group which enhances charge transfer) and the “phantom” state P∗ on the trans⇄cis isomerization pathway (twisted double bond, low polar, non-fluorescent). In this study, we observed the formation of the CRICT excited state on a time scale ranging from 2 to 20 ps depending on the solvent characteristics (polarity, viscosity and hydrogen bonding ability).

Real-time terahertz color scanner for moving objects

Terahertz time-domain spectroscopic (THz-TDS) imaging is an interesting new tool for nondestructive testing and other applications. However, the current speed of image acquisition is relatively low, making it difficult to use for moving objects. In this paper, we propose a real-time THz-TDS line scanner based on electro-optical time-to-space conversion and line focusing of a THz beam. The proposed system functions as a color scanner in the terahertz spectral region with fast line-scanning and has been successfully used to image objects, which are moved on a translation stage. The achieved THz-TDS imaging rate is 23,200 pixels per second. This proposed THz-TDS line scanner has the potential to become a powerful tool for monitoring moving objects in various real-world applications.

Femtosecond laser-induced breakdown in water: time-resolved shadow imaging and two-color interferometric imaging

Abstract Time-resolved shadow imaging and novel two-color interferometric imaging have been developed for the investigation of optical breakdown induced in water by 100 fs laser pulses with a spatial resolution better than 5 μm. With a femtosecond time resolution, we clearly identified induced-plasma and filamentation formations, as well as the shock-wave expansion. Using the two-color interferometric imaging technique together with a conventional Mach–Zehnder interferometer, we investigated the phase shift of the interference fringes produced in water by the femtosecond optical breakdown. For different pump energies, at 40 ns after the breakdown, we estimated the water temperature to be between 30°C and 60°C and the water pressure between 10 and 40 MPa at the shock-wave front.

Three-dimensional terahertz computed tomography of human bones

Three-dimensional terahertz computed tomography has been used to investigate dried human bones such as a lumbar vertebra, a coxal bone, and a skull, with a direct comparison with standard radiography. In spite of lower spatial resolution compared with x-ray, terahertz imaging clearly discerns a compact bone from a spongy one, with strong terahertz absorption as shown by additional terahertz time-domain transmission spectroscopy.

Fast three-dimensional terahertz computed tomography using real-time line projection of intense terahertz pulse

We demonstrated fast three-dimensional transmission terahertz computed tomography by using real-time line projection of intense terahertz beam generated by optical rectification in lithium niobate crystal. After emphasizing the advantage of intense terahertz pulse generation for two-dimensional spatio-temporal terahertz imaging, peak-to-peak amplitudes of pulsed terahertz electric field have been used to obtain a series of projection images at different rotation angles. Then a standard reconstruction algorithm has been employed to perform final three-dimensional reconstruction. Test samples including a medicine capsule have been investigated with a total acquisition time to only 6 minutes.

Real-time two-dimensional imaging in scattering media by use of a femtosecond Cr 4+ :forsterite laser

An original femtosecond Cr(4+):forsterite laser source associated with a nonlinear optical correlator was used for imaging through scattering media with 1220-nm light. The system, which operates as an ultrafast optical gate by sum-frequency generation in a nonlinear crystal, was able to detect the light reflected from a resolution chart hidden in a turbid medium, at an attenuation of as much as 15 mean free paths. When the object was illuminated with a collimated beam, real-time two-dimensional images were obtained, with a maximum transverse resolution of ~20 microm.

Dual excited states in 4- dimethylamino 4′-cyanostilbene (DCS) revealed by sub-picosecond transient absorption and Kerr ellipsometry

4-dimethylamino 4′-cyanostilbene (DCS) and p-dimethylamino, p′-cyano, 1-1′-bi-indanylidene (substituted “stiff stilbene” DCS-1324) are investigated using sub-picosecond time-resolved absorption and Kerr ellipsometry experiments. The latter technique makes it possible to work at low excitation energy and low concentration, thereby avoiding intensity and concentration effects which exist in DCS derivatives. Using this technique, in non-polar solvent (cyclohexane), only the presence of a single excited state is observed for the two studied compounds. In polar solvents, DCS-1324 derivative where twisting of the anilino moiety is prevented but double-bond twisting is allowed also reveals the presence of only one excited state. On the other hand, in polar solvents, for the DCS a precursor-successor relationship is clearly observed between two different excited states. These results can be understood within a four excited states model derived from a previously suggested diagram: the delocalized excited (DE) state, the internal charge transfer (ICT) state (highly polar, nearly planar configuration, formed quasi instantaneously from the DE state by electronic reorganization), the conformational relaxed ICT (CRICT) state (highly polar and fluorescent, involves conformational geometric changes such as twisting of the anilino group which enhances charge transfer) and the “phantom” state P∗ on the trans ↔ cis isomerization pathway (twisted double bond, lowly polar, non-fluorescent).

ENHANCEMENT AND SUB-PICOSECOND DYNAMICS OF OPTICAL NON-LINEARITIES OF EXCITED-STATES : TRANS-STILBENE IN SOLUTION

Using degenerated four wave mixing techniques and optical Kerr effect we studied the optical non-linearities of excited t-stilbene solution. Strong enhancement of the non-linear signal between the excited and ground states is observed and related to the third order non-linear susceptibility ( χ(3)) taking into account resonance conditions. Sub-picosecond studies of the X(3) dynamics show, depending on the experimental conditions, a fast decay time (in the range of < 1 ps) associated to electronic processes and a slower decay time (10–100 ps), depending on the solvent, associated with the reorientation time τor of the molecules in solution. We show that vibrational relaxation and solvent cooling may control the dynamics of χ(3).