Absorption Of Short Laser Pulse Research Articles

Photoacoustic Tomography with a Ring Ultrasound Transducer: A Comparison of Different Illumination Strategies.

Photoacoustic (PA) imaging is a methodology that uses the absorption of short laser pulses by endogenous or exogenous chromophores within human tissue, and the subsequent generation of acoustic waves acquired by an ultrasound (US) transducer, to form an image that can provide functional and molecular information. Amongst the various types of PA imaging, PA tomography (PAT) has been proposed for imaging pathologies such as breast cancer. However, the main challenge for PAT imaging is the deliverance of sufficient light energy horizontally through an imaging cross-section as well as vertically. In this study, three different illumination methods are compared for a full-ring ultrasound (US) PAT system. The three distinct illumination setups are full-ring, diffused-beam, and point source illumination. The full-ring system utilizes a cone mirror and parabolic reflector to create the ringed-shaped beam for PAT, while the diffuse scheme uses a light diffuser to expand the beam, which illuminates tissue-mimicking phantoms. The results indicate that the full-ring illumination is capable of providing a more uniform fluence irrespective of the vertical depth of the imaged cross-section, while the point source and diffused illumination methods provide a higher fluence at regions closer to the point of entry, which diminishes with depth. In addition, a set of experiments was conducted to determine the optimum position of ring-illumination with respect to the position of the acoustic detectors to achieve the highest signal-to-noise ratio.

Directivity of GHz shear acoustic waves launched by the absorption of short laser pulses at the interface between a transparent and an absorbing material

When a short laser pulse is focused at the interface between a transparent and an absorbing material, an expansion source is created that generates mainly longitudinal acoustic pulses in the absorbing material. However, the interaction of these waves with the interface favours the excitation of shear acoustic pulses. In this paper, we present an analytical prediction of the directivity of shear waves launched in a semi-infinite absorbing medium, and we introduce corrections in order to account for detection in an absorbing layer. Experiments were performed with a picosecond ultrasonic set-up focusing femtosecond laser pulses to a micron spot size along the interface between sapphire and titanium. In this configuration, acoustic wave-fronts were reflected at the free surface of the micron titanium layer, and then optically detected at the interface. We also present an approach to filter and enhance the shear waves by processing measured wave-fronts recorded at multiple locations along the interface. In the resulting signals, the contribution of GHz shear acoustic waves, propagating mainly in directions that are oblique with respect to the interface, is enhanced.

Anharmonic resonance absorption of short laser pulses in clusters: A molecular dynamics simulation study

Linear resonance (LR) absorption of an intense 800 nm laser light in a nano-cluster requires a long laser pulse >100 fs when Mie-plasma frequency (ωM) of electrons in the expanding cluster matches the laser frequency (ω). For a short duration of the pulse, the condition for LR is not satisfied. In this case, it was shown by a model and particle-in-cell (PIC) simulations [Phys. Rev. Lett. 96, 123401 (2006)] that electrons absorb laser energy by anharmonic resonance (AHR) when the position-dependent frequency Ω[r(t)] of an electron in the self-consistent anharmonic potential of the cluster satisfies Ω[r(t)]=ω. However, AHR remains to be a debate and still obscure in multi-particle plasma simulations. Here, we identify AHR mechanism in a laser driven cluster using molecular dynamics (MD) simulations. By analyzing the trajectory of each MD electron and extracting its Ω[r(t)] in the self-generated anharmonic plasma potential, it is found that electron is outer ionized only when AHR is met. An anharmonic oscillator model, introduced here, brings out most of the features of MD electrons while passing the AHR. Thus, we not only bridge the gap between PIC simulations, analytical models, and MD calculations for the first time but also unequivocally prove that AHR process is a universal dominant collisionless mechanism of absorption in the short pulse regime or in the early time of longer pulses in clusters.

Real-time observation of coherent acoustic phonons generated by an acoustically mismatched optoacoustic transducer using x-ray diffraction

The spectrum of laser-generated acoustic phonons in indium antimonide coated with a thin nickel film has been studied using time-resolved x-ray diffraction. Strain pulses that can be considered to be built up from coherent phonons were generated in the nickel film by absorption of short laser pulses. Acoustic reflections at the Ni–InSb interface leads to interference that strongly modifies the resulting phonon spectrum. The study was performed with high momentum transfer resolution together with high time resolution. This was achieved by using a third-generation synchrotron radiation source that provided a high-brightness beam and an ultrafast x-ray streak camera to obtain a temporal resolution of 10 ps. We also carried out simulations, using commercial finite element software packages and on-line dynamic diffraction tools. Using these tools, it is possible to calculate the time-resolved x-ray reflectivity from these complicated strain shapes. The acoustic pulses have a peak strain amplitude close to 1%, and we investigated the possibility to use this device as an x-ray switch. At a bright source optimized for hard x-ray generation, the low reflectivity may be an acceptable trade-off to obtain a pulse duration that is more than an order of magnitude shorter.

Absorption of short laser pulses in underdense plasma by considering ohmic heating and ponderomotive force effects

The effect of the non-relativistic ponderomotive force in the interaction of an intense laser pulse with isothermal and non-isothermal underdense collisional plasmas is studied. In this work by considering the ohmic heating of plasma electrons and the ponderomotive force, the nonlinear dielectric permittivity of the plasma medium is obtained and the equation of electromagnetic wave propagation is solved. It is shown that due to the effect of the ponderomotive force in both isothermal and non-isothermal plasmas, the increasing of laser pulse intensity leads to the steepening of the electron density profile and the narrowing of electron bunching. Since the ponderomotive force modifies the electron density and temperature distribution, the spatial damping rate of laser energy and the absorption coefficient are obtained in the collisional regime of isothermal and non-isothermal plasmas. It is shown that in these plasmas, the amplitude of the electric field is increased by increasing laser pulse energy while their wavelength is decreased. Furthermore, by increasing laser pulse intensity, the absorption coefficient is decreased strongly.

Artifact removal in photoacoustic section imaging by combining an integrating cylindrical detector with model-based reconstruction

Photoacoustic section imaging reveals optically absorbing structures within a thin slice of an object. It requires measuring acoustic waves excited by absorption of short laser pulses with a cylindrical acoustic lens detector rotating around the object. Owing to the finite detector size and its limited depth of focus, various artifacts arise, seen as distortions within the imaging slice and cross-talk from neighboring areas of the object. The presented solution aims at avoiding these artifacts by a special design of the sensor and by use of a model-based reconstruction algorithm that improves section images by incorporating information from neighboring sections. The integrating property of the cylindrical detector, which exceeds in direction of the cylinder axis the size of the imaged object, avoids the lateral blurring that normally results from the finite width of a small detector. Applying a maximum likelihood reconstruction method for the inversion of the imaging system matrix to the temporal pressure signals yields line projections of the initial energy distribution, from which section images are obtained by applying the inverse Radon transform. By using data from few sections, a significant reduction of artifacts related to the imperfections of the sensor is demonstrated both in simulations and in phantom experiments.

Imaging of tumor vasculature using Twente photoacoustic systems

Photoacoustic imaging is a hybrid imaging modality based on the detection of acoustic waves generated by the absorption of short laser pulses in biological tissue. It combines the advantages of excellent contrast achieved in optical techniques with the high resolution of ultrasound imaging. In this article we present a review of the work done at the University of Twente to image tumor angiogenesis in vivo using this technique. We start with a description and the technical details of the different photoacoustic systems developed in our laboratory, with their validation on phantoms. We then discuss small-animal studies with results of serial imaging of angiogenesis over a 10-day period at the site of tumor induction in a rat. Further, we present clinical results using a photoacoustic mammoscope of breast cancer imaging based on angiogenesis-driven optical absorption contrast.

Taking into account electron-electron collisions in classical absorption of short laser pulses

The previously derived universal expression for the plasma permittivity is used to solve the problem of absorption of a short laser pulse incident on a semibounded solid-dense plasma. It is shown that the results of the exact theory may differ significantly from those obtained with the generally accepted models, especially in a weakly collisional regime corresponding to temperatures in the range 0.5-1 keV.

Absorption of Short Laser Pulses on Solid Targets in the Ultrarelativistic Regime

We report the first direct measurements of total absorption of short laser pulses on solid targets in the ultrarelativistic regime. The data show an enhanced absorption at intensities above 10(20) W/cm(2), reaching 60% for near-normal incidence and 80%-90% for 45 degrees incidence. Two-dimensional particle-in-cell simulations demonstrate that such high absorption is consistent with both interaction with preplasma and hole boring by the intense laser pulse. A large redshift in the second harmonic indicates a surface recession velocity of 0.035c.

Pulsed optoacoustic characterization of layered media

Thermoelastic waves generated by absorption of short laser pulses are used to characterize the layer structure of materials. The method is based on the analysis of the distribution of absorbed laser energy from temporal profiles of recorded acoustic signals. Particularly in view of noninvasive medical applications, optoacoustic front surface transducers are investigated in this study, where irradiation of the surface and detection of the acoustic wave take place on the same side of the sample. Front surface detection of optoacoustic waves is studied theoretically and experimentally, with special emphasis on acoustic diffraction and the differences between measurements in the acoustic near and far field. In the experiments, samples with stepwise and continuously varying depth profiles of absorption coefficient were irradiated with laser pulses of 6–8 ns duration. For the detection of the acoustic waves either an optical ultrasound sensor or an annular piezoelectric film was used. Generating the optoacoustic waves with a flat top laser beam profile and detecting the signals in the acoustic near field yields optimal conditions for direct measurements of the distribution of absorbed energy and the absorption coefficient in the medium. Far field measurements are advantageous for detecting and imaging layer boundaries at large depths in the sample.

Picosecond spectroscopy of excited levels of solved molecules

Solved, molecules, e.g. dimethylaminobenzaldehyde and stilbene, are excited by one-photon or two-photon absorption of short laser pulses, and the time dependence of the spectra of excited levels is probed by means of delayed monochromatic or broad-band pulses. Rate parameters of excited levels are given in dependence on solvent and temperature. The influence of interaction processes between solved molecules is considered. The non-exponential decrease of probe beam absorption in concentrated solutions of stilbene and its temperature dependence could be explained applying Weller's theory of diffusion controlled reactions.