We demonstrate the application of terahertz pulse imaging for the in-vivo study of human tissue, in this case the upper layers of human skin. The terahertz pulses comprise frequencies from below 100 GHz to over 2 THz and are generated using optical pulse excited semiconductor devices with a conversion efficiency of better than 10-3. The terahertz pulses are used to obtain tomographic information on the skin surface tissue. From the data the stratum corneum thickness and hydration may be mapped or cross-sectional images displayed.
We develop several new optical techniques for microscopic semiconductor diagnostics and use them for inspection of semiconductor surfaces. Short-pulse lasers (femtosecond Ti:sapphire and Cr4+:forsterite) are used for nonlinear optical studies.
We use a novel design of Cr4+:forsterite laser to generate ultrashort high energy laser pulses around 1200 - 1300 nm. Pumping with a fiber laser provides an excellent long-term stability, and semiconductor saturated absorber mirror provides a stable mechanism for self-starting. We characterize the output of this laser and discuss its application for deep tissue biological imaging and semiconductor processing.
The measurement of the second order nonlinear susceptibility of collagen in various biological tissues has potential applications in the detection of structural changes which are related to different pathological conditions. We investigate second harmonic generation in a rat-tail tendon, a highly organized collagen structure consisting of parallel fibers. Using an electro-optic modulator and a quarter-wave plate, we modulate the linear polarization of an ultra-short pulse laser beam that is used to measure second harmonic generation in a confocal microscopy setup. Phase-sensitive detection of the generated signal, coupled with a simple model of the collagen protein structures, allows us to measure a parameter (gamma) related to nonlinear susceptibility and to determine the relative orientation of the structures. Our preliminary results indicate that it may be possible to use this parameter to characterize the structure.
We present a new method of diagnosing cancer, femtosecond laser in vivo HpD (haematoporphyrin derivatives) two-photon fluorescence, and observations of in vivo HpD two-photon fluorescence of cancer tissue of little mice using excitation of femtosecond laser pulse generated by self-mode-locked Ti:sapphire laser. The narrow pulse width, the high peak power of the excitation source and the central wavelength of 810 nm that is the window wavelength of biological tissue show that biomedical signal induced by this light source must have high signal-noise ratio. This femtosecond laser pulse and the two-photon fluorescence technique do no harm to normal tissue surrounding the cancer. The result of our experiments shows that the cancer tissue can be distinguished and diagnosed with the method of in vivo HpD two-photon fluorescence excited by femtosecond laser pulses with suitable wavelength.
A micropulse-picking system attached to a free electron laser (FEL) opens a broad range of potential applications of ultrafast phenomena in medicine and biology. This paper reports the micropulse-picking system of a mid-infrared (IR) FEL at iFEL, Osaka University. We have designed the system with a germanium acousto-optic modulator (Ge-AOM), which can effectively deflect the direction of the FEL propagation due to Bragg diffraction of the FEL and radio frequency (RF) waves. The system includes a reducing optics for the FEL beam, a focusing optics onto the Ge- AOM and a micropulse-picking device (including the Ge-AOM and the RF driver). The system, which is independent on wavelength in the mid-IR region, can be realized by using the following technique: the RF frequency is carefully controlled to satisfy the Bragg angle matching over the mid-IR region. As a result, the micropulse-picking system can supply single and/or some FEL micropulses at an arbitrary repetition rate over the mid-IR region (equals 2 - 12 micrometers ) and can control the resulting peak power and average power in the ranges of approximately 1 - 2 MW and approximately 50 (mu) W - 20 mW, respectively.
Underwater laser beam welding was put forward to meet repairing of irradiated components and structures used in nuclear-power plant. Due to the interaction between laser beam and water, the laser-induced plasma or plume is rather complex than in atmosphere. This paper tries to monitor the stability of welding process by detecting the plume in underwater laser beam welding.
The content of the presented work is an experimental investigation in the border range between inert gas melt cutting and the plasma generation point. Based on spectral emission measurements, a photo detector with a wavelength sensitivity of 400 - 800 nm was found to be suitable for detecting the development of plasma. The detector was positioned behind a coated ZnSe mirror to measure the process signal coaxially. Parameter investigations show the influence of the cutting gas pressure and feed rate on the ignition of plasma. By increasing the feed rate to the maximum cutting speed, the amplitude of sudden plasma sparking increases. To avoid this effect, a concept for a closed-loop control system in order to eliminate the plasma sparking is presented.
Femtosecond laser-induced titanium plumes have been studied by emission spectroscopy and imaging, and time-of-flight measurements. The laser pulse is of around 100 fs (FWHM) duration and has a central wavelength of 800 nm. The sample was placed inside a high vacuum chamber. Time-resolved plume emission images were obtained by an ICCD camera. Compared to nanosecond laser- induced plumes, femtosecond laser-generated plumes are much more confined in space, which is due to the lack of direct laser energy absorption by the plume, as well as lower energy per pulse. The emission spectrum was studied with a combination of a monochromator and an ICCD camera. Individual emission lines from both titanium neutrals and ions were identified. Time-of-flight (TOF) experiment showed that extremely energetic ions were present in the plume. These high energies may affect the laser deposition of thin films. A TOF experiment using a nanosecond laser was carried out for comparison. Laser ablated craters were measured with an interferometric microscope, and ablation yield was expressed as a function of applied laser fluence. Two ablation regimes were identified for femtosecond laser ablation of titanium by time-of-flight and microscope measurements.
A molecular-dynamics thermal-annealing model is proposed to study the mechanisms of ablation induced in crystalline silicon by picosecond pulses. In accordance with the thermal annealing model, a detailed description of the microscopic processes resulting from the interaction of a 308 nm, 10 ps, Gaussian pulse with a Si(100) substrate has been embedded into a molecular- dynamics scheme. This was accomplished by explicitly accounting for carrier-phonon scattering and carrier diffusion. Above the predicted threshold energy for ablation, Fth equals 0.25 J/cm2, ablation is driven by subsurface superheating effects: intense heating by the pulse leads to the thermal confinement of the laser-deposited energy. As a result, the material is overheated up to its critical (spinodal) point and a strong pressure gradient builds up within the absorbing volume. At the same time, diffusion of the carriers in the bulk leads to the development of a steep temperature gradient below the surface. Matter removal is subsequently triggered by the relaxation the pressure gradient as a large--few tens of nm thick--piece of material is expelled from the surface.
Atomic kinetics and spectral modeling have revealed that level populations of plasma atoms in laser-ablated plumes may behave in a time-dependent manner, i.e. far from Local Thermodynamic Equilibrium, and that cascading population mechanisms can lead to long-lived atomic line emission. The time-scales associated to this phenomena and the interpretation of spectral data critically depend on the details of the atomic kinetics model and the quality of the rate coefficients. In order to generate accurate atomic data for neural atoms and low-charge ions present in plasma plumes, a semi-empirical techniques has been implemented in the Los Alamos atomic structure and electron scattering codes. This procedure has allowed neutrals with complex atomic structure--such as those atoms from elements often used in industrial applications--to be calculated with spectroscopic quality. Details of the atomic kinetics model for the case of a Li-Ag plasma plume and the rates generated with this new procedure are presented and discussed.
Research is underway to accelerate laser ablation plume ions for implantation (APII) into substrate. Ablation plasma ion implantation biases the deposition substrate by a large negative voltage pulse. APII has the advantages of direct acceleration and implantation of ions from metals or any other solid targets. This process is environmentally benign because it avoids the use of toxic gaseous precursors. Initial experiments are directed towards the implantation of iron ions into silicon substrates at negative voltages up to -10 kV.
The results of investigation into the use of UV-laser produced plasma for high-current commutation are summarized. It is shown that exciplex lasers can be used as efficient triggers for megavolt gas switches. If the laser spark length is comparable with the switch gap, the triggering delay time and jitter are independent on the arc position in the gap. In other cases, the shortest delay time was observed under focusing on the electrode surface. The plasma formed by exciplex lasers on the metal targets has been studied. The plasma formation thresholds and plasma expansion velocities have been measured.