Quadratic Spatial Soliton Interactions

Quadratic spatial soliton interactions were investigated in this Dissertation. The first part deals with characterizing the principal features of multi-soliton generation and soliton self-reflection. The second deals with two beam processes leading to soliton interactions and collisions. These subjects were investigated both theoretically and experimentally. The experiments were performed by using potassium niobate (KNBO3) and periodically poled potassium titanyl phosphate (KTP) crystals. These particular crystals were desirable for these experiments because of their large nonlinear coefficients and, more importantly, because the experiments could be performed under non-critical-phase-matching (NCPM) conditions. The single soliton generation measurements, performed on KNBO3 by launching the fundamental component only, showed a broad angular acceptance bandwidth which was important for the soliton collisions performed later. Furthermore, at high input intensities multi-soliton generation was observed for the first time. The influence on the multi-soliton patterns generated of the input intensity and beam symmetry was investigated. The combined experimental and theoretical efforts indicated that spatial and temporal noise on the input laser beam induced multi-soliton patterns. Another research direction pursued was intensity dependent soliton routing by using of a specially engineered quadratically nonlinear interface within a periodically poled KTP sample. This was the first time demonstration of the self-reflection phenomenon in a system with a quadratic nonlinearity. The feature investigated is believed to have a great potential for soliton routing and manipulation by engineered structures. A detailed investigation was conducted on two soliton interaction and collision processes. Birth of an additional soliton resulting from a two soliton collision was observed and characterized for the special case of a non-planar geometry. A small amount of spiraling, up to 30 degrees rotation, was measured in the experiments performed. The parameters relevant for characterizing soliton collision processes were also studied in detail. Measurements were performed for various collision angles (from 0.2 to 4 degrees), phase mismatch, relative phase between the solitons and the distance to the collision point within the sample (which affects soliton formation). Both the individual and combined effects of these collision variables were investigated. Based on the research conducted, several all-optical switching scenarios were proposed

Temperature mapping using photoacoustic and thermoacoustic tomography

Photoacoustic (PA) and thermoacoustic (TA) effects are based on the generation of acoustic waves after tissues absorb electromagnetic energy. The amplitude of the acoustic signal is related to the temperature of the absorbing target tissue. A combined photoacoustic and thermoacoustic imaging system built around a modified commercial ultrasound scanner was used to obtain an image of the target's temperature, using reconstructed photoacoustic or thermoacoustic images. To demonstrate these techniques, we used photoacoustic imaging to monitor the temperature changes of methylene blue solution buried at a depth of 1.5 cm in chicken breast tissue from 12 to 42 °C. We also used thermoacoustic imaging to monitor the temperature changes of porcine muscle embedded in 2 cm porcine fat from 14 to 28 °C. The results demonstrate that these techniques can provide noninvasive real-time temperature monitoring of embedded objects and tissue

Tissue temperature monitoring using thermoacoustic and photoacoustic techniques

Real-time temperature monitoring with high spatial resolution (~1 mm) and high temperature sensitivity (1 °C or better) is needed for the safe deposition of heat energy in surrounding healthy tissue and efficient destruction of tumor and abnormal cells during thermotherapy. A temperature sensing technique using thermoacoustic and photoacoustic measurements combined with a clinical Philips ultrasound imaging system (iU22) has been explored in this study. Using a tissue phantom, this noninvasive method has been demonstrated to have high temporal resolution and temperature sensitivity. Because both photoacoustic and thermoacoustic signal amplitudes depend on the temperature of the source object, the signal amplitudes can be used to monitor the temperature. The signal is proportional to the dimensionless Grueneisen parameter of the object, which in turn varies with the temperature of the object. A temperature sensitivity of 0.5 °C was obtained at a temporal resolution as short as 3.6 s with 50 signal averages

Quadratic soliton collisions

The details of two soliton collision processes were investigated in detail in a 1 cm long periodically poled KTP crystal for the case when the solitons were excited by inputting only the fundamental beam. The effects on the collision outcomes of the distance of the collision into the sample, collision angle and phase mismatch were measured for different relative phases between the input beams. At small angles ( around 0.4(0)) fusion, repulsion and energy transfer processes were observed, while at the collision angles approaching 3.2(0) the two output soliton beams were essentially unaffected by the interaction. The phase mismatch was varied from 3.5 to - 1.5pi for the 0.4(0) collision angle case. The output soliton separation at p input phase difference showed strongly asymmetric behavior with phase mismatch. In general, the measurements indicate a decrease in the interaction strength with increasing phase mismatch. All collision processes were performed in the vicinity of a non-critical phase matching

Performance characterization of an integrated ultrasound, photoacoustic, and thermoacoustic imaging system

We developed a novel trimodality system for human breast imaging by integrating photoacoustic (PA) and thermoacoustic (TA) imaging techniques into a modified commercial ultrasound scanner. Because light was delivered with an optical assembly placed within the microwave antenna, no mechanical switching between the microwave and laser sources was needed. Laser and microwave excitation pulses were interleaved to enable PA and TA data acquisition in parallel at a rate of 10 frames per second. A tube (7 mm inner diameter) filled with oxygenated bovine blood or 30 mM methylene blue dye was successfully detected in PA images in chicken breast tissue at depths of 6.6 and 8.4 cm, respectively, for the first time. The SNRs at these depths reached ∼24 and ∼15  dB, respectively, by averaging 200 signal acquisitions. Similarly, a tube (13 mm inner diameter) filled with saline solution (0.9%) at a depth of 4.4 cm in porcine fat tissue was successfully detected in TA images. The PA axial, lateral, and elevational resolutions were 640 μm, 720 μm, and 3.5 mm, respectively, suitable for breast cancer imaging. A PA noise-equivalent sensitivity to methylene blue solution of 260 nM was achieved in chicken tissue at a depth of 3.4 cm

Second-harmonic generation tuning curves with narrow, high-intensity beams for quasiphase-matched potassium titanyl phosphate

The tuning curves with temperature and incidence angle for second-harmonic generation were asymmetrically distorted and broadened with increasing intensity for narrow input fundamental beams in periodically poled KTiOPO4. Multiple phenomena including mutual self-focusing of the fundamental and harmonic, cascading, quadratic soliton generation, and artificially induced walk-off (for light incidence away from the poling axis) contribute, in good agreement with theory

Quantification of optical absorption coefficients from acoustic spectra with photoacoustic tomography

Optical absorption is closely associated with many physiologically important parameters, such as the concentration and oxygen saturation of hemoglobin, and it can be used to quantify the concentrations of non-fluorescent molecules. We introduce a method to quantify the absolute optical absorption based upon the acoustic spectra of photoacoustic (PA) signals. This method is self-calibrating and thus insensitive to variations in optical fluence. Factors such as the detection system bandwidth and acoustic attenuation can affect the quantification but can be canceled by measuring the acoustic spectra at two optical wavelengths. This method has been implemented on various PA systems, including optical-resolution PA microscopy, acoustic-resolution PA microscopy, and reconstruction based PA array systems. We quantified the optical absorption coefficients of phantom samples at various wavelengths. We also quantified the oxygen saturation of hemoglobin in live mice

In vivo three-dimensional photoacoustic imaging based on a clinical matrix array ultrasound probe

We present an integrated photoacoustic and ultrasonic three-dimensional (3-D) volumetric imaging system based on a two-dimensional (2-D) matrix array ultrasound probe. A wavelength-tunable dye laser pumped by a Q-switched Nd:YAG laser serves as the light source and a modified commercial ultrasound imaging system (iU22, Philips Healthcare) with a 2-D array transducer (X7-2, Philips Healthcare) detects both the pulse-echo ultrasound and photoacoustic signals. A multichannel data acquisition system acquires the RF channel data. The imaging system enables rendering of co-registered 3-D ultrasound and photoacoustic images without mechanical scanning. The resolution along the azimuth, elevation, and axial direction are measured to be 0.69, 0.90 and 0.84 mm for photoacoustic imaging. In vivo 3-D photoacoustic mapping of the sentinel lymph node was demonstrated in a rat model using methylene blue dye. These results highlight the clinical potential of 3-D PA imaging for identification of sentinel lymph nodes for cancer staging in humans

Tissue temperature monitoring using thermoacoustic and photoacoustic techniques

Real-time temperature monitoring with high spatial resolution (~1 mm) and high temperature sensitivity (1 °C or better) is needed for the safe deposition of heat energy in surrounding healthy tissue and efficient destruction of tumor and abnormal cells during thermotherapy. A temperature sensing technique using thermoacoustic and photoacoustic measurements combined with a clinical Philips ultrasound imaging system (iU22) has been explored in this study. Using a tissue phantom, this noninvasive method has been demonstrated to have high temporal resolution and temperature sensitivity. Because both photoacoustic and thermoacoustic signal amplitudes depend on the temperature of the source object, the signal amplitudes can be used to monitor the temperature. The signal is proportional to the dimensionless Grueneisen parameter of the object, which in turn varies with the temperature of the object. A temperature sensitivity of 0.5 °C was obtained at a temporal resolution as short as 3.6 s with 50 signal averages

Temperature mapping using photoacoustic and thermoacoustic tomography

Photoacoustic (PA) and thermoacoustic (TA) effects are based on the generation of acoustic waves after tissues absorb electromagnetic energy. The amplitude of the acoustic signal is related to the temperature of the absorbing target tissue. A combined photoacoustic and thermoacoustic imaging system built around a modified commercial ultrasound scanner was used to obtain an image of the target's temperature, using reconstructed photoacoustic or thermoacoustic images. To demonstrate these techniques, we used photoacoustic imaging to monitor the temperature changes of methylene blue solution buried at a depth of 1.5 cm in chicken breast tissue from 12 to 42 °C. We also used thermoacoustic imaging to monitor the temperature changes of porcine muscle embedded in 2 cm porcine fat from 14 to 28 °C. The results demonstrate that these techniques can provide noninvasive real-time temperature monitoring of embedded objects and tissue