Preparation of Materials for Deep Tissue Imaging with Slow Light

Ultrasound Optical Tomography (UOT) is a proposed technique, which combines ultrasound imaging and optical imaging in a new way to detect, e.g., blood oxygenation and tumors deep inside biological tissue, where the ultrasound provides an excellent spatial resolution, and the optical imaging gives a high contrast of different tissues according to optical properties. However, the challenges of this technique are it is hard to subtract the UOT signal from the background (diffracted light) since the UOT signal is much weaker than the background light and they are quite close in the frequency domain. Thus, a narrow band filter with a high suppression ratio is needed. Moreover, it is crucial for developing and evaluating UOT to make phantoms to mimic the properties of human tissues, and also to be able to measure the exact optical properties of the phantoms. Testing that the light propagation inside the phantoms matches Monte Carlo (MC) simulation is also necessary since Monte Carlo simulation can be used to simulate the realistic conditions of the UOT experiments in the future. In this thesis, the phantoms with a specific scattering coefficient and absorption coefficient were made by using diluted Indian ink, Intralipid 20%, and distilled water. Two spectroscopic absorption methods were used including direct absorption spectroscopy and photon time-of-flight spectroscopy. The direct absorption spectroscopy with 11% variation was used to validate the photon time-of-flight spectroscopy which can provide the scattering coefficient of the phantoms as well. With the help of the full control of the absorption coefficient and scattering coefficient, the measurement result of the light intensity transmitted through the phantoms as a function of scattering and absorption coefficient matched Monte Carlo simulations. Furthermore, a rare-earth doped crystal can be used as a spectral filter to improve the signal to background ratio (SNB). The spectral filter was burned and probed using a strongly absorbing polarization direction in the crystal. The burning procedure was decided based on simulations to give sharp edges and a good suppression between the transmission inside the center of the spectral hole and outside the spectral hole 1.5 MHz away. A 50.4 dB suppression for a 1 MHz spectral hole was achieved by optimizing the pulse parameters. The slow light effect generated inside the spectral filter can delay the UOT signal 2 μs from the background in the time domain. The lifetime of the spectral hole can be extended to more than half a minute by adding a small magnetic field of 10 mT. For the future, it is proposed that a better spectral hole with a higher suppression and shaper edges can be obtained by improving the hole burning technique. In addition, other absorbers and scatterers can also be investigated to make long-lived phantoms in the future.Are you curious about why you feel hot after lying outside for a while under the sunshine? When you work at night with the light on, do you wonder how the light bulb emits light? What’s more, have you seen the beautiful aurora in the north of Sweden? All of these happen due to the light-matter interaction! People are familiar with light, no matter whether it is natural light, e.g., the aurora or artificial light, e.g., LED light. When light meets different matters, different romantic stories (interaction) happen. This project tells a story between light and a crystal doped by rare-earth ions. However, the story only happens when light meets the matter under the right condition in a similar way as you cannot see aurora frequently. When light with a frequency close to the transition of rare-earth ions goes through the crystal, it will be absorbed. Then the rare-earth doped crystal acts as a band-pass filter which can have a variable narrow passband for the incident light. But how does it work? It will absorb the incident light but let the light with the same frequency as the passband go through. Hence, the crystals select the same frequency as its passband. How is this passband created? In brief, a passband is created inside a praseodymium (Pr) doped Y2SiO5 crystal with light which has the same frequency as the passband we want to create. The light is used to burn away the ions in the passband. Thus, when it is irradiated by the light at the same frequency, there are no longer any ions absorbing it (no story will happen). Moreover, this type of special filter will create another story, which is called ”slow light effect”. To make the story short, it is a time delay of the light like the way electrical pulses delayed in a band-pass filter in electrical circuits. Why do we need such filters? Well, our filter can slow down the light by 4 to 5 orders of magnitude! The slowed down light can be widely used in quantum memories, laser stabilization, and deep tissue imaging, etc