Thermally Induced Optical Reflection of Sound (THORS) is a novel technique that allows a user to optically manipulate sound waves in a medium by photothermally exciting that medium with infrared light, thereby creating a transient barrier due to the changes in compressibility of the medium. This phenomenon offers numerous potential applications in fields as varied as acoustic suppression and reflection, photoacoustic spectroscopy and imaging, and ultrasonic waveguiding. Previous results have revealed maximum suppression efficiencies as great as 72 ± 5% using a continuous THORS barrier system with a multi-pulsed carbon monoxide laser operating at 5.5 ± 0.25 μm and 1 ms pulses. To greater understand the spatial variations in medium compressibility between the excited medium and the surrounding environment, we have employed Raman imaging to visualize these barriers.
Thermally-induced optical reflection of sound (THORS) allows for the manipulation of sound waves without the need for traditional acoustically engineered structures. By photo-thermally exciting a medium, with infrared laser light, a barrier can be formed due to abrupt changes in compressibility of the excited medium. In this work, we demonstrate for the first time, the ability to generate THORS barriers in ambient air. Additionally, the temporal dynamics of THORS barriers, in ambient air, were characterized by using ultrasonic transducers.
During the past several decades physical vapor transport (PVT) method has been extensively used for developing laser and electronic and optical sensor materials especially for incongruent and high vapor materials. Extensive careful studies of the NASA Marshall Space Flight Center on ZnSe growth by PVT has demonstrated that both thermal and solutal convection play very important roles on the quality of crystals and can be controlled by microgravity experiments. In case, the growth is performed by sputtering or systems such as DENTON, it is very difficult to control fluid flow and both thermal and solutal convective flows. We have demonstrated that by controlling the transport path, temperature of substrate and source and using purified source micron size thick ness can be achieved. We will present the experimental results of pure and doped lead selenide (PbSe) which demonstrated various morphologies and bandgap based on size of particles based on growth condition.
In this work, we describe the phenomenon, Thermally-induced Optical Reflection of Sound (THORS), and how it can be used to optically steer acoustic waves around a 90 degree corner of a physical obstruction, where observed acoustic amplitudes are increased by a factor of 30. In addition, we discuss the introduction of ultrasonic waves to the THORS phenomenon, and preliminary results for THORS barriers generated in ambient air, using a 5.3-5.7 μm CO laser source.The manipulation and guiding of sound waves have typically required the use of physical barriers for the reflection of an incident pressure wave. With the manipulation of acoustic waves being critical for many applications in scientific and engineering fields, including subsurface tissue imaging, photoacoustic sensing, secure communications, acoustic stealth technology, and acoustic design engineering; the requirement for physical barriers often represents a significant limitation. The recently discovered phenomenon THermally-induced Optical Reflection of Sound (THORS), provides the ability to generate acoustically reflective barriers, in air, by exciting media in the path of an IR laser beam, causing abrupt changes in compressibility between the excited and surrounding media. In this work, we demonstrate the ability to efficiently reflect sound waves around physical obstructions using a laser. Additionally, this work demonstrates the ability to also manipulate ultrasonic waves via THORS barriers, where the reflection and suppression of ultrasonic pulses in the frequency range of 120-300 kHz are shown. Finally, preliminary results demonstrating the ability to employ THORS in ambient air using water vapor as the absorbing media and a 5.5 μm CO laser beam for excitation.
The ability to precisely control and manipulate acoustic waves can be highly limiting in applications and environments where placement of physical barriers for acoustic steering cannot be employed (e.g. tissues, air, etc.) In this work, we describe the ability to generate acoustic waveguides via thermally-induced optical reflection of sound (THORS) for the manipulation of acoustic waves in free space (i.e., air). Abrupt, density barriers are formed by photothermally depleting the sample in a laser beam’s path via photothermal processes, resulting in sharp differences in compressibility and significant acoustic reflection (greater than 30%). Optical waveguiding of sound can be achieved by generating THORS channels with a cylindrical (ring shaped) laser beam. By containing the acoustic waves inside a THORS cylindrical channel, a dramatically reduced acoustic decay profile of 1/r0.6 with distance is achieved. Additionally, we describe the effects that optical modulation frequency of the THORS channel has on the efficiency of acoustic waveguiding. We also show how external acoustic waves, incident to a THORS channel are suppressed, increasing the signal-to-background ratio of the internally waveguided acoustic signals. Optical waveguiding of acoustic waves offers a new paradigm in the manipulation of sound over extended distances, providing potentially significant improvements to photoacoustic sensing, secure communications, and many other applications.
The requirement for solid, rigid structures for the ability to precisely control and manipulate acoustic waves can be highly limiting in applications such as biomedical imaging where sound manipulation and control through such interfaces cannot easily be employed. In this work, we describe the ability to generate optically induced reflective sound barriers and waveguides. These barriers are generated by creating abrupt density barriers via photothermal depletion of the transport medium along the path of a laser beam, causing sharp differences in compressibility, resulting in significant acoustic reflection. Using this technique, acoustic reflection efficiencies of 30% have been demonstrated. Furthermore, employing multiple optically induced acoustic reflective barriers sequentially can result in complete suppression of incident acoustic sound wave transmission.
In addition to optically induced acoustic suppression, optical waveguiding can also be achieved using cylindrical, ring shaped laser beams. By containing the acoustic waves inside the cylindrical channel, dramatic improvements in acoustic transmission can be achieved. Optical waveguding of acoustic signals offers a new paradigm in the manipulation of sound over extended distances in various media, providing potentially significant improvements to photoacoustic sensing and imaging in many applications
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