Ujitha Abeywickrema

Holographic assessment of self-phase modulation and blooming in a thermal medium.

The use of a low-power laser beam to characterize self-phase modulation (SPM) and bubble formation during thermal blooming (TB), as well as manipulation of the bubbles, is reported. First, a low-power 633 nm laser beam is used to characterize the induced refractive index profile during SPM of a focused 514 nm pump beam in absorbing liquid media, e.g., a solution of red dye in isopropyl alcohol. The induced phase change is also characterized using digital holography via the 633 nm source as the probe and reference. During TB at higher pump powers, bubble formation occurs in the liquid. Using a modified setup, which minimizes the effects of gravity, buoyancy, and convection, stable bubbles are generated. These are characterized using in-line digital holography with the 633 nm probe beam. It is shown that the bubble size depends on exposure time of the pump and that the bubble can be steered by moving a focused low-power laser beam. Finally, possible applications of these thermally generated bubbles are discussed.

A simple optical probing technique for nonlinearly induced refractive index

Self phase modulation is a nonlinear effect that is observed when a laser beam is focused on to a high-absorbing thermal medium. A regular tea sample in a plastic cuvette is used as the nonlinear absorbing sample. The change in the refractive index of the medium occurs due to the heat generated by the focused pump beam, which in turn changes the refractive index. In this paper, self phase modulation is investigated in different ways. An Ar-Ion laser of 514 nm is used as the pump beam and a 632 nm He-Ne laser is used as the probe beam. The probe beam is introduced from the opposite side of the pump beam. Ring patterns are observed from the each side of the sample. Regular far field ring patterns are observed from the pump beam, and two sets of rings are observed with the probe beam. The behaviors of these inner and outer rings are monitored for different pump powers. The steady state heat equation is solved to obtain an exact solution for the radial heat distribution and far field ring patterns are simulated using the Fresnel–Kirchhoff diffraction integral. Ring patterns are theoretically explained using simulations results, and compared with experimental observations. Finally, an interferometric setup using the low power He-Ne laser is also used to determine the induced change in refractive index. Results are compared with those obtained directly from self-phase modulation and from the probe beam method.

High-resolution topograms of fingerprints using multiwavelength digital holography

Abstract. Fingerprint analysis is a popular identification technique due to the uniqueness of fingerprints and the convenience of recording them. The quality of a latent fingerprint on a surface can depend on various conditions, such as the time of the day, temperature, and the composition of sweat. We first developed latent fingerprints on transparent and blackened glass slides by depositing 1000-nm-thick columnar thin films (CTFs) of chalcogenide glass of nominal composition Ge28Sb12Se60. Then, we used transmission-/reflection-mode multiwavelength digital holography to construct the topograms of CTF-developed fingerprints on transparent/blackened glass slides. The two wavelengths chosen were 514.5 and 457.9 nm, yielding a synthetic wavelength of 4.1624  μm, which is sufficient to resolve pores of depths 1 to 2  μm. Thus, our method can be used to measure the level-3 details that are usually difficult to observe with most other techniques applied to latent fingerprints.

Recursive method for phase retrieval using transport of intensity and its applications

Propagation of optical fields is governed by the Helmholtz equation or the paraxial wave equation. Transport of intensity is a noninterferometric method to find the phase of an object by recording optical intensities at different distances of propagation. The transport of intensity equation results from the imaginary part of the complex paraxial wave equation and is equivalent to the principle of conservation of energy. The real part of the paraxial wave equation yields the Eikonal equation in the presence of diffraction. The amplitude and phase of the optical field must therefore simultaneously satisfy both the real and imaginary parts of the paraxial wave equation during propagation. In this paper, we demonstrate, with illustrative examples, how to exploit this to retrieve the phase through recursive calculations of the phase and intensity. This is achieved using the transport of intensity equation, which is solved using standard techniques, and the real part of the paraxial wave equation, or the transport of phase equation, which is solved using a Gauss-Seidel iterative method. Examples include calculation of the imaged phase induced through self-phase modulation of a focused laser beam in a liquid and the imaged phase of light reflected from a surface, which yields the 3D surface profile.

Referenceless 3D reconstruction of amplitude objects embedded in a liquid

Transport of intensity is used to recover the image of an amplitude object embedded in a liquid heated by a focused laser beam. Our work can find applications in 3D imaging of debris in a fireball.

Characterization and application of bubbles during thermal blooming

When a highly absorbing thermal medium is heated with a focused laser pump beam, diffraction ring patterns can be observed due to self-phase modulation. It is further observed that when the laser power increases, the usual self-phase modulation diffraction patterns change due to formation of a bubble inside the thermal lens created by the focused beam. This phenomenon, called thermal blooming, is the next step to selfphase modulation. A stable bubble is formed using a focused laser beam, and the bubble is characterized using holograms made with a probe beam. A 532 nm Argon-Ion laser is used as the pump and a 633 nm low power He-Ne laser is used as the probe. The thermal medium comprises a mixture of a red dye and isopropyl alcohol. To minimize the optical effects arising from convection, the focused pump is introduced vertically into the liquid sample. The recorded in-line holograms are numerically reconstructed to determine the size and 3d shape of the bubbles. Bubble sizes are monitored as a function of the pump intensity. Once formed, the bubbles can be steered by mechanically deflecting the pump beam or any other laser beam. Finally, Ag nanoparticles are fabricated, examined, and introduced into the thermal medium. The presence of nanoparticle agglomeration around the thermally generated bubbles is tested using a focused probe beam at 405 nm corresponding to the absorption peak of the Ag nanoparticles due to plasmonic resonance. This technique should prove useful in drug delivery systems using nanoparticles agglomerated around microbubbles.

Dynamics of thermally generated microbubbles

Forces acting on thermally generated microbubbles are studied by deriving a general force model. Microbubbles can be steered by moving the laser beam. In previous work, it has been shown that the attractive thermo-capillary force on the microbubble is much larger than the repulsive optical force. Thus the microbubble is attracted toward the laser beam, which has been also observed experimentally. In this work, dynamics of a microbubble is studied by steering microbubbles with a known constant velocity. Thermocapillary force on the microbubble is calculated using a developed model which is based on the experimental velocity values and compared with the thermo-capillary force values obtained from the theory. The effect of the viscous and friction forces are also considered. Microbubbles are generated in a thin container as in our previous work and the container is placed on a motorized translation stage. After the bubble is generated and trapped by the focused laser beam the translational stage is moved with a known velocity until the microbubble is separated from the beam. Then the two thermo-capillary forces are compared as a function of the microbubble (translation stage) velocity.

Optical trapping of metallic nanoparticles using microbubbles

In our previous work, the induced force on a microbubble due to a focused laser beam has been studied by deriving a general optical force model. It has also been shown that the microbubble, without and with agglomerated metallic nanoparticles, can be steered using a low power laser beam. An equivalent optical force for a microbubble surrounded by nanoparticles has been also calculated and presented in our previous work. While only optical forces acting on the microbubbles and nanoparticles have so far been considered, in this work the effect of surface tension force is considered and calculated using the temperature gradient generated by the focused trapping laser beam and compared with optical forces. Experiments are performed to observe the behavior of the microbubbles in the vicinity of a focused laser beam for five different wavelengths.

Polarization vector signatures for target identification

Polarization measurements can be used to distinguish different objects. In this work, polarimetry has been used as a useful tool to differentiate between two geometries, viz., a cube and a cylinder, demonstrating that polarization “signatures” can be obtained from the reflection of passive lighting. A novel approach to characterizing geometries using vector-vector space has been demonstrated to be effective in differentiating between these two geometries and lends itself well to a statistical analysis that is applicable for computer generated target identification.

Optical steering of thermally generated microbubbles in a liquid for targeted metallic nanoparticle delivery

A novel mathematical model is developed to investigate the behavior of thermally generated microbubbles in the presence of optical radiation to understand the mechanism of their steering. Forces acting on a bubble are studied in detail using a general force model. It has been proposed that these microbubbles with agglomerated metallic nanoparticles can be used for targeted drug delivery. The model can be extended to include the steering of bubbles with agglomerated silver or gold nanoparticles on their surface.