Session B2: Optics

Diode Pumped Alkali Laser for Defense

This talk will review the renewed interest in laser directed energy weapons for military applications. Current research is focused on solid state, fiber and gas lasers. A relatively new gas laser is the diode pumped alkali laser (DPAL). Gas lasers have one advantage for high powers since the gain medium can be flowed to prevent excessive heating. The DPAL CW laser operates using either cesium, rubidium or potassium vapor and converts the output of many incoherent diode lasers into a high power coherent beam. The history and recent results in diode pumped alkali lasers will be reviewed. It is possible that the 21th century might see the actual application of the Buck Rodgers ray gun.   

Multiple-particle tracking in three dimensions via temporal focusing two-photon microscopy

Multiple-particle tracking in three dimensions is of great interest in researches of molecular dynamics and interactions in living cells. The spatial and temporal resolutions of the tracked particles are the key to the development of this technology. Here we present a method of three-dimensional multiple-particle tracking based on temporally focused two-photon microscopy. The z position information of each particle is encoded in the radii of the defocused images. Based on the images, algorithms are developed to reconstruct the 3D (x,y,z) positions of these particles. In our experiment, the long-term spatial localization precision can reach 50 nm in both axial and lateral dimensions with the help of stage system stability. We demonstrate its capability of tracking living cells by videotaping microbes swimming and reconstruct their trajectories in three dimensions. In addition, two-photon dual-color imaging is achieved by simultaneously exciting two types of fluorescent nanospheres mixed with ultrafast laser to demonstrate its potential applications on studies of molecular dynamics and interactions. This method provides a simple wide-field fluorescence imaging approach for deep multiple-particle 3D tracking.   

Scattering suppression, and optimization of extremely broadband plasmonic core-shell obscurants

Plasmonic resonances of the core-shells (CSs) are controlled by their shell and geometry of the core to produce a broadband extinction (Ext). In comparison to metallic planar films where the Ext is contributed by both absorption (Abs) and reflection, the shells' is originated mainly by Abs. The considered CSs made of gold fractals grown on precipitated calcium carbonate (PCC) and silica particles are chemically synthesized. The optimization includes different core sizes, shapes, and shells. The Mie scattering resonance of 780nm diameter silica at 560nm is found to be suppressed by 75{\%} and partially replaced by the Abs in the shell which results in a clear increase of the total transmission [1]. Our experiments were supported by the effective medium theory and show that light mostly goes through the epsilon-near-zero shell with a wavelength independent absorption rate. These plasmonic structures can be optimized for the broadband Ext using mass normalization. We found that the rich surface of the PCC is the best core for the fractal shells providing the highest mass normalized Ext up to 3 m2/g over extremely broad spectral range [2]. [1] V. C. de Silva et al. Opt. Mater. Exp., 5 (2015), pp. 2491--2500 [2] V. C. de Silva et al. J. Colloid Interface Sci.,~484, (2016), pp. 116-124   

Benchmarking the Response of a Detection System Designed to Measure Hyper-Rayleigh Scattering from Helium

We use Rayleigh-scattered 266nm and 400nm light to characterize the absolute efficiency of a high-throughput detection system designed to register photons scattered out the side of a laser focus.~ Our understanding of linear Rayleigh scattering provides a calibrated photon source that mimics expected nonlinear photoemission scattered out the side of an intense laser focus in a chamber backfilled with helium.~ These measurements help to place an upper limit on possible incoherent second-harmonic generation produced by intense 800 nm light in helium.~ We use incoherent third-harmonic emission, scattered out the side of the 800nm laser, as another benchmark comparison.~ At sufficiently low pressure, direct Hyper-Rayleigh third-order emission from individual atoms is expected to be more prevalent than the less-controllable re-scattering of beam-like coherent third harmonic buildup.~   

A photonic switching mechanism using polarized light incident on dielectric surfaces.

Our experimental studies indicate that a probe laser of energy $E_{L} $ assisted by a capacitor voltage of energy $E_{C} $ induces vibrations of the dipoles located on the dielectric surface at higher frequencies as compared with the case when the probe laser is alone. This finding is consistent with a scalar addition of energies and produces a shift in the energy: $E=E_{L} +E_{C} =\;\hslash \left( {\omega_{L} +\omega_{C} } \right)$ absorbed by dipoles, and implicitly, of the refraction index (n) of the dielectric surface. Experimentally, we analyze the interaction between light and certain dielectric surfaces using the relative permittivity ($\varepsilon _{R} )$ extracted from accurate measurements of refractive indices for light incident at Brewster angle [1]. We can show that the variation of $\varepsilon_{R} $, which is proportional with n$^{2}$, is due to a competition between an increase in the vibrational frequency of the dipoles on the dielectric surface interacting with the probe laser and an increase in the polarization of the electric dipoles [2]. We find that heavy silica-based glasses, such as flint glasses do not allow the dipoles to rotate freely at low capacitor voltages due to the presence of massive PbO molecules. Therefore, lesser dense dielectrics, such as crown glasses, could be more efficient photonic switching devices. [1] C. Bahrim and W. Hsu, \textit{American Journal of Physics }\textbf{77}, 337 (2009). [2] C. Bahrim,~et al., \textit{Journal of Applied Mathematics and Physics}\textbf{\textit{~}}\textbf{2, }1105 (2014).   

Interference effects in Photo-elastic modulator when using highly coherent light.

Photo-elastic modulators (PEMs) are often used in ellipsometers and Magneto-Optical Kerr tracers to improve the signal to noise ratio. The light beam's state of polarization is modulated by a standing sound wave in the PEM's optical head allowing measurements up to the shot noise limit. When using a PEM with a coherent laser source the amplitude of the beam is modulated by an interference effect in the PEM's optical head. Here the effect of the PEM tilt angle and incident polarization is studied for a single axis PEM. The AC amplitude of the detector signal is decreasing with PEM tilt angle. Its dc, 1$\omega $, and 2$\omega $ vary periodically as a function of PEM tilt angle. Amplitude and period decrease for larger PEM angles. As the PEM angle dependence of the 1$\omega $ and 2$\omega $ signal are a quarter wave phase shifted it is not possible to null both of them simultaneously. At small relative retardation (0.25) and perpendicular incidence, the 1$\omega $ or 2$\omega $ can be nulled by adjusting the polarization angle. Direct observations of the detector signal indicate that the interference is larger for light polarized perpendicular to the modulation direction. This work was supported by DOD grant (HBCU/MI grant W911NF-15-1-0394).   

Two-photon flow cytometer with laser scanning Airy beams

Flow cytometry is an important technique in biomedical discovery. In the last ten years in vivo flow cytometers based on one-photon or two-photon excited fluorescence have been developed. One drawback of laser beam scanning two-photon flow cytometer is that two-photon excitation volume is fairly small due to short Rayleigh range of focused Gaussian beam. Hence, the sampling volume is much smaller than one-photon flow cytometer, which makes it challenging to count or detect rare circulating cells in vivo.Non-diffracting beams have narrow intensity profiles with an effective spot size (FWHM) as small as several wavelengths, making them comparable to Gaussian beams. The trade-off of using Airy beams rather than Gaussian beam is the fact that Airy beams have side lobes that contribute to background noise. Two-photon excitation can reduce this noise, as the excitation efficiency is proportional to intensity squared. Therefore, we developed a two-photon flow cytometer using scanned Airy beams to form a light sheet that could be used to intersect the blood vessel. The set up can successfully detect and count flowing micro beads and tumor cells in micro channel. According to our knowledge, this is the first application of Airy beams in flow cytometry.   

Optical properties of a reversible phase transition of sol-gel-produced La2NiO4 thin films

Many materials undergo ultrafast solid-solid phase transitions characterized by unusual changes in their physical properties across their phase transition. There is some evidence that the class of materials that take the form $\rm{Ln}_{2}\rm{MO}_{4+\delta}$ (Ln = La, Nd, Sm, Eu; M = Ti, Co, Ni, Cu) may exhibit such a transition when the materials are heated above room temperature and at certain levels of oxygen saturation ($\delta$). Earlier studies of $\rm{La}_{2}\rm{NiO}_{4+\delta}$ have shown a large change in conductivity near 250 degrees C, but the optical properties of thin films of materials fitting this pattern have gone virtually unmeasured. Here, we present the results of several electrical and optical experiments characterizing sol-gel-produced thin films of two of these materials that explore the region between 400 nm and 2.4 µm at temperatures between 20 and 400 degrees C.   

Prevention of oxidation in aluminum for space-based telescopes using fluorides, metals, and polymers

In the 2020 decadal review, NASA will likely discuss plans for the Large UV-Optical-IR (LUVOIR) space-based telescope. One of the purposes for the telescope is to aid in the detection and characterization of planets in other solar systems. In an effort to extend the range of reflectance into the far-UV, we propose using pure aluminum as a mirror. Aluminum has up to \textasciitilde 95{\%} reflectance up to \textasciitilde 15 eV and is almost completely transparent at higher energies. Barrier layers such as fluorides, polymers, and volatile metals prevent aluminum oxidation, which occurs almost instantly and causes the mirror to lose reflectance at \textasciitilde 6.2 eV. MgF$_{\mathrm{2}}$ has extended reflectance to \textasciitilde 12.4 eV. Polymers and volatile metals are removed in vacuum by hydrogen sputtering and re-evaporation. Hydrogen sputtering successfully removed polymers without interacting with the aluminum surface. Preliminary results of using Cd and Zn for re-evaporation show poor adhesion and that the method requires some revision.