Physics Commons^™

Magnetic-Field-Induced Alignment-To-Orientation Conversion In Sodium, G. Schinn, Xianming Han

Xianming Han

We report a detailed investigation of excited-state alignment-to-orientation conversion in the presence of an external magnetic field. This counterintuitive phenomenon occurs under intermediate-coupling conditions. A weak, linearly polarized, cw laser beam was used to excite and align the Na 3P3/2 state in an atomic beam along the z direction. The degree of circular polarization of the resulting fluorescence was detected along the z direction as a function of magnetic-field strength. The spectrally integrated transitions originating from individual F levels of the 3S1/2 state yield a maximum circular-polarization fraction of ∼40%; integrating the circular polarization over all the allowed 3S1/2-3P3/2 transitions …

Collision Induced Superfluorescence, A. Kumarakrishnan, Siddharaj Chudasama, Xianming Han

Xianming Han

We have studied superfluorescence (SF) in Ca vapor evolving on the 3d4s3DJ-4s4p3PJ−1 transitions at 1.9 mm by exciting the 4s21S0-4s4p1P1 with a pulsed dye laser. SF is generated following population transfer by spinchanging collisions with an inert gas Ar from the 4s4p1P1 and 3d4s1D2 levels. We show for the first time to our knowledge that the time delay for SF evolution follows the 1/ÎN dependence expected for the case of uniform excitation of the vapor column by collisional transfer. Here, N is the number of participating atoms that was measured directly from the photon yield. The measured photon yield for …

Diamond Growth Reactor Chemistry And Film Nucleation Enhancement Using Chlorinated Hydrocarbons, James Chenault, Charles Feigerle, Xianming Han, Robert Shaw

Xianming Han

The chemistry of diamond film growth from chlorinated hydrocarbons has been investigated using a hot filament reactor coupled to an orifice sampling mass spectrometer. The relative concentrations of the species present near the growth surface have been determined as a function of filament temperature for dilute mixtures of CH4, CH3Cl, CH2Cl2 and CHCl3 in H2. Mass spectral analysis indicated that chlorinated hydrocarbons are sequentially dechlorinated in the presence of hydrogen at moderate reactor temperatures. A dark film was deposited on all surfaces of the reactor during studies of this dechlorination of CHCl3. Raman analysis indicated that these deposits are small …

Collisional Dynamics Of The First Excited States Of Neon In The 590-670 Nm Region Using Laser Optogalvanic Spectroscopy, Xianming Han, M. Su, C. Haridass, P. Misra

Xianming Han

A mathematical rate equation model, incorporating the various processes contributing to the generation of optogalvanic signals in a discharge plasma, has been used to analyze the time-resolved waveforms of neon in the wavelength region 590–670 nm. Amplitudes, decay rates and the appropriate instrumental time constant have been determined using a non-linear least-squares fit of the observed time-resolved optogalvanic waveforms.

Investigations Of Superfluorescent Cascades, A. Kumarakrishnan, Xianming Han

Xianming Han

We report our studies of superfluorescent cascades in atomic calcium which result from two-photon excitation of several levels reasonably close to the ionization limit. We have observed significant conversion efficiencies for some of these transitions which result in subnanosecond pulses particularly in the visible wavelengths. We report the discovery of a novel two-photon scattering mechanism which could prove to be a useful method for determining collisional broadening rates. In addition, a hyper Raman transition near 17 μm is discovered which appears to be a promising candidate for a tunable source.

Production And Diagnosis Of A Highly Spin-Polarized Na Beam, G. Schinn, Xianming Han, A. Gallagher

Xianming Han

We describe optically pumping a beam of sodium atoms to >96% mS and >92% mS, mI state selection. (We have accurately measured the population of every mS, mI state in the optically pumped beam.) For the optical pumping both ground hyperfine states are pumped, using single-mode cw dye-laser radiation tuned to the 3S1/2–3P1/2 transition that is phase modulated in a LiTaO3 crystal to produce first-order sidebands at approximately the 1772-MHz hyperfine splitting of the ground state. The z-directed optical pumping is performed in a z-directed magnetic field of ~5 G. The state-selected atoms then move, in ~1 cm, into an …

Local Heating With Lithographically Fabricated Plasmonic Titanium Nitride Nanoparticles, Urcan Guler, Justus Ndukaife, Gururaj Naik, Agbai Nnanna, Alexander Kildishev, V. Shalaev, Alexandra Boltasseva

U. Guler

Titanium nitride is considered a promising alternative plasmonic material and is known to exhibit localized surface plasmon resonances within the near-infrared biological transparency window. Here, local heating efficiencies of disk-shaped nanoparticles made of titanium nitride and gold are compared in the visible and near-infrared regions numerically and experimentally with samples fabricated using e-beam lithography. Results show that plasmonic titanium nitride nanodisks are efficient local heat sources and outperform gold nanodisks in the biological transparency window, dispensing the need for complex particle geometries.

Accelerating Diffraction-Free Beams In Photonic Lattices, K. Makris, I. Kaminer, Ramy El-Ganainy, N. Efremidis, Zhigang Chen, M. Segev, Demetrios Christodoulides

Ramy El-Ganainy

We study nondiffracting accelerating paraxial optical beams in periodic potentials, in both the linear and the nonlinear domains. In particular, we show that only a unique class of z-dependent lattices can support a true accelerating diffractionless beam. Accelerating lattice solitons, autofocusing beams and accelerating bullets in optical lattices are systematically examined.

Enhancing Optical Isolator Performance In Nonreciprocal Waveguide Arrays, Miguel Levy, Turhan Carroll, Ramy El-Ganainy

Ramy El-Ganainy

We investigate the operation of optical isolators based on magneto-optics waveguide arrays beyond the coupled mode analysis. Semi-vectorial beam propagation simulations demonstrate that evanescent tail coupling and the effects of radiation are responsible for degrading the device’s performance. Our analysis suggests that these effects can be mitigated when the array size is scaled up. In addition, we propose the use of radiation blockers in order to offset some of these effects, and we show that they provide a dramatic improvement in performance. Finally, we also study the robustness of the system with respect to fabrication tolerances using the coupled mode …

Observation Of Accelerating Wannier-Stark Beams In Optically Induced Photonic Lattices, Xinyuan Qi, Konstantinos Makris, Ramy El-Ganainy, Peng Zhang, Jintao Bai, Demetrios Christodoulides, Zhigang Chen

Ramy El-Ganainy

We generate optical beams analogous to the Wannier–Stark states in semiconductor superlattices and observe that the two main lobes of the WS beams self-bend (accelerate) along two opposite trajectories in a uniform one-dimensional photonic lattice. Such self-accelerating features exist only in the presence of the lattice and are not observed in a homogenous medium. Under the action of nonlinearity, however, the beam structure and acceleration cannot be preserved. Our experimental observations are in qualitative agreement with theoretical predictions.

The Sign Learning Kink (Silk) Based Quantum Monte Carlo (Qmc), Xiaoyao Ma, Frank Loffler, Randall W. Hall, Karol Kowalski, Mark Jarrell, Juana Moreno

Randall W. Hall

Transport Properties Of The La1−Xcaxmno3 (0.5 ≤ X < 1), H. Zhou, R. Zheng, G. Li, S. Feng, Xiaojuan Fan, X. Lia

Xiaojuan Fan

The transport properties of the La1−xCaxMnO3 (0.5 ≤ x < 1) system in magnetic fields up to 14 T were studied. We found that the relationship between the charge ordering temperature TCO and Mn4+ content nMn4+ obeys the formula TCO/Tmax = 1−a(nMn4+ −n0)2, here n0 and a are constants and Tmax is the maximum of TCO. For x = 0.65, TCO arrives at the maximum value of 249.5 K in zero magnetic field, while the charge ordered (CO) state is most stable around x = 0.75. For x = 0.5 when H < 6 T the resistivity displays Mott’s variable-range hopping (VRH) behavior, when 6 < H < 12 T it is suggested that two kinds of conduction mechanism, i.e., VRH and magnetic polarons, coexist in the material, and when H > 12 T the resistivity shows metallic-like behavior and the transport mechanism is attributed to coexistence of magnetic polarons and free carriers. For x = 0.95, the conduction mechanism accords with the coexistence of VRH and magnetic polarons.

Interaction Of Ultrashort-Laser Pulses With Induced Undercritical Plasmas In Fused Silica, Jeremy Gulley, Sebastian Winkler, William Dennis, Carl Liebig, Razvan Stoian

Jeremy R. Gulley

Ultrafast light-material interactions near the damage threshold are often studied using postmortem analysis of damaged dielectric materials. Corresponding simulations of ultrashort pulse propagation through the material are frequently used to gain additional insight into the processes leading to such damage. However, comparison between such experimental and numerical results is often qualitative, and pulses near to but not exceeding the damage threshold leave no permanent changes in the material for postmortem analysis. In this article, a series of experiments is presented that measures the near- and far-field properties of a 140-fs laser pulse after propagation through a fused silica sample in …

Simultaneous Quantum-State Measurements Using Array Detection, Andrew Dawes, Mark Beck

Andrew M C Dawes

We have simultaneously measured the quantum states of two different spatial modes of the same optical beam by performing quantum-state tomography with an array detector. Both modes are well described by coherent states, but the projection of the signal onto the local oscillator mode contains a mean of 0.09 photons, while a more complicated mode has a mean of 4.3 photons. This demonstrates that for this particular mode the effective detection efficiency when using array detection is over 40 times greater than when using single detectors.

Optical Switching With Cold Atoms, Andrew Dawes

Andrew M C Dawes

A Viewpoint on: Efficient All-Optical Switching Using Slow Light within a Hollow Fiber M. Bajcsy, S. Hofferberth, V. Balic, T. Peyronel, M. Hafezi, A. S. Zibrov, V. Vuletic, and M. D. Lukin Phys. Rev. Lett. 102, 203902 (2009) – Published May 18, 2009

Dynamical Signature Of The Mott-Hubbard Transition In Ni(S,Se)(2), Yevgeniya Zastavker, Anke Husmann, Deborah Jin, Thomas Rosenbaum, X Yao, J Honig

Yevgeniya V. Zastavker

The transition metal chalcogenide Ni(S,Se)2 is one of the few highly correlated, Mott-Hubbard systems without a strong first-order structural distortion that normally cuts off the critical behavior at the metal-insulator transition. The zero-temperature (T) transition was tuned with pressure, and significant deviations were found near the quantum critical point from the usual T1/2 behavior of the conductivity characteristic of electron-electron interactions in the presence of disorder. The transport data for pressure and temperature below 1 kelvin could be collapsed onto a universal scaling curve.

Modeling Free-Carrier Absorption And Avalanching By Ultrashort Laser Pulses, Jeremy Gulley

Jeremy R. Gulley

In the past decade it was demonstrated experimentally that negatively-chirped laser pulses can lower the surface LIDT for wide band-gap materials by decreasing the number of photons required for photoionization on the leading edge of the pulse. Similarly, simulations have shown that positively-chirped pulses resulting from selffocusing and self-phase modulation in bulk dielectrics can alter the onset of laser-induced material modifications by increasing the number of photons required for photoionization on the leading edge of the pulse. However, the role of multi-chromatic effects in free-carrier absorption and avalanching has yet to be addressed. In this work a frequency-selective model of …

Laser Excited Population Redistribution In The 2p^53p Multiplet In Neon, Naveed Piracha, K. Duncan-Chamberlain, R. Ali, M. Kalyar, M. Mehmood, R. Ahmed, M. Baig

Naveed K. Piracha

Using neon hollow cathode lamp and employing laser optogalvanic technique, we have studied population redistribution in the 2p^53p multiplet. The spectra recorded in the laser energy region of 23300cm^-^1 -23600cm^-^1 show transitions originating from both the laser excited levels as well as from an adjacent level whose population builds up as a result of collisional deactivation. Employing the optical delay technique, we have been able to extract decay rates associated with the collisional population mixing of the p levels.

Monte Carlo Simulation Of The Spatial Resolution And Depth Sensitivity Of Two-Dimensional Optical Imaging Of The Brain, Peifang Tian, Anna Devor, Sara Sakadžić, Anders Dale, David Boas

Peifang Tian

Absorption or fluorescence-based two-dimensional (2-D) optical imaging is widely employed in functional brain imaging. The image is a weighted sum of the real signal from the tissue at different depths. This weighting function is defined as “depth sensitivity.” Characterizing depth sensitivity and spatial resolution is important to better interpret the functional imaging data. However, due to light scattering and absorption in biological tissues, our knowledge of these is incomplete. We use Monte Carlo simulations to carry out a systematic study of spatial resolution and depth sensitivity for 2-D optical imaging methods with configurations typically encountered in functional brain imaging. We …

Frequency Dependence In The Initiation Of Laser-Induced Damage, Jeremy Gulley

Jeremy R. Gulley

Numerous studies have investigated the role of photoionization in ultrafast laser-induced damage of bulk dielectrics. This study examines the role of spectral width and instantaneous laser frequency in laser-induced damage using a frequency dependent multiphoton ionization model and numerical simulation of an 800 nm laser pulse propagating through fused silica. When the individual photon wavelengths are greater than 827 nm, an additional photon is required for photoionization, reducing the probability of the event by many orders of magnitude. Simulation results suggest that this frequency dependence may significantly affect the processes of laser-induced damage and filamentation.

Ultrashort-Pulse Propagation Through Free-Carrier Plasmas, Jeremy Gulley, William Dennis

Jeremy R. Gulley

The past decade has seen frequent use of a modified nonlinear Schrödinger equation to describe ultrashort pulse propagation in materials where free-carrier plasmas are present. The optical contribution from the resulting free-current densities in this equation is often described using a classical Drude model. However, the ultimate form of this contribution in the modified nonlinear Schrödinger equation is somewhat inconsistent in the literature. We clarify this ambiguity by deriving the modified nonlinear Schrödinger equation from the classical wave equation containing a free-current density contribution. The Drude model is then used to obtain an expression for the complex free-carrier current density …

Measurements Of Electron Density, Temperature And Photoionization Cross Sections Of The Excited States Of Neon In A Discharge Plasma, Shaukat Mahmood, Nek Shaikh, M. Kalyar, Naveed Piracha, M. Baig

Naveed K. Piracha

In the present work emission and absorption spectroscopy have been used to determine the plasma parameters of neon in a hollow cathode discharge lamp. The excitation temperature is determined using the intensity ratio method and Boltzmann’s plot method whereas the electron density is determined from the Stark broadening of the spectral lines. The behavior of the optogalvanic signal as a function of laser energy has been studied for three transitions from the 2p53s [1/2]2 metastable state following ΔJ=ΔK=0, ±1 dipole selection rules. The saturation technique has been used to measure the photoionization cross section from three intermediate states 2p53p′ [1/2]1, …

The Study Of The 1s4–2pj Optogalvanic Transients In A Neon Discharge Plasma, Naveed Piracha, R. Feaver, T. Gilani, Rizwan Ahmed, R. Ali, M. Baig

Naveed K. Piracha

Time dependent optogalvanic signals induced by the 1s4→2pj laser excitations have been studied in neon DC plasma. The decay rates related to all the four 1si levels have been derived by fitting the waveforms with a mathematical rate equation model. The temporal signatures of three transitions namely 638.3, 650.7 and 724.5nm related to the 2p7, 2p8 and 2p10 upper levels, respectively, have been found to be different from the rest of the transitions. We relate these effects to the population redistribution of decaying channels and to the processes responsible for the optogalvanic effect.

Simulation Of Ultrashort Laser Pulse Propagation And Plasma Generation In Nonlinear Media. Dissertation, Jeremy Gulley

Jeremy R. Gulley

In this dissertation, a modified nonlinear Schrödinger equation is derived, which describes the propagation of ultrashort laser pulses through nonlinear materials in which plasma generation and laser-induced damage can occur. Differences between this model and models currently used in the literature are investigated and analyzed by numerical simulations. Ultrafast laser-induced material modification is investigated using this method by simulating the propagation of fully 3+1D (3 spatial plus 1 time dimension) laser pulses, which are numerically constructed from experimentally measured beam profiles and pulse shape data. The latest of these investigations reveals that standard rate-equation models for the free-electron plasma generation …

The Study Of Optogalvanic Effect Associated With The 1s States Of Neon, Naveed Piracha, K. Nesbett, S. Marotta

Naveed K. Piracha

We report on the temporal evolution of the optogalvanic signals in neon using a commercial hollow cathode lamp in conjunction with a Nd:YAG pumped dye laser system. We have recorded transitions excited from each of the neon 1s states over a range of discharge current to investigate the dominant physical processes responsible for the optogalvanic effect in neon discharges. The decay rates associated with the four 1s levels have been obtained using a mathematical rate equation model. A linear relationship between the decay rates and the discharge current has been found and the effective lifetimes and the electron collisional rate …

Simulation And Analysis Of Ultrafast-Laser-Pulse-Induced Plasma Generation In Fused Silica, Jeremy Gulley, Sebastian Winkler, W. M. Dennis

Jeremy R. Gulley

Recent experiments on optical damage by ultrashort laser pulses have demonstrated that the temporal pulse shape can dramatically influence plasma generation in fused silica. We use a modified 3+1D nonlinear Schrödinger equation for the pulse propagation coupled to a rate equation for the plasma density in the dielectric material to simulate pulse propagation and plasma formation in fused silica. We use these simulations to analyze the influence of pulse shape and beam geometry on the formation of the electron plasma and hence modification in the bulk material. In particular, we simulate the effect of pulses reconstructed from experimental data. It …

On The Time-Resolved Optogalvanic Spectra Of Neon And Krypton, Naveed Piracha, K. Nesbett, S. Moten, P. Moeller

Naveed K. Piracha

In this work time resolved optogalvanic signals associated with transitions excited from the first metastable state of neon and krypton have been studied. These gases have similar energy state configurations and it is of significant interest to study their time resolved optogalvanic waveforms resulting from transitions belonging to the states of same quantum numbers. The experimentally observed optogalvanic signals recorded for different discharge currents have been fitted to a theoretical model to obtain parameters that determine amplitudes, instrumental time constants and decay rates of the 1s levels.

Simulation And Analysis Of Ultrafast Laser Pulse Induced Plasma Generation In Dielectric Materials, Jeremy Gulley, Sebastian Winkler, William Dennis

Jeremy R. Gulley

Recent experiments on optical damage by ultrashort laser pulses have demonstrated that the temporal pulseshape can dramatically influence plasma generation in fused silica and sapphire. In this work a modified 3+1D nonlinear Schroedinger equation for the pulse propagation coupled to a rate equation for the plasma density in the dielectric material is used to simulate pulse propagation and plasma formation in a range of dielectric materials. We use these simulations to analyze the influence of pulse-width, pulse-shape and beam geometry on the formation of the electron plasma and hence damage in the bulk material. In particular, when possible, we simulate …

Infrared Depletion Spectroscopy Of Aniline–Toluene Cluster: The Investigation Of The Red Shifts Of The Nh2 Stretching Vibrations Of Aniline–Aromatic Clusters, Naveed Piracha, F. Ito, T. Nakanaga

Naveed K. Piracha

The infrared spectra of the NH2 stretching vibrations of aniline–toluene cluster and corresponding cluster cation have been measured by the infrared depletion method. The NH2 stretching vibrations of aniline–toluene cluster were observed at 3393 and 3468 cm−1. The red shifts from the corresponding bands of aniline monomer are 29 and 40 cm−1, respectively, for the NH2 symmetric and anti-symmetric stretching vibrations. The magnitudes of the red shifts are similar to those of other aniline–aromatic clusters. The structure of this cluster has been found to be a sandwich type, that is, the two aromatic rings are parallel in the cluster and …

Rapid Two-Dimensional Optical Spectroscopy Through Acousto-Optic Pulse Shaping, W. Wagner, Peifang Tian, L. Chunqiang, J. Semmlow, Warren Warren

Peifang Tian

No abstract provided.