Frequency shift in signal and idler gain peaks due to nonlinear refractive index in narrow-band optical parametric oscillators

The frequency chirp in the gain spectra for the signal and idler waves in nanosecond optical parametric oscillators (OPOs) due to the nonlinear refractive index of the gain medium at the pump wavelength is discussed. Computed results for a commonly used 355 nm pumped OPO with lithium borate and beta barium borate as a gain medium show that the frequency shift remains small compared with the gain bandwith, even for a narrow-band OPO pumped above threshold.     

Sensitivity oftls and minimum norm methods ofdoa estimation to errors due to either finite data or sensor gain and phase perturbations

In thedoa (direction of arrival) estimation problem, we encounter either finite data or insufficient knowledge in array characterisation or both. It is therefore important to study how the subspace-based methods perform under these conditions. In this paper, we first consider the finite data case and establish two results: (i) the total least-squares approach to the linear prediction method (which we refer to astls-flp method) is equivalent to the minimum norm (min. norm), method and (ii) thetls-fblp method yields a 3 dB lower mean-square error (mse) in thedoa estimates as compared to thetls-flp method. Next, we consider the asymptotic performance of the min. norm method in the presence of sensor gain and phase perturbations, and derive the expressions for themse in thedoa estimates assuming an uniform linear array. For the special case of a single source, we also obtain a simple and explicit expression for themse which, when compared with the corresponding result for themusic algorithm, shows that the min. norm method is more sensitive than themusic when the number of sensors exceeds 2. Computer simulations are included to support the theoretical predictions. This work is supported in part by the Electronics and Radar Development Establishment, Bangalore.     

Frequency stabilization of a diode-pumped Nd:Yag laser at 532 nm to iodine by using third-harmonic technique

A compact frequency standard was constructed by stabilizing the frequency of a diode-pumped Nd:YAG laser to the Doppler-free spectrum of iodine at 532 nm. The performance of the laser and the stability and repeatability of the stabilization scheme were investigated. The dependence of the laser frequency on such parameters as pressure of the iodine cell, modulation amplitude, and pressure were also studied. The results show that by using standard third-harmonic locking technique stability and repeatability comparable to more elaborate iodine-stabilized Nd:YAG laser systems can be achieved in a portable, and relatively simple and inexpensive, setup.     

Limits to remote molecular detection via coherent anti-Stokes raman spectroscopy using a maximal coherence control technique

ABSTRACT

The demand for remote molecular detection has been rising in recent years. The technique of coherent anti-Stokes Raman spectroscopy (CARS) has become one of the most optimal methods due to its high efficiency, fast response time and ease of use. In this article, we estimate the number of detectable photons from a CARS signal by using a semiclassical nonlinear optics approach. Several key parameters and their effect on the signal are studied in the following discussion. We also provide a method to prepare the maximum coherence between vibrational states in an effective two level system.     

To react or not to react? Intrinsic stochasticity of human control in virtual stick balancing

Understanding how humans control unstable systems is central to many research problems, with applications ranging from quiet standing to aircraft landing. Increasingly, much evidence appears in favour of event-driven control hypothesis: human operators only start actively controlling the system when the discrepancy between the current and desired system states becomes large enough. The event-driven models based on the concept of threshold can explain many features of the experimentally observed dynamics. However, much still remains unclear about the dynamics of human-controlled systems, which likely indicates that humans use more intricate control mechanisms. This paper argues that control activation in humans may be not threshold-driven, but instead intrinsically stochastic, noise-driven. Specifically, we suggest that control activation stems from stochastic interplay between the operator''s need to keep the controlled system near the goal state, on the one hand, and the tendency to postpone interrupting the system dynamics, on the other hand. We propose a model capturing this interplay and show that it matches the experimental data on human balancing of virtual overdamped stick. Our results illuminate that the noise-driven activation mechanism plays a crucial role at least in the considered task, and, hypothetically, in a broad range of human-controlled processes.     

Sharpening accepted thermodynamic wisdom via quantum control: or cooling to an internal temperature of zero by external coherent control fields without spontaneous emission

Cooling of internal atomic and molecular states via optical pumping and laser cooling of the atomic velocity distribution, rely on spontaneous emission. The outstanding success of such examples, taken together with general arguments, has led to the widely held notion that radiative cooling requires spontaneous emission. We here show by specific examples and direct calculation, based primarily on breaking emission-absorption symmetry as in lasing without inversion, that cooling of internal states by external coherent control fields is possible. We also show that such coherent schemes allow us to practically reach absolute zero in a finite number of steps, in contrast to some statements of the third law of thermodynamics.     

Formation of complex alkoxides to control layer structure in Sr-Bi-M-O (M: Ta or Nb) perovskite thin films

Precursors for layer-structured perovskite thin films of Sr₂Bi₂Ta₃O_12.5 or SBT223, and Sr₂Bi₂Nb₃O_12.5 or SBN223 were prepared by the reactions of Sr-Bi (2 : 2) double methoxyethoxide and Ta or Nb methoxyethoxide, followed by partial hydrolysis. By several analytical techniques, such as ¹H-, ¹³C- and ⁹³Nb-NMR (nuclear magnetic resonance) and FT-IR (Fourier transform infrared) the molecular structure of the precursors was found to be similar to the layer-structured perovskite crystal sub-lattice. As a result, the onset of crystallization in the sol-gel derived SBT223 thin films was at a low temperature of 550°C. By rapid thermal annealing in an oxygen atmosphere, the single-phase perovskite films exhibited preferred (1 1 7) orientation. In contrast to the SBT system, the observed number of the pseudo-perovskite layers of TaO₆ octahedral units between Bi₂O ₂ ^{2 +} layers in the SBT223 was three. Despite the elongation of the unit cell along the c-axis, 700°C-heated SBT223 thin film did not exhibit a saturated hysteresis loop due to the small crystallite size.     

A comparison of the quantitative methods for the analysis of the platinum-containing anticancer drug [cis-[amminedichloro(2-methylpyridine)]platinum(II)] (ZD0473) by HPLC coupled to either a triple quadrupole mass spectrometer or an inductively coupled plasma mass spectrometer

The use of high performance liquid chromatography coupled with inductively coupled plasma mass spectrometry (HPLC-ICPMS) as means for the quantitative determination of ZD0473, a platinum anticancer drug, and its related biologically active "aqua" compounds in biofluid samples is described. The performance of the resulting HPLC-ICPMS method was compared with that of a conventional HPLC-triple quadrupole mass spectrometer-based (HPLC-MS/MS) system for properties such as limit of detection, linearity, and reproducibility using spiked samples. The methods were then applied to the determination of plasma ultrafitrate concentrations of ZD0473 in dog plasma samples obtained following intravenous and oral administration at 0.5 and 6 mg/kg, respectively. These experiments showed that both methods were capable of providing accurate and precise results but that the HPLC-ICPMS method had advantages of extended linear range and superior sensitivity, providing a limit of quantification of 0.1 ng/mL for ZD0473, as compared to 5 ng/mL using the current HPLC-MS/MS method. In addition, by using a single combined HPLC-ICPMS/MS/MS system, it was possible to determine the relative MS/MS response of the aqua compounds for the first time.     

Gel- to -crystallite conversion technique for the syntheses of M-β/β″-alumina (M = Li, Na, K, Rb, Ca or Eu)

Various alkali and alkaline earths hydroxides are reacted with hydrated alumina gel Al₂O₃·nH₂O (80 < n < 120) in hydrophilic organic solvents to yield precursor(s) with metal cation entrapped in the matrix, with the general formula M_yAlO_y(OH)_3−y (where M = Li, Na, K, Rb, Ca and Eu). Depending on the cation, the precursor crystallizes as metal-inserted gibbsite or boehmite structure. The nature of the precursor depends on the size of the cation, and its concentration, solvent used, solvent/water ratio, etc. These precursors yield M-β/β″-alumina phase on calcination at elevated temperatures in the case of sodium, potassium and rubidium. In the case of lithium, LiAl₅O₈ (spinel) phase is obtained. While calcium and europium give rise to calcium hexaaluminate and EuAlO₃ (perovskite), respectively, on calcination. The need of metal ion-inserted-γ-alumina phase is emphasized in obtaining M-β/β″-alumina phase.