Non-linear ramsey resonance in the optical region

The interaction of gas particles with three separated light fields has been considered. It has been shown that a resonance with the width equal to the reciprocal transit time appears in such a system. The resonance is due to coherence conservation and change of distribution of a number of particles by nonlinear interaction with the fields. Its properties are close to the Ramsey resonance in the microwave range. This new type of resonance may have a relative width of 10⁻¹¹ to 10⁻¹².     

Observation of Ramsey resonance absorption in three separated laser fields produced by a corner reflector

Experimental observation of optical Ramsey interference in saturated absorption of methane in a low-pressure gas cell with three spacially separated laser beams at 3.39 μm is reported.     

A model for ground-state and excited-state microwave optical double resonance

A model is developed to describe the physical principles of both ground-state and excited-state microwave optical double resonance. This model uses a steady-state kinetic analysis to determine the populations of three quantum levels in the presence of two monochromatic radiation fields. The microwave transition rate and the laser transition rate are obtained from separate analyses using the semiclassical two-state transition probability averaged over the effects of collisions. The Doppler effect of the optical transition rate is explicitly included. The competing effects of laser power, microwave power, and pressure on signal intensity and lineshape are described. Calculated lineshapes and relative signal intensities based on this model are in good agreement with previous measurements on BaO and NO₂.     

A theoretical analysis on coherent double resonant absorptive lineshape in closely spaced transitions for λ − type five level system

Under Doppler free double resonant condition, an open five level system with three closely spaced upper levels and a pair of closely spaced lower levels, is allowed to interact with two electromagnetic fields. In the domain of semiclassical formulation of atom-field interaction, the optical Bloch equations involving the density matrix elements are constructed. These coupled optical Bloch equations are unsolvable in closed analytical form. We use the perturbation method for getting the approximate analytical solutions to the coupled optical Bloch equations. The absorptive signal lineshape corresponding to pump frequency Ω₁ and signal frequency Ω₂ is obtained. We also obtain the absorptive signal line shape by interchanging the pump and signal (i.e., the pump frequency is Ω₂ and the signal frequency is Ω₁) mutually. The interferences between the probability amplitudes for different energy levels involving the dipole allowed and the dipole forbidden transitions give rise to the field dependent and field independent quantum coherence respectively. With the suitable manipulation of the coherence between the two lower levels, the signal lineshapes for on-resonance and off-resonance pump positions are explored in a great detail. The off-resonance pump leads to the two-photon absorption and hence the signature of the nonlinear resonances. On the other hand, the on-resonance pump positions lead to the Rabi splitting. The shifts of the resonance peak positions are explored as a function of pump intensity and the level spacings of the closely spaced levels.     

Population inversion on transitions to the ground state of atoms upon nonresonance absorption of laser radiation

Lasing at the resonance transitions (D _1? and D _2?lines) of sodium was observed in the superradiance mode upon nonresonance optical excitation in the presence of a buffer gas. The dependences of the lasing intensity on the exciting radiation intensity and on the detuning of its frequency from the frequencies of resonance transitions were studied. It is found that, under specific conditions of the experiment (high pressure of a buffer gas and a rather high radiation intensity), in the case of a large positive detuning of the exciting radiation frequency from the resonance (“working”) transition frequency, the population inversion is produced at the “ working” transition, which results in lasing.     

Continuous-flow optical pumping NMR in a closed circuit system

In a typical continuous-flow optical pumping setup, the chemical shift of xenon in the adsorbed phase depends on the gas flow rate due to warming of the sample surface by the gas stream. Calibration of the system using the (207)Pb resonance of solid lead nitrate is necessary to determine the actual sample temperature. Optimum pulse repetition rates are strongly affected by gas flow and spin-lattice relaxation rates. The interplay of flow and pulse repetition rate alters signal intensity ratios and may lead to the complete suppression of signals.     

Rayleigh backscatter noise in integrated optical resonance gyro

Rayleigh backscatter noise in integrated optical resonance gyroscope (IORG) is researched both in theoretically and experimentally. The characteristics of Rayleigh backscatter noise in the resonator of SiO₂ on Si substrate are formulized, and the static state and dynamic state models of IORG are constructed. The relationship between the optical signal and Rayleigh backscatter noise is simulated, and the affection between the interference signal of Rayleigh backscatter noise and reverse optical signal is also calculated. The degree of degrading the performance of resonator due to Rayleigh backscatter noise is analyzed, such as finesse and fundamental detection limit. The relationship between the line-width of the tunable laser and the intensity of transmit light and Rayleigh backscatter noise is simulated. Through the theory of modulation, the method of inflicting two different frequencies to the arm of integrated optical modulator is presented. The verified experiment result showed that the saw-tooth wave modulation could effectively restrain Rayleigh backscatter noise in IORG.     

Dual structure of saturated absorption resonance at an open atomic transition

Experiments on open transitions of the D ₁ line of alkali metals (Cs and Rb isotopes) reveal the dual structure of saturated absorption resonance in the signal of a high-intensity optical wave in the presence of a low-intensity counterpropagating wave. Theoretical analysis shows that the observed shape of the resonance is associated with the openness of the atomic transition as well as with the Doppler effect for atoms in a gas. The results are of general physical significance for nonlinear spectroscopy and can also find application in metrology (frequency and time standards on open transitions).     

High-precision pulsed photoacoustic spectroscopy in NO2-N2

A method for high-precision pulsed photoacoustic spectroscopy applied to a simple system for detection of NO₂ traces in nitrogen is presented. The acoustic signal from a closed cell containing NO₂/N₂ samples irradiated by a pulsed visible laser is analyzed in the frequency domain. A signal-processing method to obtain a high-resolution Fourier spectrum of the signal was developed. An accurate fitting of the resonance peaks with Lorentzian profiles gives high-precision determination of the amplitude and width of the resonance peaks. The resonance maximum is proportional to the absorbed energy; therefore, the choice of the laser wavelength, linewidth and frequency stability are critical for a precise calibration due to the fine structure of the NO₂ optical spectrum. The method also allows high-accuracy measurement of the Q of the acoustic cavity. The dependence of Q on the buffer gas pressure is characteristic of an acoustic cavity where energy losses near the walls predominate. Consequently, an important enhancement of sensitivity takes place at high N₂ pressure. Received: 1 June 2001 / Revised version: 27 July 2001 / Published online: 7 November 2001     

High contrast Ramsey fringes with coherent-population-trapping pulses in a double lambda atomic system

We report the observation of Raman-Ramsey fringes using a double lambda scheme creating coherent population trapping in an atomic ensemble combined with pulsed optical radiations. The observation was made in a Cs vapor mixed with N2 buffer gas in a closed cell. The double lambda scheme is created with lin perpendicular lin polarized laser beams leading to higher contrast than the usual simple lambda scheme. The pulsed trapping technique leads to narrow fringe widths scaling as 1/(2T) with high contrasts which are no longer limited by the saturation effect. This technique operates in a different way from the classical Ramsey sequence: the signal is done by applying a long trapping pulse to prepare the atomic state superposition, and fringe detection is accomplished by optical transmission during a short second trapping pulse without any perturbation of the dark state.     

Chromatic monitoring of downstream microwave plasma source

The application of the chromatic sensing for monitoring of a microwave plasma source is described. The emitted radiation from the plasma excited in the argon, oxygen and CF₄ mixture was measured with three PIN-diodes with integrated optical filters. The response of the chromatic signals on variation of power and gas composition was investigated. Whereas a good sensitivity of the integrated optical signal to the power was confirmed, only a limited sensitivity to the working gas mixture was found.     

利用核磁共振测量乙醇汽油溶液浓度

利用核磁共振成像分析仪测量了不同汽油体积含量的乙醇汽油溶液的核磁共振信号,分析了横向弛豫时间、信号强度与汽油浓度的关系.结果表明乙醇汽油溶液中汽油的体积含量与信号强度呈线性关系,因此利用低磁场核磁共振可以测量乙醇汽油浓度.     

Experimental study of enhanced emission of the laser-ablated plume in backing gas

The effects of background gases on the optical emission of the excimer-laser-ablated plume from a brass target have been studied experimentally. It is found that the plume emission can be enhanced significantly in a proper gas ambient. In hydrogen, the highest peak intensity is detected, and in argon, there is a distinctive difference in the pressure-dependent emission between in He and in the other three gases, Ar, N₂ and H₂. Moreover, the monitored line peak intensity remains unchanged in Ar and N₂ and increases in H₂ within a distance above the target surface; but in He, the observed peak intensity decreases with distance like in vacuum. Furthermore, the emissions of several more atomic lines of Cu and Zn atoms from the plume are also found to be enhanced in the same manner in gas ambient. Some physical processes involved in the plume expansion and the possible mechanisms for the enhanced emission of the plume in backing gas are discussed.     

Optical CO<Subscript>2</Subscript> Sensing with Ionic Liquid Doped Electrospun Nanofibers

The first use of electrospun nanofibrous materials as highly responsive fluorescence quenching-based optical CO₂ sensors is reported. Poly(methyl methacrylate) and ethyl cellulose were used as polymeric materials. Sensing slides were fabricated by electrospinning technique. A fiber-optic bundle was used for the gas detection. CO₂ sensors based on the change in the fluorescence signal intensity of ion pair form of 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS). The sensor slides showed high sensitivities due to the high surface area-to-volume ratio of the nanofibrous membrane structures. The preliminary results of Stern-Volmer analysis show that the sensitivities of electrospun nanofibrous membranes to detect CO₂ are 24 to 120 fold higher than those of the thin film based sensors. The response times of the sensing reagents were short and the signal changes were fully reversible. The stability of ion pair form of HPTS in the employed matrix materials was excellent and when stored in the ambient air of the laboratory there was no significant drift in signal intensity after 7 months. Our stability tests are still in progress.     

Formation of Germanium–Vacancy Color Centers in CVD Diamond

We study Ge-doped polycrystalline diamond films synthesized, using microwave plasma chemical vapor deposition (CVD) in CH₄-H₂ base mixtures. We compare two sources of the dopant – gaseous monogermane (GeH₄) and solid Ge plates. We investigate the structure and phase composition of the obtained films, using scanning electron microscopy, photoluminescence (PL), and Raman spectroscopy. We vary the precursor gas composition to maximize the intensity of the Germanium–vacancy (Ge-V) PL signal at 602 nm and discover that, using [C]-rich gas mixtures ([CH₄]=20%), we are able to increase the intensity of Ge-V signal by two orders of magnitude in comparison with Ge-doped high-quality microcrystalline films of the same thickness but grown at [CH₄]=4%. The attained results may be used for the fabrication of polycrystalline diamond films and plates with high concentrations of Ge-V centers, which may serve as source material for the fabrication of submicrometer-sized luminescent diamond particles for local optical thermometry.

     

High-resolution velocity selecting separated-field excitation of a calcium atomic beam

It is shown that pulsed separated-field excitation provides a useful method for time-of-flight velocity selection of atomic beams. High-resolution nonlinear optical Ramsey fringes generated by one narrow velocity group have been observed at the (³ P ₁–¹ S ₀) intercombination line of Ca. The corresponding second-order Doppler broadening deduced from the Fourier transform of the fringe signal could be reduced by about one order of magnitude from 3.7 kHz for cw separated field excitation to 0.4 kHz.     

Physics in plasmonic and Mie scattered near-field for efficient surface-enhanced Raman scattering template

We describe the physics of the SERS based on the optical near-field intensity enhancement on the metallic (plasmonic) and the nonmetallic (Mie scattering) nanostructured substrates with two-dimensional (2D) periodic nanohole arrays. The calculation by the Finite-Difference Time-Domain (FDTD) method revealed that the optical intensity enhancement increases with the increase of the thickness of a gold film coating on the nonmetallic (dielectric) nanostructured Si, GaAs, and SiC substrates. The resonance spectrum shifts with the changes in the geometrical structure of the void diameter and inter-void distance. It was clarified that the optical intensity enhancement obtained with the gold-coated substrate is equivalent to that with a gold substrate at 70-nm thick gold coating on the dielectric substrates in this structure. The resonance spectral bandwidth for Mie scattering and plasmonic near-fields is different. Therefore, if the Stokes line of the Raman scattering is located within the resonance bandwidth, the SERS signal is enhanced proportionally to the fourth power of the electric near-field. However, if the Stokes shift is located out of the resonance bandwidth, the SERS signal enhancement is only proportional to the square of the scattered near-field.     

Signal processing system in cavity enhanced spectroscopy

The paper presents a signal processing system used for nitrogen dioxide detection employing cavity enhanced absorption spectroscopy. In this system, the absorbing gas concentration is determined by the measurement of a decay time of a light pulse trapped in a cavity. The setup includes a resonance optical cavity, which was equipped with spherical and high reflectance mirrors, the pulsed diode laser (414 nm) and electronic signal processing system. In order to ensure registration of low-level signals and accurate decay time measurements, special preamplifier and digital signal processing circuit were developed. Theoretical analyses of main parameters of optical cavity and signal processing system were presented and especially signal-to-noise ratio was taken into consideration. Furthermore, investigation of S/N signal processing system and influence of preamplifier feedback resistance on the useful signal distortion were described. The aim of the experiment was to study potential application of cavity enhanced absorption spectroscopy for construction of fully optoelectronic NO₂ sensor which could replace, e.g., commonly used chemical detectors. Thanks to the developed signal processing system, detection limit of NO₂ sensor reaches the value of 0.2 ppb (absorption coefficient equivalent = 2.8 × 10⁻⁹ cm⁻¹).     

Influence of the buffer layer properties on the intensity of Raman scattering of graphene

Using a model of oscillating dipoles, we simulate the intensity of the G‐band in the Raman signal from structures consisting of graphene, separated by an arbitrary buffer layer from a substrate. It is found that a structure with an optimized buffer layer refractive index and thickness exhibits a Raman signal which is nearly 50 times more intense than that from the same structure with a non‐optimized buffer layer. The theoretical simulations are verified by Raman measurements on structures consisting of a layer of graphene on SiO₂ and Al₂O₃ buffer layers. The optical contrast of the single graphene layer is calculated for an arbitrary buffer layer. It was found that both the Raman intensity and optical contrast can be maximized by varying the buffer layer thickness. Copyright © 2013 John Wiley & Sons, Ltd.