Linewidth Studies Research Articles

Quantum cascade lasers as broadband sources via strong RF modulation

In this work, we demonstrate that in a regime of strong modulation, by generating pulses of the length of the order of a few cavity lifetimes (hundreds of ps), a broadband quantum cascade laser can be driven to lase on a bandwidth (250 cm−1) limited by the gain. In addition, the amplitude noise of the radiation was shown to be limited by the detector. A laser linewidth study has been performed under different operating conditions, finding values spanning from 20 to 800 MHz, indicating a trade-off between emission bandwidth, amplitude stability, and coherence. The pulsed intensity of the output arises from the gain switching dynamics of the laser, while the self-seeded nature of the emission is responsible for the low amplitude noise and the relatively narrow linewidth of the individual spectral lines of the multimode emission.

Studies of temperature dependent Raman spectroscopy of two nematic liquid crystalline compounds of homologous series

Raman spectroscopy is an important tool to understand the structural and molecular behaviour of the liquid crystals when they undergo through different temperatures. It also helps to understand the different phase changes of the liquid crystal material as temperature changes. In this work, the structural properties of two nematic liquid crystals having relatively high clearing temperature namely 4 butylcyclohexyl-3, 5-difluoro-4- isothiocyanato biphenyl and 4-pentylcyclohexyl-3, 5-difluoro-4-isothiocyanato biphenyl are studied. The study is done using temperature dependent Raman spectroscopy. From the studies of the two compounds it has been found that the experimental values are agree well with the various functional groups and different bond assignments recorded in literature. This agreement validates the presence of different functional groups and different stretching bonds in the two studied liquid crystal compounds. Deformations of some of the peak positions of the two liquid crystal compounds have been observed with the change in phase at different temperature. Also to understand the behaviour of the Raman peak near the clearing temperatures of the liquid crystal compounds the linewidth of the different peak values at different temperature have also been studied. From the line width study, the various phase transition temperatures of the two liquid crystalline compounds can be confirmed.

Linewidth study of pixelated aluminum nanowire gratings on polarization performance.

Nowadays, nanowire gratings are widely used in various applications such as imaging sensors and high-resolution microscopes. Structure parameters are the main factors that affect the optical performance of the gratings. This work aims to present the influence of the linewidth of pixelated aluminum nanowire gratings with a fixed period on the transmittance and extinction ratio in the visible region. By controlling the exposure doses of electron beam lithography (EBL), different linewidths of pixelated aluminum nanowire gratings with a period of 170 nm were fabricated. The significant effects of linewidth difference on the polarization performance were verified by the simulations of finite-difference time-domain (FDTD) software. The simulations were divided into two parts: the discussion of the pure aluminum without considering oxidation and the discussion of the surface aluminum being oxidized into the aluminum oxide. An optical system was built to evaluate the performance of the fabricated structures. The results show that the trends of the measurement results are consistent with that of simulation. This work will give a guide to the fabrication and evaluation of the nanowire gratings.

Uncertainty in linewidth quantification of overlapping Raman bands

Spectral linewidths are used to assess a variety of physical properties, even as spectral overlap makes quantitative extraction difficult owing to uncertainty. Uncertainty, in turn, can be minimized with the choice of appropriate experimental conditions used in spectral collection. In response, we assess the experimental factors dictating uncertainty in the quantification of linewidth from a Raman experiment highlighting the comparative influence of (1) spectral resolution, (2) signal to noise, and (3) relative peak intensity (RPI) of the overlapping peaks. Practically, Raman spectra of SiGe thin films were obtained experimentally and simulated virtually under a variety of conditions. RPI is found to be the most impactful parameter in specifying linewidth followed by the spectral resolution and signal to noise. While developed for Raman experiments, the results are generally applicable to spectroscopic linewidth studies illuminating the experimental trade-offs inherent in quantification.

Laser linewidth and fiber nonlinearity tolerance study of C-16QAM compared to square 16QAM in coherent OFDM system

Abstract We introduced an effective modulation format circle 16 quadrature amplitude modulation (C-16QAM) to improve the laser linewidth induced phase noise and fiber nonlinear effects tolerance in coherent orthogonal frequency division multiplexing (OFDM) system without other losses compared to square 16QAM. Although C-16QAM has improved the performance of single channel system with Viterbi–Viterbi carrier phase estimation, C-16QAM using in coherent OFDM system has not been performed and such configuration of system may solve many problems in the next generation access networks. Here we numerically studied two separate manifestations of phase noise generated by laser linewidth and fiber nonlinear effects. We take these two kinds of phase noise into consideration separately by investigating the influence of laser linewidth with fixed launch power into transmission fiber and the influence of fiber nonlinear effects with fixed laser linewidth. We find that the C-16QAM improves the laser linewidth induced phase noise significantly and improves fiber nonlinear effects tolerance to a certain degree compared to square 16QAM. This coherent C-16QAM OFDM system may have great prospects for the next generation access networks for these significantly improvements.

Study of structural defects and crystalline perfection of near stoichiometric LiNbO3 crystals grown from flux and prepared by VTE technique

Near-stoichiometric LiNbO3 (SLN) single crystals were grown/prepared by top seeded solution growth/vapor transport equilibration (VTE) technique, and investigated for stoichiometry, disorder and structural defects. The optical absorption and Raman line-width studies revealed higher stoichiometry (i.e., higher Li/Nb) for SLN prepared by vapor transport equilibration (SLN_V) technique in comparison to SLN grown from K2O flux (SLN_K) and Li-rich melt (SLN_L). The nearly symmetric single diffraction curve (DC), though broad, as observed for SLN_L specimen in high resolution X-rays diffraction (HRXRD) analysis depicted lesser low angle grain boundaries. On the other hand, relatively sharp DC with lowest full-width at half-maximum (FWHM ∼45 arc-sec) in HRXRD and lesser Urbach energy (∼80meV) in the absorption spectra for SLN_V crystal revealed less structural defects with respect to other SLN crystals. The higher FWHM of DCs in HRXRD for SLN_L and SLN_K is attributed to growth related imperfections usually observed in solution growth. Though, VTE process results in SLN crystals with better stoichiometry and lesser structural defects but the limitation being that samples up to ∼1mm thickness can be prepared with this technique. For bulk SLN, growth from K2O flux resulted in better stoichiometry whereas Li-rich flux resulted in better structural quality. The absorption spectra of the grown SLN crystals depicted oxygen vacancy induced electronic defects (Nb4+, polarons), which was further authenticated by X-ray absorption near-edge structure (XANES) analysis at Nb K edge revealing lesser Nb4+ defects in SLN with respect to congruent lithium niobate (CLN) crystal.

The lidar equation solution depending on the laser radiation line width studies

The lidar equation solving versus laser radiation line width dependence have been studied numerically. This problem was solved at first for the Mi nano aerosol scattering.

Linewidth study of the frequency-modulated laser based on the delayed self-heterodyne scheme

We investigate the linewidth of the frequency-modulated laser (FML) by using the delayed self-heterodyne (DSH) method. We deduce the expression of the linewidth theoretically and arrive at the conclusion that the linewidth of the FML measured by DSH method is the convolution of the spectrum of the output of the heterodyne interferometer with an ideal monofrequency laser under modulation and the natural linewidth of the laser. Experiments are carried out and the testing linewidths are in agreement with the theoretical results. The properties of the linewidth of the FML are analyzed based on the conclusion obtained and its guidance for application is discussed.

Phase transition between cubic and monoclinic polymorphs of the tetracyanoethylene crystal: the role of temperature and kinetics

Prediction of the relative stabilities and phase transition behavior of molecular crystalline polymorphs is highly coveted as distinct phases can possess different physical and chemical properties while having similar energies. Crystalline tetracyanoethylene (TCNE, C6N4) is known to exhibit rich solid state phase behavior under different thermodynamic conditions, as demonstrated by a wealth of experimental studies on this system. Despite this fact, the role of temperature and kinetics on the phase diagram of TCNE remains poorly understood. Here, first-principles calculations and high-resolution Fourier-transformed infrared (HR-FTIR) spectroscopy experiments are used to study the relative stabilities of the cubic and monoclinic phases of TCNE as a function of temperature. Specifically, density-functional theory with the van der Waals interactions method of Tkatchenko and Scheffler (DFT+vdW) is employed. The accuracy of this approach is demonstrated by the excellent agreement between the calculated and experimental structures. We find that the cubic phase is the most stable polymorph at 0 K, but becomes less favorable than the monoclinic phase at 160 K. This temperature-induced phase transition is explained on the basis of varying close contacts and vibrational entropies as a function of temperature. These findings are supported by a temperature-dependent HR-FTIR linewidth study of the CN vibrons.

Optical lifetime and linewidth studies of the [formula omitted] transition in [formula omitted]: A potential material for quantum memory applications

The effect of substituting deuterium for hydrogen in EuCl3·6H2O on the lifetime and homogeneous and inhomogeneous linewidths of the 7F0→5D0 transition at below 4K are investigated. It is observed that the transition lifetime increases from 116μs to 2.6ms and the homogeneous linewidth decreases from 4.1kHz to 430Hz. The sensitivity of the inhomogeneous broadening to the residual hydrogen concentration is estimated to be 91MHz/atm%. The potential of utilizing this crystal for quantum memory applications is discussed.

Coherent spectroscopy of a Λ atomic system and its prospective application to tunable frequency offset locking

We investigate the coherent pump–probe spectroscopy of a three-level Λ system, , in the hyperfine manifold of D2 transition (852 nm) of cesium with particular reference to the sub-Doppler linewidth resonance arising from Aulter–Townes (AT) splitting and the possibility of using it for realizing a scheme for tunable atomic frequency offset locking (AFOL). We discuss here the theoretical framework for a Λ system interacting with a coherent pump and probe and use it to describe the process of modulation transfer in the AT and electromagnetically induced transparency regimes. We further employ an experimental scheme consisting of a strong pump and a pair of weak probes to resolve the sub-Doppler linewidth (∼8 MHz) AT resonance and study its dependence on pump intensity and detuning. In order to explore the possibility of using such a sub-Doppler linewidth resonance for AFOL, we use its first derivative signal as a frequency discriminator to stabilize the probe laser. The frequency stability of the probe is characterized by means of error signal analysis. This study reveals that while the frequency stability of the AT locked laser is limited by the pump laser, the tuning range of the offset frequency lock can cover the entire Doppler profile and its immediate neighbourhood, thereby providing a simple and cost effective alternative to the external modulator. The study described in this paper contributes to the discussion on the subtle link between dressed state spectroscopy and AFOL, which is relevant for developing a master–slave-type laser system in the domain of coherent photon–atom interaction.

Scaling of Surface Plasmon Resonances in Triangular Silver Nanoplate Sols for Enhanced Refractive Index Sensing

The plasmonic spectra of solution phase ensembles of triangular silver nanoplates have been analysed in order to examine the fundamental properties underlying their size-dependent enhanced refractive index sensitivities. Linewidth studies highlight variations in the response of these solution phase nanostructures to those previously reported for single immobilized triangular nanostructures. The observation of insignificant broadening of the resonance linewidth for larger edge length nanoplates highlights minimal contribution of radiative damping processes at these dimensions. Comparative single nanoplate studies using discrete dipole approximations were performed to analyse the dephasing processes contributing to these reduced linewidths and to determine the key parameters defining the underlying plasmonic response. These single nanoplate approximations highlight the dominance of absorption processes over radiative processes and demonstrate that this dominance can be attributed to the platelet nature/geometry of the nanoplates. These calculations indicate that the higher aspect ratio allows for the maintenance of coherent plasmon oscillations as the edge length of the triangular platelet increases within the sols. Thickness studies verify that this reduction in radiation damping is due to high aspect ratio and can act to confine electromagnetic fields at the nanoplate surface, thereby increasing near-field enhancement and hence the resultant plasmonic refractive index sensitivity.

Multiparametric Optimization of 31P NMR Spectroscopic Analysis of Phospholipids in Crude Tissue Extracts. 2. Line Width and Spectral Resolution

The quality of NMR spectra in general and of spectra to be used for analysis of compound mixtures in particular is essentially defined by two basic parameters: signal-to-noise ratio and spectral resolution. The latter is determined by signal dispersion (chemical shift differences) and line widths. The present study focuses on multiparametric optimization of spectral resolution in (31)P NMR spectra of phospholipids from brain tissue extracts. This report presents, for the first time, a systematic and comprehensive study of phospholipid (31)P NMR line widths as a function of four experimental parameters: (i) extract concentration, (ii) concentration of a chelating agent, (iii) pH of the aqueous component of the solvent system, and (iv) temperature of the NMR measurement. Theoretical underpinnings of observed line width variations (transversal relaxation effects) are briefly discussed. In conjunction with an analogous, concurrently published report on chemical shift effects in the same tissue extract system, this multiparametric line width study provides a complete set of methodological guidelines for (i) generating well-defined tissue extracts, and (ii) choosing matched and optimized measurement conditions for highly reproducible and well-resolved (31)P NMR spectra of brain phospholipids. This study also offers a comprehensive database and a strategy for rational and efficient optimization of phospholipid spectra from other tissue extracts.

Chemomechanical Nanolithography: Nanografting on Silicon and Factors Impacting Linewidth

We present a two-fold extension of previous work on Atomic Force Microscope-based chemomechanical functionalization: (1) chemomechanical nanografting, which extends chemomechanical functionalization to a more stable initial surface, and (2) linewidth studies that show the impact of force and Atomic Force Microscope probe tip wear on patterning resolution. Alkene, alcohol, and alkyl halide molecules were nanografted to silicon and imaged with in situ atomic force microscopy, time-of-flight secondary ion mass spectrometry with Automated eXpert Spectrum Image Analysis, and scanning electron microscopy. Chemomechanical nanografting demonstrated linewidths down to 50 nm. Lines written on hydrogen-terminated silicon were used to explore the impact of tip radius and tip wear on linewidth when using Si3N4 coated tips.

Copper-63 NMR line width study of the copper(I)-acetonitrile system.

The principal focus of this study is the (63)Cu NMR line widths in Cu(I)-acetonitrile (AN) solutions. The variations with the concentrations of Cu(I) salts (trifluoromethanesulfonate and perchlorate), added salts, water, chloride ion, and temperature have been studied. A quantitative analysis shows that the anomalous temperature dependence of the line widths is not due to ion pairing or anion complexation but results primarily from formation of a species with a different coordination number or less symmetrical arrangement of AN ligands than in the normal tetrahedral Cu(AN)(4)(+) ion. Solvent viscosity and ion pairing (with triflate) also are identified as factors having the expected effects on the line widths. The results of earlier studies also are discussed and analyzed by the current model where possible.

7Li/31P NMR Studies of Lithiated Arylacetonitriles in THF−HMPA Solution: Characterization of HMPA-Solvated Monomers, Dimers, and Separated Ion Pairs

The structures of lithiated phenylacetonitrile and 1-naphthylacetonitrile were studied in THF and HMPA−THF solution. In pure THF, 7Li NMR line width studies suggest that these species exist as contact ion pairs; HMPA titration studies confirm this conclusion. In the presence of 0.25−2 equiv of HMPA, HMPA-solvated monomeric and dimeric contact ion pairs are detected by 31P and 7Li NMR spectroscopy. Finally, at 4−6 equiv of added HMPA, 7Li NMR spectra provide direct evidence for the formation of HMPA-solvated separated ion pairs.

Linewidth Study of Electric Dipole induced Spin Resonance in Uniaxially Stressed n-InSb for Far-Infrared Region: Theoretical

Experimental investigation on electric dipole induced ESR in n-InSb was reported in the preceding paper (Experimental). Far-infrared laser with λ=220 µ m∼84 µ m and the samples with N D - N A =10 15 cm -3 were employed in the experiment. In this article we mainly focus our attentions to the origin of the observed linewidth. Linewidth is nearly constant at low temperature and increases with T above ∼20 K: 0.02 Tesla at 4.2 K and 0.1 Tesla at 60 K. From the relation of \(|g^{\ast}|\mu_{\rm B}\Delta H=\hbar/T_{1}\), we obtain T 1 ∼10 -12 sec at T =60 K, where | g * |=44 is obtained from the absorption data for λ=119 µm. Such a short relaxation time T 1 cannot be expected from any spin-lattice relaxation process. Rashba-Sheka's Hamiltonian: H ′ =δ 0 ( E ) σ·κ , σ : Pauli spin matrix, κ x = k y k x k y - k z k x k z etc. , is responsible for the linewidth broadening. This interaction stems from the lack of inversion symmetry in InSb. Linewidth calculation based on the Kubo-Tomita's theory qualitatively explain...

Linewidth Study of Electric Dipole Induced Spin Resonance in Uniaxially Stressed n-InSb in Far Infrared Regions: Experimental

We have carried out the linewidth measurements of the electric dipole induced spin resonance (EDSR) of conduction electrons in uniaxially stressed n-InSb in the far-infrared (FIR) regions. Our employed samples of n-InSb are highly doped with donors (6.0×10 15 cm -3 and 8.5×10 15 cm -3 ) and thus have metallic properties due to degenerate electrons. The linewidths of the EDSR have been obtained as the functions of the magnetic field (2.10∼6.39 T), the temperature (4.2∼70 K) and the donor concentration. The main features of the experimental results are the followings: (1) It is found that the minimum of the linewidth appears on the magnetic field dependence at 4.2 K. (2) In the lower resonance magnetic field (2.10 T and 2.74 T) and lower temperature regions ( T<∼20 K), we have recognized a slight reduction of the linewidth with the increasing of temperature. (3) For moderately high temperatures ( T>∼20 K), the increment of the linewidth is found on our employed experimental conditions for all samples. (4) For the sample with higher donor concentration, the enhancement of the linewidth is also found in comparison with the sample with lower donor concentration. In this paper, the linewidth of the EDSR spectrum is mainly discussed in respect of the experimental results. The detailed theoretical discussion will be presented in the following paper by Sugihara et al.

A Monte-Carlo analysis of the picosecond Raman spectroscopy of germanium

A former investigation of one of the present authors (JK) and coworkers, concerning the role of isotopic scattering of optic zone-center phonons in natural and isotopically enriched Ge crystals, had revealed that the phonon-population lifetimes, as obtained from the signal decay in two-pulse picosecond anti-Stokes Raman spectroscopy, are systematically longer than the phonon lifetimes derived from linewidth studies of 1st-order Raman spectra. The authors explained this discrepancy as arising from the interference of the phonon-decay dynamics with the phonon-generation dynamics during the laser pulses. The present work critically re-examines and confirms this original interpretation through a detailed k -space Ensemble-Monte-Carlo simulation of the two-pulse Raman experiment

NMR line width, relaxation and diffusion studies of cyclohexane confined in porous silica

The rotational and translational dynamics of cyclohexane confined within silica pores of 4–50 nm in diameter, have been studied as a function of temperature by measuring 1 H NMR line widths, self-diffusion coefficients, spin–spin relaxation times, and spin–lattice relaxation times, and the results are compared with values obtained for the bulk material. The confinement in the pores gives rise to substantial changes in the molecular dynamics and phase behaviour. The line shape and T 2 measurements revealed a two-phase system, consisting of a liquid-like component at the surface and a plastic phase in the centre of the pore. The highly mobile surface layer is observable far below the reduced transition temperature of the confined cyclohexane. A high diffusion rate of the adsorbed cyclohexane is observed over a wide temperature range — even well below the region of the depressed freezing point. The diffusion coefficient of the molecules in the surface layer is three orders of magnitude larger than in the plastic phase of bulk cyclohexane.