Weak Sidebands Research Articles

Tunable Microwave Frequency Multiplication by Injection Locking of DFB Laser With a Weakly Phase Modulated Signal

We have demonstrated in this paper a novel tunable microwave frequency multiplication by injecting a weakly phase-modulated optical signal into a DFB laser diode. Signals with multiple weak sidebands are generated by cross-phase modulation of a continuous wave (CW) with short pulses from mode-locked fiber laser. Then, frequency multiplication is achieved by injection and phase locking a commercially available DFB laser to one of the harmonics of the phase modulated signal. The multiplication factor can be tuned by changing the frequency difference between the CW and the free oscillating wavelength of the DFB laser. The experimental results show that, with an original signal at a repetition rate of 1 GHz, a microwave signal with high spectral purity and stability is generated with a multiplication factor up to 60. The side-mode suppression ratio over 40 dB and phase noise lower than -90 dBc/Hz at 10 kHz are demonstrated over a continuous tuning range from 20 to 40.

Narrow spectral linewidth of single zinc-blende GaN/AlN self-assembled quantum dots

We study by microphotoluminescence the optical properties of single self-assembled zinc-blende GaN/AlN quantum dots grown by plasma-assisted molecular beam epitaxy. As opposed to previous reports, the high quality of such zinc-blende GaN quantum dots allows us to evidence a weak acoustic phonon sideband as well as a limited spectral diffusion. As a result, we report on resolution-limited quantum dot linewidths as narrow as 500 ± 50 μeV. We finally confirm the fast radiative lifetime and high-temperature operation of such quantum dots.

Photoluminescence of macroporous-alumina prepared by polyacrylamide gel technique

Macroporous -Al2O3 is synthesized by polyacrylamide gel technique. X-ray diffractor combined with a photoluminescence spectrophotometer is used to investigate the formation of Al2O3 phase and the light emission properties of xerogel sintered at different temperatures. It is demonstrated that high-purity -Al2O3 can be obtained at a sintering temperature of 1150 ℃. Scanning electron microscopy images reveal that the synthesized -Al2O3 has a monolithic structure. Photoluminescence spectra show that a major emission band around 365 nm and a weaker side band around 330 nm are observed when the excitation wavelength is 228 nm. The pore-formation and luminescence mechanisms of porous alumina are discussed based on the experimental results.

Fault diagnosis of motor drives using stator current signal analysis based on dynamic time warping

Electrical motor stator current signals have been widely used to monitor the condition of induction machines and their downstream mechanical equipment. The key technique used for current signal analysis is based on Fourier transform (FT) to extract weak fault sideband components from signals predominated with supply frequency component and its higher order harmonics. However, the FT based method has limitations such as spectral leakage and aliasing, leading to significant errors in estimating the sideband components. Therefore, this paper presents the use of dynamic time warping (DTW) to process the motor current signals for detecting and quantifying common faults in a downstream two-stage reciprocating compressor. DTW is a time domain based method and its algorithm is simple and easy to be embedded into real-time devices. In this study DTW is used to suppress the supply frequency component and highlight the sideband components based on the introduction of a reference signal which has the same frequency component as that of the supply power. Moreover, a sliding window is designed to process the raw signal using DTW frame by frame for effective calculation. Based on the proposed method, the stator current signals measured from the compressor induced with different common faults and under different loads are analysed for fault diagnosis. Results show that DTW based on residual signal analysis through the introduction of a reference signal allows the supply components to be suppressed well so that the fault related sideband components are highlighted for obtaining accurate fault detection and diagnosis results. In particular, the root mean square (RMS) values of the residual signal can indicate the differences between the healthy case and different faults under varying discharge pressures. It provides an effective and easy approach to the analysis of motor current signals for better fault diagnosis of the downstream mechanical equipment of motor drives in the time domain in comparison with conventional FT based methods.

Unitary and singular value decompositions of parametric processes in fibers

Parametric (four-wave mixing) processes in fibers, driven by one or two strong pumps, couple the evolution of two weak sidebands. These processes are governed by the (spatial) evolution equations dzX=AX+BX†t and their associated input–output relations X(z)=M(z)X(0)+N(z)X†t(0), where X is the (sideband) amplitude vector, A and B are coefficient matrices, and M and N are transfer matrices. In principle, one can use unitary decompositions (UDs) to facilitate the mathematical analyses of evolution equations, or singular value decompositions (SVDs) to facilitate the physical interpretations of input–output relations (by diagonalizing them). In these notes, the SVD method is reviewed, and applied to studies of modulation interaction (MI), phase conjugation (PC) and Bragg scattering (BS). The UD method is also reviewed. It works for BS driven by continuous-wave (CW) or pulsed pumps. Although it works for MI and PC driven by CW pumps, it does not work for MI and PC driven by pulsed pumps (because the coefficient matrices are not simultaneously diagonalizable). Another decomposition method, based on eigenvectors of an extended coefficient matrix, and their adjoints, works for CW and pulsed pumps. Further work is required, to clarify the relation between the SVD and adjoint methods, and apply the latter method to processes of current interest.

Multicolor multipartite entanglement produced by vector four-wave mixing in a fiber

Multipartite entanglement is a resource for quantum communication and computation. Vector four-wave mixing (FWM) in a fiber, driven by two strong optical pumps, couples the evolution of four weak optical sidebands (modes). Depending on the fiber dispersion and pump frequencies, the mode frequencies can be similar (separated by less than 1 THz) or dissimilar (separated by more than 10 THz). In this report, the discrete- and continuous-variable entanglement produced by vector FWM is studied in detail. Formulas are derived for the variances of, and correlations between, the mode quadratures and photon numbers. These formulas and related results show that the modes are four-partite entangled.

Single-walled carbon nanotubes show stable emission and simple photoluminescence spectra with weak excitation sidebands at cryogenic temperatures

Low-temperature microphotoluminescence (PL) maps were measured over a broad range of emission and excitation wavelengths for various samples of individual single-walled carbon nanotubes (SWNTs). These included nanotubes produced by chemical vapor deposition which were probed either in direct contact with a sapphire substrate or suspended between supports (on top of a carbon nanotube ``forest''). Whereas surfactant-coated SWNTs manifest PL intermittency and spectral diffusion, the surfactant-free samples show stable PL over minute time scales at temperatures down to $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This indicates that PL intermittency and spectral diffusion are not intrinsic properties of SWNTs, but can be caused by interactions with a surfactant coating. We also demonstrate that interactions between spatially close (presumably contacting) nanotubes may have contributed to the complexity of previously reported low-temperature PL spectra. Apart from such multipeak PL maps characterized by intense excitation sidebands, we also observed simple single-peak PL maps which can be attributed to only one SWNT within the laser excitation spot. In addition, we compare typical PL intensity (quantum efficiency) and structural assignment of PL peaks for the various types of samples studied.

Evidence for Specificity in Lipid-Rhodopsin Interactions

The interaction of bovine rhodopsin with poly- and monounsaturated lipids was studied by (1)H MAS NMR with magnetization transfer from rhodopsin to lipid. Experiments were conducted on bovine rod outer segment (ROS) disks and on recombinant membranes containing lipids with polyunsaturated, docosahexaenoyl (DHA) chains. Poly- and monounsaturated lipids interact specifically with different sites on the rhodopsin surface. Rates of magnetization transfer from protein to DHA are lipid headgroup-dependent and increased in the sequence PC < PS < PE. Boundary lipids are in fast exchange with the lipid matrix on a time scale of milliseconds or shorter. All rhodopsin photointermediates transferred magnetization preferentially to DHA-containing lipids, but highest rates were observed for Meta-III rhodopsin. The experiments show clearly that the surface of rhodopsin has sites for specific interaction with lipids. Current theories of lipid-protein interaction do not account for such surface heterogeneity.

Observations concerning light promoted electronic delocalization in covalently linked transition metal complexes

The emission spectral band shapes of several polypyridine-ligand (PP) bridged bis-ruthenium(II) complexes imply that Ru(II)/Ru(III) electronic coupling is weaker in their lowest energy metal to ligand charge transfer (MLCT) excited states than in their corresponding mixed valence ground states. In general, the amplitudes of the vibronic contributions to emission band shapes decrease markedly with the excited state-ground state energy differences, and it is expected that complexes with degenerate, or mixed valence excited states will have very weak vibronic side bands if configurational mixing of the degenerate MLCT excited states is substantial. However, the bimetallic PP-bridged ruthenium complexes emit at significantly lower energy than their monometallic analogs, but the vibronic contributions to their 77 K emission spectra are very similar to those of their monometallic complexes analogs. This indicates that the mixed valence excited states of the bimetallic complexes are electronically localized.

Photoconductivity Spectra of Single-Carbon Nanotubes: Implications on the Nature of Their Excited States

We have measured the photoconductivity excitation spectra of individual semiconducting carbon nanotubes incorporated as the channel of field-effect transistors. In addition to the pronounced resonance that correlates with the second van Hove transition (E(22)) in semiconducting carbon nanotubes, a weaker sideband at about 200 meV higher energy is observed. Electronic structure calculations that include electron-phonon coupling indicate that the spectra originate from the simultaneous excitation of an exciton (main resonance) and a C-C bond stretching phonon (sideband). The spectral features are not compatible with an interband interpretation of the excitation involved.

Luminescence of Rare Earth Doped Si/ZrO2 Co-Sputtered Films

AbstractEr3+, Nd3+ and Tm3+ doped Si/ZrO2 thin films have been prepared by rf co-sputtering. The films are 5 inches long and divided into 50 sections or positions, labeled from P1 to P50. The target configuration is such that the main target is ZrO2 (181.46 cm2), the rare earth (RE) oxide pellet (1.43 cm2) is placed on the main target below the middle of section of the film and the Si chip (6.67 cm2 or 6.00 cm2) is placed on the main target below one end of the film (P40 to P50). The films were annealed to 700°C. The Er3+4I13/2→4I15/2 emission was detected but no 4I13/2→4I15/2 emission was detected. The 4I13/2→4I15/2 emission shows a narrow peak at 1527 nm (FWHM = 6.5 nm for P20) with two weaker side bands from 1430 nm to 1500 nm and from 1550 nm to 1600 nm. The Nd3+4F3/2→4I9/2, 4F3/2→4I11/2, and 4F3/2→4I13/2 emissions were detected being the 4F3/2→4I11/2 peaked at 1065 nm (FWHM = 39.5 nm for P30) the strongest. The 4F3/2→4I9/2 emission is relatively weak, less than one-fourth the peak intensity of the 4F3/2→4I1/2 emission but is broad (FWHM = 92 nm for P30). No Er3+4I13/2→4I15/2 emission or Nd3+4F3/2 emissions were detected for the Si rich ends of the respective films, being the emission stronger from the Si poor end of the film towards the middle of the film. The maximum Er3+4I13/2→4I15/2 emission is for P20 and the maximum peak intensity for the Nd3+4F3/2→4I11/2 emission is for P30. The excitation wavelength dependence behavior for the Nd3+4F3/2→4I11/2 emission is that typical of energy transfer from the Si nanoparticles to the emitting Nd3+ ions. The excitation wavelength behavior for the Er3+4I13/2→4I13/2 emission reflects a mix of energy transfer from the Si nanoparticles and strong absorption for excitation wavelengths of 488.0 nm and 514.5 nm. The Tm3+ doped Si/ZrO2 thin film does not exhibit infrared (IR) PL from the 3H4 emission or the 3F4→3H6 emission. The intense band from 500 nm to 800 nm observed for all of the RE doped Si/ZrO2 films, due to defects in ZrO2, barely permits the detection of the Tm3+3H4→3H6 emission which is best observed for P35.

Absorption and fluorescence spectra in diode laser spectrometry affected by relaxation oscillations

Relaxation sidebands were observed in the cavity mode spectra of laser diodes. Strong sidebands amounting to some percent of the central mode intensity affect the resolution and structure of absorption and fluorescence spectra. Very weak sidebands which are not visible in cavity mode spectra or by absorption spectrometry can be identified by saturation of a transition.

Laser-induced luminescence of xenon implanted natural diamond

We report here on laser-induced luminescence studies of the Xe ion related optical center in diamond. The sample with the Xe center was a natural diamond (type Ia) irradiated with 500 keV Xe ions at room temperature with the dose 1×10 13 cm −2 and annealed at 1400 C. The Xe center exhibits an isolated zero phonon line (ZPL) at 811.7 nm (1.527 eV) and a weak vibrational sideband. Growth of a second ZPL at 793.1 nm was observed at elevated temperatures T>65 K. Using Ti:sapphire laser excitation, it was established that both ZPLs observed are resonance 0–0 lines. It was concluded that the excited state of the optical transition has a double-level structure, and the ground state consists of a single level. The excited state energy level splitting of about Δ E=236±20 cm −1 was found by studying the temperature behavior of these ZPLs. This splitting is different from the spectroscopic splitting of 289 cm −1. Possible reasons of the discrepancy are discussed. The vibronic sideband of the ZPL at 811.7 nm with several phonons and quasilocal peaks was observed. The Huang–Rhys factor was determined to be exp(− S)=0.9±0.05 at temperature 8 K.

Multi-mode competition in an FEL oscillator at perfect synchronism of an optical cavity

The sustained saturation in a short pulse free electron laser (FEL) oscillator at perfect synchronism of an optical cavity has been observed recently by Japan Atomic Energy Research Institute (JAERI) FEL group by using their super-conducting linac (Phys. Rev. Lett., in preparation). The experiments have clearly shown that FEL efficiency becomes maximum at perfect synchronism, although it has been considered that only a transient state exists at perfect synchronism due to the lethargy effect. Through careful analyses of the experimental condition of JAERI FEL, we found that, in spite of the short length of the electron micro-bunch, the saturation appears due to the following features, which were different from other FEL experiments: (1) very large ratio of the small signal gain to losses, (2) very long electron macro-bunch which can tolerate a slow start up. The saturation and high efficiency at perfect synchronism were benefited from the contribution of the weak sideband instability. In order to analyse these new experimental phenomena, we expanded our three-dimensional time-dependent FEL code applicable to longitudinal mode competition, and obtained satisfactory numerical results, which were qualitatively in agreement with the experimental results.

Normal modes and propagation dynamics in a strongly driven Raman medium

We study the collinear propagation of two weak sideband fields in a far-off-resonance Raman medium driven by a strong field. We show the existence of two sideband-field superpositions called normal modes that propagate independently at different group velocities, one at the vacuum speed of light and one at a reduced velocity and with an induced relative phase shift. We find that the effect of slow group velocity on nonlinear conversion occurs via the relative group delay as well as the relative phase shift.

Stimulated Raman scattering with slow light

We study the propagation dynamics of weak Raman sideband fields in a far-off-resonance Raman medium driven by a strong coupling field. We show that the interaction of the system with the strong field, under the conditions of high density, narrow Raman-transition width, and small two-photon detuning, results ina slow group velocity and a substantial enhancement of the injected anti-Stokes sideband field as well as an efficient generation of a Stokes sideband field. We find that the effective group velocity is the same for the two weak fields and is proportional to the field frequency difference instead of the frequency of the corresponding field. We also discuss the condition for exponential growth of the two sideband fields in the medium. We perform numerical calculations for solid hydrogen, a realistic system where the requirements for high density and small Raman width can be met. We demonstrate that the group velocity can be slowed down by several orders, and that the slow light plays a key role for the stimulated Raman scattering process in solid hydrogen.PACS Nos.: 42.50Gy, 42.50Hz, 42.65Ky, 42.79Nv

High-spin states in the odd-oddN=Znucleus50Mn

High-spin states in the odd-odd $N=Z$ nucleus ${}^{50}\mathrm{Mn}$ have been identified. At low spin, the $T=1$ isobaric analogue states of ${}^{50}\mathrm{Cr}$ are established up to ${I}^{\ensuremath{\pi}}{=4}^{+}.$ At high spin, the $T=0$ band built on the low-lying ${I}^{\ensuremath{\pi}}{=5}^{+}$ isomer in ${}^{50}\mathrm{Mn}$ becomes energetically favored and this band is observed up to its ${f}_{7/2}$-shell termination at ${I}^{\ensuremath{\pi}}{=15}^{+}.$ Spherical shell model calculations in the full $\mathrm{pf}$ shell are found to be in excellent agreement with the experimental results. In addition, a weak side band in ${}^{50}\mathrm{Mn}$ is observed to spin ${(12}^{\ensuremath{-}})$ and is interpreted as a possible octupole vibrational structure.

Hexagonal optical structures in photorefractive crystals with a feedback mirror

A nonlinear theory is presented for the formation of hexagonal optical structures in a photorefractive medium equipped with a feedback mirror. Oppositely directed beams in photorefractive crystals are unstable against the excitation of sideband waves. It is shown here that as this instability evolves to its nonlinear stage, the three-wave interaction between weak sideband beams does not stabilize it, but rather leads to explosive growth of the amplitudes of beams whose transverse wave vectors form angles that are multiples of π/3. As a result, sideband beams at these angles are found to be correlated. A range of parameters is found in which four-wave interactions saturate the explosive instability, which explains the appearance of stable hexagons in the experiment. Outside this region, nonlinearities of higher order saturate the explosive instability, and the process of hexagon generation must be studied numerically. Matrix elements are obtained for the three-and four-wave interactions as functions of the distance to the feedback mirror, and an equation for the time evolution of the sideband wave amplitudes is derived that describes the hexagon generation. A comparison is made with experimental results for the photorefractive crystals KNbO3 and BaTiO3.

An adaptable NMR broadband decoupling scheme

A broadband heteronuclear decoupling scheme for high resolution liquid phase NMR spectroscopy is described, based on adiabatic fast passage with a linear frequency sweep. It is shown that the flexible choice of shaping function for the radiofrequency amplitude profile allows the method to be adapted to fit a wide range of sample parameters and experimental conditions. The practical limitations on phase cycling are emphasized. Expressions are derived to guide the choice of operating parameters so as to achieve a large effective bandwidth, low radiofrequency power dissipation, or weak cycling sidebands in the decoupled spectrum.

Effect of isotopic disorder on the Fu modes in crystalline C60.

The reflectance of undoped single crystals of natural ${\mathrm{C}}_{60}$ and isotopically pure $^{12}\mathrm{C}_{60}$ and $^{13}\mathrm{C}_{60}$ has been measured from 400 to 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at high resolution (0.2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) above and below the orientational-ordering transition. In natural ${\mathrm{C}}_{60}$, the ${\mathit{F}}_{\mathit{u}}$(1) mode shows evidence for isotopic splitting at 295 K; all the other modes are singlets. Below the orientational-ordering transition, the ${\mathit{F}}_{\mathit{u}}$(1) mode splits into five resolvable components, the ${\mathit{F}}_{\mathit{u}}$(3) and ${\mathit{F}}_{\mathit{u}}$(4) modes split into quartets, and the ${\mathit{F}}_{\mathit{u}}$(2) mode appears to remain a singlet. In the isotopically pure materials all the modes are singlets at 295 K; below the orientational-ordering transition, the ${\mathit{F}}_{\mathit{u}}$(1) mode splits into only four components; the ${\mathit{F}}_{\mathit{u}}$(3) and ${\mathit{F}}_{\mathit{u}}$(4) modes split into triplets; the ${\mathit{F}}_{\mathit{u}}$(2) mode shows only weak evidence for splitting. In those modes which display crystal-field splitting, isotopic disorder generates only a weak side band. The ${\mathit{F}}_{\mathit{u}}$(1) mode displays fine structure which may be related to the intrinsic merohedral disorder in the ground state. It is proposed that the anomalous increase in the oscillator strength of the ${\mathit{F}}_{\mathit{u}}$(4) mode is due to a modulation of the charge density along the 〈110〉 intermolecular bonding directions.