Sideband Intensity Research Articles

Energy transfer properties and temperature-dependent luminescence of Ca14Al10Zn6O35: Dy3+, Mn4+ phosphors

Dy3+-doped or Dy3+/Mn4+ co-doped Ca14Al10Zn6O35 phosphors were prepared by the solid-state reaction method. The phosphors were characterized by means of XRD, SEM, luminescence analysis, and thermal stability analysis. The obtained phosphors can be excited efficiently by NUV light and exhibit blue/yellow emission and deep red emission, which result from 4F9/2 → 6H15/2/4F9/2 → 6H13/2 transitions of Dy3+ ions and 2E → 4A2 transition of Mn4+ ions, respectively. The efficient energy transfer from Dy3+ ions to Mn4+ ions was confirmed, which was realized via the electric dipole–dipole interaction. The thermal quenching temperature of the phosphors is higher than 463 K, indicating that the phosphors have an excellent thermal stability. The temperature-induced electron population variation in the vibrational states caused the different influence on the emission intensity of Stokes and anti-Stokes sidebands of Mn4+ ions. By adding Na+ ion as a charge compensator and changing its doping concentration, the emission intensity of Ca13.88Al9.99Zn6O35: 0.12Dy3+, 0.01Mn4+ sample can be enhanced and the color tunability can be achieved. Ca14Al10Zn6O35: Dy3+, Mn4+, Na+ phosphors have potential application in solid-state lighting field.

Quantum Hamiltonian theory of an electro-optical modulator

A Quantum Hamiltonian formalism is proposed for the description of an electro-optical modulator based on the linear Pockels effect. Optical photons interact with photons of a microwave mode in a combined high-Q cavity made of a LiNbO3 crystal. The microwave photons occupy a coherent state, while optical photons have an arbitrary density matrix. The spectrum of a photodetected modulated signal is analyzed as a function of the frequency of a tunable optical filter. Numerical estimates are obtained, and quantum effects in the spectrum, such as the red shift of the central frequency and sidebands, the possibility of modulation of the optical signal by the microwave field vacuum, and the asymmetry of the intensity of the spectral sidebands, are discussed.

Theory of effect of near-infrared laser polarization direction on high-order terahertz sideband generation in semiconductors

In a semiconductor illuminated by a strong terahertz (THz) field, the electron-hole pairs excited by linear polarized near-infrared (NIR) laser can recombine to emit high-order THz sideband. Previous experimental results have shown, under the same condition of excitation intensity, the polarization direction of the NIR laser could affect the sideband intensity. In this letter, we theoretically investigate the effect of the NIR laser polarization direction on high-order terahertz sideband generation in bulk GaAs.

Correlations in pulsed resonance fluorescence.

We investigated the first and second-order correlations of the light scattered near-resonantly by a quantum dot under excitation by a frequency comb, i.e., a periodically pulsed laser source. In contrast to its monochromatic counterpart, the pulsed resonance fluorescence spectrum features a superposition of sidebands distributed around a central peak with maximal sideband intensity near the Rabi frequency. Distinguishing between the coherently and incoherently scattered light reveals pulse-area dependent Rabi oscillations evolving with different phase for each component. Our observations, which can be reproduced theoretically, may impact schemes for remote entanglement based on pulsed two-photon interference.

Optical sideband generation up to room temperature with mid-infrared quantum cascade lasers.

Mid-infrared (MIR) sideband generation on a near infrared (NIR) optical carrier is demonstrated within a quantum cascade laser (QCL). By employing an externally injected NIR beam, E(NIR), that is resonant with the interband transitions of the quantum wells in the QCL, the nonlinear susceptibility is enhanced, leading to both frequency mixing and sideband generation. A GaAs-based MIR QCL (E(QCL) = 135 meV) with an aluminum-reinforced waveguide was utilized to overlap the NIR and MIR modes with the optical nonlinearity of the active region. The resulting difference sideband (E(NIR) - E(QCL)) shows a resonant behavior as a function of NIR pump wavelength and a maximum second order nonlinear susceptibility, χ((2)), of ~1 nm/V was obtained. Further, the sideband intensity showed little dependence with the operating temperature of the QCL, allowing sideband generation to be realized at room temperature.

Iterative intersymbol interference cancellation in vestigial sideband Nyquist–subcarrier modulation system

The intersymbol interference caused by dispersion, chirp, and a vestigial sideband filter in intensity modulation and a direct detection single carrier system is analyzed theoretically and numerically. An iterative nonlinear intersymbol interference cancellation technique is proposed and experimentally demonstrated in a 40-Gbps 16-QAM Mach-Zehnder modulator-based vestigial sideband intensity modulation and direct detection half-cycle Nyquist–subcarrier modulation system over a 100-km uncompensated standard single-mode fiber transmission for the first time. The experimental results show that 2.2-dB receiver sensitivity improvement is obtained at the forward error correction limit by using the iterative technique.

Research of an integrative technique with polarization modulation for generating high-quality multi-wavelength optical source

With the development of society, the need of fiber optical communication expansion and upgrades is increasing. Multi-wavelength optical source has been popularly researched as one of the key component of DWDM. A integrative technique with polarization modulation for high-quality multi-wavelength optical source has been proposed theoretically and experimentally. A comprehensive and systematic overview of how to get a high-quality multi-wavelength optical source, based on in-depth and detailed experiment research. It is found that the optical intensity of the sidebands are not only related to the microwave source and DC bias voltage loaded at EOIM, but also affected by the polarization state of the input light. Five perfect sidebands with equal peak value can be realized by regulating the polarization state of the input light, in addition to regulating the microwave source and DC bias voltage loaded at EOIM as well as. The peak–peak difference of intensity of sidebands can be down to 0.00dBm in the best situation. Polarization state of the input light plays a good role in generating wonderful sidebands with equal space, balanced peak value, high signal-to-noise ratio. The number of sidebands can be selected according to specific application. The findings may have important implications for high-quality multi-wavelength optical source.

40-Gbps vestigial sideband half-cycle Nyquist subcarrier modulation transmission experiment and its comparison with orthogonal frequency division multiplexing

We experimentally demonstrate the superior performance of a 40-Gbps 16-QAM half-cycle Nyquist subcarrier modulation (SCM) transmission over a 100-km uncompensated standard single-mode fiber using dual-drive Mach-Zehnder modulator-based vestigial sideband intensity modulation and direct detection. The impact of modulator chirp on the system performance is experimentally evaluated. This Nyquist-SCM technique is compared with optical orthogonal frequency division multiplexing in both back-to-back and 100-km transmission experiments, and the results show that the Nyquist system has a better performance.

A Novel Measurement Approach for the Half-wave Voltage of Phase Modulator based on PM-MZI Photonic Link

This paper presents a new method for measuring the half-wave voltage $V_{\pi}$ of an electro-optic phase modulator based on a phase-modulated photonic link with interferometric demodulation. By using this method, the $V_{\pi}$ can be obtained with the RF voltage amplitude input required to achieve 1-dB gain compression of link and the differential delay of a Mach-Zehnder interferometer. We measure the $V_{\pi}$ of a commercial phase modulator by using the presented method and the carrier/the first sideband intensity ratio method. Furthermore, we compare the two measurements with the typical value provided by the manufacturer. The experiment shows that this novel measurement method is feasible, straightforward, and accurate.

Silica/Electro-Optic Polymer Optical Modulator With Integrated Antenna for Microwave Receiving

A silica/electro-optic (EO) polymer phase modulator is proposed for microwave radiation receiver with the use of a bowtie antenna. Waveguide design optimization is presented for a waveguide with an EO polymer core and silica/sol-gel cladding. Electrode effects on the insertion loss and poling efficiency are also analyzed and conditions for low-loss and high poling efficiency established. The bowtie antenna is simulated and shows a broadband response with a maximum at 5 GHz and a 3 dB-bandwidth of approximately 12 GHz. A fiber splicing technique is presented that reduces coupling loss between SMF-28 and the waveguide. Experimental results for a fabricated device with microwave-response between 10-14 GHz are shown with carrier to first sideband intensity difference of up to -36 dB.

Exciton-magnon transitions in the frustrated chromium antiferromagnets CuCrO2,α-CaCr2O4, CdCr2O4, and ZnCr2O4

We report on optical transmission spectroscopy of the Cr-based frustrated triangular antiferromagnets CuCrO${}_{2}$ and $\ensuremath{\alpha}$-CaCr${}_{2}$O${}_{4}$, and the spinels CdCr${}_{2}$O${}_{4}$ and ZnCr${}_{2}$O${}_{4}$ in the near-infrared to visible-light frequency range. We explore the possibility to search for spin correlations far above the magnetic ordering temperature and for anomalies in the magnon lifetime in the magnetically ordered state by probing exciton-magnon sidebands of the spin-forbidden crystal-field transitions of the Cr${}^{3+}$ ions (spin $S=3/2$). In CuCrO${}_{2}$ and $\ensuremath{\alpha}$-CaCr${}_{2}$O${}_{4}$ the appearance of fine structures below ${T}_{\text{N}}$ is assigned to magnon sidebands by comparison with neutron-scattering results. The temperature dependence of the linewidth of the most intense sidebands in both compounds can be described by an Arrhenius law. For CuCrO${}_{2}$ the sideband associated with the ${}^{4}{A}_{\mathsf{2}}\ensuremath{\rightarrow}{}^{2}{T}_{\mathsf{2}}$ transition can be observed even above ${T}_{\text{N}}$. Its linewidth does not show a kink at the magnetic ordering temperature and can alternatively be described by a ${Z}_{2}$ vortex scenario proposed previously for similar materials. The exciton-magnon features in $\ensuremath{\alpha}$-CaCr${}_{2}$O${}_{4}$ are more complex due to the orthorhombic distortion. While for CdCr${}_{2}$O${}_{4}$ magnon sidebands are identified below ${T}_{\text{N}}$ and one sideband excitation is found to persist across the magnetic ordering transition, only a weak fine structure related to magnetic ordering has been observed in ZnCr${}_{2}$O${}_{4}$.

High order sideband generation in terahertz quantum cascade lasers

We demonstrate the generation of high order terahertz (THz) frequency sidebands (up to 3rd order) on a near infrared (NIR) optical carrier within a THz quantum cascade laser (QCL). The NIR carrier is resonant with the interband transition of the quantum wells composing the QCL, allowing the nonlinearity to be enhanced and leading to frequency mixing. A phonon depopulation based QCL with a double metal cavity was used to enhance the intracavity power density and to demonstrate the higher order sidebands. The 1st order sideband intensity shows a linear dependence with THz power corresponding to a single THz photon, while the second order sideband has a quadratic dependence implying a two THz photon interaction and hence a third order susceptibility. These measurements are compared to the photoluminescence and the QCL bandstructure to identify the states involved, with the lowest conduction band states contributing the most to the sideband intensity. We also show that the interaction for the second order sideband corresponds to an enhanced direct third order susceptibility χ(3) of ∼7 × 10−16(m/V)2, two orders of magnitude greater than the bulk value.

Evolution of dark–dark soliton pairs in a dispersion managed erbium-doped fiber ring laser

The evolution of dark–dark soliton pairs is numerically investigated in a dispersion managed erbium-doped fiber ring laser. It is found that the fluctuation of the background, the sideband intensity of the optical spectrum, the distance and the width of dark–dark soliton pairs are related to the orientation of the polarization controllers and the pump power of the fiber ring laser. The evolution of dark–dark soliton pairs is caused by the modulation instability and the balance between gain and loss in the fiber ring laser.

Structural relaxation in tetrahedrally coordinated Co2+ along the gahnite-Co-aluminate spinel solid solution

The structural relaxation around the Co2+ ion along the gahnite (ZnAl2O4)-Co-aluminate (CoAl2O4) join was investigated by a combined X-ray diffraction (XRD) and electronic absorption spectroscopy (EAS) approach. Monophasic spinel samples (Zn1-yCoyAl2O4 with y = 0, 0.25, 0.5, 0.75, and 1 apfu) were obtained through solid-state reaction (1300 °C with slow cooling). The cobalt incorporation induces a linear increase of the unit-cell parameter (a) accompanied by an increasing inversion parameter (up to 0.07) so that the Co2+ for Al3+ substitution in the octahedral site is, at a first approximation, the cause of the lattice expansion. However, a careful consideration of T-O distances highlights the role played by an enhanced covalence degree of Zn-O bonds. The optical spectra are characterized by the occurrence of electronic transitions of Co2+ in tetrahedral coordination affected by a strong spin-orbit coupling, causing a threefold splitting of spin-allowed bands. Further complications stem from mixing of quadruplet and doublet states (leading to a consistent intensity gain of spin-forbidden bands) and vibronic effects (producing intense sidebands). Crystal field strength goes from 4187 to 4131 cm-1 with increasing cobalt amount, while the Racah B parameter is in the 744-751 cm-1 range (C ∼3375 cm-1). To achieve a reliable estimation of the local Co-O distance, the tetrahedral distance evolution was recast to eliminate the effects of the inversion degree. By this way, a relaxation coefficient as low as ε = 0.47 was obtained, i.e., significantly smaller than literature data for other spinel systems. The gahnite-Co-aluminate join seems to be constrained by the strong preference of Zn2+ for the tetrahedral site in which its enhanced covalency can be exerted, limiting the cation exchange between tetrahedral and octahedral sites as well as the lattice flexibility.

Experimental observation of electron–hole recollisions

An intense laser field can remove an electron from an atom or molecule and pull the electron into a large-amplitude oscillation in which it repeatedly collides with the charged core it left behind. Such recollisions result in the emission of very energetic photons by means of high-order-harmonic generation, which has been observed in atomic and molecular gases as well as in a bulk crystal. An exciton is an atom-like excitation of a solid in which an electron that is excited from the valence band is bound by the Coulomb interaction to the hole it left behind. It has been predicted that recollisions between electrons and holes in excitons will result in a new phenomenon: high-order-sideband generation. In this process, excitons are created by a weak near-infrared laser of frequency f(NIR). An intense laser field at a much lower frequency, f(THz), then removes the electron from the exciton and causes it to recollide with the resulting hole. New emission is predicted to occur as sidebands of frequency f(NIR) + 2nf(THz), where n is an integer that can be much greater than one. Here we report the observation of high-order-sideband generation in semiconductor quantum wells. Sidebands are observed up to eighteenth order (+18f(THz), or n = 9). The intensity of the high-order sidebands decays only weakly with increasing sideband order, confirming the non-perturbative nature of the effect. Sidebands are strongest for linearly polarized terahertz radiation and vanish when the terahertz radiation is circularly polarized. Beyond their fundamental scientific significance, our results suggest a new mechanism for the ultrafast modulation of light, which has potential applications in terabit-rate optical communications.

Interferometric scanning microscopy for the study of disordered materials

We demonstrate an interferometric optical technique that probes pair-pair correlations in disordered materials. Fraunhofer diffraction patterns, using coherent double-probe illumination, exhibit Young’s interference fringes whose strength is influenced by structural correlations between the two probed regions. Fourier transforms of diffraction patterns exhibit holographic sidebands, and the strength of correlations is proportional to the sideband intensity. Autoregression analysis of the correlation strength provides a direct measure of the characteristic ordering length scales. This technique is extendable in principle to x-ray and electron probes for studying materials at atomic length scales.

Observation of large contrast electromagnetically induced absorption resonance due to population transfer in a three-level Λ-system interacting with three separate electromagnetic fields

We describe a new scheme to induce large contrast (nearly 50%) absorption resonances using three co-propagating fields which interact with a three-level Λ-system (obtained by the D(2) transition of (87)Rb atoms) in an N-configuration scheme. A single mode laser which couples the upper ground state to the excited state of (87)Rb is phase modulated at half the hyperfine splitting frequency. The resultant three line spectrum interacts with the atomic vapor yielding a population transfer which increases the absorption by an amount which depends on the carrier to modulation side band intensity ratio.

Technical aspects of fast magic-angle turning NMR for dilute spin-1/2 nuclei with broad spectra

For obtaining sideband-free spectra of high-Z spin-1/2 nuclei with large (>1000 ppm) chemical-shift anisotropies and broad isotropic-shift dispersion, we recently identified Gan's modified five-pulse magic-angle turning (MAT) experiment as the best available broadband pulse sequence, and adapted it to fast magic-angle spinning. Here, we discuss technical aspects such as pulse timings that compensate for off-resonance effects and are suitable for large CSAs over a range of 1.8 γB 1; methods to minimize the duration of z-periods by cyclic decrementation; shearing without digitization artifacts, by sharing between channels (points); and maximizing the sensitivity by echo-matched full-Gaussian filtering. The method is demonstrated on a model sample of mixed amino acids and its large bandwidth is highlighted by comparison with the multiple- π-pulse PASS technique. Applications to various tellurides are shown; these include GeTe, Sb 2Te 3 and Ag 0.53Pb 18Sb 1.2Te 20, with spectra spanning up to 190 kHz, at 22 kHz MAS. We have also determined the 125Te chemical shift anisotropies from the intensities of the spinning sidebands resolved by isotropic-shift separation.

Thermalization of coupled atom-light states in the presence of optical collisions

The interaction of a two-level atomic ensemble with a quantized single mode electromagnetic field in the presence of optical collisions (OC) is investigated both theoretically and experimentally. The main accent is made on achieving thermal equilibrium for coupled atom-light states (in particular dressed states). We propose a model of atomic dressed state thermalization that accounts for the evolution of the pseudo-spin Bloch vector components and characterize the essential role of the spontaneous emission rate in the thermalization process. Our model shows that the time of thermalization of the coupled atom-light states strictly depends on the ratio of the detuning and the resonant Rabi frequency. The predicted time of thermalization is in the nanosecond domain and about ten times shorter than the natural lifetime at full optical power in our experiment. Experimentally we are investigating the interaction of the optical field with rubidium atoms in an ultra-high pressure buffer gas cell under the condition of large atom-field detuning comparable to the thermal energy in frequency units. In particular, an observed detuning dependence of the saturated lineshape is interpreted as evidence for thermal equilibrium of coupled atom-light states. A significant modification of sideband intensity weights is predicted and obtained in this case as well.

Strong-field terahertz optical sideband generation for wavelength conversion in asymmetric double quantum wells

Terahertz (THz) optical sideband generation in asymmetric double quantum wells in strong THz fields is studied theoretically. Both monotonic THz-power dependence and saturation of the first-order sideband intensity can occur, depending on the near infrared (NIR) and THz photon energies. In both respects the calculated sideband intensity behavior agrees with experimental findings [Appl. Phys. Lett. 75, 2728 (1999); Phys. Rev. B 70, 115312 (2004)]. The nonmonotonic THz-power dependence and saturation of the sideband intensity are due to the excitonic Stark splitting. While the sideband saturation imposes limitations on the wavelength conversion efficiency, the simultaneous broadening of sideband resonances benefits wavelength conversion by increasing the number of NIR frequencies available for wavelength conversion.