Detailed model and investigation of gain saturation and carrier spatial hole burning for a semiconductor optical amplifier with gain clamping by a vertical laser field

A detailed model for semiconductor linear optical amplifiers (LOAs) with gain clamping by a vertical laser field is presented, which accounts the carrier and photon density distribution in the longitudinal direction as well as the facet reflectivity. The photon iterative method is used in the simulation with output amplified spontaneous emission spectrum in the wide band as iterative variables. The gain saturation behaviors and the noise figure are numerically simulated, and the variation of longitudinal carrier density with the input power is presented which is associated with the ON-OFF state of the vertical lasers. The results show that the LOA can have a gain spectrum clamped in a wide wavelength range and have almost the same value of noise figure as that of conventional semiconductor optical amplifiers (SOAs). Numerical results also show that an LOA can have a noise figure about 2 dB less than that of the SOA gain clamped by a distributed Bragg reflector laser.

Low-frequency noise properties of GaN Schottky barriers deposited on intermediate temperature buffer layers

Metal-semiconductor-metal (MSM) structures were fabricated by RF-plasma-assisted MBE using different buffer layer structures. One type of buffer structure consists of an AlN high-temperature buffer layer (HTBL) and a GaN intermediate temperature buffer layer (ITBL), another buffer structure consists of just a single A IN HTBL. Systematic measurements in the flicker noise and deep level transient Fourier spectroscopy (DLTFS) measurements were used to characterize the defect properties in the films. Both the noise and DLTFS measurements indicate improved properties for devices fabricated with the use of ITBL and is attributed to the relaxation of residue strain in the epitaxial layer during growth process. (C) 2003 Elsevier Ltd. All rights reserved.

Photon-assisted Fano resonance and corresponding shot noise in a quantum dot

We have studied the Fano resonance in photon-assisted transport through a quantum dot. Both the coherent current and the spectral density of shot noise have been calculated. It is predicted that the shape of the Fano profile will also appear in satellite peaks. It is found that the variations of Fano profiles with the strengths of nonresonant transmissions are not synchronous in absorption and emission sidebands. The effect of interference on photon-assisted pumped current has also been investigated. We further predict the current and spectral density of shot noise as a periodic function of the phase, which exhibits an intrinsic property of resonant and nonresonant channels in the structures.

Spontaneous relaxation of a charge qubit under electrical measurement

In this work we first derive a generalized conditional master equation for quantum measurement by a mesoscopic detector, then study the readout characteristics of qubit measurement where a number of remarkable new features are found. The work would, in particular, highlight the qubit spontaneous relaxation effect induced by the measurement itself rather than an external thermal bath.

Effect of spontaneous and piezoelectric polarization on intersubband transition in AlxGa1-xN-GaN quantum well

A self-consistent solution of conduction band profile and subband energies for AlxGa1-xN-GaN quantum well is presented by solving the Schrodinger and Poisson equations. A new method is introduced to deal with the accumulation of the immobile charges at the AlxGa1-xN-GaN interface caused by spontaneous and piezoelectric polarization in the process of solving the Poisson equation. The effect of spontaneous and piezoelectric polarization is taken into account in the calculation. It also includes the effect of exchange-correlation to the one electron potential on the Coulomb interaction. Our analysis is based on the one electron effective-mass approximation and charge conservation condition. Based on this model, the electron wave functions and the conduction band structure are derived. We calculate the intersubband transition wavelength lambda(21) for different Al molar fraction of barrier and thickness of well. The calculated result can fit to the experimental data well. The dependence of the absorption coefficient a on the well width and the doping density is also investigated theoretically. (C) 2004 American Vacuum Society.

Calculation of the current noise spectrum in mesoscopic transport: A quantum master equation approach

Based on our recent work on quantum transport [X. Q. Li , Phys. Rev. B 71, 205304 (2005)], we show how an efficient calculation can be performed for the current noise spectrum. Compared to the classical rate equation or the quantum trajectory method, the proposed approach is capable of tackling both the many-body Coulomb interaction and quantum coherence on an equal footing. The practical applications are illustrated by transport through quantum dots. We find that this alternative approach is in a certain sense simpler and more straightforward than the well-known Landauer-Buttiker scattering matrix theory.

Spontaneous emission lifetime distribution of infinite line antennas in two-dimensional photonic crystals with finite size

The authors calculate the lifetime distribution functions of spontaneous emission from infinite line antennas embedded in two-dimensional disordered photonic crystals with finite size. The calculations indicate the coexistence of both accelerated and inhibited decay processes in disordered photonic crystals with finite size. The decay behavior of the spontaneous emission from infinite line antennas changes significantly by varying factors such as the line antennas' positions in the disordered photonic crystal, the shape of the crystal, the filling fraction, and the dielectric constant. Moreover, the authors analyze the effect of the degree of disorder on spontaneous emission. (c) 2007 American Institute of Physics.

A direct digital frequency synthesizer with fourth-order phase domain Delta Sigma noise shaper and 12-bit current-steering DAC

This paper presents a direct digital frequency synthesizer (DDFS) with a 16-bit accumulator, a fourth-order phase domain single-stage Delta Sigma interpolator, and a 300-MS/s 12-bit current-steering DAC based on the Q(2) Random Walk switching scheme. The Delta Sigma interpolator is used to reduce the phase truncation error and the ROM size. The implemented fourth-order single-stage Delta Sigma noise shaper reduces the effective phase bits by four and reduces the ROM size by 16 times. The DDFS prototype is fabricated in a 0.35-mu m CMOS technology with active area of 1.11 mm(2) including a 12-bit DAC. The measured DDFS spurious-free dynamic range (SFDR) is greater than 78 dB using a reduced ROM with 8-bit phase, 12-bit amplitude resolution and a size of 0.09 mm(2). The total power consumption of the DDFS is 200)mW with a 3.3-V power supply.

Theory and experiment of two-photon 'direct' interference for type-II spontaneous parametric down-conversion with an ultrafast pulse laser pump

Fourth-order spatial interference of entangled photon pairs generated in the process of spontaneous parametric down-conversion pumped by a femtosecond pulse laser has been performed for the first time. In theory, it takes into account the transverse correlation between the two photons and is used to calculate the dependence of the visibility of the interference pattern obtained in Young's double-slit experiment. In this experiment, a short focal length tens and two narrow band interference filters were adopted to eliminate the effects of the broadband pump laser and improve the visibility of the interference pattern under the condition of nearly collinear light and degenerate phase matching.

Size limit and spontaneous emission factor for equilateral triangle semiconductor microlasers

The size of equilateral triangle resonator (ETR) needed for confining the fundamental mode is investigated by the total reflection condition of mode light rays and the FDTD numerical simulation. The confinement of the TM modes can be explained by the total reflection of mode light rays, and the confinement of the TE modes requires a larger ETR than the TM modes, which may be caused by excess scattering or radiation loss for the TE modes. With the multilayer staircase approximation, it is found that the spontaneous emission factor of the ETR lasers has the same form as that of strip waveguide lasers.

Spatial interference effect of two-photon in femtosecond-pulse pumped spontaneous parametric down-conversion

We have used the transverse correlated properties of the entangled photon pairs generated in the process of spontaneous parametric down-conversion, which is pumped by a femtosecond pulse laser, to perform Young's interference experiment. Unlike the case of a continuous wave laser pump, a broadband pulse laser pump can submerge an interference pattern. In order to obtain a high visibility interference pattern, we used a lens with a tunable focal length and two interference filters to eliminate the effects of the broadband pump laser. It is proven that the process of two-photon direct interference is a post-selection process.

Effects of piezoelectricity and spontaneous polarization on electronic and optical properties of wurtzite III-V nitride quantum wells

A theoretical model accounting for the macropolarization effects in wurtzite III-V nitrides quantum wells (QWs) is presented. Energy dispersions and exciton binding energies are calculated within the framework of effective-mass theory and variational approach, respectively. Exciton-associated transitions (EATs) are studied in detail. An energy redshift as high as 450 meV is obtained in Al0.25GaN0.75/GaN QWs. Also, the abrupt reduction of optical momentum matrix elements is derived as a consequence of quantum-confined Stark effects. EAT energies are compared with recent photoluminescence (PL) experiments and numerical coherence is achieved. We propose that it is the EAT energy, instead of the conduction-valence-interband transition energy that is comparable with the PL energy. To restore the reduced transition rate, we apply an external electric field. Theoretical calculations show that with the presence of the external electric field the optical matrix elements for EAT increase 20 times. (C) 2001 American Institute of Physics.

Random telegraph noise in the photoluminescence of individual GaxIn1-xAs quantum dots in GaAs

We have investigated random telegraph noise in the photoluminescence from InGaAs quantum dots in GaAs. Dots switching among two and three levels have been measured. The experiments show that the switching InGaAs dots behave very similarly to switching InP dots in GaInP. but differently from the more commonly investigated colloidal dots. The switching is attributed to defects, and we show that the switching can be used as a monitor of the defect.

Characterization of hot-electron effects on flicker noise in III-V nitride based heterojunctions

We report experiments on hot-electron stressing in commercial III-V nitride based heterojunction fight-emitting diodes. Stressing currents ranging from 100 mA to 200 mA were used. Degradations in the device properties were investigated through detailed studies of the I-V characteristics, electroluminescence, Deep-Level Transient Fourier Spectroscopy and flicker noise. Our experimental data demonstrated significant distortions in the I-V characteristics. The room temperature electroluminescence of the devices exhibited 25% decrement in the peak emission intensity. Concentration of the deep-levels was examined by measuring the Deep-Level Transient Fourier Spectroscopy, which indicated an increase in the density of deep-traps from 2.7 x 10(13) cm(-3) to 4.21 x 10(13) cm(-3) at E-1 = E-C - 1.1eV. The result is consistent with our study of 1/f noise, which exhibited up to three orders of magnitude increase in the voltage noise power spectra. Our experiments show large increase in both the interface traps and deep-levels resulted from hot-carrier stressing.

A high-gain high-linearity wide-band low noise amplifier for terrestrial and cable receptions

This paper presents the design of a wide-band low-noise amplifier (LNA) implemented in a 0.35 mu m SiGe BiCMOS technology for cable (DVB-C) and terrestrial (DVB-T) tuner applications. The LNA utilizes current injection to achieve high linearity. Without using inductors, the LNA achieves 0.1-1GHz wide bandwidth and 18.8-dB gain with less than 1.4-dB gain variation. The noise figure(NF) of the wideband LNA is 5dB, its 1-dB compression point is -2dBm and IIP3 is 8dBm. The LNA dissipates 120mW power with a 5-V supply.