Electric-field switching of exciton spin splitting in coupled quantum dots

We investigate theoretically the spin splitting of the exciton states in semiconductor coupled quantum dots (CQDs) containing a single magnetic ion. We find that the spin splitting can be switched on/off in the CQDs via the sp-d exchange interaction using the electric field. An interesting bright-to-dark exciton transition can be found and it significantly affects the photoluminescence spectrum. This phenomenon is induced by the transition of the ground exciton state, arising from the hole mixing effect, between the bonding and antibonding states. (C) 2008 American Institute of Physics.

Room temperature photoluminescence of tensile-strained Ge/Si0.13Ge0.87 quantum wells grown on silicon-based germanium virtual substrate

We report a room temperature study of the direct band gap photoluminescence of tensile-strained Ge/Si0.13Ge0.87 multiple quantum wells grown on Si-based germanium virtual substrates by ultrahigh vacuum chemical vapor deposition. Blueshifts of the luminescence peak energy from the Ge quantum wells in comparison with the Ge virtual substrate are in good agreement with the theoretical prediction when we attribute the luminescence from the quantum well to the c Gamma 1-HH1 direct band transition. The reduction in direct band gap in the tensile strained Ge epilayer and the quantum confinement effect in the Ge/Si0.13Ge0.87 quantum wells are directly demonstrated by room temperature photoluminescence.

Magnetic field switching in parallel quantum dots

We show that the Coulomb blockade in parallel dots pierced by magnetic flux Phi completely blocks the resonant current for any value of Phi except for integer multiples of the flux quantum Phi(0). This non-analytic (switching) dependence of the current on Phi arises only when the dot states that carry the current are of the same energy. The time needed to reach the steady state, however, diverges when Phi -> n Phi(0). Copyright (C) EPLA, 2009

Whispering-gallery microcavity semiconductor lasers suitable for photonic integrated circuits and optical interconnects

The characteristics of whispering-gallery-like modes in the equilateral triangle and square microresonators are introduced, including directional emission triangle and square microlasers connected to an output waveguide. We propose a photonic interconnect scheme by connecting two directional emission microlasers with an optical waveguide on silicon integrated circuit chip. The measurement indicates that the triangle microlasers can work as a resonance enhanced photodetector for optical interconnect.

Temperature-induced switching-over of the luminescence transitions in GaInNAs/GaAs quantum wells

Photoluminescence (PL) spectra of the GaInNAs/GaAs single quantum well (SQW) with different N compositions are carefully studied in a range of temperatures and excitation power densities. The anomalous S-shape temperature dependence of the PL peak is analysed based on the competition and switching-over between the peaks related to N-induced localized states and the peak related to interband excitonic recombination. It is found that with increasing N composition, the localized energy increases and the turning point of the S-shape temperature dependence occurs at higher temperature, where the localized carriers in the bandtail states obtain enough thermal activation energy to be dissociated and delocalized. The rapid thermal annealing (RTA) effectively reduces the localized energy and causes a decrease of the switching-over temperature.

Rearrangeable nonblocking thermo-optic 4 x 4 switching matrix in silicon-on-insulator

A rearrangeable nonblocking thermo-optic 4 x 4 switching matrix is demonstrated. The matrix, which consists of five 2 x 2 multimode interference-based Mach-Zehnder interferometer (MMI-MZI) switch elements, is fabricated in silicon-on-insulator waveguide system. The average excess loss for the optical path experiencing 2 and 3 switch elements is 6.6 and 10.1 dB respectively. The crosstalk in the matrix is measured to be between -12 and -19 dB. The switching time of the device is less than 30 mu s.

Passive Q-switching modelocked Yb3+-doped fibre laser with GaAs absorber grown at low temperature

GaAs absorber was grown at low temperature (550degreesC) by metal organic chemical vapour deposition (MOCVD) and was used as an output coupler with which we realized Q-switching modelocked Yb3+-doped fibre laser. The shortest period of the envelope of the Q-switched modelocking is about 3mus. The modelocking threshold is 4.27W and the highest average output pulse power is 290 mW. The modelocking frequency is 12 MHz.

Simultaneous Q switching and mode locking with narrow envelope width in a microchip Nd : YVO4 laser with a composite semiconductor absorber

By using a composite semiconductor absorber and an output coupler, we demonstrated a Q-switched and mode-locked diode-pumped microchip Nd:YVO4 laser. With a 350-mu m-thick crystal, the width of the Q-switched envelope was as short as 12 ns; the repetition rate of the mode-locked pulses inside the Q-switched pulse was more than 10 GHz. The average output power was 335 mW at a maximum pump power of 1.6 W. Q-switched envelope widths of 21 and 31 ns were also achieved with crystals 0.7 and 1.0 mm thick, respectively.

Design of two-dimensional photonic crystal edge emitting laser for photonic integrated circuits

An edge emitting laser based on two-dimensional photonic crystal slabs is proposed. The device consists of a square lattice microcavity, which is composed of two structures with the same period but different radius of air-holes, and a waveguide. In the cavity, laser resonance in the inner structure benelits from not only the anomalous dispersion characteristic of the first band-edge at the M point in the first Brillouin-zone but also zero photon states in the outer structure. A line defect waveguide is introduced in the outer structure for extracting photons from the inner cavity. Three-dimensional finite-difference time-domain simulations apparently show the in-plane laser output from the waveguide. The microcavity has an effective mode volume of about 3.2(lambda/eta(slab))(3) for oscillation -mode and the quality factor of the device including line defect waveguide is estimated to be as high as 1300.

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.

Mode analysis of photonic crystal polarization beam splitter and its application in integrated circuits design - art. no. 698437

In this work, the guided modes of a photonic crystal polarization beam splitter (PC-PBS) are studied. We demonstrate that the transmission of a low-loss photonic crystal 120 degrees waveguide bend integrated with the PBS will be influenced if the PBS is multi-moded. We propose a single-moded PC-PBS structure by introducing deformed structures, and it shows twice the enhancement of the transmission. This device with remarkable improvement of performance is promising in the use of photonic crystal integrated circuits design.

Low Power Integrated Circuits for Wireless Neural Recording Applications

A group of prototype integrated circuits are presented for a wireless neural recording micro-system. An inductive link was built for transcutaneous wireless power transfer and data transmission. Power and data were transmitted by a pair of coils on a same carrier frequency. The integrated receiver circuitry was composed of a full-wave bridge rectifier, a voltage regulator, a date recovery circuit, a clock recovery circuit and a power detector. The amplifiers were designed with a limited bandwidth for neural signals acquisition. An integrated FM transmitter was used to transmit the extracted neural signals to external equipments. 16.5 mW power and 50 bps - 2.5 Kbps command data can be received over 1 MHz carrier within 10 mm. The total gain of 60 dB was obtained by the preamplifier and a main amplifier at 0.95Hz - 13.41 KHz with 0.215 mW power dissipation. The power consumption of the 100 MHz ASK transmitter is 0.374 mW. All the integrated circuits operated under a 3.3 V power supply except the voltage regulator.

A 12-bit 300 MHz CMOS DAC for high-speed system applications

This paper describes a 12-bit 300 MHz CMOS DAC for high-speed system applications. The proposed DAC consists of a unit current-cell matrix for 8 MSBs and a binary-weighted array for 4 LSBs. In order to ensure the linearity of DAC, a double Centro symmetric current matrix is designed by using the Q(2) random walk strategy. To minimize the feedthrough and improve the dynamic performance, the drain of the switching transistors is isolated from the output lines by adding two cascoded transistors.

A direct digital frequency synthesizer with single-stage delta-sigma interpolator and current-steering DAC

This paper presents a direct digital frequency synthesizer (DDFS) with a 16-bit accumulator, a 4th-order single-stage pipelined delta-sigma interpolator and a 300MS/s 12-bit current-steering DAC based on Q(2) Random Walk switching scheme. The delta-sigma interpolator is used to reduce the phase truncation error and the ROM size. The measured spurious-free dynamic range (SFDR) is greater than 80 dB for 8-bit phase value and 12-bit sine-amplitude output. The DDFS prototype is fabricated in a 0.35um CMOS technology with core area of 1.11mm(2).

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.