Control over hysteresis curves and thresholds of optical bistability in different semiconductor double quantum wells

The effects of optical field on the phenomenon of optical bistability(OB) are investigated in a K-type semiconductor double quantum well(SDQW) under various parametric conditions. It is shown that the OB threshold can be manipulated by increasing the intensity of coupling field. The dependence of the shift of OB hysteresis curve on probe wavelength detuning is then explored. In order to demonstrate controllability of the OB in this SDQW, we compare the OB features of three different configurations which could arise in this SDQW scheme, i.e., K-type, Y-type, and inverted Y-type systems. The controllability of this semiconductor nanostructure medium makes the presented OB scheme more valuable for applications in all-optical switches, information storage, and logic circuits of all optical information processing.

Enhancement of four-wave mixing process in a four-level double semiconductor quantum well

The time-dependent four-wave mixing（FWM） is analyzed in a four-level double semiconductor quantum well. The results show that both the amplitude and the conversion efficiency of the FWM field are enhanced with increasing the strength of two-photon Rabi frequency. Interestingly, when the one-photon detuning becomes stronger the control field corresponding to the maximum efficiency increases. Such a controlled enhanced FWM may be used to generate coherent short-wave length radiation, and it can have potential applications in quantum control and communications.

Investigation of GaN-based dual-wavelength light-emitting diodes with p-type barriers and vertically stacked quantum wells

Designs of p-doped in quantum well （QW） barriers and specific number of vertically stacked QWs are proposed to improve the optical performance of GaN-based dual-wavelength light-emitting diodes （LEDs）. Emission spectra, carrier concentration, electron current density, and internal quantum efficiency （IQE） are studied numerically. Simulation results show that the efficiency droop and the spectrum intensity at the large current injection are improved markedly by using the proposed design. Compared with the conventional LEDs, the uniform spectrum intensity of dual-wavelength luminescence is realized when a specific number of vertically stacked QWs is adopted. Suppression of electron leakage current and the promotion of hole injection efficiency could be one of the main reasons for these improvements.

High power red-light GalnP/AlGalnP laser diodes with nonabsorbing windows based on Zn diffusion-induced quantum well intermixing

The layer structure of GaInP/AlGaInP quantum well laser diodes （LDs） was grown on GaAs substrate using low-pressure metalorganic chemical vapor deposition （LP-MOCVD） technique. In order to improve the catastrophic optical damage （COD） level of devices, a nonabsorbing window （NAW）, which was based on Zn diffusion-induced quantum well intermixing, was fabricated near the both ends of the cavities. Zn diffusions were respectively carried out at 480, 500, 520. 540, and 580 ℃ for 20 minutes. The largest energy blue shift of 189.1 meV was observed in the window regions at 580 ℃. When the blue shift was 24.7 meV at 480 ℃, the COD power for the window LD was 86.7% higher than the conventional LD.

InGaN/GaN laser diode characterization and quantum well number effect

The effect of quantum well number on the quantum efficiency and temperature characteristics of In- GaN/GaN laser diodes （LDs） is determined and investigated. The 3-nm-thick In0.13Ca0.87N wells and two 6-am-thick GaN barriers are selected as an active region for Fabry-Perot （FP） cavity waveguide edge emitting LD. The internal quantum efficiency and internal optical loss coefficient are extracted through the simulation software for single, double, and triple InGaN/GaN quantum wells. The effects of device temperature on the laser threshold current, external differential quantum efficiency （DQE）, and output wavelength are also investigated. The external quantum efficiency and characteristic temperature are improved significantly when the quantum well number is two. It is indicated that the laser structures with many quantum wells will suffer from the inhomogeneity of the carrier density within the quantum well itself which affects the LD performance.

20 Mbps wireless communication demonstration using terahertz quantum devices

A wireless terahertz （THz） communication link is demonstrated, in which a THz quantum cascade laser and a THz quantum-well photo-detector （QWP） serve as the emitter and receiver, respectively. With the help of the well-matched THz QWP, the optical collection efficiency has greatly been improved. A data signal transmitted over 2.2 m with a low bit error rate （≤1 × 10^-8） and data rate as high as 20 Mbps is achieved, which are almost 1 order of magnitude higher than that previously reported.

Influence of the upper waveguide layer thickness on optical field in asymmetric heterostructure quantum well laser diode

Asymmetric broad-waveguide separate-confinement heterostructure（BW-SCH） quantum well（QW） laser diode emitting at 808 nm is analyzed and designed theoretically.The dependence of the optical field distribution, vertical far-field angle,and internal loss on different thicknesses of the upper waveguide layer is calculated and analyzed.Calculated results show that when the thicknesses of the lower and upper waveguide layers are 0.45 and 0.3μm,respectively,for the devices with 100-μm-wide stripe and 1000-μm-long cavity,an output power of 7.6 W at 8 A,a vertical far-field angle of 37°,a slope efficiency of 1.32 W/A, and a threshold current of 189 mA can be obtained.

Detection of a directly modulated terahertz light with quantum-well photodetector

We demonstrate a wireless transmission link at 3.9 THz over a distance of 0.5 m by employing a terahertz （Hz） quantum-cascade laser （QCL） and a THz quantum-well photodetector （QWP）. We make direct voltage modulation of the THz QCL and use a spectral-matched THz QWP to detect the modulated THz light from the laser. The small signal model and a direct voltage modulation scheme of the laser are presented. A square wave up to 30 MHz is added to the laser and detected by the THz detector. The bandwidth limit of the wireless link is also discussed.

Dependence of third-order nonlinear susceptibility on strain induced piezoelectric field in In_xGa_(1-x)N/GaN quantum well

The third-order susceptibility of InxGa1-xN/GaN quantum well (QW) has been investigated by taking into account the strain-induced piezoelectric (PZ) field, and the effective-mass Schrodinger equation is solved numerically. It is shown that the third-order susceptibility for third harmonic generation (THG) of InxGa1-xN/GaN QW is related to indium content in QW and the intensity of the PZ field. Thecharacteristics of xTHG(3) (-3ω, ω, ω,ω ) as the function of the wavelength of incident beam, well width and indium content, have been analyzed.

Fabrication of an electro-absorption modulated distributed feedback laser by quantum well intermixing with etching ion-implantation buffer layer

We report the fabrication details of a monolithically integrated electro-absorption modulated distributed feedback laser （EML） based on the ion-implantation induced quantum well intermixing （QWI） technique. To well-preserve material quality in the laser region, thermal-oxide SiO2 is deposited before implantation and the ion-implantation buffer layer is etched before annealing. Thirteen pairs quantum well and barrier are employed to compensate deterioration of the modulator＇s extinction ratio （ER） caused by the QWI process. The fabricated EML exhibits an 18 dB static ER at 5 V reverse bias. The 3 dB small signal modulation band- width of modulator is over 13.5 GHz indicating that this EML is a suitable light source for over 16 Gb/s optical transmission links.

Photoluminescence study of the annihilation process of donor-bound excitons in ZnO:observation of quantum mechanical interference

We report a photoluminescence observation of the coupling of donor-bound excitons and longitudinal optical phonons in high-quality ZnO crystals at 5 K. The first-order phonon Stockes line of donor-bound excitons exhibits a distinct asymmetric line shape with a clear dip at its higher energy side, suggesting that quantum mechanical interference occurs during the annihilation of donor-bound excitons. The donor binding energy is determined to be 49.3 meV from spectral featural.

Room temperature continuous wave operation of 1.33-um InAs/GaAs quantum dot laser with high output power

Continuous wave operation of a semiconductor laser diode based on five stacks of InAs quantum dots （QDs） embedded within strained InGaAs quantum wells as an active region is demonstrated. At room temperature, 355-mW output power at ground state of 1.33-1.35 μm for a 20-μm ridge-waveguide laser without facet coating is achieved. By optimizing the molecular beam epitaxy （MBE） growth conditions, the QD density per layer is raised to 4 × 10^10 cm^-2. The laser keeps lasing at ground state until the temperature reaches 65 ℃.

Passively mode-locked Tm,Ho:YVO_4 laser based on a semiconductor saturable absorber mirror

We report the demonstration of passively continuous-wave mode-locking （CWML） of diode-pumped Tm,Ho：YVO4 laser using an InGaAs/GaAs multiple quantum-well （MQW） structure semiconductor as the saturable absorber. Stable mode-locking pulses at the central wavelength of 2 041 nm are obtained. The maximum output power is 151 roW. The pulse duration is 10.5 ps at the repetition rate of 64.3 MHz.

High performance 1689-nm quantum well diode lasers

1689-nm diode lasers used in medical apparatus have been fabricated and characterized. The lasers had pnpn InP current confinement structure, and the active region consisted of 5 pairs of InGaAs quantum wells and InGaAsP barriers. Stripe width and cavity length of the laser were 1.8 and 300μm, respectively. After being cavity coated and transistor outline （TO） packaged, the lasers showed high performance in practice. The threshold current was about 13 ± 4 mA, the operation current and the lasing spectrum were about 58 ± 6 mA and 1689 ± 6 nm at 6-mW output power, respectively. Moreover, the maximum output power of the lasers was above 20 roW.

Narrow-linewidth single-polarization frequency-modulated Er-doped fiber ring laser

We demonstrate a 1550-nm narrow-linewidth fiber ring laser with stable single polarization by using single-mode Er-doped fiber as active fiber and saturable absorber. A polarization-maintaining circulator is used to acquire single-polarization laser light with the degree of polarization of 99.8%--99.9%. The linewidth measured using a delayed self-heterodyne method is less than 0.5 kHz. Frequency of the fiber laser can be modulated by driving the waveguide phase modulator with proper voltage. A Mach-Zehnder interferometer with the optical path difference between two arms of about 36 km is used to study the long-distance coherent detection of the fiber laser for frequency-modulated continuous-wave application.

1.3-μm uncooled 10 Gb/s directly modulated MQW AlGaInAs/InP laser diodes

In this paper, we report a novel 1.3-μm uncooled AlGaInAs/InP multiple quantum well （MQW） ridge waveguide laser diodes. By optimizing the design of MQW structure and facet coatings, together with the application of reversed-mesa ridge waveguide （RM-RWG） structure, polyimide planarization, and lift-off processes technology, an uncooled 1.3-μm, 10-Gb/s directly modulated MQW ridge waveguide laser diode was successfully fabricated. The threshold current and the slope efficiency were 7 mA and 0.48 mW/mA, respectively. The directly modulated bandwidths of 11 and 9.2 GHz were achieved at room temperature and 80℃, respectively.

Modeling and identification on nonlinear saturable and reverse-saturable absorptions of gold nanorods using femtosecond Z-scan technique

An improved Z-scan analysis approach is proposed by establishing and solving the saturable absorption （SA） and reverse-SA （RSA） models, respectively. Near-infrared femtosecond Z-scans are carried out on the synthesized gold nanorods （NRs） possessing the average length of 46 nm using a femtosecond laser operated at the wavelength of 800 nm, which is close to the peak position of longitudinal surface plasmon resonance （SPR） （710 nm） of gold NRs. At lower input intensity of less than 400 GW/cm^2, the normalized transmission exhibits only SA phenomenon; however, when it exceeds 400 GW/cm , both SA and RSA are observed. By using the presented Z-scan modeling and theory, the three-photon absorption （3PA） is identified in the material, and the 3PA cross-section is determined to be 1.58× 10-^71 cm^6s^2.

High speed and wide temperature range uncooled 1.3-μm ridge waveguide DFB lasers

A 1.3-μm wavelength vertical-mesa ridge waveguide mulitple-quantum-well （MQW） distributed feedback （DFB） laser with high directly modulated bandwidth and wide operation temperature range is reported. With the optimization of the strained-layer MQWs in the active region, the surrounding graded-index separated-confinement-heterostructure waveguide layers, together with the optimization of the detuning and coupling coefficient of the DFB grating, high directly modulation bandwidth of 16 GHz at room temperature and wide working temperature range from -40 to 85 ℃ are obtained. The mean time to failure （MTTF） is estimated to be over 2×10^6 h. The device is suitable as light source of high-bit-rate optical transmitters with small size and reduced cost.

Growth and optical characteristics of ZnCdSe/ZnSe QWs on Si substrate with ZnO buffer

In this paper, the growth and characteristics of ZnCdSe/ZnSe quantum wells (QWs) prepared on ZnO-Si (111) templates are reported. An oriented ZnO thin film with a smooth surface was employed to be the buffer layer for the ZnCdSe/ZnSe QWs growth. Scanning electron microscopy (SEM) patterns showed that the ZnO buffer layer improved the smoothness of the ZnCdSe/ZnSe sample. Up to the 3rd longitudinal optical phonon of Zn0.56Cd0.44Se observed in Raman spectra suggests that the crystal quality of ZnCdSe/ZnSe QWs is reasonably good. The influence of quantum confinement effect on exciton characters of the QWs was also demonstrated.

Dielectric coatings for optimized low-loss saturable absorbers for high-power ultrafast laser

With the development of high power ultrafast laser passively mode-locked by a semiconductor saturable absorber mirror （SESAM）, the damage threshold and degeneration mechanism of the SESAM become more and more important. One way to reduce the maximum electric field inside the active part of the SESAM is the use of a dielectric coating on the top of the semiconductor structure. With Presnel formula, optical transfer matrix, and optical thin film theory, the electric field distribution and reflectance spectrum can be simulated. We introduce the design principles of SESAM including the dependence of reflectance spectrum on dielectric function of absorber, and investigate the dependences of the electric field distribution, modulation depth, reflectance spectrum, and the relative value of incident light power at the top quantum well of SESAM on the number of SiO2/Ta2O5 layers.