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Detailed theoretical analysis of the effect of the carrier density disorder on the optical conductivity of graphene-based vertical-cavity optical modulator is presented. Analytical expression for the optical conductivity of disordered graphene is derived by assuming Gaussian distribution for the carrier density variation. We theoretically demonstrate a simple ultrafast optical modulator using graphene-based vertical-cavity and double dielectric DBR mirrors. Two graphene flakes connected to a gate voltage are placed inside a resonant cavity to control the optical conductivity of graphene and to detune the resonant wavelength of the optical cavity. The bandpass spectrum of the device can be tuned by varying the gate voltage at an ultrahigh speed. Switching speed as high as 63 GHz is predicted for two flakes structures separated by a 100 nm SiO2 layer with a wavelength range as high as ~ 5 nm around the resonant wavelength. The transmission characteristics of the modulator are studied for different disorder levels. Our analysis reveals that disorder significantly reduces both the contrast ratio and the insertion loss of the modulator. [ABSTRACT FROM AUTHOR]

We propose and theoretically demonstrate a novel type of an all-optical switch using two vertically coupled cavities. The top passive cavity contains 2 graphene flakes that are used to control the resonant behavior of the dual cavity structure while the bottom active cavity contains multiquantum well gain material. Our analysis reveals that the optical reflectivity of the dual cavity structure exhibits switchable high-Q resonance. At low optical input, the resonance is governed by the active bottom cavity where the resonance exhibits amplification, and at a higher optical input the resonance is governed by the passive top cavity where the resonance exhibits absorption. We show that this feature can be utilized to realize efficient all-optical switching with contrast ratio as high as 53 dB. The effect of detuning the wavelength of the pump and probe signals as well as changing the graphene chemical potential on the switching behavior is numerically studied and an analytical model has been derived. [ABSTRACT FROM AUTHOR]

A novel approach for achieving ultrafast and wide tunable vertical-cavity surface-emitting lasers (VCSELs) is presented. The design is based on incorporating multiple graphene flakes separated by SiO2 dielectric in a λ/2 cavity and placing multiquantum wells in one of the quarter wave layers of the bottom mirrors. The device concept enhances both the wavelength tuning range and tuning speed which is attractive for many photonic applications. By applying a gate voltage on the graphene flakes, the effective refractive index of the cavity can be changed with ultrahigh speed. We have shown that 2 graphene flakes inside a 5 µm-diameter passive cavity provides tuning speed as high as 32 GHz and wavelength tuning range of 3.8 nm. The tuning speed increases to 52 GHz when the device diameter is reduced to 3 µm. The effect of spacer thickness and the number of graphene flakes on the performance characteristics of the device is investigated. Our analysis shows that tuning speed as high as 2 GHz and wavelength tuning range of ~ 12 nm is obtained for VCSEL that incorporates 6 graphene flakes separated by 10 nm SiO2 spacers and biased in parallel. The tuning speed can be increased to ~ 16 GHz when the flakes are biased in series at the expense of higher gate voltage. A tradeoff between the wavelength tuning range and the tuning speed is obtained. [ABSTRACT FROM AUTHOR]

A novel approach for enhancing the tuning speed of vertical-cavity surface-emitting lasers (VCSEL) is presented. The design is based on incorporating gated multiple graphene flakes in one of the DBR layers that is adjacent to the cavity to tune the resonance wavelength of the device. Different design structures have been studied. By applying a gate voltage on the graphene flakes, the effective refractive index of the cavity is changed which detunes the resonant wavelength of the cavity. The unique features of graphene ensure fast tuning speed. Our analysis on a 1.3 μm InAs quantum dot-based VCSELs predicts that inserting double graphene flakes separated by 20 nm SiO2 spacer in a λ/4 layer yields ~ 0.6 nm wavelength tuning range and ~ 6.3 GHz tuning speed. The threshold current variation within the tuning range is less than 43%. The wavelength tuning range can be increased to 1.2 nm by using 4 graphene flakes which reduces the tuning speed to ~ 1.9 GHz and increases the threshold current variation to 84%. In another design, we find that inserting 4 graphene flakes separated by 80 nm SiO2 spacer in a 3λ/4 layer yields ~ 0.8 nm tuning range and 14.5 GHz tuning speed. For fine wavelength tuning, the device is capable of providing > 39 GHz tuning speed. [ABSTRACT FROM AUTHOR]

A novel semi-analytical model for cross-gain modulation (XGM) in quantum dot vertical-cavity semiconductor optical amplifier has been derived. The derived model, which is simple, requires small computational time compared with numerical simulation and can efficiently be used to investigate the optical bistable/stable characteristics of the device. Large XGM bandwidth can also be obtained when the device operates in the bistable region and in the stable region close to the bistable edge. A figure of merit is proposed to optimize the complicated tradeoff between the XGM bandwidth, efficiency, applied current and the input pump to probe ratio. We find that optimum characteristics can be obtained in our investigated device when the probe wavelength is detuned by 9 Å above the resonant wavelength and when a specific CW probe density is applied to the input of the device. [ABSTRACT FROM AUTHOR]

The characteristics of optical bistability in quantum dot vertical cavity semiconductor optical amplifier integrated with MEMS membrane have been investigated, and a closed-form model is derived taking into account the effect of the quantum dot discrete states and the membrane deflection and reflectivity. We show that small membrane deflection significantly adjusts the resonant wavelength, the contrast ratios and the hysteresis width. The shape of the input-output characteristics of the device can be adjusted to display clockwise, butterfly and counterclockwise hysteresis loops depending on the membrane deflection and the initial wavelength detuning. Our analysis reveals that the contrast ratios of the upper and lower bistable levels are totally different. Also, it has been shown that the characteristics of the bistability are strong function of the active layer linewidth enhancement factor and the membrane reflectivity. [ABSTRACT FROM AUTHOR]

The peculiarities of cross-gain modulation (XGM) in optically bistable quantum-dot vertical-cavity semiconductor optical amplifiers (QD-VCSOA) are theoretically studied. We find that the optical hysteresis can be controlled by XGM which for example can be utilized in resting an all-optical flip-flops using single QD-VCSOA. Our analysis reveals that intrinsic high-speed XGM response up to 200 GHz can be obtained in QD-VCSOA. [ABSTRACT FROM PUBLISHER]

We propose and theoretically demonstrate a novel type of tunable bistable device. Detuning is achieved by introducing deflectable microelectromechanical systems (MEMS) membrane suspended above the top distributed Bragg reflector (DBR). By applying external voltage at the MEMS, the membrane deflects, which detunes the resonant wavelength of the vertical-cavity semiconductor optical amplifiers, and consequently changes the bistability of the device. Therefore, the hysteresis width and the shape of the input–output characteristics of the device (clockwise, butterfly, and counterclockwise hysteresis loops) can be controlled electronically. [ABSTRACT FROM PUBLISHER]

The spectral characteristics of the linear gain and the small-signal crosstalk of positively detuned and negatively detuned quantum dot gain-clamped semiconductor optical amplifiers (QDGC-SOA) have been studied. Our analysis reveals that the linear gain of the ground-state in positively detuned QDGC-SOA can be enhanced by doping the dots by p-type concentration, while the linear gain of the excited-state in negatively detuned QDGC-SOA can be enhanced by doping the dots by n-type concentration. The small-signal crosstalk is found to be proportional to the amplification where we find that positively detuned QDGC-SOA exhibits less crosstalk (by at least 3 dB) than negatively detuned QDGC-SOA. In addition, we find that positively detuned QDGC-SOA is capable of providing larger optical gain. [ABSTRACT FROM AUTHOR]

The polarization characteristics of doped spin-injected quantum dot vertical cavity surface emitting laser (QD-VCSEL) have been studied using multi-population spin-flip coupled rate equation model. The stability conditions of spin-injected VCSEL are also investigated for different device parameters. Our analysis reveals that QD-VCSEL emitting at high lasing energies, close to the first excited state energy, demonstrates high output ellipticity and high ellipticity gain compared to QD-VCSEL emitting at the ground state energy. Also, we find that increasing the p-type doping concentration of the quantum dots increases the output ellipticity gain of the device and significantly alters the stability conditions of the VCSEL. Large instability is observed when the VCSEL is emitting near $$\sim \!\!70\hbox {meV}$$ above the ground state energy and when the quantum dots are doped with $$\sim \!\!4\times 10^{10}\,\hbox {cm}^{-2}$$ of p-type concentration. Moreover, we find that QD-VCSEL exhibits good stability when its emission energy is approximately 10-20 meV higher than the ground state energy and when the dots are doped with more than $$4\times 10^{10}\,\hbox {cm}^{-2}$$ of p-type concentration. [ABSTRACT FROM AUTHOR]

The linear optical gain of gain-clamped quantum dash semiconductor optical amplifiers (GCSOAs) has been investigated using the rate equation model. The gain spectrum of GCSOA for different wavelength detuning and different doping has been studied. Our analysis shows that the linear gain can be increased as the laser wavelength is detuned to high wavelength where the peak of the optical gain, which is found at wavelengths below the ground state wavelength, is shifted to lower wavelength as the laser wavelength is increased. We find that doping the dashes by either N-type or P-type enhances the linear optical gain and shifts the gain peak to lower wavelength. Moreover, we found that GCSOA with lightly N-type doping demonstrates large separation between the laser and the amplifier wavelength. Also we find that small inhomogeneous line broadening enhances the linear gain peak, shifts the gain peak to wavelength lower than the GS wavelength and widens the gain spectrum. [ABSTRACT FROM AUTHOR]

We have developed a novel analytical model, which describes the dynamic characteristics of optical pulse amplification and saturation in three state quantum dot (QD) semiconductor optical amplifiers (SOAs). The model takes into account the effect of the ground state, the excited state and the 2-dimensional wetting layer. The model is simple, accurate and fast, which makes it suitable for device design and characterization. The derived model has been utilized to study large-signal cross-gain modulation and crosstalk in multi-channel QD-SOA. Analytical expressions for large signal cross-gain modulation and crosstalk in multichannel SOA are derived. The effect of the dot relaxation/escape lifetimes and energy separation between QD states on cross-gain modulation and crosstalk are also studied. Our calculations show that by reducing QD energy state separation, via engineering the dot size and composition, one can reduce the cross-gain modulation efficiency and reduce crosstalk in multi-channel QD-SOAs. [ABSTRACT FROM AUTHOR]

Novel closed-form model for multiple-state quantum-dot semiconductor optical amplifiers (QD-SOAs) is derived. The model takes into account the effect of the ground state, excited state and the wetting layer. The model is simple, accurate and exhibits negligible computational time compared with numerical simulation. In addition, the derived model is valid for arbitrary applied current and input photon density and is interesting for device design and optimization. Analytical expressions for the optical gain, effective saturation density, maximum output density and the transparency current are also derived. Our model revealed that the effective saturation density of QD-SOAs strongly depends on the photon density and the applied current. [ABSTRACT FROM AUTHOR]

A simple, accurate, and fast multilayer feedforward artificial neural network (ANN) model for quantum-dot semiconductor optical amplifiers (QD-SOAs) is developed. The developed ANN model has demonstrated excellent precision and accurately Models the physical characteristics of QD-SOAs such as pulse amplification and four-wave mixing characteristics. Furthermore, the developed ANN model requires a very small computational time compared with numerical models, which is very attractive for computer-aided-design applications. The model is used to create very interesting design curves for QD-SOAs. In addition, the model is suitable for physical-parameter extraction from available measured data. [ABSTRACT FROM AUTHOR]

The characteristics of short optical pulse four-wave mixing (FWM) and amplification in quantum dot semiconductor optical amplifiers (QD-SOAs) are investigated taken into account the effect of the multi-discrete QD energy levels. Different saturation and recovery response for the electron and hole states are observed, which is attributed to different energy spacing between the energy states. We found that the 3 dB saturation energy of QD-SOA depends on the pulse width for short input pulses. Also, the optimum time delay between the probe and pump pulses in QD-SOAs, which provides maximum FWM efficiency in QD-SOAs, is smaller than the optimum delay in quantum well SOA. [ABSTRACT FROM AUTHOR]

The linewidth enhancement factor (α) of inhomogenously broadened InAs/GaAs quantum dot (QD) lasers is studied taking into account the effect of the multi-discrete QD energy levels for both electrons and holes. The effect of size-fluctuation and the separation between the energy states on α for different applied bias is studied. Very small value of α (∼0.3) is obtained at high-applied bias. This value is smaller than the linewidth enhancement factor in quantum well lasers. An empirical formula for the linewidth enhancement factor is also derived. [ABSTRACT FROM AUTHOR]

Detailed theoretical analysis of the size fluctuation in InAs-GaAs quantum dot (QD) lasers is presented. Analytical expressions for the inhomogeneous line broadening and the optical gain are derived for a Gaussian size fluctuation distribution. The effect of size fluctuations on the QD carrier density, modal gain, and differential gain is studied. Red shifts in the gain peak is observed when size fluctuations increases. The energy detuning between the gain peak and the differential gain peak for a pyramidal quantum dot system having an average base length of 130 Å and standard deviation of 7 Å is about 12 meV. [ABSTRACT FROM AUTHOR]

Detailed theoretical analysis of the gain characteristics of quantum-dot semiconductor optical amplifiers (QD-SOA) is presented. An analytical expression for the optical gain is derived from the quantum dot and wetting layer rate equations. Due to the better confinement of carriers in the quantum dots, our calculation shows that large unsaturated optical gain can be obtained at low operating current. Also, we found that the output saturation intensity of QD-SOA is higher than the output saturation intensity of bulk-SOA. This fact lends itself to the design of efficient low-power SOAs. [ABSTRACT FROM AUTHOR]

Presents a study to calculate the absorption coefficient of Si-Ge-Si quantum wells (QWs) as a function of transverse bias. Examination of symmetric and asymmetric QWs; How the electroabsorption and electrooptic effect in these QWs could prove to be the basis for the realization of efficient Si-based electrooptic modulators; Discussion on the device designs.

Novel closed-form model for the threshold conditions and the optical gains of vertical dual-cavity semiconductor optical amplifier (VDC-SOA) is derived. Expressions for the transfer functions that relate the average photon density inside the two cavities to the incident photon density are derived. The model is simple, accurate and easy to use for designing dual-wavelength laser amplifiers. Good agreement is obtained between the analytical model and numerical calculations. The transmitted and reflective bistability characteristics of quantum dot VDC-SOA have been investigated. The threshold condition of each mode is expressed in terms of the top-cavity and the bottom-cavity currents. The dual modes λ o + and λ o - show different bistability behavior as a function of the top cavity current when the device operates at fixed operating point below the threshold curve. We find that the contrast ratio of mode λ o + is a weak function of the device currents, and the hysteresis width increases when the top cavity current is increased. For mode λ o - large contrast ratio and hysteresis width are obtained when the two cavities exhibit equal currents. Our analysis also reveals that the spectral characteristics of mode λ o + are different from that of mode λ o - . [ABSTRACT FROM AUTHOR]

Examines efficient wavelength switching in an In[sub 0.4]Ga[sub 0.6]As/GaAs self-organized quantum dot laser with an intracavity absorber section. Wavelength shift caused by a shift of lasing between the bound states of the quantum dots; Heterostructure of the quantum dot laser; Light-current characteristics.

A novel type of multichannel and multistate all-optical switch using a single sample-grating quantum-dot-distributed feedback semiconductor optical amplifier has been proposed and theoretically demonstrated. The multichannel device, which operates below threshold, utilizes cross-gain modulation and the sample-grating technique. The multichannel outputs are strongly coupled and are utilized to get multistability at several wavelength channels. Three logic states can be obtained when the inputs are properly detuned to the sample-grating comb modes. The three logic states, which exhibit reasonable gain, are separated by wide hysteresis width and can be tuned to a different wavelength channels. The device characteristics are very useful for building all-optical logic gates, flip-flops, and decision circuits. [ABSTRACT FROM AUTHOR]

We propose and theoretically demonstrate a novel type of all-optical multistable device using single quantum dot distributed feedback semiconductor optical amplifier. The device utilizes cross-gain modulation with proper wavelength detuning to obtain multistability in a conventional DFB semiconductor optical amplifier. We find that dual bistability, which can be adjusted vertically or horizontally by applying small probe input, can be obtained in both the pump and the probe characteristics. Adjustable switching with contrast ratio as high as 14 dB is obtained from the probe signal. [ABSTRACT FROM AUTHOR]

A detailed analysis of the efficiency of four-wave mixing (FWM) is performed for a closely spaced energy state quantum dash semiconductor optical amplifier (SOA). The analysis takes into account the effect of all discrete energy states, the gain dispersion of the quantum dash, and the effect of doping on the FWM characteristics. Our analysis reveals that large FWM efficiency can be obtained when the energy of the pump signal is equal to the first excited state. We find that at low-applied current doping the dashes by p-type concentration enhances the FWM efficiency. While at high applied current, we find that doping the SOA with p-type concentration slightly enhances the FWM efficiency of the ground state and the first excited state and lowers the FWM efficiency of the higher excited states. Also we find that doping the dashes by n-type concentration slightly modifies the FWM efficiency spectrum. [ABSTRACT FROM AUTHOR]

Far-infrared spontaneous emission at 300 K and lower temperatures, due to intersubband transitions in self-organized In[sub 0.4]Ga[sub 0.6]As/GaAs quantum dots, has been characterized. Measurements were made with a multidot layer near-infrared (∼1 μm) interband laser. The far-infrared signal, centered at 12 μm, was enhanced after the interband transition reached threshold at 300 K. The results are explained in terms of the carrier dynamics in the dots. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the effect of the quasi-Fermi level discontinuity and the electrostatic potential nonlinearity on the transconductance, junction capacitance, base transit time and the emitter charge time of heterojunction bipolar transistors. Effect of the termionic emission boundary condition at the heterojunction interface.