Photonic integrated circuits fabricated using ion implantation

Intermixing the wells and barriers of quantum-well (QW) laser heterostructures generally results in an increase in the bandgap energy and is accompanied by changes in the refractive index. A technique, based on ion implantation-induced QW intermixing, has been developed to enhance the quantum-well intermixing (QWI) rate in selected areas of a wafer. Such processes offer the prospect of a powerful and simple fabrication route for the integration of discrete optoelectronic devices and for forming photonic integrated circuits     

Modal characteristics and bistability in twin microdisk photonic molecule lasers

We demonstrate coupled-mode characteristics and bistability in photonic molecule lasers composed of evanescent-coupled GaInAsP twin microdisks. First, we show room-temperature continuous-wave operation by photopumping and discuss the unique behavior of coupled modes, i.e., the anticrossing and splitting characteristics of bonding and antibonding modes. Next, we present the clear bistability, which is observed for the antibonding mode by nonuniform pumping with an effective power of nearly 40 /spl mu/W. It is explained by rate equation analysis, which assumes saturable absorption. The analysis also predicts mode switching by the carrier-induced refractive index change. A micron-sized device with a very low power consumption will allow large scale integration of optical memories, optical flip-flops, and so forth.     

Influences of unconfined states on the optical properties ofquantum-well structures

The population of the unconfined states, with energies above the band edge of the barrier layers, can be significant in some regions of the active volume in high power lasers and amplifiers. This paper analyzes the influences of these states on optical properties, such as gain, refractive index, differential gain, and linewidth enhancement factor, for different quantum-well (QW) structures. Our results show that at high excitation levels, the unconfined band contributions to the real part of the optical susceptibility can be significant, especially in structures with weak quantum confinement potentials. This is in agreement with recent measurements of peak gain and carrier-induced refractive index change versus carrier density, for InGaAs-GaAs QW laser structures     

Quantitative model for the kinetics of compositional intermixing inGaAs-AlGaAs quantum-confined heterostructures

A quantitative atomic-scale model for the kinetics of intermixing in GaAs-AlGaAs quantum-confined heterostructures is presented. The model takes into account the statistical nature of the defect diffusion through heterostructures and calculates its effect on the Ga-Al interdiffusion across the associated interfaces. The model has been validated by successfully predicting the observed amounts of bandgap shift induced by the process of hydrogen plasma induced defect layer intermixing, as well as for the process of impurity-free vacancy disordering using SiO₂ caps. Good agreement between calculated and measured bandgap shifts has been observed. Values of the group-III vacancy diffusion coefficient, where the agreement took place, are between 2 and 3×exp[-2.72/k_BT] cm²·s^-1     

Theoretical characterization of multilayer photonic crystals having a 2D periodicity

We present a new, very accurate and fast model of photonic bandgap (PBG) structure characterized by a two‐dimensional (2D) periodic change of the refractive index and finite height, therefore named quasi 3D PBG. The new model is based on the Floquet–Bloch formalism and allows to find all the propagation characteristics including the space harmonics and the total field distribution, the propagation constants, the guided and radiated power and modal loss induced by the 2D grating. A clear explanation of the physical phenomena occurring when a wave propagates inside the 2D periodic structure is presented, including the photonic bandgap formation and the radiation effects. The approach does not require any theoretical approximation, and can be applied to study rigorously any PBG‐based multilayer structures. We have applied the model to investigate several structures for both optical and microwave applications. Copyright © 2005 John Wiley & Sons, Ltd.     

Focused ion-beam implantation induced thermal quantum-wellintermixing for monolithic optoelectronic device integration

By focused ion beam implantation induced thermal intermixing the bandgap of quantum-well layer structures can be selectively changed. This allows lateral bandgap engineering and gives a new degree of freedom for lateral structuring. The principle technological aspects like the dependence of the bandgap shift on implantation parameters and the spatial resolution are investigated and applied to the fabrication of photonic and optoelectronic devices. Lateral waveguiding in InP-based materials, the possibility of monolithic integration of bandgap shifted waveguide areas into active devices and the improvement of the lateral carrier confinement in ridge waveguide lasers are demonstrated. Due to the high spatial resolution, modulated bandgap gratings could be realized with periods down to 90 mn. These bandgap gratings were used to create gain-coupled distributed-feedback lasers in different material systems with well controlled single-mode emission     

Plasma-induced quantum well intermixing for monolithic photonic integration

Plasma-induced quantum well intermixing (QWI) has been developed for tuning the bandgap of III-V compound semiconductor materials using an inductively coupled plasma system at the postgrowth level. In this paper, we present the capability of the technique for a high-density photonic integration process, which offers three aspects of investigation: 1) universality to a wide range of III-V compound material systems covering the wavelength range from 700 to 1600 nm; 2) spatial resolution of the process; and 3) single-step multiple bandgap creation. To verify the monolithic integration capability, a simple photonic integrated chip has been fabricated using Ar plasma-induced QWI in the form of a two-section extended cavity laser diode, where an active laser is integrated with an intermixed low-loss waveguide.     

Ultrafast all-optical switching in multiple-quantum-well Y-junctionwaveguides at the band gap resonance

An integrated Y-junction switch is used to demonstrate all-optical switching due to two mechanisms in GaAs-AlGaAs multiple quantum well structures at room temperature. Carrier-induced nonlinearities are compared to nonlinearities due to the optical Stark effect for ultrafast operation. In the first case, device geometry is exploited by a two-pulse switching process, whereby one control pulse turns the device on and a second control pulse turns the device off. Time gating of signal pulses within an 8 ps window was realized in our experiments. In the second case, the nonlinearity is ultrafast, and hence switching and recovery take place during the control pulse evolution. Consecutive switching events spaced 1.7 ps apart have been achieved. In our measurements, two-photon absorption played a significant role in the switching characteristics of the device. In order to access the carrier-induced nonlinearity, the pulse energy that is needed for switching (9 pJ in this case) results in a very high peak intensity that leads to two-photon absorption. This is observed as a strongly induced absorption exactly at the precise zero time delay between the control and signal pulses. In the second case, two-photon absorption competes directly with the optical Stark effect since both are instantaneous intensity dependent effects. Furthermore, the optical Stark effect appears to saturate at a low intensity level (0.9 pJ or 200 MW/cm² ); consequently only incomplete switching with a 2:1 contrast ratio was observed, whereas a 9:1 switching contrast was achieved with the carrier-induced nonlinearity     

Quasi-one-dimensional photonic crystal as a compact building-block for refractometric optical sensors

We report the fabrication and the characterization of the refractometric and thermo-optical properties of a quasi-one-dimensional waveguide photonic crystal-a strong, 76-/spl mu/m-long Bragg grating. The transmission spectra (around 660 nm) of the structure have been measured as a function of both the cladding refractive index and the temperature. The transmission stopband was found to shift by 0.8-nm wavelength for either a cladding refractive index change of 0.05 or a temperature change of 120 K. The steep stopband edges provide a sensitive detection method for this band shift, by monitoring the transmitted output power.     

Origin of high-speed modification of refractive index in fusedquartz by vacuum ultraviolet laser irradiation

We report on high-speed modification and a large change of the refractive index on the order of 10^-2 of fused quartz upon vacuum ultraviolet (VUV) laser irradiation. The origin of the large refractive index change is discussed based on the laser-induced color center and surface morphology changes. The VUV-UV absorption spectrum of modified samples indicates the formation of a color center at around 163 nm (≡Si-Si≡defect), which is attributed to bond scission of fused quartz by VUV laser-induced electron excitation. On the other hand, simultaneous UV laser irradiation in our present experimental scheme is responsible for the generation of a surface damage at the large number of pulses, which causes scattering and deterioration of the optical properties of the irradiated regions     

Low-energy ion-implantation-induced quantum-well intermixing

In this paper, we present the attractive characteristics of low-energy ion-implantation-induced quantum-well intermixing of InP-based heterostructures. We demonstrate that this method can fulfil a list of requirements related to the fabrication of complex optoelectronic devices with a spatial control of the bandgap profile. First, we have fabricated high-quality discrete blueshifted laser diodes to verify the capability of low-energy ion implantation for the controlled modification of bandgap profiles in the absence of thermal shift. Based on this result, intracavity electroabsorption modulators monolithically integrated with laser devices were fabricated, for the first time, using this postgrowth technique. We have also fabricated monolithic six-channel multiple-wavelength laser diode chips using a novel one-step ion implantation masking process. Finally, we also present the results obtained with very low-energy (below 20 keV) ion implantation for the development of one-dimensional and zero-dimensional quantum confined structures.     

Thermal analysis of blood undergoing laser photocoagulation

The temperature of blood undergoing laser-induced photocoagulation during long-pulse (10 ms) 532 nm irradiation was measured in a time- and spatially-resolved manner using a novel technique. This method is based on the change in reflectivity of a solid-liquid interface given a dynamically changing refractive index in the liquid phase. In our case, the temperature-dependent change in the refractive index of blood was utilized, and the reflectivity at a glass-blood interface was measured. Measurements were compared to predictions from a finite-element model incorporating the effects of time-dependent changes in the absorption coefficients of the blood, and phase changes representing coagulation and the liquid/vapor transition. Previous studies have linked the onset of blood coagulation to a sharp rise in the 532-nm reflectance of the blood. Based on the thermal measurements and the results of an Arrhenius analysis, we postulate that the reflectance rise is a combination of protein denaturation and red blood cell conformal changes     

Redshifting and broadening of quantum-well infrared photodetector'sresponse via impurity-free vacancy disordering

The partial intermixing of the well and barrier materials offers unique opportunities to shift locally the bandgap of quantum-well (QW) structures. We have demonstrated redshifting and broadening of the wavelength responses of bound-to-continuum GaAs and InP based quantum-well infrared photodetectors (QWIP's) after growth via impurity-free vacancy disordering (IFVD). A comprehensive set of experiments is conducted on QWIP's fabricated from both as-grown and multiple-quantum-well (MQW) structures. Compared to the as-grown detector, the peak spectral responses of the disordered detectors were shifted to longer wavelengths. The peak absolute response of the disordered GaAs based QWIP is lower by almost a factor of four. However, the responsivity characteristics of the disordered InP based QWIP show no major degradation. In general, with the spectral broadening taken into account, the overall performance of the disordered QWIP's has not dropped significantly. Thus, the postgrowth control of the QW composition profiles by impurity-free vacancy disordering offers unique opportunities to fine tune various aspects of a photodetector's response. Theoretical calculations of the absorption coefficient spectrum are in excellent agreement with the experimental data     

II-VI semiconductors on InP for green-yellow emitters

Novel II-VI compound materials such as MgZnCdSe, BeZnCdSe, BeZnTe, and related superlattices grown on InP substrates have been investigated for yellow-green emitters employing molecular beam epitaxy. MgZnCdSe was grown in the Mg composition range of 0/spl sim/0.6 to clarify the compositional dependency of the bandgap and refractive index. MgSe-ZnCdSe and MgSe-ZnSeTe short-period superlattices were investigated; the superlattices behaved as quasi-quaternaries (QQs), so that their bandgap energies were controlled by the layer thickness combination of superlattices. For realizing strong lattice hardness, Be-contained H-VI compounds, such as BeZnCdSe and BeZnTe bulk crystals, and MgSe-BeZnCdSe, ZnCdSe-BeZnTe, and MgSe-BeZnTe short-period superlattices were investigated. The superlattices also behaved with QQ properties, by use of which multilayered heterostructures could be grown without growth interruption. Applying the superlattices, visible LEDs were fabricated emitting at the wavelengths from 554 (yellow-green) to 644 nm (red) at room temperature. For yellow (575 nm) LEDs, a long lifetime more than 3500 h was demonstrated even for defect densities as high as 10/sup 5/ cm/sup -2/. The BeZnTe buffers were effective in suppressing the defect density to less than 7 /spl times/ 10/sup 3/ cm/sup -3/. Finally, MgZnCdSe-based II-VI LDs were successfully operated with yellow-green lasing emissions around 560 nm at 77 K.     

A density functional theory study of the structural,electronic, and optical properties of XGaO3 (X = V,Nb) perovskites for optoelectronic applications

An ab initio study using density functional theory (DFT) is carried out to explore the structural, electronic, and optical properties of vanadium gallate (VGaO₃) and niobium gallate (NbGaO₃). The structural properties of these compounds are determined by using the full-potential linearized augmented plane wave (FP-LAPW) technique as implemented in WIEN2k with a standard functional, i.e., the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA). In addition, the local density approximation plus Hubbard parameter (LDA?+?U) is employed to calculate the electronic bandgap and total and partial density of states (TDOS and PDOS), to overcome the limitation of the PBE-GGA functional in terms of underestimation of the electronic bandgap. The values computed for the indirect bandgap of VGaO₃ and NbGaO₃ are 0.45 and 0.51 eV, respectively, indicating that both materials are semiconductors in nature. The PDOS of the studied materials reveal that 3d-states of vanadium atoms, 4d-states of niobium atoms, and 2p-states of oxygen atoms form the valence band. Moreover, the Kramer–Kronig relations are used to compute the optical properties of the title compounds. The dielectric functions, refractive index, optical conductivity, absorption coefficient, extinction coefficient, energy loss function, and reflectivity of these materials are also computed. The results for the studied properties reveal that NbGaO₃ exhibits better properties than VGaO₃ for use in optoelectronic applications.

     

Amplified spontaneous emission spectroscopy in strainedquantum-well lasers

Amplified spontaneous emission spectroscopy is used to extract the gain and refractive index spectra systematically. We obtain the gain and differential gain spectra using the Hakki-Paoli method. The refractive index profile, the induced change in refractive index by an incremental current, and the linewidth enhancement factor are measured from the Fabry-Perot peaks and the current-induced peak shifts in the amplified spontaneous emission spectra. The measured optical gain and refractive index are then compared with our theoretical model for strained quantum-well lasers. We show that a complete theoretical model for calculating the electronic band structure, the optical constant, and the linewidth enhancement factor agrees very well with the experiment. Our approach demonstrates that amplified spontaneous emission spectroscopy can be a good diagnostic tool to characterize laser diodes, extract the optical gain and index profiles, and confirm material parameters such as the strained quantum-well band structure parameters for a semiconductor structure under carrier injection     

Blueshifting of InGaAsP-InP laser diodes using a low-energyion-implantation technique: comparison between strained andlattice-matched quantum-well structures

Blueshifted InGaAsP-InGaAs-InP laser diodes have been fabricated using a technique that includes a low-energy ion implantation, used to generate point defects near the surface of the structure, followed by a thermal anneal which causes the diffusion of these defects through the quantum wells (QWs). This diffusion of point defects induces a local intermixing of atoms in the QWs and barriers, which results in a decrease in the emission wavelength of the devices. Results obtained with strained and lattice-matched QW structures are compared. For lattice-matched structures, electroluminescence wavelength shifts as large as 76 nm were obtained. Strained QW structures presented a much smaller blueshift (≈10 nm). In both cases, we observed no significant change of the threshold current caused by the intermixing process     

Strain-overcompensated GaInP-AlGaInP quantum-well laser structuresfor improved reliability at high-output powers

Strain-overcompensated multiple-quantum-well (MQW) laser structures have been analyzed theoretically as well as experimentally for the first time. Strain overcompensation reduces the bandgap shrinkage that normally takes place at the facets of compressively strained layers because of strain relaxation. This results in a lower absorption of the laser spot leading to a remarkable improvement of the reliability of high-power laser diodes     

Postgrowth control of the quantum-well band edge for the monolithic integration of widely tunable lasers and electroabsorption modulators

We describe a quantum-well intermixing process for the monolithic integration of various devices, each with a unique band edge. The process involves a single ion implant followed by multiple etch and anneal cycles. We have applied this method to design and fabricate widely tunable sampled-grating distributed Bragg reflector lasers with integrated electroabsorption modulators. The devices employ three unique band edges, and demonstrate exceptional tuning, gain, and absorption characteristics.     

基于莫尔偏折的低温等离子体气体温度参数研究

放电等离子体有着非常广泛的实际工程应用价值,其内部温度特性是表征等离子体性质的一个重要参数。为了探明大气压下放电等离子体的气体温度空间分布特性参数,搭建了一套基于莫尔偏折原理的光学测试系统,对铜电极下小电流(0.1A)放电产生的低温等离子体进行了诊断。首先利用傅里叶变换和Abel逆变换,根据莫尔条纹图像提取了等离子体折射率分布曲线。然后通过求解等离子体物理方程,建立等离子体折射率与温度之间的对应关系。最后通过将折射率反演变换,重建出距下电极不同位置的等离子体气体温度分布,研究并分析温度随位置变化的趋势及原因。结果表明:在同一径向位置上,等离子体中心处温度最高,并向两边迅速递减。不同径向位置下的中心处最高温度范围约为1 400~1 600K。在竖直方向上,随着距下电极位置的距离增加,等离子体气体温度分布和直径都略微增加。