We report on photoluminescent properties of ultrafine ZnO nanorods and ZnO/Zn0.8Mg0.2O nanorod quantum-well structures. The catalyst-free metalorganic chemical vapor deposition (MOCVD) technique enables control of ZnO nanorod diameters in the range of 5 to 150 nm. From the PL spectra of ultrafine ZnO nanorods with a mean diameter smaller than 10 nm, a systematic blue-shift in their PL peak position was observed by decreasing their diameter, presumably due to the quantum confinement effect along the radial direction in ZnO nanorods. In addition, we obtained time-integrated and time-resolved PL spectra of ZnO/Zn0.8Mg0.2O nanorod single-quantum-well structures (SQWs) in the temperature range of 10 K to 300 K. The nanorod SQWs also showed a PL blue-shift and the energy shift was dependent on ZnO well layer width. The PL peak position shift originates from the quantum confinement effect of carriers in nanorod quantum structures. Furthermore, we investigated spatially-resolved PL spectra of individual nanorod SQWs using scanning near-field optical microscopy.
Based on mechanical replication, nanoimprint lithography is an emerging technology that can achieve lithographic resolutions beyond the limitations set by light diffractions or beam scatterings in conventional lithographic techniques, while promising high-throughput patterning. This tutorial paper reviews the status and some of the recent progress in the commercial applications of this technology.
The effect of quantum nanostructure interdiffusion or intermixing using impurity free vacancy diffusion (IFVD) and impurity induced disordering (IID) techniques, on the various types of GaAs- and InP-based quantum-well (QW) and quantum-dot (QD) structures for postgrowth photonic device integration is presented Using IFVD, we have demonstrated the fabrication of photonic integrated circuits such as multi-wavelength laser chips and broadband superluminescent diodes in GaAs/AlGaAs QW structures. Postgrowth bandgap engineering using low energy neutral ion implantation technique has been developed in InGaAs/InGaAsP QW systems. Using this interdiffusion technique, 10-channel multi-wavelength lasers and 8-channel integrated optical performance monitors have been demonstrated. The effects of IFVD and IID on the interdiffusion of InGaAs/GaAs QDs have been investigated. High spatial bandgap selectively processes with differential bandgap shifts between the interdiffused and non-interdiffused sections of over 100 meV have been observed from both IFVD and IID techniques in InGaAs/GaAs QD laser structure. At theoretical modeling level, we have developed a unified three-dimensional model for the electronic states calculation of the interdiffused quantum nanostructures with arbitrary shape, which is universally applicable to various quantum structures such as QW, quantum wire, quantum dash, and QDs. This model serves as a valuable tool to gain a more physical understanding of interdiffusion or intermixing techniques suitable for the integration technology in quantum nanostructures.
Solid-state thermal neutron detectors are generally fabricated in a planar configuration by coating a layer of neutron-to-alpha converter material onto a semiconductor. The as-created alpha particles in the material are expected to impinge the semiconductor and create electron-hole pairs which provide the electrical signal. These devices are limited in efficiency to a range near (2-5%)/cm2 due to the conflicting thickness requirements of the converter layer. In this case, the layer is required to be thick enough to capture the incoming neutron flux while at the same time adequately thin to allow the alpha particles to reach the semiconductor. A three dimensional matrix structure has great potential to satisfy these two requirements in one device. Such structures can be realized by using PIN diode pillar elements to extend in the third dimension with the converter material filling the rest of the matrix. Our strategy to fabricate this structure is based on both "top-down" and "bottom-up" approaches. The "top down" approach employs high-density plasma etching techniques, while the "bottom up" approach draws on the growth of nanowires by chemical vapor deposition. From our simulations for structures with pillar diameters from 2 micron down to 100 nm, the detector efficiency is expected to increase with a decrease in pillar size. Moreover, in the optimized configuration, the detector efficiency could be higher than 75%/cm2. Finally, the road map for the relationship between detector diameter and efficiency will be outlined.
The two major trends in the development of photodetectors (PDs) are improving bandwidth-efficiency product and obtaining high output power-bandwidth product. In this invited paper we review our recent work on PDs, which can meet these two challenges simultaneously, for the applications of 40-Gb/s long-haul and 10-Gb/s short-reach fiber communication.
Design, fabrication, and characterization of high-performance AlxGa1-xN-based photodetectors for solar-blind applications are reported. AlxGa1-xN heterostructures were designed for Schottky, p-i-n, and metal-semiconductor-metal (MSM) photodiodes. The resulting solar-blind AlGaN detectors exhibited low dark current, high detectivity, and high bandwidth.
Geiger mode Avalanche Photodiodes fabricated using complementary metal-oxide-semiconductor (CMOS) fabrication technology combine high sensitivity detectors with pixel-level auxiliary circuitry. Radiation Monitoring Devices has successfully implemented CMOS manufacturing techniques to develop prototype detectors with active diameters ranging from 5 to 60 microns and measured detection efficiencies of up to 60%. CMOS active quenching circuits are included in the pixel layout. The actively quenched pixels have a quenching time less than 30 ns and a maximum count rate greater than 10 MHz. The actively quenched Geiger mode avalanche photodiode (GPD) has linear response at room temperature over six orders of magnitude. When operating in Geiger mode, these GPDs act as single photon-counting detectors that produce a digital output pulse for each photon with no associated read noise. Thermoelectrically cooled detectors have less than 1 Hz dark counts. The detection efficiency, dark count rate, and after-pulsing of two different pixel designs are measured and demonstrate the differences in the device operation. Additional applications for these devices include nuclear imaging and replacement of photomultiplier tubes in dosimeters.
In this paper the design and performance of novel micromechanically-tunable vertical-cavity semiconductor optical amplifiers (VCSOAs) are presented. Theoretical design issues include overviews of the signal gain, wavelength tuning characteristics, saturation properties, and noise figure of these unique devices. Using general Fabry-Perot relationships it is possible to model both the wavelength tuning characteristics and the peak signal gain of tunable VCSOAs, while amplifier rate equations are used to describe the saturation and noise properties. It is found that these devices follow many of the same design trends as fixed-wavelength VCSOAs. However, with tunable devices, the tuning mechanism is found to result in varying amplifier properties over the wavelength span of the device. Experimental results for three generations of devices are given. The culmination of this work is a new bottom-emitting design in which the optical cavity is inverted and the MEMS-tuning structure serves as the high-reflectivity back mirror. By suppressing the variation in mirror reflectance with tuning, this configuration exhibits a two-fold increase in the effective tuning range as compared with our initial devices-with a minimum of 5 dB fiber-to-fiber gain (12 dB on-chip gain) over a wavelength span of roughly 21 nm, from 1557.36 nm to 1536.43 nm. Furthermore, these devices exhibit saturation, bandwidth and noise properties similar to state-of-the-art fixed-wavelength VCSOAs, including a fiber-coupled saturation output power of -1.36 dBm and an average gain bandwidth and noise figure of 65.2 GHz and 7.48 dB.
We report on the first monolithic 1310 nm Vertical Cavity Surface Emitting Lasers (VCSELs) with top and bottom InGaAsP/InP distributed Bragg reflectors (DBRs). The lasers show single mode powers over 1.0 mW at room temperature and single mode powers up to 0.5 mW at 85oC. The lasers, designed to be single mode, have side mode suppression ratios exceeding 45 dB over all temperatures and all powers.
This paper investigates internal physical mechanisms that have thus far prevented current-injected InGaN/GaN verticalcavity surface-emitting lasers (VCSELs) from lasing. Advanced device simulation is applied to a realistic VCSEL design. Several obstacles to lasing are identified, including current leakage, lateral carrier non-uniformity, and selfheating during pulsed operation.
A master-oscillator power-amplifier system at λ=1083 nm with 5.3 Watt output power and a narrow spectral linewidth was realised. The master oscillator was a distributed Bragg reflector (DBR) laser with a 3-μm wide ridge waveguide (RW) and a total length of 2 mm. The power amplifiers were a 4 mm long antireflection coated tapered laser diodes with 500 μm or 1000 μm long straight RW sections. At a temperature of 40oC and an injection current of 160 mA, the DBR laser had a wavelength of 1083 nm. The emitted light of the DBR laser was focused into the tapered amplifier with a seed power of up to 36 mW. At 10oC and at a current through the tapered amplifier of 8.6 A, a maximum output power of 5.3 W was measured. Over the full operating range single longitudinal mode operation at a wavelength of λ=1083 nm was maintained with a side mode suppression ratio better than 35 dB. The vertical far field angle was below 22o (FWHM).
LEDs using light conversion have gained importance for a variety of applications. As an example, white conversion LEDs were introduced by Osram OS in 1998 as dashboard illumination devices. Since then they have evolved into a powerful light source for projection and lighting applications. Today the quality of white LED light can be adjusted from a cool bluish white to a warm white with very good color rendering. Combining several wavelengths emitted by both chips and phosphors makes a variety of LED colors accessible. The concept of realizing any customized color by light conversion is called "Color on Demand" (COD). Another advantage for certain colors is, that the conversion LED can be more efficient than the purely chip based solution. In particular conversion LEDs with a dominant wavelength of 563 nm can be offered due to the strong converter R&D platform of Osram. The efficiencies of these LEDs, which consist of a blue emitting chip and a green emitting nitride phosphor, are 10 times higher than the ones of the corresponding InGaAlP-based solution. All COD LEDs have proven an excellent stability even in accelerated life time tests and thus are suitable for automotive applications.
Data rates above 250 MBit/s via polymer optical fiber (POF) require specially designed light emitting diode (LED) chips. Because of the absorption minimum of the POF, the red wavelength region around 650 nm is of special interest. LED chips in this region comprise active regions out of the AlGaInP material system. To achieve low rise and fall times in the optical output, the current density has to be increased to levels about 400A/cm2. Hence the chip design needs ways to confine the current injection to a region significantly smaller than the chip itself. Conventional methods use epitaxially grown layers to ensure the lateral current spreading over the active region. But to confine the current to this region, the current spreading has to be eliminated locally. Typically, this is achieved by ion-implantation, mesa etching or selective oxidation of an AlGaAs layer of high Al-content. In this paper we present a new, planar, and very cost effective chip design for data rates around 250 MBit/s. No current spreading layer is included in the epitaxial growth, but is supplied during chip process using a transparent conductive oxide. Optical power around 2 mW at 20 mA without epoxy encapsulation, and rise and fall times around 2.5 ns have been reached.
A novel theoretical approach combining scattering theory with supercritical angle transmission is introduced for treating light incidence on nanotextured surfaces. The theory is used to evaluate enhanced light extraction from interfaces with sub-wavelength feature sizes, where the ray tracing approach breaks down. A unified analytic formula covering the transition from periodic to random surface texturing is obtained. The results will be compared with experimental enhanced light extraction results from GaN textured interfaces. The extraction efficiency is studied as a function of the average feature size and the rms deviation from the average values. It is argued that enhanced extraction occurs due to both supercritical transmission for single wave incidence, and the quick randomization of the incident wave-vector directions via internal scattering.
We present here results from a uniquely designed InP modulator chip combined with advanced packaging concepts, which enables high-end applications in optical data communications. An electroabsorption (EA) modulator, with a strained InGaAsP or InGaAlAs multiple quantum well structure, is monolithically integrated with a semiconductor optical amplifier. This design offers broad wavelength tunability while maintaining high extinction ratio, high optical output power, and high dispersion tolerance. The amplified EA modulator chip is co-packaged with a distributed feed back (DFB) laser ensuring separate optimization of the laser and modulator sections. The optical isolator, placed between the laser and modulator, completely eliminates adiabatic chirp. This Telcordia-qualified laser integrated modulator platform enables superior performance previously not thought possible for InP absorption based modulators. 11dB of dynamic extinction ratio, 5dBm of modulated output power, and ±1200ps/nm or +1600ps/nm dispersion tolerance can be simultaneously achieved in un-amplified 10Gb/s data transmission. Full C-band tunability using a single device is also demonstrated with the LIM module. Extensive simulations and transmission system evaluations shows that with the controllable chirp, the cost-effective LIM performs as well as a Mach-Zehnder modulator in dispersion managed and amplified long-haul WDM systems. Lastly, the first uncooled 10Gb/s long-reach operation at 1550nm was demonstrated with LIM packages. Using a simple control algorithm, a constant modulated output power of 1dBm with less than 1dB dispersion penalty over 1600ps/nm single mode fiber is achieved in an 80 degrees environmental temperature range without any module temperature control. Utilizing the Al-based material system, also allows a reduced variation of the extinction ratio.
The accurate detection of minute amounts of chemical and biological substances has been a major goal in bioanalytical technology throughout the twentieth century. Fluorescence dye labeling detection remains the effective analysis method, but it modifies the surroundings of molecules and lowering the precision of detection. An alternative label free detecting tool with little disturbance of target molecules is highly desired. Theoretical calculations and experiments have demonstrated that many biomolecules have intrinsic resonance due to vibration or rotation level transitions, allowing terahertz (THz)-probing technique as a potential tool for the label-free and noninvasive detection of biomolecules. In this paper, we first ever combined the THz optoelectronic technique with biochip technology to realize THz biosensing. By transferring the edge-coupled photonic transmitter into a thin glass substrate and by integrating with a polyethylene based biochip channel, near field THz detection of the biomolecules is demonstrated. By directly acquiring the absorption micro-spectrum in the THz range, different boiomecules can then be identified according to their THz fingerprints. For preliminary studies, the capability to identity different illicit drug powders is successfully demonstrated. This novel biochip sensing system has the advantages including label-free detection, high selectivity, high sensitivity, ease for sample preparation, and ease to parallel integrate with other biochip functionality modules. Our demonstrated detection capability allows specifying various illicit drug powders with weight of nano-gram, which also enables rapid identification with minute amounts of other important molecules including DNA, biochemical agents in terrorism warfare, explosives, viruses, and toxics.
The purpose of this paper is to present the design of an electro-optic polymer traveling-wave waveguide photodetector operating in the 1.55 μm wavelength optical telecom window; this component seems to be completely new. Thanks to equal light and microwave speeds in the polymer material waveguides, microwave photonic mixing at frequencies as high as 60 GHz should be reached by the traveling-wave device. The component is based on three waves mixing technique; therefore it is a band-pass photodetector. The physical effect of the three waves mixing will be analyzed, and then a design of the component will be proposed.
Interest in silicon as a material for optoelectronics has increased year after year. We propose numerical analysis of an integrated waveguide-vanishing-based modulator realized by ion implantation in SOI wafer. The active region is 3×3 μm2 and the lateral confinement is guaranteed by two highly-doped As (8×1019cm-3) and B (2×1019cm-3) implanted regions 1-μm-deep. This type of structure allows to obtain a planar device, avoiding structural steps which are harmful for photolithography processes. The resulting channel waveguide shows single mode operation and propagation losses of about 1.8 dB/mm, which are acceptable for short structures.
The modulation is based on a lateral p-i-n diode, which injects free carriers into the rib volume between the doped regions. We have optimized the device for maximum injection efficiency for a given applied voltage. The resulting optical behavior can be explained by the lateral confinement vanishing that transforms the rib waveguide in a slab waveguide, once the rib is full of free carriers. This phenomenon occurs at driving voltage of about 1.0 V, with electrical power consumption below 1 mW, and implies a rapid variation of the propagating characteristics, and as consequence an optical beam lateral redistribution into the structure. Results show that an optical modulation depth close to 100% can be reached with a switching time of about 30 ns. A set of numerical simulations has been performed in order to evaluate the thermal response of the device and thus to estimate the thermo-optic effect related to the biasing of the device itself. The main advantages of this device are the low cost and full integrability with electronic devices; thus the device can be suitable in many application fields.
The implementation of more complex diode laser concepts also increases the demands for improved measurement technology and the need for new analytical tools. In particular concerning the thermal properties of novel high-power devices, there are several established experimental methods. Micro-Raman spectroscopy as well as reflectance techniques, such as photo- and thermo-reflectance measurements, provide information on facet temperatures, whereas emission wavelength shifts enable for the determination of averaged temperatures along the laser axis. Here we report on the successful application of a complementary technique, namely imaging thermography in the 1.5-5 μm wavelength range using a thermocamera, to diode laser analysis. The use of this known technique for the purpose of device analysis became possible due to the enormous technical progress achieved in the field of infrared imaging. We investigate high-power diode lasers and laser arrays by inspecting their front facets. We find raw data to be frequently contaminated by thermal radiation traveling through the substrate, which is transparent for infrared light. Subtraction of this contribution and re-calibration allows for the determination of realistic temperature profiles along laser structures, however, without spatially resolving the facet heating at the surface of the laser waveguide. Furthermore, we show how hot spots at the front facet can be pinpointed. Thus our approach also paves the way for an advanced methodology of device screening.
In order to realize high productivity and reliability of the modern semiconductor fabrication, defects inspection techniques for the layers of SOI (Silicon On Insulator) wafer become more essential. The layers on SOI wafer have only about 100~200nm thickness, at present, the technique of evaluating appropriately the defects in such thin layers does not exist. Novel optical defect measurement method to detect and classify the nano-defects that exist in the layers of SOI wafers by using evanescent field is proposed. In this method, propagating infrared laser in the layer generates evanescent field, which is affected by the defects existing on and below the surface. To execute light coupling into the layer, the prism coupler that is made of single crystal silicon was developed. In this paper, first the electromagnetic field in near-field of the wafer surface vicinity was analyzed by using FDTD (Finite Difference Time Domain) simulation tool. The results show this method can detect and classify the nano-meter scale defects on the surface and in the layers. Next the fundamental experiment was carried out to detect defects like sub-micrometer diameter hole that was fabricated on SOI wafer using FIB (Focused Ion Beam) machining. The experimental result also shows this method can detect the defects on the surface of SOI wafers sensitively.
A numerical approach based on the finite element - artificial transmitting boundary method is newly formulated for suppressing the sidelobe of integrated acousto-optic tunable filters with weighted coupling on a piezoelectric substrate. We use cosine and Gaussian weighting functions and obtained relevant response curves of TE-TM mode conversion efficiency. The result shows that the sidelobe can be efficiently suppressed by transforming the weighting function. Conversion of Gaussian weighting function is better than the one of cosine weighting function. The result is agreed with other references.
We propose a robust, multi-mode interferometer-based, 2x2 photonic switch, which demonstrates high tolerance to typical fabrication errors and material non-uniformity. This tolerance margin is dependent upon the properties inherent to the MMI design and benefits from the high symmetry of the switch. The key design parameter of the device is to form a pair of well defined self-images from the injected light in the exact center of the switch. In allowing the index modulated regions to precisely overlap these positions, and by creating identical contact features there, any refractive index change induced in the material due to electrical isolation will be duplicated in both self-images. Since the phase relation will remain unchanged between the images, the off-state output will be unaltered. Similarly, offset and dimension errors are reflected symmetrically onto both self-images and, as a result, do not seriously impact the imaging. We investigate the characteristics of the switch under different scenarios using the finite difference beam propagation method. Crosstalk levels better than -20 dB are achievable over a wavelength range of 100 nm when utilizing this configuration. Polarization independence is maintained during device operation.