The graphene market continues to expand across a range of applications including consumer electronics, sensors, flexible wearables, supercapacitors, conductive inks and coatings. Thanks to graphene’s extraordinary electrical and mechanical properties a new generation of rapid, sensitive, low-cost bio/chemical sensors can be envisaged with impact upon healthcare, drug discovery, and bio/chemical detection applications.
Here we present the graphene-related R&D work at RISE Acreo with focus upon three objectives: graphene materials including wafer scale graphene-on-SiC, chemically synthesized graphene oxide (GO), reduced graphene oxide (RGO) and graphene quantum dots (or carbon nano particles); design and fabrication of the graphene devices, especially on their multiplexed sensing capability for enabling detection of multiple targets on a miniature integrated chip; and analysis of sensing mechanisms.
A few sensor examples will be described in this work, one is a graphene sensor to monitor glucose for diabetes. Another is a dopamine (DA) sensor utilizing graphene/ZnO-tetrapod hybrids for early diagnosis of Parkinson diseases (PD). DA is an important biomarker in the serum of patients with PD. The third one is a proton transmission detector utilizing 3D graphene onto SiC, which can initiate a new application for the detection of ionizing particle irradiation onto living cells. Finally, graphene sensors for forensic applications will be addressed; for instance, detection of amphetamine and TNT has been explored, aiming at rapid onsite crime scene analysis. In addition, a comprehensive analysis of the market and commercial opportunities of these devices will be presented.
Lasers, modulators and photodetectors are key components to configure transmitter and transceiver in optical communication links or optical information processing systems. There are a few optical transmission windows with low attenuation (< 0.2 dB/km) located around 1550nm, 1060nm and 850nm. The 1550nm and 850nm bands are wider explored than 1060nm window owing to more available components and optics in these regimes that used nowadays in telecom/datacom network facilities.
This work presents design, fabrication and characterizations of III-V semiconductor quantum structures based optoelectronic components produced at RISE Acreo’s ISO9001 certified clean-room laboratory. The 850nm and 1060nm devices have been designed and fabricated using the quantum structures grown on GaAs substrate, while the 1550nm components are based on the epi structures lattice matched to InP substrate. Desired operating wavelength can be turned by bandgap engineering, and the format of the devices can be arranged according customized specifications. As a demonstration, a 2x64 spatial light modulator (SLM) array with gradually increase operating wavelengths of each pixel at about 850nm will be detailed, which was used to achieve planar-integrated free-space optics for signal splitting, interconnection and processing. For 1550nm window, two components will be presented, one is a small aperture electroabsorption modulator (EAM) with diameter of 150µm that reached data rate at about 1 Gbps in a FSO link, another is an avalanche photodiode (APD) linear array. In addition, the lasers operating at about 1060nm utilizing quantum wells or quantum dots will be addressed.
The satellite market is shifting towards smaller (micro and nanosatellites), lowered mass and increased performance platforms. Nanosatellites and picosatellites have been used for a number of new, innovative and unique payloads and missions. This trend requires new concepts for a reduced size, a better performance/weight ratio and a reduction of onboard power consumption. In this context, disruptive technologies, such as laser-optical communication systems, are opening new possibilities. This paper presents the C3PO1 system, “advanced Concept for laser uplink/ downlink CommuniCation with sPace Objects”, and the first results of the development of its key technologies. This project targets the design of a communications system that uses a ground-based laser to illuminate a satellite, and a Modulating Retro-Reflector (MRR) to return a beam of light modulated by data to the ground. This enables a downlink, without a laser source on the satellite. This architecture suits well to small satellite applications so as high data rates are potentially provided with very low board mass. C3PO project aims to achieve data rates of 1Gbit/s between LEO satellites and Earth with a communication payload mass of less than 1kilogram. In this paper, results of the initial experiments and demonstration of the key technologies will be shown.
Zinc oxide tetrapods (ZnO-Ts) were synthesized by flame transport synthesis using Zn microparticles. This work herein reports a systematical study on the structural, optical and electrochemical properties of the ZnO-Ts. The morphology of the ZnO-Ts was confirmed by scanning electron microscopy (SEM) as joint structures of four nano-microstructured legs, of which the diameter of each leg is 0.7-2.2 μm in average from the tip to the stem. The ZnO-Ts were dispersed in glucose solution to study the luminescence as well as photocatalytic activity in a mimicked biological environment. The photoluminescence (PL) intensity in the ultraviolet (UV) region quenches with linear dependence to increased glucose concentration up to 4 mM. The ZnO-Ts were also attached with glucose oxidase (GOx) and over coated with a thin film of Nafion to form active layers for electrochemical glucose sensing. The attachment of GOx and coating of Nafion were confirmed by infrared spectroscopy (FT-IR). Furthermore, the current response of the active layers based on ZnO-Ts was investigated by cyclic voltammetry (CV) in various glucose concentrations. Stable current response of glucose was detected with linear dependence to glucose concentration up to 12 mM, which confirms the potential of ZnO-Ts for biomolecule sensing applications.
The epitaxial graphene-on-silicon carbide (SiC-G) has advantages of high quality and large area coverage owing to a natural interface between graphene and SiC substrate with dimension up to 100 mm. It enables cost effective and reliable solutions for bridging the graphene-based sensors/devices from lab to industrial applications and commercialization. In this work, the structural, optical and electrical properties of wafer-scale graphene grown on 2’’ 4H semi-insulating (SI) SiC utilizing sublimation process were systemically investigated with focus on evaluation of the graphene’s uniformity across the wafer. As proof of concept, two types of glucose sensors based on SiC-G/Nafion/Glucose-oxidase (GOx) and SiC-G/Nafion/Chitosan/GOx were fabricated and their electrochemical properties were characterized by cyclic voltammetry (CV) measurements. In addition, a few similar glucose sensors based on graphene by chemical synthesis using modified Hummer’s method were also fabricated for comparison.
Three types of electroabsorption modulators (EAMs) based on III-V semiconductor multiple quantum wells (MQW) are presented in this work. One is a novel monolithic integration traveling-wave EAM for an analog optical transmitter/transceiver to achieve integrated photonic mm-wave functions for broadband connectivity. Another one is composed of an integrated EAM 1D array in a photonic beam-former as a Ku-band phased array antenna for seamless aeronautical networking through integration of data links, radios, and antennas. The third one addresses the use of MQW EAMs in free space optical links through biological tissue for transcutaneous communication.
Quantum structures base on type-II In(Ga)Sb quantum dots (QDs) embedded in an InAs matrix were used as active
material for achieving long-wavelength infrared (LWIR) photodetectors in this work. Both InAs and In(Ga)Sb are
narrow band semiconductor materials and known to possess a large number of surface states, which apparently play
significant impact for the detector’s electrical and optical performance. These surface states are caused not only by
material or device processing induced defects but also by surface dangling bonds, oxides, roughness and contaminants.
To experimentally analyze the surface states of the QD structures treated by different device fabrication steps, atomic
force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray
photoelectron spectroscopy (XPS) measurements were performed. The results were used to optimize the fabrication
process of the LWIR photodetectors in our ongoing project. The dark current and its temperature dependence of the
fabricated IR photodetectors were characterized in temperature range 10 K to 300 K, and the experiment results were
analyzed by a theoretic modeling obtained using simulation tool MEDICI.
We report on the device characterization of In(Ga)Sb/InAs quantum dots (QDs) based photodetectors for long wave IR
detectors. The detection principle of these quantum-dot infrared photodetectors (QDIPs) is based on the spatially indirect
transition between the In(Ga)Sb QDs and the InAs matrix, as a result of the type-II band alignment. Such photodetectors
are expected to have lower dark currents and higher operating temperatures compared to the current state of the art InSb
and mercury cadmium telluride (MCT) technology.
The In(Ga)Sb QD structures were grown using metal-organic vapour-phase epitaxy and explored using structural,
electrical and optical characterization techniques. Material development resulted in obtaining photoluminescence up to
10 μm, which is the longest wavelength reported in this material system. We have fabricated different photovoltaic IR
detectors from the developed material that show absorption up to 8 μm. Photoresponse spectra, showing In(Ga)Sb QD
related absorption edge, were obtained up to 200 K. Detectors with different In(Ga)Sb QDs showing different cut-off
wavelengths were investigated for photoresponse. Photoresponse in these detectors is thermally activated with different
activation energies for devices with different cut-off wavelengths. Devices with longer cut-off wavelength exhibit higher
activation energies. We can interpret this using the energy band diagram of the dots/matrix system for different QD sizes.
Narrow bandgap semiconductors GaSb, InAs, and InSb are important building blocks for infrared photodetectors based
on type-II InSb quantum dots or an InAs/GaSb strained layer superlattice. Understanding the surface chemical
composition of these materials can provide valuable information that enables optimization of device surface passivation
techniques leading towards surface leakage free IR photodetectors. We report on an investigation into Ga-, In-, Sb-, and
As-oxides and other chemical species on the surface of untreated, dry etched and thermally treated GaSb, InAs and InSb
samples by x-ray photoelectron spectroscopy. The experimental results reveal the presence of Sb- and Ga-oxides on the
surfaces of the untreated and treated GaSb samples. Both Sb- and In-oxides were observed on the surface of all InSb
samples, and especially the dry etched sample had thicker oxide layers. In the case of the InAs samples, not only In- and
As-oxides XPS signals were obtained, but also AsCl species were found on the ICP dry etched sample. These results
helped to analyze the dark current of our fabricated IR detectors.
We report on the optical and structural characterization of InSb QDs in InAs matrix, grown on InAs (100) substrates, for
infrared photodetection. InSb has 7% lattice mismatch with InAs forming strained QDs, which are promising for longwave
IR applications, due to their type-II band alignment. This report contains material development results of InSb QDs
for increasing their emission wavelength towards long-wave IR region. Samples were grown by two techniques of MBE
and MOVPE, with different InSb coverage on InAs (100) substrates. Structures grown by MBE reveal QD related
photoluminescence at 4 μm. AFM investigations of the MBE grown structures showed uncapped dots of ~ 35 nm in size
and ~ 3 nm in height, with a density of about 2 x 1010 cm-2. Cross-section TEM investigations of buried InSb layers
grown by MBE showed coherently strained QDs for nominal InSb coverage in the range of 1.6 - 2 monolayers (MLs).
Layers with InSb coverage more than 2MLs contain relaxed QDs with structural defects due to large amount of strain
between InSb and InAs. Samples with such large dots did not show any InSb related luminescence. The MOVPE grown
InSb samples exhibit a strong QD related emission between 3.8 to 7.5 μm, depending on the amount of InSb coverage
and other growth parameters. We report the longest wavelength observed so far in this material system.
InSb nanostructures embedded in InAs and InAsSb matrices were grown on InAs (001) and GaAs (001) substrates by
molecular beam epitaxy. The diameter and height of InSb quantum dots (QDs) on InAs with 2ML-InSb coverage grown
by Stranski-Krastanov (S-K) are ~36.8 nm and ~3.1 nm, respectively. The density of QDs is ~2.5×1010 cm-2. The size
distribution of InSb QDs on InAs with 2ML-InSb coverage grown by migration enhanced epitaxy (MEE) was larger than
that of its S-K counterpart. Unique InSb quantum dashes (Q-dashes) on InAsSb elongated along two directions were
found on an AlSb-buffered GaAs substrate. InSb Q-dashes grown by migration enhanced epitaxy (MEE) were ~159 nm
in length, ~63 nm in width, and ~11 nm in height. A large reduction of volume of InSb structures between those in the
matrix and those on the surface was found. Threading disl°Cations resulting from the Q-dash structures were also
observed. This may be attributed to As-Sb exchange.
Qin Wang, David Rihtnesberg, Andreas Bergström, Susanne Almqvist, Andy Zhang, Wlodek Kaplan, Jan Andersson, Abhilash Sugunan, Xuran Yang, Muhammet Toprak
ZnO nanorods (NRs) sensors utilizing hybrid or monolithic integration of the NRs on nanoscale or microscale
interdigitated electrodes (IDEs) were fabricated and characterized. The IDEs with their finger electrode width ranging
from 50 nm to 3 μm were formed on SiO2/Si substrates by nanoimprint lithography or conventional photolithography
and metallization techniques, whereas the ZnO NRs were grown by chemical synthesis method. The average diameter of
the ZnO NRs is about 100 nm, and their length can be varied from 2 to 5 μm by controlling growth time. When sensing
targets, such as molecules or nanoparticles, bind onto the ZnO NRs, the conductance between IDEs will change. As
probing test, II-VI quantum dots (QDs) were attached on the ZnO NRs, and clear responses were obtained by measuring
and comparing current-voltage (I-V) characteristic of the sensor before and after binding the QDs.
Nowadays novel micro-fabrication and wafer-based manufacturing approach allows realizing micro-optics in a way
scientists have dreamt for generations, in particular, utilizing nano-imprint lithography as fabrication tooling enables
greatly accelerating the micro-optics technology to its frontier. In this report, we present wafer-scale fabrication of
various types of micro-optical elements based on photoresist, benzocyclobutene, photocurable imprint resist, and
semiconductor materials by using thermal reflow, reactive ion etching, and imprint techniques. Especially, several
concave or convex 3-dimensional micro-optical structures shaped by imprint method are detailed. These micro-optical
elements can be monolithically or hybrid integrated onto optoelectronics devices, such as photodetectors and emitters as
optical beam focuser, collimator, filter, or anti-reflectance elements. As application examples, polymer microlenses were
integrated directly on the top of UV dual functional devices and quantum dot long wavelength infrared photodetectors,
respectively.
Performance, advantages and drawbacks of GaN- and SiC-based ultraviolet (UV) photodetectors are analyzed and
compared. This includes metal-semiconductor-metal photodetectors, p-i-n photodiodes and avalanche photodiodes.
Design, process and characterization of these devices are described. Fabricated p-i-n InGaN/GaN quantum well
photodetectors are shown to exhibit a quantum efficiency of about 50% at 365 nm with a peak ultraviolet to visible
rejection ratio more than 3 orders of magnitude. A novel SiC avalanche photodiode design is shown to produce stable
avalanche breakdown characteristics for devices up to 2mm in diameter. A significant increase of its photo responsivity
is also demonstrated when the avalanche voltage is applied.
We have demonstrated surface normal detecting/filtering/emitting multiple functional ultraviolet (UV) optoelectronic devices based on InGaN/GaN, InGaN/AlGaN and AlxGa1-xN/AlyGa1-yN multiple quantum well (MQW) structures with operation wavelengths ranging from 270 nm to 450 nm. Utilizing MQW structure as device active layer offers a flexibility to tune its long cut-off wavelength in a wide UV range from solar-blind to visible by adjusting the well width, well composition and barrier height. Similarly, its short cut-off wavelength can be adjusted by using a GaN or AlGaN block layer on a sapphire substrate when the device is illuminated from its backside, which further provides an optical filtering effect. When a current injects into the device under forward bias the device acts as an UV light emitter, whereas the device performs as a typical photodetector under reverse biases. With applying an alternating external bias the device might be used as electroabsorption modulator due to quantum confined Stark effect. In present work fabricated devices have been characterized by transmission/absorption spectra, photoresponsivity, electroluminescence, and photoluminescence measurements under various forward and reverse biases. The piezoelectric effect, alloy broadening and Stokes shift between the emission and absorption spectra in different InGaN- and AlGaN-based QW structures have been investigated and compared. Possibilities of monolithic or hybrid integration using such multiple functional devices for biological warfare agents sensing application have also be discussed.
In this study, bias mediated tuning of the detection wavelength within the infrared wavelength region is demonstrated for quantum dots-in-a-well (DWELL) infrared photodetectors. In DWELL structures, intersubband transitions in the conduction band occur from a discrete state in the quantum dot to a subband in the quantum well. Compared to "conventional" quantum dot infrared photodetectors, where the transitions take place between different discrete bands in the quantum dots, new possibilities to tune the detection wavelength window are opened up, partly by varying the quantum dot energy levels and partly by adjusting the width and composition of the quantum well. In the DWELL structure used, an asymmetric positioning of the InAs quantum dot layer in a 8 nm wide In0.15Ga0.85As/GaAs QW has been applied which enables tuning of the peak detection wavelength within the long wavelength infrared (LWIR; 8 - 14 µm) region. When the applied bias was reversed, a wavelength shift from 8.5 to 9.5 µm was observed for the peak position in the spectral response. For another DWELL structure, with a well width of 2 nm, the tuning range of the detector could be shifted from the medium wavelength infrared (MWIR; 3-5 µm) region to the LWIR region. With small changes in the applied bias, the peak detection wavelength could be shifted from 5.1 to 8 µm. These tuning properties of DWELL structures could be essential for applications such as modulators and two-colour infrared detection.
FTTH networks require implementing a diplexer at each user termination. According to most of the standards, this
diplexer detects a download signal beam at 1.49μm and emits an upload signal beam at 1.31μm on the same single
mode fibre. Both signals exhibit datarate speed below 2.5Gbps. Today, most of the diplexers are obtained by actively
aligning a set of individual optoelectronic components and
micro-optics. However, new manufacturing solutions
satisfying very low cost and mass production capability requirements of this market would help to speed the massive
spreading of this technology. In this paper, we present an original packaging design to manufacture Diplexer Optical
Sub-Assembly for FTTH application. A dual photodiode is stacked over a VCSEL and detects both the download
signal beam at 1.49μm passing through the laser and one part of the upload signal beam at 1.31μm for monitoring.
To satisfy this approach, an innovative VCSEL has been designed to have a very high transmission at 1.49μm. All
these components are mounted on a very small circuit board on glass including also integrated circuits such as
transimpedance amplifier. So, the device combines advanced optoelectronic components and highly integrated
Multi-Chip-Module on glass approach using collective wafer-level assembling technologies. For the single mode
fibre optical coupling, active and passive alignment solutions are considered.
The Detuning Loading Effect, i.e., the effects of the modulation performance on the position of the lasing mode relative
to the Bragg reflection peak, is investigated in a Modulated Grating Y-branch laser. By proper adjustment of the lasing
mode position, simultaneous chirp reduction and modulation bandwidth enhancement can be obtained. The lasing mode
position is also crucial for side mode suppression ratio and output power.
KEYWORDS: Modulation, Modulators, Eye, Sensors, Signal to noise ratio, Signal detection, Transceivers, Free space optics, Retroreflectors, Optical communications
An optical communication link with a modulated retro-reflector in a cat's eye optics arrangement has been evaluated. A
transmissive multiple quantum well modulator with a modulation bandwidth in excess of 10 MHz was placed in a wide
field-of-view cat's eye optic system. A laser transceiver with one line and one broadband source around 1550 nm was
used to interrogate the modulated retro-reflector and detect the reflected and modulated signal. Multi-level digital
encoding was evaluated as a way to increase the information transfer rate in conditions with relatively high signal-to-noise
ratio. The links were evaluated in indoor as well as outdoor conditions.
In this paper, some devices were reviewed to be used in quantum communications. We presented a low density of
Quantum Dots, which could be used to get single quantum dot as light emitting source for generating single photons. An
analytical model to study the thermal behavior of a solid media in interaction with one, two or three laser beams was
developed using the classical heat equation. Integrated optic micro-ring resonators and its simulated result also are
presented. Development of active micro-ring in silicon is at an early stage, where both vertical and horizontal techniques
are feasible. With the epitaxy growth techniques, a possibility for achieving controllable QD density, size and good
uniformity are proposed. A low density of QDs in range of 108 cm-2 has demonstrated through successive adjustment of
the growth parameters. Details among the devices are presented and discussed.
Retro-communication by the joint use of light modulators and retro-reflecting devices has been proposed and demonstrated as a promising technique to further extend the use of free-space optical communication (FSO) to mobile communication. We have developed and demonstrated high-speed surface-normal GaAs-based and InP-based multiple quantum well electroabsorption modulators and arrays operating at different near-infrared wavelengths for FSO link applications. In this paper we report on the structure and device design issues, and trade-offs when GaAs-based or InP-based modulators are needed in the FSO links. In particular, the modulator structure properties, the device fabrication techniques and the device static and dynamic performance are investigated and compared.
We report on a quantum dots-in-a-well infrared photodetector (DWELL QDIP) grown by metal organic vapor phase epitaxy. The DWELL QDIP consisted of ten stacked InAs/In0.15Ga0.85As/GaAs QD layers embedded between n-doped contact layers. The density of the QDs was about 9 x 1010 cm-2 per QD layer. The energy level structure of the DWELL was revealed by optical measurements of interband transitions, and from a comparison with this energy level scheme the origin of the photocurrent peaks could be identified. The main intersubband transition contributing to the photocurrent was associated with the quantum dot ground state to the quantum well excited state transition. The performance of the DWELL QDIPs was evaluated regarding responsivity and dark current for temperatures between 15 K and 77 K. The photocurrent spectrum was dominated by a LWIR peak, with a peak wavelength at 8.4 μm and a full width at half maximum (FWHM) of 1.1 μm. At an operating temperature of 65 K, the peak responsivity was 30 mA/W at an applied bias of 4 V and the dark current was 1.2×10-5 A/cm2. Wavelength tuning from 8.4 μm to 9.5 μm was demonstrated, by reversing the bias of the detector.
We report on a quantum dot (QD) structure grown on a 4'' GaAs substrate by metal organic vapor phase epitaxy (MOVPE), which consists of five stacked InAs/InGaAs/GaAs QD layers embedded in the center of a typical in-plane waveguide. The density of the QDs is about 2.5 x 1010 cm-2 per QD layer. The photoluminescence (PL) peak wavelength at 1322 nm corresponding to the interband transition of the QD ground states was observed at room temperature with a full width at half-maximum of 49 meV. A good uniformity of the QD structure across the 4'' wafer was verified with a variation of the PL peak wavelength of 0.9 % from the wafer center to the edge. Top p-contacts and a bottom n-contact were processed on the QD structure, and electroluminescence (EL) spectra were measured at different temperatures. An EL peak corresponding to the QD ground states emission was obtained at 1325 nm at room temperature.
We report on the design, fabrication and characterization of 1550 nm electroabsorption modulators based on InGaAs/InAlAs coupled quantum wells grown on InP substrate by MBE. Large and small single modulators and modulator arrays have been fabricated on a wafer scale with an optimized device fabrication technology. The modulator size, shape, contact arrangements, and the array configurations have been varied to achieve suitable device performance for different retro-reflective free-space optical communication links. The device electrical and optical properties have been characterized by I-V, photoluminescence, absorption, transmittance and reflectance measurements. Modulators exhibit contrast ratios of 2:1 at a 3V driving bias and contrast ratios of 2:1 over a 30 nm bandwidth at 6V. A maximum contrast ratio of 4:1 is obtained at a 12 V driving voltage.
A novel retrocommunication link utilizing reflective multiple quantum well (MQW) optical modulators and nonmechanical beam steering and tracking is demonstrated. Large aperture reflective MQW modulators using AlGaAs/GaAs are optimized and manufactured. The modulators exhibit a contrast ratio larger than 4:1 and a modulation bandwidth of 10 MHz. Nonmechanical beam steering and tracking are studied using nematic liquid crystal (NLC) spatial light modulators (SLMs). The communication link is comprised of a retromodulating array with four MQW modulators and a transceiver using a NLC SLM for beam steering and tracking. Transfer of audio, real-time image data and pseudorandom bit sequences over 100-m range while tracking the moving retromodulator is shown. The link is capable of transferring data at approximately 8 Mbps.
Retrocommunication is a new technique for asymmetric free-space optical communication that has attracted interest during recent years. Novel technologies such as multiple quantum well (MQW) optical modulators and non-mechanical laser beam steering and tracking have been studied for implementation in a retrocommunication link. Large and small aperture reflective AlGaAs/GaAs MQW modulators were optimised and fabricated. The modulators exhibit high contrast ratios (from 5 to 100) and high modulation rates (up to 16 Mbit/s). A retroreceiver consisting of four large aperture MQW modulators, associated optics and drive electronics was fabricated. Nematic liquid crystal spatial light modulators have been evaluated, characterised for beam steering and tracking and implemented in a transceiver. Small area MQW modulators, used in focal plane configurations, were studied for static communication links. Results from a novel retrocommunication link utilising a retroreceiver and non-mechanical laser beam steering and tracking will be presented. Bit rates of 8 Mbit/s were observed during non-mechanical tracking of a moving retroreceiver over 100 m range. The demonstrator system was capable of transferring audio-, real-time images or bit streams. The demonstrated principles show promising features for future low weight free-space communication links. Performance calculations including requirements for a retrocommunication link using MQW modulators and non-mechanical beam steering are discussed.
We report on the development of different surface-normal multiple quantum well (MQW) modulator devices. Owing to their unique speed advantage, arrays of surface-normal MQW modulators are very well suited for fast and parallel signal processing, and can be developed for both digital or analogue signals. We present the design and fabrication process for single surface-normal MQW modulators, as well as for high fill factor (80-90%) 1-D arrays of such modulators. Design issues and trade-offs in terms of modulator size, speed and contrast ratios are described. Contrast ratios in excess of 100:1 have been demonstrated. 3-dB frequency modulation bandwidths in excess of 1GHz have been obtained for single modulator devices of 125μm in diameter. Performances of 1-D arrays of 64 and 128 modulators (pixel size: 2mmx80μm and 2mmx40μm, respectively) are also presented, with response time for programmable filtering in the order of 10 ns.
We present design and fabrication methods for surface normal monolithic amplitude modulators with an aperture up to 14 x 14 mm2, a contrast ratio of 6:1 and for low driving voltages (≤8 V). The modulators consist of undoped GaAs/AlGaAs quantum wells embedded in a Fabry-Perot (FP) resonance cavity grown by MOVPE. To improve the device performance the FP cavity, the period and thickness of the quantum well and doping concentration were optimised. Also, the dimension of the modulator were varied from 0.5 x 0.5 to 14 x 14 mm2. The results show that the yield of the modulators increases significantly when decreasing the size of the modulators. To remedy the low yield issue for wide aperture modulator, a pixelated approach was used to divide the mono pixel in a monolithic modulator into several pixels, for example from 4 to 48. The modulation speed of the modulators with different dimensions was characterised by electro-optic (EO) response measurements. The temporal optical response of the large modulators was satisfactory up to the order of MHz modulation frequency where the RC constant limited the performance. A few of the modulators with wide apertures are to be assembled into an optical link system for free-space communication.
Multiple quantum well spatial light modulators (MQW SLMs) are promising devices for future high-speed applications. We present results obtained with a single-pixel amplitude modulator. We discuss the status of our work on a 128x128-pixel ternary SLM. This SLM will run at 10 kHz and have one low-reflectance level and two high reflectance levels with a phase difference of pi. We also present a study of the relation between the coding domain and the structural design of modulators.
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