With the advent of optical methods for stimulation and functional recording of neuronal activity in the brain, there is a growing need for fully flexible, ultracompact photonic devices for light delivery and light collection in brain tissue. In this paper, we will discuss our recent advances in designing a flexible optoelectronic neural implant platform that integrates passive and active optical components with electrical recording functionality. We leverage the exquisite optical and electrical insulation properties Parylene C, a biocompatible and flexible polymer to realize a fully functional optoelectrical neural interface.
Engineered semiconductor quantum structures that enforce carrier confinement in all three spatial dimensions have recently become of interest for potential applications in the sensing of infrared radiation via intersub-level transitions. These structures, most often called quantum dots, may offer a viable alternative to the mercury cadmium telluride semiconductor and GaAs/(Al,Ga)As quantum-well structures for infrared detection. Their major advantages for detection include (i) operation under normal-incidence illumination, (ii) a predicted high responsivity due to a long electron lifetime in the excited states, and (iii) a potential for high-temperature operation. This paper will review the current-state-of-development of (In,Ga)As/GaAs quantum-dot infrared detectors that are sensitive to light in the middle wavelength infrared (3-5 μm) region of the electromagnetic spectrum. The paper will also discuss some of the leading edge experimental results that suggest that quantum-dot active regions may offer a route to elevated device operating temperatures (> 150 K).
This work reports on theoretical studies of optical properties of gold nanospheres and nanorods with various aspect ratios, embedded in a porous anodic alumina matrix. When the alumina template is made under certain conditions, nanostructures with diameters in the 15-25 nm range can be synthesized. These nanoparticles have potential applications in optical filters and sensors.
The results of optical phenomena investigations in quantum dot and quantum well structures under interband optical pumping are presented. Interband and intraband light absorption in nanostructures with quantum dots has been studied experimentally and theoretically. Photoluminescence and interband light absorption in stepped quantum wells have been investigated including PL studies under picosecond optical pumping. Experimental results have been compared with results of calculation of energy spectrum and transition probabilities. It is shown that inversion of population exists between the third and second excited levels of stepped quantum well.
Numerous efforts are directed at investigating the use of optics at short distances--for example, at the chip-to-chip and board-to-board levels of the interconnection hierarchy. This book provides an overview of the state of the art in heterogeneous integration of electronics, optoelectronics, and micro-optics for short-distance optical interconnections.
Intersubband carrier transitions in either the conduction or valence bands of III-V semiconductors are important for the design of a new generation of optoelectronic devices. These transitions have been successfully used to demonstrate the operation of a new class of infrared lasers and detectors. In this paper, we discuss the use of intersubband transitions in quantum dot nanostructures for infrared sensing. The quantum dot structures in our work allow the detection of normal-incidence light. This is promising for the design of future focal plane arrays that do not require a grating structure to scatter incident light into the correct polarization for detection via intersubband transitions. The quantum dot infrared photodetectors in this work also exhibit an intrinsic photovoltaic effect. Both photoconductive and photovoltaic operation has been demonstrated at low temperatures (40 K) with responsivities on the order of tens of mA/W for bias voltages less than 0.5 V. Detectivities ranging from 2 X 108 cmHz1/2/W to 7 X 109 cmHz1/2/W have been measured in devices operating in either the photovoltaic or photoconductive mode. We have demonstrated that the quantum dot structures have the capability to detect infrared light in the 9 to 13 micrometers spectral band.
In this paper, we review our research efforts on RCE high- speed high-efficiency p-i-n and Schottky photodiodes. Using a microwave compatible planar fabrication process, we have designed and fabricated GaAs based RCE photodiodes. For RCE Schottky photodiodes, we have achieved a peak quantum efficiency of 50% along with a 3-dB bandwidth of 100 GHz. The tunability of the detectors via a recess etch is also demonstrated. For p-i-n type photodiodes, we have fabricated and tested widely tunable devices with near 100% quantum efficiencies, along with a 3-dB bandwidth of 50 GHz. Both of these results correspond to the fastest RCE photodetectors published in scientific literature.
Resonant cavity enhanced (RCE) photodiodes are promising candidates for applications in optical communications and interconnects where ultrafast high-efficiency detection is very desirable. In RCE structures, the electrical function of the photodiode is largely unchanged, but optically it is subject to the effects of the cavity, mainly wavelength selectivity and a large enhancement of the resonant optical field. The increased optical field allows photodetectors to be made thinner and therefor faster in the transit-time limited operation, while simultaneously maintaining a high quantum efficiency at the resonant wavelengths. The combination of RCE detection scheme with Schottky photodiodes allows for fabrication of high-performance photodetectors with relatively simple material structure and fabrication process. In RCE Schottky photodiodes, a semi-transparent metalization can be used simultaneously as the electrical contact and the top reflector for the resonant cavity. Device performance is optimized by varying the thickness of the Schottky metalization and utilizing a dielectric matching layer. We present theoretical and experimental results on spectral and high-speed properties. We have demonstrated RCE Schottky photodiodes in (Al, In)GaAs/GaAs material system with temporal response of 10 ps full-width-at-half-maximum. These results were measurement setup limited and a conservative estimation of the bandwidth corresponds to more than 100 GHz. The photodiodes were designed and fabricated for 900 nm and 840 nm resonant wavelengths. The best measured quantum efficiency is around 50% which is slightly less than the theoretical prediction for these devices.
An analysis of the second-order nonlinear polarizability of III-V, -oriented heterostructures is presented. This analysis is used as a basis for the discussion of visible second-harmonic light-emitters. It is then shown experimentally that active blue-green light-emitters can be fabricated from (In,Ga)As/GaAs quantum well laser structures.
Intersubband transitions in GaAs/(Al,Ga)As quantum wells have been successfully used in the design of novel infrared detectors for over a decade now. Both conduction- and valence-band based detectors have been investigated. In general, the conduction-band based detectors fabricated from direct gap GaAs/(Al,Ga)As heterostructures are not sensitive to normal-incidence light. This is a consequence of the quantum mechanical rules that govern light absorption in these structures. In order to detect normal-incidence light, a grating structure which scatters the incident light into higher order, transverse magnetic modes is used. To avoid the use of gratings, research is being carried out in (In,Ga,Al)As/(Al,Ga)As conduction-band quantum well structures that can absorb normal-incidence light. This paper reviews recent progress in such detectors.
Multiple quantum well (MQW) electro-optic modulators grown on both GaAs and InP substrates have been designed and characterized. Strained-layer (In,Ga)As/GaAs p-i-n diodes grown on (100) GaAs substrates were found to have a differential absorption coefficient of 3.7 X 103 cm-1 for an applied electric field of 6.6 X 104 V/cm. These devices were also grown on (110) GaAs substrates and exhibited polarization sensitive electroabsorption. In addition, InGaAs/InAlAs asymmetric coupled MQWs were designed and fabricated. Real charge transfer kinetics between the coupled MQWs were exhibited by these devices.
The photoluminescent properties of GaAs/AlxGa1 - xAs multiple quantum well structures grown on (100) GaAs substrates by molecular beam epitaxy have been investigated as a function of the AlAs mole fraction. It is found that as the AlAs mole fraction in the AlxGa1 - xAs barriers is increased, the corresponding photoluminescent peak intensity of the quantum well structures increases and reaches a maximum at an AlAs mole fraction of about 45%. Further increases in the Al content of the barriers beyond 45% results into a decrease in the luminescence efficiency. At the maximum intensity, the peak luminescence from the quantum well structures is enhanced by about two orders of magnitude (at room temperature) when compared to the luminescence from such structures with an AlAs mole fraction of 20% in the barriers.