We have designed new THz metastructure waveguides on Si wafers, aimed for low propagation loss and integration with
Si-based integrated circuits. The waveguide has a round cross-sectional hollow-core, surrounded by high reflectioncladding-
walls formed by high-contrast metastructure gratings. We developed a new fabrication technique to fabricate
such a 3D metastructure cage waveguide structure. The waveguide is built using the entire wafer thickness which
involves deep Si etching of periodically spaced holes and using isotropic undercut etching to create a connecting a line
of etched spheres in the middle of the wafer to form the waveguide’s hollow core, then deep etch the high-contrast
grating through the entire wafer thickness to form the cladding for the waveguide. We have successfully modeled and
fabricated such a waveguide structure. The next step is to experimentally test and characterize the waveguide in the THz
We have developed a new type of Si-based 3D cage-like high-contrast metastructure waveguide with both “slow-light”
and low-loss properties, which has applications in providing a long time-delay line or a high Q cavity in chip-scale optoelectronic integrated circuits (OEIC). Traditional semiconductor optical waveguides always have high loss when used in a high dispersion (slow-light) region. A preliminary computational model has predicted that there is a slow-light and low propagation loss region within cage-like hollow-core waveguide formed by 4 high-contrast-gratings walls/claddings. Using our new processing technique, we fabricated several such waveguides on a Si wafer with different core sizes/shapes and different HCGs for 1550 operation wavelength. We have conducted experimental waveguide delay test measurements using a short optical pulse which indicate that the group velocity of these metastructure waveguides are in the range of 20- 30% of the speed of the light. Using a waveguide “cut-back” method, we have experimentally determined the propagation loss of these waveguides are in the range of 2-5dB/cm. We are also developing this type of high-contrast metastructure hollow-core waveguide for different operating wavelength/frequency such as THz for different applications.
We present a new type of Si-based, metastructure, hollow-core waveguide that has highly desirable "slow-light" and
low-loss properties for providing time-delays or high-Q cavities in chip-scale integrated OE circuits. This waveguide has
high contrast grating (HCG) metastructures as the 4 claddings/walls of a squared hollow-core structure. We have
successfully fabricated this 3-D metastructure waveguide using a new nano-fabrication techniques including one selfaligned,
cycled, modified Bosch etch process. Our computational modeling indicates that there is a slow-light region
with very little propagation loss. We will report our preliminary experimental waveguide test results for propagation loss
and group velocity.
We have designed and developed a new, simplified 3-dimensional (3D) Photonic Crystal (PhC) fabrication
technique that can be used to fabricate a nanoscale 3D structure from the 2D surface of a Si (or SOI) wafer with a
single lithography and self-aligned etching sequence. This technique produces deep trenches with controlled width
variation along the vertical direction. Using an alternating sequence of Bosch etches, a combined cryogenic etching
and/or chemical etching process, allows the Bosch etched layers to maintain the width defined by the mask, while
the cryogenic/chemical etched layer creates a lateral undercut that decreases the width beneath the surface. The
result is a 3D lattice structure with a stack of vertical square grids. This paper reports the experimental procedures
and results of fabrication of a 3D lattice structure that forms an array of hollow-core waveguides. We also compare
several different etch recipes for the attempt to produce a uniform structure with smooth walls. These techniques
will reduce overall fabrication cost, increase yield and are compatible with CMOS processing. Using this method,
one can fabricate a variety of Si/SOI based 3D PhC structures including hollow-core, high contrast grating,
A demonstration of non-mechanical beam steering of high speed VCSEL arrays is reported. 980 nm oxide-confined
VCSELs were grown on a GaAs substrate with AlGaAs distributed Bragg reflectors (DBRs) and an InGaAs active
region. Up to 24 separate VCSEL arrays can be independently switched to sweep the beam across the illumination area.
Three array designs with varying total areas were fabricated with 97, 137, and 278 elements. The corresponding mesa
diameters were 24, 14 and 10 μm with pitches of 52, 40 and 26 μm. The relative output power, speed, and thermal
performance of an array design is reported.
Authors report the demonstration of the emission wavelength tuning of InAs quantum-dashes within InAlGaAs
quantum-wells grown on InP substrate, that gives the initial wavelength emission at ~1.65 &mgr;m. The impurity-free
dielectric cap annealing and the nitrogen ion-implantation induced intermixing techniques have been implemented to
spatially control the group-III intermixing in the structure, which produces differential bandgap shift of 80 nm and 112
nm, respectively. Transmission electron microscopy, optical and electrical characterizations have been performed to
evaluate the quality of the intermixed QD material and bandgap tuned devices. Compared to the control (nonintermixed)
lasers, the light-current characteristics for the over 125 nm wavelength shifted QD lasers are not
significantly changed suggesting that the quality of the intermixed material is well-preserved. The intermixed lasers
exhibit the narrow linewidth as compared to the as-grown due to the improved QD homogeneity. The integrity of the QD
material is retained after intermixing suggesting the potential application for the planar integration of multiple
active/passive QD-based devices on a single InP chip.
We present the development of a fabrication technique for a semiconductor-based photonic crystal (PhC) nano-membrane device with reconfigurable active waveguides using micro-electro-mechanical systems (MEMS) technology. This device can be used as a basic building block for optoelectronic integrated circuits that can be reprogrammed for different functionalities such as switches, modulators, time delay lines, resonators, etc. The device is fabricated three-dimensionally on GaAs/Al<sub>x1</sub>GaAs/Al<sub>x2</sub>GaAs epitaxial layers on a GaAs substrate. The device has a top PhC membrane layer structure composed of hexagonal holes in a triangular lattice. Below that, a separate suspended bridge layer can insert a line of posts into the PhC holes to create a defect line. This MEMS feature can generate/cancel a section of the waveguide in the PhC platform, or (by partial removal) it can change the dispersion of the waveguide. Therefore, the same structure can be used as different types of devices. In this paper, we will discuss detailed fabrication processes for such a multi-layer 3D device structure, including e-beam lithography, inductively coupled plasma reactive ion etching, and multiple steps of regular photolithography and selective wet chemical etching. The unique processing sequence allows us to fabricate the multi-layer 3D device structure from one top surface without regrowth, wafer bonding, or access from the back surface. This simplifies the device processing and reduces the fabrication cost.
We present our design and fabrication of a semiconductor based photonic bandgap (PBG) nano-membrane device with MEMS features. This device could be used as a basic building block for a reconfigurable optoelectronic integrated circuit that can be reprogrammed for different functionalities. We combine a PBG platform with a MEMS feature to build such a reconfigurable device. The device has a top PBG membrane layer structure composed of hexagon holes in a triangular lattice. Below that, a separate suspended bridge layer can insert a line of posts into the photonic crystal holes to create a defect line. This MEMS feature can generate/cancel a section of the waveguide in the PBG platform, or it can change the dispersion of the waveguide. Therefore, the same structure can be used as different types of devices such as switches, modulators, time delay lines, etc. This device is fabricated on GaAs/Al<sub>x1</sub>GaAs/Al<sub>x2</sub>GaAs/GaAs-substrate epi-layers grown by MBE. We have developed the fabrication technique for such a device using e-beam lithography, inductively coupled plasma (ICP) reactive ion etching, and multiple steps of regular photolithography and selective wet chemical etching. The fabricated PBG membranes are 60 nm to 300 nm thick, with a thin wall between the holes of ~120 nm. A line of mushroom shaped MEMS posts are inserted into the ~1 μm PBG holes. We are fine tuning each of these processing steps toward the fabrication of a workable device.
We report on our design and fabrication of a semiconductor based photonic bandgap nano-membrane device with MEMS features. This device could be used as a basic building block for a reconfigurable optoelectronic integrated circuit that can be reprogrammed for many different functionalities.
The Army Research Laboratory is researching system architectures and components required to build a 32x32 pixel scannerless ladar breadboard. The 32x32 pixel architecture achieves ranging based on a frequency modulation/continuous wave (FM/cw) technique implemented by directly amplitude modulating a near-IR diode laser transmitter with a radio frequency (RF) subcarrier that is linearly frequency modulated (i.e. chirped amplitude modulation). The backscattered light is focused onto an array of metal-semiconductor-metal (MSM) detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency (IF) signal (a product of the mixing process) whose frequency is proportional to the target range. Pixel read-out is achieved using code division multiple access techniques as opposed to the usual time-multiplexed techniques to attain high effective frame rates. The raw data is captured with analog-to-digital converters and fed into a PC to demux the pixel data, compute the target ranges, and display the imagery. Last year we demonstrated system proof-of-principle for the first time and displayed an image of a scene collected in the lab that was somewhat corrupted by pixel-to-pixel cross-talk. This year we report on system modifications that reduced pixel-to-pixel cross-talk and new hardware and display codes that enable near real-time stereo display of imagery on the ladar's control computer. The results of imaging tests in the laboratory will also be presented.
We report on the fabrication of quantum grid infrared photodetector (QGIP) arrays and demonstrate their feasibility for use as multi-channel long wavelength infrared spectrometers. The quantum well infrared photodetector (QWIP) material structure was designed to exhibit broadband absorption in the wavelength range of 7 μm to 16 μm. By fabricating QGIP devices with this QWIP material, scattering of light at an individual wavelength of interest within the material absorption range can create narrow band detection in each device. Arrays of QGIP devices with varying geometry, each tailored to respond to a discrete wavelength were fabricated. Details of the epi-growth, processing steps taken to fabricate required device features for narrow band absorption of the QGIP devices, and characterization methods will be discussed.
The Army Research Laboratory is researching a focal plane array (FPA) ladar architecture that is applicable for smart munitions, reconnaissance, face recognition, robotic navigation, etc.. Here we report on progress and test results attained over the past year related to the construction of a 32x32 pixel FPA ladar laboratory breadboard. The near-term objective of this effort is to evaluate and demonstrate an FPA ladar using chirped amplitude modulation; knowledge gained will then be used to build a field testable version with a larger array format. The ladar architecture achieves ranging based on a frequency modulation/continuous wave technique implemented by directly amplitude modulating a near-IR diode laser transmitter with a radio frequency (rf) subcarrier that is linearly frequency modulated (chirped amplitude modulation). The diode's output is collected and projected to form an illumination field in the downrange image area. The returned signal is focused onto an array of optoelectronic mixing, metal-semiconductor-metal detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency (IF) signal resulting from the mixing process whose frequency is proportional to the target range. This IF signal is continuously sampled over a period of the rf modulation. Following this, a signal processor calculates the discrete fast Fourier transform over the IF waveform in each pixel to establish the ranges and amplitudes of all scatterers.
We report on temporal response measurements of InGaAs metal-semiconductor-metal photodetectors (MSM-PDs) under high-illumination conditions. The peak current efficiency of the MSM-PDs decreases as the optical pulse energy increases due to space-charge-screening effects. The screening effects begin to occur at an optical pulse energy as low as 1.0 pJ/pulse, as predicted by a recent two-dimensional model. The fall time and full width at half maximum of the impulse response increase as the optical pulse energy increases and decrease as the bias voltage increases. For optical pulse energies between 1.0 pJ and 100 pJ, the rise time displays a U-shaped behavior as the bias voltage increases. This may be associated with the shape of the electron velocity-field characteristic in conjunction with screening of the dark field by optically generated carriers.
The design of the next generation of vertical-cavity surface-emitting lasers (VCSELs) will greatly depend on the availability of accurate modeling tools. Comprehensive models of semiconductor lasers are needed to predict realistic behavior of various laser devices, such as the spatially nonuniform gain that results from current crowding. Advanced physics models for VCSELs require benchmark quality experimental data for model validation. This paper presents preliminary results of a collaborative effort at ARL to fabricate and experimentally characterize test optoelectronic structures and VCSEL devices, and at CFDRC to develop comprehensive multiphysics modeling, design and optimization tools for semiconductor lasers and photodetectors. Experimental characterization procedure and measurements of optical and electrical data for oxide-confined intracavity VCSELs are presented. A comprehensive multiphysics modeling tools CFD-ACE+ O’SEMI has been developed. The modeling tool integrates electronic, optical, thermal, and material gain data models for the design of VCSELs and edge emitting lasers (EELs). This paper presents multidimensional simulation analysis of current crowding in oxide-confined intracavity VCSELs. Computational results helped design the test structures and devices and are used as a guide for experimental measurements performed at ARL.
Finite difference analysis was used to determine the thermal characteristics of continuous wave (CW) 850 nm AlGaAs/GaAs implant-apertured vertical-cavity surface-emitting lasers. A novel flip-chip design was used to enhance the heat dissipation. The temperature rise in the active region can be maintained below 40 °C at 4 mW output power with 10 mA current bias. In contrast, the temperature rise reaches above 60 °C without flip-chip bonding. The transient-temperature during turn-on of a VCSEL was also investigated. The time needed for the device to reach the steady-state temperature was in the range of a few tenths of a milli-second, which is orders of magnitude larger than the electrical or optical switch time. Flip-chip bonding will reduce the shift of the wavelength, peak power, threshold current and slope efficiency during VCSEL operations.