Choonsup Lee

Penetrating 3-D Imaging at 4- and 25-m Range Using a Submillimeter-Wave Radar

We show experimentally that a high-resolution imaging radar operating at 576-605 GHz is capable of detecting weapons concealed by clothing at standoff ranges of 4-25 m. We also demonstrate the critical advantage of 3-D image reconstruction for visualizing hidden objects using active-illumination coherent terahertz imaging. The present system can image a torso with <1 cm resolution at 4 m standoff in about five minutes. Greater standoff distances and much higher frame rates should be achievable by capitalizing on the bandwidth, output power, and compactness of solid state Schottky-diode based terahertz mixers and multiplied sources.

A Frequency-Multiplied Source With More Than 1 mW of Power Across the 840–900-GHz Band

We report on the design, fabrication, and characterization of an 840-900-GHz frequency multiplier chain that delivers more than 1 mW across the band at room temperature with a record peak power of 1.4 mW at 875 GHz. When cooled to 120 K, the chain delivers up to 2 mW at 882 GHz. The chain consists of a power amplifier module that drives two cascaded frequency triplers. This unprecedented output power from an electronic source is achieved by utilizing in-phase power-combining techniques. The first stage tripler uses four power-combined chips while the last stage tripler utilizes two power-combined chips. The source output was analyzed with a Fourrer transform spectrometer to verify signal purity.

A thermal inkjet printhead with a monolithically fabricated nozzle plate and self-aligned ink feed hole

A monolithic thermal inkjet printhead has been developed and demonstrated to operate successfully by combining monolithic growing of a nozzle plate on the silicon substrate and electrochemical etching of silicon for an ink feed hole. For the monolithic fabrication, a multiexposure and single development (MESD) technique and Ni electroplating are used to form cavities, orifices, and the nozzle plate. Electrochemical etching, as a back-end process, is applied to form an ink feed hole through the substrate, which is accurately aligned with the frontside pattern without any backside mask. The etch rate is nearly proportional to the current density up to 50 /spl mu/m/min. Experiments with a 50-/spl mu/m-diameter nozzle show ink ejection up to the operating frequency of 11 kHz with an average ink dot diameter of about 110 /spl mu/m for 0.3-A, 5-/spl mu/s current pulses.

Leak-tight piezoelectric microvalve for high-pressure gas micropropulsion

This paper describes the results of our development of a leak-tight piezoelectric microvalve, operating at extremely high upstream pressures for microspacecraft applications. The device is a normally closed microvalve assembled and fabricated primarily from micromachined silicon wafers. The microvalve consists of a custom-designed piezoelectric stack actuator bonded onto silicon valve components (such as the seat, boss, and tether) with the entire assembly contained within a stainless steel housing. The valve seat configurations include narrow-edge seating rings and tensile-stressed silicon tethers that enable the desired, normally closed, leak-tight operation. Leak testing of the microvalve was conducted using a helium leak detector and showed leak rates of 5/spl times/10/sup -3/ sccm at 800 psi (5.516 MPa). Dynamic microvalve operation (switching rates of up to 1 kHz) was successfully demonstrated for inlet pressures in the range of 0/spl sim/1000 psi. The measured static flow rate for the microvalve under an applied potential of 10 V was 52 sccm at an inlet pressure of 300 psi. The measured power consumption, in the fully open state, was 3 mW at an applied potential of 30 V. The measured dynamic power consumption was 180 mW for 100 Hz continuous operation at 100psi.

Demonstration of a room temperature 2.48–2.75 THz coherent spectroscopy source

We report the first demonstration of a continuous wave coherent source covering 2.48-2.75 THz, with greater than 10% instantaneous tuning bandwidth and having 1-14 μW of output power at room temperature. This source is based on a 91.8-101.8 GHz synthesizer followed by a power amplifier and three cascaded frequency triplers. It demonstrates for the first time that purely electronic solid-state sources can generate a useful amount of power in a region of the electromagnetic spectrum where lasers (solid state or gas) were previously the only available coherent sources. The bandwidth, agility, and operability of this THz source have enabled wideband, high resolution spectroscopic measurements of water, methanol, and carbon monoxide with a resolution and signal-to-noise ratio unmatched by any other existing system, providing new insight in the physics of these molecules. Furthermore, the power and optical beam quality are high enough to observe the Lamb-dip effect in water. The source frequency has an absolute accuracy better than 1 part in 10(12) and the spectrometer achieves sub-Doppler frequency resolution better than 1 part in 10(8). The harmonic purity is better than 25 dB. This source can serve as a coherent signal for absorption spectroscopy, a local oscillator for a variety of heterodyne systems and can be used as a method for precision control of more powerful but much less frequency agile quantum mechanical terahertz sources.

In-Phase Power-Combined Frequency Triplers at 300 GHz

We report on the design, fabrication and characterization of a 300 GHz Schottky-diode frequency tripler made of two mirror-image integrated circuits that are power-combined in-phase in a single waveguide block using compact Y-junctions at the input and output waveguides. Each chip features six anodes on a 5 thick GaAs membrane. The tripler has 5-15% conversion efficiency measured across the 265-330 GHz band when driven with 50-250 mW of input power at room temperature. At 318 GHz it delivers a peak power of 26 mW with 11% conversion efficiency. The power-combined frequency multiplier is compared with a single-chip tripler designed for the same band using the same integrated circuit.

A Broadband 835–900-GHz Fundamental Balanced Mixer Based on Monolithic GaAs Membrane Schottky Diodes

The development of a 835-900-GHz biasable fundamental balanced mixer using planar GaAs Schottky diodes is presented. The monolithic microwave integrated circuit integrates two planar Schottky anodes in a balanced configuration, stripline filtering elements, and on-chip capacitor on a thin GaAs membrane. At 850 GHz, double side-band (DSB) mixer noise temperature of 2660 K and conversion loss of 9.25 dB are measured, respectively, at room temperature. When the mixer is cooled to 120 K, the DSB mixer noise temperature and conversion loss improve to 1910 K and 8.84 dB, respectively.

Design and Characterization of a Room Temperature All-Solid-State Electronic Source Tunable From 2.48 to 2.75 THz

We report on the design, fabrication and test of an all-solid-state, frequency agile source that produces over across the 2.48-2.75 THz band at room temperature. This frequency-multiplied source is driven by a W-band synthesizer followed by a power amplifier that delivers 350-450 mW (25.5-26.5 dBm) and a cascade of three balanced frequency triplers. The first stage tripler is based on four power-combined six-anode GaAs Schottky diode devices, and the second stage tripler is based on two four-anode GaAs devices. The output tripler uses a single unbiased device featuring two anodes monolithically integrated onto a thin GaAs membrane. The source delivers a record at 2.58 THz at room temperature. This frequency multiplied source is analyzed with a Fourier transform spectrometer (FTS) and the unwanted harmonics are found to be at least 29 dB below the desired signal. This source, when used as the local oscillator for a hot-electron bolometer mixer, will enable heterodyne instruments for future space missions to map the cosmologically-important 2.675 THz HD molecular line.

Multiple-channel GaAs/AlGaAs high electron mobility transistors

Multiple-channel high electron mobility transistors (HEMTs) have been designed and fabricated on GaAs/AlGaAs heterostructural material grown by molecular beam epitaxy (MBE). The sheet carrier density of the two-dimensional electron gas (2-DEG) measured at 77 K was linearly proportional to the number of high mobility electron channels, and reached 5.3 × 1012cm-2for six-channel HEMT structures. Depletion-mode devices of the double-heterojunction HEMT were operated between negative pinchoff voltage and forward-biased gate voltage without any transconductance degradation. A peak extrinsic transconductance of 360 mS/mm at 300 K and 550 mS/mm at 77 K has been measured for a 1-µm gate-length double-heterojunction enhancement-mode device. An extremely high drain current of 800 mA/mm with a gate-to-drain avalanche breakdown voltage of 9 V was measured on six-channel devices.

A wafer-level diamond bonding process to improve power handling capability of submillimeter-wave Schottky diode frequency multipliers

We have developed a robust wafer-level substrate bonding process that has allowed us to bond CVD diamond to GaAs membrane-based submillimeter-wave Schottky diode frequency multipliers. The high thermal conductivity of CVD diamond allows the chip to dissipate heat more efficiently thus increasing the power handling capability of the chips. This process has resulted in single-chip multiplier devices working in the submillimeter-wave range that can handle hundreds of milliwatts of input power. Output powers of 40 mW at 250 GHz and 27 mW at 300 GHz from a single chip have been demonstrated with this method. It is expected that by power combining these chips it is now possible to achieve a wideband 300 GHz signal with more than 100 mW of power. This represents a dramatic improvement in the current state of the art and allows one to begin realizing submillimeter-wave radar applications.