W-D. Zhang

Observation of terahertz absorption signatures in microliter DNA solutions

Strong, repeatable Terahertz (THz) signatures are measured in small samples of DNA solution using a THz photomixing spectrometer with custom conical-horn coupling. The DNA specimen is linear, single-stranded (ss), 13-mer polynucleotide, and is diluted in buffer solution to have a concentration of 0.15 μmol/ml. The strongest signature is centered ∼717 GHz, and has a strength of 2.5 × 1016 cm−1/mol, linewidth 48 GHz, and damping time of 6.6 ps. The origin of the signature is attributed to the hydrogen bond of locally cross-linked ssDNAs.

THz behavior of indium-tin-oxide films on p-Si substrates

This paper reports broadband THz free-space transmission measurements and modeling of indium-tin-oxide (ITO) thin films on p-doped Si substrates. Two such samples having ITO thickness of 50 and 100 nm, and DC sheet conductance 260 and 56 Ω/sq, respectively, were characterized between 0.2 and 1.2 THz using a frequency-domain spectrometer. The 50-nm-film sample displayed very flat transmittance over the 1-THz bandwidth, suggesting it is close to the critical THz sheet conductance that suppresses multi-pass interference in the substrate. An accurate transmission-line-based equivalent circuit is developed to explain the effect, and then used to show that the net reflectivity and absorptivity necessarily oscillate with frequency. This has important implications for the use of thin-film metallic coupling layers on THz components and devices, such as detectors and sources. Consistent with previous reported results, the sheet conductance that best fits the THz transmittance data is roughly 50% higher than the DC values for both samples.

A Nonlinear Circuit Simulation of Switching Process in Resonant-Tunneling Diodes

A large-signal circuit model is used to compute the switching time for double-barrier resonant-tunneling diodes. The model consists of linear circuit elements plus a nonlinear I-V characteristic. The linear elements include a series resistor, a capacitor, and an inductor. The capacitance considers the charge accumulation, depletion in spacer layers, as well as charging-discharging of the quantum-well (QW) region. The inductance accounts for the delay of the current with respect to the voltage across the QW during the abrupt switching transition through the negative differential resistance region. A second-order Runge-Kutta method is used to solve for the switching transient, and then fit to experimental data for a high-quality InGaAs/AlAs resonant tunneling diode (RTD) using the QW inductance as a fitting parameter. Excellent agreement is found for the 10%-90% switching time with a calculated capacitance of 98 fF and a fitted inductance of 1300 pH. This large-signal inductance is approximately 7× greater than the small-signal inductance that has been successfully used to predict the fmax of RTDs such as the one tested here.

Demonstration of a GaAs-based 1550-nm continuous wave photomixer

An Er:GaAs-based 1550-nm CW photomixer is demonstrated. The related mechanism is extrinsic photoconductivity with optical absorption between the localized deep levels created by the Er and the extended states above the conduction band edge of GaAs. With the power boost made possible by a fiber-coupled erbium-doped-fiber amplifier, the Er:GaAs photomixers, operating at 1550 nm, radiate THz power levels easily measured by a Golay cell, and display a power spectrum having a −3 dB roll-off frequency of 307 GHz. This corresponds to a photocarrier lifetime of 520 fs, in good agreement with a previous measurement of the bandwidth of the same material in a photoconductive switch.

Ultrafast photoconductive devices based upon GaAs:ErAs nanoparticle composite driven at 1550 nm

This paper reports progress on a type of ultrafast photoconductive source that can be driven at 1550 nm but exhibits the robustness of GaAs (e.g., low-temperature-grown GaAs) driven at 780 nm. The approach is GaAs doped heavily with Er (≈4x1020 cm-3 or 2% atomic-Er-to-Ga fraction) such that ErAs nanoparticles form spontaneously during epitaxial growth by MBE. The nanoparticles are mostly spherical with a diameter of a few nm while the packing density is estimated as high as ~2.2x1019/cm3. Yet, the Er-doped GaAs epilayer maintains excellent structural quality and smooth surface morphology. A photoconductive switch coupled to a 4-turn square spiral antenna is fabricated and characterized. At least ~40 μW average THz power is generated when the device is biased at 75 V and pumped with a 1550-nm 90-fs-short pulsed laser having average power ~85 mW. This research is significant for 1550-nm-technologycompatible, cost-effective THz sources.

Abrupt dependence of ultrafast extrinsic photoconductivity on Er fraction in GaAs:Er

We present a study of room-temperature, ultrafast photoconductivity associated with a strong, sub-bandgap, resonant absorption around λ = 1550 nm in three MBE-grown GaAs epitaxial layers heavily doped with Er at concentrations of ≈2.9 × 1018 (control sample), 4.4 × 1020, and 8.8 × 1020 cm−3, respectively. Transmission-electron microscopy reveals lack of nanoparticles in the control sample, but abundant in the other two samples in the 1.0-to-3.0-nm-diameter range, which is consistent with the previously known results. We measure very high photoelectron (Hall) mobility (2.57 × 103 cm2/V-s) and terahertz power (46 μW average) in the 4.4 × 1020 sample, but then, an abrupt decay in these properties as well as the dark resistivity is seen as the Er doping is increased just 2 times. The Er doping has little effect on the picosecond-scale, 1550-nm photocarrier lifetime.

Accurate MM-wave-to-THz power measurements with large-area pyroelectric detectors

We present an improved method for measuring power at ∼100 GHz and above using a large-area (5-mm diam) LiTaO3 pyroelectric detector. The pyroelectric element is packaged in a lid-free TO-5 can, which in turn is mated to one end of a 1/2-inch-diam brass tube whose opposite end is flared for improved coupling efficiency. This “light-pipe”-like arrangement is cross-calibrated against a thermistor-based waveguide Anritsu ML83A power sensor using a 104-GHz Gunn oscillator and precision attenuator. It also stabilizes the pyroelectric element against background (IR) radiation and air currents. The resulting power detector is useful in the ∼10-nW-to-1-uW region where waveguide power sensors do not operate and free-space sensors like Golay cells and pyroelectrics have always been uncalibrated and unstable.

Non-contact, antenna-free probe for characterization of THz devices and components

This paper presents fabrication and testing of a contact-free, antenna-free probe for characterizing planar semiconducting devices in the THz region.

Imaging the hydration level of human skin with a millimeter-wave reflectometer

The hydration level of skin has become an important factor in evaluating the state of human health. We have demonstrated a hydration sensor that can be scanned over large regions of human body potentially in seconds. It is based on a millimeter-wave reflectometer centered at either W-Band or Ka-band. Ka-band provides higher hydration accuracy (<1%) and greater depth of penetration (>1 mm), thereby allowing access to the important dermis layer of skin. W-band provides less depth of penetration but finer spatial resolution (∼2 mm).

A millimeter-wave reflectometer for whole-body hydration sensing

This paper demonstrates a non-invasive method to determine the hydration level of human skin by measuring the reflectance of W-band (75-110 GHz) and Ka-band (26-40 GHz) radiation. Ka-band provides higher hydration accuracy (<1%) and greater depth of penetration (> 1 mm), thereby allowing access to the important dermis layer of skin. W-band provides less depth of penetration but finer spatial resolution (~2 mm). Both the hydration sensing concept and experimental results are presented here. The goal is to make a human hydration sensor that is 1% accurate or better, operable by mechanically scanning, and fast enough to measure large areas of the human body in seconds.