Qin Hua

Tunable terahertz plasmon in grating-gate coupled graphene with a resonant cavity

Plasmon modes in graphene can be tuned into resonance with an incident terahertz electromagnetic wave in the range of 1–4 THz by setting a proper gate voltage. By using the finite-difference-time-domain (FDTD) method, we simulate a graphene plasmon device comprising a single-layer graphene, a metallic grating, and a terahertz cavity. The simulations suggest that the terahertz electric field can be enhanced by several times due to the grating–cavity configuration. Due to this near-field enhancement, the maximal absorption of the incident terahertz wave reaches up to about 45%.

Extraction of terahertz emission from a grating-coupled high-electron-mobility transistor

In a grating-coupled high-electron-mobility transistor, weak terahertz emission with wavelength around 400 μm was observed by using a Fourier-transform spectrometer. The absolute terahertz emission power was extracted from a strong background blackbody emission by using a modulation technique. The power of terahertz emission is proportional to the drain—source current, while the power of blackbody emission has a distinct relation with the electrical power. The dependence on the drain—source bias and the gate voltage suggests that the terahertz emission is induced by accelerated electrons interacting with the grating.

Characterization of a room temperature terahertz detector based on a GaN/AlGaN HEMT

We report on the characterization of a room temperature terahertz detector based on a GaN/AlGaN high electron mobility transistor integrated with three patch antennas. Experimental results prove that both horizontal and perpendicular electric fields are induced in the electron channel. A photocurrent is generated when the electron channel is strongly modulated by the gate voltage. Despite the large channel length and gate-source/drain distance, significant horizontal and perpendicular fields are achieved. The device is well described by the self-mixing of terahertz fields in the electron channel. The noise-equivalent power and responsivity are estimated to be and 3 mA=W at 292 K, respectively. No decrease in responsivity is observed up to a modulation frequency of 5 kHz. The detector performance can be further improved by engineering the source-gate-drain geometry to enhance the nonlinearity.

Fabrication and Characterization of a Single Electron Transistor Based on a Silicon-on-Insulator

A single electron transistor based on a silicon-on-insulator is successfully fabricated with electron-beam nano-lithography, inductively coupled plasma etching, thermal oxidation and other techniques. The unique design of the pattern inversion is used, and the pattern is transferred to be negative in the electron-beam lithography step. The oxidation process is used to form the silicon oxide tunneling barriers, and to further reduce the effective size of the quantum dot. Combinations of these methods offer advantages of good size controllability and accuracy, high reproducibility, low cost, large-area contacts, allowing batch fabrication of single electron transistors and good integration with a radio-frequency tank circuit. The fabricated single electron transistor with a quantum dot about 50 nm in diameter is demonstrated to operate at temperatures up to 70 K. The charging energy of the Coulomb island is about 12.5 meV.

A sensitive charge scanning probe based on silicon single electron transistor

Single electron transistors (SETs) are known to be extremely sensitive electrometers owing to their high charge sensitivity. In this work, we report the design, fabrication, and characterization of a silicon-on-insulator-based SET scanning probe. The fabricated SET is located about 10 μm away from the probe tip. The SET with a quantum dot of about 70 nm in diameter exhibits an obvious Coulomb blockade effect measured at 4.1 K. The Coulomb blockade energy is about 18 meV, and the charge sensitivity is in the order of 10−5−10−3 e/Hz1/2. This SET scanning probe can be used to map charge distribution and sense dynamic charge fluctuation in nanodevices or circuits under test, realizing high sensitivity and high spatial resolution charge detection.

Mapping an on-chip terahertz antenna by a scanning near-field probe and a fixed field-effect transistor*

In the terahertz (THz) regime, the active region for a solid-state detector usually needs to be implemented accurately in the near-field region of an on-chip antenna. Mapping of the near-field strength could allow for rapid verification and optimization of new antenna/detector designs. Here, we report a proof-of-concept experiment in which the field mapping is realized by a scanning metallic probe and a fixed AlGaN/GaN field-effect transistor. Experiment results agree well with the electromagnetic-wave simulations. The results also suggest a field-effect THz detector combined with a probe tip could serve as a high sensitivity THz near-field sensor.