July 03, 2009

Carbon Nanotube Quantum Dot Terahertz Detectors and On-Chip High Resolution near-field terahertz detector

Two types of emerging terahertz detectors are based on novel nanoelectronic technologies. Future work to combine the two will enable a real time terahertz video camera.

1. A highly sensitive and frequency tunable terahertz detector based on a carbon nanotube (CNT) quantum dot (QD).

Observations have been made of electron tunneling via terahertz-photon detection, called photon-assisted tunneling. This result means that the CNT-QD structure can be utilized as a frequency tunable terahertz detector. CNT-QD detector functions properly up to approximately 7 K. Higher-temperature operation of the CNT-QD terahertz detector is also possible with more refined fabrication techniques.

The next important step is to improve detector performance in two important ways: sensitivity and frequency selectivity. A much more sensitive readout of the terahertz-detected signal could be achieved by capacitively coupling a CNT-QD with a quantum point contact device on a GaAs/AlGaAs heterostructure, which makes it possible to observe single-electron dynamics. And frequency selectivity could be improved by using a double-coupled CNT-QD, in which photon-assisted tunneling takes place as a result of electron transitions between two well-defined discrete levels.

2. A near-field terahertz detector for high-resolution imaging.

Contrary to the situation in the microwave and visible-light region, the development of near-field imaging in the terahertz region has not been well established. Japan RIKEN has developed a new device for near-field terahertz imaging in which all components—an aperture, a probe, and a detector—are integrated on one gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) chip. This scheme allows highly sensitive detection of the terahertz evanescent field alone, without requiring optical or mechanical alignment.

Two approaches can be used to achieve high spatial resolution in optical imaging: a solid immersion lens and near-field imaging. Though we have previously constructed a terahertz imaging setup based on a solid immersion lens, its resolution is restricted by the diffraction limit.3 A powerful method for overcoming the diffraction limit is the use of near-field imaging. This technique has been well established in visible and microwave regions using either a tapered, metal-coated optical fiber or a metal tip, and either a waveguide or a coaxial cable. However, the development of near-field imaging in the terahertz region has been hindered by the lack of terahertz fibers or other bulk terahertz-transparent media suitable for generating near-field waves, as well as the low sensitivity of commonly used detectors in the terahertz region.

In conventional near-field imaging systems, the propagation field arising from the scattering of the near-field (evanescent) wave is measured with a distant detector, which requires detecting very weak waves (and the influence of far-field waves is unavoidable). In contrast, our near-field terahertz imager places the aperture, probe, and detector in close proximity. The 8-µm-diameter aperture and planar probe, each of which is insulated by a 50-nm-thick silicon dioxide (SiO2) layer, are deposited on the surface of a GaAs/AlGaAs heterostructure chip.

An optical micrograph (left) and a schematic representation (right) shows the design of a highly sensitive on-chip near-field THz detector. The 8-µm-diameter aperture and planar metallic probe, each of which is insulated by a 50-nm-thick silicon dioxide (SiO2) layer, are deposited on the surface of a GaAs/AlGaAs heterostructure chip. (Courtesy of RIKEN)

Because integration with the CNT-QD detector requires improvements in the device fabrication process (specifically, by using higher-performance electron-beam lithography equipment), a two-dimensional electron gas (2DEG)—located only 60 nm below the chip surface—is used as the terahertz detector.

Why Terahertz Detection is Tough

The photon energy of the terahertz wave, on the order of millielectron volts (meV), is two to three magnitudes lower than that of the visible light, making the development of a high-performance terahertz detector a difficult task. Another problem with terahertz detection is low spatial resolution of terahertz imaging, which results from the longer wavelengths of terahertz radiation compared to that of visible light.

Work to Combine the Carbon Nanotube Quantum Dot Detector for Near Field Detection

One of the challenges for future terahertz sensing technology is to achieve high detection sensitivity and high spatial resolution simultaneously. To realize this, we are now trying to combine the two techniques described above; namely to modify the CNT-QD terahertz detector into a similar structure for near-field detection. Compared to the 2DEG detector, the CNT detector exhibits much higher sensitivity and has a much smaller sensing area (approximately 200 nm compared to 8 µm for the 2DEG detector). This detector, integrated with an aperture and a probe, would show ultrahigh sensitivity and nanometer resolution simultaneously.

We further expect that when many CNTs are integrated in a two dimensional configuration, the resulting device will serve as a real-time, high-resolution terahertz imaging detector; in effect, a terahertz video camera.

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