Field-effect Self-mixing Terahertz Detectors

In this chapter, the basic characteristics of terahertz electromagnetic wave and terahertz applications are briefly introduced. Challenges in general, to develop solid-state terahertz devices including emitters and detectors are discussed by comparing the existing principles and actual devices. The focus of this thesis is then refined to the understanding of the self-mixing mechanism and the development of practical detectors based on field-effect transistors or those alike. The outline of this thesis is given at the end of this chapter.

Self-mixing of terahertz electromagnetic wave occurs in a field-effect electron channel when the terahertz electric field modulates both the local electron density and the drift velocity. In order to realize sensitive terahertz detection, asymmetry in the electric field and/or the charge density is required for generation of a unidirectional photocurrent/voltage. Existing hydrodynamic detection theories are reviewed and discussed. A detector model taking into account the spatial distributions of both the terahertz electric field and the electron density in the gated electron channel is developed in this chapter. The model presents full detector characteristics when both a source–drain bias and a gate voltage are applied. The model suggests that an asymmetric distribution of terahertz electric field is preferred for high-responsivity terahertz detection without a source–drain bias. The strength of terahertz photoresponse is characterized by the self-mixing factor and the field-effect factor. The former factor can be optimized by a strongly asymmetric and enhanced terahertz near field by using asymmetric terahertz antennas. Simulations based on the FDTD method confirm the effectiveness of asymmetric antenna design and the low-pass filter to isolate the antenna blocks from the electrical bonding pads for the detector.

In this chapter, the fabrication, characterization, and optimization of self-mixing terahertz field-effect detectors based on AlGaN/GaN 2DEG are introduced in details. By fabrication, five different detectors are made to uncover the self-mixing mechanism and search for an optimized detector design. By characterization, we not only obtain the \(I-V\) characteristics, the responsivity, the noise-equivalent power, the response spectrum, the response speed, the polarization effect, etc, but also we probe the localized self-mixing photocurrent based on which the quasi-static detector model and the design of asymmetric antenna are verified. Under the guidance of the detector model, we focus on the design of terahertz antennas and field-effect gate to improve the detector responsivity and sensitivity. An asymmetric antenna with three dipole blocks is found to be the most effective antenna among the five different designs. A design rule for high-sensitivity terahertz detectors is given.

The effect of symmetries in the terahertz field distribution and the field-effect channel on terahertz photocurrent is examined and compared to the quasi-static field-effect detector model. Resonant excitation of cavity plasmon modes and nonresonant self-mixing of terahertz waves are demonstrated in an AlGaN/GaN two-dimensional electron gas with symmetrically designed nanogates, antennas, and filters. We found that the self-mixing signal can be effectively suppressed by the symmetric design and the resonant response benefits from the residual asymmetry. The findings further confirm the quasi-static field-effect detector model. The findings also suggest that a single detector may provide both a high sensitivity from the self-mixing mechanism and a good spectral resolution from the resonant response by optimizing the degree of geometrical and/or electronic symmetries.

In the terahertz 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 that a field-effect terahertz detector combined with a probe tip could serve as a high-sensitivity terahertz near-field sensor.

Nonresonant self-mixing terahertz detectors have a few advantages concerning the room-temperature operation, high sensitivity, wide response spectrum, high speed, and high signal-to-noise ratio over other slow bolometric detectors for room-temperature applications. Terahertz imaging based on single-pixel detectors and a \(1 \times 9\) linear detector array are demonstrated with a high spatial resolution and a high signal-to-noise ratio. The single-pixel detector is successfully implemented in a Fourier transform spectrometer and a fast Fourier transform spectroscopy of a 900 GHz terahertz signal is demonstrated. Although the demonstrated self-mixing detectors are far from optimal in sensitivity, speed, stability, and reproducibility, this technology has a great potential to be further developed and optimized to bring many terahertz applications at room temperature into reality.

In this chapter, we emphasize that plasmon devices based on 2DEG materials ways become more and more important for terahertz applications. The main results and the conclusions are listed for non-resonant self-mixing detection of terahertz electromagnetic wave. The future trends are developing heterodyne plasmon detectors and resonant plasmon detectors are discussed. Questions concerning the ultimate limit in plasmon detection are raised for further studies.