Imaging technology in the sub-THz frequency range (f = 0.1~1 THz) has various applications, such as body scanning in the security and medical areas. Recently, some research groups have reported a multi-pixel THz detector for real-time and large-area THz imaging [1,2]. However, since sub-THz waves have a relatively long wavelength (λ=. 0.3~3 mm), they cause a spatial resolution limit in THz imaging with their diffraction limit (λ/2).
In this work, we experimentally demonstrate the improved photoresponse and spatial resolution of the novel nanoscale aperture THz detector by near-field microscopy technology. FET layout with 1 μm aperture width and 280 nm probe distance, which is designed in a single copper layer for increasing and focusing the electric field when the THz wave arrives. By raster scanning the key-shaped structure at 0.5 THz, the photoresponse of the probe-type aperture detector is 7 times improved than without aperture.
Moreover, the spatial resolution, which is the minimum detectable distance between the boundary of materials, of the aperture detector is 2 times higher than the apertureless detector. Therefore, our probe-type aperture design in the THz detector can provide the possibility of high-performance and high-resolution THz imaging system.
THz technology
The detector usually acts as a transit mode, and the gate length limit is almost 10nm, so the operating frequency limit is below 1THz. However, the price of a high electron mobility transistor is higher than that of a silicon-based field effect transistor. Plasma wave transistor (PWT) is described with two-dimensional electron gas (2DEG) collected by gate overdrive voltage.
The electron bunch passes through the channel at the plasma wave speed which is much higher than the local electron speed. Plasma wave transistors (PWT) based on field effect transistor (FET) have been proposed for terahertz (THz) wave detector. In the two-dimensional electron gas (2DEG) channel, electrons flow at the plasma wave speed, which is much higher than the local electron speed.
In the case of detector, resonant detector can provide the maximum responsivity in the resonant mode with underdamped plasma wave through the channel when the plasma wave forms a standing wave. The high frequency regime has two mechanisms short gate (L< sτ) and long gate (L>> sτ). Hence, short gate house operation as resistive mixer and long gate house operation as non-resonant mode.
Non-resonant mode
The detectors listed generally have an output voltage below millivolts, so a gain stage should be required for about 40 dB of gain, so a multi-pixel detector like the one shown on the previous slide has a presentation limit.
Motivation
Near field microscopy technology
The main point of near-field microscopy is how to make the near-field wave. In figure 2-3, the emitter emits the THz wave that hit the object and the distance between sample and aperture is close we can detect the wave coming out by scattering towards sample. If we change the position of sample and detector, the emitter emits the wave and passes the aperture, then near-field wave occurs and the sample surrounding the aperture is scattered.
The advantage of the aperture type is that the design and array structure can be easily created on-chip, so it is suitable for real-time imaging. This scattering creates a near-field wave and detects the sample, as shown in Figure 2-4. The resolution of the probe type is the same as the edge of the probe, so it can be much smaller than the aperture type.
However, the optical system is necessary because of the locking mechanism, and it is difficult to make an array structure. In Figure 2-5, the waveguide has a diameter D, and the distance from the edge of the waveguide is x. Therefore, around that point the electric field decreases exponentially, and around the wavelength the electric field decreases linearly.
Enhancing the E-field by probe tip
The top plot in Figure 2-7 is a substrate-only simulation that had no device and no metal substrate, so there is only absorption with the substrate. The two graphs are almost the same, then we know that the tile and the device do not have much effect on the antenna simulation. A near-field wave is excited at the probe aperture, but decays rapidly, so it cannot affect the antenna gate.
Layer design and cross-section view of device
The THz emitter is a diode armature that emits a frequency of 100 GHz to 500 GHz, as shown in Figure 3-4. The top picture is the structure for near field confirmation and the bottom picture is the structure for the imaging system. In Figure 3-3 (b), the aperture and the detector are designed on one chip, so the distance between the aperture and the detector is less than 1μm.
Confirmation the near-field by experiment
First detector and second detector have the same MOSFET detector, but the first detector has a near-field antenna and the second detector has electrical antenna. The results in figure 4-2, we can see the improvement of resolution by near-field antenna. The photoresponse of near-field antenna with detector is 7 times greater than electric antenna with detector.
With this result, if the distance between the near-field antenna and the detector is closed, the photo response can be larger than that of the electric antenna. To measure the resolution of two detectors, normalize each photo response and compare the distance between 80% and 20% of the local maximum value. The point of the imaging system is the position of the near field source and the position of the object and the distance between the source and the object.
In the case of the classical apertured near-field microscopy method, the near-field occurs after passing the aperture. And the near field scatters the object, the transmission wave is detected in the THz detector. The incident wave is also an evident wave, so the new detector is detecting that wave.
We need a detector with high spatial resolution and the distance between object and detector must be small. If we can reduce the distance between object and aperture, we can improve the resolution of the imaging system more than this work in Figure 4-3. A high-resolution plasmonic THz detector has been experimentally demonstrated using near-field microscopy technology.
By using Verify the near field by experiment in Section 4.1, we can control the evanescent wave in the casting process. Decreasing the distance between the near-field probe and the detector as Figure 6-1, the photoresponse increases exponentially with Figure 6-2 if the evanescent wave occurred. New aperture design for near-field Terahertz imaging system is proposed with more improved and focused E-field based gate acting as near-field probe.
We expect the improvement of the near-field distribution by reducing the distance between near-field antenna and detector. Kawano, K Ishibashi, "An on-chip near-field terahertz probe and detector", Nature Photon, vol 2, pp.
