
- Photonics Research
- Vol. 10, Issue 4, 1090 (2022)
Abstract
1. INTRODUCTION
Terahertz (THz) waves are electromagnetic waves with frequencies between those of microwave and mid-infrared. This frequency region was previously called the “THz gap” [1] because of the difficulty in THz signal generation and detection. The rapid development of THz radiation sources and detectors [2–4] in recent decades has gradually closed the “THz gap” and demonstrated great potential in many areas, such as high-speed data transmission in ultrahigh-definition television systems [5,6]. Newly emerged applications, such as artificial intelligence and machine learning, also demand high-speed data transmission among advanced processing units, such as CPU, GPU, and TPU, in inter/intrachip systems [7,8]. The mismatch between heavy data computation and transmission among advanced processing units has created a new gap, the “interconnect gap” [8]. Advances in the THz region may help bridge the newly emerged “interconnect gap.” For example, a THz on-chip photonic waveguide based on a valley Hall photonic crystal (VPC) structure has been reported recently, which supports error-free transmission at a data transfer rate of up to
To develop a fully integrated THz topological inter/intrachip communication system, it is necessary to integrate various functionalities such as waveguides, switches, and modulators on the same topological photonic crystal platform. However, most existing VPCs in the THz regime are limited to passive and static configurations, where the behavior of THz signals cannot be tuned dynamically at will. Experimental implementation of tunable topological VPC structures is, thus, a crucial step toward fully integrated topological THz communication systems. Here, we demonstrate a simple and low-cost method for dynamically tunable THz topological waveguides, based on which we realized effective on/off switching of THz waves. A VPC structured THz waveguide based on high-resistivity silicon with four 60° sharp corners operating at
2. THEORY AND SIMULATION
We designed photonic crystal structures on a high-resistivity silicon substrate, as shown in Fig. 1(a). The photonic crystal adopts a honeycomb lattice with a lattice constant
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Figure 1.(a) THz topological photonic waveguide switch based on valley kink state. (b) Unit cell of the valley photonic crystal containing two types of triangular holes A and B with different sizes of a nonzero
3. MATERIALS AND METHODS
One of the most intriguing properties of topological valley kink states is that they are backscattering-free against sharp corners and preserve the valley pseudospin. To demonstrate this property, we employed photolithography and deep reactive-ion etching to fabricate a straight and a highly bent (trapezoidal) domain wall on a high-resistivity (
Figure 2.(a) Optical image of the fabricated straight and bent VPC waveguide. (b) Optical microscopy image of the domain wall with opposite
The pumping light (
Figure 2(b) shows the optical microscopy image of the lattice, which includes a “kink”-type domain wall between two VPCs with opposite
The transmission spectra (S11) and reflection spectra (S12) are presented in Figs. 3(a) and 3(b). By comparing the reflected signals in the devices with straight and bent domain walls, one can conclude that the sharp bends do not introduce additional backscattering. The relatively large backscattering signals in the measured results mainly originate from the terminals of the valley kink states. The backreflection around 285 GHz in Fig. 3(b) mainly occurs at (1) the interface between WR-3 hollow waveguide and (2) the interface between the line-defect photonic crystal waveguide and the valley kink states. It should be noted that such backscattering is not due to the sharp turns of the valley kink states because sharp turns do not introduce large intervalley scatterings. This can be confirmed by the experimental results in Fig. 3(c), which shows the measured reflection spectrum of S11 and S22 when the topological channels have been turned off by a strong visible light. One can clearly find that (1) S11 and S22 have different spectra because of the different relative positions between the WR-3 hollow waveguide and the tapered structures at the left and right terminals and (2) the energy levels of S11 and S22 remain almost unchanged even when the topological channels are turned off [see S12 in Fig. 3(c)]. The experimental results confirm that the measured backscattering mainly occurs outside the topological channels, which are also consistent with previous experiments using near-field mapping [35]. This can be confirmed by the fact that the Fabry–Perot-like interference in S11 can be greatly repressed when the valley kink states are turned off by the visible light, as shown in Fig. 3(c). The propagation loss of the VPC waveguide is low because (1) high-resistivity silicon shows low absorption in the THz regime, which was also reported in Ref. [9], and (2) the valley kink states lie below the light line and, thus, theoretically have zero radiation loss into the free space [36]. For these reasons, we expect that the propagation loss of the VPC waveguide mainly comes from imperfect fabrication processes, which is quite small and cannot be accurately quantified from our current experimental results.
Figure 3.(a) Transmission spectra of the straight VPC for different S parameters. (b) Transmission spectra of the bent VPC for different S parameters. (c) Transmission spectra of the bent VPC with pumping light at
4. RESULTS AND DISCUSSION
A fiber-coupled continuous-wave laser (
Figure 4.(a) Photo of the experimental setup of the VPC waveguide switch. The pumping light is a fiber-coupled laser at 447 nm. The blue dashed double-head arrow indicates that the pumping light can be adjusted vertically. (b) The light spot was scanned from position 1 to position 5 to locate the best pumping position to the VPC waveguide. (c) Transmission spectra of various pumping positions at the same pumping intensity of
Figure 5 illustrates the application scenario of our VPC waveguide optical switch in a multiprocessor on-chip communication system. To achieve high-speed communication among different processors, we should make full use of the 20 GHz bandwidth in our VPC design. For example, when Chip 1 communicates with Chip 2, the desired data transmission path is marked by the solid black arrow in Fig. 5. However, the signal would also be transmitted to Chip 4 by following the dashed black path, which interferes with the communication frequency channel between Chips 5 and 4 (marked by the solid green arrow), leading to the 20 GHz bandwidth resource not being fully used. Techniques such as wavelength-division multiplexing can be used to overcome channel interference, but they lower the data transmission speed and complicate the system. Instead, by focusing an external pumping beam on the topological waveguide (as marked by the round spot in Fig. 5), interference with the communication channel to Chip 4 can be eliminated. In practical applications, light from an array of semiconductor laser diodes can be allocated to different topological waveguide channels, which form a programmable information network among the processors in an interchip communication system. Therefore, our method provides a low-cost solution to the channel interference issue while maintaining the communication rate in the THz interconnect system.
Figure 5.Schematic of an interchip communication system based on VPC waveguides. The pumping light can switch off the undesired channel to eliminate the interference. The black dashed line represents the undesired data transmission from Chip 1 to Chip 4, which interferes with the communication between Chip 5 and Chip 4 (marked by the solid green arrow).
Theoretically, the switching speed of our design is limited by the lifetime of the photoexcited free carriers in the silicon substrate. We could not measure the on/off switching speed directly from 270 to 290 GHz owing to the limited functionalities of our VNA. We did not perform time-resolved THz spectroscopy because the lifetime of free carriers in high-resistivity silicon is usually beyond its measurable range. The modulation effect from the free carriers is broadband to the THz signal [30]. Thus, it is reasonable to use a continuous-wave THz source (100 GHz, from Terasense Group Inc.) to measure the photoexcited carrier lifetime. The THz source was normally incident to the VPC device surface, and the pumping light was shone on the device at an oblique angle [Fig. 6(a)]. The pumping light was triggered by a transistor-transistor-logic (TTL) signal at various frequencies ranging from 0 to 110 kHz. The details of the experimental setup are provided in Fig. 6(b). Using this method, we could calculate the lifetime of the free carriers and estimate the switching speed of the VPC waveguide. The normalized modulation depth was calculated based on the modulation results at a triggered frequency of zero. The 3 dB modulation bandwidth in the transmission configuration was
Figure 6.(a) Schematic of photoexcited free-carrier lifetime measurement. The THz signal transmitted through the VPC device and was attenuated by the photoexcited carriers, which were generated by a modulated pumping beam shone at an oblique angle. (b) Experimental setup for carrier lifetime measurement.
Figure 7.Modulation depth as a function of modulation frequency, showing the 3 dB modulation bandwidth of
5. CONCLUSION
We proposed and experimentally demonstrated an optical approach for effectively switching on/off the VPC waveguide. The bent VPC waveguide based on a high-resistivity silicon substrate shows immunity to backscattering at sharp bends and has an operating band from 270 to 290 GHz. THz-absorbing free carriers were generated by a 447 nm continuous-wave laser to effectively switch off the waveguide. With a pumping intensity of
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