Conference Agenda

Quantum technologies are experiencing considerable growth in the microwave and optical domains, while their development in the THz spectral range is still in its infancy, but promises significant technological impact1. In this context, developing a novel technology to realize two-level quantum systems at THz frequencies compatible with direct on-chip integration would represent a major breakthrough 1. To this aim, graphene quantum dots are very attractive due to their high flexibility in engineering electronic states through their size, shape, and edges2. Here, we present a two-level system based on a hBN-encapsulated graphene double quantum dot (DQD) exhibiting a tunable transition frequency at THz frequencies. Using low temperature transport measurements combined with non-equilibrium Green’s functions model, we demonstrate a two-level system with resonance frequency of up to 0.14 THz. We further show that a single graphene QD exhibits a large THz electric dipole with a length of ≈230 nm, revealed by photon-assisted tunnelling phenomenon3. We also present original hybrid THz resonators4 that combine relatively high-quality factors (Q37) with a deep subwavelength mode volume (V3.2x10−4λ3). Coupling graphene DQDs to these Tamm resonators opens new avenues for generating and detecting non-classical THz light states, essential building blocks of quantum technologies.

This contribution presents a novel Schottky-diode-based Single-Pole Single-Throw (SPST) waveguide switches operating at mm/submm-waves. This technology offers unmatched low insertion loss and very fast switching performance. Moreover, Schottky-based SPST Switches are suitable for operation in regime of extremely fast power limiters and electronically controlled attenuators. These findings suggest great perspectives of Schottky-based WG switches for applications in radar technologies and wireless telecommunication. Further development of this technology towards multi-channel switches will also enable a new family of Schottky-diode-based switches, opening the way for more compact mm-wave radiometer instruments for Earth Observation and other space applications.

The integration of Silicon Nitride TriPleX waveguides based optical beamforming network (OBFN) chips with THz photodiodes (PDs) has predominantly relied on optical butt coupling [1]. However, this approach restricts the integration to chip’s edge, with a limitation of integrating the chip with linear PD arrays for 1D beam steering. To enable applications demanding integration of multiple THz-PDs like 2D beam steering, a vertical integration approach is considered, allowing the integration of a PD matrix onto the surface of an OBFN chip. This approach aims at eliminating the need for complex bonding processes and facilitates large-scale integration of PDs [2]. To achieve this integration, a successful transfer of epitaxial layers of THz-PDs from the InP substrate onto the TriPleX chip is developed. This transfer procedure is developed to achieve prism coupling from TriPleX waveguide to PD. A TriPleX chip with a bonding window for the subsequent bonding process is developed. This window is created by dry etching the top SiO₂ cladding layer over the TriPleX waveguides. Aluminum oxide (Al₂O₃) layer is chosen as bonding layer due to their transparency and intermediate refractive index at a wavelength of 1.55µm [3]. The InP substrate is successfully bonded on the Si substrate with the Al₂O₃ bonding layer thickness of ~23 nm, verifying this hybrid vertical integration technology. Additionally, the result indicates the potential of scaling up integrations of the THz-PDs with a tunable dual-mode laser to realize on-chip THz source.

Reference:

[1] C. G. Roeloffzen et al., "Low-loss Si3N4 TriPleX optical waveguides: Technology and applications overview," IEEE journal of selected topics in quantum electronics, vol. 24, no. 4, pp. 1-21, 2018.

[2] M. R. Billah et al., "Hybrid integration of silicon photonics circuits and InP lasers by photonic wire bonding," Optica, vol. 5, no. 7, pp. 876-883, 2018.

[3] P. Kumar, M. K. Wiedmann, C. H. Winter, and I. Avrutsky, "Optical properties of Al₂O₃ thin films grown by atomic layer deposition," Applied Optics, vol. 48, no. 28, pp. 5407-5412, 2009.

Reconfigurable intelligent surfaces (RISs) are envisioned to enable future millimetre wave (mm-wave) and terahertz (THz) communication and sensing systems [1]. They consist of many tunable radiating elements that redirect impinging waves to desired directions or focal spots. Liquid crystal (LC) offers a scalable, low-power tuning option for large-scale RISs. Unlike typical resonance-based LC-RIS approaches, delay line architectures (see Fig. 1) offer advantages in jointly optimizing response time, loss and bandwidth. In [2], a delay line based LC-RIS operating around 60 GHz demonstrated response times <100 ms, unit cell losses <7 dB, a bandwidth of more than 10% and beam steering between ±50°.

LC-RIS are also well suited for THz applications, as LC losses typically remain stable with rising frequency. As frequency increases, more radiating elements fit into the same physical dimensions. However, scaling the LC-RIS towards higher frequencies poses challenges, particularly related to decreasing substrate thickness and feature sizes. In this context, an LC-RIS operating around 240 GHz has been designed [3], and is currently being manufactured. The glass thickness is in the 50 to 100 µm range, and the minimum feature size corresponds to 10 µm. Notably, substrates with thicknesses around or below 100 µm also enable mechanical flexibility. Simulations demonstrate beam steering beyond ±50°, a bandwidth exceeding 10%, aperture efficiency above 25% and response times in the millisecond range. Beyond frequency scaling, research on LC-RIS also targets higher efficiency, broader bandwidths, faster response times and novel functionalities, paving the way for new communication and sensing applications.

In this paper, a graphene-based higher odd-order harmonic generator implemented in 0.13-μm SiGe HBT technology with f_t/f_max of 470/650GHz is presented, providing an approach to near field graphene-based signal generation.