Takashi Arikawa

Abstract As a key component for next-generation wireless communications (6 G and beyond), terahertz (THz) electronic oscillators are being actively developed. Precise and dynamic phase control of ultrafast THz waveforms is essential for high-speed beam steering and high-capacity data transmission. However, measurement and control of such ultrafast dynamic process is beyond the scope of electronics due to the limited bandwidth of the electronic equipment. Here we surpass this limit by applying photonic technology. Using a femtosecond laser, we generate offset-free THz pulses to phase-lock the electronic oscillators based on resonant tunneling diode. This enables us to perform phase-resolved measurement of the emitted THz electric field waveform in time-domain with sub-cycle time resolution. Ultrafast dynamic response such as anti-phase locking behaviour is observed, which is distinct from in-phase stimulated emission observed in laser oscillators. We also show that the dynamics follows the universal synchronization theory for limit cycle oscillators. This provides a basic guideline for dynamic phase control of THz electronic oscillators, enabling many key performance indicators to be achieved in the new era of 6 G and beyond.

An insulating bulk state is a prerequisite for the protection of topological edge states(1). In quantum Hall systems, the thermal excitation of delocalized electrons is the main route to breaking bulk insulation(2). In equilibrium, the only way to achieve a clear bulk gap is to use a high-quality crystal under high magnetic field at low temperature. However, bulk conduction could also be suppressed in a system driven out of equilibrium such that localized states in the Landau levels are selectively occupied. Here we report a transient suppression of bulk conduction induced by terahertz wave excitation between the Landau levels in a GaAs quantum Hall system. Strikingly, the Hall resistivity almost reaches the quantized value at a temperature where the exact quantization is normally disrupted by thermal fluctuations. The electron localization is realized by the long-range potential fluctuations, which are a unique and inherent feature of quantum Hall systems. Our results demonstrate a new means of effecting dynamical control of topology by manipulating bulk conduction using light.

We demonstrate coherent control of cyclotron resonance in a two-dimensional electron gas (2DEG). We use a sequence of terahertz (THz) pulses to control the amplitude of coherent cyclotron resonance oscillations in an arbitrary fashion via phase-dependent coherent interactions. We observe a self-interaction effect, where the 2DEG interacts with the THz field emitted by itself within the decoherence time, resulting in a revival and collapse of quantum coherence. These observations are accurately describable using single-particle optical Bloch equations, showing no signatures of electron-electron interactions, which verifies the validity of Kohn's theorem for cyclotron resonance in the coherent regime.