arXiv subjects

Safumi Suzuki

Publications and source records attributed to Safumi Suzuki.

6 recordsLinked to original sources

arXiv2025

Microcomb-referenced photonic stabilization of resonant tunneling diode terahertz oscillators

We demonstrate a compact stabilization scheme for terahertz (THz) sources by exploiting the complementary advantages of microresonator-based optical frequency combs (microcombs) and resonant tunneling diodes (RTDs). A microcomb-driven photomixing THz signal is employed as the master for injection locking of an RTD, enabling faithful transfer of the microcomb stability into the RTD. Using this approach, the free-running RTD linewidth of 50 MHz was narrowed to 165 Hz, while the single-sideband phase noise reached -80 dBc/Hz at a 10 kHz offset with a locking range of 80 MHz. Compared with conventional electronic frequency multiplier or fiber-comb-based schemes, this method avoids high-order frequency multiplication and associated noise penalties, offering a compact and practical alternative. The dual functionality of linewidth narrowing and power scalability highlights the potential of microcomb-assisted injection locking as a route toward chip-scale, spectrally pure THz sources for beyond-5G/6G wireless communication and radar, with prospects for future extension to time-frequency metrology and precision sensing.

physics.optics↗

arXiv2025

Theoretical analysis of frequency variation tolerance between elements of RTD-THz oscillator arrays for mutual locking

Array configuration is one of the effective ways to increase the output power of terahertz oscillators using resonant tunneling diodes. Mutual locking between the array elements results in coherent single-spectrum oscillation and a narrow radiation beam. However, frequency variation between elements disturbs the mutual locking. In this paper, the frequency variation that can be tolerated for mutual locking is approximately derived for an array with arbitrary coupling configuration. Using this result, the element-number dependence of the frequency variation tolerance for 1D and 2D arrays is calculated. In 1D arrays, the frequency variation tolerance decreases inversely proportional to the square root of the element number. In 2D arrays, as the number of columns is increased while keeping the number of elements per column constant, the frequency variation tolerance increases for small number of columns, reaches a maximum at square configuration, and then decreases with increasing number of columns. The element-number dependence in 2D arrays is smaller than that in 1D arrays.

physics.app-ph↗

arXiv2024

High-power even- and odd mode emission from linear arrays of resonant-tunneling-diode (RTD) oscillators in the 0.4- to 0.8-THz frequency range

Resonant tunneling diode (RTD) oscillators possess the highest oscillation frequency among all electronic THz emitters. However, the emitted power from RTDs remains limited. Here, we propose linear RTD-oscillator arrays capable of supporting coherent emission from both odd and even coupled modes. Both modes exhibit constructive interference in the far field, enabling high power emission. Experimental demonstrations of coherent emission from 11-RTD-oscillator linear arrays are presented. The odd mode oscillates at approximately 450 GHz, emitting about 0.5 mW, while the even mode oscillates at around 750 GHz, emitting about 1 mW. Moreover, certain RTD-oscillator arrays demonstrate dual-band oscillation under different biases, allowing for controllable switching between two coupled modes. In addition, during bias sweeping in both directions, a notable hysteresis feature is observed in the switching bias for the odd and even modes. Our linear RTD-oscillator array represents a significant step forward in the realization of high-power large RTD-oscillator arrays and enables large-scale applications of RTD devices.

physics.optics↗

arXiv2023

Theoretical Analysis of Terahertz Detection of Resonant Tunneling Diodes

We analyze the terahertz detection characteristics of resonant tunneling diodes (RTDs) using a set of simple equations that covers three detection modes; (i) direct detection, (ii) amplified detection, and (iii) self-homodyne (coherent) detection. (i) and (ii) are based on the square-law detection, and (iii) is on the homodyne detection with the RTD used as an injection-locked local oscillator. The calculated results exhibit small- and large-signal areas depending on irradiation power. In the small-signal area, the detection current is proportional to irradiated power for (i) and (ii), and to square root of irradiated power for (iii). The detection current has a peak at the bias voltage at the boundary between (ii) and (iii). Effect of frequency fluctuation of irradiated wave is analyzed for (iii), and it is shown that the detection current is proportional to irradiated power if the fluctuation becomes wider than injection-locking range. The analytical results in this paper reasonably explain the reported experiments.

physics.app-ph↗

arXiv2023

Simple model for frequency response of a resonant tunneling diode caused by potential change of quantum well due to electron charge

The frequency dependence of negative differential conductance (NDC) is an important property for the resonant-tunneling-diode terahertz source. Among several phenomena determining the frequency dependence, this paper shows that the effect of potential change of the quantum well due to electron charge can be analyzed with a simple and tractable model based on the tunneling admittance and capacitance. The result is identical to that of Feiginov's analysis based on more fundamental equations, showing a one-to-one correspondence between the parameters of the two analyses. Similar to Feiginov's analysis, our analysis also shows that NDC remains finite even at infinitely high frequency. It is shown in our model that this result is attributed to neglecting the tunneling time at the emitter barrier. Comprehensive analysis of the frequency dependence of NDC will be possible by incorporating the tunneling time into the present model.

cond-mat.mes-hall↗

arXiv2022

Intrinsic Frequency Limit of Direct Modulation of Resonant-Tunneling-Diode Terahertz Emitters and Effect of External Feedback Injection

Output power of resonant-tunneling-diode (RTD) terahertz (THz) emitters can be modulated by the bias modulation similar to a semiconductor laser. This property is useful for applications of the THz waves to wireless communications and radars. In this paper, we theoretically analyze the modulation-frequency dependence of the output response using an equivalent circuit of the RTD-THz oscillator. It is shown that there exists an intrinsic cutoff frequency of modulation in RTD, which is independent of the external circuit that supplies the modulation signal to the oscillator. This cutoff frequency is determined by the time constant given by the capacitance of RTD divided by the absolute value of negative differential conductance minus loss conductance of the oscillator, and is about 100 GHz for typical RTD-THz oscillators. We also analyze the effect of external feedback injection on the modulation characteristics. If the period of the modulation frequency is equal to an integral multiple of the round-trip time of the feedback, a dip or peak occurs in the modulation response of the output, depending on whether the THz carrier components in the output and the feedback are in phase or out of phase. We also discuss the possibility of increase in cutoff frequency by the feedback with short round-trip time.

physics.app-ph↗