Atsushi Ishizawa

On‐chip erbium‐doped optical amplifiers and lasers are essential for realizing fully integrated active silicon photonic circuits, but their performance has been limited by the low gain in predominantly used amorphous host materials and by the difficulty of direct integration on silicon. Here, we demonstrate optical amplification and lasing from erbium‐doped single‐crystal gadolinium oxide (Er:) thin films epitaxially grown on silicon. Optical gain measurements on waveguides fabricated on this platform exhibit a large material gain of dB/cm and an on‐chip net gain exceeding 13 dB in a 6‐mm‐long waveguide at 2.3 K, while a measurable gain is maintained up to room temperature. Continuous‐wave lasing with low threshold, narrow linewidth, and large side‐mode suppression ratio is also demonstrated in Er: microring resonators. These results establish Er: as the first monolithic crystalline gain medium directly integrated on silicon, providing a scalable route toward high‐performance silicon and quantum photonic integrated circuits.

We propose a method for measuring laser frequency using only one optical frequency comb (OFC), eliminating the need for high-precision wavelength meters. By leveraging a Vernier effect within the OFC, we geometrically increase the comb-mode-number difference and sequentially stabilize the OFC. This method enables an accurate and reliable determination of the comb-mode number. Because the method is robust against relative fluctuations in mode spacing up to 3.6 × 10⁻¹⁰, it significantly relaxes fluctuation requirements compared with traditional methods, thus broadening practical applications in metrology standards and precision spectroscopy.

Dissipative Kerr soliton microcombs have inspired various intriguing applications such as spectroscopy, ranging, telecommunication, and high purity microwave generation. Mechanically actuated soliton microcombs provide enhanced controllability and flexibility for Kerr solitons, thus enabling technological progress to be made on such practical applications. Here, we present architectures for coherent dual-comb techniques and ultralow-noise microwave generation by exploiting the mechanical actuation of ultrahigh-Q crystalline microresonators. By unifying a pump laser, we demonstrate highly coherent dual-soliton combs using distinct resonators with slightly different repetition rates. We also report significant phase-noise reduction achieved by directly generating Kerr solitons from a sub-Hz linewidth ultrastable laser. This study paves the way for further advancements in a wide variety of applications based on Kerr soliton microcombs.

We demonstrated a highly efficient optical memory operation using an atomic frequency comb (AFC) created from a spectrally flattened optical frequency comb with an isotopically pure ¹⁶⁷Er³⁺:Y₂SiO₅ crystal under zero magnetic field. Compared with standard AFC creation methods such as pulse train method, our method, called comb transfer method, can create a broadband and high-efficiency AFC at lower power and with less effort. This technique can make a significant improvement in the quality of AFCs in crystals doped with low concentrations of rare-earth ions.

Acoustically induced dressed states of long-lived erbium ions in a crystal are demonstrated. These states are formed by rapid modulation of two-level systems via strain induced by surface acoustic waves whose frequencies exceed the optical linewidth of the ion ensemble. Multiple sidebands and the reduction of their intensities appearing near the surface are evidence of a strong interaction between the acoustic waves and the ions. This development allows for on-chip control of long-lived ions and paves the way to highly coherent hybrid quantum systems with telecom photons, acoustic phonons, and electrons.

Acousto-optic measurement of sound (AOMoS) has been widely utilized in various acoustic studies owing to its noninvasive nature. However, the presence of significant measurement noise often degrades the acousto-optic signals and thus hinders promising applications. This is partly because the characteristics of the noises in AOMoS have not been fully clarified, which prevents us from adequately predicting and controlling them. To address this challenge, we theoretically and experimentally investigate three specific measurement noises in AOMoS: optical intensity noise, optical phase noise, and environmental noise. Convenient notations for evaluating optical noises in acoustic units are provided, which make it easier to understand the AOMoS noises from an acoustic point of view. Our findings showed that among the six noise sources we investigated, optical shot noise defines the white noise floor at high frequencies while background sound noise determines the low-frequency noise spectrum. The analysis provided in this paper will serve as an exhaustive guide for designing and implementing an AOMoS system with a desired noise level.

A simpler and more accurate measurement of absolute optical frequencies (AOFs) is very important for optical communications and navigation systems. To date, an optical reference has been needed for measuring AOFs with twelve-digit accuracy because of the difficulty in measuring them directly. Here, we focus on an electro-optics-modulation comb that can bridge the vast frequency gap between photonics and electronics. We demonstrate an unprecedented method that can directly measure AOFs to an accuracy of twelve digits with an RF frequency counter by simply delivering a frequency-unknown laser into an optical phase modulator. This could open up a new horizon for optical-referenceless optical frequency metrology. Our method can also simultaneously achieve a 100-fold phase-noise reduction in a conventional signal generator. This corresponds to an increase in the transmission speed of wireless communications of by about seven times.

We propose a method of supercontinuum light generation enhanced by multimode excitation in a precisely dispersion-engineered deuterated SiN (SiN:D) waveguide. Although a regularly designed SiN-based nonlinear optical waveguide exhibits anomalous dispersion with the fundamental and first-order multimode operation, the center-symmetric light pumping at the input edge has so far inhibited the full potential of the nonlinearity of SiN-based materials. On the basis of numerical analysis and simulation for the SiN:D waveguide, we intentionally applied spatial position offsets to excite the fundamental and higher-order modes to realize bandwidth broadening with flatness. Using this method, we achieved an SNR improvement of up to 18 dB at a wavelength of 0.6 µm with an offset of about 1 µm in the Y-axis direction and found that the contribution was related to the presence of dispersive waves due to the excitation of TE₁₀, and TE₀₁ modes.

We created high-resolution atomic frequency combs (AFCs) and optimized the pulse sequence used in AFC memory to realize highly efficient AFC quantum memory using the ¹⁶⁷Er³⁺:Y₂SiO₅ hyperfine level system. As a result, the formation and relaxation times of AFCs were revealed, and AFCs with a resolution of 800 kHz were created by using an appropriate pump light intensity. An efficiency of 0.16% with a memory time of 0.5 µs was achieved even under a zero magnetic field and a 10 ppm Er concentration sample. This result paves the way for realizing highly efficient communication-wavelength AFC quantum memory.

We have achieved the simultaneous generation of a 2.6-octave-wide supercontinuum (SC) spectrum over 400–2500 nm and third-harmonic light solely by a dispersion-controlled silicon-nitride waveguide (SiNW). To increase the visible intensity of the SC light component, we fabricated low-loss 5-mm-long deuterated SiNWs with spot-size converters by low-temperature deposition. We succeeded in measuring the carrier-envelope-offset (CEO) signal with a 34-dB signal-to-noise ratio because this short deuterated SiNW provides a large temporal overlap between the f and 3f components. In addition, we have demonstrated this method of CEO locking at telecommunications wavelengths with f-3f self-referencing generated solely by the SiNW without the use of highly nonlinear fiber and an additional nonlinear crystal. Compared with the method of CEO locking with a highly nonlinear fiber and a standard f-2f self-referencing interferometer, this method is not only simple and compact but also stable.