Research

Integrated hybrid silicon photonic devices with nanomaterials

Deterministic Integration of hBN Single-Photon Emitters on SiN Waveguides via Femtosecond Laser Processing

arXiv:2504.19477 (2025).
https://doi.org/10.48550/arXiv.2504.19477

Under writing...

Hybrid silicon all-optical switching devices integrated with two-dimensional material

Adv. Opt. Mater. 13, 2402531 (2024).
https://doi.org/10.1002/adom.202402531

As the demand for faster and more energy-efficient optical communications increases, all-optical switching, which handles the routing and modulation of optical signals, plays a crucial role in photonic integrated circuits (PICs). Conventional all-optical switching devices based on silicon substrates allow for seamless integration with other electronic and optical devices. However, the carrier lifetime of silicon imposes limitations on response speed, constraining both speed and efficiency. In contrast, two-dimensional (2D) materials such as transition metal dichalcogenides exhibit unique optoelectronic properties, including rapid carrier recombination and strong light-matter interactions, making them promising candidates for next-generation photonic devices. This study aims to overcome the performance limitations of all-optical switching by leveraging a hybrid structure that combines silicon with 2D materials, thereby harnessing the advantages of silicon as a platform for PICs.

We developed a hybrid all-optical switching device that integrates a 2D semiconductor, molybdenum ditelluride (MoTe₂), with a silicon nanocavity. This device achieves switching by leveraging the refractive index change in MoTe₂ induced by optical excitation. To enable efficient light-matter interaction, we designed a photonic crystal nanobeam cavity and transferred MoTe₂ flakes onto it. By irradiating MoTe₂ with optical pulses, the resonant wavelength of the cavity shifts, enabling the switching operation.

The cavity integrated with MoTe₂ retained a high Q-factor necessary for low-energy operation. Additionally, it was observed that defects introduced by the degradation of MoTe₂ promoted non-radiative recombination, further enhancing the switching speed. By tuning the wavelengths of the signal and pump, an optimal modulation contrast was achieved, with the optimal pump wavelength determined to be approximately 1000 nm. Experimental results demonstrated high-speed (tens of picoseconds) and low-energy (hundreds of femtojoules) switching (Fig. 3(b)). Compared to devices based solely on silicon, this hybrid switch exhibited significantly improved switching speeds and reduced energy consumption.

This hybrid all-optical switching device overcomes the performance limitations of silicon, achieving energy-efficient and high-speed switching. It holds great promise for future applications in integrated photonic systems.

Quantization of mode shifts in nanocavities integrated with atomically thin sheets

Adv. Opt. Mater. 10, 2200538 (2022).
https://doi.org/10.1002/adom.202200538

Two-dimensional (2D) layered materials such as graphene, transition metal dichalcogenides (TMDs), and hexagonal boron nitride have attracted considerable attention due to their exotic physical properties and potential for diverse applications. With atomically precise thickness over macroscopic areas and extensive compatibility offered by van der Waals interface, photonic devices controlled through single atomic layers would present a new direction in nanotechnology. It is however challenging to control the microcavities by TMDs, and the cavity modes are barely affected.

Here we specially design the nanobeam cavity to be the air mode to enhance the interaction with 2D materials. We fabricate the nanobeam cavity from a silicon-on-insulator substrate using electron beam lithography and inductively-coupled plasma etching. Tungsten diselenide (WSe2) flakes are then transferred on the cavities using a polymer stamp method.

The interaction with the 2D material is evaluated with a home-built confocal photoluminescence microscopy system at room temperature by comparing the fundamental mode before and after the transfer of WSe2. A single sharp peak is observed in the spectrum and is identified as the fundamental mode. The air-mode cavity shows a redshift of 26.0 nm after the transfer, which is attributed to a change in the average dielectric constant. The large shift indicates the enhanced responsivity for the air mode cavities, consistent with the simulations showing strong fields within the WSe2 flake.

By controlling the thickness of WSe2 flakes with atomic precision, we demonstrate extreme sensitivity down to the monolayer limit. The wavelength shift decreases by 3 nm as the thickness of WSe2 is reduced layer by layer. We observe the clear resolved steps, indicating that the mode shifts are indeed quantized.

Waveguide coupled cavity-enhanced light emission from individual carbon nanotubes

APL Photonics 6, 031302 (2021).
https://doi.org/10.1063/5.0042635

Silicon photonics has enabled on-chip integration of various optical components, expanding the capabilities of monolithic photonic circuits. Single-walled carbon nanotubes (CNTs) are promising candidates as nanoscale light emitters in silicon photonics because they exhibit photoluminescence in telecom-wavelength regime at room temperature and can be grown directly on silicon substrates. By utilizing silicon optical microcavities, it has been possible to enhance the emission from CNTs and to narrow the emission spectrum, which is important in optical communications. The next step toward on-chip devices is to couple the light enhanced by the cavity to an optical waveguide, connecting the light emitter and other optical components for mutual access. In addition, in order to harness the unique optical properties of CNTs such as single photon emission, it is important to isolate individual CNTs and keep cleanliness of CNTs and devices.

In this work, we demonstrate an individual single-walled CNT light emitter integrated onto a microcavity and a waveguide operating in the telecom wavelength regime. Using finite-difference time-domain simulations, we have modified an air-mode photonic crystal nanobeam cavity to have one thin end mirror for guiding the light into the waveguide. CNTs are grown on a SiO2/Si substrate and transferred on the cavities through an all-dry process ensuring cleanliness of CNTs and devices. The light emission from the identified CNT is enhanced at the cavity resonance and extracted from the waveguide facet. The waveguide-coupled light can easily be connected to various optical components on a monolithic chip and optical fibers.

(a) Schematic of the device. (b) Simulated spatial distribution of the y-component of the electric field.

Nanocavity Si Raman lasers

Our group has developed a Raman Si laser based on a high- quality- (high-Q)-factor photonic-crystal (PC) nanocavity with a resonator size of 10 µm that enables an ultralow threshold of ~1 µW. Such a small, low-threshold device is suited for dense integration on Si photonic circuits, which can be employed for applications such as cw laser sources and all-optical switching devices.

I have been studying the lasing characteristics of the nanocavity Raman Si lasers with experimental and computer skills, which enables suited device design of this laser for the future applications. Details are below.

Precise measurement of a Raman shift of Si

Optics Express, Vol. 23, pp 3951 (2015).

Our nanocavity Raman Si laser utilizes two high-Q nanocavity modes to confine the pump light and Stokes Raman scattered light, which will hereafter be referred to as the pump mode and the Stokes mode, respectively. In this work, a precise measurement of the Raman shift of PC Si heterostructure nanocavities for Raman laser applications is demonstrated.

One of the key requirements for higher performance is that the frequency spacing between these two modes matches the Raman shift of Si well, with an error less than 1.0×10−2THz taking into account for the full width at half-maximum of the Raman gain of Si which is ∼0.1THz. However, it is well known that the reported values for the Raman shift of Si vary within a certain error range due to local sample heating caused by absorption of excitation laser light, long measurement times, and temperature fluctuations of the surrounding air. Previously reported values for the Raman shift of Si lie in the range of 15.59±0.03THz (520±1.0 cm−1). Therefore, in this work we utilize near-infrared excitation by a laser with a wavelength of 1.42 um, which allows us to avoid local sample heating. Additionally, we exploit the two high-Q nanocavity modes to calibrate the Raman frequency. The obtained precise value for the Raman shift of Si in the PC nanocavity is 15.606THz (520.71 cm−1) with a small uncertainty of 1.0×10−3THz.

Band structure of a hetero-nanocavity.

Lasing dynamics of nanocavity Raman Si lasers

Phys. Rev. Applied 10, 024039 (2018).

In this work, the lasing dynamics of a nanocavity Raman Si laser are investigated.

It is commonly accepted that the output of a Raman Si laser tends to saturate for higher excitation powers because of free-carrier absorption (FCA). The measurements in this work reveal that the free carriers, which are generated by two-photon absorption (TPA), induce dynamic effects during the initial lasing process. These effects can be confirmed even at the very low threshold power of 0{.}12 uW. At higher excitation powers, the Raman laser signal exhibits a significant reduction within a few hundreds of nanoseconds after the initial rise, followed by clear oscillations. The presented data show that the temporal behavior of the laser signal strongly depends on the excitation wavelength. The numerical simulations presented in this thesis indicate that the oscillations reflect the dynamical shift of the resonant wavelength of the nanocavity. The oscillation of the shift originates from the competition between the thermo-optic and the carrier-plasma effects, which are induced by free carriers generated via TPA.

Schematic of TPA and FCA in Si.

Optical gain shape of nanocavity Raman Si lasers

Optica 5, 1256 (2018).

In this work, the excitation-wavelength dependence of the optical gain in a nanocavity Raman Si laser is reported.

In order to improve the performance of semiconductor lasers in terms of threshold, output power or energy efficiency, it is important to clarify the spectral shape of the optical gain. This optical gain spectrum determines the optimum operating point at a given excitation power, and thus a convenient technique to obtain the optical gain spectrum of a nanocavity Raman Si laser is required.

This work demonstrates the so-called stimulated-Raman-scattering excitation (SRE) spectroscopy, which allows us to reveal the range of excitation wavelengths enabling laser operation, the excitation condition for maximum output, the shift of the gain peak, and the enhancement of the Raman gain including nonlinear optical losses. It is shown that the laser output remarkably decreases in the long-wavelength region of the cavity resonance as the excitation power increases, which has important implications for devices. Numerical simulations suggest that the optical loss due to FCA induced by TPA grows substantially above a certain threshold.

SRE map.

Perovskite solar cells

Charge carrier dynamics in perovskite solar cells

J. Phys. Chem. Lett. 7, 3186 (2016).

This work was carried out at Kanemitsu Lab. in Kyoto Univ. as a visiting PhD student.

Charge carrier dynamics in perovskite CH3NH3PbI3 solar cells were studied by means of microscopic photoluminescence (PL) and photocurrent (PC) imaging spectroscopy. The PL intensity, PL lifetime, and PC intensity varied spatially on the order of several tens of micrometers. Simultaneous PL and PC image measurements revealed a positive correlation between the PL intensity and PL lifetime, and a negative correlation between PL and PC intensities. These correlations were due to the competition between photocarrier injection from the CH3NH3PbI3 layer into the charge transport layer and photocarrier recombination within the CH3NH3PbI3 layer.

PL and PC imaging of a perovskite solar cell.

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