Waveguide coupled cavity-enhanced light emission from individual carbon nanotubes

APL Photonics 6, 031302 (2021).

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.

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.

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.

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.