Yongzhuo Li

Thermo-optic switch based on transmission-dip shifting in a double-slot photonic crystal waveguide

Optical switch based on an ultra-compact double-slot photonic crystal waveguide (DS-PCWG) with a titanium/aluminum microheater is demonstrated. The operating principle relies on shifting a transmission-dip caused by the defect mode coupling in photonic band gap (PBG). Based on the unique mode coupling in PBG, low switching power of 9.2 mW and high extinction ratio of 17 dB are achieved experimentally while the length of DS-PCWG is only 16 μm.

Room-temperature continuous-wave lasing from monolayer molybdenum ditelluride integrated with a silicon nanobeam cavity

Monolayer transition-metal dichalcogenides (TMDs) have the potential to become efficient optical-gain materials for low-energy-consumption nanolasers with the smallest gain media because of strong excitonic emission. However, until now TMD-based lasing has been realized only at low temperatures. Here we demonstrate for the first time a room-temperature laser operation in the infrared region from a monolayer of molybdenum ditelluride on a silicon photonic-crystal cavity. The observation is enabled by the unique combination of a TMD monolayer with an emission wavelength transparent to silicon, and a high-Q cavity of the silicon nanobeam. The laser is pumped by a continuous-wave excitation, with a threshold density of 6.6 W cm-2. Its linewidth is as narrow as 0.202 nm with a corresponding Q of 5,603, the largest value reported for a TMD laser. This demonstration establishes TMDs as practical materials for integrated TMD-silicon nanolasers suitable for silicon-based nanophotonic applications in silicon-transparent wavelengths.

Temperature dependence of ministop band in double-slots photonic crystal waveguides

We proposed and fabricated a double-slots photonic crystal waveguides (PCWGs) structure formed by introducing two slots into PCWGs with air-bridge structure on silicon-on-insulator substrate. The mode characteristics of double-slots PCWGs were investigated theoretically and experimentally. The transmission spectra present a sharp and deep dip (22 dB with bandwidth of 6 nm) caused by ministop band in the proposed structure, which is 15 dB deeper than that in the W3 PCWG. Additionally, dependence of the dip on temperature in the double-slots PCWG was measured and a temperature coefficient 0.159 nm/°C can be concluded.

Strong Optomechanical Coupling in Nanobeam Cavities based on Hetero Optomechanical Crystals

A hetero optomechanical crystal nanobeam cavity with high mechanical frequency of 5.88 GHz is proposed. By enhancing the overlap between optical and strain field, an optomechanical coupling rate as high as 1.31 MHz is achieved.

Optomechanical crystal nanobeam cavity with high optomechanical coupling rate

An optomechanical crystal nanobeam cavity, only by increasing the radius of air holes in the defect region, is proposed and optimized for a high optomechanical coupling rate. In our proposed cavity, the photonic and phononic defect modes are simultaneously confined by each corresponding bandgap, and the overlap of the optical and mechanical modes can be improved simply by adjusting the radius of the air holes. Accordingly, an optomechanical coupling rate (g) as high as 1.16 MHz is obtained, which is the highest coupling rate among the reported optomechanical crystal cavities. What’s more, the proposed cavity also exhibits a high mechanical frequency of 4.01 GHz and a small effective mass of 85 fg for the fundamental mechanical mode.

Ultralow Propagation Loss Slot-Waveguide in High Absorption Active Material

A slot waveguide formed by high absorption active material is proposed to reduce the propagation loss for monolithic integration. The low propagation loss is attained by concentrating the optical field inside the low-index slot region without absorption. The simulation results show that the propagation loss at 1.55 μm for the slot waveguide can be as low as 1.5 dB/cm in active material with an absorption coefficient of 3000 cm-1, whereas the optical profile is set to be confined within 650 nm.

Photonic Crystal Nanobeam Cavity With Stagger Holes for Ultrafast Directly Modulated Nano-Light-Emitting Diodes

A photonic crystal nanobeam cavity with stagger holes in InP/InGaAsP/InP heterostructure is proposed for ultrafast directly modulated nano-light-emitting diodes (nanoLEDs). With stagger holes, the quality factor <i>Q</i> can be engineered in the range of 10² ~ 10⁴ while keeping a small mode volume (<i>V</i>_eff). As a result, the modulation speed of nanoLEDs can be dramatically improved by a small <i>V</i>_eff to enhanced spontaneous emission (SpE) rate and a moderate <i>Q</i> to counterbalance SpE lifetime and photon lifetime of the cavity. In our simulation, the direct modulation bandwidth could be higher than 60 GHz with optimal <i>Q</i> value of 2150 and <i>V</i>_eff of 2.3( λ₀/2<i>n</i>)³.

Fabrication of high-aspect-ratio double-slot photonic crystal waveguide in InP heterostructure by inductively coupled plasma etching using ultra-low pressure

Double-slot photonic crystal waveguide (PCW) in InP heterostructure is fabricated by inductively coupled plasma (ICP) etching. Due to using an ultra-low pressure of 0.05 Pa, etch depths up to 3.5 μm for holes with diameter of 200 nm and 1.8 μm for slots of ∼40 nm are achieved, which indicate a record-high aspect-ratio, i.e. 45, for such narrow slots in InP heterostructure. Moreover, etching quality is evaluated based on both the transmission performance and the linewidth of micro-photoluminescence (μ-PL). In our measurement, a structure-dependent transmission-dip about 17 dB is obtained from a 17-μm-long W3 PCW, and a PL widening as small as 19 nm compared to the corresponding wafer is observed. These promising experimental results evidence the high etching quality realized in this work and confirm the feasibility of etching small-feature-size patterns by ICP technology for InP based devices in future mono-/hetero-integrated photonic circuits.

Demonstration of hetero optomechanical crystal nanobeam cavities with high mechanical frequency

Optomechanical crystal is a combination of both photonic and phononic crystal. It simultaneously confines light and mechanical motion and results in strong photon-phonon interaction, which provides a new approach to deplete phonons and realize on-chip quantum ground state. It is promising for both fundamental science and technological applications, such as mesoscopic quantum mechanics, sensing, transducing, and so on. Here high optomechanical coupling rate and efficiency are crucial, which dependents on the optical-mechanical mode-overlap and the mechanical frequency (phonon frequency), respectively. However, in the conventional optomechanical-crystal based on the same periodical structure, it is very difficult to obtain large optical-mechanical mode-overlap and high phonon frequency simultaneously. We proposed and demonstrated nanobeam cavities based on hetero optomechanical crystals with two types of periodic structure. The optical and mechanical modes can be separately confined by two types of periodic structures. Due to the design flexibility in the hetero structure, the optical field and the strain field can be designed to be concentrated inside the optomechanical cavities and resemble each other with an enhanced overlap, as well as high phonon frequency. A high optomechanical coupling rate of 1.3 MHz and a high phonon frequency of 5.9 GHz are predicted theoretically. The proposed cavities are fabricated as cantilevers on silicon-on-insulator chips. The measurement results indicate that a mechanical frequency as high as 5.66 GHz is obtained in ambient environment, which is the highest frequency demonstrated in one-dimensional optomechanical crystal structure.

Small-feature-size Etching of InP/InGaAsP by inductively coupled plasma at ultra-low pressure

Deep etching for InP/InGaAsP based slotted photonic crystal by inductively coupled plasma at ultra-low pressure was studied. High-aspect-ratio of 28 for 60-nm-wide slots and 17 for air-holes with diameter of 200 nm was achieved, respectively.