Tomohiro Tetsumoto

CMOS compatible high-Q photonic crystal nanocavity fabricated with photolithography on silicon photonic platform

Progress on the fabrication of ultrahigh-Q photonic-crystal nanocavities (PhC-NCs) has revealed the prospect for new applications including silicon Raman lasers that require a strong confinement of light. Among various PhC-NCs, the highest Q has been recorded with silicon. On the other hand, microcavity is one of the basic building blocks in silicon photonics. However, the fusion between PhC-NCs and silicon photonics has yet to be exploited, since PhC-NCs are usually fabricated with electron-beam lithography and require an air-bridge structure. Here we show that a 2D-PhC-NC fabricated with deep-UV photolithography on a silica-clad silicon-on-insulator (SOI) structure will exhibit a high-Q of 2.2 × 105 with a mode-volume of ~1.7(λ/n)3. This is the highest Q demonstrated with photolithography. We also show that this device exhibits an efficient thermal diffusion and enables high-speed switching. The demonstration of the photolithographic fabrication of high-Q silica-clad PhC-NCs will open possibility for mass-manufacturing and boost the fusion between silicon photonics and CMOS devices.

Ultrasmall in-plane photonic crystal demultiplexers fabricated with photolithography

We demonstrate ultrasmall demultiplexers based on photolithographic photonic crystals. The footprint of the demultiplexers is 110 μm2 per channel. Our in-plane demultiplexers are clad with silica, which makes them stable and easy to integrate with other silicon photonic devices. We describe two types of demultiplexers with spacings of 136 and 267 GHz between channels for application to dense wavelength division multiplexing. Integrated titanium nitride heaters allow us to precisely control the channel wavelength. We report a 2.5 Gbps transmittance experiment with sufficiently small crosstalk and discuss ways of achieving even lower crosstalk between channels.

Observation of energy oscillation between strongly-coupled counter-propagating ultra-high Q whispering gallery modes

We report the first experimental observation of an energy oscillation between two coupled ultra-high Q whispering gallery modes in the time domain. Two counter-propagating whispering gallery modes in a silica toroid microcavity were employed for this purpose. The combination of a large coupling coefficient between the two modes and an ultra-high Q factor, which creates a large Γ value of > 10, results in a clear energy oscillation. Our measurement is based on a drop-port measurement technique, which enables us to observe the light energy in the two modes directly. The oscillation period measured in the time domain precisely matched that inferred from mode splitting in the frequency domain, and the measured results showed excellent agreement with results calculated with the developed numerical model.

All-optical tunable buffering with coupled ultra-high Q whispering gallery mode microcavities

All-optical tunable buffering was recently achieved on a chip by using dynamically tuned coupled mode induced transparency, which is an optical analogue of electromagnetically induced transparency. However, the small Q s of about 105 used in those systems were limiting the maximum buffering time to a few hundred ps. Although employing an ultra-high Q whispering gallery mode (WGM) microcavity can significantly improve the maximum buffering time, the dynamic tuning of the WGM has remained challenging because thermo-optic and pressure tunings, which are widely used for WGM microcavities, have a very slow response. Here we demonstrate all-optical tunable buffering utilizing coupled ultra-high Q WGM cavities and the Kerr effect. The Kerr effect can change the refractive index instantaneously, and this allowed us to tune the WGM cavity very quickly. In addition, from among the various WGM cavities we employed a silica toroid microcavity for our experiments because it has an ultra-high Q factor (>2 × 107) and a small mode volume, and can be fabricated on a chip. Use of the Kerr effect and the silica toroid microcavity enabled us to observe an on-chip all-optical tunable buffering operation and achieve a maximum buffering time of 20 ns.

Compact resonant electro-optic modulator using randomness of a photonic crystal waveguide.

We fabricate and demonstrate an electro-optic modulator that utilizes the randomness in a photonic crystal waveguide. We exploit a way of using random photonic crystals for device application that involves restricting the area influenced by the randomness. Our random photonic crystal waveguide is in a diffusive regime and the confinement of light is observed only for a W0.98 waveguide (98% of the original width) placed between W1.05 photonic crystal waveguides, where we obtained a transmittance spectrum with an ultra-high Q of 2.4 × 105. A numerical investigation revealed that the experimental yield rate of the appearance of the high-Q confined mode is larger than 80%, by properly designing the length of W0.98. Since the confinement location is predictable, we integrate a p-i-n structure and demonstrate a GHz electro-optic modulation.

High-Q coupled resonances on a PhC waveguide using a tapered nanofiber with high coupling efficiency.

We experimentally demonstrate high-Q cavity formation at an arbitrary position on a silicon photonic crystal waveguide by bringing a tapered nanofiber into contact with the surface of the slab. An ultrahigh Q of 5.1 × 10(5) is obtained with a coupling efficiency of 39%, whose resonant wavelength can be finely tuned by 27 pm by adjusting the contact length of the nanofiber. We also demonstrate an extremely high coupling efficiency of 99.6% with a loaded Q of 6.1 × 10(3). We show that we can obtain a coupled resonances, which has the potential to be used for slow light generation.

High-Q silica zipper cavity for optical radiation pressure driven MOMS switch

We design a silica zipper cavity that has high optical and mechanical Q (quality factor) values and demonstrate numerically the feasibility of a radiation pressure driven micro opto-mechanical system (MOMS) directional switch. The silica zipper cavity has an optical Q of 4.0 × 104 and an effective mode volume Vmode of 0.67λ3 when the gap between two cavities is 34 nm. The mechanical Q (Qm) is determined by thermo-elastic damping and is 2.0 × 106 in a vacuum at room temperature. The opto-mechanical coupling rate gOM is as high as 100 GHz/nm, which allows us to move the directional cavity-waveguide system and switch 1550-nm light with 770-nm light by controlling the radiation pressure.

Kerr-induced controllable adiabatic frequency conversion in an ultrahigh Q silica toroid microcavity

In this Letter, we report, based on our knowledge, the first demonstration of Kerr-induced adiabatic frequency conversion in a silica toroid microcavity. Taking advantage of the instantaneous response of the Kerr effect, we achieved adiabatic frequency conversion with a controllable amount of frequency shift and time width. In addition, thanks to the combination of the Kerr effect and the ultrahigh Q (>107) of the silica toroid microcavity, we also observed multiple frequency conversion within a photon lifetime. Furthermore, use of the Kerr effect allowed us to investigate the influence of the relative phase between the original and converted light.

Investigation of the influence of the proximity effect and randomness on a photolithographically fabricated photonic crystal nanobeam cavity

Recent progress on the fabrication techniques used in silicon photonics foundries has enabled us to fabricate photonic crystal (PhC) nanocavities using a complementary metal-oxide-semiconductor (CMOS) compatible process. A high Q two-dimensional PhC nanocavity and a one-dimensional nanobeam PhC cavity with a Q exceeding 100 thousand have been fabricated using ArF excimer laser immersion lithography. These are important steps toward the fusion of silicon photonics devices and PhC devices. Although the fabrication must be reproducible for industrial applications, the properties of PhC nanocavities are sensitively affected by the proximity effect and randomness. In this study, we quantitatively investigated the influence of the proximity effect and randomness on a silicon nanobeam PhC cavity. First, we discussed the optical properties of cavities defined with one- and two-step exposure methods, which revealed the necessity of a multi-stage exposure process for our structure. Then, we investigated the impact of block structures placed next to the cavities. The presence of the blocks modified the resonant wavelength of the cavities by about 10 nm. The highest Q we obtained was over 100 thousand. We also discussed the influence of photomask misalignment, which is also a possible cause of disorders in the photolithographic fabrication process. This study will provide useful information for fabricating integrated photonic circuits with PhC nanocavities using a photolithographic process.

Adiabatic frequency conversion in an ultra-high-Q silica microcavity using the Kerr effect

We experimentally demonstrate adiabatic frequency conversion in an ultra-high-Q silica toroid microcavity using the Kerr effect. We show that the amount of conversion, conversion time width, and number of conversions can be freely controlled.