Felippe A. S. Barbosa

Ultra-low-loss on-chip resonators with sub-milliwatt parametric oscillation threshold

On-chip optical resonators have the promise of revolutionizing numerous fields including metrology and sensing; however, their optical losses have always lagged behind their larger discrete resonator counterparts based on crystalline materials and flowable glass. Silicon nitride (Si3N4) ring resonators open up capabilities for optical routing, frequency comb generation, optical clocks and high precision sensing on an integrated platform. However, simultaneously achieving high quality factor and high confinement in Si3N4 (critical for nonlinear processes for example) remains a challenge. Here, we show that addressing surface roughness enables us to overcome the loss limitations and achieve high-confinement, on-chip ring resonators with a quality factor (Q) of 37 million for a ring with 2.5 {\mu}m width and 67 million for a ring with 10 {\mu}m width. We show a clear systematic path for achieving these high quality factors. Furthermore, we extract the loss limited by the material absorption in our films to be 0.13 dB/m, which corresponds to an absorption limited Q of at least 170 million by comparing two resonators with different degrees of confinement. Our work provides a chip-scale platform for applications such as ultra-low power frequency comb generation, high precision sensing, laser stabilization and sideband resolved optomechanics.

Photonic Needles for Light Delivery in Deep Tissue-like Media

We demonstrate a new platform for minimally invasive, light delivery probes leveraging the maturing field of silicon photonics, enabling massively parallel fabrication of photonic structures. These Photonic Needles probes have sub-10 μm cross-sectional dimensions, lengths greater than 3 mm–surpassing 1000 to 1 aspect ratio, and are released completely into air without a substrate below. We show the Photonic Needles to be mechanically robust when inserted into 2% agarose. The propagation loss of these waveguides is low–on the order of 4 dB/cm.

Microphotonic needle for minimally invasive endoscopic imaging with sub-cellular resolution

Ultra-compact micro-optical elements for endoscopic instruments and miniaturized microscopes allow for non-invasive and non-destructive examination of microstructures and tissues. With sub-cellular level resolution such instruments could provide immediate diagnosis that is virtually consistent with a histologic diagnosis enabling for example to differentiate the boundaries between malignant and benign tissue. Such instruments are now being developed at a rapid rate; however, current manufacturing technologies limit the instruments to very large sizes, well beyond the sub-mm sizes required in order to ensure minimal tissue damage. We show here a platform based on planar microfabrication and soft lithography that overcomes the limitation of current optical elements enabling single cell resolution. We show the ability to resolve lithographic features that are as small as 2 μm using probes with a cross section that is only 100 microns in size. We also show the ability to image individual activated neural cells in brain slices via our fabricated probe.

High resolution microphotonic needle for endoscopic imaging (Conference Presentation)

GRIN (Graded index) lens have revolutionized micro endoscopy enabling deep tissue imaging with high resolution. The challenges of traditional GRIN lenses are their large size (when compared with the field of view) and their limited resolution. This is because of the relatively weak NA in standard graded index lenses. Here we introduce a novel micro-needle platform for endoscopy with much higher resolution than traditional GRIN lenses and a FOV that corresponds to the whole cross section of the needle. The platform is based on polymeric (SU-8) waveguide integrated with a microlens micro fabricated on a silicon substrate using a unique molding process. Due to the high index of refraction of the material the NA of the needle is much higher than traditional GRIN lenses. We tested the probe in a fluorescent dye solution (19.6 µM Alexa Flour 647 solution) and measured a numerical aperture of 0.25, focal length of about 175 µm and minimal spot size of about 1.6 µm. We show that the platform can image a sample with the field of view corresponding to the cross sectional area of the waveguide (80x100 µm2). The waveguide size can in principle be modified to vary size of the imaging field of view. This demonstration, combined with our previous work demonstrating our ability to implant the high NA needle in a live animal, shows that the proposed system can be used for deep tissue imaging with very high resolution and high field of view.

On-chip thermo-optic tuning of suspended microresonators

Suspended optical microresonators are promising devices for on-chip photonic applications such as radio-frequency oscillators, optical frequency combs, and sensors. Scaling up these devices demands the capability to tune the optical resonances in an integrated manner. Here, we design and experimentally demonstrate integrated on-chip thermo-optic tuning of suspended microresonators by utilizing suspended wire bridges and microheaters. We demonstrate the ability to tune the resonance of a suspended microresonator in silicon nitride platform by 9.7 GHz using 5.3 mW of heater power. The loaded optical quality factor (QL ~92,000) stays constant throughout the detuning. We demonstrate the efficacy of our approach by completely turning on and off the optical coupling between two evanescently coupled suspended microresonators.

High quality factor Si 3 N 4 ring resonators achieved by surface roughness reduction

We demonstrate high-confinement Si3N4 ring resonators with a quality factor of 15.6 million. We show that ultra-high quality factors are achievable by using a process that addresses surface roughness on all interfaces of the waveguides.

Quantum Noise Revisited: Complete Measurement of Spectral Field Modes

We show that quantum noise in the spectral domain usually corresponds to a mixed quantum measurement, and cannot attain complete information about the quantum state of spectral modes [PRL 111, 200402 (2013).]