Deniz K. Armani

Ultra-high-Q toroid microcavities on a chip

The circulation of light within dielectric volumes enables storage of optical power near specific resonant frequencies and is important in a wide range of fields including cavity quantum electrodynamics, photonics, biosensing and nonlinear optics. Optical trajectories occur near the interface of the volume with its surroundings, making their performance strongly dependent upon interface quality. With a nearly atomic-scale surface finish, surface-tension-induced microcavities such as liquid droplets or spheres are superior to all other dielectric microresonant structures when comparing photon lifetime or, equivalently, cavity Q factor. Despite these advantageous properties, the physical characteristics of such systems are not easily controlled during fabrication. It is known that wafer-based processing of resonators can achieve parallel processing and control, as well as integration with other functions. However, such resonators-on-a-chip suffer from Q factors that are many orders of magnitude lower than for surface-tension-induced microcavities, making them unsuitable for ultra-high-Q experiments. Here we demonstrate a process for producing silica toroid-shaped microresonators-on-a-chip with Q factors in excess of 100 million using a combination of lithography, dry etching and a selective reflow process. Such a high Q value was previously attainable only by droplets or microspheres and represents an improvement of nearly four orders of magnitude over previous chip-based resonators.

Re-configurable fluid circuits by PDMS elastomer micromachining

We report on a microfabrication technique for realizing re-configurable micro fluidics devices using polymethylsiloxane material (PDMS). The mechanical characteristics of the material, including the Youngs modulus and the adhesion energy have been determined experimentally. The magnitude of Youngs modulus ranges from 8.7/spl times/10/sup 5/ Pa to 3.6/spl times/10/sup 5/ Pa. The adhesion energy is a function of the PDMS composition as well as chemical treatment. A method for efficiently developing flow interconnects has been demonstrated.

Microfabrication technology for polycaprolactone, a biodegradable polymer

In this paper, we report the development of micromachining techniques for a biodegradable polymer for the first time. By virtue of their ability to naturally degrade in tissues, biodegradable polymers hold immense promise as new materials for implantable biomedical microdevices. This work focuses on the establishment of microfabrication processes for biodegradable microstructures and microdevices. Three unique fabrication processes have been established: (1) a micro-molding process to form 3D microstructures in polycaprolactone (PCL) via a silicon micromachined mold, (2) a method of transferring metal patterns to surfaces of PCL substrates, (3) techniques for sealing both dry and liquid-filled PCL micro-cavities with a metal thin film (e.g. gold). The chemical compatibility of PCL with common micromachining chemicals have been investigated.

Ultralow-threshold microcavity Raman laser on a microelectronic chip

Using ultrahigh-Q toroid microcavities on a chip, we demonstrate a monolithic microcavity Raman laser. Cavity photon lifetimes in excess of 100 ns combined with mode volumes typically of less than 1000 (microm)3 significantly reduce the threshold for stimulated Raman scattering. In conjunction with the high ideality of a tapered optical fiber coupling junction, stimulated Raman lasing is observed at an ultralow threshold (as low as 74 microW of fiber-launched power at 1550 nm) with high efficiency (up to 45% at the critical coupling point) in good agreement with theoretical modeling. Equally important, the wafer-scale nature of these devices should permit integration with other photonic, mechanical, or electrical functionality on a chip.

Fabrication and coupling to planar high-Q silica disk microcavities

Using standard lithographic techniques, we demonstrate fabrication of silica disk microcavities, which exhibit whispering-gallery-type modes having quality factors (Q) in excess of 1 million. Efficient coupling (high extinction at critical coupling and low, nonresonant insertion loss) to and from the disk structure is achieved by the use of tapered optical fibers. The observed high Q is attributed to the wedged-shaped edge of the disk microcavity, which is believed to isolate modes from the disk perimeter and thereby reduce scattering loss. The mode spectrum is measured and the influence of planar confinement on the mode structure is investigated. We analyze the use of these resonators for very low loss devices, such as add/drop filters.

Fiber-coupled erbium microlasers on a chip

An erbium-doped, toroid-shaped microlaser fabricated on a silicon chip is described and characterized. Erbium-doped sol-gel films are applied to the surface of a silica toroidal microresonator to create the microcavity lasers. Highly confined whispering gallery modes make possible single-mode and ultralow threshold microlasers.

Ultra-high-Q microcavity operation in H2O and D2O

Optical microcavities provide a possible method for boosting the detection sensitivity of biomolecules. Silica-based microcavities are important because they are readily functionalized, which enables unlabeled detection. While silica resonators have been characterized in air, nearly all molecular detections are performed in solution. Therefore, it is important to determine their performance limits in an aqueous environment. In this letter, planar microtoroid resonators are used to measure the relationship between quality factor and toroid diameter at wavelengths ranging from visible to near-IR in both H2O and D2O, and results are then compared to predictions of a numerical model. Quality factors (Q) in excess of 10^8, a factor of 100 higher than previous measurements in an aqueous environment, are observed in both H2O and D2O.

Electrical thermo-optic tuning of ultrahigh-Q microtoroid resonators

The ability to tune resonant frequency in optical microcavities is an essential feature for many applications. Integration of electrical-based tuning as part of the fabrication process has been a key advantage of planar microresonant devices. Until recently, the combination of these features has not been available in devices that operate in the ultrahigh-Q regime where device quality factors (Q) can exceed 100 million. In this letter, we demonstrate an electrically tunable resonator on a chip with ultrahigh-quality factors. Futhermore, the devices have demonstrated tuning rates in excess of 85 GHz/V2 and are capable of tuning more than 300 GHz.

Replica-molded high-Q polymer microresonators.

Ultrahigh-Q microtoroids on a chip are applied as replication masters to demonstrate replica-molded high-Q microresonator arrays. Replica Q factors are nearly material loss limited, affirming the integrity of the replication process, and are as high as 5 x 10(6), or nearly a factor of 40 greater than previous polymer-based devices. Because the molding process is nondestructive, both the master and the molds can be reused. Additionally, by using a novel optical polymer (Vicast), we demonstrate storage of high-Q microresonators in the mold for weeks, providing a method to preserve the whispering-gallery Q factor.

Micro-Molded High Q Polymer Resonators for Optical Loss Determination

Replica molding of ultra-high-Q toroidal microresonators can produce polymer microresonators with material-limited quality factors (Q). This was demonstrated previously using polymers which cure thermally. In this work, high Q polymer microresonators were fabricated using the replica molding technique from previously uncharacterized polymers which require either a thermal or UV cure. The quality factor and effective refractive index of whispering gallery modes was measured at wavelengths ranging from the visible (680nm) through the near-IR (1550nm). The optical absorption coefficient (material absorption) of these previously uncharacterized polymers was determined from the quality factor and effective refractive index of the polymer.