Research Highlights

Silicon-On-Anything Photonics We have developed a novel yet general method to integrate silicon photonic devices onto arbitrary substrates without using any adhesives that may compromise silicon’s optical performance. As a proof of the concept, we used the method to fabricate, for the first time, silicon on calcium fluoride (CaF2) photonic devices for mid-infrared photonics. The new silicon-on-CaF2 mid-infrared devices, including waveguides and micro-ring resonators, have shown superior optical quality, which exceeds or is on par with devices fabricated with other more complicated methods, and have a vast potential for improvement. This result proved the new fabrication method’s capability of integrating silicon photonic devices and systems on arbitrary foreign substrates while maintaining the pristine optical performance To demonstrate the strength of the new devices, we use the device to achieve on-chip cavity-enhanced mid-infrared spectroscopic sensing of chemicals with a detection limit below 0.1 nanogram. The technology also predicts further reduced detection limit down to the unprecedented picogram level simply by aligning the sensing wavelength to the main analyte absorption bands. In addition, the devices have the unique capability to measure both absorption and refractive indices of the chemicals in a single measurement. The acquisition of both characteristics of the chemicals allowed us to experimentally identify and quantify the concentration of them in liquid mixtures. Comparison with the result from traditional infrared spectroscopy instruments confirmed the accuracy of the new chip-scale sensing devices.

Reference: ACS Nano (2014). [link].

Optomechanical Relay A cavity optomechanical device with two optical channels is capable of amplifying RF signals transmitted by light. The control laser in the first waveguide (top) excites the resonance of the micro-donut resonator, generating a strong optical force to displace the second waveguide (bottom). In it, a signal laser with much higher power is mechanically modulated by the weaker control laser so signal amplification is achieved. The device also show novel and tunable mechanical nonlinearity that is completely induced by the cavity optomechanical effect.

Reference: Nature Communications (2012). [link].

Flexible Silicon Photonics Silicon photonics now goes flexible on plastics! We have developed a process to transfer waveguide circuits, including interferometers and ring resonators, as a whole from the standard SOI substrate on to a plastic one. The optical properties of the devices are not affected or degraded by the transfer process: the interferometers have high extinction ratios and the ring resonators have high quality factor. The results are highly compliant photonic devices that are mechanically tunable and can be applied as optical sensors to forces and strain.

Reference: Scientific Reports (2012). [link].

Optomechanical Memory Cavity optomechanics now is made into a new type of non-volatile memory as this burgeoning research topic's first practical application. Here, the digital bits "0" and "1" are registered as one of the bi-stable mechanical states of a nanomechanical beam which is embedded inside an optical ring resonator and is buckling up or down due to the stress built in the material. The two mechanical states are stable thus the memory is non-volatile. The memory can be written and erased all-optically so that it can become an important element for further optical information processing system. Furthermore, the double potential well in this system and the demonstrated ability to fully control its mechanical states could lead to study of intriguing quantum effects such as quantum tunneling and quantum state entanglement.

Reference: Nature Nanotechnology (2011). [link]. News and Views by Garrett Cole and Markus Aspelmeyer. [link] Science magazine news by Adrian Cho: Using Light to Flip a Tiny Mechanical Switch. [link] IEEE Spectrum Magazine von Neil Savage: Laser makes memory mechanical. [link]

High-speed lightrail Lightrail guides light (instead of passenger trains). A racetrack loop of a double rail (i.e. a slot waveguide) makes a high quality optical resonator. A very short portion (~2 micron) of rail is suspended from the ground to make a bridge with a tunnel beneath it. The bridge is driven to vibrate at its characteristic frequencies by the doubly enhanced optical force in-between the rails. This high-speed lightrail is the latest example of the burgeoning cavity optomechanical system at a record high frequency of 760 MHz.

Reference: Applied Physics Letters (2010). [link].

NEMS-GC gas analyzer An array of NEMS cantilever resonators are packed into the very small volume of a microfluidic gas channel. Each of the cantilevers is chemically functionalized with a thin polymer coating and thus absorbs gas molecules with chemical selectivity. The whole device is connected to an ultra-fast gas chromatography (GC) system to perform chemical analysis and recognition. The high speed and sensitivity of these NEMS detectors enable the system to resolve a mixture of 13 different chemical components within 5 seconds. This is a first-step in developing a miniature real-time chemical analysis system with resolution rivaling bench-top instruments.

Reference: Nano Letters (2010). [link], Lab on a Chip Highlight.

Reactive cavity backaction The optical force between a micro-disk resonator and a waveguide consists of both dispersive and reactive contributions from the optomechanial cavity backaction. Conterintuitively, the total optical force is not maximal at the optical resonance, but rather at some detuned frequency where both force terms are attractive. The understanding of this new reactive backaction force will play an important role in further exploitation of cavity optomechanics.

Reference: Physical Review Letters 103, 223901 (2009). [link]

Repulsive and attractive optical forces. Fundamentally similar to other electromagnetic forces, the gradient optical force is intrinsically biopolar. Two coupled waveguides attract or repel each other depending on the relative optical phase of the lightwave that each carries. Using a Mach-Zehnder interferometer configuration, the optical force can be tuned between repulsive or attractive by changing the input laser wavelength. Its direction is inferred from the vectorial response of the waveguides' nanomechanical motion.

Reference: Nature Photonics 3, 464 (2009). [link] [featured on cover] MIT Tech Review story [link].

Top 100 Stories of 2009 by Discover Magzine [link].

Nature Nanotech 2009

Optical cantilever array All optical control and multiplexing of nano cantilevers. Light can hop over the small gap between the ends of two head-to-head cantilevers/waveguides. The cantilevers' relative motion changes the transmission of light across the gap and thus can be detected by measurement of the amplitude of transmitted optical signal. Multiplexing is achieved by using properly designed integrated optical components (called multi-mode interferometers -MMI) that split and combine light between multiple pairs of those cantilevers.

Reference: Nature Nanotechnology 4, 377 (2009). [link] News & Views, by Mark Freeman. [link]

Nature 2008

Demonstration of on-chip optical force Optical forces can drive on-chip nanomechanical devices. Highly confined optical field in nanophotonic devices generates significant gradient optical force, just like the force in optical tweezers that traps dielectric beads. This optical force is utilized to actuate a silicon waveguide which is suspended from the substrate (see inset) and becomes a nanomechanical resonator. The amplitude of the optical force is carefully quantified by measuring the excited motion of the device. This experiment is the first demonstration of a nanomechanical device actuated by light in an integrated photonic circuitry.

Reference: Nature 456, 480, (2008). [link] News and Views by Tobias Kippenberg [link]. MIT Tech Review story. [link]

Nature 2008Self-sensing nanocantilevers Resonant motion of the nanocantilevers was read-out with integrated piezoresistive transducers made of thin metal films. The nanocantilevers are only 500 nm long and their resonance frequencies are beyond 100 MHz. Mass sensitivity demonstrated with these nanocantilevers is below 1 attogram (1e-18 gram) in air and room temperature conditions.

Reference: Nature Nanotechnology, 2, 114 (2007). [link] News and Views by John Mamin. [link]