We investigate the use of integrated optical circuits for novel applications in optomechanics, quantum optics and telecommunications. To get a feeling of what this is all about check out the following research highlights.
Diamond optomechanical circuits
Diamond provides attractive material properties for both optical applications and mechanical devices. We built the first nanoscale circuits from polycrystalline diamond that use both in a single device. Using polycrystalline diamond deposited by chemical vapour deposition allows for fabricating large-scale circuits.
Nature Communications 4, 1690 (2013) [link]
News in Optik & Photonik: [link]
Article in Materials Today: [link]
Press release: [link]
Waveguide coupled single photon detectors
Superconducting nanowires allow for realizing single photon detectors with high timing resolution and quantum efficiency. We demonstrate the first nanophotonic circuits that incorporate such devices on chip. By fabricating nanowire detectors directly on top of waveguides we achieve near perfect detection efficiency, high timing resolution and a miniature footprint all in the same device. The results are the first step towards fully scalable single photon circuits at telecoms wavelengths.
Nature Communications 3, 1325 (2012) [link].
Optomechanical interactions are often used to cool mechanical resonators with the goal of reaching the quantum ground state of a macroscopic object. Optomechanics can on the other hand also be employed to pump energy from an optical field into the mechanical resonator and thus amplify its motion. Using both principles we demonstrate an all-optical memory element based on nano-mechanical resonators. Bits are written and read out using opto-mechanical interactions to switch between different buckled states: optical heating is used to overcome the potential barrier between the 0 and 1 states, while optical cooling is used to select the desired final state.
Because bot the 0 and 1 states are mechanically stable, our memory element is non-volatile. Furthermore, its implementation allows us to observer the zero-frequency anomaly, predicted earlier theoretically. Such a phenomenon can be used to tune the mechanical resonance frequency over a wide range with applications in signal processing and routing.
Nature Nanotechnology, Advance Online Publication (2011) [link]. Science magazine news von Adrian Cho: Using Light to Flip a Tiny Mechanical Switch. IEEE Spectrum Magazine von Neil Savage: Laser makes memory mechanical.
Second Harmonic Generation in GaN on Silicon
Non-linear optics is particularly effective in integrated photonic circuits, because light is very tightly confined and therefore the optical intensities can be enormous. To exploit intensity enhancement, materials with high non-linear coefficients are required. We used a new approach to realize photonic components in gallium nitride (GaN) thin films on silicon substrates, using a robust bonding technique and nano-fabrication. GaN features strong second order non-linearity, on the same order as lithium niobate. Through indirect phase-matching in micro-ring resonators we achieve strong frequency doubling or second harmonic generation (SGH) on chip, tunable over a wide wavelength range. The results hold promise for the realization of visible light sources on chip.
Optics Express 19, 10462 (2011) [link].
Ultra-high frequency opto-mechanical resonators
For various applications it is desirable to operate mechanical resonators at high frequencies to reduce air damping and noise. Using cavity feedback in high-Q slot ring resonators we demonstrated opto-mechanical actuation of doubly-clamped beam resonators. In slot resonators the gradient of the optical field is enhanced and thus leads to increased optical forces. Cavity feedback can also be employed to enhance the optical interaction further.
The resulting strong opto-mechanical interactions allow us to drive short nano-mechanical resonators with a length below 2 microns. These devices show remarkable displacement sensitivity of 350 am/rt Hz, at room temperature.
Applied Physiks Letters 97, 183110 (2010) [link].
Reactive optomechanical backaction
Gradient optical forces can be enhanced by employing optical cavities, in which the circulating optical power is "recycled" and thus increase the optical intensity. When coupling a nano-mechanical waveguide resonator to an optical cavity, the resulting optical force consists of several contributions stemming from dispersive, dissipative and also reactive coupling. We used a high-Q microdisc resonator to illustrate this novel coupling mechanism: counterintuitively, the optical force is not maximum at the cavity resonance but rather at a detuned wavelength, where the force components are attractive. The results will be important for future exploitation of opto-mechanical interactions in a chip-scale framework.
Physical Review Letters 103, 223901 (2009) [link].
Attractive and repulsive optical forces revealed
Full control of optical drive on a chip is highly desirable. Using complex photonic circuits allowed us to not only control the amplitude of the gradient optical force, but also the sign. This requires coupling of two opto-mechanical resonators in-plane. To achieve this we use a double-Mach Zehnder interferometer in which the phase of the interacting light-waves can be varied by tuning the wavelength. Because the phase difference controls the sign of the optical force, we can pull the nano-beams together or push them apart - by just changing the laser settings.
Full control of optical interactions on chip will be useful for applications ranging from all-optical routing to chipscale sensing and quantum measurement.
For sensing applications it is often preferable to work with cantilevers rather than doubly clamped beam because of lower stiffness and larger mechanical displacements. Optomechanical cantilevers are a convenient way of actuating and sensing mechanical motion. We realized such devices by fabricating head-to-head cantilevers - with a miniature vertical offset. These devices are driven by the gradient optical force: but no phase-sensitive circuit (like a Mach-Zehnder Interferometer or a cavity) is required. Therefore the devices respond to broadband optical drive and can be used with cheap incoherent optical sources.
Several cantilevers can be read out in parallel by using optical multiplexer: multi-mode interference couplers (MMIs). Such a device focuses light into a number of defined optical spots, which are coupled to 10 opto-mechanical cantilevers in our case. All devices are read out at the same time and distinguished by their mechanical resonance frequency.
Optical forces on a chip
Optical forces are usually believed to be too weak for practical applications. However, when used in nano-scale devices, even small forces can have a big effect. We demonstrated that nano-mechanical resonators can be efficiently driven by gradient optical forces, that work in much the same way as optical tweezers - on a chip. Our resonator is a silicon photonic waveguide, which is released from the underlying substrate using nano-fabrication techniques.
By modulating the optical intensity within the circuit the mechanical resonator can be set into motion and acts as a highly sensitive displacement detector. Linear and non-linear mechanical behavior is readily observed with a displacement sensitivity of 18fm/rtHz.