A universal 3D imaging sensor on a silicon photonics platform.

Accurate three-dimensional (3D) imaging is essential for machines to map and interact with the physical world1,2. Although numerous 3D imaging technologies exist, each addressing niche applications with varying degrees of success, none has achieved the breadth of applicability and impact that digital image sensors have in the two-dimensional imaging world3-10. A large-scale two-dimensional array of coherent detector pixels operating as a light detection and ranging system could serve as a universal 3D imaging platform. Such a system would offer high depth accuracy and immunity to interference from sunlight, as well as the ability to measure the velocity of moving objects directly11. Owing to difficulties in providing electrical and photonic connections to every pixel, previous systems have been restricted to fewer than 20 pixels12-15. Here we demonstrate the operation of a large-scale coherent detector array, consisting of 512 pixels, in a 3D imaging system. Leveraging recent advances in the monolithic integration of photonic and electronic circuits, a dense array of optical heterodyne detectors is combined with an integrated electronic readout architecture, enabling straightforward scaling to arbitrarily large arrays. Two-axis solid-state beam steering eliminates any trade-off between field of view and range. Operating at the quantum noise limit16,17, our system achieves an accuracy of 3.1 millimetres at a distance of 75 metres when using only 4 milliwatts of light, an order of magnitude more accurate than existing solid-state systems at such ranges. Future reductions of pixel size using state-of-the-art components could yield resolutions in excess of 20 megapixels for arrays the size of a consumer camera sensor. This result paves the way for the development and proliferation of low-cost, compact and high-performance 3D imaging cameras that could be used in applications from robotics and autonomous navigation to augmented reality and healthcare.

α-Actinin links extracellular matrix rigidity-sensing contractile units with periodic cell-edge retractions.

During spreading and migration, the leading edges of cells undergo periodic protrusion-retraction cycles. The functional purpose of these cycles is unclear. Here, using submicrometer polydimethylsiloxane pillars as substrates for cell spreading, we show that periodic edge retractions coincide with peak forces produced by local contractile units (CUs) that assemble and disassemble along the cell edge to test matrix rigidity. We find that, whereas actin rearward flow produces a relatively constant force inward, the peak of local contractile forces by CUs scales with rigidity. The cytoskeletal protein α-actinin is shared between these two force-producing systems. It initially localizes to the CUs and subsequently moves inward with the actin flow. Knockdown of α-actinin causes aberrant rigidity sensing, loss of CUs, loss of protrusion-retraction cycles, and, surprisingly, enables the cells to proliferate on soft matrices. We present a model based on these results in which local CUs drive rigidity sensing and adhesion formation.

High-Throughput Mechanobiology Screening Platform Using Micro- and Nanotopography.

We herein demonstrate the first 96-well plate platform to screen effects of micro- and nanotopographies on cell growth and proliferation. Existing high-throughput platforms test a limited number of factors and are not fully compatible with multiple types of testing and assays. This platform is compatible with high-throughput liquid handling, high-resolution imaging, and all multiwell plate-based instrumentation. We use the platform to screen for topographies and drug-topography combinations that have short- and long-term effects on T cell activation and proliferation. We coated nanofabricated "trench-grid" surfaces with anti-CD3 and anti-CD28 antibodies to activate T cells and assayed for interleukin 2 (IL-2) cytokine production. IL-2 secretion was enhanced at 200 nm trench width and >2.3 µm grating pitch; however, the secretion was suppressed at 100 nm width and <0.5 µm pitch. The enhancement on 200 nm grid trench was further amplified with the addition of blebbistatin to reduce contractility. The 200 nm grid pattern was found to triple the number of T cells in long-term expansion, a result with direct clinical applicability in adoptive immunotherapy.

CD28 and CD3 have complementary roles in T-cell traction forces.

Mechanical forces have key roles in regulating activation of T cells and coordination of the adaptive immune response. A recent example is the ability of T cells to sense the rigidity of an underlying substrate through the T-cell receptor (TCR) coreceptor CD3 and CD28, a costimulation signal essential for cell activation. In this report, we show that these two receptor systems provide complementary functions in regulating the cellular forces needed to test the mechanical properties of the extracellular environment. Traction force microscopy was carried out on primary human cells interacting with micrometer-scale elastomer pillar arrays presenting activation antibodies to CD3 and/or CD28. T cells generated traction forces of 100 pN on arrays with both antibodies. By providing one antibody or the other in solution instead of on the pillars, we show that force generation is associated with CD3 and the TCR complex. Engagement of CD28 increases traction forces associated with CD3 through the signaling pathway involving PI3K, rather than providing additional coupling between the cell and surface. Force generation is concentrated to the cell periphery and associated with molecular complexes containing phosphorylated Pyk2, suggesting that T cells use processes that share features with integrin signaling in force generation. Finally, the ability of T cells to apply forces through the TCR itself, rather than the CD3 coreceptor, was tested. Mouse cells expressing the 5C.C7 TCR exerted traction forces on pillars presenting peptide-loaded MHCs that were similar to those with α-CD3, suggesting that forces are applied to antigen-presenting cells during activation.

Graphene metallization of high-stress silicon nitride resonators for electrical integration.

High stress stoichiometric silicon nitride resonators, whose quality factors exceed one million, have shown promise for applications in sensing, signal processing, and optomechanics. Yet, electrical integration of the insulating silicon nitride resonators has been challenging, as depositing even a thin layer of metal degrades the quality factor significantly. In this work, we show that graphene used as a conductive coating for Si3N4 membranes reduces the quality factor by less than 30% on average, which is minimal when compared to the effect of conventional metallization layers such as chromium or aluminum. The electrical integration of Si3N4-Graphene (SiNG) heterostructure resonators is demonstrated with electrical readout and electrostatic tuning of the frequency by up to 0.3% per volt. These studies demonstrate the feasibility of hybrid graphene/nitride mechanical resonators in which the electrical properties of graphene are combined with the superior mechanical performance of silicon nitride.

Cells test substrate rigidity by local contractions on submicrometer pillars.

Cell growth and differentiation are critically dependent upon matrix rigidity, yet many aspects of the cellular rigidity-sensing mechanism are not understood. Here, we analyze matrix forces after initial cell-matrix contact, when early rigidity-sensing events occur, using a series of elastomeric pillar arrays with dimensions extending to the submicron scale (2, 1, and 0.5 µm in diameter covering a range of stiffnesses). We observe that the cellular response is fundamentally different on micron-scale and submicron pillars. On 2-µm diameter pillars, adhesions form at the pillar periphery, forces are directed toward the center of the cell, and a constant maximum force is applied independent of stiffness. On 0.5-µm diameter pillars, adhesions form on the pillar tops, and local contractions between neighboring pillars are observed with a maximum displacement of ∼60 nm, independent of stiffness. Because mutants in rigidity sensing show no detectable displacement on 0.5-µm diameter pillars, there is a correlation between local contractions to 60 nm and rigidity sensing. Localization of myosin between submicron pillars demonstrates that submicron scale myosin filaments can cause these local contractions. Finally, submicron pillars can capture many details of cellular force generation that are missed on larger pillars and more closely mimic continuous surfaces.

Absorption bleaching by stimulated emission in erbium-doped silicon-rich silicon nitride waveguides.

Stimulated emission from sensitized erbium ions in silicon-rich silicon nitride is demonstrated by pump-probe measurements carried out in waveguides. A decrease in the photoinduced absorption of the probe at the wavelength of erbium emission is observed and is attributed to stimulated emission from erbium excited indirectly via localized states in the silicon nitride matrix.

On-chip supercontinuum optical trapping and resonance excitation of microspheres.

We demonstrate the simultaneous optical manipulation and analysis of microscale particles in a microfluidic channel. Whispering gallery modes (WGMs) in dielectric microspheres are excited using the evanescent field from a silicon nitride waveguide. A supercontinuum source is used to both trap the microspheres to the surface of the waveguide and excite their resonant modes. All measurements are in plane, thus providing an integrated optofluidic platform for lab-on-a-chip biosensing applications.

Oxidized silicon-on-insulator (OxSOI) from bulk silicon: a new photonic platform.

We demonstrate a bulk silicon alternative to the conventional silicon-on-insulator photonics platform, using common CMOS process-based Si(3)N(4) masking and oxidation techniques. We show waveguide losses as low as 2.92 dB/cm with a technique that can be implemented on the front-end of a typical CMOS fabrication line.

Minimizing temperature sensitivity of silicon Mach-Zehnder interferometers.

We present a novel design approach for integrated Mach-Zehnder interferometers to minimize their temperature sensitivity and demonstrate, for the first time, near zero spectral shifts with temperature (approximately 0.005 nm/K) in these devices. This could lead to fully CMOS-compatible passively compensated athermal optical filters and modulators.

Controlling photonic structures using optical forces.

The use of optical forces to manipulate small objects is well known. Applications include the manipulation of living cells by optical tweezers and optical cooling in atomic physics. The miniaturization of optical systems (to the micro and nanoscale) has resulted in very compliant systems with masses of the order of nanograms, rendering them susceptible to optical forces. Optical forces have been exploited to demonstrate chaotic quivering of microcavities, optical cooling of mechanical modes, actuation of a tapered-fibre waveguide and excitation of the mechanical modes of silicon nano-beams. Despite recent progress in this field, it is challenging to manipulate the optical response of photonic structures using optical forces; this is because of the large forces that are required to induce appreciable changes in the geometry of the structure. Here we implement a resonant structure whose optical response can be efficiently statically controlled using relatively weak attractive and repulsive optical forces. We demonstrate a static mechanical deformation of up to 20 nanometres in a silicon nitride structure, using three milliwatts of continuous optical power. Because of the sensitivity of the optical response to this deformation, such optically induced static displacement introduces resonance shifts spanning 80 times the intrinsic resonance linewidth.

High confinement micron-scale silicon nitride high Q ring resonator.

We demonstrate high confinement, low-loss silicon nitride ring resonators with intrinsic quality factor (Q) of 3*10(6) operating in the telecommunication C-band. We measure the scattering and absorption losses to be below 0.065dB/cm and 0.055dB/cm, respectively.

Deposited silicon high-speed integrated electro-optic modulator.

We demonstrate a micrometer-scale electro-optic modulator operating at 2.5 Gbps and 10 dB extinction ratio that is fabricated entirely from deposited silicon. The polycrystalline silicon material exhibits properties that simultaneously enable high quality factor optical resonators and sub-nanosecond electrical carrier injection. We use an embedded p(+)n(-)n(+) diode to achieve optical modulation using the free carrier plasma dispersion effect. Active optical devices in a deposited microelectronic material can break the dependence on the traditional single layer silicon-on-insulator platform and help lead to monolithic large-scale integration of photonic networks on a microprocessor chip.

Low modal volume dipole-like dielectric slab resonator.

We propose a novel geometry in a silicon planar resonator with an ultra-small modal volume of 0.01(lambda/2n)(3). The geometry induces strong electric field discontinuities to decrease the modal volume of the cavity below 1(lambda/2n)(3) The proposed structure and other common resonators such as 1D and 2D photonic crystal resonators are compared for tradeoffs in confinement and quality factors.

Spontaneous emergence of periodic patterns in a biologically inspired simulation of photonic structures.

We simulate an evolutionary process in the lab for designing a novel high confinement photonic structure, starting with a set of completely random patterns, with no insight on the initial geometrical pattern. We show a spontaneous emergence of periodical patterns as well as previously unseen high confinement subwavelength bowtie regions. The evolved structure has a Q of 300 and an ultrasmall modal volume of 0.112 (lambda/2n)3. The emergence of the periodic patterns in the structure indicates that periodicity is a principal condition for effective control of the distribution of light.