Optical calorimetry in microfluidic droplets.

A novel microfluidic calorimeter that measures the enthalpy change of reactions occurring in 100 µm diameter aqueous droplets in fluoropolymer oil has been developed. The aqueous reactants flow into a microfluidic droplet generation chip in separate fluidic channels, limiting contact between the streams until immediately before they form the droplet. The diffusion-driven mixing of reactants is predominantly restricted to within the droplet. The temperature change in droplets due to the heat of reaction is measured optically by recording the reflectance spectra of encapsulated thermochromic liquid crystals (TLC) that are added to one of the reactant streams. As the droplets travel through the channel, the spectral characteristics of the TLC represent the internal temperature, allowing optical measurement with a precision of ≈6 mK. The microfluidic chip and all fluids are temperature controlled, and the reaction heat within droplets raises their temperature until thermal diffusion dissipates the heat into the surrounding oil and chip walls. Position resolved optical temperature measurement of the droplets allows calculation of the heat of reaction by analyzing the droplet temperature profile over time. Channel dimensions, droplet generation rate, droplet size, reactant stream flows and oil flow rate are carefully balanced to provide rapid diffusional mixing of reactants compared to thermal diffusion, while avoiding thermal "quenching" due to contact between the droplets and the chip walls. Compared to conventional microcalorimetry, which has been used in this work to provide reference measurements, this new continuous flow droplet calorimeter has the potential to perform titrations ≈1000-fold faster while using ≈400-fold less reactants per titration.

Monolithic integration of germanium-on-insulator p-i-n photodetector on silicon.

A germanium-on-insulator (GOI) p-i-n photodetector, monolithically integrated on a silicon (Si) substrate, is demonstrated. GOI is formed by lateral-overgrowth (LAT-OVG) of Ge on silicon dioxide (SiO(2)) through windows etched in SiO(2) on Si. The photodetector shows excellent diode characteristics with high on/off ratio (6 × 10(4)), low dark current, and flat reverse current-voltage (I-V) characteristics. Enhanced light absorption up to 1550 nm is observed due to the residual biaxial tensile strain induced during the epitaxial growth of Ge caused by cooling after the deposition. This truly Si-compatible Ge photodetector using monolithic integration enables new opportunities for high-performance GOI based photonic devices on Si platform.

Polarization-sensitive broadband photodetector using a black phosphorus vertical p-n junction.

The ability to detect light over a broad spectral range is central to practical optoelectronic applications and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here, we demonstrate a broadband photodetector using a layered black phosphorus transistor that is polarization-sensitive over a bandwidth from ∼400 nm to 3,750 nm. The polarization sensitivity is due to the strong intrinsic linear dichroism, which arises from the in-plane optical anisotropy of this material. In this transistor geometry, a perpendicular built-in electric field induced by gating can spatially separate the photogenerated electrons and holes in the channel, effectively reducing their recombination rate and thus enhancing the performance for linear dichroism photodetection. The use of anisotropic layered black phosphorus in polarization-sensitive photodetection might provide new functionalities in novel optical and optoelectronic device applications.

Transparent metallic fractal electrodes for semiconductor devices.

Nanostructured metallic films have the potential to replace metal oxide films as transparent electrodes in optoelectronic devices. An ideal transparent electrode should possess a high, broadband, and polarization-independent transmittance. Conventional metallic gratings and grids with wavelength-scale periodicities, however, do not have all of these qualities. Furthermore, the transmission properties of a nanostructured electrode need to be assessed in the actual dielectric environment provided by a device, where a high-index semiconductor layer can reflect a substantial fraction of the incident light. Here we propose nanostructured aluminum electrodes with space-filling fractal geometries as alternatives to gratings and grids and experimentally demonstrate their superior optoelectronic performance through integration with Si photodetectors. As shown by polarization and spectrally resolved photocurrent measurements, devices with fractal electrodes exhibit both a broadband transmission and a flat polarization response that outperforms both square grids and linear gratings. Finally, we show the benefits of adding a thin silicon nitride film to the nanostructured electrodes to further reduce reflection.

A submicron plasmonic dichroic splitter.

Spectral imaging and sensing techniques, new solar cell designs and wavelength-division multiplexing in optical communication rely on structures that collect and sort photons by wavelength. The strong push for chip-scale integration of such optical components has necessitated ultracompact, planar structures, and fomented great interest in identifying the smallest possible devices. Consequently, novel micro-ring, photonic crystal and plasmonic solutions have emerged. Meanwhile, the optical coupling of subwavelength plasmonic structures supporting a very limited number of modes has also enabled new functionalities, including Fano resonances and structural electromagnetically-induced transparency. Here we show how two similarly sized subwavelength metal grooves can form an ultracompact submicron plasmonic dichroic splitter. Each groove supports just two electromagnetic modes of opposite symmetry that allows independent control of how a groove collects free-space photons and directs surface plasmon polaritons. These results show how the symmetry of electromagnetic modes can be exploited to build compact optical components.