Single mode thermal emission

Broadband near-infrared emission in silicon waveguides

Silicon photonic integrated circuit foundries enable wafer-level fabrication of entire electro-optic systems-on-a-chip for applications ranging from datacommunication to lidar to chemical sensing. However, silicon's indirect bandgap has so far prevented its use as an on-chip optical source for these systems. Here, we describe a fullyintegrated broadband silicon waveguide light source fabricated in a state-of-the-art 300-mm foundry. A reverse-biased p-i-n diode in a silicon waveguide emits broadband near-infrared optical radiation directly into the waveguide mode, resulting in nanowatts of guided optical power from a few milliamps of electrical current. We develop a one-dimensional Planck radiation model for intraband emission from hot carriers to theoretically describe the emission. The brightness of this radiation is demonstrated by using it for broadband characterization of photonic components including Mach-Zehnder interferometers and lattice filters, and for waveguide infrared absorption spectroscopy of liquid-phase analytes. This broadband silicon-based source can be directly integrated with waveguides and photodetectors with no change to existing foundry processes and is expected to find immediate application in optical systems-on-a-chip for metrology, spectroscopy, and sensing.

Laser-type cooling with unfiltered sunlight

Cooling of systems to sub-Kelvin temperatures is usually done using either a cold bath of particles or spontaneous photon scattering from a laser field; in either case, cooling is driven by interaction with a well-ordered cold (i.e., low-entropy) system. However, there have recently been several schemes proposed for "cooling by heating," in which raising the temperature of some mode drives the cooling of the desired system faster. We discuss how to cool a trapped ion to its motional ground state using unfiltered sunlight at 5800K to drive the cooling. We show how to treat the statistics of thermal light in a single-mode fiber for delivery to the ion and show experimentally how the blackbody spectrum is strongly modified by being embedded in quasi-one-dimension. Quantitative estimates for the achievable cooling rate with our measured fiber-coupled low-dimensional sunlight show promise for demonstrating this implementation of cooling by heating.

Thermal radiation control from hot graphene electrons coupled to a photonic crystal nanocavity

Controlling thermal radiation is central in a range of applications including sensing, energy harvesting, and lighting. The thermal emission spectrum can be strongly modified through the electromagnetic local density of states (EM LDOS) in nanoscale-patterned metals and semiconductors. However, these materials become unstable at high temperature, preventing improvements in radiative efficiency and applications such as thermophotovoltaics. Here, we report stable high-temperature thermal emission based on hot electrons (>2000 K) in graphene coupled to a photonic crystal nanocavity, which strongly modifies the EM LDOS. The electron bath in graphene is highly decoupled from lattice phonons, allowing a comparatively cool temperature (700 K) of the photonic crystal nanocavity. This thermal decoupling of hot electrons from the LDOS-engineered substrate opens a broad design space for thermal emission control that would be challenging or impossible with heated nanoscale-patterned metals or semiconductor materials.

Coupling between multimode fibers and slab waveguides

In guided-wave optics, using gratings to couple between single mode waveguides and single mode fibers and vice versa is well-established. In contrast, the coupling between multimode waveguides is more complex and a much less understood topic, even though multimode coupling is essential for the excitation of guided modes from spatially incoherent sources or for the extraction of spatially incoherent radiation from a guided-wave platform. Here, we present the design for a grating that couples multiple modes of a 2D slab waveguide into a multimode fiber and vice versa and discuss the corresponding challenges. We highlight the importance of matching mode numbers and scattering angles and show that the coupling efficiency can readily drop to low values. We present a rudimentary design that illustrates the key issues by demonstrating the coupling from a multimode fiber into a waveguide slab and back into another fiber, which achieves a total efficiency of -34 dB. By modeling the same geometry, we achieve good agreement, which allows us to explain the physics of the coupler and to suggest improvements. Future options are discussed to improve the coupling elements with a better directivity in order to achieve a maximal coupling efficiency. Our findings can be exploited for improving the multimode light injection into and out of integrated guided-wave optical systems.