Using an atom interferometer method, we measure the recoil velocity of cesium due to the coherent scattering of a photon. This measurement is used to obtain a value of h/M Cs and the fine structure constant, α. The current fractional uncertainty is ∆α/α = 7.4 × 10-9 .

The insulator-to-metal transition (IMT) in vanadium dioxide (VO 2 ) can enable a variety of optics applications, including switching and modulation, optical limiting, and tuning of optical resonators. Despite the widespread interest in VO 2 for optics, the wavelength-dependent optical properties across its IMT are scattered throughout the literature, are sometimes contradictory, and are not available at all in some wavelength regions. Here, the complex refractive index of VO 2 thin films across the IMT is characterized for free-space wavelengths from 300 nm to 30 µm, using broadband spectroscopic ellipsometry, reflection spectroscopy, and the application of effective-medium theory. VO 2 films of different thicknesses are studied, on two different substrates (silicon and sapphire), and grown using different synthesis methods (sputtering and sol-gel). While there are differences in the optical properties of VO 2 synthesized under different conditions, these differences are surprisingly small in the 2-11 µm range where the insulating phase of VO 2 also has relatively low optical loss. It is anticipated that the refractive-index datasets from this article will be broadly useful for modeling and design of VO 2 -based optical and optoelectronic components, especially in the mid-wave and long-wave infrared.

We have constructed a system that isolates a key element of our experimental setup from vertical motions of the ground and the surrounding apparatus. This system combines the passive isolation of mechanical springs and an optical table floating on compressed air with an active system that measures the acceleration of the mass to be isolated and feeds back to a solenoid actuator to cancel this motion. Passive isolation alone reduces the acceleration error signal by a factor of 30–1000 from 10 to 100 Hz and by as much as a factor of 1000 above 100 Hz. With the feedback path closed, the system acts like a spring-mass system with a natural resonance frequency of 0.033 Hz. The acceleration error signal is reduced by an additional factor of up to 300 from 0.1 to 20 Hz. This system has enabled us to make precision atom interferometric measurements that would have been impossible without vibration isolation.

Thermophotovoltaics (TPV) is the process by which photons radiated from a thermal emitter are converted into electrical power via a photovoltaic cell. Selective thermal emitters that can survive at temperatures at or above ∼1000°C have the potential to greatly improve the efficiency of TPV energy conversion by restricting the emission of photons with energies below the photovoltaic (PV) cell bandgap energy. In this work, we demonstrated TPV energy conversion using a high-temperature selective emitter, dielectric filter, and 0.6 eV In 0.68 Ga 0.32 As photovoltaic cell. We fabricated a passivated platinum and alumina frequency-selective surface by conventional stepper lithography. To our knowledge, this is the first demonstration of TPV energy conversion using a metamaterial emitter. The emitter was heated to >1000°C, and converted electrical power was measured. After accounting for geometry, we demonstrated a thermal-to-electrical power conversion efficiency of 24.1 0.9% at 1055°C. We separately modeled our system consisting of a selective emitter, dielectric filter, and PV cell and found agreement with our measured efficiency and power to within 1%. Our results indicate that high-efficiency TPV generators are possible and are candidates for remote power generation, combined heat and power, and heat-scavenging applications.

The basic physical principles behind atom interferometers based on optical pulses of light are summarized. This method of atom interferometry is based on measurements in the time and frequency domain and is an inherently precise measurement technique. After a brief discussion of some of the important technical requirements for good fringe accuracy and visibility, we describe an interferometer that has measured the acceleration of an atom due to gravity with a resolution better than one part in 10 10 . We project that the absolute accuracy of our measurement will be of the order of a few parts in 10 9 . We also describe an interferometer experiment that measures the recoil energy shift of an atom when it absorbs a photon. When combined with the value of the Rydberg constant and the mass ratios M Cs /m p and m p /m e , one can obtain a value for α, the fine structure constant. Currently, we have an experimental resolution ∆α/α ∼ 10 −8 after two hours of integration time and are studying the systematic effects that affect the measurement.

Towards a future lab-on-a-chip spectrometer, we demonstrate a compact chip-scale air-clad silicon pedestal waveguide as a Mid-Infrared (Mid-IR) sensor capable of in situ monitoring of organic solvents. The sensor is a planar crystalline silicon waveguide, which is highly transparent, between λ = 1.3 and 6.5 μm, so that its operational spectral range covers most characteristic chemical absorption bands due to bonds such as C-H, N-H, O-H, C-C, N-O, C=O, and C≡N, as opposed to conventional UV, Vis, Near-IR sensors, which use weaker overtones of these fundamental bands. To extend light transmission beyond λ = 3.7 μm, a spectral region where a typical silicon dioxide under-clad is absorbing, we fabricate a unique air-clad silicon pedestal waveguide. The sensing mechanism of our Mid-IR waveguide sensor is based on evanescent wave absorption by functional groups of the surrounding chemical molecules, which selectively absorb specific wavelengths in the mid-IR, depending on the nature of their chemical bonds. From a measurement of the waveguide mode intensities, we demonstrate in situ identification of chemical compositions and concentrations of organic solvents. For instance, we show that when testing at λ = 3.55 μm, the Mid-IR sensor can distinguish hexane from the rest of the tested analytes (methanol, toluene, carbon tetrachloride, ethanol and acetone), since hexane has a strong absorption from the aliphatic C-H stretch at λ = 3.55 μm. Analogously, applying the same technique at λ = 3.3 μm, the Mid-IR sensor is able to determine the concentration of toluene dissolved in carbon tetrachloride, because toluene has a strong absorption at λ = 3.3 μm from the aromatic C-H stretch. With our demonstration of an air-clad silicon pedestal waveguide sensor, we move closer towards the ultimate goal of an ultra-compact portable spectrometer-on-a-chip.

We demonstrate selective emission from a heterogeneous metasurface that can survive repeated temperature cycling at 1300 K. Simulations, fabrication, and characterization were performed for a cross-over-a-backplane metasurface consisting of platinum and alumina layers on a sapphire substrate. The structure was stabilized for high temperature operation by an encapsulating alumina layer. The geometry was optimized for integration into a thermophotovoltaic (TPV) system, and was designed to have its emissivity matched to the external quantum efficiency spectrum of 0.6 eV InGaAs TPV material. We present spectral measurements of the metasurface that result in a predicted 22% optical-to-electrical power conversion efficiency in a simplified model at 1300 K. Furthermore, this broadly adaptable selective emitter design can be easily integrated into full-scale TPV systems.

Tunable quantum cascade lasers operating in the terahertz frequency range are demonstrated. By using an external cavity based on the reflection from a movable mirror, both broad and fine tuning of the frequency are achieved by varying the cavity length. Coarse tuning up to 3cm−1 is obtained near the center frequency of 4.8THz (∼160cm−1), and continuous mode-hop-free tuning is observed over ∼ 0.4cm−1, nearly corresponding to the cavity free spectral range.