Session E2: Atomic, Molecular and Optical Physics II

A Novel Diffuse Reflecting Material for Applications in Integrating Cavity Spectroscopy

Integrating cavities are a common and indispensable tool in the modern optics laboratory. The diffuse reflecting walls of the cavity allow it to collect and spatially integrate the radiant flux emitted from a source, making them ideal for applications in radiometry and photometry. In addition, integrating cavities have also been used to make very sensitive absorption measurements. Recently, we have developed a new diffuse reflecting material with the highest diffuse reflectivity ever measured in the visible and UV portions of the spectrum. The material is a packed fumed silica powder (i.e. quartz powder), and can be used to make high-reflectivity integrating cavities. We have used these quartz powder cavities in a variety of spectroscopic applications including: the detection of organic toxins via Raman spectroscopy, the detection of water contaminants through fluorescence spectroscopy, and measurements of the absorption coefficient of pure water in the UV. Furthermore, we have developed a variation of cavity ring-down spectroscopy (CRDS) that we call integrating cavity ring-down spectroscopy (ICRDS). ICRDS allows for direct measurements of the absorption in a sample, even in the presence of strong scattering. Currently, the commercially available diffuse reflectors have insufficient reflectivity for ICRDS, but our high-reflectivity fumed silica cavities have made ICRDS a reality.   

Improved Design for Cosine Collectors

A cosine collector is a detector whose response to incident light is proportional to the cosine of the angle between the incident light and collector's surface normal. These detectors have been used for many years to measure relative and absolute spectral intensity of a multitude of radiant sources. After close inspection of commercial cosine collectors we noticed a systematic problem with most of the collectors offered - they do not adequately measure the cosine value they claim to measure. We have designed a new cosine collector that accurately measures the cosine of incident light with unprecedented accuracy. The strength of this detector lies in the geometry used for the detector aperture. It allows over 99{\%} of incident light to be collected while acting as a slit to produce the expected cosine behavior.   

Simple but efficient optical condensers based on liquid droplets

Optical condensers used in microscopes illuminate the object under observation at inclined angles, thus providing enhanced resolution. However, these condensers contain many parts such as diaphragms, lenses etc. that make them bulky. Recently, we have found that liquid droplets over the surface of the object under observation can be used as efficient condensers that can provide sub-wavelength resolution. Also, an interesting way to analyze and characterize these droplet-based condensers using Fourier plane imaging microscopy (FPIM) technique and the potential for achieving deeper sub-wavelength resolution shall be discussed in detail.   

Active mode-locking of mid-infrared quantum cascade lasers

Active mode locking, i.e. modulation of gain or losses at the cavity round-trip frequency is one of the methods for generating ultrashort pulses in lasers. For Quantum Cascade Lasers (QCLs), it is believed that their short gain recovery time $\sim$ 1\,ps as compared to a much longer cavity round-trip time ($\sim$ 50\,ps) prohibits generation of mode-locked pulses. We perform space-time domain simulations of QCL dynamics solving coupled density-matrix equations and Maxwell's equations with a realistic transport model of the active region. We find that active gain modulation of a short section of a two-section monolithic laser cavity leads to robust mode locking and generation of picosecond pulses over a broad range of laser parameters. This finding shows a viable path towards achieving ultrashort pulse generation in QCLs.   

Flexible, Lightweight Terahertz Photodetector and Polarizer Based on Carbon Nanotube Fibers

Carbon nanotubes (CNTs) have some extraordinary properties such as ultrabroadband (ultraviolet to far-infrared) absorption, ultrahigh electrical and thermal conductivities, ultralight weights and ultrahigh mechanical strengths, which have attracted the interest of researchers in diverse fields. In particular, the photonic and optoelectronic properties of CNTs are ideally suited for terahertz (THz) technologies which have applications in astronomy, communications, and sensing. The THz detectors and polarizers that we currently use are mostly bulky operated at very low temperatures, and have complex structures. Here, we explore the optoelectronic properties of high-performance multifunctional fibers of CNTs towards the development of lightweight, flexible THz photodetectors and polarizers. We present a photothermoelectric-effectbased flexible CNT-fiber photodetector fabricated using a novel technique. The spatial variation of doping in CNT fibres creates a Seebeck coefficient gradient, leading to a photothermoelectric signal. This CNT-fiber photodetector shows polarization-sensitive detection in the THz regime. Under illumination by a 3.52THz beam, the I-V characteristic of the fiber photodetector produced a finite short-circuit current (I$_{\mathrm{SC}})$ and open-circuit voltage (V$_{\mathrm{OC}})$. The device showed responsivities up to 2.1 mA/W. We also fabricated a CNT-fiber-based polarizer, which exhibited ideal polarizer properties with $\sim$ 100{\%} degree of polarization and extinction ratios of better than 30 dB in the THz range.   

Changing the optical and electrical properties of a dielectric surface

The optical response of a dielectric surface to an incident laser radiation can be shifted when the surface is illuminated with a thermal source of radiation or when a uniform electric field is set up along the surface. Using a blackbody source one can generate an entire curve of dispersion for wavelengths lesser than the wavelength of the probe laser. A low capacitor voltage across the dielectric can shift the wavelength of the probe laser as perceived by the dielectric surface toward smaller values. This shift is due to an increase of the vibrational frequency of the electric dipoles located on the dielectric surface. The change in the polarization properties of the dielectric surface suggests the use of this configuration as an optoelectronic switch driven by a relatively small capacitor voltage. We report results of relative permittivity for flint and crown glasses illuminated with a diode laser of 532 nm. Our measurements indicate that the reflection of the light by the dielectric surface is done within one chemical bond.   

Electronically Controlled Condensers for Sub-Wavelength Microscopy

We are exploring the use of hemispherical Electronically Controlled Condensers (ECCs) with no moving parts, lenses, or mirrors, to improve the sub-wavelength resolution capabilities of optical microscopes. ECCs consist of a number of light emitting diodes (LED) placed inside a hollow hemisphere. Electronically controlling individual or groups of LEDs spatially and temporally gives the condenser numerous advantages over common optical condensers. ECCs are a simple solution to achieve a variable numerical aperture (NA) depending on the illumination incident angle. Our approach allows for the realization of bright field (NA$_{\mathrm{O}}$ \textgreater NA$_{\mathrm{c}})$ and dark field (NA$_{\mathrm{O}}$ \textless NA$_{\mathrm{c}})$ optical microscopy in the same setup. We anticipate a number of applications for ECCs. LEDs emitting at different wavelengths, will enable the realization of quasi-monochromatic or polychromatic ECCs where the wavelength selection will be determined by the microscopy application. Near IR optical Tomography and panoramic microscopy can be realized by controlling the spatial and temporal illumination of the LEDs. ECCs can also be used to perform Fourier Ptychography to achieve both wide-field view and high resolution.   

Spectroscopic analysis of gas separated from liquid

After the Deepwater Horizon oil spill, large concentrations of methane were observed dissolved in the Gulf of Mexico corresponding to a huge release of methane along with the discharge of oil from the site. It has been proposed that quantifying the amount of released methane could help quantify the magnitude of such a spill. We propose to build a system for extracting dissolved trace gasses from collected sea water samples that is capable of operating on board a research vessel. This system will enable on-site analysis of gas content without the need to transport large volumes of water samples back to the laboratory. The system is designed to continuously circulate a sample volume of water through a membrane filter that will extract dissolved gasses into an evacuated collection cylinder. The extracted gasses can then be analyzed on-site with infrared cavity ringdown spectroscopy. We present our initial results on the gas-liquid separation and on the ringdown spectroscopy of methane. The high sensitivity cavity ringdown spectrometer enables isotopic analysis of $^{13}$C, which can also serve to differentiate between carbon sources of natural and anthropogenic sources.