THz Spectroscopy · Laboratory Astrophysics & Chemical Dynamics · Astronomy

THz Spectroscopy

Comprehensive spectroscopic studies in the electromagnetic spectrum between ~100 GHz to over 10 THz have historically been scarce due to few existing methods in terahertz (THz) generation and detection. The THz regime captures intrinsic resonances from collective oscillations in all phases of matter which are typically challenging to observe. The Blake group is chiefly concerned with the use of THz time domain spectroscopy (THz-TDS) to investigate a wide array of fundamental phenomena in gases, liquids, and solids. Knowledge of complex organic chemistry and chemical abundances in the interstellar medium (ISM), for instance, can be acquired and compared to astronomical data. The THz spectral region is valuable for reduced line density when compared to the millimeter wave spectrum, for the existence of high-resolution observatories, and potentially for strong transitions resulting from the lowest-lying vibrational modes of large molecules.

The heart of the THz time-domain spectrometer is the ultrafast laser. Due to the femtosecond duration of ultrafast laser pulses and an energy-time uncertainty relationship, the pulses typically have a several-THz bandwidth. By use of optical rectification, the optical pulse carrier envelope shape, i.e. intensity-time profile, can be transferred to the phase of the resulting THz pulse. The phase, along with THz pulse amplitude, allows measurement of the full electric field interaction with our samples in a time-resolved manner. Additionally, time-dependent transient excitations can be observed through conventional pump-probe spectroscopy techniques; extremely relevant for dynamics studies on semiconductors, biological molecules and more.

Chirped pulse amplifiers produce ultrafast laser pulses <50 fs in duration with several mJ energy. With this amount of energy per pulse, one can access a wide variety of non-linear optical processes. We use this light to generate THz photons via two different processes. Optical rectification produces THz light via a second order process in a crystal such as ZnTe. This process can be used to generate light in the ~0.1-3 THz range. In a slightly different process, we use a two-color plasma to generate a broader bandwidth of THz light. Here, the fundamental and second harmonic frequencies of the laser light are focused together to make a plasma in the air. When the free electrons in the plasma are pulled back-and-forth by the electric field of the second harmonic light, classical electromagnetism dictates THz light emission.

We then detect the THz light generated via either process using electro-optic sampling. This technique requires the output of the laser to be split in generation and detection beams. The detection beam is delayed and then recombined with the THz light on a crystal. The THz light can be detected by the effect it has on the polarization of the detection beam; the magnitude of the change in polarization is linearly proportional to the electric field of the THz light. In the Blake lab we also develop instrumentation to help us answer important questions in chemical physics and astrochemistry. Recent efforts have yielded a method to collect THz spectra in a single shot. Current projects include working to implement a plasma detection scheme.

These above techniques are used in our projects focused on fundamental chemical physics. We are currently looking into investigating liquid solvent dynamics, and charge transfer processes in both liquids and semiconductors.

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Laboratory Astrophysics and Chemical Dynamics

Amino acids, life-essential biomolecules, have been found in a number of meteoritic samples, as well as recently in pristine samples returned from the comet Wild 2 as part of the STARDUST mission. The question of how and where these molecules form can only be answered through a dedicated collaborative effort between observational astronomers, astrochemical modelers, and laboratory astrophysics.

Laboratory Astrophysics research in the Blake group focuses on the development of instruments which have applications both towards the spectroscopic characterization of molecules of astrophysical interest as well as to more fundamental chemical physics (e.g. cluster formation, reaction dynamics, and molecular interactions). The Blake Group employs a variety of techniques spanning the frequency spectrum from the microwave (1-100 GHz), through the sub-mmm (100-1000 GHz), and into the THz (1-10 THz), both in the gas-phase and in the solid-phase.

Microwave spectroscopy has been an invaluable tool for elucidating molecular structure for over 50 years. Until recently microwave spectrometers were only capable of measuring one to a few microwave transitions simultaneously due to their narrow bandwidth (~1 MHz). In the last decade the Pate lab developed a broadband microwave spectrometer that is capable of exciting more than 10 GHz of bandwidth, opening the door for new studies of gas phase structure and dynamics. We are currently constructing a broadband spectrometer to study the large amplitude motions of gas phase isolated molecules and small clusters. This work is useful for understanding the fundamental physical chemistry of gas phase molecules as well as providing laboratory spectra for identification of molecules in the interstellar medium.

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There is an intimate connection between the early solar nebula and the interstellar medium from which it formed. Thus, the study of current star-forming environments can tell us much about how we came to be. The combination of rapidly improving observational tools and increasingly sophisticated theory has provided, for the first time, a broad outline of the physical processes associated with the assembly of Sun-like stars and their attendant planetary systems. Despite these impressive gains, many crucial details are poorly understood. In addition, the scope of the problem of stellar and planetary formation has broadened drastically with the burgeoning field of extra-solar planet detection, imaging and spectroscopy (for up to the date information on this exciting research, the Exoplanet Data Explorer is a great, and interactive, place to start) Furthermore, stars are by their nature gregarious. Whether they are part of a multiple system or members of an association, the majority of stars are born in environments that are considerably more complex than that outlined below. The characterization of star-forming regions therefore presents considerable challenges both observationally and theoretically.

A particular focus of our research over the past decade has been the observational and radiative transfer analysis of the dust and gas composition in protoplanetary disks. The coupling of broadband spectroscopy with the Spitzer and Herschel Space Telescopes with deep ground-based high angular and spectral resolution studies at the Keck and Very Large Telescopes has been especially fruitful. We have discovered the molecular photospheres of such disks, that emit principally at infrared through far-IR and sub(mm) wavelengths, and developed means to locate critical snow/frost lines in the disk. Detailed non-Local Thermodynamic Equilibrium radiative transfer codes a critical step in our analysis of the high dynamic range observations from space and the ground, as are collaborative efforts with groups around the world to understand the chemical networks at play. With the advent of shared risk observing at the Atacama Large Millimeter Array, or ALMA, the direct chemical imaging of such snow lines and various chemical species should soon lead to a first characterization of growing protoplanets.

The high dynamic range spectroscopy tools developed for the study of gas in circumstellar disks can also be applied to the characterization of exo-planet atmospheres. Typically this has been done for exo-planets that transit and/or that are eclipsed by their host stars, we are developing tools that can study non-transiting systems as well.

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