Session Q27: Focus Session: New Trends in Spectroscopy III

Coherent frequency combs and spectroscopy

Optical frequency combs maintain precise phase coherence across the entire visible spectrum and they have profoundly changed optical frequency metrology and ultrafast science, with breakthrough developments in optical atomic clocks, optical frequency synthesis, direct frequency comb spectroscopy (DFCS), high-resolution quantum control, coherent pulse synthesis and amplification, and control of sub-femtosecond electron dynamics in atoms and molecules. DFCS [1] is a new spectroscopic approach that realizes simultaneously broad spectral coverage, high spectral resolution, many parallel detection channels, ultrahigh sensitivity, and real-time analysis [2]. These powerful capabilities have been demonstrated in a series of experiments where identification and quantification of many different molecular states or species are achieved in a massively parallel fashion [3].\\[4pt] [1] A. Marian et al., Science 306, 2063 (2004). \\[0pt] [2] M. J. Thorpe et al., Science 311, 1595 (2006). \\[0pt] [3] M. J. Thorpe {\&} J. Ye, Appl. Phys. B 91, 397 (2008).   

Enhancement of wideband terahertz-beam signals by a CW laser beam, in time-domain terahertz spectroscopy

In time-domain terahertz spectrometry (TDTS) a wide terahertz beam is produced by irradiating a femtosecond laser pulse onto a terahertz emitter, typically a semiconductor slab or an electro-optic material. Since a compact TDTS uses a small femtosecond laser, its terahertz signal is weak, and the resulting terahertz spectrum is often ambiguous. Our recent experiments have shown that irradiating the emitter with an additional CW laser beam along with a femtosecond laser beam can enhance the wideband terahertz signal. Such an enhancement is possible, because an additional CW laser beam increases the photocurrents in the terahertz emitter, which in turn increases the terahertz output -- a time derivative of the photocurrents. While the power of the wideband terahertz beam increases with the addition of CW laser beam power, small variations in the terahertz spectrum have also been observed with the addition of the CW beam. The variations in the terahertz spectrum could be due to the power fluctuations of the CW laser beam, or it could be due to the overall spectral modification in the combined laser beams as the spectrum of CW laser was slightly different from that of the femtosecond laser. Details of the terahertz-beam variations, as well as various applications of this enhanced TDTS, will be discussed in the presentation.   

Molecular spectroscopy with comb-stabilized diode lasers

We have constructed a comb-stabilized extended cavity diode laser-based spectrometer designed for precision spectroscopic measurements of chemical intermediates. Initial experiments have concentrated on the detection of sub-Doppler absorptions in acetylene near 1.5 $\mu m$ in order to characterize the instrument. In the immediate future, we plan to attempt a detection of the $X_2 - X_1$ fine structure transition in lead fluoride (PbF) at 1.2 $\mu m$, a molecule targeted because of its potential as a vehicle for a future measurement of the electron's electric dipole moment.   

Beller Lectureship Talk: Real-time broadband spectroscopy with laser frequency combs

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Adiabatic Control of Two-Photon Transitions via Optical Frequency Comb

We show that a phase modulated frequency comb can be used to perform two-photon transitions between molecular vibrational levels forming three-level $\lambda$-system. The phase across a single pulse in the pulse train is modulated by a sin-function with a carefully chosen amplitude and modulation frequency. Partial adiabatic population transfer to the final state is fulfilled by each pulse in the applied pulse train providing a controlled population accumulation in the final state. Detuning the pule train parameters to less than the frequency difference between the initial and final states in the $\lambda$-system changes the time scale of molecular dynamics but leads to the same complete population transfer. The proposed scheme may be used to form ultracold molecules, e.g., KRb.   

Terahertz spectra of ionized air produced by various sources

Experiments have shown that terahertz spectroscopy is a useful tool for the identification and characterization of chemicals and biological materials, mostly in their solid or liquid phases. Recently, we carried out terahertz spectroscopy experiments on ambient air, which contains gas molecules that may be ionized by various ionization sources, such as background radiation, high-voltage transmission lines, and corona discharge. Our experiments, which were performed with several different ionization sources, including a corona discharge device and nuclear isotopes, indicated that terahertz spectra of ambient air depend on the degree of air ionization, as well as on the ionization source. In this presentation we will discuss the details of our experiments and the implications of our experimental results.   

Molecular Spectroscopy with Frequency Combs

Pulsed femtosecond frequency combs are rapidly developing as a powerful spectroscopic tool. As a spectroscopic source stabilized frequency combs potentially offer broad spectral coverage, near perfect frequency accuracy, low timing jitter and broadband compatibility with resonant cavities. This talk will focus on the first three advantages in a dual comb spectroscopic technique that is highly analogous to traditional Fourier transform spectroscopy. In the dual comb approach, (pioneered in the THz by Keilmann, Van der Weide and coworkers under the name multi-heterodyne spectroscopy), one comb is used to sample a gas and a second frequency comb serves as a local oscillator (LO) that samples the first comb. The LO is held at a slightly different repetition rate than the first comb. When viewed in the time domain, the comb sources each emit a train of pulses. With the difference in repetition rates, for each successive pair of pulses, the timing between the sample and LO laser pulses shifts slightly. Through successive measurements, the LO pulses read out the entire time domain structure of the transmitted sample pulse. Through a Fourier transform, we recover the broadband, complex, absorption profile of the sample gas. In analogy to a Fourier transform spectrometer the LO serves as a scanning interferometer arm. The removal of moving parts from the system along with the addition of high brightness collimated sources brings new flexibility to FTIR spectroscopy. This talk will focus on strengths and limitations of the dual comb technique. Specifically we focus on comb stabilization techniques that allow for long averaging periods, firmware based averaging techniques that keep data sizes manageable and allow for realtime data processing, time domain multiplexing of signal and reference data for continuous removal of system drift, and difference frequency generation techniques to extend this system into the mid IR. We will also discuss methods to improve the sensitivity of this technique as well as tradeoffs between sensitivity and resolution, while maintaining the inherent frequency accuracy of this system.   

Characterization of molecular crystal phonons

The terahertz frequency vibrational modes of molecular crystals can be used to determine polymorphism in pharmaceuticals and fundamental coupling between intramolecular and intermolecular motions [1]. As the molecular complexity increases, assignment of these modes becomes problematic with inhomogeneous broadening and increasing density of modes. Recently inhomogeneous broadening has been overcome by forming crystalline films on the surface of waveguides[2], however the inherent mode overlap for large systems is still problematic as the films formed are poly crystalline. Here we achieve mode separation by using single crystals and rapid modulation of the relative alignment of the terahertz polarization and the crystal axes by rotating the sample. The signal at the rotation frequency allows the rapid mapping out the optically active vibrational resonances along the different molecular and crystal orientations. 1. C.J. Strachan, P.F. Taday, D.A. Newnham, K.C. Gordon, J.A. Zeitler, M. Pepper, and T. Rades, J. Pharm. Sci., 2005. 94: p. 837-846. 2. J.S. Melinger, N. Laman, S.S. Harsha, S. Cheng, and D. Grischkowsky, J. Phys. Chem. A, 2007. 111: p. 10977-10987.   

Terahertz Time-Domain Spectroscopy of D$_{2}$O

The dielectric spectrum of D$_{2}$O between 15 GHz and 2 THz was measured using Terahertz Time-Domain Spectroscopy. The motivation of this work is to gain an understanding of liquid water dynamics on a molecular level. To achieve this, we use a correction to the dielectric response of polar molecules known as the reduced polarization. This correction allows us to relate the macroscopic quantity of the permittivity to the microscopic correlation function in a manner appropriate for polar liquids. Similar to previous studies on H$_{2}$O, evidence is shown of correlated and anti-correlated dipole-dipole interactions in liquid D$_{2}$O. More interestingly, the spectra also reveal dynamics that could be intimately related to the density anomaly of water.