Quantum State-Resolved Spectroscopic Techniques

Summary

Dual electro-optical frequency comb methods are being developed to enhance detection sensitivity and speed for spectroscopic studies ranging from the terahertz (THz) to the ultraviolet (UV) regions. These methods are used in state resolved spectroscopic studies to investigate the structure and dynamics of biomolecules and their response to the solvent, water. The experimental data from these studies are compared with predictions from state-of-the-art classical and quantum chemical methods to elucidate the fundamental forces and motions that define biomolecular structure and function and to expose the strengths and deficiencies inherent in the underlying models.

In the THz region, phase coherent chirped pulses are used to drive an electro-optic modulator in the near-IR and generate flat optical frequency combs in the THz region by mixing with an offset carrier in a photo-conductive switch. A second chirped pulse with a slightly different bandwidth is used to down-convert the THz comb to the radiofrequency (RF) region below 1 MHz by mixing with a slightly different offset carrier in a photoconductive receiver. The optical crossover method effectively eliminates the phase noise between the two near-IR driving lasers in the RF domain, enabling highly coherent RF combs that uniquely map the THz spectral features and dynamics. These methods are used in state resolved spectroscopic studies to investigate the structure and dynamics of biomolecules and their response to the solvent, water. The experimental data from these studies are compared with predictions from state-of-the-art classical and quantum chemical methods to elucidate the fundamental forces and motions that define biomolecular structure and function and to expose the strengths and deficiencies inherent in the underlying models.

Extension of the DOFC method into the UV region has been achieved by continuous-wave sum-frequency mixing in a nonlinear crystal for generation of combs over the 400 nm to 440 nm region. Phase coherent chirped pulses with slightly different bandwidths are used to drive phase modulators in separate legs on an interferometer, each equipped with an acousto-optic modulator for sideband separation at the detector. Two novel spectral interleaving schemes for UV comb generation are demonstrated by either separately up-converting each near-IR comb or by combining the two near-IR combs prior to sum frequency mixing in a non-linear crystal. In both cases, the dual UV combs are detected using photon counting technique where the UV spectral information is uniquely down converted to the RF domain by coherent mixing at a photomultiplier tube.

Description

Terahertz radiation interrogates the lowest frequency vibrational (phonon) modes of biomolecules. These modes characterize the incipient motions for large-scale conformational changes responsible for the backbone flexibility of protein, polynucleotide and polysaccharide. Thus, terahertz spectral features provide a sensitive probe of the subtle forces that impact the collective nuclear motions that extend over a large portion of the biomolecular framework. Experimental THz data on biomolecular systems that may also include water can provide validation and improvement of current force field and biomolecular dynamics models important in drug design. Further, because the energy available at terahertz frequencies is comparable to the thermal energy at room temperature, temperature dependent studies can quantify the vibrational anharmonicities of the hydrogen bonding networks associated with these large amplitude motions which are properties most challenging to accurately predict.

For examples, see State-resolved terahertz spectroscopy of biomolecules.

The UV region provides vital information in numerous frontier areas that include the emerging fields of quantum sensing and imaging. Recently, two novel spectral interleaving schemes for UV generation together with a faint sensing method based on photon counting have been used to investigate rubidium atoms important in quantum sensing and optical clocks, and other chemicals. For rubidium, the UV optical frequency combs obtained from the two differential chirped-pulse down conversion processes (i.e., the combined and separated beam approaches) lead to severely distorted line shape functions that arise from temporal magnification of the spectral response function in the frequency domain. However, it is shown that the natural (unperturbed) line shapes are easily recovered through the back-transformation to the time domain. Methods for DOFC wavelength extension to the deeper UV region are currently being explored using cavity enhanced nonlinear mixing techniques for quantum applications that may enhance the coincidence counting rates using entangled photons.

Major Accomplishments

• Dual comb THz spectra of formic acid in the temporally magnified frequency domain and unmagnified time domain obtained a function of sample pressure.
• THz spectra of dipeptide nanotubes have revealed the importance of mode coupling and decoupling on the permeability dynamics of water through hydrophobic pore regions.
• THz spectra undergo a dramatic transformation when water binds to hydrophilic sites in peptide crystals.
• Temperature dependent studies in the THz region quantify the anharmonic character of phonon modes in biomolecular crystals.

Associated Product(s)

JB95 Spectral fitting program