Frequency-domain Optical Spectroscopy Research Articles

Lifting restrictions on coherence loss when characterizing non-transparent hypersonic phononic crystals

Hypersonic phononic bandgap structures confine acoustic vibrations whose wavelength is commensurate with that of light, and have been studied using either time- or frequency-domain optical spectroscopy. Pulsed pump-probe lasers are the preferred instruments for characterizing periodic multilayer stacks from common vacuum deposition techniques, but the detection mechanism requires the injected sound wave to maintain coherence during propagation. Beyond acoustic Bragg mirrors, frequency-domain studies using a tandem Fabry–Perot interferometer (TFPI) find dispersions of two- and three-dimensional phononic crystals (PnCs) even for highly disordered samples, but with the caveat that PnCs must be transparent. Here, we demonstrate a hybrid technique for overcoming the limitations that time- and frequency-domain approaches exhibit separately. Accordingly, we inject coherent phonons into a non-transparent PnC using a pulsed laser and acquire the acoustic transmission spectrum on a TFPI, where pumped appear alongside spontaneously excited (i.e. incoherent) phonons. Choosing a metallic Bragg mirror for illustration, we determine the bandgap and compare with conventional time-domain spectroscopy, finding resolution of the hybrid approach to match that of a state-of-the-art asynchronous optical sampling setup. Thus, the hybrid pump–probe technique retains key performance features of the established one and going forward will likely be preferred for disordered samples.

Feasibility of direct digital sampling for diffuse optical frequency domain spectroscopy in tissue

Frequency domain optical spectroscopy in the diffusive regime is currently being investigated for biomedical applications including tumor detection, therapy monitoring, exercise metabolism and others. Analog homodyne or heterodyne detection of sinusoidally modulated signals has been the predominant method for measuring phase and amplitude of photon density waves that have traversed through tissue. Here we demonstrate the feasibility of utilizing direct digital sampling of modulated signals using a 3.6 gigasample/second 12 bit analog to digital converter. Digitally synthesized modulated signals between 50 MHz and 400 MHz were measured on tissue-simulating phantoms at six near-infrared wavelengths. An amplitude and phase precision of 1% and 0.6° were achieved during drift tests. Amplitude, phase, scattering and absorption values were compared with a well-characterized network analyzer-based diffuse optical device. Optical properties measured with both systems were within 3.6% for absorption and 2.8% for scattering over a range of biologically relevant values. Direct digital sampling represents a viable method for frequency domain diffuse optical spectroscopy and has the potential to reduce system complexity, size and cost.

Congruent MRI and near-infrared spectroscopy for functional and structural imaging of tumors.

We present a combined near-infrared diffuse optical spectroscopy (DOS) and Magnetic Resonance Imaging (MRI) system for the study of animal model tumors. A combined broadband steady-state and frequency domain optical spectroscopy apparatus was integrated with the MRI. The physiological properties of tissue rendered by MRI, including vascular volume fraction and water, were compared with chromophore concentrations as determined from the parameters obtained by optical measurements. DOS measurements provided oxy-hemoglobin, deoxy-hemoglobin, and water concentration locally in tumors. A method for co-registration of the information obtained by both modalities was developed. Using Monte Carlo simulations, the optically sampled volume was superimposed on the MR images, illustrating which tissue structure was probed optically. Finally, two optical contrast agents, indocyanine green (ICG) and methylene blue (MB), were employed and their kinetics were measured by DOS system from different locations on the tumor and compared with Gd-DTPA enhancement maps obtained from MRI.

Role of frequency domain optical spectroscopy in the detection of neonatal brain hemorrhage ? A newborn piglet study

Inability of continuous wave (CW) optical spectroscopy to measure changes in scattering, and the use of an arbitrary rather than an actual baseline, makes the CW method highly susceptible to errors that can lead to a false-positive or false-negative diagnosis. Our objective was to assess whether, and to what extent, the use of quantitative frequency domain spectroscopy would improve our ability to detect and monitor the development of brain hemorrhage. A dual-channel frequency-domain tissue spectrometer (Model 96208, ISS, Inc., Champaign, IL) was used to monitor the development of experimental subcortical and periventricular-intraventricular hemorrhage (IVH) in 10 newborn piglets (blood injection model). The multidistance approach was employed to calculate the absorption and reduced scattering coefficients and hemoglobin changes from the ac, dc, and phase values acquired at four different source-detector distances and at 752 nm and 830 nm. There were significant absorption and scattering changes in the subcortical hematoma (n = 5) and the IVH groups (n = 5). The smallest detectable amount of blood in the brain was 0.04 ml. Changes associated with subcortical hematoma were several times higher than those associated with IVH, and correlated better with the estimated cross-sectional area of the hematoma than with the volume of the injected blood. As opposed to IVH, there was a significant absorption difference between the injured (subcortical hematoma) and normal side of the brain, probably because in case of IVH a significant volume of the injected blood had accumulated/spread beyond the reach of the probe. Clearly, frequency-domain spectroscopy cannot increase our ability to quantify the volume (size) or the oxygenation of the injected blood, especially in the case of IVH. However, the ability to quantify the baseline tissue absorption and scattering would significantly improve diagnostic performance, and may allow for early identification and treatment of neonatal brain hemorrhage.

Role of Frequency Domain Optical Spectroscopy in the Detection of Neonatal Brain Hemorrhage—A Newborn Piglet Study

Role of Frequency Domain Optical Spectroscopy in the Detection of Neonatal Brain Hemorrhage—A Newborn Piglet Study

Tunneling dynamics of delocalized protons in benzoic acid dimers: A study of the temperature dependence by time and frequency domain optical spectroscopy

The translational tunneling of the acid protons in benzoic acid dimers coupled to a dye molecule is investigated in single crystals and at liquid helium temperatures. The optical transitions of the dye are used to monitor the dynamics of protons delocalized over the two wells of the double-minimum potential, which governs their motion. Using transient grating as well as optical hole-burning techniques in protonated and deuterated crystals the population relaxation rate and its temperature dependence is determined between 1.35 and 4.2 K. It is found that this rate is immeasurably small at the lowest temperatures (⩽ 107 s −1) As the temperature is raised the rate increases very rapidly and attains at 4.2 K a value of 2.5 × 10 9 s −1. This value contrasts with the rates 2 × 10 −8 −4 × 10 8 s −1, measured previously in systems where the protons of the benzoic acid dimers are localized by the larger environment induced asymmetry of the double-well potential. These results demonstrate the importance of multi-(two-)photon processes in the relaxation of delocalized protons which dominate the one-phonon processes at higher temperatures.