Single-wavelength Methods Research Articles

Broadband Two-Photon Absorption Spectroscopy with Stimulated Raman Scattering as an Internal Standard.

Two-photon absorption (2PA) spectroscopy provides valuable information about the nonlinear properties of molecules. In contrast with single-wavelength methods, broadband 2PA spectroscopy using a pump-probe approach gives a continuous 2PA spectrum across a wide range of transition energies without tuning the excitation laser. This contribution shows how stimulated Raman scattering from the solvent can be used as a convenient and robust internal standard for obtaining accurate absolute 2PA cross sections using the broadband approach. Stimulated Raman scattering has the same pump-probe overlap dependence as 2PA, thus eliminating the need to measure the intensity-dependent overlap of the pump and probe directly. Eliminating the overlap represents an important improvement because intensity profiles are typically the largest source of uncertainty in the measurement of absolute 2PA cross sections using any method. Raman scattering cross sections are a fundamental property of the solvent and therefore provide a universal standard that can be applied any time the 2PA and Raman signals are present within the same probe wavelength range. We demonstrate this approach using sample solutions of coumarin 153 in methanol, DMSO, and toluene, as well as fluorescein in water.

Single-wavelength sO2 measurement based on Grüneisen-relaxation photoacoustic effect.

The photoacoustic effect-based sO2 measurement is attracting more and more attention due to its non-invasiveness and accuracy. Compared with the linear dual-wavelength method, the sO2 measurement based on single-wavelength excitation can be potentially applied with simplified system construction. However, the single-wavelength methods proposed in previous studies decreases the safety or lacks the in-depth resolution. This paper proposes a novel single-wavelength method based on the Grüneisen-relaxation (GR) nonlinear effects. It avoids the high fluence excitation with maintaining in-depth resolution and obtains the signals in hundreds of nanoseconds, simultaneously improving the safety and detection speed. The construction of a single laser source for GR effect generation makes the system stable. The sO2 quantification results of blood samples have a good consistency with the reference values. Our work provides a safer and faster measurement method, and a stable system, to promote its application in the clinical area.

A fast dual wavelength laser beam fluid-less optical CT scanner for radiotherapy 3D gel dosimetry I: design and development

Three dimensional dosimetry by optical CT readout of radiosensitive gels or solids has previously been indicated as a solution for measurement of radiotherapy 3D dose distributions. The clinical uptake of these dosimetry methods has been limited, partly due to impracticalities of the optical readout such as the expertise and labour required for refractive index fluid matching. In this work a fast laser beam optical CT scanner is described, featuring fluid-less and dual wavelength operation. A second laser with a different wavelength is used to provide an alternative reference scan to the commonly used pre-irradiation scan. Transmission data for both wavelengths is effectively acquired simultaneously, giving a single scan process. Together with the elimination of refractive index fluid matching issues, scanning practicality is substantially improved. Image quality and quantitative accuracy were assessed for both dual and single wavelength methods. The dual wavelength scan technique gave improvements in uniformity of reconstructed optical attenuation coefficients in the sample 3D volume. This was due to a reduction of artefacts caused by scan to scan changes. Optical attenuation measurement accuracy was similar for both dual and single wavelength modes of operation. These results established the basis for further work on dosimetric performance.

A surface plasmon resonance sensing method that can improve sensitivity

Based on surface plasmon resonance sensing theory, composite films of gold (Au) and silver (Ag) are used to improve sensing properties. A differential intensity detection method is also presented. Composite films with different thicknesses of Au and Ag are used for detection. Two light sources are used for differential detection, and the reference light path is determined to eliminate fluctuation of the light source. Detection theory is analyzed, and experiments are conducted to detect the refractive indices of glycerin solutions. The sensitivity of the differential-wavelength and single-wavelength methods is simulated. Results show that the use of composite film and the differential-wavelength method can enhance detection sensitivity. The rotating angle detection device and the spectrometer are not used in the experiments. The proposed detection device is small and cheap. The detecting range can be adjusted on the basis of the incident angles and wavelengths of the light sources. Thus, the proposed system can be extensively applied to detection.

Multi-Wavelength Photoacoustic Visualization of High Intensity Focused Ultrasound Lesions.

High intensity focused ultrasound (HIFU) thermal therapies are limited by deficiencies in existing image-guidance techniques. Previous studies using single-wavelength photoacoustic (PA) imaging have demonstrated that HIFU lesions generate contrast with respect to native tissues but have not sufficiently assessed lesion extent. The purpose of this study is to demonstrate feasibility of characterization of in vitro HIFU ablation lesion dimensions using 3D multi-wavelength PA imaging. Fresh porcine cardiac and liver tissue samples were embedded in agar phantoms and ablated using a 2.5 MHz small-animal HIFU system. Both 2D and 3D multi-wavelength photoacoustic-ultrasonic (PAUS) scans were performed in the near-infrared (NIR) range to characterize the change in the absorption spectrum of tissues following ablation and were compared to stained gross pathology to assess treatment margins and lesion extent. Comprehensive 2D multi-wavelength PA imaging yielded a spectrum in ablated tissue that did not display the characteristic local maximum in the optical absorption spectrum of deoxy-hemoglobin (Hb) near 760 nm. Two-dimensional tissue characterization map (TCM) images reconstructed from 3D TCM volumes reliably characterized lesion area and showed >70% area agreement with stained gross pathology. In addition, tissue samples were heated via water bath and concurrently interrogated with 2D PAUS imaging. PA signal exhibited an initial amplitude increase across all wavelengths, corresponding to an initial temperature increase, before then exhibiting a spectral change. This study suggests that multi-wavelength PA imaging has potential to obtain accurate characterization of HIFU lesion extent and may be better suited to guide HIFU ablation therapies during clinical treatments than single-wavelength methods.

High-contrast subcutaneous vein detection and localization using multispectral imaging

Multispectral imaging has shown promise in subcutaneous vein detection and localization in human subjects. While many limitations of single-wavelength methods are addressed in multispectral vein detection methods, their performance is still limited by artifacts arising from background skin reflectance and optimality of postprocessing algorithms. We propose a background removal technique that enhances the contrast and performance of multispectral vein detection. We use images acquired at visible wavelengths as reference for removing skin reflectance background from subcutaneous structures in near-infrared images. Results are validated by experiments on human subjects.

Streak-camera-based long-distance range finder with 10^−7 resolution

A novel optical distance meter with a pulsed yttrium aluminum garnet laser and a pair of streak cameras is developed. To minimize timing errors, the time scale is marked by an optical-registration-clock pulse. This assures an intrinsic resolution of 3 mm. For field applications, comparison of multiwavelength and single-wavelength methods confirms that the latter method is much better if carefully corrected. By use of the equipotential temperature criterion for correction of the atmospheric parameters, such as the atmospheric boundary layer effect and the temperature along the ray trajectory, a resolution of better than 1 cm is achieved over a range of 30 km. This result corresponds to a relative resolution of 3 × 10(-7). Typical ranging results in a geophysical application are shown for monitoring of the relative translational shift of the Asian (Eurasian) and the Philippine plates; the predicted shift is found to be absent.

Optical performance of amorphous hydrogenated carbon layers: range of optical constants achievable

The optical properties of differently deposited amorphous carbon layers have been investigated by spectrophotometric means from the middle infrared up to the visible spectral region. The optical constants of the layers could be calculated by a numerical procedure that combines features of single-wavelength methods and multiwavelength methods. Depending on the deposition technique applied, the optical constants of the layers have been found to vary over a wide range. Thus, refractive index values could be established between 1.6 and 2.9, while IR-absorption coefficients vary between 20/cm and 40,000/cm. The optical performance of the layers is thus nonuniform and may be influenced by the selection of suitable deposition parameters. Due to the potentially wide range of optical constants of amorphous carbon, various possibilities exist for the application as an optical thin film material. Applications such as selective absorber coatings for solar physical purposes are discussed as well as mechanically and chemically resistant antireflection coatings on high-refractive index semiconducting materials (silicon, germanium).

A hybrid method for determination of optical thin film constants

A new method for optical thin film constant determination from spectrophotometric data is presented. The method combines features of single-wavelength methods as well as multiwavelength methods and thus represents a hybrid method specially designed for the analysis of amorphous thin film materials with more or less involved absorption behaviour, such as amorphous hydrogenated carbon (a-C:H), amorphous hydrogenated silicon (a-Si:H) and related materials. The unambiguity and continuity of solution contours are guaranteed without the postulation of any analytical dispersion law, by introducing specially constructed mathematical terms into the merit function to be minimized. The accuracy in determining refractive indices and film thicknesses is comparable to that achieved by curve-fitting methods. Weak inhomogeneities in film thickness may be taken into account.

Comparison of Calculated Detection Limits in Molecular Absorption, Molecular Luminescence, Raman, Molecular Ionization, and Photothermal Spectrometry*

Detection limits for absorption, luminescence, photoionization, Raman, and photothermal molecular spectrometric methods are calculated from signal-to-noise expressions. The detection limits (in m−3) represent the best possible values by the various methods, since the limiting noises in each case were chosen. Molecular luminescence methods, with laser excitation, approach 1 species/cm3. Methods relying upon two (or more) laser wavelengths generally have much poorer detection limits than the simpler single-wavelength methods.