Brett A. Hooper

A review of the Stanford Mark III infrared FEL program

Abstract The performance of the Mark III infrared FEL with a new microwave gun will be reviewed. Operation of the accelerator is now close to design values. The Mark III has provided over 2000 hours of laser time to experiments in FEL physics, materials science and medical physics. Highlights of the experimental program will be presented and the new facility at Duke will be described.

Catheter for diagnosis and therapy with infrared evanescent waves

We have developed an optical delivery device (catheter) capable of transmitting broadband infrared light (IR wavelengths from 2 to 10 microm) for both diagnostic and therapeutic applications. The catheter is 1.68 mm in outer diameter and 1 m in length. It consists of two hollow glass waveguides coupled to a high-refractive-index optic tip. The IR light interacts with the tissue at the optic-tissue interface to measure the spectral signatures and perform therapy on the tissue at this interface. Fourier-transform IR spectrophotometer light is used to obtain the spectral signatures, and an IR free-electron laser (FEL) is used to study the therapeutic interaction of evanescent waves with the tissue. We present our catheter design; preliminary IR spectroscopy of aorta, blood, fatty tissue, and muscle; and IR FEL therapy on atheroslerotic aorta.

Fourth-harmonic generation in a single lithium niobate-crystal with cascaded second-harmonic generation.

Tunable second- and fourth-harmonic radiation was generated in a single 1-cm-long lithium niobate (LiNbO(3)) crystal with the Mark III infrared free electron laser at Duke University. The fundamental laser radiation was tuned from 2 to 2.5 µm, yielding 1-1.25-µm radiation (second harmonic) and 0.5-0.625-µm radiation (fourth harmonic). A fundamental-second-harmonic energy conversion efficiency of 66% and a fundamental-fourth-harmonic energy conversion efficiency of 3.3 × 10(-6) were measured. The maximum energy in the fourth harmonic was 3.3 nJ.

Classification of Atherosclerotic Rabbit Aorta Samples with an Infrared Attenuated Total Reflection Catheter and Multivariate Data Analysis

The strongly overlapping infrared absorption features of atherosclerotic and normal rabbit aorta samples as governed by their water, lipid, and protein content render the direct evaluation of molecular characteristics obtained from infrared (IR) spectroscopic measurements challenging for classification. We have successfully applied multivariate data analysis and classification techniques based on partial least squares regression (PLS), linear discriminant analysis (LDA), and principal component regression (PCR) to IR spectroscopic data obtained by using a recently developed infrared attenuated total reflectance (IR-ATR) catheter prototype for future in vivo diagnostic applications. Training data were collected ex vivo from atherosclerotic and normal rabbit aorta samples. The successful classification results on atherosclerotic and normal aorta samples utilizing the developed data evaluation routines reveals the potential of spectroscopy combined with multivariate classification strategies for the identification of normal and atherosclerotic aorta tissue for in vitro and, in the future, in vivo applications.

Performance of a Broadband Free‐Electron Laser and Preliminary Studies on Its Application to Biology and Medicine

Recent performance improvements in the free‐electron laser (FEL) have allowed it to operate at a level suitable for most applications in laser medicine. A brief description of the operation of a free‐electron laser will be presented. The performance desired in a medical FEL will be described and the performance to date of the Mark III mid‐infrared to visible FEL at Stanford University will be summarized. A series of experiments, accessing over 400 hours of FEL beam time, have been performed at Stanford as part of the University of Utah Laser Institute medical FEL program using the Mark III for both in vitro and in vivo studies of laser tissue interactions and materials science studies in semiconductor materials. Future improvements to the laser with the purpose of making it suitable for biomedical research and applications will be described.

Applications of harmonic generation of picosecond pulses from a free electron laser

Abstract Using nonlinear crystals we have produced harmonics of the Mark III infrared free electron laser at wavelengths from 495 nm to .1 μm. Optimization of energy conversion efficiency as a function of temporal walkoff, Poynting-vector walkoff, finite line width and length will be discussed. Preliminary results on an optimized coated crystal system will be presented and compared to theory.

Conversion efficiency optimization of free electron laser light

Abstract We have been investigating numerical simulations and analytical solutions of the evolution of the fundamental and harmonics of a laser field in a nonlinear crystal. We present results from analytical solutions and second harmonic generation experiments on the Mark III Infrared Free Electron Laser at Duke University. The approximations that are typically applied to the nonlinear wave equation do not completely model our experimental results.

Oxygen saturation dependent absorption and scattering of whole blood

We report on the scattering properties of oxygenated and de-oxygenated whole blood from 250-1000 nm. We determined the complex refractive index of oxygenated and de-oxygenated hemoglobin using Kramers Kronig analysis and Optical Coherence Tomography measurements. Combining these data with Mie theory, the scattering properties of oxygenated and deoxygenated whole blood were calculated. The results show strong oxygen saturation dependent scattering effects, which should be taken into account in data analysis of optical oxymetry.

Calculations of scattering by (de-)oxygenated whole blood

We report on the scattering properties of oxygenated and de-oxygenated whole blood from 250-1000 nm. We determined the complex refractive index of oxygenated and de-oxygenated hemoglobin using Kramers Kronig analysis and Optical Coherence Tomography measurements. Combining these data with Mie theory, the scattering properties of oxygenated and deoxygenated whole blood were calculated. The results show strong oxygen saturation dependent scattering effects, which should be taken into account in data analysis of optical oxymetry.

Shaping pulses using frequency conversion with a modulated picosecond free electron laser

Abstract Computer simulations and experiments indicate that we can shape the infrared picosecond pulses of the Mark III FEL in amplitude, frequency, and phase. Strongly modulated fundamental and second harmonic pulses have been generated by operating the Mark III FEL in the regime of strong sideband growth. In this paper, we present the results of simulations and experiments for second harmonic generation with fundamental inputs near 3 μm.