Vanderbilt Free Electron Laser Research Articles

Multiphoton absorption in germanium using pulsed infrared free-electron laser radiation

We report wavelength- and intensity-dependent transmission measurements of intense mid-infrared radiation from the Vanderbilt free-electron laser in single-crystal Ge(100) in the wavelength range of 2.8--5.2 \ensuremath{\mu}m. This range accesses both the direct and indirect energy gaps in Ge, requiring in each case either two or three photons (2PA or 3PA) for absorption. Large changes in the multiphoton absorption rate are seen at the direct-to-indirect and 2PA-to-3PA transitions. Photon interactions are dominated by free-carrier absorption (FCA), primarily due to holes. The entire absorption process is modeled with the two- and three-photon absorption coefficients ($\ensuremath{\beta}$ and $\ensuremath{\gamma}$) as fitting parameters. Using newly measured values of the low-intensity FCA cross sections, we find a best fit to the data at 2.8 \ensuremath{\mu}m that is in agreement with theory and previous measurements. We report a ratio of 175 for $\ensuremath{\beta}$ across the direct-to-indirect transition, and a ratio of 5 across the same transition for $\ensuremath{\gamma}$. These ratios are independent of systematic variations in free-carrier cross sections and beam diameter.

Controlling thermal damage of incisions using diamond, copper, and sapphire heat-conducting templates with and without cooling

We investigated the reduction of thermal damage to the surrounding tissue when laser incisions were made with and without using thermal conducting templates at room temperature and cooled to 5 degrees C. We used the Vanderbilt free-electron laser (FEL) at 5.4, 6.1, 6.45, and 7.7 microns. We also used a conventional continuous wave (CW) carbon dioxide laser at 10.6 microns. Incisions were made on 5x10 mm pieces of human breast skin (in vitro) and analyzed with histology. Computer morphometrics were used to measure the amount of thermal damage. All templates produced a statistically significant reduction in the thermal damage. Additionally, we showed that cooling the templates made a statistically significant greater reduction in the thermal damage. The cooled diamond template reduced the thermal damage from the FEL to 28% of the damage observed without a template. The same cooled template reduced the thermal damage from the CO(2) laser to 56% of the damage observed without a template. Lesser reductions were observed with the copper template and even less with the sapphire template. The sapphire template reduced the thermal damage to 39 and 67% of the damage observed without a template for the FEL and the CO(2) laser, respectively. These results indicate that unwanted lateral thermal damage from laser incisions can be reduced with cooled thermally conductive templates with the best results obtained with the diamond template, which is also the best thermal conductor.

Infrared near-field microscopy with the Vanderbilt free electron laser: overview and perspectives

Scanning near-field optical microscopy (SNOM) makes it routinely possible to overcome the fundamental diffraction limit of standard (far-field) microscopy. Recently, aperture-based infrared SNOM performed in the spectroscopic mode, using the Vanderbilt University free electron laser, started delivering spatially-resolved information on the distribution of chemical species and on other laterally-fluctuating properties. The practical examples presented here show the great potential of this new technique both in materials science and in life sciences.

Very high resolution near-field chemical imaging using an infrared free electron laserPresented at the LANMAT 2001 Conference on the Interaction of Laser Radiation with Matter at Nanoscopic Scales: From Single Molecule Spectroscopy to Materials Processing, Venice, 3–6 October, 2001.

High resolution infrared imaging of thin films and biological systems is one of the most challenging experimental problems in contemporary science and technology. In this work, we have for the first time successfully tested a novel high resolution approach, based on a spectroscopic version of scanning near-field optical microscopy (SNOM). The coupling of the Vanderbilt Free Electron Laser tunable infrared radiation to SNOM apparatus enabled us to clearly reveal different chemical constituents on a growth medium for biofilm. The images were obtained by SNOM detection of reflected 6.95 μm photons, corresponding to the stretch absorption of sulfur and nitrogen compounds, constituents of the growth medium. We attained a lateral resolution of 0.2 μm (λ/35), well below the diffraction limit of classical microscopy.

Spectroscopic scanning near-field optical microscopy with a free electron laser: CH2 bond imaging in diamond films.

Hydrogen chemistry in thin films and biological systems is one of the most difficult experimental problems in today's science and technology. We successfully tested a novel solution, based on the spectroscopic version of scanning near-field optical microscopy (SNOM). The tunable infrared radiation of the Vanderbilt free electron laser enabled us to reveal clearly hydrogen-decorated grain boundaries on nominally hydrogen-free diamond films. The images were obtained by SNOM detection of reflected 3.5 microm photons, corresponding to the C-H stretch absorption, and reached a lateral resolution of 0.2 microm, well below the lambda/2 (lambda = wavelength) limit of classical microscopy.

Beam Delivery of the Vanderbilt Free Electron Laser with Hollow Wave Guides: Effect on Temporal and Spatial Pulse Propagation

Beam Delivery of the Vanderbilt Free Electron Laser with Hollow Wave Guides: Effect on Temporal and Spatial Pulse Propagation

Beam Delivery of the Vanderbilt Free Electron Laser with Hollow Wave Guides: Effect on Temporal and Spatial Pulse Propagation

Hollow Wave Guides were evaluated as a beam delivery system for the Free Electron Laser (FEL) at Vanderbilt in preparation of surgical applications. They can transmit the mid-infrared wavelength range (2µm - 9µm) and tolerate the high peak intensity (>1014 W/m2) in the micropulse of the FEL. Changes in the temporal and spatial beam characteristics induced by the transmission through 1.5 meter Hollow Wave Guides with bore radii of 250 µm and 530 µm were investigated. Temporal broadening of the micro pulses was studied using intensity autocorrelation measurements and beam profile measurements were performed with a pyroelectric camera. Results demonstrate significant pulse broadening and development of higher order modes induced by sub-optimal coupling of the beam into the Hollow Wave Guide. Bending of the Hollow Wave Guide induced additional losses and reduced propagation of higher modes responsible for broadening the pulse. Calculations with a geometrical ray model support the findings on pulse broadening. Optimal coupling conditions are extremely critical for maximal transmission performance of the Hollow Wave Guide. Design consequences for a FEL delivery system are discussed.

Effective laser ablation of bone based on the absorption characteristics of water and proteins.

Collateral thermal damage and residual char formation have severely limited the use of conventional lasers in the surgical preparation of bony tissue. Thermal damage from lasers can be minimized by selecting a wavelength that is strongly absorbed and by reducing the laser pulse duration. In contrast to the fixed wavelengths and microsecond pulse intervals of conventional lasers, the Vanderbilt free electron laser (FEL) can be set at wavelengths ranging from 2.1 to 9.8 microm, and the pulse duration can be reduced to a series of 1 to 2 picosecond (ps) micropulses delivered in succession over intervals of 4 microsecond macropulses. The purpose of this study was to compare the morphologic and chemical changes induced in the near-surface region of bone following exposure to the FEL at 3.0, 6.1, and 6.45 microm wavelengths. The selected wavelengths coincide with the vibrational modes of proteins and water within bone. Under general anesthesia, laser incisions were made in the tibias of 14 skeletally mature rabbits. Laser parameters included 22.5+/-2.5 mJ/pulse delivered in individual 4 microsecond macropulses at a repetition rate of 30 Hz, focused to 200 microm and 500 microm spot sizes. Laser incisions were made using a computer-assisted surgical program, and control incisions were created with a bone saw. Rabbits were euthanized after the final incision, tibias recovered, and non-decalcified specimens processed for light microscopy. Separate samples were prepared for FTIR (Fourier transform infrared) photoacoustic spectroscopic analysis. The light microscopy sections of the ablation defects created at the differing wavelengths showed similar features, i.e., 2 zones of collateral damage, a zone generally < 10 mm of extensive thermal damage, and a wider zone of empty lacunae. In comparing treated and untreated surfaces, the spectral differences were limited to a relative decrease in intensity of the amide II and III absorption peaks in all laser-treated surfaces. Spectroscopic and histologic results indicated minimal thermal damage to bone ablated at 3.0, 6.1, and 6.45 microm wavelengths using the FEL (Fourier transform infrared) at the specified parameters. The FTIR photoacoustic spectroscopic results suggest that the char layer is limited to an area less than approximately 6 microm from the surface.

Infrared wavelength-selective photodesorption on diamond surfaces

We report first studies of photodesorption from diamond films using the Vanderbilt Free-Electron Laser at two infrared wavelengths: 3.5 μm, corresponding to localized absorption by C–H bonds at grain boundaries, and 5 μm, corresponding to two-phonon absorption by the bulk diamond. At 3.5 μm, we observe two photodesorption thresholds at fluences well below the onset of ablation. In contrast, at 5 μm, a single photodesorption threshold is identified at a fluence which is close to the second fluence threshold, observed at 3.5 μm. These experiments shed light on the physics of wavelength selective desorption through non-electronic channels.

Effect of dispersive optical elements in the Vanderbilt Free-Electron Laser resonator

The Brewster-angle windows used in the optical resonator of the Vanderbilt FEL become dispersive at wavelengths approaching the band-edge near 10 μm. The effect of this dispersion is included in a 1-D FEL simulation code as an integral transform of the optical pulse emerging from the wiggler in each pass of the FEL. The results indicate small amounts of chirp of the optical pulse, as might have been expected beforehand. In addition, the optical-resonator length-desynchronism curve is broadened, and the relatively sharp cutoff of oscillation near exact synchronism predicted by simulations without dispersion becomes a smooth rolloff with dispersion. These results agree with experimental observations.

Laser ablation as a function of the primary absorber in dentin.

Infrared transmission spectra of dentin reveal a broad absorption band between 6.0 and 7.0 microns composed of absorption peaks of water, collagen and carbonated hydroxyapatite. The nearly constant absorption and the existence of absorption peaks of different tissue components were used to investigate ablation as a function of the primary absorber. Laser ablation of dentin as a function of fluence was studied in the wavelength range between 6.0 and 7.5 microns using the Vanderbilt Free-Electron Laser (FEL). Depth and volume of the ablation crater were determined with a silicon replica method and subsequent confocal laser topometry. SEM investigations were performed on the irradiated surfaces. For the description of the experimental data an ablation model is developed. At all applied wavelengths we found a linear increase of ablation depth as a function of fluence above a threshold fluence. The lower absorption of dentin at 7.5 microns compared to the absorption at 6.0, 6.5 and 7.0 microns results in a greater ablation threshold. At 6.0, 6.5 and 7.0 microns wavelengths the ablation thresholds are comparable. The experimental data are in good agreement with an ablation model using a mean absorption coefficient of the target material. No thermal cracking is observed after ablation in dentin. The post ablative surface structure at 6.0 and 7.0 microns looks similar whereas at 7.5 microns the surface reveals a greater roughness. The ablation efficiency and threshold depend on the mean absorption but do not depend upon the chemical identity of the primary absorber in dentin. Calculations show that heat conduction during the laser pulse leads to a thermal equalization between the heated microstructures and surrounding tissue resulting in an ablation with little dependence on the primary absorber.

Laser Decompression Of The Facial Nerve

Laser decompression of the facial nerve is an experimental approach to evaluating the parameters for laser usage at a nerve-bone interface. This article summarizes the anatomic considerations and indications for facial nerve decompression, and the advantages and disadvantages of laser decompression of the facial nerve. Preliminary studies using the Vanderbilt free electron laser are described, and future directions for research are outlined.

Infrared matrix-assisted laser desorption and ionization by using a tunable mid-infrared free-electron laser

Initial results of infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectrometry of proteins by using the Vanderbilt free-electron laser as the source of selective vibrational excitation are reported. The ability of this laser to initiate desorption and ionization by excitation of specific vibrational modes is demonstrated. For the first time it is shown that IR-MALDI mass spectrometry at wavelengths other than those available from conventional fixed-frequency IR lasers, that is, 2.79 (Er:YSGG), 2.94 (Er:YAG), and 9.3-10.6 µm (CO2), is feasible and exhibits similar performance. IR-MALDI mass spectra were taken in the wavelength ranges 2.8-4 and 5.5-6.5 µm, covering the absorption bands of the O-H and C=O stretch vibrations typical of many organic compounds such as succinic acid, fumaric acid, or nicotinic acid, which were used as matrices in these studies. A comparison between these results and Er:YAG/YSGG MALDI data are given. The potential of IR-MALDI at wavelengths near the C=O stretch vibration and the possibilities for studies of the IR-MALDI mechanisms by using this kind of tunable source are discussed.

Free-electron laser wavelength-selective materials alteration and photoexcitation spectroscopy

The free-electron laser (FEL) has become an important tool for producing high-intensity photon beams, especially in the infrared. Synchrotron radiation's primary spectral domains are in the ultraviolet and X-ray region. FEL's are therefore excellent complimentary facilities to synchrotron radiation sources. While FEL's have seen only limited use in experimentation, recently developed programs at Vanderbilt University in Nashville, TN, are swiftly rectifying this situation. This review paper examines practical experience obtained through pioneering programs using the Vanderbilt FEL, which currently hosts one of the largest FEL materials research programs. Results will be discussed in three areas: two-photon absorption in germanium, FEL-assisted internal photoemission measurements of interface energy barriers (FELIPE), and wavelength-specific laser diamond ablation.

Ultra accurate measurements of interface parameters with free-electron laser

We used optical pumping by the Vanderbilt free-electron laser (FEL) and the technique of internal photoemission (IPE) to measure with an accuracy of nearly 5 meV the conduction-band discontinuity of semiconductor heterojunction interfaces as GaAlAsGaAs and GeGaAs. Very recently we demonstrated, using a titanium-sapphire pumped laser, that spatially resolved internal photoemission measurements could be performed on a Pt n-GaP Schottky barrier by a scanning near-field optics microscope within a spatial resolution of 100 nm. Shear-force and photocurrent X- Y images at different photon energies enable us to map the topography and the Schottky barrier height on the same surface. The topography's images, compared with the internal photoemission images, revealed zones where the morphology of the metallic layer was homogeneous, whereas the photocurrent was varying from place to place. Both results opened the possibility of measuring, in a simple and direct way, the local interface properties of real devices. A novel technique with submicrometric spatial resolution could be implemented: the spatially analyzed FEL-IPE (SAN FEL-IPE).

Free-electron lasers: reliability, performance, and beam delivery

The Vanderbilt free-electron laser (FEL) is a continuously tunable source of pulsed, mid-infrared radiation. FEL applications research has been underway for a decade. Recent experimental advances in FEL ablation of soft tissue indicate the potential for FEL-based protocols in surgery and medicine. In anticipation of these medical applications, the Vanderbilt FEL is being upgraded to meet the reliability and performance standards for a medical laser. Facilities for laser surgery have been constructed and equipped and medical delivery systems are being developed for pre-clinical and clinical research.

FREE-ELECTRON LASER SPECTROSCOPY OF SURFACES AND INTERFACES

Synchrotron-radiation sources have become, since the late 1960’s, one of the fundamental experimental tools for surface and interface research. Only recently, however, a related type of photon sources - the free-electron lasers (FELs) — has begun to make important contributions to this field. For example, FELs have been used to reach unprecedented levels of accuracy and reliability in measuring semiconductor interface energy barriers. We review some of the present and proposed experiments that are made possible by the unmatched brightness and broad tunability of infrared FELs. Practical examples discussed in the review are supplied by our own programs at the Vanderbilt Free-Electron Laser. We also briefly analyze the possible future development of FELs and of their applications to surface and interface research, in particular, the possibility of x-ray FELs.

Wavelength-selective laser ablation of diamond using hydrogen-impurity vibration modes

We have observed wavelength-dependent laser ablation of diamond films demonstrating the CH bond-stretching mode at 3.5 μm, which is important in the early stages of photodamage. In particular, there is a sharp drop in the ablation threshold near 3.5 μm. The measurements were made with the Vanderbilt Free-Electron Laser (FEL). We also observed the onset of morphological changes by using an atomic force microscope (AFM).

Computer‐assisted surgical techniques using the vanderbilt free electron laser

The Vanderbilt Free Electron Laser (FEL) is capable of lasing between 2.0 and 8.0 microns with a high peak intensity pulsed structure. The FEL is used to investigate potential applications in otolaryngology. Charring of temporal bones and thermal stress patterns in Plexiglas indicate thermal buildup at 20 and 10 Hz repetition rates of the laser. Also, transient temperature changes measured with thermocouples in a gelatin model reveal that significant heat production occurs at these laser repetition rates. To utilize the fastest laser repetition rates and maintain minimal lateral thermal damage, a computer-controlled scanning system was devised. The authors have also used the computer control with the carbon dioxide laser and experienced improved ablation.

Free-electron laser spectroscopy of semiconductors and interfaces

The broad tunability (1–10 μm) and megawatt regime peak intensity available from the Vanderbilt Free-Electron Laser is opening new avenues in semiconductor research. Initial experiments in nonlinear optical absorption and heterojunction band-edge discontinuity measurements are discussed in order to illustrate the flexibility of the free-electron laser as a research tool.