Karsten Plamann

Multimodal Nonlinear Imaging of the Human Cornea

PURPOSE To evaluate the potential of third-harmonic generation (THG) microscopy combined with second-harmonic generation (SHG) and two-photon excited fluorescence (2PEF) microscopies for visualizing the microstructure of the human cornea and trabecular meshwork based on their intrinsic nonlinear properties. METHODS Fresh human corneal buttons and corneoscleral discs from an eye bank were observed under a multiphoton microscope incorporating a titanium-sapphire laser and an optical parametric oscillator for the excitation, and equipped with detection channels in the forward and backward directions. RESULTS Original contrast mechanisms of THG signals in cornea with physiological relevance were elucidated. THG microscopy with circular incident polarization detected microscopic anisotropy and revealed the stacking and distribution of stromal collagen lamellae. THG imaging with linear incident polarization also revealed cellular and anchoring structures with micrometer resolution. In edematous tissue, a strong THG signal around cells indicated the local presence of water. Additionally, SHG signals reflected the distribution of fibrillar collagen, and 2PEF imaging revealed the elastic component of the trabecular meshwork and the fluorescence of metabolically active cells. CONCLUSIONS The combined imaging modalities of THG, SHG, and 2PEF provide key information about the physiological state and microstructure of the anterior segment over its entire thickness with remarkable contrast and specificity. This imaging method should prove particularly useful for assessing glaucoma and corneal physiopathologies.

Harmonic microscopy of isotropic and anisotropic microstructure of the human cornea

In this study we present combined third-harmonic generation (THG) and second-harmonic generation (SHG) microscopy images of intact human corneas, and we analyze experimentally and theoretically the origin of the THG signal. Multiharmonic microscopy provides detailed images of the cornea microstructure over its entire thickness. A component of the THG signal originates from cellular structures and another one originates from anisotropy changes between successive collagen lamellae in the stroma. This anisotropy-related signal can be specifically detected using circular incident polarization, and provide contrasted images of the stacking and tissue-scale heterogeneity of stromal lamellae. Forward-radiated THG and SHG signals are generally anticorrelated, indicating that maximum THG is obtained from lamellar interfaces whereas maximum SHG is obtained from within lamellae. Polarization-resolved THG imaging reflects the a ernate anisotropy directions of the lamellae. We present a model for THG imaging of layered anisotropic samples and numerical calculations that account for our observations.

ALL-SOLID-STATE NEODYMIUM-BASED SINGLE-FREQUENCY MASTER-OSCILLATOR FIBER POWER-AMPLIFIER SYSTEM EMITTING 5.5 W OF RADIATION AT 1064 NM

We demonstrate a master-oscillator fiber power-amplifier system consisting of a diode-pumped monolithic nonplanar ring oscillator as the master oscillator and a Nd:glass double-clad fiber as the power amplifier. The system emits up to 5.5 W of single-frequency radiation at a wavelength of 1064 nm with an M(2) value of ~1.1 . The optical emission spectrum is investigated with respect to the background of residual amplified spontaneous emission. Spectrally resolved amplitude-noise behavior is examined. Further power-scaling possibilities are discussed.

Ultrashort pulse laser surgery of the cornea and the sclera

The strongly localized interaction process of ultrashort laser pulses with tissue makes femtosecond lasers a powerful tool for eye surgery. These lasers are now routinely used in refractive surgery and other forms of surgery of the anterior segment of the eye. Several clinical laser systems also offer options for corneal grafting and the potential use of ultrashort pulse lasers in glaucoma surgery has been the object of several recent studies which have shown promising results. While devices aimed for interventions in clear tissue may be based on available solid state or fibre laser technology, the development of tools for surgery in more strongly scattering tissue has to account for the compromised tissular transparency and requires the development of optimized laser sources. The present paper focuses on surgery of clear and pathological cornea as well as sclera. It aims to give an overview over typical medical indications for ultrashort pulse laser surgery, the optics of the tissues involved, the available laser technology, the laser–tissue interaction process, and possible future developments.

Thermal properties of C/H-, C/H/O-, C/H/N- and C/H/X-grown polycrystalline CVD diamond

Abstract Over 60 CVD diamond films with thicknesses in the range 2–600 μm, grown from C/H, C/H/O, C/H/Cl and C/H/N gas mixtures by microwave plasma CVD, combustion flame synthesis and r.f. plasma torch CVD, were compared in terms of their thermal, morphological, Raman and luminescence data. Correlation diagrams reveal that the content of sp2-hybridized carbon is the main factor determining the thermal properties of the films. Other parameters, e.g. thickness, crystallinity and defects, only influence the thermal performance by changing the phase purity. The presence of oxygen and nitrogen in the CVD gas phase restricts the thermal conductivity of the films to values well below the 2200 ± 200 W m−1 K−1 achieved for polycrystalline films, 250 μm thick, grown from methane and hydrogen. Diamond films with thicknesses of less than 4 μm and thermal conductivities of more than 700 W m−1 K−1 were grown from C/H and C/H/O mixtures.

Thermal measurements on diamond and related materials

Abstract The outstanding thermal conductivity of CVD diamond gives rise to many potential industrial applications, necessitating the availability of reliable, standardized, thermal measurement methods. Although, in general, these measurements are in agreement for traditional materials, for CVD diamond different measurement methods can yield significantly different values for comparable or even identical samples. The aim of this work is to compare the methods and estimate to what extent these variations may be caused by the inhomogeneity and anisotropy of the samples themselves. Using a six-layer model to represent a typical CVD diamond layer, the potential signals for the principal methods are calculated and the results are compared.

Photothermal examination of the heat diffusion inhomogeneity in diamond films of sub-micron thickness

Abstract We present measurements of the in-plane thermal diffusivity of free-standing diamond films of thicknesses from 0.3 to 6 μm. The photothermal method used for the measurements is non-destructive and contact-free, and does not require any sample preparation. The measured values lie in the range of 0.1 to 1.0 cm 2 s −1 , which is an order of magnitude below typical results obtained on bulk material. The results show a strong dependence of the thermal diffusivity on the film thickness and morphology. Phonon scattering at grain boundaries was identified as the dominating process limiting the heat diffusion. The availability of sets of films with different characteristics allowed the determination of the spatial dependence of the heat diffusivity even in very thin membranes. The observed effects are in good accordance with the extrapolation of measurements previously performed on much thicker samples. The data are correlated to results gained by Raman spectroscopy and spectroscopic ellipsometry.

In situ monitoring of second-harmonic generation in human corneas to compensate for femtosecond laser pulse attenuation in keratoplasty

The application of femtosecond lasers in corneal transplant surgery requires high pulse energies to compensate for the strong optical scattering in pathological corneas. However, excessive energies deteriorate the quality of the incisions. The aim of this study is to demonstrate the dependence of side effects on local radiant exposure, numerical aperture, and tissue properties, to quantify the penetration depth of the laser for individual corneas, and to provide a method for optimizing the energy in the volume of the cornea. We examine histological and ultrastructural sections of clear and edematous corneas with perforating and lamellar incisions performed at different pulse energies. We demonstrate that the augmented energies in edematous corneas may result in unwanted side effects even when using high numerical apertures. The dependence of the laser beam penetration depth on pulse energy is evaluated by histology and an exponential decrease is observed. We show that the penetration length can be determined by evaluating the backscattered second-harmonic emission associated with the nonlinear optical properties of the tissue. This approach represents a noninvasive method for the in situ quantification of the laser beam attenuation, enabling us to adapt the pulse energy accordingly. Experiments using adapted energies show that the side effects are minimized.

Wall erosion and material transport to the Mark I carbon divertor of JET

Erosion and deposition at the vessel walls of the main chamber of JET were measured with long term samples during the whole operation period of the Mark I carbon divertor from April 1994 until March 1995. Assuming toroidal symmetry, about 70 g Ni + Cr + Fe was found to be eroded from the inner torus wall and about 55 g Be from the outer torus wall due to sputtering by energetic charge-exchange neutrals. Deposition has been measured on a poloidal section of the Mark I C divertor and on tiles from the inner and outer wall limiters. Eroded material is redeposited in the divertor and in the main chamber where it is found predominantly at the sides of the poloidal limiters and at the outer vessel wall. At these deposition-dominated areas in total about 25 g Ni + Cr + Fe, 28 g Be and 370 g C are found. The carbon originates mostly from the carbon limiters; additionally some may be originating from the divertor plates.

Microscopic measurements of the local heat conduction in polycrystalline diamond films

Abstract It is generally known that the microstructure of polycrystalline CVD diamond samples has a strong impact on their thermal properties. Despite the fact that nowadays layers can be deposited with macroscopic thermal conductivities or diffusivities rivalling those of type II natural diamonds, the samples are highly thermally inhomogeneous and sometimes show local values differing by up to two orders of magnitude. To examine these phenomena more closely, we present a microscopic photothermal measuring method for the local thermal diffusivity. We demonstrate the feasibility of diffusivity measurements at a sample surface of ca. 20 × 20 μm. We show results obtained on a reference sample of known diffusivity and present measurements on a small single crystal diamond, a local measurement at the substrate side of a CVD diamond layer, and a measurement of the diffusivity inside a microcrystal at the growth side of a CVD diamond layer.