Photothermal Absorption Measurements Research Articles

Absolute Absorption Measurements in Optical Coatings by Laser Induced Deflection

Absolute measurement of residual absorption in optical coatings is steadily becoming more important in thin film characterization, in particular with respect to high power laser applications. A summary is given on the current ability of the laser induced deflection (LID) technique to serve sensitive photo-thermal absorption measurements combined with reliable absolute calibration based on an electrical heater approach. To account for different measurement requirements, several concepts have been derived to accordingly adapt the original LID concept. Experimental results are presented for prominent UV and deep UV laser wavelengths, covering a variety of factors that critically can influence the absorption properties in optical coatings e.g., deposition process, defects and impurities, intense laser irradiation and surface/interface engineering. The experimental findings demonstrate that by combining high sensitivity with absolute calibration, photo-thermal absorption measurements are able to be a valuable supplement for the characterization of optical thin films and coatings.

Measurement and compensation of laser-induced wavefront deformations and focal shifts in near IR optics

We demonstrate the feasibility of passive compensation of the thermal lens effect in fused silica optics, placing suitable optical materials with negative dn/dT in the beam path of a high power near IR fiber laser. Following a brief overview of the involved mechanisms, photo-thermal absorption measurements with a Hartmann-Shack sensor are described, from which coefficients for surface/coating and bulk absorption in various materials are determined. Based on comprehensive knowledge of the 2D wavefront deformations resulting from absorption, passive compensation of thermally induced aberrations in complex optical systems is possible, as illustrated for an F-Theta objective. By means of caustic measurements during high-power operation we are able to demonstrate a 60% reduction of the focal shift in F-Theta lenses through passive compensation.

Enhanced laser-induced deflection measurements for low absorbing highly reflecting mirrors

A new concept enhances the capability of photo-thermal absorption measurements with transversal probe beam guiding by overcoming drawbacks such as a lack of sensitivity for materials with low photo-thermal response and/or round substrate geometry. The sandwich concept using the laser-induced deflection technique is introduced and tested for the investigation of highly reflecting (HR) coatings. The idea behind the sandwich concept is based on the decoupling of the optical materials for the pump and probe beams. This is realized by either placing a HR coated rectangular substrate in between two optical (sandwich) plates or attaching a HR coated thin round substrate onto one optical plate. For both configurations, the sandwich concept results in a strong increase in sensitivity for the measurement of HR coatings deposited onto photo-thermally insensitive substrates. Experiments reveal that for a CaF2 substrate, up to two orders of magnitude enhancement in sensitivity can be achieved.

Nano-lens diffraction around a single heated nano particle

The action of a nanoscopic spherically symmetric refractive index profile on a focused Gaussian beam may easily be envisaged as the action of a phase-modifying element, i.e. a lens: Rays traversing the inhomogeneous refractive index field n(r) collect an additional phase along their trajectory which advances or retards their phase with respect to the unperturbed ray. This lens-like action has long been understood as being the mechanism behind the signal of thin sample photothermal absorption measurements [Appl. Opt. 34, 41-50 (1995)], [Jpn. J. Appl. Phys. 45, 7141-7151 (2006)], where a cylindrical symmetry and a different lengthscale is present. In photothermal single (nano-)particle microscopy, however, a complicated, though prediction-wise limited, electrodynamic scattering treatment was established [Phys. Rev. B 73, 045424 (2006)] during the emergence of this new technique. Our recent study [ACS Nano, DOI: 10.1021/nn300181h] extended this approach into a full ab-initio model and showed for the first time that the mechanism behind the signal, despite its nanoscopic origin, is also the lens-like action of the induced refractive index profile only hidden in the complicated guise of the theoretical generalized Mie-like framework. The diffraction model proposed here yields succinct analytical expressions for the axial photothermal signal shape and magnitude and its angular distribution, all showing the clear lens-signature. It is further demonstrated, that the Gouy-phase of a Gaussian beam does not contribute to the relative photothermal signal in forward direction, a fact which is not easily evident from the more rigorous EM treatment. The presented model may thus be used to estimate the signal shape and magnitude in photothermal single particle microscopy.

Novel technique for the determination of hydroxyl distributions in fused silica

Bound OH- is one major unwanted absorbing impurity causing significant attenuation for certain wavelengths of laser radiation when propagating through fused silica. A wavelength tunable laser calorimeter (LCA) is used to detect dopants and other impurities in optical material. Also, a technique for photothermal absorption measurements is combined with the LCA set-up allowing for a lateral scan of the sample retaining an absolute calibration. Regarding inhomogeneities within the distribution of impurities the combination of both techniques provides a highly sensitive method to investigate in the lateral distribution of OH- content in fused silica.

Luminescence Decay and the Absorption Cross Section of Individual Single-Walled Carbon Nanotubes

The absorption cross section of highly luminescent individual single-walled carbon nanotubes is determined using time-resolved and cw luminescence spectroscopy. A mean value of approximately 1 x 10(-17) cm2 per carbon atom is obtained for (6,5) tubes excited at their second optical transition, and corroborated by single tube photothermal absorption measurements. Biexponential luminescence decays are systematically observed, with short and long lifetimes around 45 and 250 ps. This behavior is attributed to the band edge exciton fine structure with a dark level lying a few meV below a bright one.