Session C4: Optics

Friday, October 17, 2008

High-quality dispersion management is critical for ultrafast optics. Grisms are a combination of diffraction gratings and prisms. We can use grisms for high-fidelity control of the spectral phase of ultrafast pulses, making systems much more compact and easy to adjust. While the spectral phase of a given system can be obtained with ray-tracing, analytic expressions are desirable for exploring and optimizing new designs. We show that we can analytically calculate the spectral phase of a range of grism-like structures by making a superposition of basic tilted window modules. For example, a prism pair can be described by starting with a tilted slab of glass, which defines the outer edges of the prism pair. The inner edges of the prism pair are then created by superposing a tilted slab of air, which removes glass between the prisms. We will discuss the applications of these grism designs to ultrafast amplifiers and pulse shapers.   

Friday, October 17, 2008

A number of optical applications in the deep ultra violet (DUV) range have limitations due to the absence of simple and reliable optical notch filters. This is important for resonant Raman applications that employ frequency agile laser illumination at many sequential DUV wavelengths. Our filter is based on widely known birefringent filter design originally proposed by \v{S}olc [\textit{I. \v{S}olc ``Birefringent chain filters'' JOSA }\textbf{\textit{55}}\textit{, p.621 (1965)}]. Rather than the transmission filter design of \v{S}olc, the \underline {\textbf{additive}} \v{S}olc filter (ASF) described here is suitable for narrow-line \textbf{rejection} ($<$ 1 nm), as dictated by the requirements of UVRR and other applications. We have designed and constructed such a filter and present test results. Finally, we present a design which allows fiber delivery of DUV illumination wavelengths, rejects the quartz Raman lines generated in the fiber, but then rejects the backscattered unshifted light from a target and passes the Raman lines generated by the target material.   

Friday, October 17, 2008

In applications such as measuring ionized He in the Earth's magnetosphere, it can be useful to be able to detect the relatively weak 30.4 nm line of He$^{+}$ over the stronger 58.4 nm line of neutral He. Work done previously at BYU has calculated that an aluminum/Y$_{2}$O$_{3}$ multilayer mirror should be effective at maximizing reflectance at 30.4 nm while minimizing the 58.4 nm line. We have measured the index of refraction of Y$_{2}$O$_{3}$ to improve the accuracy of these calculations and further our knowledge of Y$_{2}$O$_{3}$ between the wavelengths of 5 nm and 30 nm. We created a sample mirror using electron beam evaporation and measurements were taken using the Advanced Light Source at the Lawrence Berkeley National Laboratory. As this is the first direct measurement of the index of Y$_{2}$O$_{3}$ in this regime, it represents an improvement over previous data calculated using atomic scattering factors.   

Friday, October 17, 2008

We describe an extreme-ultraviolet (EUV) polarimeter that employs laser-generated high-order harmonics as the light source. The polarimeter is designed to characterize materials and thin films for use with EUV light. Laser high harmonics are highly directional with easily rotatable linear polarization, not typically available with other EUV sources. The harmonics have good wavelength coverage, potentially spanning the entire EUV from a few to a hundred nanometers. We have found that the polarimeter reflectance data agrees well with data obtained at the Advanced Light Source Synchrotron (Beamline 6.3.2) and with calculated data. We also describe a new measurement technique in which the ratio of p- to s-polarized reflectance is used to extract optical properties of samples, rather than the absolute reflectance.   

Friday, October 17, 2008

We suggest a simple system allowing for resonant enhancement of the refractive index under vanishing absorption. This system consists of two different three-level systems each driven by a far-detuned coherent field at one atomic transition and probed at the adjacent transition. The mechanism of index enhancement under vanishing absorption is clearly outlined in the dressed state picture. In this basis the system represents itself as a superposition of two effective absorbing and amplifying two- level schemes with atomic resonances positively and negatively detuned with respect to the probe field. Simple summation of the susceptibilities of these schemes at the frequency of the probe field results in enhanced refractive index accompanied by vanishing absorption. The experimental realization of such a system in a cell of Rb atoms is analyzed; and an upper limit estimate of the refractive index in such a system is provided.   

Friday, October 17, 2008

After photoexcitation, fluorescent molecules (fluorophores) can decay radiatively from the singlet excited state back to the ground state, or non-radiatively via a metastable triplet state. The triplet state has been associated with photobleaching whereby an irreversible photo-induced chemical change in the molecule suppresses further fluorescence. Photobleaching ultimately limits the number of fluorescence photons that can be emitted by a single fluorophore, and thus reduces the sensitivity of fluorescence microscopy. Recently it has been shown that if the triplet state is allowed to relax in the dark, the photobleaching rate can be reduced by up to a factor of $\sim $20. This was demonstrated using a picosecond pulsed laser with a variable repetition rate. We are investigating whether such a dark-state relaxation scheme can be implemented with a CW laser that is chopped on and off with microsecond-scale pulses using an acousto-optic modulator. This scheme is much simpler since it does not require the use of a pulsed laser. Preliminary data show that our dark-state relaxation scheme does lead to reduced photobleaching, and thus an increase in total fluorescence signal.   

Friday, October 17, 2008

The nonlinear propagation dynamics of 45 fs laser pulses in methanol solution has been studied with the two-photon fluorescence and the Z-scan technique. The competing nonlinear processes include self-focusing, self-phase modulation, filamention, intensity clamping and the two- photon absorption [1]. A systematic study has been performed of these nonlinear effects at different the laser powers. The simultaneous measurements of two-photon fluorescence and the transmission, displaying also continuum generation, were performed, to better understand the interplay between these effects [2]. In addition, the influence of a linear chirp of the laser pulse on the nonlinear propagation dynamics was investigated. \\[0pt] [1] H. Schroeder, S. L. Chin, Opt. Communications, 11, 1695-1703 (2002) \\[0pt] [2] M. C. Fischer, H. C. Liu, I. R. Piletic, and W. S. Warren, Opt. Express, 16, 4192-4205 (2008)