Matti Kaivola

Spectral broadening of femtosecond pulses into continuum radiation in microstructured fibers

We report on the influence of the choice of the pump wavelength relative to the zero-dispersion wavelength for continuum generation in microstructured fibers. Different nonlinear mechanisms are observed depending on whether the pump is located in the normal or anomalous dispersion region. Raman scattering and the wavelength dependence of the group delay of the fiber are found to play an important role in the process. We give an experimental and numerical analysis of the observed phenomena and find a good agreement between the two.

Supercontinuum generation in a highly birefringent microstructured fiber

We present experimental results on supercontinuum generation in a highly birefringent microstructured fiber. We show that such a fiber offers clear advantages for continuum generation over weakly birefringent fibers. In particular, the polarization is preserved along the fiber for all the spectral components. Furthermore, the two eigenpolarizations exhibit different dispersion characteristics, which provide a convenient way of tuning the properties of the generated continuum. We investigate the impact of the pump wavelength and pulse duration on the continuum and use the results to generate an ultrabroadband continuum extending from 400 to 1750 nm.

Enhanced bandwidth of supercontinuum generated in microstructured fibers.

Enhancement of the bandwidth of supercontinuum generated in microstructured fibers with a tailored dispersion profile is demonstrated experimentally. The fibers are designed to have two zero-dispersion wavelengths separated by more than 700 nm, which results in an amplification of two dispersive waves at visible and infrared wavelengths. The underlying physics behind the broad continuum formation is discussed and analyzed in detail. The experimental observations are confirmed through numerical simulations.

Laser linewidth measurements using self-homodyne detection with short delay

We compare delayed self-homodyne and self-heterodyne detection in the case of a time delay of the order of the coherence time of the laser. Both methods are found to yield similar results but the homodyne method is simpler to set up and gives a markedly better signal-to-noise ratio. With the short delay the low-frequency 1/f-noise of the laser is effectively filtered out and the measurement provides a value for the Lorentzian contribution to the laser linewidth which is often of prime interest in coherent optical communication systems.

Solid-state dye laser with modified poly(methyl methacrylate)-doped active elements

Laser generation with modified poly(methyl methacrylate) (MPMMA)-doped matrices with several different types of Rhodamine-based dyes was obtained. Pumping with a frequency-doubled Q-switched Nd:YAG laser was used. During the experiments, high conversion efficiency was achieved. The strong nonlinear dependence of the operating lifetime and the conversion efficiency of material tested on the pump-pulse-repetition rate was observed. Possible mechanisms responsible for the conversionefficiency drop and the useful lifetime of the material are discussed.

Electromagnetic multipole theory for optical nanomaterials

Optical properties of natural or designed materials are determined by the electromagnetic multipole moments that light can excite in the constituent particles. In this paper, we present an approach to calculating the multipole excitations in arbitrary arrays of nanoscatterers in a dielectric host medium. We introduce a simple and illustrative multipole decomposition of the electric currents excited in the scatterers and connect this decomposition to the classical multipole expansion of the scattered field. In particular, we find that completely different multipoles can produce identical scattered fields. The presented multipole theory can be used as a basis for the design and characterization of optical nanomaterials.

Optical Interference Lithography Using Azobenzene‐Functionalized Polymers for Micro‐ and Nanopatterning of Silicon

Azobenzene-containing polymers (azopolymers) have attracted great interest due to their potential use in various technological applications, including holographic recording, photomechanics, diffractive optics, and microand nanopatterning. [ 1–7 ] These applications are brought about by the effi cient and reversible photoisomerization of the azobenzene moieties between a rodlike trans-state and a bent cis-state, which is accompanied by various changes in the properties of the material system both at molecular and macroscopic levels. [ 7 , 8 ] Remarkably, the photoisomerization can give rise to signifi cant surface mass transport phenomena, allowing one-step inscription of high-quality, thermally stable photoinduced surface patterns onto the azopolymer fi lm. Since its fi rst demonstration in 1995, [ 9 , 10 ] the photoinduced surface-relief grating (SRG) formation has been intensively investigated in various types of azobenzene-containing materials. [ 11–15 ] The phenomenon continually keeps fi nding new potential applications. Recently, the SRGs have been combined with organic solar cells and lasers, [ 16 , 17 ] carbon-based nanomaterials, [ 18 ] and block-copolymer nanostructures. [ 19 ] Furthermore, in recent years, azopolymer-based patterns have been increasingly used as templates for fabricating periodic arrays of, e.g., titanium dioxide, [ 20–22 ] indium tin oxide, [ 23 ] and metallic [ 24 , 25 ]

Scanning heterodyne laser interferometer for phase-sensitive absolute-amplitude measurements of surface vibrations

We describe a scanning heterodyne interferometer for imaging surface vibrations with a wide frequency range, with current electronics, up to 6GHz. The heterodyne operation facilitates measurement of absolute amplitude and phase of the surface vibration without calibration. Currently, the setup allows detection of vibration amplitudes down to ∼1pm with a lateral resolution of <1μm. The interferometer is designed to accommodate the different sample types, e.g., surface and bulk acoustic wave devices and micromechanical resonators. The absolute-amplitude and phase information allows for a thorough characterization of surface vibrations in such components and provides direct information of the vibration fields not obtainable via electrical measurements.

Efficient surface-relief gratings in hydrogen-bonded polymer-azobenzene complexes.

Azobenzene-containing polymers have been extensively investigated due to the unique response of the azobenzene moiety to light fields. The trans-cis-trans photoisomerization of azobenzene can be utilized to induce molecular angular reorientation as well as macroscopic mass migration in the material. These motions can result in large and stable in-plane anisotropy, nonlinear optical activity, strong refractive-index modulation, and surface-relief gratings, showing great potential for rewritable optical recording and various photonic applications [1]. Stable control over the molecular alignment as well as the efficient inscription of surface-relief gratings typically requires chemical linkage between the photoactive chromophores and the host polymer. We demonstrate that surface-relief gratings (SRGs) with modulation depth of up to 440 nm (Fig. 1a) can be inscribed through the use of hydrogen bonding between the polymer and the chromophores [2]. Such spontaneous linkage simplifies the preparation process compared to covalent functionalization while still being strong enough to induce macroscopic motions of the polymer chains.

Photoinduced surface-relief gratings in films of supramolecular polymer–bisazobenzene complexes

We demonstrate that hydrogen-bonded polymer–azobenzene complexes, containing nonpolar bisazobenzene chromophores, yield efficient photoinduced surface-relief gratings over a wide chromophore concentration range. The efficient grating formation is thought to be enabled by the lack of spacer between the chromophores and the polymer backbone, which prevents the formation of liquid-crystalline phases even at high chromophore concentrations. The surface-modulation depth can be tuned by varying the chromophore content, and the deepest gratings (625 nm) are obtained, when every second polymer repeat unit is occupied by a chromophore. The gratings are thermally erasable by heating the samples above their glass-transition temperature, and no irreversible photodegradation of the chromophores is observed.