W. Lukosz

Integrated optical switches and gas sensors.

We have demonstrated new switching and gas-sensing effects in integrated optics using input and output grating couplers and Bragg reflector gratings with 1200 lines/mm on planar SiO(2)-TiO(2) waveguides. Switching is actuated by adsorption or desorption of water or other adsorbates on the waveguide surface through a change in the effective index of the guided modes under the grating. We derived theoretically the ultimate sensitivity limits of the grating devices employed either as switches or as gas sensors. Switching requires the adsorption and desorption, respectively, of less than one H(2)O monolayer. Sensors can detect variations in surface coverage of 1/100 of an H(2)O monolayer.

Embossing technique for fabricating integrated optical components in hard inorganic waveguiding materials.

We describe a novel embossing technique for fabricating integrated optical components in hard and resistant inorganic waveguiding materials of good optical quality. The desired structure is embossed into a dip-coated deformable gel film prepared from organometallic solutions. Subsequent heat treatment transforms it into inorganic hard oxide material. We have successfully fabricated surface-relief gratings with 1200 lines/mm on SiO(2)-TiO(2) waveguides (with refractive index n(F) approximately 1.8, thickness d(F) approximately 120 nm, and loss <1 dB/cm). The embossed gratings served as input and output grating couplers and as Bragg reflectors. We propose to fabricate other integrated optical components, for example, channel waveguides, with this method.

Preparation Of Planar Optical SiO 2 -TiO 2 And LiNbO 3 Waveguides With A Dip Coating Method And An Embossing Technique For Fabricating Grating Couplers And Channel Waveguides

Planar monomode and multimode Si02-Ti02 waveguides were prepared with a dip coating method from Liquicoat solutions supplied by E. MERCK. By varying the Si02:Ti02 mixture ratio the value of the refractive index nF of the waveguiding films on Pyrex glass substrates can be chosen to lie between nF-1.6 and nF =1.36 First results on the preparation of LiNb03 waveguides are also presented. Thicknesses, refractive indices and losses of the waveguides were determined at the blue-green Ar laser wavelengths and at the He-Ne laser wavelengths λ=632.8 nm and λ=1.153 μm. With an embossing technique we fabricated surface relief gratings on Si02-Ti02 wave-guides. We used them successfully as input grating couplers. We propose to use this emboss-ing technique to fabricate channel waveguides and other integrated optical components in inorganic hard waveguiding materials such as Si02-Ti02.

Lifetimes and radiation patterns of luminescent centers close to a thin metal film

The width d of an air gap between a 10-nm-thick luminescent europium benzoyltrifluoroacetone chelate film on a glass substrate and a 35-nm-thick layer of gold evaporated onto the base of a hemicylindrical high-index prism was varied between d >> lambda and d = 0. Thus the coupling of the luminescent centers (Eu(3+) ions, emission wavelength lambda = 612 nm) to the surface plasmons of the metal was varied. The luminescence lifetime decreased from tau = 520 microsec for d >> lambda to tau = 120 microsec for d = 0. By comparison of measured and calculated radiation patterns, i.e., of the angular distributions of the luminescent light emitted into the prism, the air gap d can be determined.

Optical bistability by photothermal displacement in prism coupling into planar waveguides.

We have identified photothermal displacement as a mechanism for optical bistability (OB) in prism coupling into absorbing planar waveguides: heating by the absorbed incoupled power causes a buckling of the waveguide, a reduction in air-gap width, and thus an increase in incoupling efficiency and a positive feedback. We developed a theory of this OB that qualitatively agrees well with experimental results obtained by coupling argon-laser light into indium-tin oxide layers as strongly absorbing waveguides. This new OB is quite different from dispersive OB in prism or grating couplers caused by a power-dependent effective guide index.

Total absorption of p-polarized light by surface plasma waves

We investigated when the reflectance R(p) for p-polarized light becomes exactly zero in an attenuated-total-reflection configuration where surface plasma waves at a metal–dielectric interface are excited. We particularly studied the reflection from coated metal mirrors at infrared wavelengths, where the complex refractive index n2 = n2′ + in2″ of the dielectric coating has a real part smaller than unity. When a certain relation between n2′ and n2″ is satisfied, the reflectance R(p) versus incidence angle has deep broad minima for a large range of coating thicknesses. Such mirrors would be efficient reflection polarizers and would have high hemispherical thermal emissivities.

Radiation-pressure-induced mechano-optical bistability.

We suggest and analyze a new radiation-pressure-induced bistability: A planar waveguide is suspended to swing as a torsional pendulum. A laser beam is coupled into the waveguide by a grating input coupler at one end of the waveguide and is outcoupled at the other end. The incoupling efficiency has a sharp angular resonance. When the pendulum is initially detuned from resonance, the torque exerted by the radiation pressure tends to drive it into resonance. The torque is proportional to the incoupling efficiency; therefore it depends on the angular position of the pendulum. This feedback leads to the mechano-optical bistability.

Self-pulsing in prism coupling into planar waveguides

Coupling argon-laser light into absorbing SiO(2)-TiO(2) waveguides, we observed at constant input power depending on the angle of incidence and the initial gap width, self-pulsing, bistability, or a special form of bistability with either a constant or a self-pulsing output. The systems dynamics is governed by two effects with different time constants, both caused by heating of the waveguide by the incoupled absorbed power: (1) thermal expansion of waveguide and prism, which reduces the coupling gap width and thus changes the incoupling efficiency, and (2) desorption of H(2)O molecules from the SiO(2)-TiO(2) layer, which changes the effective guide index.

Thermal Expulsion As A New Mechanism For Optical Bistability In Prism Coupling Into Planar Wavegaides

We investigated theoretically and experimentally a new type of cptical bistability (0B) in prism coupling into absorbing planar waveguides. Origin of this (B is the thermal expansion of the substrate (and the waveguiding film) and of the prism, due to heating by the part P = P of the incoupled power which is absorbed in the incoupling region. This expansion causes a reduction in the width d of the air gap between the prism coupler and the waveguide in the ompling region. Since the incoupling efficiency is a function of d, positive feedback, and therefore as, can occur. At the same input power Pa, the system has two bistable states, which differ in gap width d, in absorbed power Pa, and consequently in output (reflected) power PR. (see Fig. 1).

Self-Pulsing in Prism Coupling into Planar Absorbing SiO2-T1O2 Waveguides

Self-pulsing optical systems produce at constant input power P an output power PR (t)varying periodically with time t. Self-pulsing (SP) in nonlinear Fabry Perots and hybrid systems is well known. We reported the first known observation of SP in prism coupling into planar waveguides [1]. Recently we identified the physical mechanism of the SP and we shaved that SP occurs in a wider class of waveguide materials [2]. We also demonstrated that the same prism-waveguide system can exhibit SP, optical bistability (0B), and a special form of bistability (0B/SP), where at the same constant input power P the output power P is either constant in time or self-pulsing. In the present paper we present more experimental material and discuss the physical effects determining the systems behaviour.