Torsten Beck

High-Q conical polymeric microcavities

We report on the fabrication of high-Q microresonators made of low-loss, thermoplastic polymer poly(methyl methacrylate) (PMMA) directly processed on a silicon substrate. Using this polymer-on-silicon material in combination with a thermal reflow step enables cavities of conical geometry with an ultrasmooth surface. The cavity Q factor of these PMMA resonators is above 2×106 in the 1300 nm wavelength range. Finite element simulations show the existence of a variety of “whispering gallery” modes in these resonators explaining the complexity of the measured transmission spectra.

Direct laser writing for active and passive high-Q polymer microdisks on silicon

We report the fabrication of high-Q polymeric microdisks on silicon via direct laser writing utilizing two-photon absorption induced polymerization. The quality factors of the passive cavities are above 10(6) in the 1300 nm wavelength region. The flexible three-dimensional (3D) lithography method allows for the fabrication of different cavity thicknesses on the same substrate, useful for rapid prototyping of active and passive optical microcavities. Microdisk lasers are realized by doping the resist with dye, resulting in laser emission at visible wavelengths.

Low-threshold conical microcavity dye lasers

We report on lasing in rhodamine 6G-doped, conical polymeric microcavities with high quality factors fabricated on a silicon substrate. Threshold pump energies as low as 3 nJ are achieved by free-space excitation in the quasistationary pumping regime with lasing wavelengths around 600 nm. Finite element simulations confirm that lasing occurs in whispering gallery modes which corresponds well to the measured multimode laser-emission. The effect of dye concentration on lasing threshold and lasing wavelength is investigated and can be explained using a standard dye laser model.

On-chip microlasers for biomolecular detection via highly localized deposition of a multifunctional phospholipid ink

We report on a novel approach to realize on-chip microlasers, by applying highly localized and material-saving surface functionalization of passive photonic whispering gallery mode microresonators. We apply dip-pen nanolithography on a true three-dimensional structure. We coat solely the light-guiding circumference of pre-fabricated poly(methyl methacrylate) resonators with a multifunctional molecular ink. The functionalization is performed in one single fabrication step and simultaneously provides optical gain as well as molecular binding selectivity. This allows for a direct and flexible realization of on-chip microlasers, which can be utilized as biosensors in optofluidic lab-on-a-chip applications. In a proof-of-concept we show how this highly localized molecule deposition suffices for low-threshold lasing in air and water, and demonstrate the capability of the ink-lasers as biosensors in a biotin-streptavidin binding experiment.

Strongly confined, low-threshold laser modes in organic semiconductor microgoblets

We investigate lasing from high-Q, polymeric goblet-type microcavities covered by an organic semiconductor gain layer. We analyze the optical modes in the high-Q cavities using finite element simulations and present a numerical method to determine the cutoff thickness of the gain layer above which the whispering gallery modes are strongly confined in this layer. Fabricated devices show reduced lasing thresholds for increasing gain layer thicknesses, which can be explained by a higher filling factor of the optical modes in the gain layer. Furthermore, reduced lasing threshold is accompanied by a red-shift of the laser emission.

High-Q polymer resonators with spatially controlled photo-functionalization for biosensing applications

We demonstrate the applicability of polymeric whispering gallery mode resonators fabricated on silicon as biosensors. Optical measurements on the passive resonators in the visible spectral range yield Q-factors as high as 1.3×107. Local, covalent surface functionalization, is achieved by spatially controlled UV-exposure of a derivative of the photoreactive crosslinker benzophenone. Protein detection is shown using the specific binding of the biotin-streptavidin system.

PMMA-micro goblet resonators for biosensing applications

We report on a new type of whispering gallery mode (WGM) resonator made of out of low-loss polymer poly (methyl methacrylate) (PMMA). These optical cavities are fabricated using standard semiconductor processing methods in combination with a specific thermal reflow process. During this subsequent thermal treatment, surface tension leads to the goblet like geometry of the resonator and to an ultra smooth surface. The Q-factor of these goblet resonators is above 2·106 in the 1310 nm wavelength range. In order to demonstrate the applicability of the goblet resonators for bio sensing, Bovine Serum Albumin was detected by monitoring the shift of resonator modes due to protein adsorption.

Q-factor and density of optical modes in pyramidal and cone-shaped GaAs microcavities

GaAs pyramids on top of GaAs∕AlAs distributed Bragg reflectors (DBRs) are studied as candidates for microcavities with low mode volume. Photoluminescence spectra of single pyramids with embedded quantum dots show cavity modes with quality (Q-) factors of up to 700. Furthermore, to assess the complex mode structure in pyramids a finite-difference time-domain simulation with rotational symmetry is used to evaluate cavity modes in a cone on top of a DBR. A cone angle around 46° was identified for highest Q-factors. Based on our calculations, approaches to improve the light confinement are suggested which should result in microcavities with high Q-factors.

Strongly confining bare core CdTe quantum dots in polymeric microdisk resonators

We report on a simple route to the efficient coupling of optical emission from strongly confining bare core CdTe quantum dots (QDs) to the eigenmodes of a micro-resonator. The quantum emitters are embedded into QD/polymer sandwich microdisk cavities. This prevents photo-oxidation and yields the high dot concentration necessary to overcome Auger enhanced surface trapping of carriers. In combination with the very high cavity Q-factors, interaction of the QDs with the cavity modes in the weak coupling regime is readily observed. Under nanosecond pulsed excitation the CdTe QDs in the microdisks show lasing with a threshold energy as low as 0.33 μJ.

Lasing in dye-doped high-Q conical polymeric microcavities

We report on lasing in conical microcavities, which are made out of the low-loss polymer poly (methyl methacrylate) (PMMA) doped with the dye rhodamine 6G, and directly fabricated on silicon. Including a thermal reflow step during fabrication enables a significantly reduced surface roughness, resulting in low scattering losses of the whispering gallery modes (WGMs). The high cavity quality factors (above 2·106 in passive cavities) in combination with the large oscillator strength gain material enable lasing threshold energies as low as 3 nJ, achieved by free-space excitation in the quasistationary pumping regime. Lasing wavelengths are detected in the visible wavelength region around 600 nm. Finite element simulations indicate that lasing occurs in fundamental TE/TM cavity modes, as these modes have - in comparison to higher order cavity modes - the smallest mode volume and the largest overlap with the gain material. In addition, we investigate the effect of dye concentration on lasing wavelength and threshold by comparing samples with four different concentrations of rhodamine 6G. Observations are explained by modifying the standard dye laser model.