Christoph Hunziker

Configurationally locked, phenolic polyene organic crystal 2-{3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene}malononitrile : linear and nonlinear optical properties

We show that the organic polyene crystal 2-{3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene}malononitrile (OH1) is a new material with excellent nonlinear optical and electro-optical properties. The principal refractive indices and the absorption constants have been determined over a broad wavelength range from 0.6to2.2 μm. OH1 shows a large birefringence of Δn>0.55 and a low absorption of α<1 cm−1 in a large wavelength range between 800 and 1400 nm. Furthermore, the nonlinear optical tensor elements d333=120 ± 10 pm/V, d223=13 ± 2 pm/V, and d322=8.5 ± 2 pm/V have been determined by the Maker-fringe technique at the fundamental wavelength of 1.9 μm. The electro-optic coefficients r333, r223, and r113 have been measured at the wavelengths of 633, 785, 1064, and 1319 nm, and it is shown that the dispersion can be described by a one-oscillator model. OH1 is a promising material for nonlinear optical and electro-optical applications due to the large nonlinear coefficient d333, the large electro-optic coefficient r333=109 ± 4 pm/V at 632.8 nm, the favorable crystal growth properties, as well as the orthorhombic crystal symmetry (point group mm2), which renders crystal preparation and orientation much simpler than in most organic nonlinear optical crystals.

Electro-optic single-crystalline organic waveguides and nanowires grown from the melt

Organic nonlinear optical materials have proven to possess high and extremely fast nonlinearities compared to conventional inorganic crystals, allowing for sub-1-V driving voltages and modulation bandwidths of over 100 GHz. Compared to more widely studied poled electro-optic polymers, organic electro-optic crystals exhibit orders of magnitude better thermal and photochemical stability. The lack of available structuring techniques for organic crystals has been the major drawback for exploring their potential for photonic structures. Here we present a new approach to fabricate high-quality electro-optic single crystal waveguides and nanowires of configurationally locked polyene DAT2 (2-(3-(2-(4-dimethylaminophenyl)vinyl)-5,5-dimethylcyclohex-2-enylidene)malononitrile). The high-index-contrast waveguides (delta(n) = 0.54 +/- 0.04) are grown from the melt between two anodically bonded borosilicate glass wafers, which are structured and equipped with electrodes prior to bonding. Electro-optic phase modulation is demonstrated for the first time in the non-centrosymmetric DAT2 single crystalline channel waveguides at a wavelength of 1.55 microm. We also show that this technique in combination with DAT2 material allows for the fabrication of single-crystalline nanostructures inside large-area devices with crystal thicknesses below 30 nm and lengths of above 7 mm.

Extremely large nonresonant second-order nonlinear optical response in crystals of the stilbazolium salt DAPSH

We report on the extremely large nonresonant quadratic optical nonlinearity of the stilbazolium salt trans-4′-(dimethylamino)-N-phenyl-4-stilbazolium hexafluorophosphate (DAPSH). The phenyl-pyridinium chromophores in DAPSH crystals grown from acetone solution pack with a highly aligned polar order, resulting in a very large birefringence, Δn=1.17±0.06 at λ=0.83 μm and Δn=0.83±0.04 at λ=1.55 μm. More importantly, this leads to an extremely large diagonal quadratic susceptibility with the nonlinear optical coefficient for second-harmonic generation reaching up to d111=290±40 pm/V at 1.907 μm fundamental wavelength, which presents a considerable improvement with respect to the presently best material trans-4′-(dimethylamino)-N-methyl-4-stilbazolium tosyate (DAST) with d111=210±55 pm/V at λ=1.907 μm. The result is in agreement with the preferential packing of the chromophores and the previous studies demonstrating higher microscopic nonlinearity of the chromophores in DAPSH compared to that of DAST.

Amplified spontaneous emission in para-sexiphenyl bulk single crystals

Amplified spontaneous emission (ASE) in single crystals of para-sexiphenyl grown in the β phase by vapor transport is demonstrated upon photoexcitation at 355nm. Excitation with femtosecond laser pulses leads to dual wavelength ASE at 427 and 450nm, while excitation with nanosecond laser pulses leads to ASE only at 450nm. The threshold fluences for nanosecond and femtosecond pumpings are determined to be 885 and 110μJ∕cm2, respectively. Additionally, the singlet exciton annihilation rate is measured to be γss=(1.2±0.1)×10−6cm3s−1.

Highly ordered thin films of a bis(dithienothiophene) derivative

We report growth as well as structural, optical, and charge-transport properties of highly ordered thin films of a fused thiophene derivative, 6,6′-di-n-hexyl-[2,2′]bi(dithieno[3,2-b:2′,3′-d]thiophenyl) (DH-BDT). DH-BDT was synthesised with the aim of fabricating high quality, molecularly aligned thin films for organic electronic applications. Structural phase transitions are observed upon heating from room temperature, indicating the existence of three different polymorphs, denoted α, β and γ. The transition temperatures are Tα→β = 92 ± 2 °C and Tβ→γ = 140 ± 2 °C. The growth of thin films of DH-BDT can be controlled to afford either the β- or the γ-phase. Thermal annealing leads to the formation of large single crystalline grains with areas as large as 7 × 104 μm2. The molecules in the γ-phase are cofacially aligned and show horizontally layered thin film growth. The good crystallinity and the large grain size in the γ-phase lead to hole mobilities up to μγ = 0.09 cm2 V−1 s−1, based on the measurement of space-charge-limited currents (SCLC). The β-phase consists of mutually shifted molecules, resulting in a lower hole mobility of 4.4 × 10−5 cm2 V−1 s−1 but improving the relative luminescence quantum yield by 140% relative to that of the γ-phase. Field-effect transistors of DH-BDT in the γ-phase have been fabricated and yield hole mobilities which are of the same order of magnitude as the SCLC mobilities.

Electro-optic tuning and modulation of single-crystalline organic microring resonators

We present, for the first time to our knowledge, the fabrication and electro-optic (EO) tuning of single-crystalline organic microring resonators. In recent years, optical microring resonators have proven to be highly suitable building blocks for the realization of very large-scale integrated photonic circuits. In particular, microresonators based on organic materials are very promising for ultrafast EO applications, due to the electronic nature of the EO response preserving the modulation performances beyond 100 GHz. In contrast to polymer waveguiding structures realized previously, our crystalline thin-film devices feature an excellent long-term stability of the chromophore orientation and superior photochemical stability, and they do not require high-field poling prior to operation. The introduced thin-film fabrication method significantly reduces fabrication complexity of organic crystalline EO waveguides, compared to previously developed techniques. We have fabricated crystalline COANP (2-cyclo-octylamino-5-nitropyridine) microring resonators with resonance contrast up to 10 dB, ring waveguide propagation losses of about 10 dB/cm, a free spectral range of 1.6 nm, a finesse of up to 20, and a corresponding Q-factor of about 20,000, measured in the telecom wavelength range around 1.55 μm. We have demonstrated resonance wavelength tuning at the rate of 0.13 GHz/V(1.1 pm/V).

Limitations of proximity-effect corrections for electron-beam patterning of planar photonic crystals

We investigate the patterning accuracy limits of electron-beam lithography with different proximity-effect correction (PEC) methods applied to the fabrication of planar photonic crystal structures (PPCS). Energy-intensity distribution simulations reveal that conventional energy-equalization PEC techniques present a lower limit of the best attainable hole-radius variation of 1% for a generic PPCS, while a method proposed by Watson (midpoint-equalization PEC) should inherently account for beam broadening and theoretically can reach perfect accuracy. Simulation results are verified experimentally. Additionally, we introduce a new method to determine the beam-broadening parameter . We compare energy-equalization PEC and midpoint-equalization PEC regarding the impact of geometrical key parameters of PPCS on achievable patterning accuracy, and show that proximity effects impose severe limitations on the patterning of structures with large fill ratios and/or small lattice constants. Furthermore, we perform a sensitivity analysis of both PEC methods on the proximity parameters and show that overestimation of the backscatter efficiency can actually improve the lithographic accuracy of the energy-equalization method and mimic the midpoint-equalization PEC method to a certain degree.

Fabrication and phase modulation in organic single-crystalline configurationally locked, phenolic polyene OH1 waveguides

A novel and promising technique for the fabrication of electro-optically active single crystalline organic waveguides from 2-{3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene}malononitrile (OH1) is presented. OH1 is an interesting material for photonic applications due to the large electro-optic coefficients (r333 = 109+/-4 pm/V at 632.8 nm) combined with a relatively high crystal symmetry (orthorhombic with point group mm2). Due to the very favorable growth characteristics, large-area (> 150 mm(2)) single crystalline thin films with very good optical quality and thickness between 0.05-10 microm have been grown on amorphous glass substrates. We have developed and optimized optical lithography and reactive ion etching processes for the fabrication of wire optical waveguides with dimensions of w x h = 3.4 x 3.5 microm(2) and above. The technique is capable of producing low loss integrated optical waveguides having propagation losses of 2 dB/cm with a high refractive index contrast between core-cladding and core-substrate of delta n = 1.23 and 0.72, respectively at 980 nm. Electro-optic phase modulation in these waveguides has been demonstrated at 632.8 nm and 852 nm. Calculations show that with an optimized electrode configuration the half-wave voltage x length product V(pi) x L can be reduced from 8.4 Vcm, as obtained in our device, to 0.3 Vcm in the optimized case. This allows for the fabrication of sub-1 V half-wave voltage, organic electro-optic modulators with highly stable chromophore orientation.

Hybrid organic crystal/silicon-on-insulator integrated electro-optic modulators

We demonstrate electro-optic modulation in hybrid organic-crystal/silicon photonic waveguides. The organic material is the newly developed organic crystal OH1 with very high electro-optic figures of merit, n3r = 530 pm/V at 1319 nm, and the processing possibilities considerably improved compared to previous high-nonlinearity organic crystals. We have developed an epitaxial-like solution growth of OH1 on various substrates and fabricated electro-optic modulators with electro-optic functionality either directly in OH1 wire waveguides or in OH1 active cladding of silicon wire waveguides. OH1-based waveguides offer a great potential for high-bandwidth, sub-1-V half-wave voltage, hybrid organic/silicon electro-optic modulators with high electro-optic activity and stability.

Organic electro-optic single crystalline films for integrated optics

We have fabricated organic electro-optic single crystalline thin films on various inorganic substrates. A high refractive index contrast of up to Δn = +0.6 at 1.55 μm with respect to glass substrates and up to Δn = -1.9 at 1.55 μm with respect to silicon substrates has been achieved. The single crystalline films can be grown quasi-epitaxially without lattice matching and also on amorphous substrates providing appropriate interface interactions and solid-liquid phase equilibrium conditions. The thickness of the single-crystalline films can vary between less than 30 nm and above 5000 nm; they are therefore appropriate for optical waveguiding structures, as well as nano-size electro-optic structures needed for future nanophotonics. Several organic electro-optic crystalline materials have been employed using solution or melt-based processing. The techniques are suitable for the fabrication of conventional wire electro-optic waveguides, silicon-organic hybrid electro-optic waveguides, as well as more complex organic-inorganic structures such as single-crystalline electro-optic microring resonators.