Institut für Optische und Elektronische Materialien

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TUHH OEM: Organic Nonlinear Optics

Organic Nonlinear Optics

Introduction

Electro-optical (EO) and all-optical switches and modulators are important building blocks in todays and future optical communication systems. EO signal processing requires materials whose optical refractive index and or absorption can be modified by an external electric field. The Pockels effect describes a second order nonlinear optical phenomenon where the refractive index is linearly dependent on an externally applied electric field. Organic nonlinear optical (ONLO) materials [1–6] are well suited for this type of application due to the ultrafast, nonresonant response of the conjugated π-electrons (see Fig. 1a), resulting in a very high bandwidth (> 100 GHz) [7]. With the help of sophisticated theoretical models (density function theory and time-dependent density function theory) organic molecules can be designed to exhibit very high Pockels-coefficients (> 300 pm/V [8,9], see Fig. 1b), permitting drive voltages in devices, such as Mach-Zehnder modulators of below 1 V (www.lumera.com). Further advantages include low temperature solution (spin casting) processing of thin films devices, low material costs as well as compatibility with a diverse array of materials and material technologies including silicon photonics.

Fig1a Fig1b
Fig. 1a: Structures of some common electro-optic chromophores. [10] Fig. 1b: Past and projected increase of electro-optic activity (r33) of organic materials. [10]

Nano structuring of polymer films with dimensions on the scale of approximately 100 nm and aspect ratios of 1:16 is possible by using electron cyclotron resonance high-density plasma etching (ECR). A sketch of the fabrication process is shown in Figure 2. An etch mask with the final pattern is created with an electron beam lithography step into an e-beam resist (PMMA). Using Ar ion beam etching this pattern of the resist mask is transferred into the NiCr-film, which acts as a hard mask in the subsequent deep etch into the waveguide and substrate layer. The nano fabrication of our devices is carried out at the Institute of Photonic Technology in Jena, Germany (IPHT). [11]

Fig2
Fig. 2: Sketch of the overall fabrication process .

Goals

The research of the institute for Optical and Electronic Materials concentrates on designing and characterizing EO-tunable devices using ONLO materials [12–16], either by defining the devices directly in the ONLO material or by functionalizing silicon nanophotonic devices using ONLO materials.

If the device is directly written into the ONLO material, the organic material acts as core material to which the light is confined [17–20]. Therefore its spectral properties can be easily modified by changing the refractive index of the ONLO region with an external electric field [21]. The major challenge of this approach is the moderate refractive index of polymers (n ≈ 1.6), which limits its light confinement ability. In case of photonic crystal nano cavities this results in a smaller achievable bandgap and reduced Q factors (Photonic crystals), as compared to high index systems.

Silicon on the other hand offers a high refractive index (n = 3.5) and therefore excellent light confinement ability, but no intrinsic Pockels response. To combine the best properties of both worlds nanophotonic structures are written into silicon-on-insulator substrates and then covered and infiltrated with ONLO materials, which act as cladding material. A shift in the refractive index of the ONLO cladding leads to change in the effective refractive index of the optical mode, due to the evanescent optical field components in the cladding. With careful adjustment of the geometry, the fraction of optical field that is located in the ONLO region of the device can be maximized to achieve a strong sensitivity of the device to external fields. This is most effectively done by introducing a slot into the waveguide along the propagation direction [22–24].

Results

All polymer low-index contrast 2D photonic crystals can show omni-directional photonic band gap effects (see Figs. 3a and 3b)[25].

Fig3a Fig3b
Fig. 3a: Scanning electron micrograph displaying the cleaved edge of a 2D polymer triangular photonic crystal made from a P(MMA/DR1) core, with mesoporous silica as the substrate material (a = 650 nm, r = 280 nm). Holes were etched into the core material, with a slight penetration of the substrate material. Fig. 3b: Simulated (lines) and experimental (dots) transmission spectra of a bulk 40 lattice constant triangular photonic crystal in a polymer slab waveguide. The simulated and experimental results are in excellent agreement. A suppression of 15 dB represents the noise floor of the employed measurement apparatus. A band gap between 1170 and 1235 nm is clearly identifiable.

All polymer 2D photonic crystal cavitities made of nonlinear polymers (see Fig. 4a) demonstrate electrooptical modulation with sub 1 Volt sensitivity (see Fig. 4b) [21].

Fig4a Fig4b
Fig. 4a: Cleaved sample of tapered photonic crystal line-defect resonator (lattice constant a = 500 nm, hole radii r = 80...150 nm, slab thickness d = 1450 nm). The cavity is excited from the left side and transmitted light is collected from the right side Fig. 4b: Modulation amplitude at the fundamental modulation frequency (circles, f = 200 Hz) and first harmonic frequency (squares, f = 400 Hz) for low modulation voltages observed for a poled sample. Modulation sensitivity below 1V is observed. No detectable Kerr response at such low modulation fields (response at fundamental frequency).

References

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