Lukas Mutter

Linear and nonlinear optical properties of the organic crystal DSTMS

We report on the linear and nonlinear optical properties of the novel organic crystal 4-N, N-dimethylamino-4′-N′-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS). The principal refractive indices n1, n2, and n3 have been determined by a direct measurement of the group index dispersion over a broad wavelength range from 0.6 to 1.6 μm. The linear absorption coefficients for light polarized along the dielectric axes have been measured in the transparency range from 600 to 2000 nm. DSTMS crystals show a large birefringence of Δn=0.5 and low absorption α<0.7 cm−1 in the telecommunication wavelength range at 1.55 μm. Furthermore, the nonlinear optical tensor elements d111, d122, and d212 have been determined by the Maker-fringe technique at the fundamental wavelength of 1.9 μm. DSTMS crystals exhibit a very large nonlinear activity with a nonlinear optical coefficient d111=214±20 pm/V. Phase-matching curves determined from our optical data showed that parametric oscillation can be achieved in the wavelength range from 1 to 2.2 μm with effective nonlinear optical coefficients larger than 25 pm/V.

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.

Photobleaching and optical properties of organic crystal 4-N, N-dimethylamino-4′-N′-methyl stilbazolium tosylate

Photobleaching of organic optical materials can be used to structure the surface layer for integrated optics applications. Linear optical properties and absorption were studied in 4-N, N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST) organic crystals within the absorption band from 260–700 nm in order to determine the depth-range of photobleaching. The results were obtained from reflectivity measurements and bleaching experiments. The depth range of photobleaching can be varied between 0.2 and 2.6 μm by selecting a suitable wavelength.

1.3µm-wavelength phase-locked VCSEL arrays incorporating patterned tunnel junction

We report the fabrication and the performance of phase-locked VCSEL arrays emitting near 1310 nm wavelength. The arrays were fabricated using double wafer fusion by patterning a tunnel junction layer, which serves to define the individual single mode array elements. Phase-locking in both one-dimensional and two-dimensional array configurations was confirmed by means of far field and spectral measurements as well as theoretical modeling. CW output powers of more than 12 mW were achieved.

Transverse mode discrimination in long-wavelength wafer-fused vertical-cavity surface-emitting lasers by intra-cavity patterning

Transverse mode discrimination is demonstrated in long-wavelength wafer-fused vertical-cavity surface-emitting lasers using ring-shaped air gap patterns at the fused interface between the cavity and the top distributed Bragg reflector. A significant number of devices with varying pattern dimensions was investigated by on-wafer mapping, allowing in particular the identification of a design that reproducibly increases the maximal single-mode emitted power by about 30 %. Numerical simulations support these observations and allow specifying optimized ring dimensions for which higher-order transverse modes are localized out of the optical aperture. These simulations predict further enhancement of the single-mode properties of the devices with negligible penalty on threshold current and emitted power.

Intra-cavity patterning for mode control in 1.3μm coupled VCSEL arrays

We report coupled VCSEL arrays, emitting at 1.3 μm wavelength, in which both the optical gain/loss and refractive index distributions were defined on different vertical layers. The arrays were electrically pumped through a patterned tunnel junction, whereas the array pixels were realized by intra-cavity patterning using sub-wavelength air gaps. Stable oscillations in coupled modes were evidenced for 2x2 array structures, from threshold current up to thermal roll-over, using spectrally resolved field pattern analysis.

Optical injection locking of transverse modes in 1.3-µm wavelength coupled-VCSEL arrays

Optical injection locking of 1.3-µm phase-locked VCSEL arrays defined by patterned tunnel junctions and wafer fusion is investigated experimentally and theoretically. The impact of the overlap between the master laser injection beam and the injected modes is demonstrated and explained with a rate equation model that incorporates the spatial variations.

GROWTH AND PLANAR STRUCTURING OF DAST CRYSTALS FOR OPTICAL APPLICATIONS

The organic salt 4-N, N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST) is a nonlinear optical crystal with superior properties for integrated electro-optical applications and THz-devices. High quality DAST single crystals have been grown by the solution growth method. The electro-optical properties were determined in the spectral range between 700 and 1535 nm (r111=47±8 pm/V at λ=1535 nm). Several planar structuring techniques for integrated optics applications based on DAST have been investigated. The production of single crystal thin films with minimal thicknesses of 10 μm is presented as well as the channel waveguide structuring of DAST surfaces by photobleaching techniques in the depth range of 0.2 up to 2.6 μm and by femtosecond laser ablation.

Improved single-mode emission characteristics of long-wavelength wafer-fused vertical-cavity surface-emitting lasers by intra-cavity patterning

We report on transverse mode discrimination in long-wavelength wafer-fused vertical-cavity surface-emitting lasers (VCSELs) incorporating ring-shaped air gap patterns at the fused interface between the active region and the top distributed Bragg reflector (DBR). These 60-nm deep patterns were implemented with the aim of favoring the fundamental mode while preserving high output power. The VCSELs under consideration emit in the 1310-nm band and incorporate an AlGaInAs-based quantum well active region, a regrown circular tunnel junction and undoped GaAs/AlGaAs DBRs. A large batch of devices with varying pattern dimensions was investigated by on-wafer mapping, allowing significant statistical analysis leading to conclusions on their typical behavior. We observe experimentally a dependence of the side-mode suppression ratio on the geometrical parameters of the patterns. In particular, we identified a design that statistically increases the maximal single-mode emitted power by more than 20%. Numerical simulations of the patterned-cavity VCSELs based on our fully three dimensional electrical, thermal and optical VCSEL computational model support these observations. They show that patterns with a large inner diameter actually confine the first-order transverse mode and enhance its modal gain. In smaller devices, this mode is pushed out of the optical aperture and suffers larger losses. Optimized parameters were found numerically for enhancing the single-mode properties of the devices with negligible penalty on emitted power and threshold current.

UV Second Harmonic Generation at 266 nm in He + Implanted β-BaB 2 O 4 Optical Waveguides

We report on the second harmonic generation of deep UV light in β-BaB2O4 (BBO) waveguides pumped by a frequency-doubled continuous-wave Nd:YAG laser. An output power of 0.32 mW at 266 nm has been achieved for an internal pump power of 670 mW. Optical channel waveguides in BBO crystals were produced by He+ ion implantation, lithographic masking and ion etching. The linear and nonlinear optical properties and the power handling capability of these waveguides are presented.