J. Roither

Epitaxial Bragg mirrors for the mid-infrared and their applications

Bragg interference mirrors consisting of stacks of dielectric layers with an optical thickness of a quarter wavelength are of great importance for optoelectronic device applications. For the mid-infrared spectral range mirrors with high reflectivity stop bands are fabricated from combinations of Pb1� xEuxTe/EuTe materials by molecular beam epitaxy on BaF2 substrates. These mirrors designed by the transfer matrix method exhibit reflectivities in excess of 99% by only 3 Bragg mirror layer pairs and very wide stop band regions, reaching a width of up to 60% of the target wavelength. Based on these very efficient mirrors, planar microcavities are demonstrated with an ultra-high effective finesse of up to 1700. Stimulated emission between 3 and 6mm is obtained by optically pumping a vertical-cavity surface-emitting laser containing PbTe quantum wells with Pb1� xEuxTe barriers as active medium embedded between two dielectric Bragg mirrors. Depending on the design of the resonator, pulsed laser operation is observed up to 651C. The enhancement of light absorption in the cavity is used to study the absorption of superlattices containing correlated self-organized PbSe quantum dots. r 2002 Elsevier Science Ltd. All rights reserved.

Emission properties of 6.7μm continuous-wave PbSe-based vertical-emitting microcavity lasers operating up to 100K

A detailed analysis of midinfrared cw lasing of IV–VI vertical-cavity surface-emitting devices is presented. The structures, based on high-finesse microcavities containing PbSe as active medium, show optically pumped cw laser emission up to temperatures of 100K at a long wavelength of 6.7μm. Stimulated emission with a very narrow beam divergence below 1° and a large temperature tuning range of 70nm is found. The measured linewidth of the laser emission is only 0.6nm, limited by the spectrometer resolution with a strong narrowing with respect to the linewidth of the subthreshold signal. The observed cw output power amounts up to 1.2mW at 85K.

Room temperature operation of epitaxial lead-telluride detectors monolithically integrated on midinfrared filters

Epitaxial PbTe midinfrared photodetectors are monolithically integrated on optical filter structures, like λ∕4 antireflection layers or λ∕2 microcavities. The antireflection layers result in an increased photoresponsivity of the detectors by a factor of 2.2, measured at the target wavelength of 3.1μm. The microcavities, acting as efficient narrow band filters, consist of two PbEuTe∕EuTe Bragg interference mirrors separated by a PbEuTe cavity layer. The photoresponse spectra of the detectors integrated with the microcavity filters exhibit a single resonance at 3.6μm with a relative line width of 2.7%. The narrow photoresponse peak is in coincidence with a molecular absorption line typical for nonaromatic aldehydes. For all devices room temperature operation is demonstrated either in photoconductive or in photovoltaic operation mode.

Spatial and temporal frequency domain laser-ultrasound applied in the direct measurement of dispersion relations of surface acoustic waves

We present a laser-ultrasound measurement technique which combines adjustable spatial and temporal modulation of the excitation laser beam. Our method spreads the intensity of an amplitude modulated continuous wave laser over a micro-scale pattern on the sample surface to excite surface acoustic waves. The excitation pattern consists of parallel, equidistant lines and the waves generated from the individual lines interfere on the sample surface. Measurement is done in the spatial-temporal frequency domain allowing the direct determination of dispersion relations. The technique performs with high signal-to-noise-ratios and low peak power densities on the sample.

Highly directional emission from colloidally synthesized nanocrystals in vertical cavities with small mode spacing

The optical properties of light emitting devices based on colloidally synthesized CdSe/CdS core/shell semiconductor nanocrystals embedded in vertical cavities are investigated. The cavities are several micrometers, thick and formed by a metallic mirror and a dielectric Bragg interference mirror deposited on the opposite surfaces of cleaved pristine mica sheets. Due to the large cavity length, up to 30 resonances are found within the Bragg mirror stop band. The corresponding small mode spacing allows one to extract a large portion of the broad nanocrystal luminescence band from the cavity upon optical excitation. The spontaneous emission of these cavities is highly forward directed with a beam divergence smaller than 1.3°. Furthermore, the emission is linearly polarized which is a result of the birefringent properties of the mica sheets.

Para-sexiphenyl-CdSe/ZnS nanocrystal hybrid light emitting diodes

CdSe/ZnS core/shell nanocrystals (NCs) are integrated into para-sexiphenyl (p-6P) based hybrid light emitting diodes, to obtain green and red emission in addition to blue emission originated from p-6P. For the active region of the devices, ultrathin layers of p-6P and NCs are deposited by hot wall epitaxy and spin casting, respectively, resulting in current-voltage characteristics with small leakage currents and low onset voltages. The achieved electroluminescence exhibits narrow emission line widths and thus high color purity, as required for color display applications.

Nanocrystal-based microcavity light-emitting devices operating in the telecommunication wavelength range

Highly luminescent colloidally prepared HgTe nanocrystals (NCs) are used to fabricate microcavity light-emitting devices operating around 1.5μm. They consist of a Bragg interference mirror from standard optical materials deposited on glass substrates, an active layer embedding the nanocrystals, and a metallic top mirror. These devices give highly directional narrow single-mode emission with a beam divergence below 3° and a spectral width smaller by a factor of 8 than that of a NC reference sample. The emission wavelength can be tuned between 1.4 and 1.75μm by changing the cavity length and thus, the cavity finesse. The influence of the latter on output power and beam divergence is discussed. Furthermore, operation up to 75 °C is demonstrated without degradation of the NCs, which is promising for potential applications.

Two- and one-dimensional light propagations and gain in layer-by-layer-deposited colloidal nanocrystal waveguides

Optical waveguides containing high percentages of colloidal nanocrystals have been fabricated by layer-by-layer deposition on planar and patterned glass substrates. The two- and one-dimensional waveguidings in these structures are demonstrated by propagation loss experiments. The experimental results obtained for various film thicknesses and widths of the waveguide stripes together with simulations of the light propagation indicate that the losses are dominated by surface roughness. The variable stripe length method is used to determine the optical gain of 230cm−1 from the amplified spontaneous emission. This high value makes the authors’ waveguide structures very promising for applications in amplifiers and lasers with reduced threshold powers.

Enhanced color conversion from colloidal CdSe/CdS dot/rods by vertical microcavities

Colloidal CdSe/CdS dot/rods exhibit efficient photoluminescence from the spherical CdSe dots at wavelengths well below the absorption edge of the rod material. This property makes dot/rods advantageous for color conversion applications, especially when they are embedded in optical microcavities to improve light extraction in forward direction. Here, surface emitting half-wavelength microcavities are demonstrated containing films of dot/rods as active material, exhibiting luminescence enhancement factors of up to 21 at the resonator wavelengths, whereas with conventional CdSe/ZnS core-shell nanocrystals only half of this value is obtained.

Enhanced infrared emission from colloidal HgTe nanocrystal quantum dots on silicon-on-insulator photonic crystals

A two dimensional silicon-on-insulator based photonic crystal structure is used to enhance the emission from colloidal HgTe nanocrystal quantum dots embedded in a thin polymer film. The enhancement is resonant to the leaky eigenmodes of the photonic crystals due to coherent scattering effects. Transmittance and photoluminescence experiments are presented to map the leaky mode dispersion and the angle dependence of the emission enhancement factor, which reaches values up to 80 (650) for vertical (oblique) emission in the telecommunication wavelength range.