Ch. Lienau

Fabry–Perot effects in THz time-domain spectroscopy of plasmonic band-gap structures

Using terahertz time-domain spectroscopy, we study transmission in one-dimensional arrays of slits fabricated on metal plates by laser machining. The enhanced peaks of zero-order transmission spectra are attributed to the combined effects of Fabry–Perot and surface plasmon resonances. Angle dependence of transmission spectra shows that the strongly surface plasmon-enhanced peaks appear when the Fabry–Perot-type resonance is located nearby in energy. This means that surface waves traveling in the horizontal direction couple with nearest Fabry–Perot resonance to generate enhanced peaks. These results are in excellent agreement with theoretical calculations.

Invisible plasmonic meta-materials through impedance matching to vacuum

We report on perfect transmission in two-dimensional plasmonic matamaterials in the terahertz frequency range, in which zeroth order transmittance becomes essentially unity near specific resonance frequencies. Perfect transmission may occur when the plasmonic metamaterials are perfectly impedance matched to vacuum, which is equivalent to designing an effective dielectric constant around epsilonr = -2. When the effective dielectric constant of the metamaterial is tuned towards epsilonr and the hole coverage is larger than 0.2, strong evanescent field builds up in the near field, making perfect transmission possible.

Light emission from the shadows: Surface plasmon nano-optics at near and far fields

When light illuminates a thick metal film perforated with small holes, shadows appear. At the nanoscopic level, however, light can be emitted predominantly from the metal surfaces between the holes—shadows can be indeed brighter than the lighted holes. The symmetry of the near-field emission pattern is determined by the symmetry of the surface plasmon waves. Surprisingly, these nanoscopic emission patterns from the metal can be preserved to the far-field region, where the pattern becomes sinusoidal. This unusual behavior of light emission from the shadows is explained by efficient wave vector selection.

Light‐induced expansion of fiber tips in near‐field scanning optical microscopy

The emission profiles of laser diodes working at 780 nm and 1300 nm are studied by near‐field scanning optical microscopy. As the near‐field probe is scanned across the laser mirror facet, the laser emission induces a transient expansion of the probe tip which is monitored using shear force microscopy. The thermal expansion of the tips reaches absolute values of up to 100 nm per mW of emitted laser power. A fully metallized near‐field probe tip is shown to serve as a local bolometer with a spatial resolution of better than 1 μm.

Terahertz transparency at Fabry-Perot resonances of periodic slit arrays in a metal plate: experiment and theory

We report on a terahertz transparency in periodic arrays of metallic slits using terahertz time- domain spectroscopy. Experimental results and theoretical calculations reveal that Fabry-Perot resonance appearing in spectral region below first Rayleigh minimum strongly enhances terahertz transmission and a symmetric eigenmode inside the slits is excited under the condition of terahertz transparency.

Optical near‐field photocurrent spectroscopy: A new technique for analyzing microscopic aging processes in optoelectronic devices

The potential of optical near‐field photocurrent spectroscopy for analyzing microscopic aging processes in optoelectronic devices is demonstrated. The technique combines the subwavelength spatial resolution of near‐field optics with tunable laser excitation, allowing for selective investigation of specific parts of the device structure. Experiments on GaAs/(AlGa)As high power laser diodes before and after accelerated aging provide direct visualization of defect growth within the p‐i‐n junction and information on aging‐enhanced recombination processes close to the laser facet. The effect of wave guiding of the exciting light on the image formation is discussed. The nondestructiveness makes this technique a particularly attractive method for in situ analysis in high power laser diodes.

Nanoscopic measurements of surface recombination velocity and diffusion length in a semiconductor quantum well

We use a near-field microscopic technique to probe photoluminescence from the edge area of a quantum well. Near the edge, surface recombination gives rise to a gradual variation of the photoluminescence signal on a micrometer length scale. The overall shape in this transition region depends strongly on the excitation intensity. From solving two dimensional diffusion equations, we deduce the surface recombination velocity and the diffusion length. It is shown that the surface recombination velocity decreases with increasing intensity due to the saturation of nonradiative defect states.

Near‐field scanning optical microscopy of polarization bistable laser diodes

TE/TM polarization bistability in a λ=1.3 μm ridge‐waveguide InGaAsP/InP bulk laser is studied by near‐field scanning optical microscopy with an optical resolution of better than λ/8. The near‐field mode profiles of TE and TM emission show different lateral widths and distinctly different mode center positions. This lateral shift is related to a nonuniform strain distribution along the active layer. Based on this strain gradient, we present a model that accounts for the hysteresislike current dependence of the polarization resolved laser output.

Exciton transport into a single GaAs quantum wire studied by picosecond near-field optical spectroscopy

We report a time-resolved near-field luminescence study of excitonic real-space transfer into single GaAs quantum wires. Excitons generated by local optical excitation in a 250 nm spot undergo diffusive transport over a length of several microns and are subsequently trapped into the quantum wire by optical phonon emission. Local energy barriers in the vicinity of the quantum wire, originating from the epitaxial growth mechanism of the nanostructure, directly influence the real-space transfer dynamics and trapping efficiency.

Evidence for strain-induced lateral carrier confinement in InGaAs quantum wells by low-temperature near-field spectroscopy

A strain-induced lateral variation of the band edges of a 10-nm-thick In0.16Ga0.84As quantum well embedded in GaAs is achieved by patterning of a 100-nm-thick compressively strained In0.52Ga0.48P stressor layer. The strain modulation results in a splitting of the 10 K far-field photoluminescence (PL) spectra into two emission peaks. Spectrally resolved two-dimensional near-field PL images establish a clear spatial and spectral separation of the two far-field PL peaks, indicating a lateral carrier confinement with a confinement energy of about 10 meV. Finite-element calculations of the strain distribution are used to determine the lateral band-edge shifts and are well in agreement with the experimental findings.