S. Malzer

Tunable, continuous-wave Terahertz photomixer sources and applications

This review is focused on the latest developments in continuous-wave (CW) photomixing for Terahertz (THz) generation. The first part of the paper explains the limiting factors for operation at high frequencies ∼ 1 THz, namely transit time or lifetime roll-off, antenna (R)-device (C) RC roll-off, current screening and blocking, and heat dissipation. We will present various realizations of both photoconductive and p-i-n diode–based photomixers to overcome these limitations, including perspectives on novel materials for high-power photomixers operating at telecom wavelengths (1550 nm). In addition to the classical approach of feeding current originating from a small semiconductor photomixer device to an antenna (antenna-based emitter, AE), an antennaless approach in which the active area itself radiates (large area emitter, LAE) is discussed in detail. Although we focus on CW photomixing, we briefly discuss recent results for LAEs under pulsed conditions. Record power levels of 1.5 mW average power and conve...

Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses

The evolution of surface morphology of tungsten irradiated by single-beam femtosecond laser pulses is investigated. Ripplelike periodic structures have been observed. The period of these ripples does not show a simple relation to the wavelength and angle of incidence. The orientation of ripples is aligned perpendicularly to the direction of polarization for linearly polarized light. Surprisingly, we find that the alignment of the ripple structure turned left or right by 45 degrees with respect to the incident plane when using right and left circularly polarized light, respectively. The period of the ripple can be controlled by the pulse energy, the number of pulses, and the incident angle. We find a clear threshold for the formation as a function of pulse energy and number of pulses. The mechanism for the ripple formation is discussed, as well as potential applications in large-area structuring of metals.

Efficient terahertz emission from ballistic transport enhanced n-i-p-n-i-p superlattice photomixers

The authors report on photomixing terahertz sources that overcome the transit time versus RC-time trade-off and allow for independent optimization of both of them, using a n-i-p-n-i-p superlattice. Furthermore, they take advantage of ballistic transport for reduced transit times. Apart from more favorable material parameters, In(Al)GaAs photomixers benefit from the advanced telecommunication laser technology around 1.55μm as compared to GaAs. In such devices, a terahertz-power output of 1μW has been achieved at 0.4THz at a photocurrent of 3.8mA. A comparison between corresponding GaAs- and InGaAs-based n-i-p-n-i-p photomixers reveals an improvement of performance by at least an order of magnitude for the latter one.

THz-photomixer based on quasi-ballistic transport

We report on a novel concept for THz photomixers with high conversion efficiency up to several THz. In contrast to the conventional pin photomixer we can overcome the trade-off between either optimizing transit-time or RC-roll-off. Using quasi-ballistic transport in nano-pin-diodes the transport path can be optimized regarding both path length and transit time. Independently, the capacitance can be kept small by using a sufficiently large number of optimized nano-pin-diodes in series. The concept is presented in detail and first experimental results are reported which corroborate our theoretical expectations.

Self-organized tungsten nanospikes grown on subwavelength ripples induced by femtosecond laser pulses

We report on the formation of nanoscale tungsten spikes generated on subwavelength periodic ripples which built up by single beam 800 nm femtosecond laser pulses. The nanospikes have a diameter ranging from 10 to 100 nm and are up to 250 nm in length. The nanospikes orientate from the ridges toward the valleys of the ripple structures independent of the polarization of the light. The heterogeneous nucleation of the liquid phase at the irradiated surface and the inhomogeneous surface roughness are considered as the mechanism of this nanospike formation.

Coupled whispering gallery mode resonators in the Terahertz frequency range

We report on coupling of two whispering gallery mode resonators in the Terahertz frequency range. Due to the long wavelength in the millimeter to submillimeter range, the resonators can be macroscopic allowing for accurate size and shape control. This is necessary to couple specific modes of two or more resonators. Sets of polyethylene (PE) and quartz disk resonators are demonstrated, with medium (loaded) quality (Q)-factors of 40-800. Both exhibit coinciding resonance frequency spectra over more than ten times the free spectral range. Loading effects of single resonators are investigated which provide strong Q-factor degradation and red-shifts of the resonances in the 0.2% range. By coupling two resonators of the same size, we observe mode splitting, in very good agreement with our numerical calculations.

Enhancement of optical absorption and photocurrent of 6H-SiC by laser surface nanostructuring

Nanoscale ripple structures with spatial periods of up to 155nm are generated by a single beam ultrashort pulsed laser structuring of 6H-SiC surface. The period of nanoripples does not depend on the laser fluence and the number of pulses. Optical absorption and photocurrent measurements on SiC samples are investigated before and after laser nanostructuring. Nearly 40% enhancement of optical absorption and a factor of 3 increase of photocurrent are achieved after laser nanostructuring.

Interferometer measurements of terahertz waves from Bi2Sr2CaCu2O8+d mesas

We fabricated rectangular mesa structures of superconducting Bi2Sr2CaCu2O8+d (Bi2212) using e-beam lithography and Ar ion beam etching techniques for terahertz (THz) emission. c-axis resistance versus temperature (R?T), current?voltage (I?V) characteristics and bolometric THz power measurements were performed to characterize Bi2212 mesas. The emission frequency of mesas was determined using a Michelson interferometer setup which also demonstrates polarized emission. Interference patterns of THz radiation from Bi2212 mesas were detected by various detectors such as a liquid helium cooled silicon composite bolometer, a Golay cell and a pyroelectric detector. An emitted power as high as 0.06?mW was detected from Bi2212 mesas. For the first time, most of the pumped power was extracted as THz emission from a Bi2212 mesa. The radiation at 0.54?THz was detected using the Michelson interferometric setup.

Enhanced recombination tunneling in GaAs pn junctions containing low-temperature-grown-gaas and ErAs layers

We report electrical conductivity studies of highly-doped GaAs pn diodes containing a strongly n-doped low-temperature-grown (LT)–GaAs layer and pn junctions containing an approximately one monolayer thick ErAs layer. At room temperature, current densities of 1 kA/cm2 for the n-LT–GaAs samples and 6 kA/cm2 for the ErAs samples at 1 V forward bias have been measured. The I–V characteristics under forward bias for the n-LT–GaAs and ErAs samples exhibit significantly different behavior. At low temperatures, the n-LT–GaAs samples reveal a shoulder in the I–V characteristics, which can be explained by a model taking into account tunneling of carriers into LT midgap states. A similar model was able to explain the current transport in the ErAs diodes as tunneling of carriers into metallic regions inside the pn junction.

Fabrication of genuine single-quantum-dot light-emitting diodes

We present a simple approach for the fabrication of genuine single quantum-dot light-emitting diodes. A submicron wide bottom contact stripe is formed by focused ion beam implantation doping into a GaAs buffer layer. Successive overgrowth with a thin intrinsic layer incorporating self-assembled InAs quantum dots, followed by a top contact layer of complementary doping type and standard photolithographic processing, allows for electrical cross sections in the sub-μm2 range. In devices with sufficiently low dot densities, only one single dot is expected to be electrically addressed. Both the observed current versus voltage characteristics and the evolution of the electroluminescence spectra as a function of applied voltage clearly demonstrate that this goal has been achieved.