Terahertz Photomixer Sources Research Articles

High-resolution, background-free spectroscopy of shallow-impurity transitions in semiconductors with a terahertz photomixer source

We report on a method which enables background-free spectroscopy of the narrow absorption lines of shallow impurity transitions in low-doped and high-purity semiconductors, providing a much better spectral resolution than standard Fourier-transform spectrometers. To achieve this, we combined high-resolution continuous-wave terahertz spectroscopy based on a photomixer system with a bandgap excitation scheme for varying the concentration of neutral impurities. For the minority doping species, the lifetime of the excited states can be directly inferred from such a measurement by analyzing the line shape with respect to the width of the lifetime-related Lorentzian contribution. For a germanium sample with residual impurity concentrations of the order 1012 cm−3, we obtained lifetime values of 110 and 130 ps for the 2 Γ 8 − excited state of the acceptors B and Al, respectively. The method complements time-domain pump-probe measurements for moderately and highly doped samples.

A hybrid analysis method for plasmonic enhanced terahertz photomixer sources

A hybrid analysis of a continuous-wave terahertz photomixer source structure with plasmonic nano-grating electrodes is presented. Using the hybrid analysis, the enhancement of the optical power absorption due to the presence of the one-dimensional metallic nano-grating is investigated by defining an absorption enhancement factor. We show that the proposed absorption enhancement factor can be used as a design tool, whose maximization provides the optimum geometrical parameters of the nano-grating. Based on drift-diffusion model, the photocurrent enhancement due to the nano-grating electrodes is studied under three different bias configurations. Moreover, the dependence of the photocurrent on the physical parameters of the photomixer is analyzed.

Hybrid Computational Simulation and Study of Continuous Wave Terahertz Photomixers

A hybrid numerical simulation method is presented to model and analyze integrated terahertz (THz) photomixer antennas. The proposed computational method combines an optoelectronic solver and a full-wave electromagnetic solver to rigorously model continuous wave (CW) THz photomixer sources. In this hybrid computational approach, the photomixer source is modeled in a rigorous manner without any approximation. The optoelectronic solver is used to find absorbed optical intensity and optical carrier generation rate inside the fast photoconductive region through solving an optical scattering problem. Then, the equations governing the charge carrier transport inside the photoconductor are solved to give THz photo-current by considering realistic material parameters. Finally, through a full-wave electromagnetic solver, and using calculated photo-current from the optoelectronic simulator, antenna parameters and radiated THz power are obtained. Using the proposed hybrid simulation method the effects of photomixer parameters on the THz photo-current and radiated power is rigorously investigated for several geometries. Moreover, results of a parametric study on various factors such as carrier lifetime of material, incident optical power density, applied bias voltage, THz beat frequency, and the gap size are presented. The method can be used for accurate design refinement at pre-fabrication stage.

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 conversion efficiencies as high as 2 × 10−3 have been reached, about 2 orders of magnitude higher than those obtained with CW antenna-based emitters. The second part of the paper is devoted to applications for CW photomixers. We begin with a discussion of the development of novel THz optics. Special attention is paid to experiments exploiting the long coherence length of CW photomixers for coherent emission and detection of THz arrays. The long coherence length comes with an unprecedented narrow linewidth. This is of particular interest for spectroscopic applications, the field in which THz research has perhaps the highest impact. We point out that CW spectroscopy systems may potentially be more compact, cheaper, and more accurate than conventional pulsed systems. These features are attributed to telecom-wavelength compatibility, to excellent frequency resolution, and to their huge spectral density. The paper concludes with prototype experiments of THz wireless LAN applications. For future telecommunication systems, the limited bandwidth of photodiodes is inadequate for further upshifting carrier frequencies. This, however, will soon be required for increased data throughput. The implementation of telecom-wavelength compatible photomixing diodes for down-conversion of an optical carrier signal to a (sub-)THz RF signal will be required.

A Global Approach for Modeling and Analysis of Edge-Coupled Traveling-Wave Terahertz Photoconductive Sources

In this paper, a global and geometry-independent approach is proposed for accurate analysis of edge-coupled continuous wave (CW) traveling-wave terahertz photomixer sources. All major physical phenomena involved in the operation of such devices are included in three interconnected solvers, which are combined as a unified analysis tool. A photonic solver is developed to find the optical intensity across the fast photoabsorbing region from which the carrier generation rate is determined. A semiconductor solver is used to study the charge carrier transport inside the photoconductive region through drift-diffusion model, and to predict the generated photocurrent with the beat frequency of two CW lasers. An electromagnetic solver is introduced to rigorously calculate the coupled terahertz signal into the guiding transmission line through a Lorentz reciprocity theorem. Theoretical formulation behind each solver is discussed in detail, and numerical results from each solver are presented. The proposed approach is a powerful tool for global optimization of the photoconductive sources, especially for maximizing the optical-to-terahertz power conversion.

Modeling and Analysis of a Multilayer Dielectric Slab Waveguide With Applications in Edge-Coupled Terahertz Photomixer Sources

The waveguiding properties of a multilayer dielectric slab waveguide structure with applications in edge-coupled terahertz photomixer sources are studied. The structure guides two interfering laser beams, which their central frequency difference falls into the terahertz spectrum. The top layer of the dielectric waveguide structure is made of an ultrafast photoabsorbing material, wherein the power of the guided modes are being absorbed and converted into a terahertz signal. The optical field and power distributions inside the waveguide structure are studied for different physical parameters of the dielectric layers. The absorbed optical intensity and the generated terahertz photocurrent and terahertz power inside the photoabsorbing layer are calculated