Daryoosh Saeedkia

Terahertz Photonics: Optoelectronic Techniques for Generation and Detection of Terahertz Waves

Optoelectronic techniques for generation and detection of terahertz (THz) signals have been reviewed. The operation principles of THz photomixer sources and THz parametric sources have been studied and recent developments in these areas have been presented. The performances of developed THz optoelectronic sources have been discussed and compared.

The interaction of laser and photoconductor in a continuous-wave terahertz photomixer

The interaction of laser and photoconductor in an optical heterodyne conversion scheme is studied in detail. A dc biased photoconductor excited by two continuous-wave (CW) laser beams with a difference in their central frequencies falling in the terahertz spectrum is considered as the core element in all photoconductive photomixing structures. For this configuration the continuity equations for the electron and hole densities are solved in their general form along with the appropriate boundary conditions to find photocurrent distribution inside the photoconductor. It is shown that in a CW terahertz photomixing scheme the resulting photocurrent contains a dc component and a terahertz component. It is also shown that the amplitude and the phase of the terahertz component of the photocurrent are functions of the applied bias, physical parameters of the photoconductor, parameters of the lasers, and photomixer configuration. The dependency of the photocurrent on all of these parameters is explored in detail for a typical photomixer made of low-temperature-grown GaAs photoconductor.

Analysis and design of a photoconductive integrated photomixer/antenna for terahertz applications

A new terahertz (THz) photoconductive photomixer/antenna device is presented. A continuous-wave THz signal is generated in a dc-biased photoconductive film by employing optical heterodyne scheme, and at the same time, the size of the film on the grounded dielectric substrate is designed to have an efficient broadside radiation. Incorporating the photoconductive film as the photomixing media and the radiation element not only eliminates source to antenna coupling problem but also makes the proposed device attractive for THz array source configurations. Analytical expressions for the photocurrent and the radiation power are derived under high dc bias condition, and the effects of the device configuration on its performance is studied. Two possible photomixer configurations, namely longitudinal and transversal, are introduced and their photomixing efficiencies and radiation power are compared. The typical nW output power is achievable by mW laser pump power for frequencies up to 10 THz.

A comprehensive model for photomixing in ultrafast photoconductors

A comprehensive model for photomixing phenomenon in a dc-biased ultrafast photoconductor is proposed. The dependence of the carrier lifetime and carrier velocity on the electric field is taken into account, and the significance of carrier transport and carrier generation and recombination mechanisms in a terahertz photomixer are evaluated. A method for calculating the nonuniform induced electric field due to excess charge density resulting from unequal electron and hole recombination lifetimes is presented

Analysis and design of a continuous-wave terahertz photoconductive photomixer array source

An array of photoconductive photomixer/antenna elements as a continuous-wave terahertz source is proposed, and its radiation characteristic is studied. Employing photomixer/antenna elements in an array configuration increases available terahertz power, while each of the array elements consumes optical power less than its maximum sustainable power. A few microwatt terahertz power is achievable from a typical array structure. It is shown that the radiated beam can be steered by changing the angle between the two exciting laser beams.

Terahertz Generation and Detection Using Low Temperature Grown InGaAs-InAlAs Photoconductive Antennas at 1.55

Photoconductive antennas fabricated on novel low temperature (LT) beryllium (Be) doped InGaAs-InAlAs multi-quantum-well structures have been evaluated as THz emitters and detectors in a time-domain spectroscopy (TDS) system. We present system responses with THz pulses having spectral range up to 3 THz and power to noise ratio of 60 dB, making them competitive with LT-GaAs excited at 800 nm and among the highest reported to date for this material system.

\mu{\hbox{m}}

A single-pixel camera for THz imaging has been proposed in [1]. In this article, we propose a new design for a single-detector THz imaging system based on Compressive Sampling (CS), which does not need raster scanning of the object in front of the THz beam. We exploit a time-efficient and cost-effective design to acquire the CS measurements. As a result, the image acquisition time in the proposed imaging system is only limited to the speed of the THz detector. The proposed approach is applicable to other types of imaging as well.

Pulse Excitation

Photomixing phenomenon in superconductors is studied. For a photo-excited superconductor film the fundamental equations for the carriers motion inside the superconductor material have been used in connection with the two-temperature model to find an analytic expression for the generated terahertz (THz) photocurrent inside the film. The dependency of the resulting photocurrent on the material and laser parameters, temperature, and photomixer configuration for a photomixer made of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// high-temperature superconductor is studied.

A 2D camera design with a single-pixel detector

We propose, for the first time, the application of whispering gallery mode (WGM) perturbation technique in dielectric analysis of disk shape pharmaceutical tablets. Based on WGM resonance, a low-cost high sensitivity sensor in milllimeter-wave frequency range is presented. A comprehensive sensitivity analysis was performed to show that a change in the order of 10−4 in the sample permittivity can be detected by the proposed sensor. The results of various experiments carried out on drug tablets are reported to demonstrate the potential multifunctional capabilities of the sensor in moisture sensing, counterfeit drug detection and contamination screening. Analytically, two sample placement configurations, i.e. a tablet placed on top of a dielectric disk resonator and inside a dielectric ring resonator, have been studied to predict the resonance frequency and Q-factor of the combined sample-resonator structure. The accuracy of the analytical model was tested against full-wave simulations and experimental data.

Modeling and analysis of high-temperature superconductor terahertz photomixers

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.