Efficiency of Photoconductive Terahertz Generation in Nitrogen-Doped Diamonds

The efficiency of the generation of terahertz radiation from nitrogen-doped (∼0.1–100 ppm) diamonds was investigated. The synthetic polycrystalline and monocrystalline diamond substrates were pumped by a 400 nm femtosecond laser and tested for the photoconductive emitter operation. The dependency of the emitted THz power on the intensity of the optical excitation was measured. The nitrogen concentrations of the diamonds involved were measured from the optical absorbance, which was found to crucially depend on the synthesis technique. The observed correlation between the doping level and the level of the performance of diamond-based antennas demonstrates the prospects of doped diamond as a material for highly efficient large-aperture photoconductive antennas.

Silicon Lens Optimization to Create Diffuse, Uniform Illumination from Incoherent THz Source Arrays

Arrays of terahertz (THz) sources provide a pathway to overcoming the radiation power limitations of single sources. Several independent sources of THz radiation may be implemented in a single integrated circuit, thereby realizing a monolithic THz source array of high output power. Integrated THz sources must generally be backside-coupled to extended hemispherical dielectric lenses in order to suppress substrate modes and extract THz power. However, this lens also increases antenna gain and thereby produces several non-overlapping beams. This is because individual source pixels are relatively large. Hence, their spatial separation on-chip translates to angular separation in the far-field. In other words, there are gaps in their field of view into which very little THz power is projected. Therefore, they cannot homogeneously illuminate an imaging target. This article presents a simple, practical, and scalable method to convert arrays of incoherent THz sources into a diffuse, uniform illumination source without the need for reducing pixel size. Briefly, individual beam divergence is optimized by tailoring the dimensions of the extended hemispherical dielectric lens such that the far-field beams of adjacent source pixels overlap and combine to form a uniform far-field beam. We applied this method to an incoherent 8 × 8-pixel THz source array radiating 10.3 dBm at 0.42 THz as a proof of concept and thereby realized a 10.3-dBm 0.42-THz diffuse, uniform illumination source that was then deployed in a demonstration of THz active imaging.

Photoconductive Semiconductor-Based Optoelectronic Devices for Generation of Terahertz Radiation

We report on the development and fabrication of a superlattice-based InAlAs/InGaAs heterostructure featuring an ultrashort photocarrier lifetime which has been used as a material base for a photoconductive antenna (PCA)-emitter with a frequency bandwidth of 10 GHz — 3.0 THz and dynamic range above 60 dB. We show an efficiency of plasmonic electrodes embedded into the 10 μm PCA’s gap demonstrating the enhancement of the emitted THz power up to 10 μW (at bias voltage 20—30 V and average pump power 10 mW). The results could open a pathway towards the development of the Russian THz spectroscopic and imaging systems.

Integrated Dual-DFB PIC for High-Purity THz Carrier Generation Enabling Ultrafast THz Wireless Communications

With the explosive growth of global wireless data traffic, the Terahertz band (0.3–10 THz) is promising for ultrafast wireless communications, due to the enormous available bandwidth [1]. Photonic generation of THz carriers displays extremely large tunable range and modulation bandwidth, making it nearly ideal for THz communications. However, the current photonics-based THz carrier generators are based on discrete bulky components [2] with high cost and energy consumption, which hinder them from practical applications. Here, we present an injection-locked heterodyne source based on generic foundry-fabricated photonic integrated circuits (PIC) attached to a photo-mixing uni-travelling carrier photodiode (UTC-PD), generating high-purity THz carriers for high-speed and long-distance wireless communication. The generated THz carrier can span from 0 to 1.4 THz, determined by the tunable wavelength spacing between the two distributed feedback (DFB) modes within the range 0-10.7 nm. We show that a generated 0.4 THz carrier transmits a record-high single-channel net rate of 131 Gbit/s over 10.7 m of wireless distance with only − 24 dBm emitted THz power, by employing 16-QAM-OFDM modulation and a nonlinear equalization technique. To the best of our knowledge, this is the highest data rate for a single-channel THz wireless transmission and requires the lowest THz power/bitrate/distance. The scheme of the monolithic dual-DFB PIC based THz generation shows a great potential for fully integrated, cost-effective and energy-efficient THz transmitters.

An Efficient Photomixer Based Slot Fed Terahertz Dielectric Resonator Antenna

A slot fed terahertz dielectric resonator antenna driven by an optimized photomixer is proposed, and the interaction of the laser and photomixer is studied. It is demonstrated that in a continuous wave terahertz photomixing scheme, the generated THz power is proportional to the 4th power of the surface electric field of photocondutive layer. Consequently, the optical to THz conversion efficiency of the proposed photomixer has an enhancement factor of 487. This is due to the fact that the surface electric field of the proposed photomixer with a 2D-Photonic Crystal (PhC) superstrate has been improved from 2.1 to 9.9 V/m, which represents a substantial improvement. Moreover, the electrically thick Gallium-Arsenide (GaAs) supporting substrate of the device has been truncated to create a dielectric resonator antenna (DRA) that offers a typical radiation efficiency of more than 90%. By employing a traditional coplanar strip (CPS) biasing network, the matching efficiency has been improved to 24.4%. Therefore, the total efficiency has been considerably improved due to the enhancements in the laser-to-THz conversion, as well as radiation and matching efficiencies. Further, the antenna gain has been improved to 9dBi at the presence of GaAs superstrate. Numerical comparisons show that the proposed antenna can achieve a high gain with relatively smaller dimensions compared with traditional THz antenna with Si lens.

A modulation-doped heterostructure-based terahertz photoconductive antenna emitter with recessed metal contacts

We present the implementation of an efficient terahertz (THz) photoconductive antenna (PCA) emitter design that utilizes high mobility carriers in the two-dimensional electron gas (2DEG) of a modulation-doped heterostructure (MDH). The PCA design is fabricated with recessed metal electrodes in direct contact with the 2DEG region of the MDH. We compare the performance of the MDH PCA having recessed contacts with a PCA fabricated on bulk semi-insulating GaAs, on low temperature-grown GaAs, and a MDH PCA with the contacts fabricated on the surface. By recessing the contacts, the applied bias can effectively accelerate the high-mobility carriers within the 2DEG, which increases the THz power emission by at least an order of magnitude compared to those with conventional structures. The dynamic range (62 dB) and bandwidth characteristics (3.2 THz) in the power spectrum are shown to be comparable with the reference samples. Drude-Lorentz simulations corroborate the results that the higher-mobility carriers in the MDH, increase the THz emission. The saturation characteristics were also measured via optical fluence dependence, revealing a lower saturation value compared to the reference samples. The high THz conversion efficiency of the MDH-PCA with recessed contacts at low optical power makes it an attractive candidate for THz-time domain spectroscopy systems powered by low power fiber lasers.