Research on Efficient Photoconductive Antennas (PCAs) and Wideband Circular Polarizers Operating at Terahertz Frequencies


Student thesis: Doctoral Thesis

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Award date12 Jul 2023


In past decades, the THz regime (0.1-10THz) has drawn the attention of researchers due to its broad spectrum of possible applications. The leading THz research areas are biomedicine, earth science, space science, communication and information technology, security, process industries, defense, etc. The potential attributes such as low photon energy (0.4meV to 40meV) and nonionized penetration in human skin, clothing, paper, wood, plastic, etc., make the THz wave beneficial for hazardless disease diagnosis (e.g., skin cancer), dental imaging, food processing, and various material characterizations. Due to the short wavelength of the THz, high-resolution imaging is possible. Besides this, THz exhibits unique spectral signatures of many explosives and toxic, dangerous illegal substances, making THz waves advantageous in security and screening. On the other hand, the absorption of molecules with vibrational frequency in THz offers a wide range of applications in remote sensing and imaging. For example, absorption of THz in water or a moist environment helps the paper manufacturing industry distinguish the hydrated and dried substances. Despite such diverse, explored, and new applications, efficient and compact THz equipment remains a matter of research. Various THz devices have recently been developed based on their operating range and applications, which are linear and nonlinear. The operating frequency range, output power, or working environment limit some. Some devices are bulky, huge, and require a specific operating environment. The expensive and complex manufacturing requirements decelerate the evolution of THz devices.

Among recent developments, a photoconductive antenna-based terahertz time-domain spectrometer (THz-TSD) is an efficient and compact setup for material characterization and imaging. In this down-conversion approach, the photoconductive antennas (PCAs) radiate and detect the THz wave. A PCA consists of a pair of metallic electrodes patterned on a photoconductive substrate that can emit a wide range of THz waves (0.1 to 30THz) when fed with sub-picosecond photocurrent pulses. These ultrashort photocurrent pulses flow through the biased metallic electrodes when the sub-picosecond optical pulses illuminate the PCA. These sub-picosecond current pulses cause the ultrashort freely propagate THz electric field pulses (with <1ps pulse width, i.e., sub-picosecond electromagnetic wave) from the THz emitter. The radiation and detection of the electric field E(t) happen as a function of time, t; hence, the spectrometer is called a time-domain spectrometer. The radiated THz electric field pulse E(t), in sub-picosecond temporal resolution, offers the amplitude and phase information of the radiation. The Fourier transform of this radiated electric field E(t) generates the electric field as a function of frequency, i.e., E(ω). The usable bandwidth of the PCA-based THz-TDS is limited to ~3.5THz in the air (in our EKSPLA system).

Numerous approaches are available in the literature to increase PCA efficiency. Techniques such as engineering various plasmonic nanostructures, depositing antireflection coating (ARC), inserting quantum dots (QDs), thin films, and textured photoconductive substrates, etc., enhance the carrier dynamics of the PCA. But such nanoscale engineering requires advanced and complex fabrication facilities. Therefore, easy, straightforward, and inexpensive techniques are developed in this work. This research presents PCAs based on graphene and gallium arsenide (GaAs) nanowires. The significant power and bandwidth enhancement in separate investigations with graphene and nanowires, especially at high frequencies, are reported. The PCA emitters are characterized in THz-TDS with a 780nm center wavelength laser. In this THz-TDS, a coplanar stripline and a dipole PCA serve as the emitter and detector, respectively. Therefore, during the characterization, the commercial coplanar stripline PCA emitter is replaced by the in-lab-developed PCAs. The low-temperature gallium arsenide (LT-GaAs) substrate is a typical material for the THz PCAs.

The graphene-based PCAs are developed and measured in two sets. The first set demonstrates three types of PCAs (coplanar stripline and bowtie with 5μm and 20μm antenna gap). In this set, 1μm LT-GaAs (Powerway CN) substrate is used. Compared to the without graphene PCAs, the measured radiated electric field enhancement from graphene-based PCAs are 40%, 73%, and 20%. In another set of PCAs, bowties with a 20μm gap are fabricated on the 1μm LT-GaAs (Powerway CN) and 3μm LT-GaAs (BATOP GER). Here, 1μm and 3μm represent the epitaxial layer thickness deposited on the LT-GaAs substrate. For further justification, bowtie PCAs on both substrates are separately measured and compared (with and without graphene). The measured radiated electric field enhancement from the graphene-based 1μm LT-GaAs (Powerway CN) bowtie PCA is 27% more than the without 1μm LT-GaAs (Powerway CN) bowtie PCA. Similarly, the radiated electric field enhancement from the graphene-based 3μm LT-GaAs (BATOP GER) is 32% compared to the without graphene-based 3μm LT-GaAs (BATOP GER) bowtie PCA.

Next, two sets of PCAs with gallium arsenide nanowires (GaAs nanowires) are presented. In the first set, 1μm LT-GaAs (Powerway CN) substrate is used to make the PCAs: coplanar stripline and bowtie. The measured radiated electric field enhancements from these nanowires-based PCAs, compared to the without nanowires, are 8.8% and 46%. However, a semi-insulated gallium arsenide (SI-GaAs) substrate is used in the second set of nanowires-based PCAs. Replacing the expensive LT-GaAs substrate with an inexpensive SI-GaAs substrate (with nanowires) doubled the electric radiation field from PCA. Compared to the commercial coplanar stripline PCA emitter of the THz-TDS, the power increment at the lower and higher frequency region is recorded.

Further, a novel circularly polarized THz photoconductive antenna (CP PCA) emitter combining bowtie PCA and a wideband circular polarizer is presented. The proposed thin-film circular polarizer consists of three grid metal layers, which allow high transmission with a 3dB axial ratio (AR) bandwidth of 54.7% from 0.57 to 1THz. The in-house fabrication facility fabricates the PCA and the circular polarizer. The THz-TDS characterize the performance of the CP PCA. The successful realization of the CP PCA in this work can find wide-spreading applications, such as THz circular dichroism spectroscopy, THz Muller imaging, and ultra-high-speed THz wireless communications.

In this thesis, Chapter 1 explains the THz spectrum, its properties, and applications. Various THz sources, including a THz-TDS, are also described in this chapter. Chapter 2 is dedicated to the PCA, including its literature and the research methodology for the reported research work. Chapters 3 and 4 demonstrate the integration of graphene and GaAs nanowires with PCAs for performance enhancement. A novel compact circularly polarized THz photoconductive antenna (CP PCA) emitter combining bowtie PCA and a wideband circular polarizer is illustrated in Chapter 5. And at last, the outcomes of the research work are concluded in Chapter 6.