Photoconductive Dipole Research Articles

Theoretical investigations on the influence of photonic crystal shapes on the performance of graphene-based terahertz photoconductive bowtie dipole antenna

Graphene-based photoconductive bowtie dipole antenna for terahertz radiation is investigated under different geometrical shapes of photonic crystal employed in the semi-conductor substrate. The antenna is designed using graphene radiator and Gallium Arsenide substrate to operate at 1.4 THz, 1.9 THz and 2.1 THz frequencies. Cylindrical, square, triangular and hexagonal shaped photonic crystal substrates are investigated. The analysis is carried out for various antenna parameters such as reflection coefficient, VSWR, directivity and radiation efficiency. Further, polarisation aspects such as axial ratio and cross-polarisation level are also discussed. Opto-electronic simulation is performed to determine the influence of photonic crystal shapes on the emission characteristics of photoconductive antenna. Simulation results indicate that hexagonal-shaped photonic crystals provide directivity of 15 dBi, axial ratio of 29 dB, efficiency of 98% and emission spectrum bandwidth of 220 GHz, which ascertain its suitability for terahertz photoconductive antenna.

Photoconductive bowtie dipole antenna incorporating photonic crystal substrate for Terahertz radiation

In this work, a graphene-based Photoconductive bowtie dipole antenna on a photonic crystal substrate is investigated for Terahertz radiation. Basic graphene radiator involving uniform substrate operates at dual bands of 1.4 THz and 1.9 THz with an efficiency of 80% and directivity of 3.3 dBi. Photonic crystal structure is created by implanting periodic arrangement of cylindrical air holes in the Gallium Arsenide (GaAs) substrate to enhance the performance of the basic antenna design. Due to the suppression of surface wave modes by the photonic crystals, additional resonance is created at 2.1THz, making the antenna to operate at triple bands of 1.4 THz, 1.9 THz and 2.1 THz. Opto-electronic simulation is carried out and emission intensity spectrum is also determined for the first time, in a graphene-based terahertz bowtie dipole antenna involving photonic crystals in it. The directivity of the proposed antenna with photonic crystal is found to be 13.7 dBi, which is 10 dB improvement over the conventional design and efficiency of 95% is achieved.

A simple equivalent circuit model of photoconductive dipole antenna for the study of Terahertz intensity modulation

In this paper, a simple equivalent circuit model has been proposed for Ti–Au photoconductive dipole antenna to analyse its intensity modulation and radiation characteristics. The photocurrent and the terahertz emission intensity obtained using the proposed circuit model is verified using numerical method. Further, the equivalent circuit model is integrated with an electromagnetic tool to analyse its radiation characteristics and the photoconductive antenna is found to provide a directivity of 5.89 dBi at 1.58 THz. Intensity modulation of terahertz radiation is analysed using the equivalent circuit model by applying a bias voltage in the form of pulses. The modulated photocurrent output would produce the corresponding emission intensity which can be used for terahertz data transmission.

Enhancement of the intensity and bandwidth of terahertz radiation in photoconductive dipole antennas

In this paper, in order to amplify terahertz output pulses and achieve higher frequency peaks, a scheme of spiral lines is proposed to be used in the electrodes of photoconductive dipole antennas. This scheme is simulated with the help of the CST software. At first, a photoconductive antenna is simulated and compared with the experimental results. In order to improve the effectiveness of the antenna, certain changes are made in its shape. These changes are necessary because the intensity and frequency bandwidth of terahertz radiation in photoconductive antennas are less effective than other sources of terahertz waves and the shape of electrodes plays a fundamental role in the output of the radiation. As a result, adding spiral lines to antenna electrodes increases the intensity of terahertz radiation and shifts the frequency peak of that radiation toward higher frequencies.

Effect of Gallium Arsenide Charge Carrier Life Time on the Generation and Detection Efficiency of Continuous and Pulsed Terahertz Radiation

A configuration and test samples of photoconductive dipole antennas based on SI-GaAs:Cr and LT-GaAs for generation and detection of terahertz radiation are developed. Their operating characteristics in the pulsed mode and in the mode of operation as photomixers are experimentally investigated.

Определение влияния времени жизни носителей заряда арсенида галлия на эффективность непрерывной и импульсной генерации и детектирования излучения терагерцового диапазона частот

A configuration and test samples of photoconductive dipole antennas based on SI-GaAs: Cr and LT-GaAs for terahertz radiation generation and detection were developed. Their operating characteristics in pulsed mode and in the mode of operation as photomixers was experimentally investigated.

Beam steering characteristics of highly directive photoconductive dipole phased array antenna for terahertz imaging application

In this paper, the beam-steering characteristics of photoconductive dipole phased array antenna configuration at 1.95 THz is presented. The proposed array antenna configuration with frequency selective surface favourably improves its gain and directivity which is useful to upsurge the imaging capabilities to address the deliberations such as limited depth-of-field (DoF) and size-weight-and-power of the THz source for imaging applications. These are important considerations for applications like stand-off imaging and surveillance of moving targets where the high angular resolution as well as extended DoF are the important parameters for successful detection of concealed explosives. The projected planar profile and compact highly directive $$\left( {2 \times 2} \right)$$ small-gap photoconductive dipole phased array antenna can be castoff for the exposure of concealed explosives such as RDX, TNT, and HMX which illustrate their substantial spectral absorption fingerprints in terahertz (1.4–2.2 THz) regime of the spectrum. A simple method of beam-steering has been explored based on phase controlled optical excitation of highly directive small-gap photoconductive dipole array antenna. Further, the effects of uniform progressive phase shift on the beam-steering of uniform linear array (along x-axis) as well as planar array (x-axis and y-axis) is investigated.

Design of highly directive lens-less photoconductive dipole antenna array with frequency selective surface for terahertz imaging applications

In this paper, a highly directive photoconductive dipole array antenna is presented for imaging system operating at terahertz (THz) frequencies. The array antenna is formed on a single photoconductive substrate to make it compact and suitable for THz imaging applications. The presented photoconductive dipole array antenna improves the gain as well as directivity and therefore, is useful to enhance the imaging capabilities to address the considerations such as limited depth-of-field (DoF) that is the distance over which an object is considered in focus and size-weight-and-power (SWaP) of THz source for imaging applications. These are important considerations for applications like stand-off imaging and surveillance of moving targets where the high angular resolution as well as extended DoF is the key for successful detection of hidden explosives and illicit drugs. In addition to this, a technique of using frequency selective surface with photoconductive array antenna is also presented to further enhance the directivity of the radiating structure to yield high image resolution. Moreover, by using this technique, the radiation energy has been confined to the desired frequency band rather than spreading over a wide spectrum range.

Design of highly directive terahertz photoconductive dipole antenna using frequency-selective surface for sensing and imaging applications

The prospects for improving the gain and directivity of a photoconductive dipole antenna (PCA) using a bandpass frequency-selective surface (FSS) as a superstrate at terahertz frequencies for imaging and sensing applications are presented. The physical parameters of the proposed bandpass FSS for the PCA are determined using a simple synthesis technique based on a single square loop bandpass FSS. Furthermore, the effect of the unit-cell periodicity of the FSS on the resonance condition of the antenna structure is studied, and its effect on the gain, directivity, and front-to-back lobe ratio of the proposed antenna is also presented. Moreover, control over the transmission characteristics of the FSS in a particular frequency band is also discussed.

Terahertz antenna technology for imaging applications: a technical review

AbstractThe terahertz (THz) regime of the electromagnetic spectrum is rich with the emerging possibilities in imaging applications with unique characteristics to screening for weapons, explosives and bio-hazards, imaging of concealed objects, water content, and skin, and these advantages can be harnessed by using the effective THz sources and detectors. In THz imaging systems, the pulsed THz sources and detectors find unique applications and thus we have emphasized on re-visiting these kinds of systems. Several novel imaging techniques which exploit the distinctive properties of the THz systems have been presented. Moreover, the THz antenna is one of the most important components of a THz imaging system as it plays a significant role in both impedance matching and power source. Therefore, the recent developments in THz antenna design for imaging applications are reviewed and the potential challenges of such THz systems are investigated. The photoconductive antennas form the basis of many THz imaging and spectroscopy systems and finds promising applications in various scientific fields. However, for the imaging applications, there is a requirement of planar and compact THz antenna sources with on-chip fabrication and high directivity in order to achieve large depth-of-field for better image resolution. Therefore, the key modalities of improving photoconductive dipole antennas performance are identified for imaging applications. Also, the ways to improve the directivity of the photoconductive dipole antenna are discussed. The main purpose of this review is to provide an assortment of all relevant literature to bring researchers up-to-date on the current state-of-the-art and potential challenges of THz antenna technology for imaging applications.

Analytical framework of small-gap photoconductive dipole antenna using equivalent circuit model

A compact planar antenna sources with on-chip fabrication and high directivity in order to achieve large depth-of-field for better image resolution is the prospective demand for THz imaging application. Therefore, the small-gap photoconductive dipole antennas have been explored to fulfil such applications demand. However, there are certain modalities for improving the photoconductive dipole antenna performance which need to identify to accomplish high THz average radiated power and improved total efficiency. The unit-cell small-gap photoconductive dipole antenna radiation power enhancement methods need to optimize the design parameters with photoconductive material selection from theoretical simulation. Further, the potential improvement of coupling efficiency of THz wave with air as well as femto-second laser incident efficiency is also important parameters to enhance the radiation power of small-gap photoconductive dipole antenna. In this paper, we have presented an analytical procedure employing explicit mathematical expression leading to the physical behaviour of small-gap photoconductive dipole antenna. The effects of biased lines on the antenna performance parameters are discussed with the help of proposed equivalent circuit model. We have explored the effect of gap-size on the THz radiated power and on total radiation efficiency from the proposed photoconductive dipole antennas.

Analysis of highly directive photoconductive dipole antenna at terahertz frequency for sensing and imaging applications

In this paper, we have analyzed a photoconductive dipole antenna at terahertz (THz) frequency for sensing and imaging applications. For these applications, to detect powdered explosives, there is a need to have an image of the object under detection with high resolution to distinguish suspicious items from the normal background. The THz antenna is one of the most important components in a THz sensing and imaging system and there is a need to have such a transmitting THz source with high directivity and optimum radiation efficiency. Therefore, to achieve this, we have explored three photoconductive dipole antenna configurations to enhance the directivity and radiation efficiency. With the use of a simple dipole antenna and silicon lens, we are able to achieve the directivity of 10.7dBi and radiation efficiency of 91.59% in both E-plane and H-plane.

Photoconductive dipole antennas for efficient terahertz receiver

We designed various photoconductive antennas applicable to efficient terahertz (THz) receivers and experimentally investigated their detection characteristics. Three different antennas based on Grischkowsky (H-), I-, and bowtie shapes were fabricated on a 1.2-μm-thick low-temperature GaAs layer that was grown on a semi-insulating GaAs substrate and subsequently attached to extended hemispherical silicon lenses. The experimental results showed different characteristics for detection responsivity and agreed well with the theoretical prediction. Measurements of the peak-to-peak amplitudes of the detected THz photocurrent were approximately 67, 42, and 59nA for the H-shaped, I-shaped, and bowtie-shaped antennas, respectively. The I- and bowtie-shaped antennas provided higher THz detection sensitivities than the H-shaped antenna in the low-frequency region, i.e., below 0.6THz. At a frequency of 0.2THz, the I- and bowtie-shaped antennas offered an approximately 3.6-fold and 6-fold enhancement, respectively, in THz detection sensitivity compared to the H-shaped antenna. The bowtie-shaped antenna produced better peak amplitude and a wider spectral bandwidth than the I-shaped antenna. The observed detection peak frequencies of the I-shaped and bowtie-shaped antennas possessing very long dipole arms indicate that the lowest limit of the frequency detected in a typical THz-TDS using a GaAs photoconductive antenna as emitter/detector is around 0.2THz.

Application of broadband terahertz spectroscopy in semiconductor nonlinear dynamics

Semiconductor nonlinearity in the range of terahertz (THz) frequency has been attracting considerable attention due to the recent development of high-power semiconductor-based nanodevices. However, the under- lying physics concerning carrier dynamics in the presence of high-field THz transients is still obscure. This paper introduces an ultrafast, time-resolved THz pump/THz probe approach to study semiconductor properties in a nonlinear regime. The carrier dynamics regarding two mechanisms, intervalley scattering and impact ionization, was observed for doped InAs on a sub-picosecond time scale. In addition, polaron modulation driven by intense THz pulses was experimentally and theoretically investi- gated. The observed polaron dynamics verifies the interaction between energetic electrons and a phonon field. In contrast to previous work which reported optical phonon responses, acoustic phonon modulations were addressed in this study. A further understanding of the intense field interacting with solid materials will accelerate the development of semiconductor devices. This paper can be divided into 4 sections. Section 1 starts with the design and performance of a table-top THz spectrometer, which has the advantages of ultra-broad bandwidth (one order higher bandwidth compared to a conventional ZnTe sensor) and high electric field strength ( >100 kV/cm). Unlike the conventional THz time- domain spectroscopy, the spectrometer integrated a novel THz air-biased-coherent-detection (THz-ABCD) techni- que and utilized gases as THz emitters and sensors. In comparison with commonly used electro-optic (EO) crystals or photoconductive (PC) dipole antennas, the gases have the benefits of no phonon absorption as existing in EO crystals and no carrier life time limitation as observed in PC dipole antennas. In Section 2, the newly development THz-ABCD spectrometer with a strong THz field strength capability provides a platform for various research topics especially on the nonlinear carrier dynamics of semiconductors. Two mechanisms, electron intervalley scattering and impact ionization of InAs crystals, were observed under the excitation of intense THz field on a sub-picosecond time scale. These two competing mechanisms were demonstrated by changing the impurity doping type of the semiconductors and varying the strength of the THz field. Another investigation of nonlinear carrier dynamics in Section 3 was the observation of coherent polaron oscillation in n-doped semiconductors excited by intense THz pulses. Through modulations of surface reflection with a THz pump/THz probe technique, this work experimentally verifies the interaction between energetic electrons and a phonon field, which has been theoretically predicted by previous publications, and shows that this interaction applies for the acoustic phonon modes. Usually, two transverse acoustic (2TA) phonon responses are inactive in infrared measurement, while they are detectable in second-order Raman spectroscopy. The study of polaron dynamics, with nonlinear THz spectroscopy (in the far- infrared range), provides a unique method to diagnose the overtones of 2TA phonon responses of semiconductors, and therefore incorporates the abilities of both infrared and Raman spectroscopy. Finally, some conclusions were presented in Section 4. In a word, this work presents a new milestone in wave-matter interaction and seeks to benefit the industrial applications in high power, small scale devices.

三路太赫兹光导天线的功率合成技术

For the limited power capacity problem of single photoconductive antenna,we use photoconductive antenna power combining technology to research high power and ultra-broadband terahertz radiation. By constructing a power combining system of three-way photoconductive antenna,using500 μm aperture dipole photoconductive antenna as the object,we studied the power combining technology. When the optical path differences of terahertz emitted by three-way 500 μm aperture photoconductive dipole antenna are the same,the power combined time-domain spectral reaches to maximum peak,the degree of temporal coherence up to 90. 6%,the output power of terahertz is improved efficiently.

Photoconductive THz Antenna Designs With High Radiation Efficiency, High Directivity, and High Aperture Efficiency

Several linear polarized photoconductive terahertz antennas are reported and compared. A bowtie-shaped antenna, which is a variation on the standard photoconductive dipole antenna, is presented. It has satisfactory terahertz (THz) radiation performance characteristics. This bowtie-shaped antenna is combined with a silicon-based lens and with an artificial magnetic conductor (AMC) to enhance its directivity properties. These antennas provide the baseline designs in our study. A capacitively loaded dipole antenna is introduced, which naturally leads to an array implementation. The simulation results show that the peak directivity of a two-element array can be increased by 2.0 dB. The directivity and front-to-back ratio of the capacitively loaded dipole array are further enhanced by introducing a meta-film superstrate. Superdirective and high-efficiency performance characteristics are demonstrated. A THz grid antenna and its generalization to a grid antenna array are also developed. Comparisons demonstrate that these planar designs significantly outperform the lens-based reference case.

Characterization of a hollow core fibre-coupled near field terahertz probe

We describe the design and performance of a freely positionable THz near field probe based on a hollow core photonic crystal fibre-coupled photoconducting dipole antenna with an integrated sub-wavelength aperture. Experimental studies of the spatial resolution are compared with detailed finite element electromagnetic simulations and imaging artefacts that are a particular feature of this type of device are discussed. We illustrate the potential applications with descriptions of time domain near field studies of surface waves on a metamaterial and multimode propagation in a parallel plate waveguide.

Enhancement mechanism of terahertz radiation from coherent longitudinal optical phonons in undoped GaAs/n-type GaAs epitaxial structures

We have investigated the characteristics of monochromatic terahertz electromagnetic waves emitted from coherent longitudinal optical (LO) phonons in undoped GaAs/n-type GaAs epitaxial structures with various thicknesses, which range from 200 nm to 1200 nm, of the undoped GaAs layer. The terahertz waves were measured at room temperature using an optical gating method with a photoconductive dipole antenna. It was found that the intensity of the terahertz waves markedly depends on the thickness of the undoped GaAs layer. The analysis of Franz-Keldysh oscillations observed with photoreflectance spectroscopy demonstrates that the built-in electric field strength in the undoped GaAs layer, which results from the Fermi-level pinning at the surface, increases from 6.1 kV/cm to 28.4 kV/cm with a decrease in the thickness. It was revealed that the amplitude of the terahertz wave is proportional to the electric field strength in the undoped GaAs layer. This fact indicates that the efficiency of the terahertz radiation is dominated by the initial polarization of the LO phonons induced by the built-in electric field, which provides us a simple strategy for the enhancement of the terahertz radiation from the coherent LO phonons.

A hybrid time-domain model for pulsed terahertz dipole antennas

In this paper we study the effect of the length of dipole antennas on the spectrum of the radiated THz signal in pulse-excited opto-electronic terahertz systems. In particular, we investigate the origin of the commonly observed sharp dips that occur in the spectra of photoconductive dipole antennas, and explain them on the basis of reflections of the excitation current pulse that take place at the ends of the antenna. We develop a hybrid time-domain model for the system and show that the predictions of our model are in good agreement with experimental results.

Interactions between coherent optical phonons and excitonic quantum beats in GaAs/AlAs multiple quantum wells: strategy for enhancement of terahertz radiation from coherent optical phonons

AbstractWe have investigated the dynamical properties of coherent longitudinal optical (LO) phonons and excitonic quantum beats in GaAs/AlAs multiple quantum wells with various layer thicknesses, utilizing a reflection‐type pump‐probe technique and an optical gating method with a photoconductive dipole antenna. It is found that the pump‐probe signal of the coherent GaAs‐like LO phonon is markedly enhanced under the energy‐resonance condition of the LO phonon and the quantum beat due to the fundamental heavy‐hole and light‐hole excitons. This enhancement is caused by the coupling between the coherent LO phonon and excitonic quantum beat via the longitudinal polarization; namely, the quantum beat acts as a driving force for the coherent LO phonon. On the basis of this finding, we have demonstrated that the terahertz radiation from the coherent LO phonon is considerably intensified at room temperature by the longitudinal polarization coupling described above. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)