Nathan M. Burford

Computational modeling of plasmonic thin-film terahertz photoconductive antennas

This work presents a computational approach for investigating terahertz photoconductive antennas with enhanced performance via thin-film plasmonic electrode configurations. The commercially available finite element method solver COMSOL Multiphysics is implemented to solve Maxwell’s wave equations along with the coupled drift-diffusion/Poisson’s equations. The proposed approach is compared with other computational and experimental results from the literature, showing good agreement. A nanodisk array is deposited on top of a 120 nm LT-GaAs layer with the antenna electrodes located below the photoconductive layer. A femtosecond optical pump is utilized to excite the photoconductive antenna. The obtained results demonstrated significant increase in the conversion of optical energy to photocurrents as compared with conventional antennas and other plasmonic antennas from the literature. The proposed thin-film antenna with plasmonic nanostructures showed the greatest improvement in peak photocurrent—almost 336 times higher than the conventional antenna. Additionally, the thin-film antenna demonstrates a fast device response time even at a long carrier lifetime of 48 ps. The results support the capability of the proposed design to yield high optical-to-terahertz conversion efficiency, addressing the problem of low output power in terahertz photoconductive antennas.

Review of terahertz photoconductive antenna technology

Abstract. Photoconductive antennas (PCAs) have been extensively utilized for the generation and detection of both pulsed broadband and single frequency continuous wave terahertz (THz) band radiation. These devices form the basis of many THz imaging and spectroscopy systems, which have demonstrated promising applications in various industries and research fields. The development of THz PCA technology through the last 30 years is reviewed. The key modalities of improving device performance are identified, and literature is reviewed to summarize the progress made in these areas. The goal of this review is to provide a collection of all relevant literature to bring researchers up to date on the current state and remaining challenges of THz PCA technology.

Optimization of Silver Nanotoroid Arrays for the Absorption Enhancement of Silicon Thin-Film Solar Cells

The enhancement of absorbed electromagnetic energy of thin-film silicon photovoltaics due to toroid-shaped plasmonic nanoparticles is computationally investigated. Using Ansys® HFSS, infinite arrays of silver nanotoroids of various sizes are tuned to maximize the photocurrent generation of the photovoltaic. The obtained results show that larger nanotoroid arrays can be tuned to provide enhanced photocurrent generation that is comparable to traditional sphere-shaped nanoparticles. The highly tunable nature of the resonant frequencies of plasmonic nanotoroid geometries is investigated here, which hold potential advantage over nanoparticles in their ability to enhance electromagnetic energy absorption in the longer wavelength regime of the solar spectrum. The obtained results show that larger nanotoroid arrays can be tuned to provide enhanced photocurrent generation comparable to traditional nanoparticles.

Field enhancement due to surface structuring during aluminum induced crystallization of amorphous silicon

Aluminum induced crystallization (AIC) of amorphous silicon (a-Si) may potentially be causing the formation of plasmonic nanostructures on the silicon surface. Field enhancement within the silicon layer will be quantified in order to develop an understanding of the observed enhancement. Computer simulations using HFSS are presented here. The electric fields absorbed inside the silicon are obtained as a function of the incident wavelength due to irregular nanostructures of aluminum patches to simulate the induced aluminum-silicon patches.

Enhancement of terahertz imaging of packaged power electronic devices

Nondestructive imaging of packaged silicon carbide power transistors was performed using terahertz (THz) time-domain reflection imaging. Techniques were developed to process the data gathered by these measurements and enhance the quality of the THz time-domain images. It was found that applying high-pass error function filters in the frequency domain provided the best balance of improving image clarity and minimizing distortion of the time-domain signal. The results indicate that with proper data processing, THz imaging can be a viable nondestructive method for inspecting packaged power electronic devices.

Optimization of nanotoroid arrays for plasmonic solar cell applications

This work presents the optimization of silver plasmonic nanotoroids in an infinite square array configuration located on top of an amorphous silicon substrate. Using the computational electromagnetics software Ansys® HFSS, the electromagnetic energy absorption enhancement in the silicon layer is optimized by varying the geometric configuration. Percentage enhancement of the generated photocurrent is approximated and is used to compare the performance of various nanotoroid designs, as well as comparison to other geometries.

Absorption enhancement in silicon solar cells due to surface plasmons of nanotoroids

Enhanced absorption in silicon is needed to improve the efficiency of solar cells. Silver nanotoroids placed on top of a silicon substrate demonstrate larger absorption, will be presented. The method of moments is employed to compute the near electric fields absorbed in silicon substrate. The results show surface waves represented by large electric fields normal to the silicon surface and decayed as it gets deeper in the medium.

Simulation, fabrication, and measurement of a plasmonic-enhanced terahertz photoconductive antenna

In this work a new plasmonic thin-film based terahertz photoconductive antenna is proposed. The computational method utilized to design the antenna is outlined, as well as the steps and preliminary results for the fabrication and characterization of the device. The model predicted over two orders of magnitude increase in the peak photocurrent as compared to a conventional device design, while slightly reducing the width of the induced current pulse. This indicates that the proposed design will be effective as a high efficiency terahertz emitter. In addition to the computational modeling, preliminary results demonstrating the proposed fabrication processes and experimental characterization are presented. It is demonstrated that when using a pyroelectric detector to quantify the output terahertz power it is important to first quantify the power of the IR photons generated by thermal relaxation in the device.

Modeling of plasmonic terahertz antennas using COMSOL ® multiphysics

A computational model allowing for the study of terahertz photoconductive antennas is presented. This model allows for nanoscale geometry variations in three dimensions. The model is validated using measurements data. The preliminary results illustrate the effectiveness of the model in calculating the time-dependent photocurrent collected by the antenna as well as the total photocurrent enhancement of the terahertz antenna as compared to the non-plasmonic design. This model will be used as a tool to design a more enhanced plasmonic terahertz photoconductive antenna as will be presented in the conference.

Time-domain terahertz imaging and spectroscopy of x-ray blocking and scattering coatings

The terahertz (THz) band of the electromagnetic spectrum is the gap between the microwave/millimeter wave band and the far infrared band. This band has been historically defined to be around 0.1 to 10 THz, due to the long-standing difficulties in efficient generation and detection of these frequencies. Only recently have efficient and practical commercial THz systems become available. Imaging with THz waves offers several unique advantages. THz waves can penetrate several millimeters into non-conducting materials. This allows for imaging of features that are covered in an optically opaque coating. Since THz waves have a shorter wavelength than microwaves, they are able to resolve smaller features. Unlike the x-ray imaging that is often associated with imaging into materials, the low photon energy of THz waves cannot ionize materials. This negates the risk of irreversible material damage inherent with x-ray imaging.