Frontiers of Terahertz Technologies - 115DAB/WS2024 GitHub Wiki

Ava Asmani (405581652), Amanda Hacker (105483827) Department of Electrical and Computer Engineering University of California, Los Angeles Los Angeles, California, United States of America Email: {ava24, amandahacker}@ucla.edu

I. INTRODUCTION

With recent advancements in high frequency circuits har- nessing the singular properties of sub-mm waves, terahertz (THz) technology are now being considered a promising frontier with the potential to transform imaging, security, and wireless communications. The high operating frequency range of terahertz technologies possesses unique qualities that enable capabilities not achievable by microwave and radio frequency circuits. To provide a comprehensive overview of Terahertz technologies, this article will discuss the electromagnetic the- ory of terahertz waves, the history and development of modern terahertz technology, current and potential applications and existing challenges or limitations.

II. THEORY AND HISTORY

Terahertz technologies are broadly categorized as electronic systems utilizing electromagnetic waves between 100 GHZ to 10 THz, otherwise known as the Terahertz gap between mi- crowaves and infrared light. The frequency range of terahertz waves correspond to sub-mm wavelength enabling passage through many materials except water and air, production of high resolution images, accurate spectroscopy capabilities of various compounds, and non-ionizing scans of biological tissue, making them less likely to cause cancer and genetic mutations than X-rays [1]. Initial interest in terahertz was piqued by the unique prop- erties and potential benefits of utilizing such wavelengths [2]. Interest in exploring a gap of the electromagnetic spectrum was explicitly noted by H. Rubens and E.F. Nichols in 1897. Microwave radars developed during World War II enabled terahertz spectroscopy, which assisted in the discovery of tera- hertz frequencies present in the thermal black body spectrum. Notable advancements in the development of terahertz wave sources were made in the 1980s with the creation of accurate sources still used today and detectors for terahertz waves. In 1993, a basis for generating terahertz radiation serves as the blueprint for modern spectroscopy or the analysis of molecular composition through their absorption and emission patterns of electromagnetic radiation and imaging done with these tech- nologies. In the 90s, researchers began exploring the terahertz response in transistor, thus starting the exploration of circuit design for frequencies in the terahertz gap [3]. In present day, researchers have made significant progress in imaging in medicine, security, astronomy, and environmental fields, and Fig. 1: Transistor cutoff frequency vs Transistor density for SiGe BiCMOS and Silicon CMOS technologies [4]. are even exploring terahertz wireless communication to utilize untouched bandwidth. Fig. 1: Transistor cutoff frequency vs Transistor density for SiGe BiCMOS and Silicon CMOS technologies [4].

III. APPLICATION AND LIMITATION IN WIRELESS

COMMUNICATIONS

As the use of electronic devices expands so will demands for increasing data rates. Since more users are occupying frequencies in modern networks, the amount of available frequencies must grow. Terahertz technologies can operate at frequencies in the terahertz range and all sub-frequencies. With the ability to access additional frequencies unable to be previously accessed, terahertz technologies can alleviate frequency range limitations by broadening the bandwidth, or available frequencies for use. Wider bandwidths can theoretically alleviate demands and open up possibilities beyond state-of-the-art 5G networks\cite{Elayan2018}. This is because the additional high frequencies available to the network can decrease latency and increase download speeds, as frequency is directly proportional to wave speed. Moving the operational frequency band to the untouched terahertz band can also increase data transmission significantly to at least 1TB per second and achieve the speed and power required of a wireless network to support fully autonomous systems or applications like holograms and multi-sense communication \cite{HUANG2023}. Operation at such high frequencies necessitate design considerations to ensure functional operation. In order to curtail the increased noise and distortion resulting from wide bandwidth communications systems, high carrier frequencies are used in terahertz technologies. Many silicon technologies needed at the transmitter and receiver of wireless communication networks cannot be scaled to properly operate at those high frequencies, however SiGe BiCMOS has shown promise in achieving terahertz frequencies, as shown in Figure \ref{Fig:diffCMOS}. SiGe BiCMOS is a semiconductor technology made of a Silicon-Germanium alloy that merges bipolar junction and MOS transistor technologies that has the ability to integrate analog, RF and digital signals on a single chip \cite{Payam2021}. Another concern is that of preserving signal power of terahertz waves during transmission. According to the Friis transmission equation seen in (1), received signal power is inversely proportional to the square of the signal’s frequency and the communication range, or distance between the transmitter and the receiver. This formula quantifies the performance and directivity of a transmitting and receiving antenna pair, where $P_r$ and $P_t$ are the power at and into the receiving and transmitting antennas, $A_r$ and $A_t$ are the effective aperture areas of the receiving and transmitting antennas, $d$ is the distance between the antennas, and $\lambda$ is the wavelength of the radio frequency.

P_r/P_t = (A_rA_t)(d^2lambda^2) (1)

Higher frequency signals are also more susceptible atmospheric absorption and polarization mismatch between the transmitting and receiving antennas \cite{Payam2021}. One proposed method of increasing signal power while still preserving communication range is increasing antenna gain by using directional antennas or beam-switching antennas \cite{Naqvi2018}. Another design challenges facing terahertz technologies are the limitations of higher modulation schemes. Wide bandwidths offered by incorporating frequencies of the terahertz gap may be taken advantage of with higher modulation schemes such as 2048 QAM, 4096 QAM in order to improve data rates. QAM modulation schemes allow for compression of data by assigning data points to certain avaialable frequencies, amplitudes or phase shifts for optimal communication\cite{Besnoff2015}. However, the data converters required to perform modulation and demodulation such as Analog to Digital Converters (ADCs) and Digital to Analog Converters (DACs) exhibit poor capabilities at high sampling rates required by proper modulation of terahertz waves and large power consumption at high frequencies \cite{Payam2021}.

IV. APPLICATIONS IN IMAGING

Fig. 2: (a) Terahertz image with pixel size of 500 um and (b) Hematoxylin-eosin stained image of fresh rat brain tissue showing the calculated probability of finding cancerous tissue (red) versus normal tissue (blue) at a given point [8].

Advancements in terahertz technology have contributed greatly to medical imaging, especially in detecting and iden- tifying cancerous tumors. Earlier imaging techniques can identify tumors but often only in later cancer stages and with an unclear margin, increasing difficulty of local removal and increasing the risk for repeat surgeries or chances of metastasis. However, in addition to the higher resolution and non-ionizing radiation achieved by the terahertz frequency band, engineers have made use of the high attenuation, or loss, through water of terahertz radiation in cancer diagnosis research. Medical studies have shown that cancerous tumors have increased blood flow due to the cells’ rapid multiplication and therefore higher cell density and water content. The difference in passage of terahertz radiation through normal and cancerous tissue has allowed scientists to study the refractive indices and absorption coefficients in spectroscopy and imag- ing of specific tumors with respect to surrounding, healthy tissue. This application of terahertz technologies have seen much success. For example, [8] used terahertz imaging and spectroscopy on gliomas, a type of primary brain tumor that is difficult to treat and whose borders can only be determined preoperatively though MRI or fluorescent dye with minimal certainty, in rat brain tissue which can be seen in Fig. 2. By determining the refractive indices and absorption coefficients, researchers were able to develop an algorithm to determine the probability of tissue being cancerous or normal in a high resolution while improving detection of tumor margins and surrounding edema [8].

Fig. 3: Thz images of xenograft mice tumor (human tumor transplanted into immunocompromised mice) (a) pathology image of hematoxylin-eosin stained tissue, (b) THz image of freshly excised tissue, (c) pathology image of sample in (a), (d) Thz image of formalin-fixed, paraffin-embedded (FFPE) block tissue, and (e) statistical classification [9].

Another example of successful imaging using terahertz technologies is detection of breast cancer tumors, which collectively accounts for over 10 percent of all global can- cer cases. Margin assessment of breast cancer tumors with conservative tissue excision is most critical as studies showed that patients preferred lumpectomy (local removal of the tumor) over a mastectomy (removal of one or both breasts) as the latter improves upon recurrence prevention and cosmetic Fig. 2: (a) Terahertz image with pixel size of 500 um and (b) Hematoxylin-eosin stained image of fresh rat brain tissue showing the calculated probability of finding cancerous tissue (red) versus normal tissue (blue) at a given point [8]. Fig. 3: Thz images of xenograft mice tumor (human tumor transplanted into immunocompromised mice) (a) pathology image of hematoxylin-eosin stained tissue, (b) THz image of freshly excised tissue, (c) pathology image of sample in (a), (d) Thz image of formalin-fixed, paraffin-embedded (FFPE) block tissue, and (e) statistical classification [9]. damage. Spectroscopy measurements successfully determined the differences in refractive indexes between collagen and fat in healthy breast tissue versus cancerous tissue at ranges of terahertz radiation between 0.5 - 4THz. Similar success was seen in terahertz imaging, where tumor borders against healthy fat are more clearly defined in high resolution spectral graphs than in hematoxylin-eosin (HE) stained pathology samples, the current golden standard of margin assessment as seen in Fig. 3 [9].

V. CONCLUSION

Biomedical imaging and wireless communications provide just a glimpse of the capabilities of terahertz technology. Other applications include spectroscopy of particles in the environment to determine pollutants or drugs and explosives and advanced security imaging from airport screenings to military-grade, anti-stealth radars [1]. However, challenges remain for terahertz technology as costs remain extremely high to mass-produce and miniaturize terahertz chips, sources, and detectors, many standard circuit components cease to function in the terahertz band, and the lossy nature of terahertz waves in air and water, preventing long range communication. Though these continue to hinder terahertz technology’s accessibility to the general public and entrance to industry in consumer applications, research and development continues to show its great promise across multiple disciplines that will improve all aspects of everyday life.

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REFERENCES

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