Abstract-Prediction of leaf water potential and relative water content using terahertz radiation spectroscopy

Marvin Browne, Nezih Tolga Yardimci, Christine Scoffoni, Mona Jarrahi, Lawren Sack,

https://onlinelibrary.wiley.com/doi/full/10.1002/pld3.197

Increases in the frequency and severity of droughts across many regions worldwide necessitate an improved capacity to determine the water status of plants at organ, whole plant, canopy, and regional scales. Noninvasive methods have most potential for simultaneously improving basic water relations research and ground‐, flight‐, and space‐based sensing of water status, with applications in sustainability, food security, and conservation. The most frequently used methods to measure the most salient proxies of plant water status, that is, water mass per leaf area (WMA), relative water content (RWC), and leaf water potential (Ψ_leaf), require the excision of tissues and laboratory analysis, and have thus been limited to relatively low throughput and small study scales. Applications using electromagnetic radiation in the visible, infrared, and terahertz ranges can resolve the water status of canopies, yet heretofore have typically focused on statistical approaches to estimating RWC for leaves before and after severe dehydration, and few have predicted Ψ_leaf. Terahertz radiation has great promise to estimate leaf water status across the range of leaf dehydration important for the control of gas exchange and leaf survival. We demonstrate a refined method and physical model to predict WMA, RWC, and Ψ_leaf from terahertz transmission across a wide range of levels of dehydration for given leaves of three species, as well as across leaves of given species and across multiple species. These findings highlight the powerful potential and the outstanding challenges in applying in vivo terahertz spectrometry as a remote sensor of water status for a range of applications.

Abstract-Nanostructure‐Enhanced Photoconductive Terahertz Emission and Detection

Nezih Tolga Yardimci,  Mona Jarrahi, 

https://onlinelibrary.wiley.com/doi/abs/10.1002/smll.201802437

Photoconductive antennas are commonly used for terahertz wave generation and detection. However, their relatively low radiation power and detection sensitivity often place limitations on the signal‐to‐noise ratio and operation bandwidth of terahertz imaging and spectroscopy systems. Several different techniques are attempted to address these limitations. The most promising ones take advantage of the unique tools provided by nanotechnology. In this review, the recent nanotechnology‐enabled advances in photoconductive antennas, which use nanostructures, such as optical nanoantennas, plasmonic structures, and optical nanocavities, to increase the interaction of the optical pump beam with the photoconductive semiconductor, are discussed. All of these techniques are experimentally demonstrated to be efficient tools for enhancing the performance of photoconductive antennas for terahertz wave generation and detection.

Abstract-High Sensitivity Terahertz Detection through Large-Area Plasmonic Nano-Antenna Arrays

Nezih Tolga Yardimci,  Mona Jarrahi,

https://www.nature.com/articles/srep42667

Plasmonic photoconductive antennas have great promise for increasing responsivity and detection sensitivity of conventional photoconductive detectors in time-domain terahertz imaging and spectroscopy systems. However, operation bandwidth of previously demonstrated plasmonic photoconductive antennas has been limited by bandwidth constraints of their antennas and photoconductor parasitics. Here, we present a powerful technique for realizing broadband terahertz detectors through large-area plasmonic photoconductive nano-antenna arrays. A key novelty that makes the presented terahertz detector superior to the state-of-the art is a specific large-area device geometry that offers a strong interaction between the incident terahertz beam and optical pump at the nanoscale, while maintaining a broad operation bandwidth. The large device active area allows robust operation against optical and terahertz beam misalignments. We demonstrate broadband terahertz detection with signal-to-noise ratio levels as high as 107 dB.

Abstract-A High-Power Broadband Terahertz Source Enabled by Three-Dimensional Light Confinement in a Plasmonic Nanocavity

Nezih Tolga Yardimci, Semih Cakmakyapan, Soroosh Hemmati,  Mona Jarrahi,

https://www.nature.com/articles/s41598-017-04553-4

The scope and potential uses of time-domain terahertz imaging and spectroscopy are mainly limited by the low optical-to-terahertz conversion efficiency of photoconductive terahertz sources. State-of-the-art photoconductive sources utilize short-carrier-lifetime semiconductors to recombine carriers that cannot contribute to efficient terahertz generation and cause additional thermal dissipation. Here, we present a novel photoconductive terahertz source that offers a significantly higher efficiency compared with terahertz sources fabricated on short-carrier-lifetime substrates. The key innovative feature of this source is the tight three-dimensional confinement of the optical pump beam around the terahertz nanoantennas that are used as radiating elements. This is achieved by means of a nanocavity formed by plasmonic structures and a distributed Bragg reflector. Consequently, almost all of the photo-generated carriers can be routed to the terahertz nanoantennas within a sub-picosecond time-scale. This results in a very strong, ultrafast current that drives the nanoantennas to produce broadband terahertz radiation. We experimentally demonstrate that this terahertz source can generate 4 mW pulsed terahertz radiation under an optical pump power of 720 mW over the 0.1–4 THz frequency range. This is the highest reported power level for terahertz radiation from a photoconductive terahertz source, representing more than an order of magnitude of enhancement in the optical-to-terahertz conversion efficiency compared with state-of-the-art photoconductive terahertz sources fabricated on short-carrier-lifetime substrates.

Abstract-Impact of the Metal Adhesion Layer on the Radiation Power of Plasmonic Photoconductive Terahertz Sources

Deniz Turan, Sofia Carolina Corzo-Garcia, Nezih Tolga Yardimci, Enrique Castro-Camus, Mona Jarrahi

https://link.springer.com/article/10.1007%2Fs10762-017-0431-9

The use of plasmonic contact electrodes in a photoconductive terahertz source offers high optical-to-terahertz conversion efficiencies. The high efficiency is because plasmonic contact electrodes concentrate a large portion of the incident optical pump beam in close proximity to the contact electrodes. By reducing the average transport path length of the photo-generated carriers from the contact electrodes, a larger number of the photocarriers drift to the terahertz radiating elements of the photoconductive source within a sub-picosecond time scale. Therefore, higher terahertz radiation power levels are achieved compared to a similar photoconductive source without plasmonic contact electrodes. Au is a preferred metal for plasmonic contact electrodes because of the strong plasmonic enhancement factors it offers at near-infrared optical wavelengths. However, it requires an adhesion layer to stick well to most III–V semiconductor substrates used in photoconductive terahertz sources. In this paper, we analyze the impact of the Au adhesion layer on the performance of plasmonic photoconductive sources fabricated on a GaAs substrate. Our analysis suggests that Cr is the most promising adhesion layer for plasmonic contact electrodes. We show that the use of a Cr adhesion layer instead of Ti, which is used in previously demonstrated plasmonic photoconductive sources, offers up to an 80% enhancement in the generated terahertz powers. We report record-high terahertz power emissions of up to 6.7 mW from plasmonic photoconductive sources with Cr/Au contacts.

Abstract-Three-dimensional plasmonic light concentrators for efficient terahertz generation

Nezih Tolga Yardimci,  Semih Cakmakyapan, Soroosh Hemmati, Mona Jarrahi

http://ieeexplore.ieee.org/document/7999516/

Photoconductive antennas are extensively used in time-domain terahertz imaging and spectroscopy systems to generate terahertz radiation [1, 2]. These emitters consist of a terahertz antenna fabricated on a photoconductive semiconductor. When the semiconductor is pumped by a femtosecond laser and a bias voltage is applied to the antenna arms, an ultrafast photocurrent is generated. As this photocurrent drives the antenna, a pulsed terahertz radiation is generated. However, only the carriers that drift to the antenna arms in a sub-picosecond time scale can efficiently contribute to the generation of terahertz radiation. The rest of the photocarriers, namely the slow photocarriers, cause extra thermal dissipation and degrade device reliability. To improve device reliability, short carrier lifetime semiconductors are often used, which recombine the slow carriers and prevent early thermal breakdown. However, short carrier lifetime semiconductors cannot offer high carrier drift velocities. Therefore, the radiation efficiency of photoconductive emitters fabricated on short carrier lifetime substrates is limited. In this work, we present a highly reliable and efficient photoconductive terahertz emitter that circumvents the use of short carrier lifetime substrates by utilizing three-dimensional plasmonic light concentrators.

UCLA engineers develop high-performance terahertz detectors

Nanoantenna array’s higher signal-to-noise ratios mean it can find faint target signals

       UCLA electrical engineering graduate student Nezih Tolga Yardimci.

By Matthew Chin
http://newsroom.ucla.edu/releases/ucla-engineers-develop-high-performance-terahertz-detectors

Researchers from the UCLA Henry Samueli School of Engineering and Applied Science have developed a new antenna array that greatly expands the operation bandwidth and level of sensitivity for imaging and sensing systems that use terahertz frequencies.

Terahertz frequencies are an underused part of the electromagnetic spectrum that lies between the infrared and microwave bands. The unique features of this part of the spectrum could be useful for biological sensing and medical imaging, chemical identification and material characterization.

“For example, a terahertz-based imaging system could allow doctors to see how wounds are healing underneath bandages,” said Mona Jarrahi, associate professor of electrical engineering in the UCLA Henry Samueli School of Engineering and Applied Science and the principal investigator of the research. The study was published in Scientific Reports, an open-access journal from Nature.

However terahertz technology is not yet mature. One component researchers are aiming to make more efficient is a terahertz detector, which receives the terahertz signals, much like photodetectors in a camera that sense light to produce an image.

By operating across a broader bandwidth, the new nanoscale antenna array developed by Jarrahi and Nezih Tolga Yardimci, a UCLA graduate student in electrical engineering, can extract more information about material characteristics. The device’s higher signal-to-noise ratios mean it can find faint target signals. For example, the new terahertz detector can be tuned to detect certain chemicals even when target molecules are present in miniscule amounts. It can also be used to image both the surface of the skin, and deeper tissue layers.

The unique nanoscale geometry of the antenna array addresses the bandwidth and sensitivity problems of previously used terahertz detectors, the researchers said. 

“Up close, it looks like a row of small grates,” Yardimci said. “We specifically designed the dimensions of the nanoantenna elements and their spacing such that an incoming terahertz beam is focused into nanoscale dimensions, where it efficiently interacts with a stream of optical pump photons to produce an electrical signal proportional to the terahertz beam intensity.”

Jarrahi said: “The broad operation bandwidth and high sensitivity of this new type of terahertz detector extends the scope and potential uses of terahertz waves for many imaging and sensing applications.”

The research was supported by financial support from Moore Inventor Fellowship and the Presidential Early Career Award for Scientists and Engineers.

Abstract-High Sensitivity Terahertz Detection through Large-Area Plasmonic Nano-Antenna Arrays

• Nezih Tolga Yardimci
•  & Mona Jarrahi
• • 
    

http://www.nature.com/articles/srep42667

Plasmonic photoconductive antennas have great promise for increasing responsivity and detection sensitivity of conventional photoconductive detectors in time-domain terahertz imaging and spectroscopy systems. However, operation bandwidth of previously demonstrated plasmonic photoconductive antennas has been limited by bandwidth constraints of their antennas and photoconductor parasitics. Here, we present a powerful technique for realizing broadband terahertz detectors through large-area plasmonic photoconductive nano-antenna arrays. A key novelty that makes the presented terahertz detector superior to the state-of-the art is a specific large-area device geometry that offers a strong interaction between the incident terahertz beam and optical pump at the nanoscale, while maintaining a broad operation bandwidth. The large device active area allows robust operation against optical and terahertz beam misalignments. We demonstrate broadband terahertz detection with signal-to-noise ratio levels as high as 107 dB.