Nezih Tolga Yardimci

High-Power Terahertz Generation Using Large-Area Plasmonic Photoconductive Emitters

In this paper, we present a novel design of large-area photoconductive emitters which incorporates plasmonic contact electrodes to offer significantly higher optical-to-terahertz conversion efficiencies compared with conventional designs. Use of plasmonic contact electrodes enables a more efficient separation and acceleration of photocarriers, enhancing the effective dipole moment induced within the device active area in response to an incident optical pump. At an optical pump power level of 240 mW, we demonstrate broadband, pulsed terahertz radiation with radiation power levels as high as 3.8 mW over the 0.1-5-THz frequency range, exhibiting an order of magnitude higher optical-to-terahertz conversion efficiency compared with conventional designs.

Impact of substrate characteristics on performance of large area plasmonic photoconductive emitters

We present a comprehensive analysis of terahertz radiation from large area plasmonic photoconductive emitters in relation with characteristics of device substrate. Specifically, we investigate the radiation properties of large area plasmonic photoconductive emitters fabricated on GaAs substrates that exhibit short carrier lifetimes through low-temperature substrate growth and through epitaxially embedded rare-earth arsenide (ErAs and LuAs) nanoparticles in superlattice structures. Our analysis indicates that the utilized substrate composition and growth process for achieving short carrier lifetimes are crucial in determining substrate resistivity, carrier drift velocity, and carrier lifetime, which directly impact optical-to-terahertz conversion efficiency, radiation power, radiation bandwidth, and reliability of large area plasmonic photoconductive emitters.

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

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.

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

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.

High power telecommunication-compatible photoconductive terahertz emitters based on plasmonic nano-antenna arrays

We present a high-power and broadband photoconductive terahertz emitter operating at telecommunication optical wavelengths, at which compact and high-performance fiber lasers are commercially available. The presented terahertz emitter utilizes an ErAs:InGaAs substrate to achieve high resistivity and short carrier lifetime characteristics required for robust operation at telecommunication optical wavelengths. It also uses a two-dimensional array of plasmonic nano-antennas to offer significantly higher optical-to-terahertz conversion efficiencies compared to the conventional photoconductive emitters, while maintaining broad operation bandwidths. We experimentally demonstrate pulsed terahertz radiation over 0.1-5 THz frequency range with the power levels as high as 300 μW. This is the highest-reported terahertz radiation power from a photoconductive emitter operating at telecommunication optical wavelengths.

A polarization-insensitive plasmonic photoconductive terahertz emitter

We present a polarization-insensitive plasmonic photoconductive terahertz emitter that uses a two-dimensional array of nanoscale cross-shaped apertures as the plasmonic contact electrodes. The geometry of the cross-shaped apertures is set to maximize optical pump absorption in close proximity to the contact electrodes. The two-dimensional symmetry of the cross-shaped apertures offers a polarization-insensitive interaction between the plasmonic contact electrodes and optical pump beam. We experimentally demonstrate a polarization-insensitive terahertz radiation from the presented emitter in response to a femtosecond optical pump beam and similar terahertz radiation powers compared to previously demonstrated polarization-sensitive photoconductive emitters with plasmonic contact electrode gratings at the optimum optical pump polarization.

All-optical machine learning using diffractive deep neural networks

All-optical deep learning Deep learning uses multilayered artificial neural networks to learn digitally from large datasets. It then performs advanced identification and classification tasks. To date, these multilayered neural networks have been implemented on a computer. Lin et al. demonstrate all-optical machine learning that uses passive optical components that can be patterned and fabricated with 3D-printing. Their hardware approach comprises stacked layers of diffractive optical elements analogous to an artificial neural network that can be trained to execute complex functions at the speed of light. Science, this issue p. 1004 All-optical deep learning can be implemented with 3D-printed passive optical components. Deep learning has been transforming our ability to execute advanced inference tasks using computers. Here we introduce a physical mechanism to perform machine learning by demonstrating an all-optical diffractive deep neural network (D2NN) architecture that can implement various functions following the deep learning–based design of passive diffractive layers that work collectively. We created 3D-printed D2NNs that implement classification of images of handwritten digits and fashion products, as well as the function of an imaging lens at a terahertz spectrum. Our all-optical deep learning framework can perform, at the speed of light, various complex functions that computer-based neural networks can execute; will find applications in all-optical image analysis, feature detection, and object classification; and will also enable new camera designs and optical components that perform distinctive tasks using D2NNs.

1550 nm large-area plasmonic photoconductive terahertz sources

We demonstrate that large-area plasmonic photoconductive sources fabricated on ErAs:InGaAs can offer record-high pulsed terahertz radiation power levels as high as 300 μW over 0.1-5 THz frequency range at telecommunication optical pump wavelengths.

3.8 mW terahertz radiation generation through plasmonic nano-antenna arrays

We present a high-performance photoconductive terahertz emitter based on a two dimensional array of plasmonic nano-antennas. The array of plasmonic nano-antennas is specifically designed to offer high power terahertz radiation by introducing a strong time-varying dipole moment in response to an incident optical pump. We experimentally demonstrate terahertz radiation power levels as high as 3.8 mW over 0.1-5 THz bandwidth in response to a 240 mW optical pump beam, exhibiting the highest reported terahertz power levels with one order of magnitude higher optical-to-terahertz conversion efficiencies compared to conventional designs.

High-power pulsed terahertz radiation from terahertz nanoantenna arrays based on plasmonic nanocavities (Conference Presentation)

We present a photoconductive terahertz source that offers broadband pulsed terahertz radiation with enhanced optical-to-terahertz conversion efficiencies compared to photoconductive terahertz sources based on short-carrier-lifetime semiconductors. The performance enhancement is achieved by utilizing a plasmonic nanocavity that tightly confines optical pump photons inside a photoconductive layer near the terahertz radiating elements. The plasmonic nanocavity is implemented by sandwiching the photoconductive layer between a distributed Bragg reflector and plasmonic metallic structures, which are optimized to be resonant at the optical pump wavelength. The plasmonic structures are also designed as a broadband terahertz nanoantenna array. A thin undoped GaAs film is used as the photoconductive layer offering much higher carrier drift velocities compared to short-carrier-lifetime GaAs substrates. The tight confinement of the optical pump photons and the use of a low-defect photoconductive semiconductor layer allow drift of almost all of the photo-generated carriers to the terahertz nanoantennas in a sub-picosecond time scale to efficiently contribute to pulsed terahertz radiation. We experimentally demonstrate that the presented terahertz source offers 60 times higher optical-to-terahertz conversion efficiency compared to a similar terahertz nanoantenna array fabricated on a short-carrier-lifetime semiconductor. We demonstrate pulsed terahertz radiation with powers exceeding 4 mW over 0.1-4 THz frequency range.