Dr. Mohammad Taghinejad of Stanford University

The terahertz (THz) band, often referred to the 0.3-10 THz frequency range, lies between the microwave and infrared regions of the electromagnetic spectrum. This frequency band is filled with numerous characteristic spectral features associated with fundamental physical processes including rotational transitions in molecules, vibrational motions of organic compounds, lattice vibrations in solid media, intraband transitions in semiconductors, and energy gaps in superconductors. As such, the development of terahertz sources and detectors plays a pivotal role in the advancement of condensed matter physics, biology and medical sciences, global environmental monitoring, metrology, information and communication technology. However, the lack of access to suitable technologies across the 0.3-10 THz frequency range has led to the formation of a technological “THz gap,” hindering the facile and widespread use of THz waves in emerging applications. For the specific case of THz sources, the general practice is to benefit from the advanced microwave technology to shrink the gap from the low-frequency side and employ established optical methods to enable radiation at the high-frequency end. 

In this talk, I will start with a concise survey of existing methods for the generation of THz waves and will discuss existing challenges inherent to each technique, from a material as well as technological standpoint. Then I will introduce a new approach for the generation of ultra-broadband, ultra-compact, and highly efficient THz sources by employing the concept of hot-electron generation/transport in hybrid plasmonic platforms. We will discuss how the transport of hot-electrons at the interface of nanostructured metal electrodes and semiconductors brings the best of conventional techniques together for the radiation of electromagnetic waves from 0.1 to 50 THz, in a device as thin as 75 nanometers. In the last part of the talk, I will briefly explain viable approaches to capitalized on our new technique for the synthesis of terahertz waves with a desired electric-field pattern, also known as structured THz field (i.e., vortexes and singularities), for the spectroscopy of quantum materials.

Biography:

Mohammad received his PhD from Georgia Institute of Technology (Atlanta, Georgia, USA) in 2020 and currently is continuing his research as a postdoctoral scholar in Geballe Laboratory for Advanced Materials (GLAM), the department of materials science and engineering in Stanford University. During his PhD, Mohammad explored the out-of-equilibrium dynamics of optically excited carriers in solid media, envisioned for the implementation of ultrafast all-optical switches, sub-picosecond opto-electronic devices, and optical symmetry breaking in centrosymmetric media. Mohammad is the recipient of Roger P. Webb Graduate Research Assistant Excellence Award (2020), SPIE Optics and Photonics Education Scholarship (2020), and Sigma Xi Best PhD Thesis Award (2021). Terahertz behavior of out-of-equilibrium solid media for the realization of ultracompact, broadband, and integrated terahertz sources and detectors is his current research interest. Mohammad serves as a guest editor for Electronics and Symmetry journals. 

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