Terahertz Patch Antenna Microcavity Lasers with Integrated Beam Control

Terahertz (THz) refers to the region of the electromagnetic spectrum that lies in between the infrared and microwaves. This frequency range possesses great potential to host several applications in wide-ranging fields, such as wireless communications, astronomy, non-invasive imaging and security scanning. However, despite sustained progress over the past decade, THz technology has not yet reached the level of maturity and flexibility of the neighboring radio frequency (RF) and optical range. One missing key aspect is the ability to integrate advanced beam control functionalities within a monolithic platform. A promising approach to achieve this goal is to combine within a single device two features of the neighboring ranges: optical microcavities, that can sustain efficient lasing operation; and antenna arrays, providing a high level of beam control. In this thesis, we investigate via simulations fabrication and characterization the emission properties of arrays of patch antenna-coupled microcavities embedding quantum cascade active regions. The geometrical configuration of the array allows independent and simultaneous tuning of the losses governing the microcavities as well as beam shaping by constructive interference in the far-field. We show that optimized arrays emit THz with unprecedented low beam divergence and robust lasing in single frequency and spatial mode. Additionally, we demonstrate polarization functionalization by coupling the patch antenna microcavities with plasmonic wires. This feature introduces an additional degree of freedom to adjust the relative emission from the cross-polarized modes of the patch, allowing the device to radiate with any coherent polarization state from linear to circular. Finally, we discuss how this design can further enable other advanced functionalities such as active beam steering and control of THz non-linearities. The successful implementation of integrated advanced functionalities and sources on-a-chip demonstrates the ability of our platform to replicate in the THz range the beam control concepts used in the RF and optics, thus paving the way towards establishing a mature technology in this range of the electromagnetic spectrum.Okinawa Institute of Science and Technology Graduate Universit

Terahertz photoconductivity and Photocarrier dynamics in few-layer hBN/WS2 van der Waals heterostructure laminates

Van der Waals (vdW) heterostructures is a rapidly emerging field that promises to produce on-demand properties for novel optoelectronic devices. Assembly of dissimilar two-dimensional atomic crystals in the vdW heterostructure enables unique features and properties which are fundamentally different from individual 2-dimensional (2D) crystals. Currently, most growth and fabrication methods prohibit large scale, micron-thick and robust heterostructures. An alternative approach is the one based on liquid phase exfoliation allowing the possibility of scalable thin films and composites. Such thin films have inherent and predicted advantages: they can display new behaviors due to their extremely high surface area and, as free-standing laminates, can be manipulated with mixing of nanosheets and other materials for novel device attributes. We use the aforementioned route to prepare spray-coated and few microns thick WS2 and hBN/WS2 heterostructure laminates. A combination of photoluminescence and transmission electron microscope measurements show that, despite the disordered layer stacking inherent to the fabrication process, the laminates preserve the few layer optical response. In particular, the hBN/WS2 heterostructure laminates exhibit a 3-layer average distribution. Using optical pump-terahertz probe (OPTP) measurement to access the photocarrier dynamics and photoconductivity, we study and compare the photocarrier dynamics and photoconductivity of pure WS2 and hBN/WS2 samples. The hBN/WS2 samples show an unusual response that is different from what has been previously reported for pure transition metal dichalcogenides. After photoexcitation, instead of a monotonic decay as in pure WS2, an initial fast decay is followed by a rise of the negative differential terahertz (THz) transmission dominating the dynamics for the following 50 ps. By analyzing the time resolved THz complex photoconductivity, we attribute this effect to the presence of free carriers as well as dipoles at the hBN/WS2 interfaces. As previously reported in hBN/Graphene heterostructures, interfacial dipoles can form along with free carriers at the instant of photoexcitation. Whereas free carriers cause a decrease in the transient THz transmission due to Coulomb screening, dipoles can provide an increase in the pump-induced change in transmission. In terms of complex photoconductivity, free carriers have both real and imaginary components while dipoles probed off-resonance provide almost a purely imaginary response. Our results provide a deeper understanding of the photoconductive response of large van der Waals heterostructure laminates fabricated by liquid phase exfoliation, and will enable their use in future optoelectronic applications.by Mariserla Bala Murali Krishna, Julien Madeo, Joel Perez Urquizo Xing Zhu, Soumya Vinod, Chandra Sekhar Tiwary, Pulickel M. Ajayan and Keshav M. Dan