Abstract-Enhanced terahertz emission bandwidth from photoconductive antenna by manipulating carrier dynamics of semiconducting substrate with embedded plasmonic metasurface

Arkabrata Bhattacharya, Dipa Ghindani, and S. S. Prabhu

Schematic showing the fabricated PCA on SI-GaAs substrate. Inset shows the embedded metasurface in the photoconductive gap of the PCA. The TiO2 antireflection coating has not been shown here. (b) SEM image of the fabricated devices. The First and the third devices have nanostructured PC gap, while the second and the fourth are bare PCAs for reference.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-21-30272

In this article, we demonstrate a technique to enhance the Terahertz (THz) emission bandwidth from photo-conductive antenna (PCA) based on semiconducting substrates by manipulating the surface carrier dynamics of the semiconductor. Bandwidths in PCAs are limited by the decay of the photogenerated charge carriers, which in case of SI-GaAs is in the orders of 50 picoseconds. We show, with an embedded design of plasmonic meta-surface in the photoconductive gap of a PCA, it is possible to enhance the emission bandwidths by more than 50 percent. This is due to the fact that these nano-structures act as local recombination sites for the photogenerated carriers, effectively reducing the carriers’ lifetime. Additionally, the defect sites reduce the terminal current, thereby reducing the Joule heating in the device. Furthermore, the meta-surface also facilitates higher in-coupling of the exciting infrared light on to the PCA, thereby increasing the optical-to-THz conversion efficiency of the device.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Rotation of terahertz radiation due to phonon-mediated magnetoelectric coupling in chiral selenium

Anirban Pal, Sharmila N. Shirodkar, Prathamesh Deshmukh, Harshad Surdi, Bagvanth R. Sangala, S. S. Prabhu, Kailash C. Rustagi, Umesh V. Waghmare, and Pushan Ayyub

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.235141

The trigonal form of selenium (t-Se) has an unusual, quasi-one-dimensional, chiralcrystal structure. First-principles calculations have helped us uncover a polar, optic phonon in t-Se that exhibits a diagonal magnetoelectric coupling with the electric field of the incident THz radiation, and induces a parallel magnetic field due to its inherent chirality. We show that this phonon-mediated, magnetoelectric mechanism is predicted to cause optical rotation in t-Se in the 1–3 THz range and is quite distinct from the high frequency (>80 THz) activity due to electronic excitations that has been reported previously. In the second part of the paper, we report our experimental results based on THz time-domain spectroscopy as well as direct measurements of optical rotation that confirm this prediction in an aligned, monocrystalline array of t-Se microrods. The Se microrod array not only exhibits a large birefringence ($\Delta n=1.3$) but also rotates the polarization of THz radiation by ∼3°/mm. To our knowledge, this is the only elemental solid known to rotate THz polarization, because the currently available THz rotators are based mainly on liquid crystals or metamaterials. The identification of new THz-active materials and a better understanding of the underlying physics are both clearly essential to the development of better sources, detectors and components.

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Abstract-“Pattern and Peel” method for fabricating mechanically tunable terahertz metasurface on an elastomeric substrate

S. C. Ambhire, S. Palkhivala, A. Agrawal, A. Gupta, G. Rana, R. Mehta, D. Ghindani, A. Bhattacharya, V. G. Achanta, S. S. Prabhu,

Fig. 2 Schematic of the entire fabrication process starting from bottom left to bottom right in a clockwise fashion. The inset shows the molecular structure of the MPTMS monolayer (shown in purple) which works as an molecular adhesive between the gold (shown in yellow) and PDMS layer (shown in grey). The green spheres represent Silicon atoms, blue represents Carbon, yellow for Sulphur, red for Oxygen and grey represents Hydrogen atoms.

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-8-11-3382

In this article, we explore a mechanically tunable metasurface on an elastic polydimethylsiloxane (PDMS) membrane operating at Terahertz (THz) frequencies synthesized using a “pattern and peel fabrication” technique. The tunability of the metasurface is based on the change of physical dimensions of the individual micro-structures due to the strain caused by mechanical stretching. The novelty of this technique is the ability to use high resolution e-beam patterning in contrast to established screen-printing techniques reported in the literature. The metasurface studied in this article is a periodic lattice of split-ring structures resonant at THz frequencies. The effect of mechanical stretching on the response of the metasurface is investigated thoroughly through experiments and numerical simulations. The response of the metamaterial to stretching manifests as a shift in the higher order mode by ∼ 12% for an applied strain of ∼ 25%. This tunability of the spectral response with macroscopic strain is not only substantial for the given structure, but also follows a linear behavior. This device can have potential applications in communications technology, remote strain sensing, chemical and biological sensing.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Enhanced optical-to-THz conversion efficiency of photoconductive antenna using dielectric nano-layer encapsulation

Abhishek Gupta, Goutam Rana, Arkabrata Bhattacharya, Abhishek Singh, Ravikumar Jain, Rudheer D. Bapat, S. P. Duttagupta,   S. S. Prabhu,

https://aip.scitation.org/doi/abs/10.1063/1.5021023


Photoconductive antennas (PCAs) are among the most conventional devices used for emission as well as detection of terahertz (THz) radiation. However, due to their low optical-to-THz conversion efficiencies, applications of these devices in out-of-laboratory conditions are limited. In this paper, we report several factors of enhancement in THz emission efficiency from conventional PCAs by coating a nano-layer of dielectric (TiO2) on the active area between the electrodes of a semi-insulating GaAs-based device. Extensive experiments were done to show the effect of thicknesses of the TiO2 layer on the THz power enhancement with different applied optical power and bias voltages. Multiphysics simulations were performed to elucidate the underlying physics behind the enhancement of efficiency of the PCA. Additionally, this layer increases the robustness of the electrode gaps of the PCAs with high electrical insulation as well as protect it from external dust particles.

Abstract-Terahertz spectroscopic evidence of low-energy excitations in NdNi O 3

Rakesh Rana, Parul Pandey, V. Eswara Phanindra, S. S. Prabhu, and D. S. Rana

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.045123

The charge-density waves (CDW) in the framework Landau theory are visualized to manifest in complex $R\mathrm{Ni}{O}_3$ ($R$ = rare-earth) nickelates in which the structure controls the incipient charge order in the weak localization limit. Any consequent effect demonstrating these nickelates in the rare category of CDW conductors with controlled charge-lattice interactions has been elusive so far. Employing terahertz time-domain spectroscopy along selective crystal axes, we present evidence of the CDW in epitaxial strain-modulated crystal structures of prototypical $\mathrm{NdNi}{O}_3$. A finite peak structure at 5 meV in the terahertz conductivity displays all the characteristics of a CDW condensate in (110)- and (111)-oriented $\mathrm{NdNi}{O}_3$ thin films. Contrasting charge dynamics of collective CDW mode and Drude conductivity emerging across dissimilar orientations helps disentangle charge ordering from the metal-insulator transition and establish a structure-property cause-effect relationship which may propose novel attributes in emerging oxide electronics.

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Abstract-Carbon irradiated semi insulating GaAs for photoconductive terahertz pulse detection

Carbon irradiated semi insulating GaAs for photoconductive terahertz pulse detection Abhishek Singh, Sanjoy Pal, Harshad Surdi, S. S. Prabhu, S. Mathimalar, Vandana Nanal, R. G. Pillay, and G.H. Döhler  »View Author Affiliations

Optics Express, Vol. 23, Issue 5, pp. 6656-6661 (2015)
http://dx.doi.org/10.1364/OE.23.006656

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http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-23-5-6656

We report here a photoconductive material for THz detection with sub-picosecond carrier lifetime made by C¹² (Carbon) irradiation on commercially available semi-insulating (SI) GaAs. We are able to reduce the carrier lifetime of SI-GaAs down to sub-picosecond by irradiating it with various irradiation dosages of Carbon (C¹²) ions. With an increase of the irradiation dose from ~10¹² /cm² to ~10¹⁵ /cm² the carrier lifetime of SI-GaAs monotonously decreases to 0.55 picosecond, whereas that of usual non-irradiated SI-GaAs is ~70 picosecond. This decreased carrier lifetime has resulted in a strong improvement in THz pulse detection compared with normal SI-GaAs. Improvement in signal to noise ratio as well as in detection bandwidth is observed. Carbon irradiated SI-GaAs appears to be an economical alternative to low temperature grown GaAs for fabrication of THz devices.

© 2015 Optical Society of America

Abstract-Microlensless interdigitated photoconductive terahertz emitters

Microlensless interdigitated photoconductive terahertz emitters Abhishek Singh and S. S. Prabhu  »View Author Affiliations

Optics Express, Vol. 23, Issue 2, pp. 1529-1535 (2015)
http://dx.doi.org/10.1364/OE.23.001529

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http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-23-2-1529

We report here fabrication of interdigitated photoconductive antenna (iPCA) terahertz (THz) emitters based on plasmonic electrode design. Novel design of this iPCA enables it to work without microlens array focusing, which is otherwise required for photo excitation of selective photoconductive regions to avoid the destructive interference of emitted THz radiation from oppositely biased regions. Benefit of iPCA over single active region PCA is, photo excitation can be done at larger area hence avoiding the saturation effect at higher optical excitation density. The emitted THz radiation power from plasmonic-iPCAs is ~2 times more than the single active region plasmonic PCA at 200 mW optical excitation, which will further increase at higher optical powers. This design is expected to reduce fabrication cost of photoconductive THz sources and detectors.

© 2015 Optical Society of America

Abstract-Observation and Structural Control of Charge-Density-Waves resonating with Terahertz Frequencies in NdNiO3

Rakesh Rana, Parul Pandey, S. S. Prabhu, D. S. Rana

(Submitted on 10 Dec 2014)

http://arxiv-web3.library.cornell.edu/abs/1412.3244

Formation of charge-density-waves in ordered electronic phases (such as charge-order) is an emergent phenomenon in the perovskite class of correlated oxides. This scenario is visualized to prevail in exotic RNiO3 (R=rare-earth) nickelates in which the structure controls the incipient charge-order to form in weak localization limit. However, any consequent effect demonstrating these nickelates in rare category of charge-density-waves conductors with controlled charge-lattice interactions has been a fundamental challenge so-far. Here, we present first evidence of the charge-density-waves in a prototypical NdNiO3 system employing terahertz time-domain spectroscopy along selective crystal axes. A finite peak structure at 5meV in the terahertz conductivity displays all the characteristics of a charge-density-wave condensate. Contrasting charge-dynamics of collective charge-density-waves mode and Drude conductivity emerging, respectively, from orthorhombic and cubic symmetries disentangle charge-ordering from the insulating state, establish a novel structure-property cause-effect relationship, and present opportunities to harness these diverse attributes in oxide electronics.

Subjects:	Strongly Correlated Electrons (cond-mat.str-el)
Cite as:	arXiv:1412.3244 [cond-mat.str-el]
	(or arXiv:1412.3244v1 [cond-mat.str-el] for this version)

Submission history

From: Rakesh Rana [view email]
[v1] Wed, 10 Dec 2014 10:12:40 GMT (1766kb)

Abstract-Microlensless Interdigitated Photoconductive Terahertz Emitters

Abhishek Singh, S. S. Prabhu

http://arxiv-web3.library.cornell.edu/abs/1409.0311

We report here fabrication  of interdigitated photoconductive antenna (iPCA) terahertz (THz) emitters based on plasmonic electrode design. Novel design of this iPCA enables it to work without microlens array focusing, which is otherwise required for photo excitation of selective photoconductive regions to avoid the destructive interference of emitted THz radiation from oppositely biased regions. Benefit of iPCA over single active region PCA is that photo excitation can be done at larger area, hence avoiding the saturation effected at higher optical excitation density. The emitted THz radiation power from plasmonic-iPCAs is ~ 2 times more than the single active region plasmonic PCA at 200 mW optical excitation, which will further increase at higher optical powers. This design is expected to reduce fabrication cost of photoconductive THz sources and detectors.