Abstract-Impact of pump wavelength on terahertz emission of a cavity-enhanced spintronic trilayer

 

Applied Physics Letters

R. I. Herapath,  S. M. Hornett,   T. S. Seifert, G. Jakob,  M. Kläui,  J. Bertolotti, T. Kampfrath,  E. Hendry

Schematic of a spintronic trilayer with added dielectric cavity, grown on 0.5 mm of sapphire (Al2O3). The near-infrared pump pulse, incident through the substrate, is partially absorbed in the metallic layers, launching a spin current from the ferromagnetic (FM) layer into the nonmagnetic (NM) layers. The inverse spin Hall effect converts this ultrashort out-of-plane spin current into an in-plane charge current resulting in the emission of THz radiation into the optical far-field. A weak in-plane magnetic field (B) determines the magnetization direction and the linear polarization of the emitted THz field.

https://aip.scitation.org/doi/abs/10.1063/1.5048297

We systematically study the pump-wavelength dependence of terahertz pulse generation in thin-film spintronic THz emitters composed of a ferromagnetic CoFeB layer between adjacent nonmagnetic W and Pt layers. We find that the efficiency of THz generation is essentially flat for excitation by 150 fs pulses with center wavelengths ranging from 900 to 1500 nm, demonstrating that the spin current does not depend strongly on the pump photon energy. We show that the inclusion of dielectric overlayers of TiO2 and SiO2, designed for a particular excitation wavelength, can enhance the terahertz emission by a factor of up to two in field.

Abstract-Non-invasive, near-field terahertz imaging of hidden objects using a single pixel detector

R. I. Stantchev, B. Sun, S. M. Hornett, P. A. Hobson, G. M. Gibson, M. J. Padgett, E. Hendry

http://arxiv.org/abs/1509.03138#

Terahertz (THz) imaging has the ability to see through otherwise opaque materials. However, due to the long wavelengths of THz radiation ({\lambda}=300{\mu}m at 1THz), far-field THz imaging techniques are heavily outperformed by optical imaging in regards to the obtained resolution. In this work we demonstrate near-field THz imaging with a single-pixel detector. We project a time-varying optical mask onto a silicon wafer which is used to spatially modulate a pulse of THz radiation. The far-field transmission corresponding to each mask is recorded by a single element detector and this data is used to reconstruct the image of an object placed on the far side of the silicon wafer. We demonstrate a proof of principal application where we image a printed circuit board on the underside of a 115{\mu}m thick silicon wafer with ~100{\mu}m ({\lambda}/4) resolution. With subwavelength resolution and the inherent sensitivity to local conductivity provided by the THz probe frequencies, we show that it is possible to detect fissures in the circuitry wiring of a few microns in size. Imaging systems of this type could have other uses where non-invasive measurement or imaging of concealed structures with high resolution is necessary, such as in semiconductor manufacturing or in bio-imaging.

Abstract-Controlling the generation of THz radiation from metallic films using periodic microstructure

D. K. Polyushkin, 

E. Hendry, 

W. L. Barnes

http://link.springer.com/article/10.1007/s00340-015-6096-y

We report on THz frequency generation via irradiation of microstructured semicontinuous silver films by femtosecond laser pulses. By patterning the film so as to produce an array of microstrips, we show that one can use periodic microstructure to control the way nanostructured metal films produce THzradiation when illuminated by femtosecond infrared laser pulses. A simple analytical model based on the field distribution arising from an array of THz dipole emitters is used to assess the experimental data, allowing us to explain some of the main features of the generated THz radiation patterns, including the strongly resonant features of the emission spectrum.

Abstract-Inverse magneto-refraction as a mechanism for laser modification of spin-spin exchange parameters and subsequent terahertz emission from iron oxides

R. V. Mikhaylovskiy, E. Hendry, A. Secchi, J. H. Mentink, M. Eckstein, A. Wu, R. V. Pisarev, V. V. Kruglyak, M. I. Katsnelson, Th. Rasing, A. V. Kimel

(Submitted on 22 Dec 2014)

Inverse magneto-refraction as a mechanism for laser modification of spin-spin exchange parameters and subsequent terahertz emission from iron oxides

Ultrafast non-thermal manipulation of magnetization by light relies on either indirect coupling of the electric field component of the light with spins via spin-orbit interaction or direct coupling between the magnetic field component and spins. Here we propose a novel scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects, the strength of which can reach 1000 Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be called the inverse magneto-refraction, is allowed in a material of any symmetry. Its existence is corroborated by the experimental observation of THz emission by magnetic-dipole active spin resonances optically excited in a broad class of iron oxides with a canted spin configuration. From its strength we estimate that a sub-picosecond laser pulse with a moderate fluence of ~ 1 mJ/cm^2 acts as a pulsed effective magnetic field of 0.01 Tesla, arising from the optically perturbed balance between the exchange parameters. Our findings are supported by a low-energy theory for the microscopic magnetic interactions between non-equilibrium electrons subjected to an optical field which suggests a possibility to modify the exchange interactions by light over 1 %.

Subjects:	Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
Cite as:	arXiv:1412.7094 [cond-mat.str-el]
	(or arXiv:1412.7094v1 [cond-mat.str-el] for this version)

Submission history

From: Rostislav Mikhaylovskiy [view email]
[v1] Mon, 22 Dec 2014 18:55:13 GMT (2198kb)

Abstract-Terahertz emission spectroscopy of laser-induced spin dynamics in TmFeO3 and ErFeO3 orthoferrites

R. V. Mikhaylovskiy, E. Hendry, V. V. Kruglyak, R. V. Pisarev, Th. Rasing, and A. V. Kimel
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.184405

Using the examples of laser-induced spin-reorientation phase transitions in TmFeO3 and ErFeO3 orthoferrites, we demonstrate that terahertz emission spectroscopy can obtain novel information about ultrafast laser-induced spin dynamics, which is not accessible by more common all-optical methods. The power of the method is evidenced by the fact that, in addition to the expected quasi-ferromagnetic and quasi-antiferromagnetic modes of the iron sublattices, terahertz emission spectroscopy enables detection of a resonance optically excited at an unexpected frequency of ∼0.3–0.35 THz. By recording how the amplitude and phase of the excited oscillations depend on temperature and applied magnetic field, we show that the unexpected mode has all the features of a spin resonance of the Fe3+ ions. We suggest that it can be assigned to transitions between the multiplet sublevels of the 6A1 ground state of the Fe+3ions occupying rare-earth positions.

DOI: http://dx.doi.org/10.1103/PhysRevB.90.184405

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• Published 5 November 2014
• Received 22 July 2014
• Revised 23 September 2014

©2014 American Physical Society