Abstract-Terahertz hyper-Raman time-domain spectroscopy of gallium selenide and its application in terahertz detection

Applied Physics Letters

Sen Mou, Andrea Rubano, Domenico Paparo

Experimental setup: BS, beam splitter; L, lens; CH, triggered chopper; DL, delay line; PL, air plasma; F, BG39 colored filter; MC, monochromator; PM, photomultiplier tube; Si, silicon plate.

https://aip.scitation.org/doi/abs/10.1063/1.5115986

We report the observation of Terahertz (THz) hyper-Raman generation in a gallium selenide crystal. This nonlinear optical process derives from the four- and five-wave-mixing of femtosecond optical pulses and intense, subps, broadband terahertz pulses. The wavelength spectrum of the resulting signal displays two pronounced frequency sidebands close to the optical second-harmonic central frequency 2ωL$2{\omega }_L$, where ωL is the optical central frequency of the fundamental beam. The two sidebands develop around the central frequency at the (anti-) Stokes side of ωs,a=2ωL∓ωT${\omega }_{s,a}=2{\omega }_L\mp {\omega }_T$, where ωT is the THz central frequency. This nonlinear optical process is used for the coherent detection of intense and broadband terahertz waves. The proposed technique shows a good linear response of up to 90 kV/cm and a better efficiency in detecting the lowest terahertz frequencies, as compared to the standard electro-optic sampling performed in two different nonlinear crystals.

The authors acknowledge funding from the “Ministero Istruzione Università e Ricerca” and “Consiglio Nazionale delle Ricerche.”

Abstract-Terahertz Hyper-Raman Time-Domain Spectroscopy

Andrea Rubano,  Sen Mou, Lorenzo Marrucci, Domenico Paparo

https://pubs.acs.org/doi/abs/10.1021/acsphotonics.9b00265

A new spectroscopic method has been demonstrated on the benchmark crystal α-SiO2. The new technique makes use of femtosecond optical pulses and intense, sub-ps, broadband terahertz (THz) pulses to generate a THz-optical Four Wave Mixing in the investigated material. The spectrum of the generated signal is resolved in wavelength and displays two pronounced frequency sidebands close to the optical Second Harmonic central frequency 2ωL, where ωL is the optical central frequency of the fundamental beam. The two sidebands develop around the central frequency at the anti-Stokes(Stokes) side of ω(a;s)= 2ωL ± ωT, where ωT is the THz central frequency, thus resembling the spectrum of standard Hyper-Raman scattering, and hence we named this effect “THz Hyper-Raman” - THYR. Due to the large laser and THz bandwidths, it is not possible to resolve the THYR signal in the frequency domain. Nonetheless, by taking advantage of the same principle at work in THz Time-Domain Spectroscopy, it is possible to follow the evolution of the THYR signal in time and access the frequency domain again by Fourier Transform. In this way we were able to observe pronounced oscillations in time of the THYR signal whose frequencies correspond to a large variety of material excitations including Γ-point phonons, polaritons, and phonons out of the Γ-point which are usually observed only by neutron scattering techniques. To complement the richness of these observations we will show that the selection rules of the THYR process allow the simultaneous observation of both IR-and Raman-active material modes, thus highlighting the potential of this innovative experimental method.

Abstract-Broadband Terahertz Spectroscopy of Imidazolium-Based Ionic Liquids

Sen Mou, Andrea Rubano, Domenico Paparo

https://cdn-pubs.acs.org/doi/abs/10.1021/acs.jpcb.7b10886

Ionic liquids are liquid salts at ambient temperature composed of organic cations and organic/inorganic anions. Outstanding physical and chemical properties of ionic liquids lead to increasing application in scientific and industrial field. Ionic liquids have been already investigated by different spectroscopic techniques, including terahertz (THz) time-domain spectroscopy. The usual THz frequency range extends up to 2-3 THz, a relatively narrow band, which can only show the intermolecular vibrational modes. Here we report about broadband THz spectroscopy of ILs up to 13 THz. Bandwidth of intermolecular absorption band presents unexpected behavior and strong sharp intramolecular absorptions are shown. In addition, we found violation of the approximation of harmonic oscillator used to predict the peak shift of intermolecular absorption band.

Abstract-Complex Permittivity of Ionic Liquid Mixtures Investigated by Terahertz Time-Domain Spectroscopy

Sen Mou, Andrea Rubano, and Domenico Paparo

http://pubs.acs.org/doi/abs/10.1021/acs.jpcb.7b04706

Ionic liquids are salts found in their liquid state at ambient temperature. The physicochemical properties of ionic liquids can be tailored by selecting constituent cation and anion from numerous available ions. The physicochemical properties can be further tuned by mixing different neat ionic liquids. Reported data of ionic liquid mixtures reveal that frequently investigated properties such as density, viscosity and thermal stability follow corresponding mixing laws. Complex permittivity in the interval of terahertz frequencies is of great importance to understand the molecular interactions and the solvation dynamics which drive the macroscopic properties of ionic liquids, however, to the best of our knowledge, there are scarce reports about the mixing behavior of complex permittivity in ionic liquid mixtures. In this contribution, binary mixtures of 1-butyl-3-methylimidazoulium iodide ([C4C1im]I) and 1-butyl-3- methylimidazoulium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) are investigated in the terahertz spectral range, and the resulting low-energy spectra are analyzed in order to clarify the mixing laws at play. The results show that the complex permittivity of mixtures of [C4C1im]I and ([C4C1im][NTf2] obeys a linear mixing law.

Abstract-Subcycle control of terahertz waveform polarization using all-optically induced transient metamaterials

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N Kamaraju^1,2, Andrea Rubano^1,3, Linke Jian⁴, Surajit Saha⁴, T Venkatesan⁴, Jan Nötzold¹, R Kramer Campen¹, Martin Wolf¹ and Tobias Kampfrath¹

1. ¹Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany
2. ²Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
3. ³CNR-SPIN, Dipartimento di Fisica, Università di Napoli Federico II, Napoli, Italy
4. ⁴NUSNNI-Nanocore, National University of Singapore, Singapore

Correspondence: Dr N Kamaraju, Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA. E-mail: nkamaraju@gmail.com; Dr T Kampfrath, Fritz Haber Institute of the Max Planck Society, Faradayweg 4–6, 14195 Berlin, Germany. E-mail: kampfrath@fhi-berlin.mpg.de

Received 26 July 2013; Revised 18 October 2013; Accepted 13 November 2013, Published online February 28 2014

http://www.nature.com/lsa/journal/v3/n2/full/lsa201436a.html

Coherent radiation with frequencies ranging from 0.3 to 30 THz has recently become accessible using femtosecond laser technology. These terahertz (THz) waves have already been applied in spectroscopy and imaging and can be manipulated using static optical elements such as lenses, polarizers and filters. However, ultrafast modulation of THz radiation is required as well, for instance, in short-range wireless communication or for preparing shaped THz transients for the coherent control of numerous material excitations. Here, we demonstrate an all-optically created transient metamaterial that allows us to manipulate the polarization of THz waveforms with subcycle switch-on times. The polarization-modulated pulses are potentially interesting for controlling elementary motions such as the vibration of crystal lattices, the rotation of molecules and the precession of spins.