Abstract-Quantifying the photothermal conversion efficiency of plasmonic nanoparticles by means of terahertz radiation

APL Photonics

 H. Breitenborn^,   J. Dong^,  R. Piccoli^,   A. Bruhacs^,   L. V. Besteiro^,  , Z. M. Wang^,   A. O. Govorov^,   L. Razzari^,   F. Vetrone^,   R. Naccache^,  R. Morandotti

(a) Schematic diagram of THz temperature measurements based on a THz-TDS system set in reflection geometry. The 786-nm NIR beam used to plasmonically heat the GNR dispersions is overlapped with the THz focusing spot. The power of the NIR illumination transmitted through the cuvette is recorded with a power meter. The inset depicts the THz raster-scans of the front- and back-side of the cuvette. During the scanning process, the THz beam is fixed and the cuvette moves together with the IR beam pixel-by-pixel. (b) Example of a recorded THz signal reflected from the cuvette. The well-separated first and second echoes after sparse deconvolution are also plotted, in which the influence of the “ringing” due to the ambient water vapor is eliminated. (c) Temperature-dependent reflectivity of the second THz echo as a function of temperature for GNR10. (d) Calibration curves featuring a linear relationship between THz amplitude and temperature for each of the GNR dispersions.

https://aip.scitation.org/doi/abs/10.1063/1.5128524

The accurate determination of the photothermal response of nanomaterials represents an essential aspect in many fields, such as nanomedicine. Specifically, photothermal cancer therapies rely on the precise knowledge of the light-to-heat transfer properties of plasmonic nanoparticles to achieve the desired temperature-induced effects in biological tissues. In this work, we present a novel method for the quantification of the photothermal effect exhibited by nanoparticles in aqueous dispersions. By combining the spatial and temporal thermal dynamics acquired at terahertz frequencies, the photothermal conversion efficiency associated with the geometry of the plasmonic nanoparticles can be retrieved in a noncontact and noninvasive manner. The proposed technique can be extended to the characterization of all those nanomaterials which feature a temperature-dependent variation of the refractive index in the terahertz regime.

Abstract-Terahertz control of air lasing

M. Clerici, A. Bruhács, D. Faccio, M. Peccianti, M. Spanner, A. Markov, B. E. Schmidt, T. Ozaki, F. Légaré, F. Vidal,  R. Morandotti,

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.99.053802

The coherent emission from ionized nitrogen molecules is of interest for remote sensing and astronomical applications. To initiate the lasing process, we used an intense ultrashort near-infrared (NIR) pulse overlapped with a terahertz (THz) single-cycle pulse. We observed that coherent emission could be seeded and modulated by the amplitude of the THz field, which is the result of a combined effective second-order nonlinear polarization and the nonlinear effects induced by the NIR pump. Our results shed light on the role of intense transient fields in the coherent emission from photoexcited gas molecules.

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Abstract-Terahertz control of air lasing

M. Clerici, A. Bruhács, D. Faccio, M. Peccianti, M. Spanner, A. Markov, B. E. Schmidt, T. Ozaki, F. Légaré, F. Vidal, and R. Morandotti

https://journals.aps.org/pra/accepted/8007bY2dMa71016661fa2ff421161a92659e17711

The coherent emission from ionized nitrogen molecules is of interest for remote sensing and astronomical applications. To initiate the lasing process, we used an intense ultrashort near-infrared (NIR) pulse overlapped with a terahertz (THz) single-cycle pulse. We observed that coherent emission can be seeded and modulated by the amplitude of the THz field, which is the result of a combined effective second-order nonlinear polarization and the nonlinear effects induced by the NIR pump. Our results shed new light on the role of intense transient fields in the coherent emission from photoexcited gas molecules. One of the key phenomena accompanying the focusing of an intense laser pulse in air is the fluorescence from gas molecules.()()()..~[1–3]. Under appropriate excitation conditions, those molecules produce coherent radiation, which is appealing for standoff spectroscopy applications, especially when emitted in the opposite direction of the ionizing laser pulse..().()()..~[4,5]. Owing to nitrogen’s abundance in our atmosphere, one of the most investigated effects has been the ultraviolet (UV) forward emission from photoexcited molecular nitrogen ions, first described as lasing by Luo and co-workers in 2003..()…….()..~[6]. A number of experiments report narrowband coherent emission at 391 nm and 428 nm, corresponding to the transitions ${N_2^{+}(B}^2{\Sigma }_u^{+}(\nu =0))\to {N_2^{+}(X}^2{\Sigma }_g^{+}({\nu }^{\prime }=0,1\left)\right)$ (see also Fig. 1(c) for potential energy diagrams). Such observations show narrowband amplification at the frequencies corresponding to molecular transitions. Those are seeded, for instance, by harmonics of the near-infrared (NIR) pump pulse…()….(..)(()())(..)…….()..~[7–9], white light…..()…()..~[10], or self-seeded by the pump’s supercontinuum itself…..()()()..~[10–12]. Such reports have reinforced the idea that a lasing process is at the origin of the coherent emission. However, the mechanism responsible for the gain is still not fully understood, although it is likely that laser-driven couplings between electronic states in the ion..().().()……..(..)…..()..~[13,14] and rotational effects()..~[15 –21] play a role Here, we report our experimental observation and analysis of the effect of a strong terahertz (THz) electric field on the coherent emission from photoexcited nitrogen ion molecules, as outlined in Figs. 1(a) and 1(b). We show a correlation between the THz field amplitude and the coherent emission at both 391 nm and 428 nm wavelengths. We interpret our results as consequence of the THz electric field induced symmetry-breaking of the gas molecules. This is turn leads to a THz-controlled seeding of the coherent emission by means of the THz induced second-harmonic of the near-infrared pump pulse. In our investigations we used a single-cycle pulse at THz frequencies as a strong electric field. The THz transient was generated by the transverse photocurrents induced through gas ionization via a laser pulse at 1.8 $$m carrier combined with its second harmonic. 

Abstract-Invited Article: Ultra-broadband terahertz coherent detection via a silicon nitride-based deep sub-wavelength metallic slit

APL Photonics

A. Tomasino, R. Piccoli, Y. Jestin, S. Delprat1, M. Chaker,   M. Peccianti,   M. Clerici4, A. Busacca, L. Razzari,  R. Morandotti,

FIG. 1.3D sketch of the deep sub-λ slit (G) device embedded in a thin layer (T) of SiN, deposited on a quartz substrate. L and W are the length and the width of the metal pads, respectively.

https://aip.scitation.org/doi/abs/10.1063/1.5052628

We present a novel class of CMOS-compatible devices aimed to perform the solid-state-biased coherent detection of ultrashort terahertz pulses, i.e., featuring a gap-free bandwidth at least two decades-wide. Such a structure relies on a 1-µm-wide slit aperture located between two parallel aluminum pads, embedded in a 1-µm-thick layer of silicon nitride, and deposited on a quartz substrate. We show that this device can detect ultra-broadband terahertz pulses by employing unprecedented low optical probe energies of only a few tens of nanojoules. This is due to the more than one order of magnitude higher nonlinear coefficient of silicon nitride with respect to silica, the nonlinear material employed in the previous generations. In addition, due to the reduced distance between the aluminum pads, very high static electric fields can be generated within the slit by applying extremely low external bias voltages (in the order of few tens of volts), which strongly enhance the dynamic range of the detected THz waveforms. These results pave the way to the integration of solid-state ultra-broadband detection in compact and miniaturized terahertz systems fed by high repetition-rate laser oscillators and low-noise, low-voltage generators.

Abstract-Terahertz absorption by cellulose: Application to ancient paper artifacts

M. Peccianti, R. Fastampa, A. Mosca Conte, O. Pulci, C. Violante, J. Łojewska, M. Clerici, R. Morandotti, and M. Missori

https://journals.aps.org/prapplied/accepted/ce07fYc6Le011c53637216614e26e35315d5a921c

Artifacts made of cellulose, such as ancient documents, pose a significant experimental challenge in the THz transmission spectra interpretation due to their small optical thickness. In this Letter we describe a method to recover the complex refractive index of cellulose fibers from the THz transmission data obtained on single freely standing paper sheets in the 0.2\textendash 3.5~THz range. By using our technique, we were able to eliminate Fabry-Perot effects and recover the absorption coefficient of the cellulose fibers. The obtained THz absorption spectra are explained in terms of absorption peaks of the cellulose crystalline phase superimposed to a background contribution due to a disordered hydrogen bonds network. The comparison between the experimental spectra with THz vibrational properties simulated by density functional theory calculations confirms this interpretation. In addition, evident changes in the THz absorption spectra are produced by natural and artificial aging on paper samples, whose final stage is characterized by a spectral profile with only two peaks at about 2.1~THz and 3.1~THz. These results could be used to provide a quantitative assessment of the state of preservation of cellulose artifacts.