Direct Optical Tuning of the Terahertz Plasmonic Response of InSb Subwavelength Gratings

Abstract

Research on terahertz (THz) electromagnetic radiation has been booming in the past two decades in view of its unique properties and important applications in sensing, imaging, and spectroscopy. [1][2][3] The recent advancement of plasmonics and metamaterials has brought new approaches for manipulating THz waves. Signifi cant progress has been achieved in THz component development, such as ultra-high refractive index THz metamaterials, [ 4 ] beam collimated THz quantum cascade laser, [ 5 ] high effi ciency THz photomixer, [ 6 ] near fi eld on-chip THz detector, [ 7 ] THz modulators, [8][9][10][11][12] high extinction ratio THz polarizer [ 13 ] and THz spoof surface plasmon waveguides. [14][15][16] The electromagnetic response of plasmonic and metamaterial structures arise from the combined contributions of subwavelength structuring and the dielectric materials properties. As the properties of metals can hardly be tuned, THz metamaterial and plasmonic devices based on metallic subwavelength structures are normally tuned or modulated only through adjusting the properties of ambient or active dielectric elements. On the other hand, low-bandgap semiconductors, such as InSb, are excellent candidates for plasmonic materials in the THz range, owing to their small band gap, high electron mobility, small effective mass and low electron density. More importantly, the properties of semiconductors can be easily tuned by thermal, electrical or optical ways. Very recently, broadband THz plasmonic absorption in InSb touching disks was demonstrated, which showed consistency with the theoretical predictions of transformation optics for gold nanostructures at visible frequencies. [ 17 ] InSb was also used as the material to study coherent interference induced THz transparency tuned via an external magnetic fi eld. [ 18 ] By taking advantage of the fast response time of optical processes, semiconductor-based optically tunable THz active plasmonics is a very suitable concept for high-speed THz modulation. In this paper, we report the direct optical tuning of the THz plasmonic response of InSb subwavelength gratings. The potential modulation speed was characterized through carrier lifetime study by optical-pump THz-probe (OPTP) spectroscopy. Theoretical calculations of the subwavelength grating plasmonic structure show full-scale operation agility in modulation frequency and depth.In THz frequency range InSb can be modeled as a classic solid-state plasma, with its permittivity given by g InSb (T) = g ∞ [1 − T 2 p / T 2 + iT( ] and plasma frequency T p = ne 2 / g 0 g ∞ m * , where n is the carrier concentration. [ 19 ] At room temperature, the plasma frequency of InSb lies within the THz frequency range and its permittivity resembles that of noble metals in the optical frequencies, enabling plasmonic resonances of subwavelength structures with incident THz waves. [ 20,21 ] As indicated by the above equations, the permittivity of InSb depends strongly on carrier concentration, which allows the permittivity and consequently plasmonic res...

Optically and thermally controlled terahertz metamaterial via transition between direct and indirect electromagnetically induced transparency

“…The same assumption was introduced to Ref. 25 as well and it turned out that simulation and experiments showed excellent consistency with each other. Next, the transmission spectrum of the indirect and direct EIT coupling metamaterials under different conditions is investigated, as shown in fig.…”

“…The electron diffusion length of InSb can be estimated to be 12.7 µm, which is far larger than the thickness of InSb, and so assume that the two SRRs made of InSb hold a uniform conductivity of σ InSb =29120 s/m according to Ref. 25. The same assumption was introduced to Ref.…”

“…Ref. 25 demonstrated that experimentally that the recovery time of photo-excited electrons in InSb was 1360 ps and the results were measured by optical-pump THz time domain spectroscopy (THz-TDS) with 800 nm, 29.3 µJ/cm 2 , and 35 fs fetosecond laser. Fig.…”

All-integrated terahertz modulators

“…Interestingly, because of the smaller band gap of germanium (0.66 eV) and higher carrier mobilities compared to silicon, it is possible to achieve considerable modulation depths at lower power levels and, mostly, with a 1.55 μm-wavelength laser, thus providing a direct link with the optical communication technology. InSb gratings on a semi-insulating (SI)-GaAs substrate have also demonstrated a THz response which could be tuned by a cw optical pump [46]. In this experiment, the pump irradiation allows the dynamic modification of the plasma frequency of the localized surface plasmon sustained by the gratings, rather than just an all-optical induced change in conductivity.…”

“…By implementing narrow-bandwidth, frequency-selective InSb gratings instead, a modulation depth of 46.7% for TM polarization at 1.5 THz has been reported in Ref. [46] for 120 mW pump power by using a 405 nm cw laser. The modulation depth under optical pumping for both cw and pulsed THz radiation was reported to improve after spinning a polymer on the silicon substrate [43].…”