Tunability of the dielectric function of heavily doped germanium thin films for mid-infrared plasmonics

Abstract

Heavily-doped semiconductor films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work we investigate heavily n-type doped germanium epilayers grown on different substrates, in-situ doped in the 10 17 to 10 19 cm −3 range, by infrared spectroscopy, first principle calculations, pump-probe spectroscopy and dc transport measurements to determine the relation between plasma edge and carrier density and to quantify mid-infrared plasmon losses. We demonstrate that the unscreened plasma frequency can be tuned in the 400 -4800 cm −1 range and that the average electron scattering rate, dominated by scattering with optical phonons and charged impurities, increases almost linearly with frequency. We also found weak dependence of losses and tunability on the crystal defect density, on the inactivated dopant density and on the temperature down to 10 K. In films where the plasma was optically activated by pumping in the near-infrared, we found weak but significant dependence of relaxation times on the static doping level of the film. Our results suggest that plasmon decay times in the several-picosecond range can be obtained in ntype germanium thin films grown on silicon substrates hence allowing for underdamped mid-infrared plasma oscillations at room temperature.The recent push towards applications of spectroscopy for chemical and biological sensing in the mid-infrared (mid-IR)1-8 has prompted the need for conducting thin films displaying values of the complex dielectric functionǫ(ω) = ǫ ′ (ω) + iǫ ′′ (ω) that can be tailored to meet the needs of novel electromagnetic designs exploiting the concepts of metamaterials, transformation optics and plasmonics 9 . In the design of metamaterials, where subwavelength sized conducting elements are embedded in dielectric matrices, if the values of ǫ ′ of the metal and the dielectric are of the same order, but have opposite sign, the geometric filling fractions of the metal and dielectric can be readily tuned to achieve subwavelengthresolution focusing of radiation 10 . Such requirement is met by silver for wavelengths λ around 400 nm. The same condition cannot be achieved in the IR range by using elemental metals, however, because metals possess an extremely high negative value of ǫ ′ not equaled, in

Germanium plasmonic nanoantennas for third-harmonic generation in the mid infrared

“…Simulations considering the antenna geometry and the material dielectric function close to the plasma edge determine the exact resonance frequency. [5] The optical characterization system is driven by a Yb:KGW femtosecond laser with a repetition rate of 50 kHz and a pulse energy of 2 mJ. It is equipped with an optical parametric amplifier (OPA) delivering broadband pulses in the near infrared [6].…”

Mid-infrared plasmonic platform based on n-doped Ge-on-Si: Molecular sensing with germanium nano-antennas on Si

“…1B) and Kramers-Kronig approach [2], the optical constants and scattering times are calculated. The average electron scattering rate is dominated by scattering with optical phonons and charged impurities and increases almost linearly with frequency [3]. We employ frequency-and time-domain simulations to design single-and double-arm plasmonic antennas in the form of 2.0 m long, 0.8 m wide and 1.0 m thick blocks (Fig.…”

n-Ge on Si for mid-infrared plasmonic sensors

“…We have compared the plasmonic performance of n-Ge to the more common plasmonic metals of Au, Ag and Al. The higher electrical conductivity of Au and Ag do provide lower losses than n-Ge in the mid-infrared region investigated but this is counterbalanced by a significantly higher degree of field confinement in n-Ge since the doped semiconductor demonstrates true plasmonic behaviour [5].…”

“…Fig. 1 demonstrates the increase in the wavenumber (decrease in wavelength) of the plasmon edge as the doping density increased for n-Ge epilayers of thicknesses between 600 nm and 1000 nm [2][4] [5]. The lowest wavelength of ∼ 3.1 µm was achieved for an activated doping density of 2.1×10 20 cm −3 at 300 K. This is sufficient to enable electron beam lithography and reactive ion etching from the 1000 nm thick n-Ge material [4] into arrays over a 5 × 5 mm 2 array.…”

“…As demonstrated using analysis of time and frequency dependent n-Ge measurements, only electron-phonon and electron-charged impurity scattering dominate the decay of the plasmons and underdamped plasmon excitations are observed in the mid-infrared even at room temperature [5]. We have compared the plasmonic performance of n-Ge to the more common plasmonic metals of Au, Ag and Al.…”