Transient dielectric functions of Ge, Si, and InP from femtosecond pump-probe ellipsometry

Abstract

Transient dielectric functions with a 120 fs time resolution of Ge, Si, and InP were acquired from 1.7 to 3.5 eV with a femtosecond pump-probe rotating-compensator ellipsometer. The intensity of the pump laser (with 1.55, 3.10, or 4.65 eV photon energy) was adjusted to create an initial near-surface carrier density of 1020 cm−3. In Ge, there is a significant (∼15%) decrease in the E1 and E1 + Δ1 critical point absorption and a Kramers–Kronig consistent change in the refractive index because photoexcited electrons at L block these transitions and reduce their amplitudes. Only a small redshift of the E1 critical point is observed, which we attribute to lattice heating and exchange-correlation effects. Minimal changes were found for Si and InP, where electrons near Δ and Γ do not participate in interband transitions between 1.7 and 3.5 eV.

Broadband optical ultrafast reflectivity of Si, Ge and GaAs

“…3. These finding are in line with recent femtosecond pump-probe ellipsometry measurements 21 of the Ge dielectric constant. In present measurements, the negative reflectivity change of germanium in the vicinity of the E 1 ,E 1 + range is observed to survive up to the maximum delay times under investigation (up to ∼ −0.5% at 300 ps delay times).…”

“…These energies correspond to and transitions of germanium at 300 K, as discussed in Refs. 21 , 25 , 26 . On the other hand, for probing wavelengths in the 620–760 nm range, germanium transient reflectivity is more similar to that of silicon (see also Fig.…”

“…Several transient reflectivity works 5,15,17,18 regarded also GaAs and specific models taking into account the filling of valence band, band modification and the free carrier absorption were developed 19,20 . To the best of our knowledge, there are no previous pump-probe reflectivity works on germanium, but time-resolved dielectric function data in the visible range 21 and models for the interpretation have been proposed 22,23 . All those previous studies on Si, Ge, GaAs have been performed generally using different pump and probe wavelengths, pulsewidths, fluences, so a general picture of their ultrafast properties under conditions similar to the present work is still lacking.…”

Observation of second sound in a rapidly varying temperature field in Ge

“…Although electron-hole pairs can also be excited at the Γ point, the joint density of excited states at this point is much lower as compared to the L point (see section S3 of Ref. 24), which is also evidenced by the onset of the absorption spectra in Ge at E≈2 eV (25).…”

Femtoszekundumos pumpa-próba ellipszometria

“…állandó tömb anyagok esetén a vizsgált minta dielektromos állandójának felel meg, rétegrendszerek esetén a rétegrendszer tulajdonságait is magában hordozza. A mért ellipszometriai adatsorokat átváltáva pszeudó dielektromos állandóvá megállapítható, hogy a lézeres gerjesztés hatására a kristályos germánium sáv-sáv átmeneteire jellemző éles csúcsok[5] a gerjesztést követő 1-2 ps-ban elmosódottabbá válnak, és a görbék csak ~száz ps után nyerik vissza eredeti alakjukat (4. a) ábra).4.ábra: a)A különböző késleltetési időkhöz tartozó 2 spektrumok. b) A 2 változásai 2,1 eV fotonenergián (590 nm-es hullámhosszon).…”