Few picosecond dynamics of intraband transitions in THz HgTe nanocrystals

Abstract

Optoelectronic devices based on intraband or intersublevel transitions in semiconductors are important building blocks of the current THz technology. Large nanocrystals (NCs) of Mercury telluride (HgTe) are promising semiconductor candidates owing to their intraband absorption peak tunable from 60 THz to 4 THz. However, the physical nature of this THz absorption remains elusive as, in this spectral range, quantum confinement and Coulomb repulsion effects can coexist. Further, the carrier dynamics at low energy in HgTe NCs, which strongly impact the performances of THz optoelectronic devices, is still unexplored. Here, we demonstrate a broad THz absorption resonance centered at ≈4.5 THz and fully interpret its characteristics with a quantum model describing multiple intraband transitions of single carriers between quantized states. Our analysis reveals the absence of collective excitations in the THz optical response of these self-doped large NCs. Furthermore, using optical pump-THz probe experiments, we report on carrier dynamics at low energy as long as 6 ps in these self-doped THz HgTe NCs. We highlight evidence that Auger recombination is irrelevant in this system and attribute the main carrier recombination process to direct energy transfer from the electronic transition to the ligand vibrational modes and to nonradiative recombination assisted by surface traps. Our study opens interesting perspectives for the use of large HgTe NCs for the development of advanced THz optoelectronic devices such as emitters and detectors and for quantum engineering at THz frequencies.

Guided-Mode Resonator Coupled with Nanocrystal Intraband Absorption

“…Then, the electron cools from 1P e to 1S e (end of regime I); this relaxation time τ P→S has been found to be 3.5 ps. This duration is in line with the intraband relaxation dynamics in CdSe or HgTe − nanocrystals for which the relaxation decay is found to be in a 1–10 ps range . Note that in the case of pristine HgTe, only a negative signal is observed (middle panel of Figure a).…”

Vanishing Confinement Regime in Terahertz HgTe Nanocrystals Studied under Extreme Conditions of Temperature and Pressure

“…Later, Apretna et al investigated the carrier dynamics in such THz-absorbing HgTe NCs. 26 They found that intraband relaxation occurs within a few picoseconds and with a decay rate of 0.5 eV ps −1 , which is close to the values observed in more strongly confined HgTe NCs with sparser densities of states. 4,27 The THz HgTe NCs offer an interesting playground for probing an electronic state between the bulk and the strongly confined regimes.…”

Intraband dynamics of mid-infrared HgTe quantum dots

“…Furthermore, negligible fluence dependence on the cooling rates suggests that multiparticle scattering processes play only a minor role in carrier cooling which is consistent with a very recent report of intraband dynamics in THz HgTe quantum dots. 39 This conclusion is further supported by the unaltered cooling rate found for excitations in the MEG regime.…”

“…Consequently, multiparticle scattering appears to play only a minor role in intraband relaxation in these quantum dots as also reported for much larger THz-gap HgTe crystallites. 39 In the following discussion, the mean value of the cooling times for each excitation wavelength in Fig. 3 is used as a more precise value.…”

“…This seems unlikely given the large difference of level spacings in the conduction and valence bands, and much slower electron intraband relaxation times reported for doped MIR HgSe quantum dots in the range of tens to hundreds of picoseconds 9 and intraband lifetimes of more than 4 picoseconds reported for self-doped Terahertz-gap HgTe nanoparticles with roughly ten-fold smaller intraband gaps. 39 Fig. 4 Top left.…”