Undressing a collective intersubband excitation in a quantum well

All-THz pump-probe spectroscopy of the intersubband AC-Stark effect in a wide GaAs quantum well

We report the observation of the intersubband AC-Stark effect in a single wide GaAs/AlGaAs quantum well. In a three-level configuration, the n = 2 to n = 3 intersubband transition is resonantly pumped at 3.5 THz using a free-electron laser. The induced spectral changes are probed using THz time-domain spectroscopy with a broadband pulse extending up to 4 THz. We observe an Autler-Townes splitting at the 1 - 2 intersubband transition as well as an indication of a Mollow triplet at the 2 - 3 transition, both evidencing the dressed states. For longer delay times, a relaxation of the hot-electron system with a time constant of around 420 ps is measured.

Ultrawide electrical tuning of light matter interaction in a high electron mobility transistor structure

The interaction between intersubband resonances (ISRs) and metamaterial microcavities constitutes a strongly coupled system where new resonances form that depend on the coupling strength. Here we present experimental evidence of strong coupling between the cavity resonance of a terahertz metamaterial and the ISR in a high electron mobility transistor (HEMT) structure. The device is electrically switched from an uncoupled to a strongly coupled regime by tuning the ISR with epitaxially grown transparent gate. The asymmetric potential in the HEMT structure enables ultrawide electrical tuning of ISR, which is an order of magnitude higher as compared to an equivalent square well. For a single heterojunction with a triangular confinement, we achieve an avoided splitting of 0.52 THz, which is a significant fraction of the bare intersubband resonance at 2 THz.

Ultrashort electromagnetic pulse control of intersubband quantum well transitions

: We study the creation of high-efficiency controlled population transfer in intersubband transitions of semiconductor quantum wells. We give emphasis to the case of interaction of the semiconductor quantum well with electromagnetic pulses with a duration of few cycles and even a single cycle. We numerically solve the effective nonlinear Bloch equations for a specific double GaAs/AlGaAs quantum well structure, taking into account the ultrashort nature of the applied field, and show that high-efficiency population inversion is possible for specific pulse areas. The dependence of the efficiency of population transfer on the electron sheet density and the carrier envelope phase of the pulse is also explored. For electromagnetic pulses with a duration of several cycles, we find that the change in the electron sheet density leads to a very different response of the population in the two subbands to pulse area. However, for pulses with a duration equal to or shorter than 3 cycles, we show that efficient population transfer between the two subbands is possible, independent of the value of electron sheet density, if the pulse area is Π.

Resonant correlation-induced optical bistability in an electron system on liquid helium

We show that electrons on liquid helium display intrinsic bistability of resonant intersubband absorption. The bistability occurs for comparatively weak microwave power. The underlying giant nonlinearity of the many-electron response results from the interplay of the strong short-range electron correlations, the long relaxation time, and the multisubband character of the electron energy spectrum.

INTERSUBBAND SPECTROSCOPY IN QUANTUM WELL STRUCTURES AT HIGH NONEQUILIBRIUM CARRIER DENSITIES

In the present paper, the electronic intersubband transitions in semiconductor quantum well structures are investigated using transient mid infrared absorption spectroscopy after interband photoexcitation with intense picosecond pulses in the visible spectral range. The focus of our investigations is on the e2–e3 intersubband transition in an asymmetric undoped GaAs / AlGaAs quantum well (QW) structure at room temperature. At injected nonequilibrium carrier densities of 1×1013 cm -2 per QW, an e2–e3 absorption band at 99 meV is found which is blue-shifted with increasing carrier density. Intersubband absorption signals are distinguished from free-carrier absorption signals in the mid infrared (MIR) by their characteristic time behavior.

Direct observation of depolarization shift of the intersubband resonance

We have studied the intersubband resonance of GaAs/AlGaAs multi-quantum-well systems by comparing photon drag and absorption spectra obtained by in-plane photocurrent and photoconduction measurements. The peak absorption at room temperature is found to be blueshifted from the photon drag resonance by as much as 33 cm(-1). We argue that this difference gives directly the depolarization shift, since the resonant photon drag current is driven by the Doppler effect, which is a k-vector dependent single particle process.