We present the first evidence for a distinct optical phonon progression in the linear and nonlinear intersubband absorption spectra of electrons in a GaN=Al 0:8 Ga 0:2 N heterostructure. Femtosecond twocolor pump-probe experiments in the midinfrared reveal spectral holes on different vibronic transitions separated by the LO-phonon frequency. These features wash out with a decay time of 80 fs due to spectral diffusion. The remaining nonlinear transmission changes decay with a time constant of 380 fs. All results observed are described by the independent boson model.The optical line shapes of electronic transitions in condensed matter reflect the ultrafast dynamics of the elementary excitations to which the electrons are coupled. Of particular interest for a broad range of phenomena is the coupling between electrons and nuclear motions, i.e., local vibrational modes and/or phonons.(i) Coupling to underdamped nuclear motions gives rise to spectrally distinct sidebands in electronic spectra [1]. In molecules, such an interaction with intramolecular modes underlies the vibronic structure of electronic absorption and emission bands, which are additionally broadened by dephasing and spectral diffusion processes originating from a coupling to (overdamped) motions of the fluctuating surroundings [2,3]. In crystalline solids with much more delocalized electronic wave functions and phonon modes, phonon sidebands of electronic transitions have been observed mainly for interband spectra of impurities [1], for quantum-dot-like structures [4][5][6][7][8], and through coherent phonon oscillations modulating the optical reflectivity [9,10].(ii) Electron-phonon coupling strongly influences the nonequilibrium dynamics of electrons, both in the quantum-kinetic [11] and in the incoherent scattering regimes [12]. Such phenomena occur on femtosecond to picosecond time scales and have been investigated extensively by ultrafast nonlinear spectroscopy.So far, the majority of investigations of electron-phonon coupling focused on interband transitions in solids. The influence of electron-phonon coupling on intraband transitions was mainly studied in the context of polaron physics [13,14]. Intraband transitions provide direct access to the nonequilibrium dynamics of electrons. Theoretical studies suggest a prominent role of electron-phonon coupling for the line shape of intraband (free-carrier) absorption and emission. So far, clear experimental signatures of electron-phonon coupling are missing for the line shape of intersubband (IS) transitions (dipole-allowed transitions between quantized conduction subbands [15]), although first theoretical studies predict such polaronic signatures [16].Because of their strong electron-phonon coupling, nanostructures made from strongly polar materials like group-III nitrides are promising systems for the occurrence of IS phonon sidebands. Because they have large conduction band discontinuities and thus allow IS transitions at short wavelengths, GaN=AlGaN multiple quantum wells have received much interes...

The authors present evidence for a distinct optical phonon progression in the nonlinear intersubband absorption spectra of electrons in a GaN / AlN superlattice. Femtosecond two-color pump-probe experiments in the near infrared show spectral holes separated by the longitudinal optical ͑LO͒ phonon frequency and a homogeneous line broadening of approximately 50 meV. The nonlinear bleaching signal decays with a time constant of 160 fs due to intersubband scattering of delocalized electrons, followed by a weak picosecond component attributed to the relaxation of electrons from longer-lived localized states.Optical intersubband ͑IS͒ excitations in semiconductor nanostructures provide direct insight into the ultrafast dynamics of coherent optical polarizations and nonequilibrium carriers and play a key role for devices such as quantum cascade lasers.1,2 Because of the very large conduction band discontinuity, nanostructures made of group-III nitrides have recently attracted considerable interest. In particular, intersubband absorption of quasi-two-dimensional electrons in GaN / AlGaN multiple quantum wells ͑MQWs͒ has been observed at the communication wavelength Ϸ 1.55 m ͑pho-ton energy of 800 meV͒.3,4 First time-resolved experiments on MQWs have obtained intersubband relaxation times between 150 and 300 fs. 5,6 More recent femtosecond two-color pump-probe studies on similar structures have given IS relaxation times of 140 fs and a ͑sub͒picosecond intraband redistribution of carriers by thermalization and energy transfer processes of the two-dimensional ͑2D͒ electron gas. 7So far, the line shape of IS absorption in GaN / AlGaN nanostructures is not fully understood. In addition to inhomogeneous broadening due to structural imperfections, one expects a substantial homogeneous broadening caused by the ultrafast decoherence of the IS polarization. Moreover, the strong coupling between electrons and longitudinal optical ͑LO͒ phonons in group-III nitrides may give rise to spectrally distinct sidebands in electronic spectra. 8,9 Very recently, a pronounced optical phonon progression was found in the nonlinear IS absorption of a GaN / AlGaN heterostructure. 10In this letter we show that the IS transitions of electrons in GaN / AlN superlattices display a considerable homogeneous broadening of the order of 50 meV and couple strongly to LO phonons as observed through distinct sidebands in the nonlinear optical response. Transient IS spectra recorded in femtosecond two-color pump-probe experiments exhibit spectral holes on different vibronic transitions spectrally separated by the LO phonon frequency. More than 95% of the nonlinear decrease of IS absorption decay with = 160 fs due to IS scattering of delocalized n = 2 electrons, whereas a small residual bleaching decaying within several picoseconds may be attributed to excited electrons in localized states.We investigated a GaN / AlN superlattice consisting of 20 periods of 2.2 nm thick GaN potential wells doped with Si and 2.2 nm thick AlN barriers ͑for details of the sample s...

Ultrafast electron dynamics in the inversion layer of an AlGaN/GaN transistor is studied in pump-probe experiments with 50 fs mid-infrared pulses. Two-colour pump-probe measurements show an instantaneous transmission increase for all spectral positions of the probe, which demonstrates that homogeneous broadening is an important contribution to the total linewidth in this material. We observe the maximum of the induced transmission change around 5 µm. This large Stokes shift might be caused by the extremely large electron-LO-phonon scattering rate.Optical intersubband excitations in semiconductor nanostructures provide direct insight into the ultrafast dynamics of coherent optical polarizations and nonequilibrium carriers and play a key role for novel devices such as quantum cascade lasers. Because of the huge conduction band discontinuity and thus the possibility of fabricating quantum cascade lasers at short wavelengths, nanostructures made of group-III nitrides have recently attracted considerable interest. In particular, intersubband absorption in GaN/AlGaN multiple quantum wells (MQWs) was observed in the spectral range up to the communication wavelength λ ≈ 1.55 µm [1]. The first time-resolved experiments on MQWs (intersubband absorption at λ = 4.5 µm) gave a rough upper limit of 150 fs for intersubband relaxation [2]. Two-dimensional electron subbands can also be formed in the inversion layer of an AlGaN/GaN-based high-electronmobility transistor (HEMT). Recently, e1-e2 intersubband absorption was observed for the first time in AlGaN/GaNbased HEMTs [3] showing a broad absorption centred at λ = 4 µm [ figure 1(a)]. This HEMT (GS1282 in [3]) was grown on a c-face sapphire substrate using MBE. The layer sequence is 20 nm pure AlN, 1.2 µm nominally undoped GaN, 13 nm undoped Al 0.8 Ga 0.2 N, and finally 2 nm undoped GaN as a contact layer. For reflection a gold layer was sputtered onto the contact layer. In this sample, a large piezoelectric field at the heterointerface is present due to the lattice mismatch and the hexagonal crystal structure. The piezoelectric field together with the heterointerface results in a deep triangular well.Here, we present the first time-resolved study of the two-dimensional electron plasma in this Al 0.8 Ga 0.2 N/GaNbased HEMT, using direct intersubband excitation and probing of electrons via the e1-e2 intersubband transition. The intersubband absorption spectrum at a temperature of 300 K is plotted in figure 1(a). The broad absorption band is due to transitions of electrons from the n = 1 to the n = 2 subband in the triangular potential well.In our time-resolved experiments, electrons are excited from the e1 to the e2 subband by 50 fs mid-infrared pulses resonant with the optical IS transition. Pump and probe pulses at 1 kHz repetition rate are derived from the output of an optical parametric amplifier by difference frequency mixing in a 1 mm thick GaSe crystal [4]. After interaction with the sample, the probe pulses are spectrally dispersed in a monochromator (3 meV reso...

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The authors present evidence for a distinct optical phonon progression in the nonlinear intersubband absorption spectra of electrons in a GaN / AlN superlattice. Femtosecond two-color pump-probe experiments in the near infrared show spectral holes separated by the longitudinal optical ͑LO͒ phonon frequency and a homogeneous line broadening of approximately 50 meV. The nonlinear bleaching signal decays with a time constant of 160 fs due to intersubband scattering of delocalized electrons, followed by a weak picosecond component attributed to the relaxation of electrons from longer-lived localized states.Optical intersubband ͑IS͒ excitations in semiconductor nanostructures provide direct insight into the ultrafast dynamics of coherent optical polarizations and nonequilibrium carriers and play a key role for devices such as quantum cascade lasers.1,2 Because of the very large conduction band discontinuity, nanostructures made of group-III nitrides have recently attracted considerable interest. In particular, intersubband absorption of quasi-two-dimensional electrons in GaN / AlGaN multiple quantum wells ͑MQWs͒ has been observed at the communication wavelength Ϸ 1.55 m ͑pho-ton energy of 800 meV͒.3,4 First time-resolved experiments on MQWs have obtained intersubband relaxation times between 150 and 300 fs. 5,6 More recent femtosecond two-color pump-probe studies on similar structures have given IS relaxation times of 140 fs and a ͑sub͒picosecond intraband redistribution of carriers by thermalization and energy transfer processes of the two-dimensional ͑2D͒ electron gas. 7So far, the line shape of IS absorption in GaN / AlGaN nanostructures is not fully understood. In addition to inhomogeneous broadening due to structural imperfections, one expects a substantial homogeneous broadening caused by the ultrafast decoherence of the IS polarization. Moreover, the strong coupling between electrons and longitudinal optical ͑LO͒ phonons in group-III nitrides may give rise to spectrally distinct sidebands in electronic spectra. 8,9 Very recently, a pronounced optical phonon progression was found in the nonlinear IS absorption of a GaN / AlGaN heterostructure. 10In this letter we show that the IS transitions of electrons in GaN / AlN superlattices display a considerable homogeneous broadening of the order of 50 meV and couple strongly to LO phonons as observed through distinct sidebands in the nonlinear optical response. Transient IS spectra recorded in femtosecond two-color pump-probe experiments exhibit spectral holes on different vibronic transitions spectrally separated by the LO phonon frequency. More than 95% of the nonlinear decrease of IS absorption decay with = 160 fs due to IS scattering of delocalized n = 2 electrons, whereas a small residual bleaching decaying within several picoseconds may be attributed to excited electrons in localized states.We investigated a GaN / AlN superlattice consisting of 20 periods of 2.2 nm thick GaN potential wells doped with Si and 2.2 nm thick AlN barriers ͑for details of the sample s...