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The Landau level laser was proposed long ago as a unique source of monochromatic radiation that would be widely tunable in the THz and infrared spectral ranges using a magnetic field. However, despite many efforts, this appealing concept never progressed to the design of a reliable device. This is because of the efficient Auger scattering of Landau-quantized electrons, an intrinsic non-radiative recombination channel that eventually gains over cyclotron emission in all materials studied so far (conventional semiconductors with parabolic bands, but also in graphene with massless electrons). Auger processes are favoured in these systems because the Landau levels (or their subsets) are equally spaced in energy. Here, we show that this scheme does not apply to massless Kane electrons in gapless HgCdTe, where undesirable Auger scattering is strongly suppressed and sizeable cyclotron emission is observed. The gapless HgCdTe thus appears as a material of choice for future Landau level lasers.

The R-lines of the Mn4+fluorescence, associated with 2transitions in gadolinium gallium garnet exhibits large broadening as compared with other ions with d3shell. The laser excitation and fluorescence spectra without magnetic field show that this is partially inhomogeneous broadening related to presence of charge-compensating Ca2+ions. Remaining part of the broadening is due to antiferromagnetic exchange interaction between Mn4+and surrounding them six Gd3+ions. The high magnetic field up to 20 T has been used to resolve the spectra broadened by spin-spin interaction. The estimated value of the spin-spin exchange constant is equal to j ≈ 1.9 cm-1.

Local laser excitation and temperature readout from the intensity ratio of Stokes to anti-Stokes Raman scattering signals are employed to study the thermal properties of a large graphene membrane. The concluded value of the heat conductivity coefficient κ ≈ 600 W/(m·K) is smaller than previously reported but still validates the conclusion that graphene is a very good thermal conductor.

We report the photoluminescence experiments on high quality GaN samples in high magnetic fields. A detailed analysis of the recombination due to excitons bound to neutral donors and acceptors is presented. Special attention is focussed on transitions for which the impurity is left in the excited state (so called two electron satellites). These results show a rich energetic structure of excited states of the impurity involved in such a recombination process. The magnetic field dependence of the energy structure of the shallow neutral donor and acceptor in GaN is then discussed. The validity of the effective mass approximation for shallow impurities in wide gap semiconductors is examined. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

There are three exciton branches in wurzite-GaN: A, B and C. Here, time-resolved photoluminescence and magneto-optic spectroscopy of the B-exciton are presented, both free (FXB) and bound to donor (D⁰XB) in homoepitaxial GaN. The free B-exciton has an energy of about 3.483 eV and a very short lifetime of about 20 ps. The B-exciton bound to donor (D⁰XB) has an energy of 3.475 eV and a lifetime longer than the free excitons but shorter than the lifetime of the A-exciton bound to donor. It is shown that a simple model based on an invariant expression describes properly the behavior of D⁰XA and D⁰XB in a magnetic field. Results of resonance excitation experiments suggest that in many cases the process of binding a free exciton to a donor conserves the exciton type and that the relaxation of B- to A-exciton takes place later. Moreover, thermal excitation of D⁰XA to D⁰XB has been observed.

Magneto-luminescence experiments performed on a thick, freestanding GaN layer in magnetic fields up to 28 T are presented. The photoluminescence spectrum of this material shows narrow emission lines due to neutral donor-bound excitons (D⁰X). In addition to the principal recombination channel of D⁰X in which the donor is left in its ground state, two-electron satellites (TES) involving different excited donor states, as well as LO phonon replica of the principal and satellite transitions are observed. For the magnetic field range in which internal donor excitations are tuned into resonance with the LO phonon excitations, the intensity of TES is strongly enhanced. A clear resonant interaction between TES corresponding to the highly excited donor states (n > 4) and LO phonon replica of D⁰X is observed. The observed effect is discussed in terms of electron–phonon interaction.

There are three exciton branches in wurzite-GaN: A, B and C. Here, time-resolved photoluminescence and magneto-optic spectroscopy of the B-exciton are presented, both free (FXB) and bound to donor (D0XB) in homoepitaxial GaN. The free B-exciton has an energy of about 3.483 eV and a very short lifetime of about 20 ps. The B-exciton bound to donor (D0XB) has an energy of 3.475 eV and a lifetime longer than the free excitons but shorter than the lifetime of the A-exciton bound to donor. It is shown that a simple model based on an invariant expression describes properly the behavior of D0XA and D0XB in a magnetic field. Results of resonance excitation experiments suggest that in many cases the process of binding a free exciton to a donor conserves the exciton type and that the relaxation of B- to A-exciton takes place later. Moreover, thermal excitation of D0XA to D0XB has been observed.

Magneto-luminescence experiments performed on a thick, freestanding GaN layer in magnetic fields up to 28 T are presented. The photoluminescence spectrum of this material shows narrow emission lines due to neutral donor-bound excitons (D0X). In addition to the principal recombination channel of D0X in which the donor is left in its ground state, two-electron satellites (TES) involving different excited donor states, as well as LO phonon replica of the principal and satellite transitions are observed. For the magnetic field range in which internal donor excitations are tuned into resonance with the LO phonon excitations, the intensity of TES is strongly enhanced. A clear resonant interaction between TES corresponding to the highly excited donor states (n > 4) and LO phonon replica of D0X is observed. The observed effect is discussed in terms of electron–phonon interaction.

We report on the results of high-magnetic-field (up to 28T) inelastic light scattering measurements of coupled plasmon–phonon modes in metallic GaAs samples with electron concentrations ranging from 1017to 2×1018cm−3. The zero-field Raman spectra of the investigated samples show peaks related to the transverse optical phonon (TO) as well as the coupled plasmon–phonon ω−(below TO frequency) and ω+(above LO frequency) modes. In the backscattering Voigt geometry (k⊥B) in which the modes propagating across the field direction are probed, the magnetic field does not influence the TO resonance but leads to a splitting of both ω+and ω−branches. The experimental data are approximated using the dielectric function formalism. For a highly doped sample, the interaction between coupled plasmon–phonon modes with the Bernstein mode is observed.

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We present interband magneto-absorption spectra up to 20 T taken at 2 K for 100 Å CdTe and CdMnTe single and multiple quantum wells containing a two-dimensional electron gas with concentrations ne up to 4 × 1011 cm—2. Without magnetic field B the absorption spectrum is characterized by two strong peaks, whose energy difference increases linearly with the electron concentration ne. This is interpreted in the framework of the many-body theory proposed by Hawrylak. In a magnetic field B, the spectra of high ne (4 × 1011 cm—2) quantum wells show circularly polarized absorption lines. We find that: (i) asBis increased there is a dominant, strongly enhanced absorption transition whose position is related to the zigzags of the Fermi level on the conduction band Landau levels; (ii) at integer values of the filling factorν, in absorption spectra, there is a sudden redshift of this line equal to the cyclotron energy of the electron ħωeh ; (iii) between the discontinuities, in emission spectra, the values of the transition slope are nħωeh (i.e. not the expected values (n + 1/2) ħωeh) where n is the Landau level index and ħωeh represents the sum of the cyclotron energy of the electron and the hole.

Magnetic field studies of circularly polarised emission due to free and bound excitons in high quality homoepitaxial GaN layers are presented. The conventional analysis of the band structure of the wurtzite-type symmetry allowed us to determine both the signs and the values of the effective Landé g-factors for electrons and holes participating in the recombination of free excitons originating from Γ9 and Γ7 valence band components as well as for electrons and holes weakly bound to donor and acceptor centres. The magnetic field splitting scheme of the conduction and the valence band is examined. The fine structure of the excitonic emission is also discussed.

We have studied the temperature dependence of the spin-flip processes of electrons in p-type modulation-doped GaAs quantum wells. We have found that at low temperature (T ≤ 30 K) the exchange interaction between electrons and holes is the main mechanism responsible for the spin-flip of the electrons, whereas at higher temperatures the so-called Dyakonov-Perel mechanism dominates. Furthermore, we have found that the recombination time increases considerably with increasing lattice temperature from a value of τr  200 ps at 5 K to τr  400 ps at 60 K.

Recent magnetoluminescence results obtained for homoepitaxial GaN layers are presented. The neutral impurity-bound excitons and donor–acceptor pair emission lines have been studied in magnetic fields up to 27 T. Low-temperature luminescence spectra have been measured with the magnetic field parallel and perpendicular to the hexagonal c-axis of the GaN layers. Experimental results allowed us to evaluate diamagnetic shifts, effective g-factors of electrons and holes involved in neutral donor and neutral acceptor complexes as well as the electron–hole exchange constant for close donor–acceptor pairs. Both the fine structure of the neutral acceptor-bound exciton emission and the specific properties of donor–acceptor pair spectra observed in magnetoluminescence experiments are tentatively attributed to the internal structure of the acceptor state.

The laser excitation and luminescence spectra of the R-lines of the Mn4+ in gadolinium gallium garnet show that the large broadening which they exhibit is partially related to presence of charge-compensating Ca2+ ions, even for the case of manganese center without close compensation. Remaining part of the broadening is due to antiferromagnetic exchange interaction between Mn4+ and surrounding them six Gd3+ ions. The spectra remain broadened in magnetic fields below 6 T and the higher magnetic fields up to 20 T removes the broadening related to the spin-spin interaction. Luminescence and excitation spectra at 2.2 K show that the transitions occur between levels which magnetic numbers differ by 0 or ± 1. The estimated value of the spin-spin exchange constant for the ground state of Mn4+ is approximately equal to J ≈ 1.9 cm-1.

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The luminescence of n-type modulation doped GaAs/GaAlAs single quantum well structures has been used to probe the relaxation processes in the presence of quasi two dimensional electron gas under high magnetic fields. It was found that the magnetic field induced modulation of the Landau levels separation and occupation strongly alters the relaxation processes. In the Quantum Hall Effect regime the nonradiative relaxation processes through the localized states between Landau levels are strongly enhanced due to the reduction of the screening.

MBE grown Ga1−xMnxAs layers were investigated by means of magnetic resonance techniques. Two phases can be distinguished: an almost isotropic ferromagnetic phase in insulating layers and an anisotropic ferromagnetic phase in the metallic Ga1−xMnxAs. Under a strong magnetic field the field-induced insulator-to-metal transition is accompanied by the change from the ferromagnetic to the ferrimagnetic phase.

The free-electron laser FELBE at the Helmholtz-Zentrum Dresden Rossendorf enables experiments with spectral, temporal, and, by means of near-field microscopy, also high spatial resolution. FELBE delivers picosecond IR and THz pulses in a wavelength range from 4 m to 280 m. Here we review the potential of the laser and focus on two highlight pump-probe experiments. In the first experiment, the relaxation dynamics in self assembled InGaAs quantum dots at energies below the Reststrahlen band is studied. Long intradot relaxation times (1.5 ns) are found for level separations of 14 meV (3.4 THz), decreasing very strongly to ~ 2 ps at 30 meV (7 THz). The results are in very good agreement with our microscopic theory of the carrier relaxation process, taking into account polaron decay via acoustic phonons. In the second experiment, the relaxation dynamics in graphene is investigated at photon energies E = 20 - 250 meV. For excitations below the energy of the optical phonon (G mode), the relaxation times are more than one order of magnitude longer as compared to the relaxation times observed for near infrared excitation.

Epitaxial graphene (EG) is a new material that consists of one or more graphene layers that are grown on single crystal silicon carbide. EG research has for the past 6 years focused on material production and processing. High quality material can now be produced and processing methods are promising. Progress is reviewed here with an emphasis on the recently discovered multilayered epitaxial graphene (MEG) which is multilayered yet exhibits the properties of a single layer. The status of this rapidly developing field is briefly reviewed with an emphasis on properties and effects that are relevant for electronics applications.