Nikolai Georgiev

Above room temperature operation of short wavelength (λ=3.8μm) strain-compensated In0.73Ga0.27As–AlAs quantum-cascade lasers

We demonstrate the design and implementation of a broad-gain and low-threshold (Jth=860A∕cm2 at 8K) quantum-cascade laser emitting between 3.7 and 4.2μm. The active region design is based on strain-compensated In0.73Ga0.27As–AlAs on InP. Laser operation in pulsed mode is achieved up to a temperature of 330K with maximum single-facet output peak powers of 6W at 8K and 240mW at 296K. The temperature coefficient T0 is 119K.

Short-wavelength intersubband absorption in strain compensated InGaAs/AlAs quantum well structures grown on InP

We have studied intersubband absorption in strain compensated InxGa1−xAs/AlAs/InyAl1−yAs multiple quantum wells and superlattices grown on InP. X-ray diffraction shows that the layers are pseudomorphically strained and exhibit slight compositional grading of the interfaces. Owing to the high AlAs barriers, the intersubband absorption can be tailored to wavelengths shorter than 2 μm. In some samples, a small, but non-negligible absorption is also observed with s-polarized light.

On the nature of coherent artifact

The coherent interaction of femtosecond laser pulses in the pump-probe regime has been experimentally studied in the time domain by monitoring light reflection from a tellurium single crystal. The optical response of the probed medium exhibits periodic variations at a frequency equal to that of the exciting laser radiation. Experimental dependences of the observed “coherent artifact” on the pump/probe intensity ratio, the number of accumulated pulses, and the mutual orientation of the polarization vectors of electromagnetic fields and the crystallographic axes are well described by the proposed phenomenological model.

Intersubband transitions in GaP–AlP heterostructures

Intersubband optical transitions in doped AlP–GaP multiple quantum wells are investigated using midinfrared absorption. Strong p-polarized absorption corresponding to the transition from the first to the fourth electronic subband in the Xz valley is measured at wavelengths between 5 and 8μm. Additional absorption peaks related to transitions between confined donor states are also observed. The measured intersubband transition energies are consistent with an X valley conduction band offset between AlP and GaP of 280meV and a value of 1.1m0 for the longitudinal effective mass for the X valleys of AlP (where m0 is a free electron mass). These values, the role of donors and confined two-dimensional continuum states, and applications of this system for terahertz intersubband devices are discussed.

Near-infrared intersubband transitions in InGaAs-AlAs-InAlAs double quantum wells

Intersubband optical transitions at short wavelengths in strain-compensated In0.70Ga0.30As—AlAs double quantum wells are investigated by means of mid-infrared absorption. Trade-offs between achieving a high transition energy and a large oscillator strength of the two highest-energy intersubband transitions using our strain-compensation approach are analyzed as a function of the widths of the two wells. Two design strategies leading to relatively strong intersubband optical transitions at 800 meV, 1.55μm, are described and the corresponding structures grown using gas-source molecular-beam epitaxy on (001)InP are investigated. The strongest intersubband transitions obtained experimentally are generally between 300 and 600 meV, 2–4μm. Significant oscillator strength, however, also extends out to 800 meV, 1.55μm.

High-power short-wavelength quantum cascade lasers

We describe the design and implementation of a broad-gain and low-threshold (Jth = 860 A/cm2 at 8 K) quantum-cascade laser based on strain-compensated In0.73Ga0.27As-AlAs on InP. Laser emission between 3.7 and 4.2 um is achieved because of the very large Gamma-valley conduction band discontinuity with narrow quantum wells, allowing large intersubband energy differences. Furthermore, the design inhibits carrier loss from the upper lasing state into the continuum even at elevated operating temperatures, resulting in room-temperature operation. Laser operation in pulsed mode is achieved up to a temperature of 330 K with maximum single-facet output peak powers of 6 W at 8 K and 240 mW at 296 K. The temperature coefficient T0 is 119K. The 30-period structure exhibits an external differential efficiency of 13 (40% per period) at low temperatures and a maximum wall-plug efficiency of 24%. The lasing transition takes place from several upper states to several lower states, resulting in a relatively broad (300 cm-1) gain spectrum and could allow the design to be used in external tuning configurations.

Coherent phonons in NdBa2Cu3O7-x single crystals : optical-response anisotropy and hysteretic behavior

Femtosecond pump-probe measurements of reflection from crystallographic planes are performed to investigate lattice relaxation dynamics in the NdBa2Cu3O7− x high-temperature superconductor. Ultrafast phonon response is examined over a wide temperature range for various orientations of the pump and probe polarization vectors with respect to particular crystallographic axes. The initial phases of coherent phonons are measured, and hysteretic behavior is revealed in the transition between two temperature regions above Tc for the ac plane.

A time-resolved optical study of the paramagnetic dielectric-ferromagnetic metal transition in La0.7Ca0.3MnO3

The relaxation dynamics of charge carriers in an epitaxial La0.7Ca0.3MnO3 film was studied by the light reflection method with a femtosecond time resolution in a wide temperature range. The relaxation of time-resolved photoinduced reflections was shown to be two-component in the whole temperature range including the paramagnetic dielectric-ferromagnetic metal transition at T ≈ 150–160 K. The fast relaxation component had a maximum lifetime (τ ∼ 500 ps) in the transition region, and its contribution to relaxation increased as temperature decreased. The lifetime of the slow component was minimum (τ ∼ 15 fs) in the transition region. In addition, fast oscillations assigned to coherent phonons appeared in photoinduced responses at T < 200 K. The dephasing time of these oscillations increased as temperature decreased, whereas their frequency changed insignificantly.