A. Mycielski

Subpicosecond Faraday effect in Cd1−xMnxTe and its application in magneto-optical sampling

We present our studies on a subpicosecond Faraday effect in diluted magnetic semiconducting Cd1−xMnxTe single crystals and its application in a magneto-optical (MO) sampling system for time-resolved detection of ultrafast current pulses. The measurements were performed at 10 K. We used the Cd0.38Mn0.62Te crystal as the active MO medium and a low-temperature-grown free-standing GaAs photoconductive switch integrated into a coplanar transmission line as the picosecond electrical pulse generator. We observed subpicosecond MO transients that correspond to the intrinsic MO low-temperature response in the Cd1−xMnxTe system with the high Mn concentration. The current sensitivity of our MO system was found to be ∼0.1mA. We have demonstrated that the MO sampling technique using the Cd0.38Mn0.62Te transducer is as fast as the standard LiTaO3, electro-optical sampling technique and allows for a complementary (magnetic-field component) characterization of electrical picosecond transients in ultrafast switching device...

Time-resolved dynamics of coherent acoustic phonons in CdMnTe diluted-magnetic single crystals

The authors report on observation of pronounced coherent acoustic phonon (CAP) oscillations in diluted-magnetic Cd1−xMnxTe (x=0.09) single crystals by means of femtosecond, optical single-color, and two-color pump-probe spectroscopy. The observed properties of CAP oscillations are consistent with the propagating-strain-pulse model, with electronic stress as the main CAP generation mechanism. The experimental dispersion relation between the CAP oscillation frequency and the probe-beam wave vector allowed them to extract the Cd0.91Mn0.09Te refractive index dependence on wavelength, while the dependence of oscillation amplitudes on the photon energy near the Cd0.91Mn0.09Te band gap edge indicated that the material energy gap is 1.63eV. Two-color pump-probe studies showed that the speed of sound in our material is 3590m∕s and that the absorption depth of probe light, rather than the decay time of CAPs, is the dominant source of the experimentally observed oscillation dephasing time. The intrinsic lifetime of ...

CdMnTe Crystals for X-ray and Gamma-ray Detection

CdMnTe (CMT) can be a good candidate for producing gamma-ray detectors because of its wide band-gap, high resistivity, and good electron transport properties. Further, the ability to grow CMT crystals at relatively low temperatures ensures a high yield for manufacturing detectors with good compositional uniformity and few impurities. Groups at Brookhaven National Laboratory and Institute of Physics are investigating several CMT crystals, selecting a few of them to make detectors. In this paper, we discuss our initial characterization of these crystals and describe our preliminary results with a gamma-ray source.

Studies of coherent acoustic phonons in CdMnTe diluted-magnetic single crystals

We report our studies of coherent acoustic phonons (CAPs), generated and time-resolved–-detected in semimagnetic Cd1-xMnxTe single crystals by means of femtosecond pump-probe spectroscopy. On crystals with concentrations x = 0.01, 0.05 and 0.09, we implemented one-colour spectroscopy, where photon energies of both the pump and probe beams were the same and either just above or below the energy gap of the crystals. For Cd0.91Mn0.09Te we have also performed two-colour spectroscopy, in which the pump/probe photon energies were far above/below the CdMnTe bandgap, respectively. The dependences of both the frequency and dephasing time of CAP oscillations on the probe wavelength were consistent with the propagating-strain–-pulse model. In one-colour experiments, we observed a nonlinear dispersion relationship between the measured CAP frequency and the probe wave vector, which was due to a strong wavelength dependence of the probe beam on the refractive index n near the CdMnTe bandgap. The functional dependence of n, extracted from our experiments, could be fitted very well by the Schubert model. Our two-colour pump-probe measurements showed that the penetration depth of probe light, rather than the intrinsic decay time of CAPs, was the dominant source of the oscillation dephasing time. The lifetime of CAPs in our crystals was found to be at least on the order of nanoseconds. We experimentally validated that the electronic stress was the dominant generation mechanism of CAPs in CdMnTe crystals and observed that the value of the sound velocity increased with the increase of Mn-ion concentration in our samples.

Cd/sub 1-x/Mn/sub x/Te semimagnetic semiconductors for ultrafast spintronics and magnetooptics

We present ultrafast optical characterization of Cd/sub 1-x/Mn/sub x/Te single crystals with high (x>0.5-Mn) concentration, studied by magnetooptical sampling and time-resolved magnetization modulation spectroscopy. We have demonstrated that the dynamics of both Mn spins and carrier spins in Cd/sub 1-x/Mn/sub x/Te is extremely fast (in the subpicosecond range), making the nanostructures based on this material very promising for applications in spintronics and magnetooptics.

Sub-picosecond time-resolved carrier, phonon, and spin dynamics in Cd(Mn)Te diluted magnetic semiconductors

We present here our research on time-resolved carrier, phonon, and spin dynamics in the diluted-magnetic semiconductor Cd1-xMnxTe [Cd(Mn)Te] system. Our test samples were the high-quality single crystals with the Mn doping concentrations ranging from 9% to 12%, grown by a modified Bridgeman method. Femtosecond optical pump-probe spectroscopy experiments allowed us to study time-resolved dynamics of both the excited carriers and coherent acoustic phonons. Using pump photons with the energy just exceeding the Cd(Mn)Te energy gap, we observed the bleaching effect as excited carriers occupied essentially all available states at the bottom of the conduction band. With the increase of the pump photon energy, the normalized differential reflectivity (ΔR/R) signal changed sign to positive, being dominated by the electron-phonon relaxation process. All our ΔR/R traces, on the delay-time scale well above 100 ps, exhibited very regular oscillations, which were identified, as the signature of coherent acoustic phonons, generated by an electronic and thermal stress introduced at the sample surface by the pump photons. We have also excited our samples with sub-picosecond magnetic transients, generated by a low-temperature-grown GaAs photoconductive switch, and observed the sub-picosecond magneto-optic (Faraday) effect (Mn-ion spin dynamics). The sub-picosecond Faraday response makes this semimagnetic semiconductor an excellent candidate for practical applications in magneto-optics, such as in time-resolved magneto-optical sampling and imaging techniques, or in ultrafast magneto-optical transducers and modulators. In addition, Cd(Mn)Te is a very promising material for ultrafast spintronic and magnetic memory-type devices.

Room temperature photoinduced Faraday rotation in Hg1−xMnxTe alloys at 1550 nm

Faraday rotation of the polarization for a 1550 nm probe beam induced by a pump beam at shorter wavelength is studied in Hg1−xMnxTe semimagnetic semiconductors. Photoinduced rotation decays in a nanosecond timescale and it is related to carrier-induced refractive index saturation. Observed decay is discussed in terms of photoexcited carrier dynamics.

Photoconductive and electro-optic effects in (Cd,Mg)Te single crystals measured in an experiment-on-chip configuration

We present our studies on both photoconductive (PC) and electro-optic (EO) responses of (Cd,Mg)Te single crystals. In an In-doped Cd0.92Mg0.08Te single crystal, subpicosecond electrical pulses were optically generated via a PC effect, coupled into a transmission line, and, subsequently, detected using an internal EO sampling scheme, all in the same (Cd,Mg)Te material. For photo-excitation and EO sampling, we used femtosecond optical pulses generated by the same Ti:sapphire laser with the wavelengths of 410 and 820 nm, respectively. The shortest transmission line distance between the optical excitation and EO sampling points was 75 μm. By measuring the transient waveforms at different distances from the excitation point, we calculated the transmission-line complex propagation factor, as well as the THz frequency attenuation factor and the propagation velocity, all of which allowed us to reconstruct the electromagnetic transient generated directly at the excitation point, showing that the original PC transi...

Efficient thermoelectric energy conversion in Pb0.95Mn0.05Te p-n couple

We demonstrate an efficient energy conversion in a p-n thermoelectric couple built of bulk Pb0.95Mn0.05Te crystals grown by the Bridgman method and heavily doped n-type with Bi ( n=1.9×1019cm−3) or p-type with Na ( p=2.3×1019cm−3). Substitution of Mn2+ ions for Pb2+ ions at the rock-salt lattice cation sites increases the band gap of Pb1−xMnxTe and decreases the energy separation between the light hole L-band and the heavy hole Σ-band. It results in a large increase of thermoelectric power and improved thermoelectric parameters of p-type Pb1−xMnxTe. Applying the Harman method for samples of various lengths, we experimentally determined the radiation correction factors and found the thermoelectric figure of merit parameter ZT=0.75–0.8 at T=650 K for both n- and p-type materials with good thermoelectric matching of the couple. We report on thermoelectric performance of a p-n thermoelectric couple assembled of these materials and tested over the T=300–670 K temperature region.

Femtosecond electro-optic effect in (Cd,Mn)Te single crystals

We present our studies on the femtosecond electro-optic (EO) effect in high quality Cd0.88Mn0.12Te (CMT) single crystals, grown using a modified vertical Bridgman method. Our time-resolved experiments were performed using a coplanar transmission line setup, which incorporated a free-standing, low-temperature-grown GaAs photoconductive switch as a sub-picosecond electrical pulse generator and the tested CMT crystal as an active EO transducer. We determined the r-coefficient (r41) of CMT at the high (THz) frequency range to be 29.4 pm/V, an order of magnitude greater than that measured for the same sample at low (MHz) frequencies. The observed suppression of the EO coefficient at low frequencies was due to effective electric field screening by free holes in the crystal volume. Comparison to the standard EO sampling scheme based on the LiTaO3 transducer showed that CMT exhibited the same sub-picosecond temporal resolution with significantly greater voltage sensitivity. We also did not observe, the so-called dielectric loading typical for LiTaO3-based samplers.