Measurement of Coherence Decay in GaMnAs Using Femtosecond Four-wave Mixing

The aim of this experiment is to measure the defacing time in gallium manganese arsenide with high temporal resolution using the technique of full wave mixing. This is achieved using two optical pulses incident on the sample with wave vectors, K one and K two. Together, these pulses excite optical transitions that are resonant with the laser pulse resulting in the excitation of a coherent electronic polarization density.

This polarization emits light in the direction 2K two minus K one. This is the four wave mixing signal. The time resolution of this technique is determined by the optical pulse duration.

This duration is minimized by measuring the pulse auto correlation width while adjusting a pulse compressor. The auto correlation is detected through the measurement of the second harmonic intensity generated by the para pulses as a function of the interpulse delay. The samples for these experiments were grown by molecular beam epitaxy by EK Ena and his coworkers at the University of Notre Dame.

They are prepared for experiments by gluing the surface to a sapphire window and removing the substrate with the sample at the laser focus measurement is made of the intensity of the four wave mixing emitted light as a function of wavelength through spectral resolution using a monochrome as well as interpulse delay. The temporal envelope of the four wave mixing signal is also measured using a third laser pulse and performing a cross correlation of this pulse called a gate with the four wave mixing signal through some frequency generation. Ultimately one obtains a measurement of the defacing time, also called the T two or coherence decay time for the electron hole pairs.

Excited on the resonant interband transitions in the semiconductor sample. One can use these results to distinguish between an isolated two level system and a collection of closely spaced transitions. The latter leads to a so-called photon echo and the time resolved four wave mixing response.

With this technique, we can time resolve the fastest scattering processes of electrons and holes in the semiconductor system. Under study such scattering processes govern both the transport and coherence decay, and so this technique provides a useful tool for studying semiconductor materials of interest for novel optical, electronic and electronic devices. We have shown here that the coherence decay in gallium manganese arsenide occurs via spin flip scattering between the holes and the manganese ions, allowing us to obtain a direct measurement of the timescale of this important process.

This gives new insight into the exchange coupling responsible for FARO magnetic order. In this system, Gallium manganese, arsenide, and other materials within the family of three five diluted magnetic semiconductors may ultimately enable the integration of logic and memory functions within computer hardware. This is part of the broader research field of semiconductor spintronics, so our results have implications for the long term engineering of these materials by illustrating the power and flexibility of four wave mixing techniques for understanding their fundamental properties, The optical setup for time and spectrally resolved four wave mixing consists of a set of mirrors and irises that are used each day to recover the basic alignment of the optical beams within the setup.

This is followed by a pulse compressor that introduces negative group velocity dispersion to compensate for the positive group velocity dispersion caused by lenses and optical windows on the way to the sample After the compressor, the beam splitter is used to divide the incoming pulses from the laser into two excitation pulses, E one and E two. The path for E one contains a pair of mirrors mounted on a manual translation stage that is used to make course adjustments of the delay between pulses E one and E two. The path for pulse E two travels down onto a lightweight retro reflector on the cone of a speaker.

Together with a function generator and current amplifier, the speaker allows the interpulse delay to be rapidly scanned. During the experiment, the two beams are then focused using a single lens onto the sample, which is contained in an optical cryostat. This allows the sample to be held at any desired temperature between four kelvin and 300 kelvin.

For spectrally resolved time integrated experiments, the four wave mixing emission is passed through a lens onto a monochrome and detected using a high speed photo multiplier tube. The photo multiplier signal is measured versus interpulse delay while the speaker position is scanned. Using an oscilloscope for time resolved four wave mixing.

A third laser pulse is split off using another beam splitter and is sent through a separate optical delay line that contains another speaker. This pulse is mixed with the four wave mixing signal in a beta barium bate or BBO nonlinear crystal and the resulting sum frequency light is detected by another photomultiplier tube. The duration of the excitation pulses is measured and optimized by redirecting both the E one and E two excitation pulses to a second focus here.

Another BBO crystal is placed the second harmonic signal resulting from the overlap of pulses. E one and E two is measured as a function of interpulse delay. The pulse duration is measured by first inserting a large planar mirror to redirect the focal point of the two excitation beams from the sample to a secondary focus.

A nonlinear crystal that has been cut for type one phase matching for 800 nanometers at normal incidents is placed at the focal point. The crystal holder allows adjustment along all three spatial axes as well as rotation an optical filter that blocks 800 nanometers and passes the 400 nanometer. Second harmonic light from the excitation pulses is placed after the nonlinear crystal.

The angle of the crystal that optimizes the second harmonic from the combination of both excitation pulses together is normal incidents or halfway between the angles giving the maximum brightness of each of the two outer spots. The path length of pulse E one is adjusted until the central second harmonic spot is seen on the white card. This central spot is the auto correlation.

A fast photo diode used to detect the auto correlation is aligned to the central beam. The function generator and amplifier controlling the speaker motion are turned on resulting in the periodic variation in the path length of pulse. E two.

The resulting auto correlation signal may be observed by connecting the photo diode to the oscilloscope. The oscilloscope time axis may be converted to pulse delay in femtoseconds by correlating the path length of pulse E one adjusted by the manual translation stage with the temporal position of the peak second harmonic signal on the oscilloscope. The pulse duration is minimized by adjusting the prisms in the pulse compressor while viewing the auto correlation signal on the oscilloscope, making it as narrow as possible.

The minimum pulse duration achieved is approximately 20 femtoseconds. Our samples consist of a 500 micron gallium arsenide substrate, a 175 nanometer aluminum gallium arsenide layer, and a 750 nanometer thin film of gallium manganese arsenide. For experiments in the transmission geometry, the substrate must be removed.

First, the sample is placed sample side down on a small bead of optical adhesive on a sapphire window. The sample is pressed uniformly to remove excess glue from beneath the sample surface to cure the optical adhesive, the sample is placed under an ultraviolet lamp for approximately six hours. Between 300 and 400 micrometers of the substrate is then removed by grinding.

The remaining substrate is removed by immersion in a solution of citric acid and hydrogen peroxide. When the stop etch layer is reached, the sample appearance will transition from a dull gray to a mirror-like finish, indicating that it can be removed from the solution. After rinsing in deionized water, the sample is ready to be mounted in the cryostat for four wave mixing experiments.

Once the auto correlation signal is optimized and the sample has been placed in the cryostat, the large flat mirror used for pulse measurement is removed. For spectrally resolved, but time integrated four wave mixing experiments, the signal beam is focused into a monochrome. An alignment tool helps locate the four wave mixing signal spatially by aligning the beam spots corresponding to pulses E one and E two.

On the crosses a photo multiplier tube or PMT is placed after the monochrome and connected to the oscilloscope. Note, the high voltage bias on the PMT must only be turned on with the detector in the dark data acquisition software is used to record the oscilloscope trace, which is the four wave mixing signal versus the delay between pulses E one and E two for each desired wavelength. For time resolved four wave mixing experiments, the lens focusing into the monochrome is removed and a mirror is inserted to pick off the four wave mixing signal beam.

The four wave mixing beam is focused together with a gating beam into a nonlinear crystal, followed by a filter to pass only the sum frequency beam. The cross correlation of the four wave mixing signal and the gait pulse is then measured as a function of the gait pulse delay. Using A PMT, the gate pulse delay is varied by connecting the function generator and amplifier to the speaker in the gate pulse path.

The time resolved signal is then recorded using the data acquisition software by recording the oscilloscope trace for fixed values of the delay between pulse's E one and E two. This interpulse delay is set using the manual translation stage in the beam path for pulse E one. Typical results of spectrally resolved four wave mixing experiments are shown for a low temperature grown gallium arsenide reference sample on the left and gallium manganese arsenide for a manganese fraction of five thousandths of a percent.

On the right, the data are shown as a function of photon energy and the time delay between pulse's E one and E two. The broadening of the signal towards higher energies in gallium manganese arsenide is attributed to an increased density of states and the valence band caused by SPD hybridization of the substitutional manganese within the gallium arsenide host crystal. Our goal is to measure the dephasing time in gallium manganese arsenide for carriers above the band gap of gallium arsenide.

In this case, we focus on the time integrated signal at a photon energy of 1.533 ev the time integrated four wave mixing signal versus interpulse delay in gallium manganese. Arsenide is shown at the left for a photon energy of 1.533 ev. The time resolved four wave mixing signal in the same sample is shown at the right versus gait time for an interpulse delay of 54 femtoseconds.

The defacing time is extracted from these results by fitting to an analytical model indicated by the solid curves. The extracted dephasing time T two in gallium manganese arsenide is 65 femtoseconds. From comparison with data in high temperature and low temperature grown gallium arsenide, the dominant dephasing process is identified as whole manganese spin, flip scattering one can use these time resolved four wave mixing experiments to distinguish between an isolated two level system, which leads to free induction decay and a collection of closely spaced transitions, which leads to a photon echo are measured results in gallium manganese arsenide exhibit, the characteristic signature of a photon echo with a signal peak that shifts to positive time with increasing pulse delay.

The peak occurs at approximately two times the interpulse delay. The good agreement between the analytical model for an in homogeneously broadened transition and the measured results indicates that many body effects do not contribute significantly, and that the dephasing time is independent of photon energy despite defect induced localization in this system. After watching this video, we hope you'll have a good understanding of the technique of femtosecond foray mixing.

This work lays the foundation for application of this technique to study key fundamental properties in diluted magnetic semiconductors, such as the electronic structure and exchange coupling, and to measure the timescale for coherence decay in other novel materials of interest for next generation electronics.