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The Study Of Ultra-Fast Relaxation Phenomena Within Nd:Yag And Hopg Crystals.

Bryant T. Wysocki

Published May 29, 2011• 0 citations
OpenAlex

Abstract

A monolithic two-section quantum dot semiconductor laser is differentially pumped to form non-uniform current injection in the gain region. It is shown that the nature of the pulse spectral content can be manipulated through differential gain. This provided a simple and inexpensive method of optimizing the semiconductor laser performance and also demonstrated that mode locked operation can be achieved with a much larger set of injection current/absorber bias voltage pairs than was previously possible with single-pad current injection. Significantly greater absorber bias voltages were available during differential pumping; thereby reducing absorber recovery time as well as the inferred pulse length. The enhanced spectral bandwidth provided by differential pumping provided an electronic method of pulse length reduction. The capacity for wavelength tuning was also demonstrated by differential pumping with a center wavelength shift of approximately 12nm. Additionally, the design and configuration of a femtosecond IR optical parametric oscillator demonstrating continuous pulsed tunability from 1040 to 1150 nm is presented along with the examination of relaxation rates for optically excited electrons from the pump3 ing level (4 F 5 ) to the metastable upper lasing level (4 F 2 ) in neodymium doped 2 Yttrium aluminum garnet (Nd:YAG) laser crystals. The 4 F 5 lifetime was the2 oretically predicted to be approximately 12 picoseconds using the Energy Gap Law. Finally, the ultrafast relaxations of photoexcited carriers in thin films of exfoliated highly ordered pyrolytic graphite (HOPG) were examined using the equal-pulse correlation technique which provides the means to efficiently extract a relatively fast relaxation process obscured by a significantly slower one. In addition, it allows for measurements on the order of one tenth the laser pulse width, extending the temporal range of existing pump probe systems without the need for customized femtosecond lasers or complicated pulse compression systems. The HOPG samples exhibited an extremely fast relaxation due to intraband carrier-carrier thermalization observed at 15 ± 10 fs followed by intraband carrier-phonon interaction on the order of 175 ± 30 fs. A modulation in the absorption depth of 1.4 percent was observed in the samples from partial saturation due to Pauli blocking.

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A monolithic two-section quantum dot semiconductor laser is differentially pumped to form non-uniform current injection in the gain region. It is shown that the nature of the pulse spectral content can be manipulated through differential gain. This provided a simple and inexpensive method of optimizing the semiconductor laser performance and also demonstrated that mode locked operation can be achieved with a much larger set of injection current/absorber bias voltage pairs than was previously possible with single-pad current injection. Significantly greater absorber bias voltages were available during differential pumping; thereby reducing absorber recovery time as well as the inferred pulse length. The enhanced spectral bandwidth provided by differential pumping provided an electronic method of pulse length reduction. The capacity for wavelength tuning was also demonstrated by differential pumping with a center wavelength shift of approximately 12nm. Additionally, the design and configuration of a femtosecond IR optical parametric oscillator demonstrating continuous pulsed tunability from 1040 to 1150 nm is presented along with the examination of relaxation rates for optically excited electrons from the pump3 ing level (4 F 5 ) to the metastable upper lasing level (4 F 2 ) in neodymium doped 2 Yttrium aluminum garnet (Nd:YAG) laser crystals. The 4 F 5 lifetime was the2 oretically predicted to be approximately 12 picoseconds using the Energy Gap Law. Finally, the ultrafast relaxations of photoexcited carriers in thin films of exfoliated highly ordered pyrolytic graphite (HOPG) were examined using the equal-pulse correlation technique which provides the means to efficiently extract a relatively fast relaxation process obscured by a significantly slower one. In addition, it allows for measurements on the order of one tenth the laser pulse width, extending the temporal range of existing pump probe systems without the need for customized femtosecond lasers or complicated pulse compression systems. The HOPG samples exhibited an extremely fast relaxation due to intraband carrier-carrier thermalization observed at 15 ± 10 fs followed by intraband carrier-phonon interaction on the order of 175 ± 30 fs. A modulation in the absorption depth of 1.4 percent was observed in the samples from partial saturation due to Pauli blocking.
Using laser pulses of duration comparable to semiconductor relaxation and dephasing times, the coherent phenomena common to two-level systems have been demonstrated in II-VI and III-V semiconductors. These light-matter interactions are characterized by electrons and holes in their initially excited states and/or a macroscopic polarization that persists after the excitation. A nonthermal carrier distribution was observed in bulk CdSe, demonstrating the extremely fast energy relaxation possible when carrier-LO-phonon scattering is included. Quantum-confined CdSe microcrystallites were then employed to limit the available decay avenues. The one and two-pair transitions were characterized, and using spectral hole-burning, energy relaxation was shown to be substantially slower than in bulk. Dipole dephasing remained very rapid. Persistent macroscopic polarization was demonstrated in semiconductors through pump and probe experiments conducted on time scales faster than the inverse linewidth of the exciton. In the first set of experiments, it was found that the exciton optical Stark shift deviates from that predicted by a steady-state theory. The shift is smaller than expected and lasts longer than the pump pulse. Both effects are explained by free evolution of the exciton's polarization. Transient exciton bleaching is observed and shown to be a result of adiabatic following. In a second study of coherent polarization, spectral oscillations at negative delay times were studied. Oscillations were measured in a wide variety of semiconductors, temporally preceding several different pump-probe phenomena. By comparison with a semiclassical theory, it was shown that spectral oscillations in the spectral region of the exciton are evidence of persistent exciton polarization. Oscillations in the band are due to four wave mixing of the pump and probe beams.
Yttrium Aluminum Garnet (YAG, Y_3 〖Al〗_5 O_12) crystals doped with Erbium have posed an interesting position in the field of rare earth solid state lasers as they possess the property of self-saturation, in which the upper energy level in a laser has a much shorter lifetime than that of the lower level. This is the case of the 2.94μm transition in Er:YAG. That property at first seems to make this material unfit for use as a laser gain medium, however further research into these classes of rare earth materials revealed interesting non-liner energy transfer mechanisms that allow it to be a useful mid wave infra-red (MWIR) coherent source in the continuous wave (CW) and pulsed regimes. This came to fruition due to decades of modeling and spectroscopic research investigating the non-linear energy transfer mechanisms of excited state absorption (ESA), energy transfer up-conversion (ETU), and cross relaxation (XR). These effects show up in the rise and fall times of the levels observed through fluorescence. Resulting in the non-exponential rise and fall characteristics and having a squared or even cubed relation to the population. The population evolution for each of the lasing levels is now affected by ions recycling energy and in turn cause the lifetimes of the levels to “effectively” change to a point where simple linear models do not adequately describe the system and its performance. The non-linear energy transfer dynamics of Er:YAG are modeled under high resonant pump conditions. By pumping with selective resonant pumps, the interaction dynamics of the (_ ^4)I_(11/2) and the (_ ^4)I_(13/2) levels in the Erbium ion reveal the contributions from generally ignored non-linear energy transfer mechanisms. Specifically, the multi-photon effect known as excited state absorption (ESA) is modeled by measuring and characterizing the cross section in single crystal Er:YAG samples utilizing a pump and probe technique coupled with transient fluorescence measurements. The measured ESA cross section is then included in the rate equation modeling.
Gain recovery dynamics are studied in electrically pumped quantum dot (QD) based semiconductor optical amplifiers (SOAs) after amplification of double femtosecond laser pulses using ultrafast pump-probe spectroscopy with heterodyne detection. The authors observe a distinct change in gain recovery in the ground state when a significant excited state population is achieved. A complete gain recovery is found when two 150fs pulses with 5ps time delay pass through the SOA in resonance to the ground state under high injection currents of 80–100mA. The obtained results open the way for ultrafast (>200GHz) operation in p-doped QD based SOAs at 1.3μm telecommunications wavelengths.
Spectral broadening and cross relaxation of an aggregate of highly carrier-injected quantum dots have been studied experimentally by evaluating the gain saturation and optical-wave mixing characteristics of traveling-wave laser amplifiers that include self-assembled 0.98-µm InGaAs dots. Measured signal-gain spectra at room temperature showed that the gain decreases over the entire gain spectra when an intense and narrow-linewidth saturating signal is injected at a higher-order subband transition of the quantum dot. Differential-gain spectrum was calculated from the saturated and unsaturated signal gain spectra, and revealed asymmetric gain saturation with respect to the wavelength of the saturating signal. These results suggest that cross relaxation is so fast that the dot aggregate has a homogeneously broadened gain spectrum. Self-saturation and cross-saturation characteristics, and the detuning characteristics of highly nondegenerate four-wave mixing were also investigated. The latter revealed that the characteristic times of two cross-relaxation processes are about 1 ps and 100 fs.
High-power, picosecond-pulse excitation was used to study the dynamics of the pumping and decay processes in Y3{\mathrm{Y}}_{3}Y3​Al5{\mathrm{Al}}_{5}Al5​O12{\mathrm{O}}_{12}O12​:Nd3+{\mathrm{Nd}}^{3+}Nd3+. The observed fluorescence spectral lines are assigned to transitions from three different metastable states excited by both single-photon and two-photon absorption. The branching ratios, lifetimes, and quantum efficiencies of the metastable states are obtained from the data as well as the two-photon absorption cross sections. The results provide useful information concerning ultraviolet pumping efficiencies of the Nd-YAG (Nd-doped yttrium aluminum garnet) laser transition and the $5d\ensuremath{-}4f$ nonradiative decay rate for this material. In addition, the time-resolved spectroscopy method employed here is shown to be an extremely sensitive technique for obtaining two-photon absorption cross sections.
The gain characteristics of strained layer InGaAs quantum well laser amplifiers grown on GaAs substrates have been characterized using a Ti-sapphire laser in both CW and self-mode-locked configuration. All-optical bistability was achieved in such a device through the nonlinear refractive index that arises as a result of the gain saturation of the diode amplifier at high optical intensities. The Ti-sapphire laser was modified to produce wavelength tunable pulses of the order of 150 fs with a 15 nm spectral bandwidth in order to measure the dynamics of the gain using a time resolved optical pump-probe technique. The gain saturation occurs on the time scale of the pulse width of the laser and the initial recovery is extremely fast. This initial speedy recovery on a time scale of 100 fs is followed by a slower recovery with a time constant of 3 ps followed by a much slower recovery of 500 ps.