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Zwei-Photonen-Photoemission an massenselektierten Silber-Clustern aufBusolt, Ulrike, Bochum, Deutschland 08 September 2000 (has links)
No description available.
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Coherent anti-Stokes Raman scattering (CARS) optimized by exploiting optical interferenceWang, Xi 2011 May 1900 (has links)
The purpose of this work is to study the interference between the coherent nonresonant
four-wave-mixing (FWM) background and the Raman-resonant signal in the
coherent anti-Stokes Raman spectroscopy (CARS). The nonresonant background is
usually considered as a detriment to CARS. We prove that the background can be
exploited in a controllable way, through the heterodyne detection due to the interference,
to amplify the signal and optimize the spectral shape of the detected Raman
signal, and hence enhance the measurement sensitivity.
Our work is based on an optimized CARS technique which combines instantaneous
coherent excitation of multiple characteristic molecular vibrations with subsequent
probing of these vibrations by an optimally shaped, time-delayed, narrowband
laser pulse. This pulse configuration mitigates the nonresonant background while
maximizing the resonant signal, and allows rapid and highly specific detection even
in the presence of multiple scattering.
We investigate the possibility of applying this CARS technique to non-invasive
monitoring of blood glucose levels. Under certain conditions we find that the measured
signal is linearly proportional to the glucose concentration due to optical interference
with the residual background light instead of a quadratic dependence, which allows
reliable detection of spectral signatures down to medically-relevant glucose levels.
With the goal of making the fullest use of the background, we study the interference between an external local oscillator (nonresonant FWM field) and the CARS
signal field by controlling their relative phase and amplitude. Our experiment shows
that this control allows direct observation of the real and imaginary components of
the third-order nonlinear susceptibility (χ(3)) of the Raman sample. In addition, this
method can be used to amplify the signal significantly.
Furthermore, we develop an approach by femtosecond laser pulse shaping to
precisely control the interference between the Raman-resonant signal and its intrinsic
nonresonant background generated within the same sample volume. This technique
is similar to the heterodyne detection with the coherent background playing the role
of the local oscillator field. By making fine adjustments to the probe field shape, we
vary the relative phase between the resonant signal and the nonresonant background,
and observe the varying spectral interference pattern. These controlled variations of
the measured pattern reveal the phase information within the Raman spectrum, akin
to holographic detection revealing the phase structure of a source.
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Carrier dynamics in semiconductor quantum dotsSiegert, Jörg January 2006 (has links)
This thesis presents results of time-resolved photoluminescence experiments conducted on several different self-assembled InGaAs/GaAs and InAs/GaAs semiconductor quantum dot (QD) structures. Depending on the application in mind, different structural, electronic or optical properties have a different weight of importance. Fast carrier capture and relaxation is critical for QD based lasers, for example. In this thesis, the influence of surplus carriers, introduced through modulation-doping, is studied. It is shown that carrier capture is essentially unaffected whereas the intradot relaxation mechanisms, at least at low carrier concentrations, are fundamentally different. The phonon mediated cascade relaxation found in the undoped reference sample is replaced by efficient scattering with the built-in carriers in the case of the doped structures. Moreover, spin relaxation also depends on presence of extra carriers. During energy relaxation via carrier-carrier scattering, the spin polarization is preserved whereas in the undoped sample the strong interaction of relaxing carriers with LO phonons causes spin relaxation. The decay of the ground state spin polarization proceeds at the same rate for doped and undoped structures and is shown to be caused by acoustic phonons, even up to 300 K. While optimizing QD growth for specific applications, it is imperative to evaluate the influence of nonradiative recombination, which is most often detrimental. While misfit dislocations, deliberately introduced in the substrate, lead to the formation of laterally ordered, uniform dots, these samples are found to suffer from strong nonradiative recombination. Structures with different barrier thicknesses and numerical simulations indicate defects in the vicinity of the QDs as main origin of fast carrier trapping. On the other hand, it is shown that direct dot doping, compared to barrier doping or undoped structures, causes only minor degradation of the optical properties. Directly doped dots even exhibit a significantly weaker photoluminescence quenching with temperature, making them prospective for devices operating at room temperature. Finally, the superior proton radiation hardness of QD structures compared to quantum wells is demonstrated, which is due to the three-dimensional confinement. The increase of photoluminescence intensity at low to moderate doses is interpreted as an enhanced carrier transfer into the dots via the defects introduced into the material by the protons. / QC 20100920
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The Study of Carrier Cooling in InN Thin FilmTseng, Yao-Gong 02 September 2011 (has links)
The thesis investigates hot carrier relaxation and carrier recombination
mechanism of a InN thin film grown on LAO(LiAlO2) substrate with a ultrafast
time-resolved photoluminescence apparatus. Carriers were excited with laser pulses of energy 1.5 eV and of pulsewidth 150 fs from a Ti:sapphire laser. The photoexcited carriers relax excessive energy mostly within 10 ps thorough carrier-LO-phonon interaction. The effective carrier-LO-phonon emission times were estimated 197 to 58 fs in the temperature range from 250 to 35 K. The Shockley-Read-Hall coefficient was found around 0.8 ns-1. The Auger recombination was trivial at 35 K and become significant at 250 K. The fitted radiative recombination was much smaller than the theoretical estimate. Both effective carrier-LO-phonon scattering times and the radiative and nonradiative decay rates of the studied m-plane InN were found to be smaller than those of c-plane InN in other reports.
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The Ultrafast Time-resolved Photoluminescence study of ZnTe/ZnSe Quantum DotsYeh, Ying-Chou 14 July 2004 (has links)
The carrier capture and relaxation of Type II ZnTe/ZnSe
quantum dots(QDs) were investigated with ultrafast photoluminescence upconversion. We found that carrier relaxation of QDs under Volmer-Weber(VW) growth mode exhibits faster decay and rise than that of QDs under Stranski-Krastanow(SK) growth mode due to the wetting layer in SK growth mode provides as a pathway for carriers to diffuse and migrate from large(small) to small (larger) QDs. The wetting layer level was found by analyze the decay time of PL with different wavelength and temperature. The PL of VW mode and SK mode by using 532nm Nd-YAG laser also prove the existence of wetting layer. We interpret our results of VW mode in terms of Auger process with large carrier density.
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The Time-Resolved Photoluminescence Study of InN Film and InAs/GaAs QDsWu, Chieh-lung 29 July 2004 (has links)
Abstract
We have extended the spectral range of the current PL-upconversion apparatus to be operated in infrared. Using the IRPL-upconversion¡Awe study the behavior of carrier cooling of InN film and the relationship between the spacer and lifetime in InAs/GaAs stacked QDs .
We excited InN film of the band gap of 0.74eV with ultrafast Ti:sapphire laser of the wavelength 404nm. We found the phonon emission time by hot carriers of InN is 14fs. The hot carriers release their excess energy to the lattice of 35K with a timescale of 100ps. We observed in InAs/GaAs QDs that the shorter life time for samples with thin spacer is due to tunneling effect.
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The Study of Carrier Relaxation in Multi-Stacked InAs/GaAs Quantum DotsLu, Shu-kai 11 August 2006 (has links)
Carrier dynamics of mullti-stacked quantum dots (MSQDs) have been studied by means of time-integrated and time-resolved photoluminescence (PL). The MSQD with different spacer thickness of 10, 15, 20 and 30 nm were grown by molecular beam epitaxy. Time-integrated PL exhibit red shift as spacer thickness increases. The red shift originated from the vertical coupling relaxes the strain in the MSQDs, leading to a decrease in the PL peak energy. From time-resolved PL, the MSQD with spacer thickness increased reveals the shorter lifetime of PL peak among samples studies. We attribute the maximum of lifetime to a better vertical alignment. We report on a measurement of the rise and decay of luminescence intensity in the MSQDs excited at 1.54 eV (808 nm) and 3.09 eV (404 nm). The results show a slow rise time of electrons from the L to the £F valley for high photoexcitation energies. The decay in luminescence is longest with photoexcitation at 3.09 eV, we demonstrate the importance of the penetration depth and carriers tunneling. In addition, the MSQDs strongly depends of on the carrier injection. The rise times decrease with increasing excitation density. The properties are characteristic features of Auger processes.
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Detection of bacterial endospores by means of ultrafast coherent raman spectroscopyPestov, Dmitry Sergeyevich 10 October 2008 (has links)
This work is devoted to formulation and development of a laser spectroscopic technique
for rapid detection of biohazards, such as Bacillus anthracis spores. Coherent anti-Stokes
Raman scattering (CARS) is used as an underlying process for active retrieval of
species-specific characteristics of an analyte. Vibrational modes of constituent molecules
are Raman-excited by a pair of ultrashort, femtosecond laser pulses, and then probed
through inelastic scattering of a third, time-delayed laser field.
We first employ the already known time-resolved CARS technique. We apply it
to the spectroscopy of easy-to-handle methanol-water mixtures, and then continue
building our expertise on solutions of dipicolinic acid (DPA) and its salts, which happen
to be marker molecules for bacterial spores. Various acquisition schemes are evaluated,
and the preference is given to multi-channel frequency-resolved detection, when the
whole CARS spectrum is recorded as a function of the probe pulse delay. We
demonstrate a simple detection algorithm that manages to differentiate DPA solution
from common interferents. We investigate experimentally the advantages and
disadvantages of near-resonant probing of the excited molecular coherence, and finally
observe the indicative backscattered CARS signal from DPA and NaDPA powders. The possibility of selective Raman excitation via pulse shaping of the preparation pulses is
also demonstrated.
The analysis of time-resolved CARS experiments on powders and B. subtilis
spores, a harmless surrogate for B. anthracis, facilitates the formulation of a new
approach, where we take full advantage of the multi-channel frequency-resolved
acquisition and spectrally discriminate the Raman-resonant CARS signal from the
background due to other instantaneous four-wave mixing (FWM) processes. Using
narrowband probing, we decrease the magnitude of the nonresonant FWM, which is
further suppressed by the timing of the laser pulses. The devised technique, referred to as
hybrid CARS, leads to a single-shot detection of as few as 104 bacterial spores, bringing
CARS spectroscopy to the forefront of potential candidates for real-time biohazard
detection. It also gives promise to many other applications of CARS, hindered so far by
the presence of the overwhelming nonresonant FWM background, mentioned above.
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A comparison between time-resolved electroluminescence mapping and time-resolved optical beam induced current mapping in large area LEDsWeng, Chin-shu 17 July 2008 (has links)
The major purpose of LED is the electroluminescence. We use the time-resolved electroluminescence (TR-EL) method to measure the response time of LED in our experiments. In addition, typical diode has optical beam induced current (OBIC) characteristic in its depletion region. Combining upon physical reaction we can compare TR-EL and OBIC in the same LED. We are using the high frequency function generator, pulsed laser with high repetition rate, laser scanning confocal microscopy and a high frequency phase sensitive lock-in loop to achieve temporal resolution. The response time of LED can be measured in two different physical characteristic.
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Time-resolved optical beam induced current mapping in InGaN LEDLin, Yu-fong 17 July 2008 (has links)
We have implemented the time-resolved technique at frequency domain on a laser scanning microscope to investigate light emitting diodes. Leds are not high-speed device, so we use e-o modulator to change its frequency of Laser and finish the experiment. In this way, temporal response of a device can be mapped at high spatial resolution. We are using a Ti : sapphire laser and a high frequency phase sensitive lock-in loop to achieve time-resolved the dynamics properties of the light emitting devices.Laser used to excite carriers in the depletion region detected form the contract signal for scanning imaging. We can observe the OBIC effect and measure the response time of light emitting devices.
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