Ultimate-fast all-optical switching of a microcavity
Başlık çevirisi mevcut değil.
- Tez No: 401566
- Danışmanlar: PROF. DR. WILLEM VOS, DR. GEORGIOS CTISTIS
- Tez Türü: Doktora
- Konular: Biyoteknoloji, Fizik ve Fizik Mühendisliği, Biotechnology, Physics and Physics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2013
- Dil: İngilizce
- Üniversite: University of Twente
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 124
Özet
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Özet (Çeviri)
In this thesis we have studied the ultrafast all-optical switching of microcavities. We have employed the electronic Kerr effect to switch a planar semiconductor microcavity resonance in the fastest possible way at telecom wavelengths. We have used the free-carrier dispersion effect to switch distinct transversal resonances of 3D micropillar cavities. To do so, we have first studied the unswitched linear response of our planar cavities with a range of quality factors and micropillar cavities with different dimensions. We have built and described our ultrafast switch microscope which is used to study our cavities. First of all we have investigated how fast a cavity resonance can be switched reversibly. We have observed the fastest possible switching of a microcavity resonance within 300 fs at telecom wavelengths using the electronic Kerr effect. The rate of the switching is only determined by the cavity storage time; as popularly said it is only limited by the speed of light [1] and not by material related relaxation properties. We identify the cavity resonance shift from the measured transient reectivity. Since the measured transient reectivity includes the slow response time of the photodiodes, we are not able to measure the realtime dynamic cavity resonance frequency shift. It would be a great improvement to use a streak camera that can resolve the fast dynamics of the cavity resonance in real-time. The effect of the temporal overlap of the pump and the probe pulses in Kerr switching experiments has been investigated. To do so, we have studied cavities with different quality factors that controls the duration of the probe pulse. We also have investigated the effect of the pump pulse duration in switching of cavity resonance with the electronic Kerr effect. Our experiments and our calculations showed that the refractive index change induced by the electronic Kerr effect is increased at the maximum temporal overlap of the pump and the probe pulses. For this reasons, fast cavities that provide an increased temporal overlap with ultra-short pump pulses are used in this work for sub-picosecond switching of cavities. Moreover, we discovered that due to the larger field enhancements in high-Q cavities the electronic Kerr effect will be hindered by the excited free carriers. We have achieved repeated switching of microcavities within 700 fs, corresponding to THz rates. Therefore to best of our knowledge, we have broken the THz clock rate barrier experimentally using semiconductor cavities for the rst time. The THz clock rate that we achieve can be a solution for information technology that is exploring ways to meet the growth rate of the computational demand. For practical applications our method requires lasers with higher repetition rates. The recent developments in compound ultrafast laser systems [2{4] promise high repetition rates that are beneficial for repeated switching on picosecond time scales. We derived and measured the non-degenerate three-photon absorption coeffi- cient of GaAs. Our results revealed that the refractive index change induced by the electronic Kerr efgfect can be increased to an extent that is limited by the increasing density of excited free carriers. Our experiments on cavities enabled us to derive the nondegenerate third-order susceptibility ((3)) of GaAs. We successfully developed a model that quantitatively describes the frequency and the intensity dependence of nondegenerate switching with pump-probe experiment. We have found the set of parameters using which the instantaneous electronic Kerr effect can be utilized as the ultimate-fast way of all-optical switching by understanding the competition between the electronic Kerr effect and the free carriers in a switched microcavity. Using the measured nondegenerate two- and three-photon absorption coeffi- cients we estimate that the trigger pulse only lose 22 aJ=cm2 at each switch event, which is a record level of minimum energy loss per switch event. Unlike the free carrier excitation schemes, we manage to avoid absorption and demonstrated recycling of trigger photons to switch the cavity again and again. Since the energy loss during the switching of the cavity is low, a recycling scheme with minimized loss during the recycling process can be developed to demonstrate that switching with the electronic Kerr effect is an efficient way to save energy. To switch a cavity resonance at sub-picosecond time scales we first chose the electronic Kerr effect as the switching mechanism due to its instantaneous response nature. Second, we chose a fast cavity with short storage time since that sets the fundamental speed limit. The cavity we chose has a storage time of cav = 300 fs, a linewidth of ! = 20 cm1, and resonates at !res = 7812 cm1. After that we have determined the material of the cavity so that it will provide a large third order susceptibility , transparent in the telecom wavelengths, and extensively used in nanofabrication facilities. This narrows down the material choice to Si and GaAs. Since excited free carriers in a switched microcavity counteract the electronic Kerr effect, the energy of the probe and pump photons together should stay below the bandgap of the semiconductor so that free carrier excitation is suppressed. Silicon has a bandgap energy of Egap = 1:1 eV. In order to achieve switching of a Si cavity, operating at the original telecom wavelength, with the electronic Kerr effect the pump energy must be Epu 6 0:14 eV. The required pump frequency for Si is outside the range of our equipment. On the other hand, the required pump frequency to stay below the bandgap energy of GaAs together with the pump and the probe is accessible with our equipment. For this reason, we have explored switching of a GaAs cavity with the electronic Kerr effect. The probe frequency is set by the cavity resonance in the telecom range while the pump frequency is centered at !pu = 4165 cm1 (pu = 2400nm). We achieved to shift the cavity resonance by ! = 7 cm1. Our calculations indicate that if the pump pulse is stretched to pu = 740 fs the resonance frequency can be increased from ! = 7 cm1 to ! = 19 cm1 so that we can achieve to shift the resonance frequency by nearly one linewidth within 300 fs. At this point we propose to use a cavity designed to resonate with both pump and probe pulses so that the temporal and spatial overlap of the pump and probe pulses can be increased. Accordingly, the refractive index change induced by the electronic Kerr effect can be increased and switching at lower pump uences can be performed since the pump light will be also resonantly enhanced. Remarkably, we showed that during the switching of cavities we achieved both up and down frequency conversion of light. The frequency conversion that we have observed is a result of the instantaneous refractive index change achieved using the electronic Kerr effect. We achieved repeated frequency conversion at THz rates and generate blue- and red-shifted light pulses at this rate. Our analytical model predicted the dynamics in quantitative agreement with our experiments. Using the electronic Kerr effect we achieved the largest rate of change of the refractive index. In order to enter the non-adiabatic frequency conversion regime with the electronic Kerr effect, a larger resonance frequency shift of the cavity resonance is required. The shift of the cavity resonance frequency achieved via the electronic Kerr effect can be increased with double resonant cavities, and by increasing the temporal overlap of pump and the probe pulses. All-optical switching of semiconductor micropillar cavities with different diameters via the excitation of free carriers has been performed during this thesis. For the first time we have shown that the distinct transverse cavity resonances of a micropillar can be switched independently. We observe that the magnitude of the frequency shift diers for all the individual micropillars resonances. Moreover, the recovery time of the excited free carriers also differs for each cavity resonance. We have managed to change the frequency of light trapped in a micropillar cavity and observed both blue- and red-shifted light during the switch process. The micropillar cavities that we use in our experiments, host embedded quantum dots in the of layer. The micropillars cavities provide an exclusive connement and guiding for emitted light from the light sources placed inside. For this reason, they offer an excellent opportunity to study \Zap", that is, switching of local density of states and the emission rate at time scales shorter than the excited state lifetime [5]. A spectacular practice would be to study Zap with a single quantum dot placed inside a micropillar. In that case, stochastic emission of single photons can be burst into a shorter time window, therefore, deterministic control on photon emission time can be achieved. The distortion of light pulses after passing through a medium can be eliminated at a later time by time reversing the light pulses. For this reason, time reversing light pulses is an elusive wave phenomena. It has been predicted by Sivan et al. that time reversal of light pulses can be realized by the dynamic control of photonic crystals. In order to achieve time reversal, it is suggested by Sivan et al. that the dynamic control should be much faster than the pulse duration or equivalent and the spectral content of the modulation should be much wider than that of the pulse [6]. The ultrafast switching times within the pump pulse duration offered by the electronic Kerr effect can therefore be a starting point to observe time reversal of light pulses.The zero-point energy stored in the modes of an electromagnetic cavity gives rise to an attractive interaction between the opposite walls, the so-called static Casimir effect. A dynamic version of the Casimir effect is shown to occur when the index of refraction of the cavity is changed. As a result, vacuum uctuations are converted to real photons at frequencies symmetrical with respect to half of the modulation frequency [7]. The fast switching rates that we achieve here are a favourable starting point for exploring dynamic Casimir effects at frequencies that have not been accessible before.
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