Although an optimal complex weighting is properly calculated, system performance could not be expected when it is applied to a real loudspeaker array. It is It is because any real sources are not perfectly pulsating monopole.
Transfer function between a source and field points is measured to mod-el the manufactured loudspeaker. After eliminating measurement error and scaling the data, the modeled source is almost the same as the measured transfer function. Chemical Engineering.
In this research, a modal approach has been adopted to analyze the dynamic characteristics of the rolling tire by a ring model. Previously, most of the researchers focused on the changes in natural frequencies with respect to the Previously, most of the researchers focused on the changes in natural frequencies with respect to the rotational speed in global coordinate only.
In this research, the change in the damping has also been taken into account by including a damping term in the ring model equations. The obtained damping values and FRFs from the analytic model are then compared with experimental results. Design sensitivity analysis of structural-acoustic problems using the wave based method more. Maximization of the directivity ratio with the desired audible gain level for broadband design of near field loudspeaker arrays more.
This paper mainly addresses design methods for near field loudspeaker arrays. These methods have been studied recently since they can be used to realize a personal audio space without the use of headphones.
From a practical view point, From a practical view point, they can also be used to form a directional sound beam within a short distance from the sources especially using a linear.
Efficient and stable model reduction scheme for the numerical simulation of broadband acoustic metamaterials more. ABSTRACT This study proposes an efficient and stable model reduction scheme for the numerical simulation of broadband, inhomogeneous, and anisotropic acoustic systems. Unlike a conventional model reduction scheme, the proposed model Unlike a conventional model reduction scheme, the proposed model reduction scheme uses the adaptive quasi-static Ritz vector AQSRV as a basis vector.
The proposed AQSRV-based model reduction scheme has the following two representative features: 1 Multiple frequency subintervals and 2 Adaptive selection of the subinterval information i. This item will only be visible to you, admins, and anyone marked as a creator. Current visibility: Friends-only. This item will only be visible in searches to you, your friends, and admins.
Description Change Notes. Add to Collection. This item has been added to your Favorites. Content Types: Mod. File Size. One is the dephasing time on the order of a femtosecond to a picosecond. The other is the population lifetime on the order of a nanosecond. Thus, the observation of two linewidths on the absorption spectrum simply means the difference in timescale.
Similar to the peak slowdown factor of QWs, there is an optimal pump intensity at which the peak slowdown factor is the maximum.
The reason for the decrease of the slowdown factor at a higher pump intensity is the same as that for QWs. However, there are differences between the two cases. First, the magnitude of the peak slowdown factor differs by two orders of magnitude. The reason is that the density of active centers is small due the limited QD surface density and is not as high as that of QWs.
Also, due to the finite size of QDs and low coverage ratio of the QD active layer, the transverse confinement of the normally incident light is smaller than that of QWs. These two factors. The pump and signal are normally incident into the active region which consists of several layers of QDs.
B, 72 23 , , This absorption spectrum reflects the emergence of the spectral-hole burning of the inhomogeneous-broadened QDs as the pump intensity increases. The coherent absorption dip due to CPO takes place at the bottom of the spectral hole with a linewidth much narrower than that of the spectral hole, as indicated by the inset. These absorption dips are due to CPO and are much narrower than those of the incoherent spectral holes in a.
Second, if we use the optimal intensity as the unit to measure the intensity, the peak slowdown factor decreases much more slowly with the pump intensity in QDs with inhomogeneous broadening than its counterpart in QWs. The reason for this slow reduction is that QDs not in resonance with the pump, though less active in CPO, are difficult to saturate. However, for practical applications, it would be better to have a room-temperature slow-light device e. The clear peaks on the spectra of transmission and phase shift indicate the presence of CPO based on QDs at room temperature.
Later, the electrical control of slow light based on CPO was also demonstrated [9,11,17]. Compared with the case of low temperature, the static spectral-hole burning is usually absent at room temperature. Therefore, it is a reasonable approximation to model the occupation numbers of QDs by two Fermi factors with respective quasi Fermi levels in the conduction and valence bands, taking into account the inhomogeneous broadening.
In addition to the optical pump, the reverse biased voltage and forward injection current can alter the bias condition of the active region. However, the two electrical controls affect the slow light based on CPO in two distinct ways.
As shown in Figure 2. The effective population loss rate is determined not only by radiative recombination but also by the pullout of carriers from QDs. This exper- imental data shows that CPO is less affected by inhomogeneous broadening. From Su, H. In addition to the red shift due to the Stark effect, the effective lifetime is reduced because the applied electric field sweeps the carriers out of the QDs.
The quasi Fermi levels of the electrons and holes are separated. The effective lifetime is fairly constant in this case, but the background absorption is saturated by the injected carriers. The saturation of the background absorption limits the depth of the absorption dip and the available slowdown factor. Quantum Electron. On the other hand, in the forward bias regime, the external field does not change the band profile of QDs significantly, but the forward bias current saturates the background absorption by injecting electrons and holes into QDs.
The population lifetime is not significantly altered in this case, but the saturated background absorption clamps the depth of the absorption dip and reduces the available slowdown factor. This picture holds good only when the injected current is below the transparency current, that is, when population inversion is not reached. In both the reverse and forward bias regimes, one can use electrical control to change the slowdown factor, which provides direct control over the optical buffer.
To explain how electrical control can be made to operate, it is necessary to develop a model to describe how the reverse bias and forward injection current change the phenomenon of slow light based on CPO in QDs.
In the reverse bias regime, the applied electric field changes the energy lev- els Stark shift and effective lifetime of the QDs. Experiment 2.
Detuning GHz 6. The absorption is red shifted due to the Stark effect. The absorption spectrum also exhibits tunneling broadening as the reverse bias voltage increases. The linewidth of the absorption increases as the reverse bias voltage increases, which indicates the shortening of the effective population lifetime.
The operating wavelength is around 1. The trend of the red shift of the absorption spectrum is similar to that of the Stark shift in QWs. With the absorption spectra at different reverse bias voltages, one can determine the necessary parameters for the simple two-level system with inhomogeneous broadening.
The corresponding group index slowdown factor is shown in Figure 2. As the reverse bias voltage increases, the magnitude of the phase shift decreases due to the broadening of the linewidth inverse of the effective population lifetime.
From the experimental data, one can calculate the corresponding slowdown factor under different reverse bias voltages. The peak slowdown factor is around 6 at zero bias. The much lower slowdown factor compared with that of the geometry of normal incidence is due to the poorer confinement factor both in the transverse and propagation directions and a lower pump intensity farther away from the input facet because of the absorption.
Also, at room temperature, the dephasing time is much shorter than that at low temperature, which degrades the magnitude of CPO. For current injection in the forward bias region, one can use the rate equations and the charge neutrality condition to relate the carrier densities nc and nh in the conduction and valence bands, respectively, to the injected current density Jin.
The subscript loss means the recombination due to various recombination mechanisms while optical indicates the generation due to the presence of an optical field. At steady state, one can relate the two carrier densities to the two quasi Fermi levels and obtain the occupation numbers in the QDs. The reduction of the phase shift is due to the saturation of the background absorption which clamps the depth of the absorption dip. Although the trend is similar to that in the reverse bias regime.
One can see the difference by extracting the linewidth of the RF phase and plotting it as a function of the reverse bias voltage injection current in the reverse bias forward bias regimes, as shown in Figure 2.
The half- width-at-half-maximum HWHM linewidth of the RF phase shift is nearly a constant in the forward bias regime, indicating no variation for the population lifetime, while it increases as the reverse bias voltage becomes higher, reflecting the shortening of the effective population lifetime.
The above experimental data and theoretical calculations are for the p-doped samples. Therefore, before the reverse bias voltage or forward injection current is applied to the system, there are pre existing holes in the QDs, which already saturated the background absorption and therefore limited. The reduction of the RF phase shifts are caused by the saturation of the background absoprtion which limits the depth of the absorption dips and variation of the refractive index.
The linewidth stays constant in the forward bias regime, indicating a constant effective population lifetime. On the other hand, the linewidth increases with reverse bias due to the shortening of the effective lifetime. If an intrinsic sample is used, the background absorption will increase and can sustain a larger absorption dip and higher slowdown factor. However, there is a trade off. Because of the increased background absorption, the signal and pump will be attenuated more when propagating in the QD sample.
Thus, even though the larger background may increase the available slowdown factor, the required pump intensity is actually higher. Also, a larger attenuation results in a smaller signal-to-noise ratio.
Due to a shorter population lifetime possibly more efficient spontaneous radiative recombination compared with the p-doped sample, the available maximum slowdown factor is only slightly better than that of the p-doped sample. The intrinsic QD sample can offer a larger background absorption and may lead to a larger slowdown factor.
Compared with the experimental data in Figures 2. From Kondratko, P. In this section, we have considered CPO at room temperature. The foregoing description is still limited to the absorption regime. From the viewpoint of system applications, the attenuated signal may have unacceptably low signal-to-noise ratio. In the gain regime, although the amplified spontaneous emission is another factor limiting the signal-to-noise ratio, the restriction is less serious than that in the absorptive regime.
The physics of fast light in the gain regime due to CPO is similar to that of slow light in the absorption regime [15,21]. The gain medium can be bulk, QWs or QDs as long as the background gain is high enough to sustain the necessary variation for fast light.
However, there are differences between systems with distinct dimensions that can cause features in the phenomena of fast light. For example, the bulk and QW gain media usually have a significant linewidth enhancement factor while the QD gain medium has a much smaller one. As a result, while the small-signal absorption due to CPO in the QD gain medium is symmetric with respect to signal—pump detuning [11], the small-signal gain in bulk or QW gain medium has a prominent asymmetry due to the finite linewidth enhancement factor [21], as shown in Figure 2.
The corresponding variation in the refractive index is shown in b. Thus, if the carrier frequency of the signal is allowed to operate on the negative side of the detuning, where the slope is positive, it is possible to have a slow-light device in the gain medium, where even the small signal gain is high.
On the other hand, if a fast-light device is desired, one simply operates at the asymmetric dip on the gain spectrum. Nevertheless, for both the slow-light and fast-light operation in the gain regime, CPO is indispensable from FWM, and the generated FWM signal usually cannot be separated from the signal at the output of the device.
This may be one of the reasons for significant distortion of the input pulse [14]. As mentioned earlier, when the bias condition is changed, some effects originally neglected in the absorption regime can no longer be neglected. For example, below the transparency current, the effective population time is fairly constant in the forward bias regime.
However, after significant n w s g w s. The finite linewidth enhancement leads to an asymmetric gain dip. The gain at the negative signal—pump detuning is enhanced. Corresponding to the enhanced gain at the negative detuning, there is an enhanced positive slope on the spectrum of the refractive index. This part of the refractive index can be utilized in the demonstration of the slow light.
On the other hand, the negative-slope feature corresponding to the gain dip in a can be utilized in the demonstration of the fast light. Due to the significant shortening of the effective population lifetime resulting from the washout of the population grating in the wetting layer, the phase shift is much smaller than that in the absorption regime.
If the pump—probe scheme of counter propagation is used, the diffusion process in the wetting layer washes out the dynamic population grating that has resulted from CPO and the standing- wave pattern due to the pump and probe. The diffusion significantly decreases the effective lifetime to tens of picoseconds [11]. The RF phase shift due to the change of the background refractive index is even more significant than that of CPO. Although the shorter effective lifetime can increase the bandwidth of fast light, the fast-light effect is also significantly reduced.
The fast light or slow light due to CPO by itself may not be significant enough in real applications. As mentioned earlier, if the copropagation scheme is used for the pump—probe experiment, the required momentum conservation for FWM is easily satisfied, and the FWM component is amplified during the propagation through the gain medium.
In this case, the effects of CPO and FWM cannot be individually identified because the two processes couple to each other during the wave propagation. The generation of linear and FWM responses and the conversion between them are not independent of each other. It is this coupling that results in a significant RF phase shift and time advance delay in the copropagation scheme [12—14,22]. If the signal is of the form of sinusoidal modulation, analytical expressions of phase advance and small- signal gain are available [22].
The solid line is the theoretical calculation and agrees well with the experimental data. A larger injection current provides more carriers to the active region and leads to higher gain for both the signal and pump. At the frequency of several gigahertz, a phase advance of a few tens of degrees is induced, which is much larger than that of CPO in the absorption regime.
Similar to the increase of the injected current, if the pump intensity becomes higher, the phase shift as a function of the RF modulation frequency also increases, as shown in Figure 2. The above experimental data are the measured frequency response of the signal. This verifies that a true time advance of a few tens of degrees is achieved. The significant increase of the RF phase shift is caused by the coupling between CPO and FWM during the nonlinear propagation of the signal through the device.
This experiment verifies that fast light is achieved in real time. Express, 14 11 , , For a real application, the input signal is usually not a continuous wave CW carrier. Thus, it is necessary to know how a real pulse is advanced delayed when it propagates through a device.
The experiment in Ref. Compared with the output with very large detuning which experiences little effect from slow or fast light, the output with the negative detuning is delayed by 0. However, the output signal corresponding to fast light is significantly distorted with a dip at the trailing edge of the pulse [14]. This mechanism resembles CPO in a few ways, e.
However, instead of the population lifetime, the timescale which is utilized will be the spin coherence time. For CPO, the long timescale is the population lifetime T1. In semiconduc- tors, there is another long timescale which can be used in a slow-light experiment.
Usually, the hole spin coherence time is much shorter than the electron spin coherence time. If there is any spin coherence in semiconductors, most of it should be contributed by electron spin coherence.
For [] QW, the electron spin coherence time can be quite long nanosecond range in some cases, e. However, in general, the electron spin coherence time in [] QWs is still in the range of a few tens of picoseconds and is not long enough to demonstrate a significant slow-light phenomenon.
If this mechanism can be efficiently suppressed, the electron spin coherence time may become long. The suppression of the DP mechanism can be done in [] QWs [38] and has been experimentally verified [43]. If we can find a suitable pump—probe scheme, this timescale may be utilized to demonstrate slow light in [] QWs. A particular pump—probe scheme which can sense the electron spin coherence is the double-V EIT.
This pump—probe scheme utilizes the light-hole-like LH-like excitons whose transitions due to both TM-polarized and TE-polarized lights are allowed [31,32]. The pump is a TE-polarized light while the signal is a TM-polarized light. The preexisting band mixing changes the selection rule of the TE-polarized light. Rather, they are individually induced by two elliptically polarized lights. Since the TM-polarized signal has to be used, it is incident into the device via a waveguide geometry.
On the other hand, the pump can be incident into the device by either the normal-incidence or waveguide geometry. The TE-polarized pump induces the cross transitions in the figure and the TM-polarized signal induces the vertical transitions.
Due to the band mixing of [] QWs, the two transitions due to the TE-polarized pump are induced independently by two elliptically polarized lights. The spin coherence in the conduction band plays a major role in inducing the coherent dip in the absorption spectrum. Since the signal is TM-polarized, it must be incident into the device via a waveguide geometry. On the other hand, the TE-polarized pump can be either incident into the device via the waveguide geometry or normal-incidence geometry.
B, 24 4 , , This precession is optically induced and its strength is proportional to the magnitude of the small signal and thus much smaller than that in the presence of an external magnetic field. Therefore, we may also think of this pump—probe scheme as one without spin precession [33]. However, the spin coherence time of the hole is much shorter than that of the electron, and the contribution is usually much smaller.
Most of the concepts applied to CPO are applicable to this scheme based on spin coherence. The tiny spin precession also generates linear and FWM responses to the signal.
Analogous to the population lifetime, the spin coherence time is the timescale within which the spin precession can follow the beating due to pump and the signal. Thus, a coherent dip with a linewidth on the order of about the inverse of the spin coherence time is present on the absorption spectrum.
Corresponding to this absorption dip, there is a sharp and positively sloped variation within the same frequency range on the spectrum of the refractive index. This positive slope can then be used to slowdown the optical wave packet. One can thus expect that most of the trends of the calculated physical quantities based on spin coherence are similar to those based on CPO. Indeed, this is the case. Interested readers can find more details in Ref. Thus, compared with the CPO case, the signal experiences a higher slowdown factor, but also higher intrinsic absorption.
The higher absorption for the signal is a drawback of slow light based on double-V EIT. One can avoid the high absorption of the signal by switching the polarizations of the pump and signal. Although the signal may suffer lower absorption in this case, the slowdown factor is also reduced, and it is the pump that experiences higher absorption. There is another drawback of the pump—probe scheme based on the LH-like exciton.
Usually, the LH-like exciton lies in the HH-like continuum. The TE-polarized pump may suffer unwanted dissipation from the HH continuum states. One way to avoid this is to use tensile strain to lift the LH-like band to the top of the HH-like band [32].
With tensile strain, the LH-like excitonic absorption will have the lowest transition energy, and the unwanted dissipation from the HH continuum can be avoided. The tensile strain can be induced by using strained material or by applying uniaxial stress along the growth direction. The uniaxial stress can induce biaxial strain, which lifts the LH-like band to the top of the HH-like band.
This energy shift leads to a smaller photon energy for the LH-like exciton and avoids the unwanted dissipation of the TE-polarized pump. The sharp transmission peak indicates that double-V EIT is a possible candidate for the demonstration of slow light in semiconductors.
From Sarkar, S. B, 72 3 , , We can see that with a large enough stress, the highest valence band becomes LH-like. This can help reduce the extra absorption experienced by the TE-polarized pump. The experiment is carried out at 50 K. We see that a sharp peak with a linewidth of about 1 GHz is present on the transmission spectrum. The spin coherence time of these [] QWs is actually long enough for some real applications for which a bandwidth below the gigahertz range is required.
There is no strain in these QWs, therefore, the extra absorption from the HH continuum can attenuate the pump and degrade the performance of the slow-light device.
The operating temperature can also be increased to room temperature if a higher pump intensity is used. This immunity has been verified by the slow-light experiment of QDs at room temperature. However, the low surface coverage, the low confinement factor, and inefficient usage of the QDs due to inhomogeneous broadening significantly lowers the slowdown factor.
The implementation of slow light in semiconductors also makes electrical control of the optical buffer possible. One can use both reverse bias voltages and forward injection currents to electrically control the slowdown factor. Nevertheless, slow light based on the pure CPO mechanism in the absorption regime suffers from high absorption which greatly reduces the signal-to-noise ratio. To solve this problem, CPO and FWM in the gain regime are utilized so that the signal is amplified rather than attenuated during the propagation through the whole device.
However, the nonlinear propagation process based on CPO and FWM makes the generated linear and nonlinear optical components inseparable. This inseparability is part of the reasons for the significant distortion of the optical wave packet.
Analogous to CPO, spin coherence based on the LH exciton can also be used as the mechanism to demonstrate slow light. In this case, it is the tiny spin precession rather than population beating that is utilized. High intrinsic absorption puts a restriction on the length of the device. Also, the LH-like exciton lies in the HH-like continuum and brings in unwanted dissipation.
This difficulty, however, can be overcome by building tensile strain into the QW system to lower the transition energy of the LH-like exciton away from the HH-like continuum.
Sedgwick, P. Palinginis, Tao Li, S. Crankshaw, Hui Su, P. Kondratko, M. Akira, D. Nielsen, and B. Pesala, whose efforts in the research of semiconductor slow and fast light contribute to the main content of this chapter. Ku, F. Sedgwick, C. Chang-Hasnain, P. Palinginis, T. Li, H. Wang, S. Chang, and S. Slow light in semiconductor quantum wells. Chang, S. Chuang, P. Ku, C. Chang-Hasnian, P.
Palinginis, and H. Slow light using excitonic population oscillation. B, 70 23 , Palinginis, S. Crankshaw, F. Sedgwick, E. Kim, M.
Moewe, C. Chang-Hasnain, H. Wang, and S. Bigelow, N. Lepeshkin, and R. Superluminal and slow light propagation in a room- temperature solid.
Science, —, Observation of ultraslow light propagation in a ruby crystal at room temperature. Palinginis, F. Sedgwick, S. Crankshaw, M. Moewe, and C.
Room temperature slow light in a quantum-well waveguide via coherent population oscillation. Express, 13 24 —, Mork, R. Kjaer, M. Slow light in a semiconductor waveguide at gigahertz frequencies. Express, 13 20 —, Su and S. Room-temperature slow light with semiconductor quantum-dot devices. Kondratko, S. Chang, H. Su, and S. Optical and electrical control of slow light in p-doped and intrinsic quantum-dot electroabsorbers. Gotoh, S. Chuang, H. Okamoto, and Y. Tunable slow light of 1.
Room temperature slow and fast light in quantum-dot semiconductor optical amplifiers. Kondratko and S. Slow-to-fast light using absorption to gain switching in quantum-well semiconductor optical amplifier.
Express, 15 16 —, You can set it down and unpack it anywhere. To craft it, you need a Science Machine, 3 papyrus, and 1 silk. You can find it under Tools. This feature is disabled by default. We use a broad set of rules for what is and isn't allowed to be packaged.
It could result in unexpected behavior. Popular Discussions View All Nei Mongol 30 Oct pm. AGrey 25 Oct am. Joseph Joseph 17 Oct pm. JoyLau 21 Sep am.
0コメント