Excitonic lasing in semiconductor quantum wires

Excitonic lasing in semiconductor quantum wires

L. Sirigu, D. Y. Oberli, L. Degiorgi, A. Rudra, and E. Kapon Department of Physics, Swiss Federal Institute of Technology-EPFL, CH-1015 Lausanne,

Switzerland

Direct experimental evidences for excitonic lasing is obtained in optically pumped V-groove quantum wire structures. We demonstrate that laser emission at a temperature of 10 K arises from a population inversion of localized excitons within the inhomogenously-broadened luminescence line. At the lasing threshold, we estimate a maximum exciton density of about 1.8 105cm-1.

Semiconductor lasers incorporating low-dimensional heterostructures, quantum wires and quantum boxes, are attracting considerable interest because of their potential for improved performance over quantum well lasers1. This prediction is based, in the single-particle picture, on the sharper density of states resulting from the confinement of the charge carriers in two or three directions. The inclusion of electron-hole Coulomb correlations in the theoretical description of the optical spectra of semiconductor quantum wires has, however, dampened these early expectations. The primary effect of Coulomb correlations is to remove the singularity of the one-dimensional (1D) joint density of states and to greatly reduce the absorption above the band edge2. In the regime of high carrier densities screening of the Coulomb interaction, band gap renormalization and phase space filling might become important3. The evolution of the optical response as the density of electron-hole pairs is increased has been intensively studied in 1D systems in recent years. However, contrasting theoretical predictions were made: in several studies4-5, gain was found to appear only at densities above the Mott density (about mid 105 cm-1) while, in another one, substantial gain was calculated at densities largely below this value when a self-consistent treatment of the electron-hole correlations was included6. The occurrence of excitonic gain that is predicted by the latter study is an interesting feature of 1D quantum structures. Laser emission from excitons confined in quantum wells has so far not been observed in III-V semiconductor heterostructures, but has been observed in their II-VI counterparts for which the exciton binding energy is much larger7. Evidence for lasing attributed to excitons has been previously reported in T-shaped quantum wire structures (QWR’s), for which only one electron 1D-subband was present8.

In the present study, we report on the observation of lasing from excitons in optically excited V-groove GaAs/AlGaAs QWR laser structures. The emission is attributed to the recombination of excitons associated with the lowest energy electron- and hole- subbands of the QWR. Moreover, we find that the emission energy remains nearly constant within the inhomogenously broadened photoluminescence line of the QWRs for both continuous wave (cw) and pulsed optical excitation over a wide range of power densities. These results corroborate the important role played by electron-hole Coulomb correlations in the optical emission from quasi-1D quantum wires in the density regime of the Mott transition.

The sample used for our study was grown by low-pressure organometallic chemical vapor deposition on a GaAs substrate patterned with a 3 μm-pitch grating. It is a semiconductor laser

structure that incorporates five vertically stacked GaAs quantum wires separated by 47 nm thick

Al

0.25Ga

0.75

As barriers placed in the core region of an optical wave guide. The core region has a

total thickness of 370 nm and is cladded with 1 μm thick Al

0.62Ga

0.38

As layers on both sides. The

center of the QWRs-stack was displaced by 72 nm from the center of the core region in order to obtain an optimal mode confinement factor9. The position of the QWRs-stack can be seen in the cross-sectional transmission electron micrograph of the core region of our QWR laser structure depicted in Fig.1. The V-shape of the dielectric waveguide yields an optical mode in the shape of a heart that is well confined in the core region of the waveguide according to our calculation of the electromagnetic field distribution of the optical modes9.

Cleaved optical cavities were mounted in a helium-flow cryostat and kept at 10 K. The laser cavities were optically pumped using 3 ps pulses from a mode-locked Ti-Sapphire laser operating at a wavelength of 720 nm (1.72eV) with a repetition rate of 80 MHz. The excitation light was incident on the growth surface and focused through two cylindrical lenses onto a stripe 46 μm wide and 1.5 mm long oriented in the direction parallel to the optical wave guide. The QWRs were perpendicular to the cleaved facets forming the mirrors of the optical cavity. The emitted light was collected from one of the cleaved facets, dispersed by a double-grating spectrometer and detected with a cooled GaAs photomultiplier tube.

Optical emission spectra of the QWR laser structure are displayed in Fig. 2 for different values of the optical power density below, at and above the threshold for lasing in the QWR. Upon increasing the pump power, we observe a nearly constant energy of the peak at 1.581 eV

that corresponds to the optical transition e

1-h

1

associated with the ground electron- and hole-

subband of the QWRs. A significant spectral narrowing is also found as the power density is increased and crosses the lasing threshold. This evidences the existence of amplified spontaneous emission within this inhomogenously broadened PL line in this density regime. A typical light output versus input power characteristic is shown in Fig. 2 b for a cavity length of about 0.9 mm. The emission intensity varies linearly at low excitation power over three orders of magnitude (from 0.1 to 100 mW). Above the lasing threshold (at 350 mW) the intensity variation is again linear, indicating that the modal gain has saturated. In Fig. 2 c, a high-resolution emission spectrum obtained above threshold features well-resolved Fabry-Perot modes that correspond to different longitudinal optical modes of the cavity within the inhomogenuous line of the QWR-PL. It is worth noting that the optical oscillations have in this

spectrum a high contrast ratio, the value of which being mainly limited, however, by the non-uniform pumping profile of the focussed laser beam.

In order to identify the regime in which lasing takes place, we now estimate the electron-hole pair density that was directly generated in each QWR by the 3 ps optical pulse. The electron-hole pair density is given by

n t Pw T ho inho

=?

αω

()()()

1R hΓΓ

where α is the excitonic absorption coefficient, t and w are respectively the effective wire thickness and width, R (0.3) is the normal incidence reflectivity at the laser energy hω, P is the power density, T is the pulse repetition period (12.5 ns). The last term in the previous expression

is the ratio between the homogeneous (Γ

ho ) to inhomogeneous (Γ

inho

) widths of the optical

transition. Assuming an absorption coefficient, α, of 16000 cm-1 in the V-groove QWR and a power density, P, of 0.6 kW/cm2 at the lasing threshold, we estimate an excited electron-hole pair density of 1.8 105 cm-1.10-11 This density represents an upper bound of the inverted population achieved for lasing. Because this density is lower than the Mott transition density we conclude that the optical emission is dominated by excitonic recombination. We emphasize that actual theoretical estimates3,4 of the Mott density yield different values between 3 and 8 x105 cm-1 for GaAs QWR structures with the same exciton binding energy. To further confirm the excitonic nature of the emission we compare, in Fig. 3, PL spectra that are obtained under pulsed excitation over a wide range of power densities with a PL spectrum obtained under cw excitation. In this latter case the power density is very low and corresponds to about one tenth the typical power density used in the PL and PL-excitation (PLE) spectra displayed in Fig. 4. The relevance of excitonic recombination at low temperature in the PL and PLE spectra has already been established in previous studies of similar QWR structures12. From the absence of any significant shift (less than 2 meV) we infer that the lasing emission originates from the recombination of excitons as it is the case for the QWR-peak of the cw-PL spectrum.

We will now address the origin of the energy blueshift that is observed as the power density is increased up to about 100 W/cm2 (below lasing threshold). It could originate from an imperfect compensation between the renormalization of the band-gap energy introducing a red-shift (BGR) and a lowering of the exciton binding energy yielding a blue-shift as the carrier density is increased. In the density range of the present experiment, these many-body effects were evaluated resulting in an absorption peak that exhibits a weak blueshift as the carrier

density approaches the Mott density3. Alternatively, the energy blueshift may be attributed to effects related to disorder. We note that, under cw excitation condition, similar shifts were indeed observed along with the appearance of a high energy tail in the PL spectrum that is indicative of a heating of the hot carrier distribution14. These latter findings can also be related to disorder effects that broaden the peaked optical density, thereby yielding a blueshift of the emission for a heated Maxwell-Boltzmann distribution of electron-hole pairs15. Beyond the lasing threshold an energy redshift is finally observed, which is caused by a local heating of the sample.

The PL and PLE spectra, which are depicted in Fig. 4, were obtained from the same sample in a standard pseudo-backscattering configuration at a temperature of 10 K16. In this sample, the upper part of the optical wave guide was chemically etched away including the top and part of the side quantum wells in order to strongly reduce their contributions to the PL spectrum. The energy position of the PL peak was again found at 1.582 eV. The PLE spectrum shows six prominent peaks corresponding to optical transitions between electron and hole 1D-subbands of the same index. The optical spectra are characterized by a full width at half maximum of 9 meV (lowest optical transition) and a Stokes shift of 6 meV. The comparison of the PL and PLE spectra demonstrates unambiguously that the lasing emission at 1.581 eV

correspond to the e

1-h

1

transition between the respective n=1 subband of electrons and holes.

The presence of a significant Stokes shift indicates that, at the energy of the lasing peak, the excitons are localized. Localization is induced by the presence of interfacial disorder in these QWRs. Disorder can strongly affect the optical properties of the QWRs : previous studies performed on similar V-groove QWRs have shown that the radiative lifetime of localized excitons is enhanced over that of free excitons17-18 and that it is more weakly dependent on temperature than the predicted dependence for free excitons15. Thus, we infer that the observed laser emission arises from a population inversion of localized excitons on the low-energy side of the absorption peak.

Excitonic gain in an inhomogenous emission line can arise if the exciton oscillator strength is large enough: otherwise, as the carrier density is increased, significant bleaching of the excitonic resonance might occur before a large enough population inversion is realized in order to reach the gain regime. It is also worth noting that the build-up of a population inversion from localized excitons does not need to rely on an effective spectral diffusion to the relevant

localized states within the inhomogenous luminescence line. However, the relaxation into these localized sites must occur on a time scale that is shorter than the average radiative lifetime of the excitonic population. Further experimental work and theoretical development are deemed necessary to address the dynamical aspect of the gain generation.

In conclusion, we have demonstrated laser emission that is sustained by excitonic gain at a temperature of 10 K in a semiconductor quantum wire structure. Laser emission is shown to arise from the population inversion of localized excitons. Whether disorder and hence localization of the excitons is necessary to obtain excitonic gain remains to be explored as our understanding of the gain generation in a weakly-disorded semiconductor gain medium is not yet complete. This study should stimulate further a theoretical description of the effects of electron-hole Coulomb correlations in the gain regime of quasi one-dimensional semiconductor systems.

We wish to acknowledge helpful discussions on the excitonic gain in 1D systems with C. Piermarocchi. We are also grateful to K. Leifer for providing the TEM micrographs. This work was supported in part by the Swiss National Foundation for Sciences.

Figures

Figure 1: Cross-sectional dark-field micrograph of the core region of the QWR laser structure obtained by transmission electron microscopy. Brighter areas correspond to regions with a larger Al composition. The location of the five QWRs is marked by an arrow.

50 nm QWR

Al0.62Ga0.38As Al0.25Ga0.75As

Figure 2: (a) Photoluminescence spectra at 10K of the QWR laser sample above, below and near the lasing threshold in TE-polarization. (b) Dependence on input excitation power of the PL output power; arrows indicate the excitation powers used for the optical spectra depicted in (a).(c) High-resolution emission spectrum above the lasing threshold showing the Fabry-Perot modes of the optical cavity (the mode linewidth is equal to 0.37 ? and the instrumental resolution is set to 0.17 ?).

5004003002001000Pump Power (mW) 1.584

1.5821.580Photon Energy (eV)

0N o r m a l i z e d I n t e n s i t y

1.641.621.601.581.56Photon Energy (eV)

Figure 3 : Evolution of the photoluminescence spectra taken under pulsed excitation with increasing power densities and cw-PL spectrum obtained from the cleaved edge at the same position. The energy shift of the PL emission remains below the value of the Stokes shift (6meV). Spectra have been shifted upwards for clarity. Estimated exciton density under cw excitation is about 10 cm -1.

N o r m a l i z e d I n t e n s i t y 1.64

1.621.601.581.56Photon Energy (eV)

Figure 4: Linearly-polarized PLE spectrum and the corresponding PL spectrum of an etched QWR laser sample at 10K. The polarization of the excitation is parallel to the wire axis. The different optical transitions e n -h n are marked by arrows.

I n t e n s i t y 1.68

1.661.641.621.601.581.56Photon Energy (eV)

References

1 Y. Arakawa and H. Sakaki, Appl. Phys. Lett. 40, 939 (1982).

2 T. Ogawa and T. Takagahara, Phys. Rev. B 44, 8138 (1991) ; F. Rossi and E. Molinari, Phys. Rev. B 53, 16462 (1996)

3 E. H. Hwang and S. Das Sarma, Phys. Rev. B 58, R1738 (1998); C. R. Bennett, K. Güven and

B. Tanatar, Phys. Rev. B 57, 3994 (1998). D. W. Wang, and S. Das Sarma, Condensed

Matter/9905038.

4 F. Rossi and E. Molinari, Phys. Rev. Lett. 76, 3642 (1996).

5 S. Benner and H. Haug, Europhys. Lett. 16, 579 (1991); Phys. Rev. B 47, 15750 (1993).

6 F. Tassone and C. Piermarocchi, Phys. Rev. Lett. 82, 843 (1999).

7J. Ding et al., Phys. Rev. Lett. 69, 1707 (1992).

8 W. Wegscheider et al., Phys. Rev. Lett. 71, 4071 (1993).

9 D. Crisinel, M.-A. Dupertuis, and E. Kapon, Opt. Quantum Electron. 31, 797 (1999).

10 H. Ando, S. Nojima, H. Kanbe, J. Appl. Phys. 74 , 6383 (1993).

11 The value of 1.6 104 cm-1 is the calculated absorption coefficient for the lowest excitonic peak of a rectangular QWR of similar aspect ratio to the measured wire (Ref. 10). The ratio of the homogeneous to inhomogeneous linewidths of the calculated and measured absorption peaks,

= 8 meV).

respectively, is equal to 1/3 (Γ

ho

12 F. Vouilloz, D. Y. Oberli, M. -A. Dupertuis, A. Gustafsson, F. Reinhardt, and E. Kapon, Phys. Rev. Lett. 78, 1580 (1997); Phys. Rev. B 57, 12378 (1998).

13 Ch. Gréus et al., Europhys. Lett. 34, 213 (1996). See also the calculated density dependence of BGR in Ref. 3.

14 Unpublished work ; see also E. Martinet, F. Reinhardt, A. Gustafsson, G. Biasiol. and E. Kapon, Appl. Phys. Lett. 72, 701 (1998).

15 D. Y. Oberli, F. Vouilloz, R. Ambigapathy, B. Deveaud and E. Kapon, Proceedings of the sixth international meeting on Optics of Excitons in Confined Systems, Ascona, 1999, phys. stat. sol. (a) in print.

16 See Ref. 12 for more experimental details.

17 J. Bellessa et al., Phys. Rev. B 58, 9933 (1998).

18 D. Y. Oberli et al., Phys. Rev. B 59, 2910 (1999).

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Join In剑桥小学英语(改编版)入门阶段Unit 1Hello,hello第1单元嗨,嗨 and mime. 1 听,模仿 Stand up Say 'hello' Slap hands Sit down 站起来说"嗨" 拍手坐下来 Good. Let's do up Say 'hello' Slap hands Sit down 好. 我们来再做一遍.站起来说"嗨"拍手坐下来 the pictures. 2 再听一遍给图画编号. up "hello" hands down 1 站起来 2 说"嗨" 3 拍手 4 坐下来说 3. A ,what's yourname 3 一首歌嗨,你叫什么名字 Hello. , what's yourname Hello. Hello. 嗨. 嗨. 嗨, 你叫什么名字嗨,嗨. Hello, what's yourname I'm Toby. I'm Toby. Hello,hello,hello.嗨, 你叫什么名字我叫托比. 我叫托比 . 嗨,嗨,嗨. I'm Toby. I'm Toby. Hello,hello, let's go! 我是托比. 我是托比. 嗨,嗨, 我们一起! Hello. , what's yourname I'm 'm Toby. 嗨.嗨.嗨, 你叫什么名字我叫托比.我叫托比. Hello,hello,hello. I'm 'm Toby. Hello,hello,let's go! 嗨,嗨,嗨.我是托比. 我是托比. 嗨,嗨,我们一起! 4 Listen and stick 4 听和指 What's your name I'm Bob. 你叫什么名字我叫鲍勃. What's your name I'm Rita. What's your name I'm Nick. 你叫什么名字我叫丽塔. 你叫什么名字我叫尼克. What's your name I'm Lisa. 你叫什么名字我叫利萨. 5. A story-Pit'sskateboard. 5 一个故事-彼德的滑板. Pa:Hello,Pit. Pa:好,彼德. Pi:Hello,:What's this Pi:嗨,帕特.Pa:这是什么 Pi:My new :Look!Pi:Goodbye,Pat! Pi:这是我的新滑板.Pi:看!Pi:再见,帕特! Pa:Bye-bye,Pit!Pi:Help!Help!pi:Bye-bye,skateboard! Pa:再见,彼德!Pi:救命!救命!Pi:再见,滑板! Unit 16. Let's learnand act 第1单元6 我们来边学边表演.

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in，on，at的时间用法和地点用法 一、in, on, at的时间用法 ①固定短语： in the morning/afternoon/evening在早晨/下午/傍晚, at noon/night在中午/夜晚, （不强调范围，强调的话用during the night） early in the morning=in the early morning在大清早， late at night在深夜 on the weekend在周末（英式用at the weekend在周末，at weekends每逢周末） on weekdays/weekends在工作日/周末， on school days/nights在上学日/上学的当天晚上， ②不加介词 this, that, last, next, every, one, yesterday, today, tomorrow, tonight，all，most等之前一般不加介词。如， this morning 今天早晨 （on）that day在那天（that day更常用些） last week上周 next year明年 the next month第二个月（以过去为起点的第二个月，next month以现在为起点的下个月） every day每天 one morning一天早晨 yesterday afternoon昨天下午 tomorrow morning明天早晨 all day/morning/night整天/整个早晨/整晚（等于the whole day/morning/night） most of the time （在）大多数时间 ③一般规则 除了前两点特殊用法之外，其他≤一天，用on，＞一天用in，在具体时刻或在某时用at（不强调时间范围） 关于on 生日、on my ninth birthday在我九岁生日那天 节日、on Teachers’Day在教师节 （注意：节日里有表人的词汇先复数再加s’所有格，如on Children’s Day, on Women’s Day, on Teachers Day有四个节日强调单数之意思，on Mother’s Day, on Father’s Day, on April Fool’s Day, on Valenti Day） 星期、on Sunday在周日，on Sunday morning在周日早晨 on the last Friday of each month 在每个月的最后一个星期五 日期、on June 2nd在六月二日 on the second (of June 2nd) 在六月的第二天即在六月二日 on the morning of June 2nd在六月二日的早晨，on a rainy morning在一个多雨的早晨 on a certain day 在某天 on the second day在第二天（以过去某天为参照） 注意：on Sunday在周日，on Sundays每逢周日（用复数表每逢之意），every Sunday每个周日，基本一个意思。 on a school day 在某个上学日，on school days每逢上学日。on the weekend在周末，on weekends每逢 周末。 关于in in June在六月 in June, 2010在2010年六月

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2时间介词in,on,at的用法

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充电，电流注入正极点，该相输出端电位仍然等同于正极点电位，输出电压U=+Vdc/2。通常标识为所谓的“1”状态，如图所示。 “1”状态“0”状态 “-1”状态 (2) 当Sa2、Sa3导通，Sa1、Sa4关断时，若负载电流为正方向，则电源对电容C1充电，电流从O点顺序流过箱位二极管Da1，主开关管Sa2:，该相输出端电位等同与0点电位，输出电压U=O;若负载电流为负方向，则电流顺序流过主开关管Sa3和箱位二极管Da2，电流注入O点，该相输出端电位等同于O点电位，输出电压U=0，电源对电容C2充电。即通常标识的“0”状态，如图所示。 (3) 当Sa3、Sa4导通，Sa1、Sa2关断时，若负载电流为正方向，则电流从负极点流过与主开关Sa3、Sa4反并联的续流二极管对电容C2进行充电，该相输出端电位等同于负极点电位，输出电压U=-Vdc/2;若负载电流为负方向，则电源对电容C2充电，电流流过主开关管Sa3、Sa4注入负极点，该相输出端电位仍然等同于负极点电位，输出电压U=-Vdc/2。通常标识为“-1”状态，如图所示。 三电平逆变器工作状态间的转换

剑桥小学英语join in五年级测试卷

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sheep 笔试部分（共80分） 一、Write the questions.将完整的句子写在下面的横线上。(10分) got it Has eyes on a farm it live sheep a it other animals eat it it Is 二、Look and choose.看看它们是谁，将字母填入括号内。(8分) A. B. C. D.

E. F. G. H. ( ) pig ( ) fox ( ) sheep ( ) cat ( ) snake ( ) lion ( ) mouse ( ) elephant 三、Look at the pictures and write the questions.看图片，根据答语写出相应的问题。(10分) No,it doesn’t. Yes,it is.

Yes,it does. Yes,it has. Yes,it does. 四、Choose the right answer.选择正确的答案。(18分) 1、it live on a farm？ 2. it fly？

3. it a cow？ 4. it eat chicken？ 5. you swim？ 6. you all right？ 五、Fill in the numbers.对话排序。(6分) Goodbye. Two apples , please. 45P , please. Thank you.

(完整版)剑桥小学英语Joinin六年级复习题(二)2-3单元.doc

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tomorrowmorning明天早晨 allday/morning/night整天/整个早晨/整晚（等于thewholeday/morning/night）mostofthetime（在）大多数时间 3、一般规则 除了前两点特殊用法之外，其他≤一天，用on，＞一天用in，在具体时刻或在某时用at（不强调时间范围） 关于 On 1 2) 3) （注意：节日里有表人的词汇先复数再加s’所有格，如 onChildren’sDay,onWomen’sDay,onTeachers’Day有四个节日强调单数之意思，onMother’sDay,onFather’sDay,onAprilFool’sDay,onValentine’sDay） 星期、onSunday在周日，onSundaymorning在周日早晨onthelastFridayofeachmonth在每个月的最后一个星期五

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三电平逆变器的主电路结构及其工作原理

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三电平逆变器的主电路结构及其工作原理 所谓三电平是指逆变器交流侧每相输出电压相对于直流侧有三种取值，正端电压 (+Vdc/2)、负端电压(-Vdc/2)、中点零电压(0)。二极管箱位型三电平逆变器主电路结构如图所示。逆变器每一相需要4个IGBT开关管、4个续流二极管、2个箱位二极管;整个三相逆变器直流侧由两个电容C1、C2串联起来来支撑并均衡直流侧电压，C1=C2。通过一定的开关逻辑控制，交流侧产生三种电平的相电压，在输出端合成正弦波。 三电平逆变器的工作原理 以输出电压A相为例，分析三电平逆变器主电路工作原理，并假设器件为理想器件，不计其导通管压降。定义负载电流由逆变器流向电机或其它负载时的方向为正方向。 (l) 当Sa1，、Sa2导通，Sa3、Sa4关断时，若负载电流为正方向，则电源对电容C1充电，电流从正极点流过主开关Sa1、Sa2，该相输出端电位等同于正极点电位，输出电压 U=+V dc/2;若负载电流为负方向，则电流流过与主开关管Sa1、Sa2反并联的续流二极管对电容C1充电，电流注入正极点，该相输出端电位仍然等同于正极点电位，输出电压U=+V dc/2。通常标识为所谓的“1”状态，如图所示。

“1”状态“0”状态 “-1”状态 (2) 当Sa2、Sa3导通，Sa1、Sa4关断时，若负载电流为正方向，则电源对电容C1充电，电流 从O点顺序流过箱位二极管D a1，主开关管Sa2:，该相输出端电位等同与0点电位，输出电压U=O;若负载电流为负方向，则电流顺序流过主开关管Sa3和箱位二极管D a2，电流注入O点，该相输出端电位等同于O点电位，输出电压U=0，电源对电容C2充电。即通常标识的“0”状态，如图所示。 (3) 当Sa3、Sa4导通，Sa1、Sa2关断时，若负载电流为正方向，则电流从负极点流过与主开 关Sa3、Sa4反并联的续流二极管对电容C2进行充电，该相输出端电位等同于负极点电位，输出电压U=-V dc/2;若负载电流为负方向，则电源对电容C2充电，电流流过主开关管Sa3、Sa4注入负极点，该相输出端电位仍然等同于负极点电位，输出电压U=-V dc/2。通常标识为“-1”状态，如图所示。

外研社剑桥小学英语Join_in四年级上册整体课时教案

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What’s your name? The teacher prompt the pupil by whispering, I’m (Alice). (3) Asks all the pupils the same question and help those who need it by whispering I’m…. 2. How are you today? I’m fine. I’m OK. (1) The teacher explains the meaning of How are you today? (2) Tell the class to ask the question all together and introduce two answers, I’m (not very) fine/I’m Ok. (3) Asks all the pupils the same question and make sure that all the students can reply. Step3: Speak English in class. 1. Tells the pupils to open their books at page3, look at the four photographs, and listen to the tape. 2. Asks the pupils to dramatise the situations depicted in the four photographs. A volunteer will play the part of the parts of the other pupils, answering as a group. 第二学时: The pupils learn to use these new words: Sandwich, hamburger, hot dog, puller, cowboy, jeans, cinema, walkman, snack bar, Taxi, clown, superstar.