Optoelectronics Devices and Applications Part 8 pot

Effects of Quantum-Well Base Geometry on Optoelectronic Characteristics of Transistor Laser 269 Quantum- Well Position (Å) τ B,spon (ps) ƒn (undamed natural ferquency)(GHz) ξ (damping ratio) Simulated -3dB Bandwidth(GHz) 150 0.54 125.8 1.7675 38.7 190 0.7 109.7 1.5595 39.3 290 1.11 87.5 1.2288 42.3 390 1.57 74 1.0365 45.2 490 2.1 64 0.9063 47.4 590 2.57 57 0.8103 48.9 690 3.03 53.5 0.7424 50.9 Table 1. Simulated device parameters for different W EQW ; No resonance peak due to QW movement when ξ≥0.7. Fig. 11. QW location effect on optical bandwidth; an HBTL with well near its collector has larger optical bandwidth while electrical bandwidth is the same. 5.1.4 Current gain We predicted previously that optical bandwidth increment would be at cost of sacrificing the current gain (β). Further investigation of QW effect on TL optoelectronic characteristics, this time electrical, leads in an interesting result that matures the previous founding. Collector current gain for an HBTL was calculated in subsection 4.2 in equations (24) and (25). As a result of QW movement towards collector, simulation at constant bias currents shows that τ TL and consequently β declines. Fig. 13 shows this change in optical bandwidth and current gain versus displacement of QW while bias voltage, i.e. v be , forces base current Optoelectronics – Devices and Applications 270 to be constant at I B =33mA for W EQW =590 Å. This bias enforcement does not disturb generality of the simulation results. The opposite dependence of BW and β to W EQW , is a trade-off between TL optoelectronic characteristics. Other experimental and theoretical works proved the described “trade-off” between β and f -3db as well (Then et al.,2009),(faraji et al., 2009). They also predict analytically the above mentioned direct dependence of f -3db on τ B,spon . In (Then et al., 2008) the authors utilized an auxiliary base signal to enhance the optical bandwidth. As a merge of their work and the present analysis we can find the optimum place for QW that leads to better results for both β and f -3db of a TL. It means we can use both method, i.e. auxiliary base signal and QW dislocation method, simultaneously. A suggestion for finding an “optimum” QW location consists of two steps . First we focus on β and make it larger by locating the QW close to emitter, e.g. W EQW <300 Å, which results in BW<43 GHz. Then we use auxiliary AC bias signal to trade some gain for BW. It should be noted that β less than unity is not generally accepted if TL is supposed to work as an electrical amplifier. Fig. 12. Calculated optical cut-off frequency (f -3db ) versus τ B,spon . Bandwidth is maximum for τ B,spon corresponding to W EQW ≈730Å. Effects of Quantum-Well Base Geometry on Optoelectronic Characteristics of Transistor Laser 271 Fig. 13. Electrical gain-optical bandwidth trade-off in an HBTL. 5.2 Quantum-well width Being an optical collector, the QW plays a significant role as it governs both optical and electrical characteristics of TL. Among QW-base geometry parameters, other than QW location, one can investigate the width of QW incorporated within the base region. For instance, we can change the well width while other geometrical parameters, like QW location, are left unchanged in order to examine how the optical frequency response and current gain alter. Base minority carrier recombination lifetime was calculated before as in equation (9) which was an independent function of well width. Charge control model and charge analysis based on this model, described in previous sections, should be completed in order to do this analysis. The base carrier lifetime, τ B , can be written as below (Then et al., 2007c) 1  ⁄     (28) Where υ th is the thermal velocity of carriers, N r is the density of possible recombination sites and σ is the cross section of carrier capture. σ is a measure of the region that an electron has the possibility to capture and recombine with a hole and is proportional to well width (W QW ). In the other hand, N r depends on the hole concentration, i.e. N A of the base region. So we can evaluate τ B as 1  ⁄     (29) where G is a proportionality factor defined by other geometrical properties of the base. Using this equation one can extract base recombination lifetime of base minority carriers for different base doping densities. Calculations exhibit an indirect relation between τ B and well Optoelectronics – Devices and Applications 272 width, agreeing with a larger QW width enhancing the capture cross section for electrons. Moreover, the larger N A , the greater the recombination and hence the smaller τ B . The results for optical frequency response based on Statz-deMars equations of section 4 can be utilized to evaluate the optical properties of a TL for varying well width. Indeed, equations (13) and (14) require τ B,spon not τ B , as described above, therefore threshold current should be calculated for different QW widths. An expression for base threshold current of TL is as below           ⁄      1         (30) where n 0 is minority carrier density in steady-state (under dc base current density of J 0 ), τ cap is the electron capture time by QW (not included in charge control model for simplicity), τ qw is the QW recombination lifetime of electron and τ rb0 is the bulk lifetime (or direct recombination lifetime outside the well, also ignored in our model). The base geometry factor, ν, gives the fraction of the base charge captured in the QW and defines as (Zhang & Leburton, 2009)     ⁄ 1    ⁄  (31) where W qw is the QW width, the factor we investigate here, W b is the base width and x qw is the QW location, similar but not equal to previously defined parameter of W EQW . By setting all the constants, one can calculate τ B,spon and then small-signal optical frequency response and bandwidth of TL for a range of QW widths. Optimization is also possible like what we did for QW location. 6. Conclusion and future prospects An analytical simulation was performed to predict dependence of TL optoelectronic characteristics on QW position in order to find a possible optimum place for QW. Simulated base recombination lifetime of HBTL for different QW positions exhibited an increase in optical bandwidth QW moved towards the collector within the base. Further investigations of optical response prove the possibility of a maximum optical bandwidth of about 54GHz in WEQW≈730 Å. Since no resonance peak occurred in optical frequency response, the bandwidth is not limited in this method. In addition, the current gain decreased when QW moved in the direction of collector. The above mentioned gain-bandwidth trade-off between optoelectronic parameters of TL was utilized together with other experimental methods reported previously to find a QW position for more appropriate performance. The investigated transistor laser has an electrical bandwidth of more than 100GHz. Thus the structure can be modified, utilizing the displacement method reported in this paper, to equalize optical and electrical cut-off frequencies as much as possible. In previous sections we consider the analysis of a single quantum well (SQW) where there is just one QW incorporated within the base region. This simplifies the modelling and math- related processes. In practice, SQWTL has not sufficient optical gain and may suffer thermal heating which requires additional heat sink. Modifications needed to model a multiple QW transistor laser (MQWTL). First one should rewrite the rate equations of coupled carrier and photon for separate regions between wells. Solving these equations and link them by applying initial conditions, i.e. continuity of current and carrier concentrations, is the next step. In addition to multiple capture and escape lifetime of carriers, tunnelling of the 2- dimensional carriers to the adjacent wells should be considered. For wide barriers one may use carrier transport across the barriers instead the mentioned tunnelling. Simulation results Effects of Quantum-Well Base Geometry on Optoelectronic Characteristics of Transistor Laser 273 for diode laser (Duan et al., 2010), as one of the transistor laser parents, demonstrate considerable enhancement in optical bandwidth and gain of the device when increasing the number of quantum wells (Nagarajan et al., 1992), (Bahrami and Kaatuzian, 2010). Like the well location modelled here in this chapter, there may be an optimum number of quantum wells to be incorporated within the base region. Due to its high electrical bandwidth (≥100 GHz), it is needed to increase the optical modulation bandwidth of the TL. Base region plays the key role in all BJT transistors, especially in Transistor Lasers. Like Quantum-Well, base structural parameters have significant effects on optoelectronic characteristics of TL which can be modelled like what performed before during this chapter. Among these parameters are base width (Zhang et al., 2009), material, doping (Chu-Kung et al., 2006), etc. For example, a graded base region can cause an internal field which accelerates the carrier transport across the base thus alters both the optical bandwidth and the current gain considerably. 7. References Basu, R., Mukhopadhyay, B. & Basu, P.K. (2009). Gain Spectra and Characteristics of a Transistor Laser with InGaAs Quantum Well in the Base. Proceeding of International Conference on Computers and Devices for Communication, India, Dec. 2009 Bahrami Yekta, V. & Kaatuzian, H. (2010). Design considerations to improve high temperature characteristics of 1.3μm AlGaInAs-InP uncooled multiple quantum well lasers: Strain in barriers. Optik, Elsevier, doi:10.1016/j.ijileo.2010.03.016 Chan, R., Feng, M., Holonyak, N. Jr., James, A. & Walter, G. (2006). Collector current map of gain and stimulated recombination on the base quantum well transitions of a transistor laser. Appl. Phys. Lett., Vol. 88, No. 143508 Chu-kung, B.F., Feng, M., Walter, G., Holonyak, N. Jr., Chung, T., Ryou, J H., Limb, J., Yoo, D., Shen, S.C. & Dupuis, R.D. (2006). Graded-base InGaN/GaN heterojunction bipolar light-emitting transistors. Appl. Phys. Lett. Vol. 89, No. 082108 Dixon, F., Feng, M. & Holonyak, N. Jr. (2010). Distributed feedback transistor laser. Appl. Phys. Lett. Vol. 96, No. 241103 Dixon, F., Chan, R., Walter, G., Holonyak, N. Jr. & Feng, M. (2006). Visible spectrum light- emitting transistors. Appl. Phys. Lett. Vol. 88, No. 012108 Duan, Z., Shi, W., Chrostowski, L., Huang, X., Zhou, N., & Chai, G. (2010). Design and epitaxy of 1.5 μm InGaAsP-InP MQW material for a transistor laser. Optics Express, Vol. 18, Issue 2, pp. (1501-1509), doi:10.1364/OE.18.001501 Faraji, B., Pulfrey, D.L. & Chrostowski, L. (2008). Small-signal modeling of the transistor laser including the quantum capture and escape lifetimes. Appl. Phys. Lett., Vol. 93, No. 103509 Faraji, B., Shi, W., Pulfrey, D.L. & Chrostowski, L. (2009). Analytical modeling of the transistor laser. IEEE Journal of Quantum Electron., Vol. 15, No. 3, pp (594-603) Feng, M., Holonyak, N. Jr. & Hafez, W. (2004a). Light-emitting transistor: light emission from InGaP/GaAs heterojunction bipolar transistors. Appl. Phys. Lett. Vol. 84, No. 151 Feng, M., Holonyak, N. Jr. & Chan, R. (2004b). Quantum-well-base heterojunction bipolar light-emitting transistor. Appl. Phys. Lett., Vol. 84, No. 11 Feng, M., Holonyak, N. Jr., Walter, G. & Chan, R. (2005). Room temperature continuous wave operation of a heterojunction bipolar transistor laser. Appl. Phys. Lett. Vol. 87, No. 131103 Optoelectronics – Devices and Applications 274 Feng, M., Holonyak, N. Jr., Chan, R., James, A. & Walter, G. (2006a). Signal mixing in a multiple input transistor laser near threshold. Appl. Phys. Lett. Vol. 88, No. 063509 Feng, M., Holonyak, N. Jr., James, A., Cimino, K., Walter, G. & Chan, R. (2006b). Carrier lifetime and modulation bandwidth of a quantum well AlGaAs/ InGaP/GaAs/InGaAs transistor laser. Appl. Phys. Lett. Vol. 89, No. 113504 Feng, M., Holonyak, N. Jr., Then, H.W. & Walter, G. (2007). Charge control analysis of transistor laser operation. Appl. Phys. Lett. Vol. 91, No. 053501 Feng, M., Holonyak, N. Jr., Then, H.W., Wu, C.H. & Walter, G. (2009). Tunnel junction transistor laser. Appl. Phys. Lett. Vol. 94, No. 041118 Kaatuzian, H. (2005). Photonics, Vol. 1, AmirKabir University of Technology press, , Tehran, Iran Kaatuzian, H. & Taghavi,I. (2009). Simulation of quantum-well slipping effect on optical bandwidth in transistor laser. Chinese optics letters. doi:10.3788/COL20090705.0435, pp. 435–436 Nagarajan, R., Ishikawa, M., Fukushima, T., Geels, R. & Bowers, E. (1992). High speed quantum-well lasers and carrier transport effects. IEEE Journal of Quantum Electron, Vol. 28, No. 10, pp (1990-2008) Shi, W., Chrostowski, L & Faraji, B. (2008). Numerical Study of the Optical Saturation and Voltage Control of a Transistor Vertical-Cavity Surface-Emitting Laser. IEEE Photonics Technology Letters, Vol. 20, No. 24 Suzuki, Y., Yajima, H., Shimoyama, K., Inoue, Y., Katoh, M. & Gotoh, H. (1990). (Heterojunction field effect transistor laser). Ellectronics Letters, Vol. 26, No. 19 Taghavi,I & Kaatuzian, H. (2010). Gain-Bandwidth trade-off in a transistor laser : quantum well dislocation effect. Springer, Opt Quant Electron. doi: 10.1007/s11082-010-9384-0, pp. 481–488 Then, H.W., Walter, G., Feng, M. & Holonyak, N. Jr. (2007a). Collector characteristics and the differential optical gain of a quantum-well transistor laser. Appl. Phys. Lett Vol. 91, No. 243508 Then, H.W., Feng, M. & Holonyak, N. Jr. (2007b). Optical bandwidth enhancement by operation and modulation of the first excited state of a transistor laser. Appl. Phys. Lett. Vol. 91, No. 183505 Then, H.W., Feng, M., Holonyak, N. Jr. & Wu, C.H. (2007c). Experimental determination of the effective minority carrier lifetime in the operation of a quantum-well n-p-n heterojunction bipolar light-emitting transistor of varying base quantum-well design and doping. Appl. Phys. Lett. Vol. 91, No. 033505 Then, H.W., Walter, G., Feng, M. & Holonyak, N. Jr. (2008). Optical bandwidth enhancement of heterojunction bipolar transistor laser operation with an auxiliary base signal. Appl. Phys. Lett. Vol. 93, No. 163504 Then, H.W., Feng, M. & Holonyak, N. Jr. (2009). Bandwidth extension by trade-off of electrical and optical gain in a transistor laser: three-terminal control. Appl. Phys. Lett. Vol. 94, No. 013509 Then, H.W., Feng, M. & Holonyak, N. Jr. (2010). Microwave circuit model of the three-port transistor laser. Appl. Phys. Lett. Vol. 107, No. 094509 Walter, G., Holonyak, N. Jr., Feng, M. & Chan, M. (2004). Laser operation of a heterojunction bipolar light-emitting transistor. Appl. Phys. Lett . Vol. 85, No. 4768 Walter, G., James, A., Holonyak, N. Jr., Feng, M. & Chan, R. (2006). Collector breakdown in the heterojunction bipolar transistor laser. Appl. Phys. Lett. Vol. 88, No. 232105 Zhang, L. & Leburton, J P. (2009). Modeling of the transient characteristics of heterojunction bipolar transistor lasers. IEEE Journal of Quantum Electron. doi:10.1109/JQE.2009.2013215, pp. (359–366) 14 Intersubband and Interband Absorptions in Near-Surface Quantum Wells Under Intense Laser Field Nicoleta Eseanu Physics Department, “Politehnica” University of Bucharest, Bucharest Romania 1. Introduction The intersubband transitions in quantum wells have attracted much interest due to their unique characteristics: a large dipole moment, an ultra-fast relaxation time, and an outstanding tunability of the transition wavelengths (Asano et al., 1998; Elsaesser, 2006; Helm, 2000). These phenomena are not only important by the fundamental physics point of view, but novel technological applications are expected to be designed. Many important devices based on intersubband transitions in quantum well heterostructures have been reported. For example: far- and near-infrared photodetectors (Alves et al., 2007; Levine, 1993; Li, S.S. 2002; Liu, 2000; Schneider & Liu, 2007; West & Eglash, 1985), ultrafast all-optical modulators (Ahn & Chuang, 1987; Carter et al., 2004; Li, Y. et al., 2007), all optical switches (Iizuka et al., 2006; Noda et al., 1990), and quantum cascade lasers (Belkin et al., 2008; Chakraborty & Apalkov, 2003; Faist et al., 1994). It is well-known that the optical properties of the quantum wells mainly depend on the asymmetry of the confining potential experienced by the carriers. Such an asymmetry in potential profile can be obtained either by applying an electric/laser field to a symmetric quantum well (QW) or by compositionally grading the QW. In these structures the changes in the absorption coefficients were theoretically predicted and experimentally confirmed to be larger than those occurred in conventional square QW (Karabulut et al., 2007; Miller, D.A.B. et al., 1986; Ozturk, 2010; Ozturk & Sökmen, 2010). In recent years, with the availability of intense THz laser sources, a large number of strongly laser-driven semiconductor heterostructures were investigated (Brandi et al., 2001; Diniz Neto & Qu, 2004; Duque, 2011; Eseanu et al., 2009; Eseanu, 2010; Kasapoglu & Sokmen, 2008; Lima, F. M. S. et al., 2009; Niculescu & Burileanu, 2010b; Ozturk et al., 2004; Ozturk et al., 2005; Sari et al., 2003; Xie, 2010). These works have revealed important laser-induced effects: i) the confinement potential is dramatically modified; ii) the energy levels of the electrons and, to a lesser extent, those of the holes are enhanced; iii) the linear and nonlinear absorption coefficients can be easily controlled by the confinement parameters in competition with the laser field intensity. The intersubband transitions (ISBTs) have been observed in many different material systems. In recent years, apart from GaAs/AlGaAs, the InGaAs/GaAs QW structures have Optoelectronics – Devices and Applications 276 attracted much interest because of their promising applications in optoelectronic and microelectronic devices: multiple-quantum-well modulators and switches (Stohr et al., 1993), broadband photodetectors (Gunapala et al., 1994; Li J. et al., 2010, Passmore, et al., 2007), superluminescent diodes used for optical coherence tomography (Li Z. et al., 2010) and metal-oxide-semiconductor field-effect-transistors, i. e. MOSFETs (Zhao et al., 2010). The optical absorption associated with the excitons in semiconductor QWs have been the subject of a considerable amount of work for the reason that the exciton binding energy and oscillator strength in QWs are considerably enhanced due to quantum confinement effect (Andreani & Pasquarello, 1990; Jho et al., 2010; Miller, R. C. et al., 1981; Turner et al., 2009; Zheng & Matsuura, 1998). As a distinctive type of dielectric quantum wells, the near-surface quantum wells (n-sQWs) have involved increasing attention due to their potential to sustain electro-optic operations under a wide range of applied electric fields. In these heterostructures the QW is located close to vacuum and, as a consequence, the semiconductor-vacuum interface which is parallel to the well plane introduces a remarkable discrepancy in the dielectric constant (Chang & Peeters, 2000). This dielectric mismatch leads to a significant enhancement of the exciton binding energy (Gippius et al., 1998; Kulik et al., 1996; Niculescu & Eseanu, 2010a) and, consequently, it changes the exciton absorption spectra as some experimental (Gippius et al., 1998; Kulik et al., 1996; Li, Z. et al., 2010; Yablonskii et al., 1996) and theoretical (Niculescu & Eseanu, 2011a; Yu et al., 2004) studies have demonstrated. The rapid advances in modern growth techniques and researches for InGaAs/GaAs QWs (Schowalter et al., 2006; Wu, S. et al., 2009) create the possibility to fabricate such heterostructures with well-controlled dimensions and compositions. Therefore, the differently shaped InGaAs/GaAs near-surface QWs become interesting and worth studying systems. We expect that the capped layer of these n-sQWs induces considerable modifications on the intersubband absorption as it did on the interband excitonic transitions (Niculescu & Eseanu, 2011). To the best of our knowledge this is the first research concerning the intense laser field effect on the ISBTs in InGaAs/GaAs differently shaped near-surface QWs. In this chapter we are concerned about the intersubband and interband optical transitions in differently shaped n-sQWs with symmetrical/asymmetrical barriers subjected to intense high-frequency laser fields. We took into account an accurate form for the laser-dressing confinement potential as well as the occurrence of the image-charges. Within the framework of a simple two-band model the consequences of the laser field intensity and carriers-surface interaction on the absorption spectra have been investigated. The organization of this work is as follows. In Section 2 the theoretical model for the intense laser field (ILF) effect on the intersubband absorption in differently shaped n-sQWs is described together with numerical results for the electronic energy levels and absorption coefficients (linear and nonlinear). In Section 3 we explain the ILF effect on the exciton ground energy and interband transitions in the same QWs, taking into account the repulsive interaction between carriers and their image-charges. Also, numerical results for the 1S- exciton binding energy and interband linear absorption coefficient are discussed. Finally, our conclusions are summarized in Section 4. 2. Intersubband transitions in near-surface QWs under intense laser field The intersubband transitions (ISBTs) are optical transitions between quasi-two-dimensional electronic states ("subbands") in semiconductors which are formed due to the confinement Intersubband and Interband Absorptions in Near-Surface Quantum Wells Under Intense Laser Field 277 of the electron wave function in one dimension. The conceptually simplest band-structure engineered system that can be fabricated is a quantum well (QW), which consists of a thin semiconductor layer embedded in another semiconductor with a larger bandgap. Depending on the relative band offsets of the two semiconductor materials, both electrons and holes can be confined in one direction in the conduction band (CB) and the valence band (VB), respectively. Thus, allowed energy levels which are quantized along the growth direction of the heterostructure appear (Yang, 1995). These levels can be tailored by changing the QW geometry (shape, width, barrier heights) or by applying external perturbations (hydrostatic pressure, electric, magnetic and laser fields). Whereas, of course, optical transitions can take place between VB and CB states, in this Section we are concerning only with ISBTs between quantized levels within the CB. 2.1 Theory Let us consider an InGaAs n-sQW embedded between symmetrical/asymmetrical GaAs barriers. It is convention to define the QW growth as the z-axis. According to the effective mass approximation, in the absence of the laser field, the time-independent Schrödinger equation is      22 2 2         self zVzVz zEz mz  . (1) where  m is the electron effective mass,   Vz is the confinement potential in the QW growth direction and   self Vz describes the repulsive interaction in the system consisting of an electron and its image-charge.  2 0 0 11 212       self e Vz d   . (2) Here  is the semiconductor dielectric constant, 22 0 /4ee  , and 0 d is the distance between the electron and its image-charge without laser field. For the three differently shaped n- sQWs studied in this work the potential   Vz reads as follows. For a square n-sQW,   Vz has the well-known form  0 , ,0 0, 0         c SQW cw w zL VzV LzandzL zL ` (3a) For a graded n-sQW, the confinement potential is  0 0 , ,0 ,0 2 ,                c c GQW rw w rw zL VLz Vz z VzL L VV zL  (3b) Optoelectronics – Devices and Applications 278 For a semiparabolic n-sQW,   Vz is given by  0 2 0 , ,0 ,0 ,                   c c sPQW rw w rw zL VLz Vz z VzL L VV zL  (3c) The quantities w L and c L are the well width and capped layer thicknesses, respectively; 0 V is the GaAs barrier height in the QW left side (with cap layer); r V is the barrier height in the QW right side and  is the barrier asymmetry parameter. Under the action of a non-resonant intense laser field (ILF) represented by a monochromatic plane wave of frequency LF  having the vector potential     0 cos   LF At uA t  , the Schrödinger equation to be solved becomes a time-dependent one due to the time- dependent nature of the radiation field. Here  u is the unit vector of the polarization direction (chosen as z-axis). By applying the translation      rr t  the equation describing the electron-field interaction dynamics was transformed by Kramers (Kramers, 1956) as      2 ,,, 2             rt Vr t rt i rt t m  . (4) The expression     0 α sin   LF tu t  (5) describes the quiver motion of the electron under laser field action. 0  is known as the laser-dressing parameter, i. e. a laser-dependent quantity which contains both the laser frequency and intensity, 0 0   LF eA m   . (6) Thus, in the presence of the laser field linearly polarized along the z-axis, the confinement potential     Vz t  is a time-periodic function for a given z and it can be expanded in a Fourier series,        0 1expexp       v kv LF kv Vz t VJ k ikz iv t  (7) where k V is the k-th Fourier component of   Vz and v J is the Bessel function of order v. In the high-frequency limit, i.e. 1 LF   , with τ being the transit time of the electron in the QW region (Marinescu & Gavrila, 1995) the electron “sees” a laser-dressed potential which is obtained by averaging the potential     Vz t  over a laser field period, [...]... 14, (1990), 89 28- 89 38, ISSN 10 98- 0121 Asano, T.; Noda, S & Sasaki, A (19 98) Absorption magnitude and phase relaxation time in short wavelength intersubband transitions in InGaAs/AlAs quantum wells on Intersubband and Interband Absorptions in Near-Surface Quantum Wells Under Intense Laser Field 303 GaAs substrates Physica E: Low-dimensional Systems and Nanostructures, vol 2, No 1-4, (July 19 98) , 111-115,... A & Buchanan, M (2007) NIR, MWIR and LWIR quantum well infrared photodetector using interband and intersubband transitions Infrared Phys & Technol vol 50, No 2-3, (April 2007), 182 - 186 , ISSN 1350-4495 Ando, Y & Ytoh, T (1 987 ) Calculation of transmission tunneling current across arbitrary potential barriers J Appl Phys vol 61, No 4, (1 987 ),1497-1502, ISSN 1 089 -7550 Andreani, L.C & Pasquarello, A (1990)... corresponding energy levels: ground state E1 (solid lines) and first excited state E2 (dashed lines) Notations a, c, e, and f stand for various laser parameter values, 0 = 0 (black), 40 Å (olive), 80 Å (purple), and 100 Å (orange), respectively QW width and cap layer thickness are L w = 150 Å and Lc = 20 Å, respectively  282 Optoelectronics – Devices and Applications Therefore, under an intense laser field... J Appl Phys., vol 74, No 8, (1993), R1-R81, ISSN 1 089 -7550 Li, J.; Choi, K.K.; Klem, J.F.; Reno, J.L & Tsui, D.C (2006) High gain, broadband InGaAs/InGaAsP quantum well infrared photodetector Appl Phys Lett vol 89 , No 8, (2006), 081 1 28, ISSN 1077-31 18 Li, S.S (2002) Multi-color, broadband quantum well infrared photodetectors for mid-, long-, and very long wavelength infrared applications International... (solid lines) and 200 Å (dashed lines) 290 Optoelectronics – Devices and Applications Fig 8 Linear absorption coefficient,    , vs pump photon energy in a n-sSQW with Lc = 50 Å under various laser dressing parameters Notations a, b, c, d, and e stand for:  0 = 0 (black), 20 Å (blue), 40 Å (olive), 60 Å (red), and 80 Å (purple), respectively The QW widths are 100 Å (solid lines) and 200 Å (dashed... as the inset of Fig 11A shows B A  = 0 .8  = 0.6 60 100 _ GQW - sPQW 0 = 80 Å 80 40 0.6 40 30  = 0 .8 25  = 0.6 20 0= 40 Å 0= 20 Å 60 20 =1 _ GQW 35 - sPQW Eh [meV] 80 0 40 SQW =1 Ee [meV] Ee [meV] SQW 100 _ GQW - sPQW 0 .8 60 80 0 [Å] 15 0= 0  1.0 100 120 0 20 40 60 0 [Å] 80 100 120 Fig 11 The single-particle energy of the electron (A) and hole (B) as functions of the laser intensity,... - 80 Å) whereas for Lc  80 Ǻ these positions are insensitive to the cap layer variation Thirdly, we note that in the asymmetrical n-sQWs (GQW and sPQW) the blue-shift of the exciton absorption peak positions for  = 1 (identical barriers) is more pronounced comparing with the case  = 0.6 302 Optoelectronics – Devices and Applications For the In0.18Ga0 .82 As/GaAs near-surface SQW with Lw = 50 Å and. .. upper part becomes less sensitive to the laser action A 70 E1,E2 [meV] 60 E2 50 Lc 40 30 E1 20 10 0 20 40 SQW Lw=200Å Lc 60 0 [Å] Fig 2 A 80 100 120 Intersubband and Interband Absorptions in Near-Surface Quantum Wells Under Intense Laser Field 283 B 90 E2 E1,E2 [meV] 80 70 Lc 60 E1 50 Lc 40 0 20 40 60 GQW Lw=200Å 80 0 [Å] 100 120 C E1,E2 [meV] 100 E2 80 Lc 60 E1 Lc 40 0 20 40 60 0 [Å] sPQW Lw=200Å 80 ... n-sQWs with Lw = 150 Å and symmetrical barriers, under various laser intensities  0 = 0, 40 Å, and 80 Å, is plotted The values of Lc are 20 Å and 200 Å 1 Fig 8 displays the variation of    on the pump photon energy in a near-surface SQW with Lc = 50 Å under various laser intensities  0 = 0, 40 Å, and 80 Å The values of Lw are 100 Å and 200 Å One can see from Figs 7 and 8 that the increasing laser... magnetic fields Phys Rev B, vol 81 , No 15 (2010), 155314, ISSN 10 98- 0121 Karabulut, I.; Atav, U.; Safak, H & Tomak, M (2007) Linear and nonlinear intersubband optical absorptions in an asymmetric rectangular quantum well Eur Phys J B, vol 55, No 3, (Febr I, 2007), 283 - 288 ISSN1434-6036 Kasapoglu E & Sökmen, I (20 08) The effects of intense laser field and electric field on intersubband absorption in a double-graded . Simulated -3dB Bandwidth(GHz) 150 0.54 125 .8 1.7675 38. 7 190 0.7 109.7 1.5595 39.3 290 1.11 87 .5 1.2 288 42.3 390 1.57 74 1.0365 45.2 490 2.1 64 0.9063 47.4 590 2.57 57 0 .81 03 48. 9 690 3.03. vicinity of z = 0 and its overlap with the first excited wave function   z 2  (which has a minimum in z = 0) reduces. Optoelectronics – Devices and Applications 288 ii. by further. intersubband transitions (ISBTs) have been observed in many different material systems. In recent years, apart from GaAs/AlGaAs, the InGaAs/GaAs QW structures have Optoelectronics – Devices and Applications