Ultrafast all-optical modulation by near-infrared intersubband transition in n-doped InGaAs/AlAsSb quantum wells

We demonstrate the ultrafast modulation of interband (IB)-resonant light (1.0–1.1 μm) by near-infrared intersubband (ISB)-resonant light (1.55–1.9 μm) in n-doped InGaAs/AlAsSb multiple quantum wells (QWs) using non-degenerate pump–probe spectroscopy. A modulation with an absorption recovery time of 1.0–2.0 ps has been observed in a planer-type modulation device due to ultrafast ISB relaxation of the carriers. The IB carrier relaxation process in the absence of an ISB-resonant light has also been investigated by time-resolved photoluminescence (PL) measurement. The modulation speed is determined by the inter- and intra-subband relaxation of the carriers in the conduction subband. The modulation speed at 1.1 μm due to an ISB-resonant pump light at 1.95 μm has been observed to be 1.4 ps at excitation energy of 500 fJ/μm².     

Ultrashort-pulse-controlled all-optical modulation by interband and intersubband transitions in doped quantum wells

The optimum condition for achieving highly efficient ultrafast all-optical pulse modulation by modification of the absorption of a pulsed interband light field by ultrasubband control-light pulses in a doped quantum well is investigated. The modulation efficiency in the femtosecond domain can be maximized by an intersubband control light with a pulse width (200 fs) that is comparable with the phase-relaxation time of the system and an intensity that is close to the intersubband saturation intensity.     

Ultrafast all optical modulation based on intersubband transition in semiconductor quantum wells

Ultrafast modulation of interband-resonant light by intersubband-resonant light in n-doped GaAs/AlGaAs and GaN/AlGaN quantum wells was investigated by femtosecond pump-probe technique. A planar-type AlGaAs/GaAs modulation device shows a modulation speed of ～1 ps at room temperature. The observed modulation efficiency indicates that 99% modulation can be achieved with a control pulse energy of ～1 pJ when a waveguide-type device structure is utilized. The feasibility of the all-optical modulation in GaN/AlGaN quantum wells is also investigated. The intersubband carrier relaxation time, which mainly determines the modulation speed, is measured and is found to be extremely fast (130–170 fs). The results indicate that the optical modulation at a bit rate of over 1 Tb/s will be possible by utilizing the intersubband transition in GaN/AlGaN quantum wells. The modulation efficiency in GaN/AlGaN quantum wells is also discussed in comparison with that in GaAs/AlGaAs quantum wells.     

Highly strained InxGa(1−x)As−InyAl(1−y)As (x>0.8,y<0.3) layers for short wavelength QWIP and QCL structures grown by MBE

Highly strained quantum cascade laser (QCL) and quantum well infrared photodetector (QWIPs) structures based on In_xGa_(1−x)As−In_yAl_(1−y)As (x>0.8,y<0.3) layers have been grown by molecular beam epitaxy. Conditions of exact stoichiometric growth were used at a temperature of 420°C to produce structures that are suitable for both emission and detection in the 2–5 μm mid-infrared regime. High structural integrity, as assessed by double crystal X-ray diffraction, room temperature photoluminescence and electrical characteristics were observed. Strong room temperature intersubband absorption in highly tensile strained and strain-compensated In_0.84Ga_0.16As/AlAs/In_0.52Al_0.48As double barrier quantum wells grown on InP substrates is demonstrated. Γ–Γ intersubband transitions have been observed across a wide range of the mid-infrared spectrum (2–7 μm) in three structures of differing In_0.84Ga_0.16As well width (30, 45, and 80 Å). We demonstrate short-wavelength IR, intersubband operation in both detection and emission for application in QC and QWIP structures. By pushing the InGaAs–InAlAs system to its ultimate limit, we have obtained the highest band offsets that are theoretically possible in this system both for the Γ–Γ bands and the Γ–X bands, thereby opening up the way for both high power and high efficiency coupled with short-wavelength operation at room temperature. The versatility of this material system and technique in covering a wide range of the infrared spectrum is thus demonstrated.     

Femtosecond semiconductor-based optoelectronic devices for optical-communication systems

Compact semiconductor-based ultrafast optoelectronic devices are crucial for networks with a throughput in the 1–10 Tb/s range. A variety of ultrafast phenomena in semiconductors are attractive for developing such new optoelectronic devices. This paper discusses the requirements of ultrafast optical-communication systems and necessary optoelectronic devices. Recent progress of ultrafast semiconductor-based optoelectronic devices are described with a focus on all-optical switching devices, including novel devices using electron spin polarization relaxation and intersubband transition both in multiple quantum well structures.     

Monolithically integrated near-infrared and mid-infrared detector array for spectral imaging

A multi-band focal plane array sensitive in near-infrared (near-IR) and mid-wavelength infrared (MWIR) is been developed by monolithically integrating a near-infrared (1–1.5 μm) p–i–n photodiode with a mid-infrared (3–5 μm) QWIP. This multiband detector involves both intersubband and interband transitions in III–V semiconductor layer structures. Each detector stack absorbs photons within the specified wavelength band, while allowing the transmission of photons in other spectral bands, thus efficiently permitting multiband detection. Monolithically grown material characterization data and individual detector test results ensure the high quality of material suitable for near-infrared/QWIP dual-band focal plane array.     

Cross-phase-modulation-based wavelength conversion using intersubband transition in InGaAs/AlAs/AlAsSb coupled quantum wells

We report an ultrafast cross phase modulation (XPM) effect in intersubband transition (ISBT) of InGaAs/AlAs/AlAsSb coupled quantum wells, where the ISBT absorption of a transverse-magnetic mode pump signal induces phase modulation of a transverse-electric mode probe signal. Using waveguide-type ISBT devices, we have achieved XPM-based 10 Gbit/s wavelength conversion with a power penalty of 2.53 dB. Also, we propose XPM-based signal processing circuits for gate switching and modulation format conversion.     

Microscopic modeling of intersubband resonances in InAs/AlSb quantum wells

Linear absorption spectra from intersubband resonance in InAs/AlSb quantum wells are analyzed theoretically using the intersubband semiconductor Bloch equation approach. Our model goes beyond the Hartree–Fock approximation and treats particle–particle correlations under the second Born approximation. Electron–electron and longitudinal optical phonon scatterings from such a treatment describe intrinsic line broadening to the intersubband resonance. Electron subbands are determined self-consistently with a spurious-state-free 8-band k·p Hamiltonian under the envelope function approximation. To compare with experimental measurements, we also included line broadening due to electron-interface roughness scattering. Excellent agreement was achieved for temperature-dependent absorption spectra in the mid-infrared frequency range, after taking into careful account the interplay of material parameters, nonparabolicity in bandstructure, and many-body effects.     

Electronic properties of intersubband transition in (CdS/ZnSe)/BeTe quantum wells

In view of the fact that the bandwidth required in optical fiber communication systems will exceed 100 Gb s^-1, ultrafast optical switching and modulation devices with high efficiency must be developed. Given that intersubband transitions (ISBT) in quantum wells (QWs) are one of the important ultrafast phenomena, a numerical study of intersubband transition (ISBT) properties in (CdS/ZnSe)/BeTe QWs is considered. The structure modeled consists of a few monolayers of CdS embedded in a ZnSe/BeTe QW. A self-consistent analysis is made to achieve the desired properties and device applications. Variation of CdS well thickness leads to tailoring of the band alignment, achieving optical transitions in the wavelength range of 1.33–1.55 μm wavelengths for applications in optical fiber transmission. To analyze the optical behavior of the heterostructure under investigation, we have calculated the CdS well thickness-dependant oscillator strengths and electron emission energy of the intersubband transition between the two first states in the well. An attempt to explain our results will be presented.     

Intraband absorption and emission of light in quantum wells and quantum dots

High-resolution spectroscopy in the mid-infrared spectral range is used to study electronic transitions between size-quantization subbands in stepped quantum wells under picosecond interband excitation. The contributions from intersubband and intrasubband absorption of light are separated by using the difference in time profiles of the absorption coefficient for these cases. For stepped quantum wells, spontaneous interband luminescence and superluminescence are studied for different excitation levels. For structures with quantum dots, the intraband absorption spectra for n-and p-type structures and the spectra of photoinduced intraband absorption and emission (for polarized radiation) for undoped structures are studied.     

Edge state equilibration in a two-subband system

In order to honor Jörg Kotthaus, I present unpublished experimental results which were obtained in 1994 when I was a postdoc in Munich.The scattering between edge states in the quantum Hall regime is strongly reduced compared to scattering in the bulk of a two-dimensional electron gas. For edge states with different Landau quantum numbers an equilibration length as long as 100μm has been determined. In the case of Landau levels with different spin quantum numbers this length may reach values of 1 mm. Here we set out to explore the equilibration between edge states with different subband quantum numbers. Using parabolic quantum wells as a tunable multi-subband system we find that intersubband scattering can reduce the equilibration length to values below 5μm.     

Capture dynamics and far-infrared response in photovoltaic quantum well intersubband photodetectors

Recent progress in photovoltaic quantum well intersubband photodetectors (QWIP) makes these devices suitable for high-performance imaging and heterodyne detection. We report on investigations concerning the basic physics of the transport mechanism, the dynamical behavior, and the further optimization of these structures. GaAs/AlAs/AlGaAs double-barrier QWIPS designed for 3-5 μm wavelength operation provide an interesting model system to study the dynamical aspects of the photovoltaic response. We find strong evidence that carrier transfer across the AlAs barriers mainly occurs due to Γ-X intervalley scattering, even for AlAs layer thicknesses of 1-2 nm. We also discuss experimental results on photovoltaic QWIPs operating in the 8-12 μm regime. In these structures, photovoltaic operation is achieved using a combination of single-barrier quantum wells with built-in space-charge fields. We report on a photovoltaic QWIP with a cutoff energy of 118 meV and a zero-bias detectivity of 2.5×10⁹ cm√Hz/W, which is only three times less than the detectivity of a photoconductive QWIP with the same cutoff energy.     

Controlling polariton coupling in intersubband microcavities

We report the external control of the intersubband polariton coupling by manipulating the carrier density in quantum wells resonantly coupled to a GaAs/AlGaAs microcavity. The electrons in the wells were tuned by means of a depletion gate bias or by utilizing charge transfer between the energetically aligned ground subbands of asymmetric tunnel-coupled quantum wells. We propose the use of tunnel-assisted control of the polariton ground state in an asymmetrically coupled quantum well for implementing ultrafast modulation of intersubband polaritons.     

Ultrafast all optical switching via tunable Fano interference

Tunneling induced quantum interference experienced by an incident probe in asymmetric double quantum wells can easily be modulated by means of an external control light beam. This phenomenon, which is here examined within the dressed-state picture, can be exploited to devise a novel all-optical ultrafast switch. For a suitably designed semiconductor heterostructure, the switch is found to exhibit frequency bandwidths of the order of 0.1 THz and response and recovery times of about 1 ps.     

Terahertz nonlinear optics with strained p-type quantum wells

Valence-subband nonparabolicity is shown theoretically to lead to nonlinearities associated with terahertz third-harmonic generation in strained p -type quantum wells. For strained InAs quantum wells it is found that the corresponding value of chi((3)) can be as large as ~10(-12)(m/V)(2). The predicted values of chi((3)) are in the range of those associated with intersubband transitions in the mid-infrared region of the spectrum.     

非线性克尔效应对飞秒激光偏振的超快调制

研究了近红外飞秒激光的偏振在太赫兹频率的超快调制.利用抽运-探测光谱技术,通过改变两个脉冲之间的延迟时间可以控制光脉冲的旋转角.在Li：NaTb（WO₄）₂磁光晶体中观察到探测光的偏振随延迟时间变化的高速振荡,振荡信号的中心频率为0.19 THz.这种超快偏振调制现象可以解释为,抽运-探测实验构置中,前向传播的抽运光诱导的光学克尔非线性引起被晶体远端表面所反射的背向传播的探测光脉冲偏振面的额外旋转.通过改变抽运光的圆偏振旋性可以控制探测光调制信号的相位和振幅.实验结果表明,非线性光学克尔效应可以作为一种全新的手段,在磁光晶体中实现近红外飞秒激光以太赫兹频率的超快偏振调控.这将在超快磁光调制器等全光器件中得以应用.实验结果将有助于偏振依赖的超快动力学过程的研究.     

Long wavelength infrared (LWIR) photodetection in a modulation doped thyristor

A modulation doped thyristor concept is described for LWIR photodetection based upon intersubband bound to continuum absorption. The intersubband absorption generates photocurrent from undoped quantum wells to modulation doped layers (MDL). Due to the lower dark current compared to conventional quantum well infrared photodetectors (QWIPs), the thyristor infrared detector operates with little or no cooling and with similar or better performance than QWIPs at low temperatures. The operating characteristics of absorption coefficient, quantum efficiency, responsivity, detectivity, infrared gain, and dark current are determined as a function of thyristor voltage and input power level in the range of 1 μW/cm².     

Demonstration of an all-optical switching operation using an optical fiber grating coupler

An optical fiber grating coupler (FGC) is a fused optical fiber coupler with a tapered region in which refractive index-modulated gratings are written. In the FGC, the light with specific wavelength satisfying the Bragg condition of the grating can be dropped to one output port and other lights are transmitted to another output port when lights with various wavelengths are launched into the input port. The FGC can operate as an all-optical switch by controlling the Bragg wavelength of the grating using a third order nonlinear optical effect caused by a control light that are launched with a signal light. In this paper, an all-optical switching operation due to a third order nonlinear optical effect in an FGC is first demonstrated for a signal light with 1.55 μm-wavelength to be changed from one port of the FGC to another one by the 720 W peak of a control light from a Nd:YAG laser with 1.06 μm-wavelength. The switching efficiency obtained was 7%. It was clarified that a longer pulse length of the control light compared to the grating length is required to obtain a large Bragg wavelength shift for the switching. It was also clarified that the Bragg wavelength shift is caused by a third order nonlinear effect and a photothermal effect. A contribution of the photothermal effect was estimated. We also estimated the switching efficiency for pump power in the FGC switch.     

Ultrafast interband pumping of quantum‐cascade structures: A feasibility study of a THz pulse amplifier

Gain spectra of a three‐well GaAs/AlGaAs quantum‐cascade structure under interband pumping by ultrashort optical pulses are studied within the framework of the density matrix theory. A mathematical model for intersubband kinetics is derived taking into account many‐body interactions and ultrafast interband optical pumping. It is found that pulsed interband pumping leads to an increase in intersubband gain by an order of magnitude, which is characterised by 1 ps rise time and 8 ps recovery time under pumping by 100 fs pulses with a peak intensity of 100 MW/cm². Possible implementations of the concept are highlighted. The results uncover a new possibility for ultrafast switching of the material gain in quantum‐cascade structures.     

Mid-infrared spectroscopic studies and lasing in GaAs–AlGaAs quantum cascade devices

Complementary intersubband and interband optical measurements have been employed in order to study GaAs–AlGaAs quantum cascade light-emitting diode and laser structures. Using these techniques, we have measured the redistribution of electrons throughout the bridging regions and upper states in the active regions of the diode device with increasing bias. The high quality of the sample gives very narrow line widths in the optical spectra, permitting the resolution of transitions involving closely spaced energy levels. This has allowed the direct observation of level alignment at the onset of current flow through the device. In addition, stimulated emission at λ=9.7 μm has been observed from a GaAs–AlGaAs laser structure under pulsed operation. A threshold current density of 6.5 kA/cm² and peak power 300 mW are measured at 10 K and lasing operation is observed up to 200 K.