Near-infrared Intersubband Transitions Research Articles

Dramatic enhancement of near-infrared intersubband absorption in c-plane AlInN/GaN superlattices

We report substantial improvement of near-infrared (2–2.6 μm) intersubband absorption in c-plane AlInN/GaN superlattices grown by molecular beam epitaxy. Progress was obtained through optimization of AlInN growth conditions using an AlInN growth rate of 0.9-nm/min at substrate temperature of 550 °C, as well as by judiciously placing the charge into two delta-doping sheets. Structural characterization suggests that AlInN crystal quality is enhanced and interface roughness is reduced. Importantly, near-infrared absorption data indicate that the optical quality of the AlInN/GaN superlattices is now comparable with that of AlN/GaN superlattices designed to exploit near-infrared intersubband transitions.

Optical Strong Coupling between near-Infrared Metamaterials and Intersubband Transitions in III-Nitride Heterostructures

We present the design, realization, and characterization of optical strong light–matter coupling between intersubband transitions within a semiconductor heterostructures and planar metamaterials in the near-infrared spectral range. The strong light–matter coupling entity consists of a III-nitride intersubband superlattice heterostructure, providing a two-level system with a transition energy of ∼0.8 eV (λ ∼1.55 μm) and a planar “dogbone” metamaterial structure. As the bare metamaterial resonance frequency is varied across the intersubband resonance, a clear anticrossing behavior is observed in the frequency domain. This strongly coupled entity could enable the realization of electrically tunable optical filters, a new class of efficient nonlinear optical materials, or intersubband-based light-emitting diodes.

Pulsed metal-organic chemical vapor deposition of high-quality AlN/GaN superlattices for near-infrared intersubband transitions

A pulsed metal-organic chemical vapor deposition technique is developed for the growth of high-quality AlN/GaN superlattices (SLs) with intersubband (ISB) transitions at optical communications wavelengths. Tunability of the AlN and GaN layers is demonstrated. Indium is shown to improve SL surface and structural quality. Capping thickness is shown to be crucial for ISB transition characteristics. Effects of barrier- and well-doping on the ISB absorption are reported.

Near-infrared two-color intersubband transitions in AlN/GaN coupled double quantum wells

A study on a four-energy-level system in asymmetric AlN/GaN coupled double quantum wells has been performed by solving Schrödinger and Poisson equations self-consistently. It is found that the transition selection rule is recovered when the first two subband pairs resonate in the four-energy-level system. The anticrossing gap between the second excited state (2odd) and the third excited state (2even) can be up to 135 meV when the Al composition of the central barrier is 0.80. The absorption coefficient of intersubband transition (ISBT) between the ground state (1odd) and the 2even subband is approximately equal to that between the first excited state (1even) and the 2odd subband. The wavelengths of the 1odd-2even and the 1even-2odd ISBTs are 1.31 and 1.55 μm, respectively. The results give possible application to the ultrafast two-color optoelectronic devices operating within optical communication wavelength range.

Intersubband absorption in AlN∕GaN∕AlGaN coupled quantum wells

Al N ∕ Ga N ∕ Al Ga N coupled quantum wells grown by molecular beam epitaxy have been developed and characterized via intersubband absorption spectroscopy. In these structures, an AlGaN layer of sufficiently low Al content is used to achieve strong interwell coupling without the need for ultrathin inner barriers. At the same time, AlN is used in the outer barriers to provide the large quantum confinement required for near-infrared intersubband transitions. The composition of the inner barriers also provides a continuously tunable parameter to control the coupling strength. Double intersubband absorption peaks are measured in each sample, at photon energies in good agreement with theoretical expectations.

Optically nonlinear effects in intersubband transitions of GaN∕AlN-based superlattice structures

We report optically nonlinear processes related to near-infrared intersubband transitions in short period GaN∕AlN superlattices. The strong piezo- and pyroelectric effects in this material lead to intrinsic asymmetries in the electronic potential of the superlattice, and thus to strong nonlinearities of the optical susceptibility. Because of the large intersubband transition energy of nearly 1eV and the short lifetime of excited electrons in the upper quantum state, these nonlinear effects can be exploited for the fabrication of room temperature operated high-frequency detectors in the telecommunication wavelength range. At the same time, saturation effects due to resonant two-photon absorption could be observed.

Nonlinear optics with local phasematching by quantum-based meta-material

We propose a novel artificial medium, which locally provides both the optical nonlinearity and simultaneously the phasematching for the macroscopic build-up of the generated field. This is achieved by a special assembly of semiconductor quantum wells, which by careful design are both nonlinear and appropriately dispersive. A critical point for a practical realization of this meta-material is its wavelength tuning capability accomplished by applying bias voltage to the semiconductor structure, employing a voltage-controlled tunnelling-induced transparency mechanism. We theoretically demonstrate the quantum-based phasematched nonlinear meta-material principles using a specific structure comprised of GaN/AlN quantum wells providing the required near-infrared intersubband transitions. For this meta-material we estimate the enhanced second-order nonlinearity to be around 10 −8 mV −1 , while the phasematching, tuned by a single-volt bias improves the nonlinear coherence length from 40 μm up to practically infinity.

Nonlinear optical waveguides based on near-infrared intersubband transitions in GaN/AlN quantum wells

We present the design, fabrication, and characterization of III-nitride quantum-well waveguides optimized for nonlinear-optical switching via intersubband transitions. A dielectric structure consisting of an AlN lower cladding and a GaN cap layer allows minimizing the propagation losses while maintaining a large modal overlap with the quantum-well active layer. A strong nonlinear saturation of the intersubband absorption near 1.55 mum is demonstrated at record low input powers for these materials; in particular, a 3-dB saturation pulse energy of less than 10 pJ with 240-fs pulses is measured. Combined with the well established sub-picosecond recovery lifetimes of intersubband absorption in III-nitride quantum wells, these results are very promising for all-optical switching applications in future ultrafast fiber-optic communication networks.

Near infrared intersubband absorption in cubic GaN/AlN superlattices

ABSTRACTRoom temperature near-infrared intersubband transitions were observed in MBE grown non-polar cubic GaN/AlN superlattice structures. The peak wavelengths of these transitions were observed in the spectral region of 1.5–2.0 μm and were theoretically supported using a transfer matrix approach. All samples were unintentionally doped and grown on 3C-SiC substrates with a 100 nm GaN buffer. Each structure consisted of a 20 periods of GaN/AlN superlattice capped with 100nm of GaN. The thickness of the AlN barrier was fixed at 1.35, while the thickness of the GaN well was varied between 1.6 and 2.1nm. Electrochemical Capacitance Voltage (ECV) measurements allowed direct measurement of the intrinsic carrier concentration in a thick unintentionally doped cubic GaN layer, which confirmed sufficient population of the ground state energy level.

Near-infrared wavelength intersubband transitions in GaN∕AlN short period superlattices

Intersubband transitions in GaN∕AlN short period superlattices prepared by molecular beam epitaxy were investigated using the optical absorption technique. The peak position wavelengths of these transitions are found to span the spectral range of 1.35–2.90μm for samples cut into 45° waveguides with GaN quantum well thicknesses ranging between 1.70 and 2.41nm. The Fermi energy levels are estimated from the carrier concentrations, which were measured using an electrochemical capacitance-voltage profiler. The well widths were inferred from comparing the measured peak position energy of the intersubband transitions and the bound state energy levels calculated using the transfer matrix method.

Near-infrared intersubband transitions in InGaAs–AlAs–InAlAs double quantum wells

Intersubband optical transitions at short wavelengths in strain-compensated In0.70Ga0.30As—AlAs double quantum wells are investigated by means of mid-infrared absorption. Trade-offs between achieving a high transition energy and a large oscillator strength of the two highest-energy intersubband transitions using our strain-compensation approach are analyzed as a function of the widths of the two wells. Two design strategies leading to relatively strong intersubband optical transitions at 800 meV, 1.55μm, are described and the corresponding structures grown using gas-source molecular-beam epitaxy on (001)InP are investigated. The strongest intersubband transitions obtained experimentally are generally between 300 and 600 meV, 2–4μm. Significant oscillator strength, however, also extends out to 800 meV, 1.55μm.

Near-infrared intersubband transitions in delta-doped InAs∕AlSb multi-quantum wells

Intersubband transitions (ISBTs) in narrow InAs∕AlSb multiple quantum wells (MQWs) were investigated for well widths, d, ranging from 5 nm down to 1.8 nm with 10, 20, or 60 periods. In order to observe a strong ISBT signal, a heavy silicon doping was made in each InAs quantum well. Delta doping was employed for the narrowest wells to prevent silicon incorporation into the AlSb barrier layers. As the well width decreased, the ISBT signal of the MQWs decreased. However, it persisted down to d=2.1nm with a sheet doping density in each quantum well of 9×1012cm−2 and 60 periods. The ISBT signal observed for d=2.1nm was peaked at an energy of 650 and 670 meV at 300 and 77 K, respectively. These are the highest energy values ever observed for ISBTs in InAs∕AlSb MQWs.

Recent Progress in GaN-Based Superlattices for Near-Infrared Intersubband Transitions

A review of the recent progress in intersubband transitions in GaN-based superlattice structures grown by plasma-assisted molecular beam epitaxy (MBE) is presented. Careful control of the growth parameters resulted in reduced threading dislocation density as well as optimized interfaces for the superlattices. Intersubband absorption has been observed from a variety of configurations involving GaN single or asymmetric double quantum wells with either thick AlxGa1—xN barriers or short-period GaN/AlxGa1—xN superlattice barriers. The peak wavelength of absorption can be varied between 1.4–4.2 μm by changing the quantum well thickness. Electron scattering times were measured by the pump–probe technique and were found to be 240–330 fs at 1.55 μm. In addition, intersubband transitions have also been observed for GaN/AlN superlattices grown with a non-polar (110) orientation.

Sub-picosecond electron relaxation of near-infrared intersubband transitions in n-doped (CdS/ZnSe)/BeTe quantum wells

We report on intersubband (ISB) absorption and ultrafast ISB energy relaxation of carriers in n-type doped (CdS/ZnSe)/BeTe quantum wells (QWs), grown by molecular-beam epitaxy. The highly n-type doped QW samples were obtained by introducing a few monolayer CdS into a ZnSe/BeTe QW, and ISB absorption with a peak wavelength as short as 1.62 μm, covering 1.55 μm within its absorption bandwidth, was achieved. The ISB carrier relaxation was investigated by means of femtosecond (∼150 fs) one-color pump and probe technique at the ISB absorption peak. The ISB carrier relaxation time of 270 fs was observed in the sample with the absorption peak at 1.82 μm. The slow decay component with a time constant of a few ps, which has been observed in ZnSe/BeTe QWs, was not observed in the (CdS/ZnSe)/BeTe QWs, indicating that the Γ(ZnSe)–X(BeTe) electron transfer is suppressed, as expected from the band alignment.

Growth and Characterization of GaN/AlGaN Superlattices for Near-Infrared Intersubband Transitions

We report the results of the growth by molecular beam epitaxy of GaN/Al x Ga 1-x N superlattice (SL) structures with single or double quantum wells and thick Al x Ga 1-x N or short-period SL barriers. The dependence of intersubband absorption on the well width for single well SL structures was systematically investigated. For GaN quantum wells interleaved with SL barriers, the upper state electron confinement was maintained by Bragg reflection from the minigaps formed within the barriers. In addition, evidence of electron transfer from doped SL barriers to undoped GaN quantum wells was obtained. Abrupt interfaces and coherent periodicities of the SL structures were confirmed by transmission electron microscopy and X-ray diffraction measurements. Intersubband absorption at peak wavelengths between 1.4 and 4.2 μm has been observed.

Growth and Characterization of GaN/AlGaN Superlattices for Near-Infrared Intersubband Transitions

We report the results of the growth by molecular beam epitaxy of GaN/AlxGa1—xN superlattice (SL) structures with single or double quantum wells and thick AlxGa1—xN or short-period SL barriers. The dependence of intersubband absorption on the well width for single well SL structures was systematically investigated. For GaN quantum wells interleaved with SL barriers, the upper state electron confinement was maintained by Bragg reflection from the minigaps formed within the barriers. In addition, evidence of electron transfer from doped SL barriers to undoped GaN quantum wells was obtained. Abrupt interfaces and coherent periodicities of the SL structures were confirmed by transmission electron microscopy and X-ray diffraction measurements. Intersubband absorption at peak wavelengths between 1.4 and 4.2 μm has been observed.

Effect of thermally induced charged-carrier transfer on near-infrared intersubband transitions inInxGa1−xAs/AlAs/AlyAs1−ySb/InPcoupled quantum-well structure

We have investigated the near-infrared intersubband transition (ISBT) influenced by thermally induced charged-carrier transfer in strongly coupled ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{y}{\mathrm{As}}_{1\ensuremath{-}y}\mathrm{Sb}$ quantum-well structures lattice matched to InP substrates. We have compared the ISBT's in symmetric and asymmetric coupled quantum-well structures by investigating the temperature-induced change in the carrier distribution in the subband states. Qualitatively different behavior is found in symmetric and asymmetric coupled quantum-well structures. In these structures, the change in the built-in dc space-charge electric field due to charged carrier transfer results in a blueshift of the ground to the first excited state ISBT with increasing temperature. This blueshift along with the associated redshift of the higher excited state ISBT on increasing the temperature is larger in near-symmetric structures indicating a stronger coupling between the adjacent wells compared to the asymmetric structures. The thermal stability of ISBT in coupled structures is more compared to that in single quantum wells. The interface quality significantly affects the ISBT energy in these structures.

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.

Intersubband transitions and ultrafast all-optical modulation using multiple InGaAs-AlAsSb-InP coupled double-quantum-well structures

InP-based InGaAs-AlAs-AlAsSb coupled double-quantum-well structures have been optimized using wave-function engineering techniques to achieve near-infrared intersubband (ISB) transitions. Intersubband transitions at communication wavelengths of 1.3 and 1.55 /spl mu/m can be achieved in both near-symmetric and asymmetric coupled quantum-well structures by tailoring the well width, the central barrier width, and the carrier population in the conduction subband states by controlling either the doping level or the carrier temperature. We demonstrate ultrafast all-optical modulation of interband-resonant light at 1.3 /spl mu/m using intersubband-resonant light pulses at 1.55 /spl mu/m. An ultrafast absorption recovery time of 1.3 ps has been observed at 1.3 /spl mu/m, which can be reduced to 800 fs by probing at a higher energy above the Fermi level in the conduction band.

Absorption saturation of near-infrared intersubband transition in lattice-matched InGaAs/AlAsSb quantum wells

We report the near-infrared intersubband absorption characteristics in In 0.53Ga 0.47As/AlAs 0.56Sb 0.44 heterostructures lattice matched to InP substrate. The intersubband transition energy increases on reducing the quantum-well width and tends to saturate at around 0.65 eV on further reducing the InGaAs layer thickness due to the influence of the interface fluctuations. We present the first report on intersubband absorption saturation in InGaAs/AlAsSb QWs using a femtosecond light source (∼200 fs) tuned resonantly to the intersubband transition energy (0.52 eV). We observe a clear absorption saturation behavior of the intersubband transitions. At higher excitation power, a strong absorption due to interband two-photon transitions occurs. The interband two-photon transitions are induced by the intersubband transitions, as the band gap of the InGaAs well layers is resonant to the two-photon transition energy of the intersubband separation of this unique high-conduction band-offset InGaAs/AlAsSb quantum-well system.