Tensile-strained Quantum Wells Research Articles

Semiconductor Optical Amplifiers with Wide Gain Bandwidth and Enhanced Polarization Insensitivity Based on Tensile-Strained Quantum Wells.

The paper presents a wide-bandwidth, low-polarization semiconductor optical amplifier (SOA) based on strained quantum wells. By enhancing the material gain of quantum wells for TM modes, we have extended the gain bandwidth of the SOA while reducing its polarization sensitivity. Through a combination of tilted waveguide design and cavity surface optical thin film design, we have effectively reduced the cavity surface reflectance of the SOA, thus decreasing device transmission losses and noise figure. At a wavelength of 1550 nm and a drive current of 1.4 A, the output power can reach 188 mW, with a small signal gain of 36.4 dB and a 3 dB gain bandwidth of 128 nm. The linewidth broadening is only 1.032 times. The polarization-dependent gain of the SOA is below 1.4 dB, and the noise figure is below 5.5 dB. The device employs only I-line lithography technology, offering simple fabrication processes and low costs yet delivering outstanding and stable performance. The designed SOA achieves wide gain bandwidth, high gain, low polarization sensitivity, low linewidth broadening, and low noise, promising significant applications in the wide-bandwidth optical communication field across the S + C + L bands.

Analysis of Threshold Current Behavior for Bulk and Quantum-Well Germanium Laser Structures

We analyze the optical gain of tensile-strained, n-germanium (n-Ge) material taking bandgap narrowing (BGN) for heavily doped Ge into account. Both the direct bandgap and indirect bandgap are narrowed by 60 meV. Our new modeling explains the wide lasing spectrum of 1520-1700 nm in electrically pumped Ge lasers. The calculated materials gain can reach 1000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> when the injected carrier density is ~mid-10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> with a combination of 0.25% tensile strain and 4.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> n-type doping. The threshold current density is estimated to be 0.53 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for an optimized edge-emitting double-heterojunction Ge device, comparable to bulk III-V lasers. We also review current progress on Ge quantum-well (QW) structures. The threshold current density of most Ge QW structures is similar to bulk Ge. Only tensile-strained QWs show reduced threshold currents.

Near-field analysis of metallic DFB lasers at telecom wavelengths

We image in near-field the transverse modes of semiconductor distributed feedback (DFB) lasers operating at λ ≈ 1.3 μm and employing metallic gratings. The active region is based on tensile-strained InGaAlAs quantum wells emitting transverse magnetic polarized light and is coupled via an extremely thin cladding to a nano-patterned gold grating integrated on the device surface. Single mode emission is achieved, which tunes with the grating periodicity. The near-field measurements confirm laser operation on the fundamental transverse mode. Furthermore--together with a laser threshold reduction observed in the DFB lasers--it suggests that the patterning of the top metal contact can be a strategy to reduce the high plasmonic losses in this kind of systems.

Loss and Gain Measurements of Tensile-Strained Quantum Well Diode Lasers for Plasmonic Devices at Telecom Wavelengths

We performed an experimental study of tensile strain quantum wells for diode lasers operating at telecom wavelengths and in transverse magnetic polarization. The optical losses of ridge waveguide resonators are measured with passive and active techniques. The results are consistent with the theory and provide a useful material parameter database for this kind of devices. With the aim of implementing active plasmonic devices we studied the effect of the proximity of the metal top contact to the laser active region.

A spot-size converter-integrated 1.3 µm TM mode LD for coupling with surface-plasmon polariton waveguides

A 1.3 µm transverse magnetic (TM) mode laser diode (LD) with a spot-size converter was fabricated and its various characteristics were investigated. At 100 mA, the power ratio of 33 dB was obtained between TM and TE mode using 0.68% tensile strained quantum wells. The device shows far field angles of 10.4° and 11.7° in horizontal and vertical directions, respectively, resulting in 3.1 dB coupling loss with a surface-plasmon polariton (SPP) waveguide (WG). The 3 dB alignment tolerance with the SPP-WG is 7.2 µm and 6.8 µm, respectively. 3 dB bandwidth is 6.3 GHz and clear eye-opening is confirmed under 7 Gbps NRZ transmission.

Tight-binding calculation of optical gain in tensile strained [001]-Ge/SiGe quantumwells

It is known that under a tensile strain of about 2% of the latticeconstant, the energy of the bottom conduction state of bulk Ge at theΓ point falls below the minimum at the L point, leading to a direct gapmaterial. In this paper we investigate how the same condition is realizedin tensile strained Ge quantum wells. By means of a tight-bindingsp3d5s* model, we studytensile strained Ge/Si0.2Ge0.8 multiple quantum well (MQW) heterostructures grown on a relaxed SiGeSn alloy bufferalong the [001] direction. We focus on values of the strain fields at the crossover betweenthe indirect and direct gap regime of the MQWs, and calculate band edge alignments,electronic band structures, and density of states. We also provide a numericalevaluation of the MQW material gain spectra for TE and TM polarization underrealistic carrier injection levels, taking into account the leakages related to theoccupation of the electronic states at the L point. The analysis of the differentorbital contributions to the near-gap states of the complete structure allows usto give a clear interpretation of the numerical results for the strain-dependentTM/TE gain ratio. Our calculations demonstrate the effectiveness of the structures underconsideration for light amplification.

The response of semiconductor optical amplifiers containing lateral composition modulation in the quantum wells

Tensile strained quantum wells are used to change the polarization sensitivity of semiconductor optical amplifiers. However, lateral composition modulation (LCM), which is present in the quantum wells of the device, is shown to impact the device performance. A broadening of the spectral gain is seen as recombinations from the conduction band to the heavy hole band in the In-rich regions of the LCM. This additional component to the gain from the quantum well will reduce the device gain efficiency at a specific wavelength for a given current, or it may be used to broaden the gain spectrum of an individual quantum well.

Band parameters and electronic structures of wurtzite ZnO and ZnO∕MgZnO quantum wells

The band structures of wurtzite ZnO are calculated by the empirical pseudopotential method (EPM). The eight parameters of the Zn and O atomic pseudopotential form factors are obtained with the formula of Schluter et al.[Phys. Rev. B 12, 4200 (1975)]. The band parameters are extracted by using a k∙p Hamiltonian to fit the EPM results. The calculated band-edge energies (Eg, EA, EB, and EC) at the Γ point are in good agreement with the experimental results. Based on the band parameters obtained, valence subbands of wurtzite ZnO∕MgxZn1−xO tensile-strained quantum wells with different well widths and Mg compositions are calculated by the six-band k∙p method.

Electronic structures of wurtzite ZnO and ZnO/MgZnO quantum well

The band structures of wurtzite ZnO are calculated using the empirical pseudopotential method (EPM). The 8 parameters of the Zn and O atom pesudopotential form factors with Schluter's formula are obtained. The effective mass parameters are extracted by using k.p Hamiltonian to fit the EPM results. The calculated band edge energies ( E g, E A, E B, and E C) at Γ point are in good agreement with experimental results. The ordering of ZnO at the top of valence band is found to be A(Γ 7)–B(Γ 9)–C(Γ 7) due to a negative spin–orbit (SO) splitting. Based on the band parameters obtained, the valence hole subbands of wurzite ZnO/Mg x Zn 1− x O tensile-strained quantum wells (QWs) with different well widths and Mg compositions are calculated using 6-band k.p method.

Tunneling injection quantum-dot lasers with polarization-dependent photon-mediated carrier redistribution and gain narrowing

A theoretical and experimental study of a particular transverse-electric (TE) mode lasing mechanism of a tunneling injection InP quantum-dot (QD) laser is reported. In the experiment, the TE mode lasing action takes place at the first excited state of InP biaxially compressively strained QDs. This QD state is coupled to the ground state of two tensile-strained InGaP quantum wells (QWs) although the tensile-strained QW structure favors the transverse-magnetic (TM) polarization light emission. The measured TE and TM modal gain spectra show a typical QW gain evolution behavior at low injection currents, which can be theoretically modeled by the quasi-equilibrium of carrier distribution. When the injection current is increased near threshold, a TE gain narrowing and a simultaneous TM gain pinning are observed in the measured modal gain spectra, which cannot be explained via the quasi-equilibrium model. We propose a polarization-dependent photon-mediated carrier redistribution in the QD-coupled-QW structure to explain this TE and TM gain evolution behavior. When the injection current is just below threshold, the strong carrier depletion via stimulated emission due to coupling between the InP QD and InGaP QW states plays an important role in carrier redistribution, which depends on the optical transition energy and polarization. This concept of the polarization-dependent photon-mediated carrier redistribution explains the TE gain narrowing and TM gain pinning behavior. In addition, a coupled rate equation model is established, and the calculated polarization power ratio based on the coupled rate equations explains the experimental observation.

Gain narrowing and output behavior of InP-InGaAlP tunneling injection quantum-dot-well laser

Polarization-resolved amplified spontaneous emission (ASE) and gain from tensile-strained multiple quantum wells (QWs) coupled to a single layer of compressively strained quantum dots (QDs) show interesting output characteristics. Low current injection reveals transverse electric polarized ASE from the QD ground state and QD-coupled-QW state. Additionally, transverse magnetic ASE from the QW state is observed. The modal gain of this laser shows coupled active state activation which is evident by spectral narrowing and change from QW-like to QD-like spectrum.

Band structure calculation of InGaAsN//GaAs, InGaAsN//GaAsP//GaAs and InGaAsN//InGaAsP//InP strained quantum wells

The authors have calculated the band structure of 1.3-μm InGaAsN/GaAs(N)/GaAs compressively strained quantum wells (QW) using the band anticrossing model and an eight-band k.p Hamiltonian. The calculated interband optical transition energies have been compared to the experimental ones deduced from photocurrent. Because of the high compressive strain in the QW, strain-compensated structures may be required in order to grow stable multiple QWs; with this in view, the band structure of InGaAsN/GaAsP/GaAs QWs emitting at 1.3 μm was studied. Dilute nitride structures also offer the possibility of growing tensile-strained QW lasers emitting at 1.55 μm on an InP substrate. In order to evaluate the potentialities of such structures, the band characteristics of InGaAsN/InGaAsP/InP heterostructures were determined with a TM-polarised fundamental transition around 1.55 μm.

MOVPE growth of 1310?nm polarizationinsensitive strained quantumwell semiconductor optical amplifiers*

The compressivelystrained quantum wells, tensilestrained quantum wells and the combination of tensilestrained and compressivelystrained quantum wells were grown by metal organic vapor phase epitaxy, the materials properties were meas ured by photoluminescence spectra and x-ray doublecrystal diffraction. A polarizationinsensitive multiplequantumwell optical amplifier for 1310nm wavelength employing both compressively strained wells and tensilestrained wells in active region is reported. The amplifier was fabricated to ridge waveguide structure with 7° tilted cavity, the two facets were coated with antireflection thin films. The amplifier exhibited an excellent polarization insensitivity (less than 0.6dB) over the entire range of wavelength (1.28-1.34μm) and a fiber to fiber gain of 21.5B at bias current of 200mA.

Theoretical analysis of diffused quantum-well lasers and optical amplifiers

Diffused quantum-well (QW) distributed feedback (DFB) lasers and optical amplifiers will be theoretically analyzed in this paper. For DFB lasers, a design rule will be proposed and the validity of the design rule will be discussed with respect to changes in the injected carrier density. The range of grating period, which can be used in the design, is discussed. As a consequence, the maximum tuning range of the emission wavelength can be estimated without involving the time-consuming self-consistent simulation. The features of polarization independence of optical amplifiers achieved by using diffused QWs are also discussed. Our theoretical results successfully explain why polarization independence can achieve in the long-wavelength tail of the modal gain and absorption coefficient but not at photon energies above the transition edge. This explanation applies to other tensile-strained QWs for polarization-independent applications. The understanding is crucial for optimizing polarization-independent devices. To conclude, our analysis of the diffused QW optical devices demonstrates that QW intermixing technology is a practical candidate for not only realizing monolithic photonic integrated circuit, but also enhancing optical device performance.

Polarization insensitive gain medium with hybrid strained quantum well

A polarization insensitive gain medium for optical amplifiers has been fabricated. The active layer is a structure with alternate tensile and compressive strain quantum wells. The waveguide is made into a taper with angled facets. In the experiment we found that the structure can suppress the lasing and decrease the polarization sensitivity. The gain imbalance between transverse electric and transverse magnetic gains is small, and 0.1 dB is obtained at a driving current of 100 mA . The full-width at half-maximum of amplified spontaneous emission is 40 nm within large current.

Excitonic Properties of ZnS Quantum Wells in ZnMgS

The excitonic properties of cubic ZnS quantum wells in ZnMgS are studied by reflectivity and magneto-optics. A remarkable improvement in the quality of the samples grown by MBE on GaP substrates has allowed the observation of heavy and light hole exciton transitions with values for the FWHM as narrow as 5 meV. The 2s state of the heavy hole exciton is identified and exciton binding energies as high as 55 meV are deduced, indicating that for quantum wells narrower than 3.5 nm the exciton-LO phonon scattering can be suppressed. Zeeman splittings of the order of 10 meV for both the light and heavy hole exciton transitions appear in magneto-reflectivity spectra in magnetic fields up to 54 T. Large light hole exciton g-values of the order of four for all quantum wells are obtained due to the light hole being the uppermost valence band in these tensile strained quantum wells. A strong reduction in the diamagnetic shifts for narrow wells is observed due to increasing quantum confinement.

Excitonic properties of ZnS quantum wells

The excitonic properties of cubic ZnS quantum wells in ZnMgS are studied by reflectivity and magneto-optics. A remarkable improvement in the quality of the samples grown by molecular-beam epitaxy on GaP substrates has allowed the observation of heavy- and light-hole exciton transitions with values for the full width at half maximum as narrow as 5 meV. The $2s$ state of the heavy-hole exciton is identified and exciton binding energies of as high as 55 meV are deduced, indicating that for quantum wells narrower than 3.5 nm the exciton-LO phonon scattering can be suppressed. Zeeman splittings of the order of 10 meV for both the light- and heavy-hole exciton transitions appear in magnetoreflectivity spectra in magnetic fields up to 54 T. Large light-hole exciton g values of the order of 4 for all quantum wells are obtained due to the light hole being the uppermost valence band in these tensile-strained quantum wells. A strong reduction in the diamagnetic shifts for narrow wells is observed due to increasing quantum confinement.

Valence Subband Structures and Optical Properties of Strain-Compensated Quantum Wells

The strain compensation effects on the valence subbands and on the optical properties of GaInAs/AlGaInAs quantum well structures are theoretically studied for the first time. In the case of compressive-strained quantum wells, where the top valence subbands are always formed with heavy hole (HH) subbands, the compensatingly tensile-strained barriers shift the first light hole (LH) subbands upward increasing the valence band mixing between them, and significantly reducing the transverse electric (TE) gain. In contrast, in the tensile-strained quantum wells whose top valence subband is formed with LH subband, the compensatingly compressive-strained barriers shift the top LH subband downward and on some occasions the top LH subband is replaced with the first HH one. The increase of the TE gain is relatively small due to the strong valence band mixing. The strain of the barrier layers is found to play an important role in the valence subband structures and optical properties.

Theoretical Analysis of Polarization Insensitive InGaAsP Multiple-Quantum-Wells Eletroabsorption Modulators with Negative Chirp

Polarization sensitivity of InGaAsP electroabsorption (EA) modulators is analyzed by the fractional-dimensional approach. Chirping parameter α is then calculated using Kramers-Krönig relations. It is found that polarization-insensitive and negative chirp operation can be realized simultaneously for InGaAsP multiple-quantum-wells (MQW) EA modulators with optimized well width and amount of strain. We propose a polarization-insensitive InGaAsP EA modulator with 9-nm-wide 0.38% tensile strained quantum wells. The chirping parameter α is estimated to be around -2 for the first 3 dB of transmission, while the modulation characteristics remain polarization-independent up to 80 kV/cm, corresponding to an extinction ratio of over 15 dB.

Exciton dynamics and valence band mixing in tensile-strained semiconductor quantum wells

We have investigated the influence of the electronic band structure on the exciton dynamics in GaAsP tensile-strained quantum wells. We have found that the exciton cooling time is notably reduced when the heavy- and light-hole excitons are degenerate. The lifetime of the heavy-hole exciton is ~300 ps whereas it is ~500 ps for the light-hole exciton. Furthermore, we have determined, from the initial degree of polarization of the emission, the valence-band mixing as a function of the energy splitting between the heavy-hole and light-hole subbands. The degree of mixing is in qualitative agreement with tight-binding calculations.