Optical Modal Gain Research Articles

Optical Gain of Vertically Coupled Cd0.6Zn0.4Te/ZnTe Quantum Dots.

The optical modal gain of Cd0.6Zn0.4Te/ZnTe double quantum dots was measured using a variable stripe length method, where large and small quantum dots are separated with a ZnTe layer. With a large (~18 nm) separation layer thickness of ZnTe, two gain spectra were observed, which correspond to the confined exciton levels of the large and small quantum dots, respectively. With a small (~6 nm) separation layer thickness of ZnTe, a merged single gain spectrum was observed. This can be attributed to a coupled state between large and small quantum dots. Because the density of large quantum dots (4 × 1010 cm-2) is twice the density of small quantum dots (2 × 1010 cm-2), the density of the coupled quantum dots is determined by that of small quantum dots. As a result, we found that the peak gain (123.9 ± 9.2 cm-1) with the 6 nm separation layer is comparable to that (125.2 ± 29.2 cm-1) of the small quantum dots with the 18 nm separation layer.

1.3 μm optically-pumped monolithic VCSEL based on GaAs with InGa(Al)As superlattice active region

Неre we report a monolitic long-wavelength vertical cavity surface-emitting laser based on GaAs with bottom GaAs/AlGaAs distributed Bragg reflectors (DBRs), metamorphic optical cavity with In0.41Ga0.59As/In0.25Ga0.75As-active region providing emission near 1.3 μm with high modal optical gain and top dielectric SiO2/Ta2O5 DBRs. We achieve continious wave single-mode operation at room temperature under optical pumping. The proposed approach has potential for high output power and high temperature (up to 200 °C) stable operation.

Room-temperature Near-infrared Excitonic Lasing from Mechanically Exfoliated InSe Microflake.

The development of chip-level near-infrared laser sources using two-dimensional semiconductors is imperative to maintain the architecture of van der Waals integrated optical interconnections. However, the established two-dimensional semiconductor lasers may have either the disadvantages of poor controllability of monolayered gain media, large optical losses on silicon, or complicated fabrication of external optical microcavities. This study demonstrates room-temperature near-infrared lasing from mechanically exfoliated γ-phase indium selenide (InSe) microflakes free from external optical microcavities at a center wavelength of ∼1030 nm. The lasing action occurs at the sub-Mott density level and is generated by exciton-exciton scattering with a high net modal optical gain of ∼1029 cm-1. Moreover, the lasing is sustained for microdisks fabricated by a simple laser printing with a reduced threshold. These results suggest that InSe is a promising material for near-infrared microlasers and can be employed in a wide range of applications, including imaging, sensing, and optical interconnects.

Polarization Angle Dependence of Optical Gain in a Hybrid Structure of Alexa-Flour 488/M13 Bacteriophage

We measured optical modal gain of a dye–virus hybrid structure using a variable stripe length method, where Alexa-fluor-488 dye was coated on a virus assembly of M13 bacteriophage. Inspired by the structural periodicity of the wrinkle-like virus assembly, the edge emission of amplified spontaneous emission was measured for increasing excited optical stripe length, which was aligned to be either parallel or perpendicular to the wrinkle alignment. We found that the edge emission showed a strong optical anisotropy, and a spectral etalon also appeared in the gain spectrum. These results can be attributed to the corrugated structure, which causes a similar effect to a DFB laser, and we also estimated effective cavity lengths.

High Optical Gain of Solution‐Processed Mixed‐Cation CsPbBr3 Thin Films towards Enhanced Amplified Spontaneous Emission

AbstractA solution‐processed thin film made of all‐inorganic CsPbBr3 perovskite is a promising candidate for low‐cost and flexible green‐color lasers. However, the amplified spontaneous emission (ASE) of solution‐processed CsPbBr3 films still experiences a high threshold owing to poor morphology and insufficient optical gain. Here, a multiple‐cation doping strategy is demonstrated to develop compact, smooth thin films of Cs0.87(FAMA)0.13PbBr3/(NMA)2PbBr4 (FA: formamidinium; MA: methylammonium; NMA: naphthylmethylammonium) with a record high net modal optical gain of ≈3030 cm−1 and low propagation loss of 1.0 cm−1. The FA and MA cations improve the crystallization kinetics to form continuous films, and the NMA cations reduce the grain dimension, increase film dispersibility/uniformity, and enhance spatial confinement to promote optical gain. Room‐temperature ASE is demonstrated under a low threshold of ≈3.8 µJ cm−2 without degradation after four months of storage in glove box or excitation by 3 × 107 laser pulses. These findings provide insights into enhancing the optical gain and lowering the threshold of perovskite lasers in terms of molecular synthesis and microstructure engineering.

Determination of the saturation length and study of its effects in optical gain measurements of organic semiconductors using the variable stripe length method

Three indicators for the determination of the saturation length limiting the validity of the variable stripe length method are presented. Depending on the maximum stripe length being used for the nonlinear fit, the resulting modal gain, its confidence interval, and the spectral full width at half maximum vary and give clear indications on the saturation length. Exemplarily, a blend of organic semiconducting spiro-compounds has been measured to present the importance of this length and demonstrate its determination. Subsequently, the optical modal gain is calculated with consideration of the saturation length. Both the saturation length and modal gain are shown to be dependent on the pump energy but result in a constant gain-length product of 2.2 ± 0.2, independent of the pump energy. Additionally, spectral gain profiles for different pump energies are presented with a maximum net modal gain of (128±4) cm−1 found at (2.98±0.09) mJ/cm2.

Optimization of optical characteristics of In0.29Ga0.71As0.99N0.01/GaAs straddled nano-heterostructure

Optical gain and Modal gain with Lasing wavelength for In 0.29 Ga 0.71 As 0.99 N 0.01 /GaAs Straddled Heterostructures. Designing of a nanoscale Quantum Well (QW) heterostructure with a well thickness of ∼60 Å is critical for many applications and remains a challenge. This paper has a detailed study directed towards designing of In 0.29 Ga 0.71 As 0.99 N 0.01 /GaAs straddled nanoscale-heterostructure having a single QW of thickness ∼60 Å and optimization of optical and lasing characteristics such as optical and mode gain, differential gain, gain compression, anti-guiding factor, transparency wavelength, relaxation oscillation frequency (ROF), optical power and their mutual variation behavior. The outcomes of the simulation study imply that for the carrier concentration of ∼2 × 10 18 cm −3 the optical gain of the nano-heterostructure is of 2100 cm −1 at the wavelength is of 1.30 μm. Though the obtained gain is almost half of the gain of InGaAlAs/InP heterostructure, but from the wavelength point of view the InGaAsN/GaAs nano-heterostructure is also more desirable because the 1.30 μm wavelength is attractive due to negligible dispersion in the silica based optical fiber. Hence, the InGaAsN/GaAs nano-heterostructure can be very valuable in optical fiber based communication systems.

(Invited) Advanced Semiconductor Hetero-Nanocrystals for Lasing

Colloidal semiconductor nanocrystals (NCs) with their highly versatile opto-electronic properties offer exciting prospects for light-emitting devices including electrically pumped light-emitting diodes and optically pumped lasers. Optical gain and lasing in these nanocrystals have been first demonstrated back in 2000, yet it has not been possible to achieve efficient optical gain until very recently. In the last years, structurally-engineered and composition-tuned nanocrystals have been developed, which could substantially suppress the undesired nonradiative loss mechanisms (e.g., Auger recombination). These innovative NC architectures have overcome the long-standing challenges inhibiting achievement of efficient optical gain in these materials and showed unprecedented optical gain performance. Here, we will highlight our recent results on the achievement of ultralow gain thresholds, record high modal gain coefficients and long gain lifetimes using interface alloyed core/shell quantum dots and heterostructured colloidal nanoplatelets [1-5]. By promoting strong excitonic properties in colloidal nanoplatelets, we realized modal gain coefficients that could exceed 1000 cm-1 (see Fig. 1) [1, 5], better than those of the conventional semiconductor materials reported to date. These materials further enabled us to realize ultralow threshold lasing under both single- and two-photon absorption regimes with thresholds of 20 µJ/cm2 [4] and 700 µJ/cm2 [2], respectively. Moreover, we showed remarkably long gain lifetimes reaching 0.5 ns [5], almost an order of magnitude better than what has been commonly observed from conventional nanocrystals. Using these materials, we developed all-solution-processed and all-colloidal lasers via employing colloidal cavities (see Fig. 1) [1, 2]. These lasers show ultralow threshold gain under two-photon absorption that is highly promising for bio imaging. Figure 1. Heterostructured colloidal nanoplatelets for record-high modal optical gain coefficients. All-colloidal nanocrystal lasers that are fabricated via solution-processing techniques.

Optical gain tuning within IR region in type-II In0.5Ga0.5As0.8P0.2/GaAs0.5Sb0.5 nano-scale heterostructure under external uniaxial strain

Quaternary and ternary alloy semiconductors offer an extra degree of flexibility in terms of bandgap tuning. Modifications in the wave functions and alterations in optical transitions in quaternary and ternary QW (quantum well) heterostructures due to external uniaxial strain provide valuable insights on the characteristics of the heterostructure. This paper reports the optical gain in strained InGaAsP/GaAsSb type-II QW heterostructure (well width = 20 Å) under external uniaxial strain at room temperature (300 K). The entire heterostructure is supposed to be grown on InP substrate pseudomorphically. Band structure, wave functions, energy dispersion and momentum matrix elements of the heterostructure have been computed. 6 × 6 diagonalised k→·p→ Hamiltonian matrix of the system is evaluated and Luttinger-Kohn model has been applied for the band structure and wavefunction calculations. TE mode optical gain spectrum in the QW-heterostructure under uniaxial strain along [110] is calculated. Optical gain of the heterostructure as a function of 2D carrier density and temperature variation is investigated. The variation of the peak optical gain as a function of As and Sb fractions in InGaAsP as a barrier and GaAsSb as a well respectively is exhibited. For a charge carrier injection of 5×1012/cm2, the TE optical gain is 3952 cm−1 at room temperature under no external uniaxial strain. Significant increase in TE mode optical gain is observed under high external uniaxial strain (1, 5 and 10 GPa) along [110] within IR (Infrared region) region.

Selective growth of strained (In)GaAs quantum dots on GaAs substrates employing diblock copolymer lithography nanopatterning

Semiconductor laser diodes (LD) were demonstrated employing a strained (In)GaAs quantum dot (QD) active region grown by metalorganic vapor phase epitaxy (MOVPE) on nominally exact (100) GaAs substrates using selective area epitaxy (SAE). The SAE QD growth employed a SiNx nano-patterned mask defined by diblock copolymer (BCP) lithography. In-situ etching using carbon tetrabromide (CBr4), prior to the SAE of the QDs, was shown to be effective to remove the processing-related damage introduced during the nanopattern transfer process, resulting in a significant reduction in the threshold current density of the LD under the optimal in-situ etching condition. Furthermore, the modal optical gain parameter and the transparency current density were extracted by the conventional cavity length analysis (CLA) on LD devices where the QD was grown with the optimal in-situ etching condition.

The Optical Gain of a Si-Based Lattice-Matched Si0.15Ge0.621Sn0.229/Si0.637Ge0.018Sn0.345 MQW Laser

We study the optical-gain characteristics of a Si-based MQW laser, in which the active region has 20 Si0.15Ge0.621Sn0.229 quantum wells separated by 20 Si0.637Ge0.018Sn0.345 barriers. We reach a maximum optical gain of 2300 cm−1 with an estimated carrier concentration of 5·1018 cm−3, which is equivalent to the transparent current density equal to 0.5 kA/cm2. Furthermore, we discuss the optical confinement factor and modal gain. The modal gain depends sensitively on the number of the quantum wells (QWs), and this fact restricts the optical confinement factor. The modal gain of the model we proposed can reach 1500 cm−1 at the injection current density equal to 3 kA/cm2. We hope that our results show the possibility to obtain a Si-based near-infrared laser.

Theoretical estimation of optical gain in Tin-incorporated group IV alloy based transistor laser

We have calculated the electronic band structure and polarization dependent optical gain in a strain balanced SixGeySn1−x−y/GezSn1−z based transistor laser (TL) with GezSn1−z single quantum well (QW) in the base. Design consideration for QW is also addressed to ensure moderate carrier and optical confinement. A significant TE mode optical gain is obtained in mid infra red region for the transition of Г valley conduction band to heavy hole valence band. Optical gain in the QW plays an important role in determining the optical characteristics of Tin (Sn) incorporated group IV material based TL.

Deep-UV optical gain in AlGaN-based graded-index separate confinement heterostructure

We propose AlGaN-based GRaded-INdex Separate Confinement Heterostructure as a candidate for electrically pumped deep-UV semiconductor laser. Strong compositional fluctuations were introduced in the active region (75 nm-thick Al0.72Ga0.28N film grown by RF Plasma- assisted Molecular Beam Epitaxy) to obtain net modal optical gain peaked at 257 nm in excess of 80 cm −1 . We measured an optical gain threshold of 14 µJ/cm 2 . Because of polarization-doping of the compositionally graded AlGaN regions, which automatically leads to the formation of p-i-n junction, these results pave the way for lasing under electrical injection.

All‐Around SiN Stressor for High and Homogeneous Tensile Strain in Germanium Microdisk Cavities

An all-around stressor approach is presented to introduce homogeneous and high tensile strain, both in-plane and vertically, into n-doped germanium microdisks. A maximum biaxial strain up to 1.5% is achieved, close to the requirement to obtain a direct band gap germanium. The possibilities to obtain in this configuration modal optical gain in germanium microdisk cavities are discussed.

Low gain threshold density of a single InGaP quantum well sandwiched by digital alloy

A single In0.49Ga0.51P quantum well sandwiched by In0.49Ga0.51P/In0.49(Ga0.6Al0.4)0.51P digital alloy structures was investigated in terms of optical modal gain, where gain saturation effects were also considered for both changes in wavelength and stripe length by using a modal gain contour map analysis. We found the gain threshold density is considerably lower by an order of magnitude when compared to the Mott density (∼2 × 1012 cm−2), which can be attributed to a carrier-harvesting effect through the mini-band of the digital alloy.

Optical gain in single tensile-strained germanium photonic wire

We have investigated the optical properties of tensile-strained germanium photonic wires. The photonic wires patterned by electron beam lithography (50 μm long, 1 μm wide and 500 nm thick) are obtained by growing a n-doped germanium film on a GaAs substrate. Tensile strain is transferred in the germanium layer using a Si₃N₄ stressor. Tensile strain around 0.4% achieved by the technique corresponds to an optical recombination of tensile-strained germanium involving light hole band around 1690 nm at room temperature. We show that the waveguided emission associated with a single tensile-strained germanium wire increases superlinearly as a function of the illuminated length. A 20% decrease of the spectral broadening is observed as the pump intensity is increased. All these features are signatures of optical gain. A 80 cm⁻¹ modal optical gain is derived from the variable strip length method. This value is accounted for by the calculated gain material value using a 30 band k · p formalism. These germanium wires represent potential building blocks for integration of nanoscale optical sources on silicon.

Band structures and optical gain of strained GaAsxP1−x−yNy/GaP quantum wells

In the framework of effective mass Hamiltonian of semiconductor quantum well structures and band anticrossing model, we investigated the band structures of fully strained GaAsxP1−x−yNy/GaP quantum wells. The GaAsxP1−x−yNy could be widely modified to be direct-band gap or indirect-band gap by changing the mole fraction of As and N in the well layer. We found that an increase in the N mole fraction in the well layer increases the TE mode optical gain very slightly.

Polarization asymmetry and optical modal gain saturation via carrier–photon interaction in ZnO

The polarization dependence of modal gain was examined in ZnO using a variable stripe length method, where the transverse electric (TE) mode gain was dominant over the transverse magnetic (TM) mode gain due to the polarization asymmetry of the wurtzite structure. Modal gain saturation was also investigated using a modal gain contour map for the wavelength and stripe length. The TE modal gain and degree of polarization increased with increasing stripe length up to a threshold length (∼100 μm). At longer stripe lengths, rapid reduction in the carrier density along the stripe resulted in gain saturation and a spectral redshift.

Gain analysis of optically-pumped Si nanocrystal waveguide amplifiers on silicon substrate

The SiO(2)/SiO(x)/SiO(2) strip-loaded waveguide on Si substrate with buried Si nanocrystals (Si-ncs) in SiO(x) layer is demonstrated to show the Si-nc dependent optical gain. The amplified spontaneous emission (ASE) spectrum at 750-850 nm is observed with central wavelength of 805 nm and 3dB spectral linewidth of 140 nm. The optical net modal gain and loss coefficients of 85.7 cm(-1) and 21 cm(-1), respectively, are determined from the waveguide length dependent ASE intensity. By attenuating 785-nm laser diode signal to inject the pumped SiO(2)/SiO(x)/SiO(2) strip-loaded waveguide, a small-signal power gain of 13.5 decibel (dB) is obtained. Increasing the laser diode power shows a significantly reduced power gain with a saturated output power due to the finite density of the optically pumped Si-ncs. The fitting of power-dependent gain with a gain-saturated amplifier model reveals a peak gain of 35 dB and a saturation power of 1.1 nW for the SiO(2)/SiO(x):Si-nc/SiO(2)/Si strip-loaded waveguide. Similar output saturation is also observed with increasing pumping power. With the presence of optical gain in the optically pumped Si-ncs, the intended application will be the monolithic integration of the Si-nc based optical waveguide amplifier with the other on-board photonic integrated circuits for the future optical interconnect communication.

Polarization dependence of optical modal gain in ZnO

Polarization dependence of optical modal gain in ZnO