Free Carrier Absorption Loss Research Articles

Investigation on the performance of deep ultraviolet edge emitting laser diodes using graded undoped AlGaN electron blocking layer (EBL)

P-AlGaN EBLs are usually used to control the overflow of electrons from the multiple quantum wells (MQWs) in AlGaN-based deep ultraviolet (DUV) edge-emitting laser diodes (EELDs). They're specially designed to prevent the excess flow of electrons from the MQW. This control is essential for maintaining the laser's efficiency and performance. Using optimized parameters of Al composition through theoretical calculations using Crosslight software helps certify the electrically driven EELD functions effectively within the desired operational range. In this study, the traditional p-AlGaN EBL within the electrically driven EELD is substituted with an undoped AlGaN EBL. This modification aims to raise the effective barrier height of the conduction band, and thus effectively stopping the electron leakage while improving the injection of holes. By optimizing the aluminum composition in the current research, efforts are made to lower the threshold current (Ith) and elevate the overall enactment of the graded undoped AlGaN EBL EELD. Various structural designs, including both conventional and undoped configurations, have been successfully created and analyzed in this study. Particular attention was given to investigating the impact of grading techniques applied to the electron-blocking layer. The graded undoped AlGaN EBL EELD performed better than the highly doped AlGaN EBL EELD due to the minimized free carrier absorption loss. Specifically, it shows higher slope efficiency (S.E) of 1.45 W/A and a significantly lower Ith of 790 mA. A new AlGaN EBL EELD design was tested with different layers. The ones with a graded undoped EBLs performed better than traditional AlGaN EBL EELD, improving power efficiency. The graded undoped EBLs setup also reduced the threshold current compared to traditional AlGaN EBL EELD.

The optical gain variation of Ge nanowires induced by L-valley splitting under the [110] direction stress

The electronic structures of Ge nanowires under the [110] direction stress are calculated via effective-mass k·p theory, and the results manifest eight equivalent L-valleys will be split into fourfold degenerate L 1-valleys and L 2-valleys. With increasing stress, the electron levels at the L 1-valleys and L 2-valleys can be pushed close to and away from those at the Γ-valley, respectively, which causes the appearance of a rising inflection point in the Γ-valley filling ratio and gain peak intensity at around 2.5 GPa stress. Moreover, we prove the positive net peak gain with small diameters is apt to be obtained considering the free-carrier absorption loss.

Harnessing self-heating effect for ultralow-crosstalk electro-optic Mach–Zehnder switches

This paper presents a novel approach to counterbalance free-carrier-absorption (FCA) in electro-optic (E-O) Mach–Zehnder interferometer (MZI) cells by harnessing the self-heating effect. We show insights on crosstalk limitations in MZIs with direct carrier-injection and provide a detailed design methodology on a differential phase shifter pair. Leveraging both free-carrier dispersion (FCD) and self-heating effects, our design enables arbitrary phase tuning with balanced FCA loss in the pair of arms, eliminating the need for additional phase corrections and creating ultralow crosstalk MZI elements. This neat design disengages from the commonly used nested structure, thus providing an opportunity of embedding tunable couplers for correcting imperfect splitting ratios given that only two are needed. We show that with the use of tunable directional couplers, a standard ±10 nm process variation is tolerated, while achieving a crosstalk ratio below −40 dB. By direct carrier injection in both arms, the proposed device operates at nanosecond scales and can bring about a breakthrough in the scalability of E-O switch fabrics, as well as other silicon integrated circuits that have stringent requirements for crosstalk leakage.

Trade‐Off between Second‐ and Third‐Order Nonlinearities, Ultrafast Free Carrier Absorption and Material Damage in Silicon Nanoparticles

AbstractReaching the optimal second‐ and third‐order nonlinear conversion efficiencies while avoiding undesirable free carrier absorption losses and material damage in ultrashort laser‐excited nanostructures is a challenging obstacle in all‐dielectric ultrafast nanophotonics. In order to elucidate the main aspects of this problem, a multi‐physical model is developed, coupling nonlinear Maxwell equations supplied by surface and bulk nonlinearities with free carrier hydrodynamic equations for electron–hole plasma kinetics and electron‐ion transfer for silicon. The maximum feasible efficiencies for a single spherical particle supporting different electric and magnetic resonances are compared, and the harmonic yields are further optimized by tuning lattice resonances in a periodic arrangement of nanoparticles. Results support the dominant role of magnetic dipole and quadrupole contributions in the enhancement of the third harmonic and the electric dipole for the second harmonic, as well as the possibility to further improve the conversion of both harmonics simultaneously at least by two orders of magnitude by designing properly the resonant metasurface.

Raytracing Modelling of Infrared Light Management Using Molybdenum Disulfide (MoS2) as a Back-Reflector Layer in a Silicon Heterojunction Solar Cell (SHJ).

The silicon heterojunction solar cell (SHJ) is considered the dominant state-of-the-art silicon solar cell technology due to its excellent passivation quality and high efficiency. However, SHJ’s light management performance is limited by its narrow optical absorption in long-wave near-infrared (NIR) due to the front, and back tin-doped indium oxide (ITO) layer’s free carrier absorption and reflection losses. Despite the light-trapping efficiency (LTE) schemes adopted by SHJ in terms of back surface texturing, the previous investigations highlighted the ITO layer as a reason for an essential long-wavelength light loss mechanism in SHJ solar cells. In this study, we propose the use of Molybdenum disulfide (MoS2) as a way of improving back-reflection in SHJ. The text presents simulations of the optical response in the backside of the SHJ applying the Monte-Carlo raytracing method with a web-based Sunsolve high-precision raytracing tool. The solar cells’ electrical parameters were also resolved using the standard electrical equivalent circuit model provided by Sunsolve. The proposed structure geometry slightly improved the SHJ cell optical current density by ~0.37% (rel.), and hence efficiency (η) by about 0.4% (rel.). The SHJ cell efficiency improved by 21.68% after applying thinner back ITO of about 30 nm overlayed on ~1 nm MoS2. The efficiency improvement following the application of MoS2 is tentatively attributed to the increased NIR absorption in the silicon bulk due to the light constructive interface with the backside components, namely silver (Ag) and ITO. Study outcomes showed that improved SHJ efficiency could be further optimized by addressing front cell components, mainly front ITO and MoS2 contact engineering.

1300 nm Semiconductor Optical Amplifier Compatible With an InP Monolithic Active/Passive Integration Technology

In monolithic photonic integrated circuits (PICs), an optimized active/passive integration is needed to provide efficient coupling and low amount of interface reflections between amplifiers and passive components. A 1300 nm semiconductor optical amplifier (SOA) on InP substrate and optimized for butt-joint reflections is investigated. Material performance were assessed from measurements of broad area lasers. Room temperature operation reveals 1.2 W single facet output power with threshold current around 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> per well. Characteristic temperatures of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 75 K and T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> = 294 K were obtained. A compact model description of the SOA, suitable for the design of PICs and rate equation analysis, was applied to parametrize the unsaturated gain measurements. Current injection efficiency of 0.65, transparency carrier density of 0.57 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> , and free carrier absorption losses up to 15 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> were extracted from fitting the data. The model is verified with measurements of optical gain saturation. A modal gain of 15 dB for a 600 μm long narrow ridge SOA leads to output saturation power higher than 30 mW at 7 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This building block contributes to the development of an InP monolithic integration technology in the 1300 nm range, which can enable the use of photonic integrated circuits in many kind of applications.

The variation of optical gain in Ge nanowires induced by the ΔEeΓ,L and symmetry of hole states under the axial stress

The electronic structures of Ge nanowires at the direct Γ-valley and indirect L-valley are calculated under the axial sterss based on the effective-mass theory. The optical gain is also investigated, and strangely, the peak gain along the z direction almost does not change, while the peak gain along the x direction goes up rapidly as the stress increases. For the first time, we clarify the variation of the peak gain in detail via the decrease of a physical quantity ΔEeΓ,L in combination with the change of the symmetry of hole states under stress. Furthermore, the net peak gain of Ge nanowires with different diameters and axial stresses is calculated after considering the free-carrier absorption loss, and the results demonstrate that the positive net peak gain along the x direction can be achieved when the stress is up to 4 GPa.

Transfer printing of AlGaAs-on-SOI microdisk resonators for selective mode coupling and low-power nonlinear processes.

The transfer printing of aluminum gallium arsenide (AlGaAs) microdisk resonators onto a silicon-on-insulator (SOI) waveguide platform is demonstrated. The integrated resonators exhibit loaded ${Q}$Q-factors reaching $ 4 \times {10^4} $4×104, and the vertical assembly approach allows selective coupling to different spatial mode families. The hybrid platform's nonlinearity is characterized by four-wave mixing with a measured nonlinear coefficient of $ \gamma = 325\;{({\rm Wm})^{ - 1}} $γ=325(Wm)-1, with the devices demonstrating minimal two-photon absorption and free-carrier absorption losses that are inherent to SOI at telecommunications wavelengths.

The theoretical direct-band-gap optical gain of Germanium nanowires

We calculate the electronic structures of Germanium nanowires by taking the effective-mass theory. The electron and hole states at the Γ-valley are studied via the eight-band k.p theory. For the [111] L-valley, we expand the envelope wave function using Bessel functions to calculate the energies of the electron states for the first time. The results show that the energy dispersion curves of electron states at the L-valley are almost parabolic irrespective of the diameters of Germanium nanowires. Based on the electronic structures, the density of states of Germanium nanowires are also obtained, and we find that the conduction band density of states mostly come from the electron states at the L-valley because of the eight equivalent degenerate L points in Germanium. Furthermore, the optical gain spectra of Germanium nanowires are investigated. The calculations show that there are no optical gain along z direction even though the injected carrier density is 4 × 1019 cm−3 when the doping concentration is zero, and a remarkable optical gain can be obtained when the injected carrier density is close to 1 × 1020 cm−3, since a large amount of electrons will prefer to occupy the low-energy L-valley. In this case, the negative optical gain will be encountered considering free-carrier absorption loss as the increase of the diameter. We also investigate the optical gain along z direction as functions of the doping concentration and injected carrier density for the doped Germanium nanowires. When taking into account free-carrier absorption loss, the calculated results show that a positive net peak gain is most likely to occur in the heavily doped nanowires with smaller diameters. Our theoretical studies are valuable in providing a guidance for the applications of Germanium nanowires in the field of microelectronics and optoelectronics.

Optical and electrical characterization of poly-Si/SiOx contacts and their implications on solar cell design

The scope of this paper lies on the phenomenon of free-carrier absorption (FCA) in heavily phosphorus-doped poly-Si layers, applied at solar cells featuring poly-Si/SiOx passivating contacts at the rear. Firstly, FCA is investigated on test structures featuring poly-Si contacts of different thickness and doping level. Secondly, these passivating contacts are integrated into the rear of solar cells featuring a boron-diffused emitter at the front. The infrared (IR) response of the solar cells is analyzed and FCA losses are quantified. In agreement with theory, it is shown that Jsc losses due to FCA increase with poly-Si doping level and thickness. For instance, a total Jsc loss of ~0.5 mA/cm² is obtained for a 145 nm thick poly-Si layer with a doping concentration of 1.9x1020 cm-3.

Efficiency analysis of 808 nm laser diode array under different operating temperatures

The 808 nm high-efficiency laser diodes have many advantages, such as high output power, high reliabilities, compact sizes, which are widely used in many areas, such as industry, communication, science, medicine and biology. In order to improve the power conversion efficiencies of 808 nm laser diodes, the following requirements must be considered, such as loss of joule heating, loss by the carrier leakage, spontaneous radiation loss below the threshold current, loss by interface voltage defect, internal losses including free-carrier absorption loss and scattering loss. These losses above are closely related to the operating temperature of laser diode. In this paper, power conversion efficiency analysis is demonstrated from the aspects of the output power, threshold current, slope efficiency, voltage, and series resistance at different temperatures.. This is the first time that the detailed study has been carried out under various temperatures (up to the lowest temperature of -40℃). And the detailed study above can be of benefit to designing the wafer epitaxial structure. High-power 808 nm laser diode arrays are mounted on conduction cooled heatsinks. And the laser chips have 47 emitters with 50% in fill factor, 100 m stripe in width and 1.5 mm in cavity length. The asymmetric broad waveguide epitaxial structure with lower absorption loss in p-type waveguide and cladding layer is designed in order to reduce the internal losses. The device performances are measured under operating temperatures ranging from -40℃ to 25℃ including the output power, threshold current, slope efficiency, series resistance, voltage, etc. Then the power conversion efficiency of 808 nm laser diode arrays are demonstrated from the output characteristics at different operating temperatures. With temperature decreasing, the series resistance gradually increases. The loss of joule heating ratio rises from 7.8% to 10.3%. In that case, the high series resistance is the major factor to prevent the efficiency from further improving at a low temperature of -40℃. As temperature decreases from 25℃ to -40℃, the carrier leakage ratio is reduced from 16.6% to 3.1%, the carrier leakage is the dominant factor for increasing efficiency, which means that it is necessary to optimize the epitaxial structure in order to reduce the carrier leakage at the room temperature. Comparing the two different work temperatures from -30℃ to -40℃, the carrier leakage ratio only changes 0.1%, which implies that the carrier leakage could be ignored under the low temperature. Meanwhile, as temperature decreases from 25℃ to -40℃, the power conversion efficiency increases from 56.7% to 66.8%.

All-Optical Modulation in Si Quantum Dot-Doped SiO Micro-Ring Waveguide Resonator

The Si quantum dot-doped SiO $_x$ rib waveguide-based free-carrier absorption (FCA) modulator with enhanced all-optical modulation depth is demonstrated by integrating with a micro-ring waveguide resonator. The micro-ring waveguide resonator with a Q-factor of 6×103 induces a throughput transfer function in wavelength domain, such a transmittance notch can be blue-shifted by varying the excited free-carrier density of Si-QD. When injecting the continuous-wave probe at central wavelength of the transmittance notch, the probe signal can be inversely modulated by optically pumping the micro-ring waveguide resonator to blue-shift the notch away from its original wavelength. By optimizing the pump wavelengths, the largest FCA loss and highest free-carrier density can be enhanced to 2.9 cm−1 and $7.83\times 10^{16}$ cm−3, respectively. With the excited free-carrier density of $\sim 1.3\times 10^{16}$ cm−3 inside the micro-ring waveguide, the maximal wavelength of the transmittance notch can be blue-shifted by 0.033 nm. The optical pumping also induces the broadened linewidth of transmittance notch from 0.25 to 0.27 nm. With the integrated micro-ring waveguide resonator, the all-optical modulation depth can be further enhanced from 52.5% to 63.5% by shifting the notched transmission spectrum of the micro-ring waveguide with the excited free-carrier density of Si-QD at probe wavelength of 1563.5 nm.

High-power, high-efficiency 808 nm laser diode array

High-power, high-efficiency 808 nm laser diode arrays for pumping solid-state lasers have been widely used in industrial, scientific, medical and biological applications. The tendency of development of 808 nm laser diode pumping with high power, high efficiency and long lifetime is well-known. Diode-pumped solid-state system with high-efficiency laser diode array has many advantages such as compact volume, lower weight and energy saving. Currently, commercial 808 nm diode laser arrays with lower power conversion efficiency of about 50%-55%, due to the optical absorption losses for GaAs-based epitaxial materials, have been reported. In order to reduce series resistance and thermal resistance, heavily doped p-type waveguide and cladding layers are employed. However, the absorption loss on the free carriers in heavily doped p-type layers is dominant, leading to a lower power conversion efficiency. In order to achieve a high efficiency, the following requirements must be considered: improving the internal quantum efficiency by reducing the carrier leakage and increasing the electron injection efficiency; minimizing the voltage drop by optimizing the operating voltage; reducing the series resistance and thermal resistance of device; minimizing the internal loss including free-carrier absorption loss and scattering loss by designing optimized waveguide and cladding structure. In this paper, optimizing the epitaxial structure and fabricating technologies are demonstrated to achieve the high efficiency and high power. The asymmetric broad waveguide epitaxial structure with lower absorption loss in p-type waveguide and cladding layer is designed in order to achieve the above goals. The high-efficiency epitaxial structure is optimized including the thickness, doping and composition for each layer structure. The strained quantum well diode laser with lower transparency current and higher differential is of benefit to achieving the high power. A novel asymmetric broad waveguide structure is designed by optimizing the waveguide thickness and component of p-waveguide so as to reduce carrier absorption loss, the optical absorption loss in this epitaxial structure is achieved to be as low as 0.63 cm-1. The wafer is grown by metalorganic chemical vapor deposition on an n-GaAs substrate. The optimized growth conditions and substrates orientation are extensively studied to improve the crystal quality and reduce the internal loss and defects. The wafer is processed using standard procedures. For the fabricated 1-cm laser diode array mounted on P-side down on copper microchannel cooled heatsink, the device shows an output power of 150 W under an operating current of 135 A with an emitting wavelength of 809 nm, an operating voltage of 1.76 V, a slope efficiency of as high as 1.25 W/A, and maximum power conversion efficiency of as high as 65.5%, which is the highest level of 808 nm diode laser array with an output power of 150 W.

Interband cascade lasers with &amp;gt;40% continuous-wave wallplug efficiency at cryogenic temperatures

Broad-area 10-stage interband cascade lasers (ICLs) emitting at λ = 3.0–3.2 μm are shown to maintain continuous-wave (cw) wallplug efficiencies exceeding 40% at temperatures up to 125 K, despite having a design optimized for operation at ambient and above. The cw threshold current density at 80 K is only 11 A/cm2 for a 2 mm cavity with anti-reflection/high-reflection coatings on the two facets. The external differential quantum efficiency for a 1-mm-long cavity with the same coatings is 70% per stage at 80 K, and still above 65% at 150 K. The results demonstrate that at cryogenic temperatures, where free carrier absorption losses are minimized, ICLs can convert electrical to optical energy nearly as efficiently as the best specially designed intersubband-based quantum cascade lasers.

Enhanced Light Trapping in Multiple Quantum Wells by Thin-Film Structure and Backside Grooves With Dielectric Interface

Insertion of multiple quantum wells (MQWs) into the i-region of GaAs p-i-n solar cells can shift the effective bandgap to the optimal value for single-junction solar cells under high sunlight concentration. The quantum efficiency corresponding to the absorption by MQWs must be sufficiently high for achieving high-efficiency single-junction MQW solar cells. Here, we report light-trapping thin-film MQW solar cells for increasing photoabsorption in MQWs. In order to suppress the free-carrier absorption loss and enhance the light trapping, only the active layers including MQWs were processed to a cell by flip-chip bonding and dissolution of a conductive GaAs substrate. The periodic grooves formed on the back side of the cell scattered photons in the subbandgap range and trapped light inside the cell. For absorption loss reduction in a back contact metal, a dielectric interlayer was introduced between the metal and GaAs. The light-trapping structure resulted in a fivefold increase in effective optical path length compared with the physical thickness of MQWs. External quantum efficiency at wavelengths longer than the GaAs edge exceeded 50% with only 20-period MQWs. As a result, we achieved thin-film light-trapping MQW solar cells with 20% conversion efficiency.

All-optical modulation based on silicon quantum dot doped SiO x :Si-QD waveguide

All-optical modulation based on silicon quantum dot doped SiOx:Si-QD waveguide is demonstrated. By shrinking the Si-QD size from 4.3 nm to 1.7 nm in SiOx matrix (SiOx:Si-QD) waveguide, the free-carrier absorption (FCA) cross section of the Si-QD is decreased to 8 × 10−18 cm2 by enlarging the electron/hole effective masses, which shortens the PL and Auger lifetime to 83 ns and 16.5 ps, respectively. The FCA loss is conversely increased from 0.03 cm−1 to 1.5 cm−1 with the Si-QD size enlarged from 1.7 nm to 4.3 nm due to the enhanced FCA cross section and the increased free-carrier density in large Si-QDs. Both the FCA and free-carrier relaxation processes of Si-QDs are shortened as the radiative recombination rate is enlarged by electron–hole momentum overlapping under strong quantum confinement effect. The all-optical return-to-zero on-off keying (RZ-OOK) modulation is performed by using the SiOx:Si-QD waveguides, providing the transmission bit rate of the inversed RZ-OOK data stream conversion from 0.2 to 2 Mbit/s by shrinking the Si-QD size from 4.3 to 1.7 nm.

Cold-cavity measurement of optical loss from oxide-confined vertical-cavity surface-emitting lasers

Microcavity laser design and performance optimization requires a quantitative knowledge of the cavity optical losses. A generalized method using sub-threshold spectral measurements matched to model calculations is demonstrated to determine optical loss in microcavity lasers. Cold-cavity spectral characteristics are used to extract the size-dependent optical loss for small diameter oxide-confined vertical-cavity surface emitting lasers. For oxide aperture diameters less than 4 μm, the oxide scattering loss can be greater than 10 cm−1, similar to the typical values of free carrier absorption loss.

How Important Is the Influence of Poisson Potential on the Band Structures of Terahertz Quantum-Cascade Lasers?

We have investigated the importance of Poisson potential induced by intentional doping on the band structures of two-well scattering injection terahertz quantum-cascade lasers, using a self-consistent Schrödinger–Poisson method. The calculated results show that the increase in doping density leads to a dramatic increase in Poisson potential, i.e., every 1010 cm-2 increase in sheet density brings about 0.58 meV Poisson potential. As the doping is increased from 3.6×1010 to 3.0×1011 cm-2, the calculated optical transition energy shows a significant shift (27% increase). By taking account of the free carrier absorption loss and the scattering injection efficiency, a narrow region doping in the wide GaAs well is recommended to minimize the influence of Poisson potential on the band structures.

Parameterization of Free Carrier Absorption in Highly Doped Silicon for Solar Cells

Free carrier absorption (FCA) is a parasitic absorption process in highly doped silicon that might significantly reduce the amount of photons, potentially generating electron-hole pairs. Existing FCA parameterizations are mostly setup by evaluating absorption data in the range λ ≥ 4 μm. If applied in the wavelength range λ = 1.0-2.0 μm, including the relevant range for silicon solar cells, most parameterizations are not appropriate to describe FCA accurately. In this paper, new parameters are presented using optical simulation on the base of experimental reflection data to enhance the quantification of FCA losses in the considered wavelength range.

Infrared absorption of n-type tensile-strained Ge-on-Si

We analyze the IR absorption of tensile-strained, n-type Ge for Si-compatible laser applications. A strong intervalley scattering from the indirect L valleys to the direct Γ valley in n+ Ge-on-Si is reported for the first time to our knowledge. The intervalley absorption edge is in good agreement with the theoretical value. On the other hand, we found that the classical λ2-dependent Drude model of intravalley free-carrier absorption (FCA) breaks down at λ<15 μm. A first-principle model has to be employed to reach a good agreement with the experimental data. The intravalley FCA loss is determined to be <20 cm(-1) for n=4×10(19) cm(-3) at λ=1.5-1.7 μm, an order lower than the results from Drude model. The strong L→Γ intervalley scattering favors electronic occupation of the direct Γ valley, thereby enhancing optical gain from the direct gap transition of Ge, while the low intravalley free-electron absorption at lasing wavelengths leads to low optical losses. These two factors explain why the first electrically pumped Ge-on-Si laser achieved a higher net gain than the theoretical prediction using λ2-dependent free-carrier losses of bulk Ge and indicate the great potential for further improvement of Ge-on-Si lasers.