Free Carrier Absorption Research Articles

Tunable nonlinear absorption and optical limiting behavior of NaBi(MoxW1−xO4)2 single crystals with ratio of Molybdenum/Tungsten

The compositional effect of Mo/W ratio on linear, nonlinear absorption and optical limiting behavior of the NaBi(MoxW1−xO4)2 single crystals grown by Czochralski technique was investigated. X-ray diffraction patterns of the studied crystals presented well-defined peaks associated with the tetragonal crystalline structure. The nonlinear absorption performance and optical limiting threshold were determined using an open-aperture Z-scan technique. A theoretical model including one photon absorption (OPA), two photon absorption (TPA) and free carrier absorption was used to determine the nonlinear absorption parameters. All of the results showed that defect states, which strongly affect nonlinear absorption (NA) and optical limiting behaviors, can be tuned with the Mo/W ratio, enabling NaBi(MoxW1−xO4)2 single crystals to be used in desired optoelectronic applications. Linear optical absorption analysis revealed that bandgap energy and defect states can be tuned by changing the Mo/W ratio in the crystal structure. The obtained results showed that all the studied crystals had NA behavior and the nonlinear absorption coefficient decreased with increasing Mo/W ratio. Sequential TPA is the main NA mechanism for these crystals due to the fact that the incident light energy is lower than the bandgap energies and the existence of the real intermediate state around 2.32 eV.

Excitation wavelength dependent nonlinear absorption mechanisms and optical limiting properties of Bi12SiO20 single crystal

Nonlinear absorption mechanisms (NA), excitation wavelength dependence, and defect states of Bi12SiO20 (BSO) single crystal were investigated. The band gap and Urbach energies were found to be 2.51 and 0.4 eV from the absorption spectra. To evaluate the effect of excitation energy on the NA mechanism of the BSO single crystal, open aperture Z-scan experiment with 4 ns laser pulse at 532 and 1064 nm wavelengths with different intensities was performed. Obtained data were analyzed with a theoretical model considering the contributions of one photon absorption (OPA), two photon absorption (TPA) and free carrier absorption (FCA) to NA. The results indicated that the NA behavior decreased with increasing of the pump intensity as the defect states at around 2.32 eV by OPA at 532 nm, and TPA at 1064 nm excitations. The dominant NA mechanisms are OPA and sequential TPA at 532 nm as compared to the 1064 nm. A higher NA coefficient was obtained at 532 nm as compared to 1064 nm excitation. This observation was attributed to higher contribution of OPA at 532 nm even at lower input intensities compared to TPA contribution at 1064 nm. Onset optical limiting thresholds were found as 0.34 and 0.68 mJ/cm2 for 532 and 1064 nm input beams, respectively. In the light of the results, the BSO single crystal may be used as a saturable absorber or an optical limiter at convenient input intensity by effectively adjusting defect states and excitation wavelength.

Dynamics of Free Carrier Absorption and Refractive Index Dispersion in Si and Si/PolySi Microrings

We report pump-probe measurements of time resolved optical transmission spectra of Si and Si/poly-Si microrings after high free carrier densities have been generated by two-photon absorption of the pump pulse. From measurements, we can extract the recovery dynamics of free carrier absorption, refractive index dispersion, generated free carriers and finally the effective initial free carrier lifetimes. The method is validated by comparing modelling and simulations with measurements; the obtained results are in very good agreement with what predicted by the Shockley-Read-Hall recombination model for trap assisted recombination. We also propose a method for determining the empirical relations for free carrier absorption and refractive index dispersion in poly-Si waveguides.

A TCAD study on the effect of process parameters on silicon optical phase shifter performance

On-chip integrated optical phase shifters are an important part of optical phase modulators. The performance of such modulators relies heavily on the phase shifter performance, which in turn depends on multiple process parameters. This paper reports the study of the effect of different process parameters on the performance of a silicon PN optical phase shifter obtained by process simulation using Silvaco® TCAD. The effect of dopant implantation dose, implantation energy, annealing temperature and time, wafer temperature, wafer tilt and rotation, and pre-amorphization on the phase and absorption of light is discussed. The 3-dB modulation bandwidth of a lumped phase shifter and the dependency of the performance metrics on different process parameters are presented. Monte Carlo numerical simulation shows that the free-carrier absorption has a much greater dependency on the process parameters than the phase shift. The study shows that ion channeling poses a limiting factor on the phase shifter performance, which can be improved by tilting the wafer or using a pre-amorphized substrate for implantation. The study shows that the 3-dB modulation bandwidth is highly dependent on the wafer tilt angle, rotation angle, and the lattice structure of the solid substrate. A bandwidth improvement of more than 5× is observed with 1.7× lower absorption for a pre-amorphized sample at −5 V compared to a crystalline sample with the same process flow.

Atomistic nonlinear carrier dynamics in Ge

An atomistic technique to successfully demonstrate the ultrafast carrier dynamics in Ge photoconductive samples is reported here. The technique is validated against the experimental findings and with the Drude conductivities. The impact of the various different scattering mechanisms is used to calibrate the experimental results. It is observed that the total scattering rate is not a constant parameter as contrast to Drude model which uses constant scattering rate as the fitting parameter to demonstrate the ultrafast carrier dynamics, but strongly dependent on the applied peak THz field strength. It also contradicts with the relaxation time approximation (RTA) method which uses scattering rate chosen on the empirical basis as the fitting parameter to demonstrate the ultrafast carrier dynamics. On the other hand the limitations and challenges offered by various types of density functional theories (DFT) pose lot of challenges. In current manuscript various types of scattering mechanisms i.e. acoustic, intervalley, Coulomb and impact ionization on the behavior of carrier conductivity are studied in details. The proposed technique has shown capability to extract low and high frequency conductivities accurately which is impossible through the Drude model or DFT based theories. It is observed that the free carrier absorption coefficient depends on the refractive index of the material at low THz frequencies. The solution of Boltzmann transport equation through Monte Carlo technique provides valuable insights for better understanding of ultrafast carrier transportation mechanism. The free carrier absorption spectra are found to be in good agreement with the experimental results at various THz field strengths.

Limits to the Energy-Conversion Efficiency of Air-Bridge Thermophotovoltaics

As the energy economy becomes increasingly decarbonized, low-cost energy storage grows ever more important. Thermal batteries in combination with thermophotovoltaic (TPV) cells are one major source of storage. The lowest-loss TPV cells utilize an air bridge (AB) with a gold back reflector. In this work, the authors determine a 55.5% thermodynamic efficiency limit for AB-TPVs at an emitter temperature of 1400 K, based on detailed balance. Including losses from nonradiative recombination, finite resistance, and free-carrier absorption, the practical efficiency limit is 48.6%. This work provides a road map for evaluating and limiting losses, leading to even higher AB-TPV efficiencies.

Temperature Evolution of Two-State Lasing in Microdisk Lasers with InAs/InGaAs Quantum Dots.

One-state and two-state lasing is investigated experimentally and through numerical simulation as a function of temperature in microdisk lasers with Stranski-Krastanow InAs/InGaAs/GaAs quantum dots. Near room temperature, the temperature-induced increment of the ground-state threshold current density is relatively weak and can be described by a characteristic temperature of about 150 K. At elevated temperatures, a faster (super-exponential) increase in the threshold current density is observed. Meanwhile, the current density corresponding to the onset of two-state lasing was found to decrease with increasing temperature, so that the interval of current density of pure one-state lasing becomes narrower with the temperature increase. Above a certain critical temperature, ground-state lasing completely disappears. This critical temperature drops from 107 to 37 °C as the microdisk diameter decreases from 28 to 20 μm. In microdisks with a diameter of 9 μm, a temperature-induced jump in the lasing wavelength from the first excited-state to second excited-state optical transition is observed. A model describing the system of rate equations and free carrier absorption dependent on the reservoir population provides a satisfactory agreement with experimental results. The temperature and threshold current corresponding to the quenching of ground-state lasing can be well approximated by linear functions of saturated gain and output loss.

Impact of the capture time on power saturation of quantum-well diode lasers

Injected electrons and holes are captured into quantum wells with a non-vanishing time. Simulation results are presented, showing that this results in a non-equilibrium of free and bound carriers in a diode laser. Analysis of an exemplary diode laser structure is used to show that if the capture time is too large, the carrier densities in regions adjacent to the quantum well rise strongly beyond their values at threshold at very high output power. This results in an increase of the free carrier absorption and the non-stimulated recombination, causing a reduction of the slope efficiency and a downwards-bending of the power-current characteristics (power saturation), consistent with experimental results. For the layer structure studied, the additional losses due to accumulation of holes in the n-doped region is predicted to be the dominant effect.

Spectroscopic ellipsometry and raman spectroscopy of Bi1-xSbxTeI solid solutions with x≤0.1

Spectroscopic ellipsometry supported by reflectivity measurements and Raman spectroscopy are applied at room temperature to n-type Rashba semiconductors BiTeI and Bi1-xSbxTeI with nominal compositions x = 0.05 and 0.1. Complementary four probe Hall measurements are made using standard ac technique and the concentration of free carriers is determined. The Raman spectra, including TO-LO resonances are found to occupy a frequency range below 250 cm−1 in the studied materials. The pseudodielectric function, retrieved at different angles of incidence is analyzed focusing on the free carrier absorption (intraband transitions) and the optical transitions between the Rashba-split branches of the conduction band (intersubband vertical transitions). The former transitions with screened plasma frequency anchored to the zero-crossing point of the real part of the pseudodielectric function are described within a simple Drude model and the important parameters such as electron effective mass, electron mobility and high frequency dielectric constant are obtained. Anchored to the Rashba energy, the intersubband transitions obtained for each material by refining the pseudodielectric function from Drude contribution, appear in the imaginary part as a broad peak in the photon energy range between 0.15 and 0.4 eV. A noticeable red shift of this peak for Bi0.9Sb0.1TeI as compared to BiTeI is proposed to be a manifestation of the reduction of the spin splitting after part of Bi atoms is replaced by lighter Sb atoms.

Nonlinear optical properties and carrier recombination lifetime of GaN.

Nonlinear optical properties of a selection of gallium nitride samples have been measured using picosecond and nanosecond duration laser pulses at 532nm. The values of the two-photon absorption coefficient, free carrier absorption cross section, and free carrier refraction cross section are determined along with the recombination lifetime of photogenerated carriers. The effect of hot isostatic pressing on these properties in samples with linear absorption at the band edge due to defects is explored.

Optical Loss in Microdisk Lasers With Dense Quantum Dot Arrays

We discuss the origin of optical losses in microdisk lasers with a dense array of InGaAs quantum dots in the active region. In particular, we study the effect of microlaser diameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {D}$ </tex-math></inline-formula> variation from 15 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$200~\mu \text{m}$ </tex-math></inline-formula> on optical losses of different nature. A strong dependence of the lasing wavelength on the diameter is observed: the blue-shift with decreasing disk size implies an increase in optical losses, although in the case of an ideal cylinder, a noticeable optical loss should appear only at diameters comparable to the wavelength of light. A comparison of the spectral characteristics of microlasers with those of broad-area stripe lasers, for which optical loss can be easily found, gives a tool to evaluate optical loss in microdisk lasers, which was found to be unexpectedly high. It changes <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\propto }\mathbf {D}^{\mathbf {-1}}$ </tex-math></inline-formula> from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 100$ </tex-math></inline-formula> cm−1 in the smallest microlasers to ~ 5 cm−1 in the largest ones. Several possible physical mechanisms of the appearance of optical losses in microlasers are considered, such as radiative loss due to the curvature of the cylindrical cavity, free carrier absorption, light scattering due to roughness of the side walls, and absorption of light in the near-surface region. The latter type of optical loss was found to be the dominant one and can explain the experimental results once the absorbing layer with a thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2~\mu \text{m}$ </tex-math></inline-formula> was suggested. Using the Gaussian approximation for Using the Gaussian approximation for the gain spectrum, the wavelength-loss relationship was simulated and a good agreement with the experimental dependence was found. The variation of the experimental results on optical loss for nominally identical microlasers was attributed to the variation of the scattering loss. The same reason can explain the scatter of the slope efficiency, which varies from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim ~0.03$ </tex-math></inline-formula> to 0.25 W/A being governed by the ratio of the scattering loss to the surface absorption loss.

Strong spectral broadening of Cherenkov-type terahertz radiation by free carrier generation

Free carrier generation is commonly viewed as detrimental to the optical-to-terahertz conversion efficiency in electro-optic crystals due to free carrier absorption of terahertz radiation. Contrary to this view, we show by numerical simulation that free carrier generation can have a positive effect on the Cherenkov-type terahertz emission from ultrashort laser pulses by strongly broadening the radiation bandwidth. The broadening is produced by terahertz emission from a surge current of the optically generated carriers, which are accelerated by the rectified electric field. Specific results are obtained for GaP pumped at a $1.7\text{\ensuremath{-}}\textmu{}\mathrm{m}$ wavelength.

Control of the sensitivity of the detection of ionized air using photonic crystal waveguides

Ionized air is used in various industries to control electrostatic charge. On the other hand, ionized air molecules can also cause various problems since electrification of materials can induce electrostatic discharges. Therefore, compact sensors that enable a quantitative detection of ionized air will help to improve industrial processes and safety. Here we report on the detection of negatively ionized air using a photonic crystal (PC) waveguide with a length of 800 µm. In this type of detector, the PC is exposed to a flux of air ions that transfers a part of the excess charges to the PC. The light transmitted through the waveguide is then attenuated by free carrier absorption due to the excess charges from the ionized air molecules. We show that the electron density in the PC can be estimated from the magnitude of the attenuation, and that this magnitude depends on the wavelength of the light propagating in the PC waveguide. Due to the wavelength dispersion of the group velocity, light at longer wavelengths is subject to stronger attenuation than light at shorter wavelengths. This property is useful for the development of ionized-air sensors with a variable detection range.

4H-SiC Auger recombination coefficient under the high injection condition

The on-resistance of bipolar devices depends on the carrier lifetime, which is determined by Shockley–Read–Hall, surface, radiation, and Auger recombination processes. Values for the Auger recombination coefficient have been previously reported, but the values were constant in each report. However, the Auger recombination coefficient should depend on the concentration of excited carriers and the presence of traps. In this study, we observed excited carrier recombination in 4H-SiC under the high injection condition using time-resolved free carrier absorption measurements. Consequently, we discovered that the Auger recombination coefficient is dependent on the excited carrier concentration and that the traps have negligible effects on the coefficient.

Pulsed heterodyne interferometry for nonlinear SOI waveguide characterization

Silicon waveguides are a promising candidate for integrated nonlinear optics applications. Nonlinear coefficients of Silicon on Insulator (SOI) waveguides have been previously measured using techniques such as Z-scan, D-scan, Four Wave Mixing (FWM) and Self-phase modulation. However, they have several drawbacks such as they operate at high power or are cumbersome to setup and require multiple measurements to determine all the coefficients. In this work, we develop a direct and single measurement technique to characterize the nonlinear processes in SOI waveguides. This is achieved by employing a heterodyne interferometric technique to accurately measure minute nonlinear response. The measured nonlinear amplitude and phase shifts are fit to extract third-order nonlinear coefficients of Two-photon absorption, Kerr nonlinear index, Free carrier absorption and Free carrier dispersion. The obtained coefficients for SOI waveguides are close to that found in literature measured using the above-mentioned techniques. The advantages of this method include easy interpretation of the output signal and relatively low power of operation. It is especially advantageous for studying materials such as Phase Change Materials (PCM) in which phase changes occur dynamically. This aspect is quite promising for characterizing emerging materials for integrated photonics applications.

Effects of ion implantation process on defect distribution in SiC SJ-MOSFET

A superjunction (SJ) structure in power devices is compatible with low specific on-resistance and high breakdown voltage. To fabricate the SJ structure in SiC power devices, the repetition of ion implantation and epitaxial growth processes is a practical method. However, the impact of ion implantation on device performance has rarely been reported. In this study, we measured the carrier lifetime distributions in a SiC MOSFET with an SJ structure using a microscopic free carrier absorption method. Furthermore, we observed the distribution of defects via cathodoluminescence and deep levels via deep-level transient spectroscopy. We observed that Al ion implantation induced defects and reduced the carrier lifetime in the SJ structure. However, N ion implantation does not significantly induce defects. Additionally, Al ion implantation at room temperature exhibited more significant effects than implantation at 500 °C. The results can aid in controlling the carrier lifetime in SiC SJ MOSFETs.

Impact of defects on the χ(3) optical nonlinearity of sputtered WSe2 thin films in the optical communication band

We present an experimental investigation on third-order nonlinear optical properties of RF-sputtered WSe2 thin films using a single-beam Z-scan technique around the optical communication window in a near-infrared spectral band. The nonlinear absorption coefficient was obtained from the open-aperture Z-scan transmission using ultrashort pulses centered at 1520nm excitation wavelength. The results show a reverse saturable absorption signature for all the WSe2 films due to two-photon absorption (TPA) mediated electronic transitions. The dependency of the TPA coefficient on film thickness is analyzed through the modification in an electronic band structure essentially caused due to defects induced in the samples. In addition, we also explored the impact of high laser intensity on the TPA process, which essentially pointed toward a significant contribution of free-carrier absorption at small laser intensities in WSe2 thin films. This investigation provides a basis for optimally tailoring the nonlinear optical properties of transition-metal-dichalcogenides via bandgap engineering for improving photonic device functionality.

1 GHz electro-optical silicon-germanium modulator in the 5-9 µm wavelength range.

Spectroscopy in the mid-infrared (mid-IR) wavelength range is a key technique to detect and identify chemical and biological substances. In this context, the development of integrated optics systems paves the way for the realization of compact and cost-effective sensing systems. Among the required devices, an integrated electro-optical modulator (EOM) is a key element for advanced sensing circuits exploiting dual comb spectroscopy. In this paper, we have experimentally demonstrated an integrated EOM operating in a wide wavelength range, i.e. from 5 to 9 µm at radio frequency (RF) as high as 1 GHz. The modulator exploits the variation of free carrier absorption in a Schottky diode embedded in a graded silicon germanium (SiGe) photonic waveguide.

Ab initio theory of free-carrier absorption in semiconductors

The absorption of light by free carriers in semiconductors results in optical loss for all photon wavelengths. Since free-carrier absorption competes with optical transitions across the band gap, it also reduces the efficiency of optoelectronic devices such as solar cells because it does not generate electron-hole pairs. In this work, we develop a first-principles theory of free-carrier absorption taking into account both single-particle excitations and the collective Drude term, and we demonstrate its application to the case of doped Si. We determine the free-carrier absorption coefficient as a function of carrier concentration and we obtain excellent agreement with experimental data. We identify the dominant processes that contribute to free-carrier absorption at various photon wavelengths, and analyze the results to evaluate the impact of this loss mechanism on the efficiency of Si-based optoelectronic devices.

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.