Free-carrier Absorption Effect Research Articles

Is Ge an Excellent Material for Mid-IR Kerr Frequency Combs Around 3-μm Wavelengths?

Mid-infrared (Mid-IR) Kerr frequency combs, which emit board, discrete, and evenly spaced laser frequencies in the spectral region of 2–20 μm, are expected to bring us tremendous applications from molecular spectroscopy to astronomy science. To develop mid-IR Kerr frequency combs, Ge has attracted significant attention due to its unique merits of a wide transparency window (2–14 μm), high refractive index (∼4), giant third-order nonlinear refractive index (∼10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−16</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /W), and fully CMOS-compatible device fabrication. However, it is still debatable whether Ge is an excellent material for developing Kerr frequency combs in the short-wavelength mid-IR region due to the harmful multiphoton and free-carrier absorption effects. Here, we seek to provide physical insights into the feasibility of developing mid-infrared Ge-based Kerr frequency combs around 3-μm wavelengths. Our method is based on a comprehensive model developing from a modified Lugiato-Lefever equation via taking into account high-order dispersion, multiphoton absorption, free-carrier absorption, free-carrier dispersion, and thermal-optical phase shift effect. The results show that, only with the pump wavelength of above 3.5 μm, a frequency comb could be generated by adjusting the carrier lifetime and pump power appropriately. The study is expected to provide useful guidance for developing mid-IR Kerr frequency combs by using the CMOS technology.

TiOx/Ti/TiOx Tri-Layer Film-Based Waveguide Bolometric Detector for On-Chip Si Photonic Sensor

In this article, we experimentally demonstrate an on-chip optical power monitoring at the near-infrared spectral region (1520&#x2013;1600 nm) using the waveguide bolometric detector with a TiO_<i>x</i>/Ti/TiO_<i>x</i> tri-layer film based on the silicon-on-insulator (SOI) platform. For the incident light absorption causing a heat generation, a heavily doped (<inline-formula> <tex-math notation="LaTeX">${n}^{+}$ </tex-math></inline-formula>) Si waveguide is introduced to utilize the mechanism of free-carrier absorption (FCA), and a metal (Au) strip is deposited onto the <inline-formula> <tex-math notation="LaTeX">${n}^{+}$ </tex-math></inline-formula> Si waveguide to further enhance the light absorption. A bolometric material of TiO_<i>x</i>/Ti/TiO_<i>x</i> tri-layer film is integrated for the thermal-to-electrical conversion. As a result, it exhibits superior performances, which are the temperature coefficient of resistance (TCR) of &#x2212;2.296&#x0025;/K and the maximum sensitivity of &#x2212;46.45&#x0025;/mW with a low wavelength-dependency. In addition, a rapid response time is obtained with the rise/fall time of <inline-formula> <tex-math notation="LaTeX">$24.6 ~\mu \text{s}/29.8 ~\mu \text{s}$ </tex-math></inline-formula>, which is enough for a variety of optical sensing applications. This work demonstrates the waveguide-based bolometric photodetection using the FCA effect, for the first time to our knowledge, which can also be simply extended for the operation wavelength range to the mid-infrared (MIR) spectral region for bio/chemical sensing applications.

Optical Properties of Bulk Silicon Germanium in the Near Infrared Evaluated By Spectroscopic Ellipsometry

Background and ObjectivesSilicon germanium (SiGe) is expected to be applied to optical devices such as infrared sensors because of its narrower band gap than Si. For this purpose, it is very important to understand the optical properties in the infrared region of SiGe. Although there have been many reports on the optical properties of SiGe [1], there are few reports on the optical properties in the near infrared region. Since the optical properties must be modulated by the applied strain, we believe that bulk SiGe is the most appropriate sample to obtain the original optical properties of SiGe without the effect of strain. In this study, we evaluated the optical properties of bulk SiGe including the infrared region by using spectroscopic ellipsometry.ExperimentsThe samples were bulk SiGe with 16% Ge composition fabricated by Czochralski (Cz) method [2], and bulk SiGe with 32, 45 and 90% Ge composition fabricated by Traveling Liquidus Zone (TLZ) method [3]. All of these samples were confirmed to be single crystals by X-ray diffraction (XRD) and electron backscattered pattern. In addition, we confirmed that the SiGe samples are strain-free by XRD and Raman spectroscopy. The spectroscopic ellipsometry measurements were performed with an incident angle of 60°, a 70 W xenon lamp as the light source, and a wavelength range of 200 to 1600 nm for the light source.Results and DiscusssionFigure 1 shows the (a) refractive index and (b) extinction coefficient of bulk SiGe with 16% Ge composition in conjunction with those of pure Si, pure Ge. Fig. 1(a), it can be confirmed that the refractive index of bulk SiGe (16%) has an anomalous increase from around 1100 nm. On the other hand, Fig. 1(b) indicates that the extinction coefficient does not show such an anomalous change. These behavior was also observed for other bulk SiGe samples. Since this increase was observed in both Cz and TLZ growth methods, it is not considered to originate from the fabrication method. The value at 1100 nm where the anomalous increase is observed, is close to the Si band gap of 1.1 eV. The characteristics are Si like before the increase, while it is Ge like after the increase. Therefore, the refractive index of Ge seems to become dominant after the Si band gap. Since the effect of free carrier absorption is expected to occur at longer wavelengths, this anomalous increase should not be due to the free carrier absorption but some unique phenomena for SiGe alloy. We believe that this unique phenomena in the refractive index of bulk SiGe is an important finding for the application of SiGe in optical devices.[1]. J. Humlicek, M. Garriga, M. I. Alonso, and M. Cardona, J. Appl. Phys. 65, 2827 (1989).[2]. I. Yonenaga, J.Cryst. Growth 275, 91 (2005).[3]. K. Kinoshita et al., Jpn. J. Appl. Phys. 54, 04DH03 (2015). Figure 1

Effects of many-body interactions on the transient optical properties of lead halide perovskites

Lead halide perovskites (LHPs) are emerging as promising candidates for use in various high-performance optoelectronic applications, yet their photophysics remains a topic of debate. Here, we theoretically investigated how the ultrafast optical properties of a few prototype LHPs are affected by many-body interactions, including the bandgap renormalization (BGR) effect, the band-filling (BF) effect, the free-carrier absorption effect, and the exciton effect, at carrier densities ranging from 1016 to 1019 cm−3. The results show that the exciton absorption becomes more obvious near the bandgap with increasing exciton energy (as the halogen component of the LHP is varied from I to Cl). Transient reflectivity results indicate that the signal has one peak below the bandgap as a result of the BGR effect at low carrier densities and one valley above the bandgap originating from the BF effect at high carrier densities. In addition, the absorbance decreases with increasing the carrier density as a result of the BF effect because the filled energy levels are observed at 2 meV above the bottom of the conduction band. The results of the present work are expected to promote the application of LHPs in solar cells, light-emitting diodes, and other photoelectric devices.

Free carrier induced dark pulse generation in microresonators.

We theoretically demonstrate a novel, to the best of our knowledge, mechanism for dark pulse excitation in normal dispersion microresonators exploiting free carrier dispersion and free carrier absorption effects due to multi-photon absorption. Dark pulses can be generated in the three- and four-photon absorption regimes in the presence or absence of external reverse bias to control the lifetime of free carriers, respectively. Direct generation of dark pulses is proven to be feasible in both regimes with a frequency fixed laser. The dynamics of their temporal and spectral evolution have also been investigated. Our findings establish a reliable path for dark pulse and Kerr microcomb generation in related platforms with simplified controlling and tuning techniques.

Realization and optimization of optical logic gates using bias assisted carrier-injected triple parallel microring resonators

We propose a p-i-n diode embedded parallel triple microring resonator (MRR) configuration to simultaneously realize optical OR and AND, or NAND and NOR logic gates using a bias-assisted carrier injection mechanism. The applied bias on the rings induces refractive index change in the intrinsic region through bandfilling, bandgap shrinkage and free carrier absorption effects, leading to intensity variation at the output ports of the MRR due to respective resonant wavelength shift. The optical logic gate operational outputs are represented as the light intensities at the output ports of the MRR with the wavelength of the input optical signal launched into the input port being at the resonant wavelength of the microring resonator, while the operands are represented as the bias applied onto the rings. The proposed microring resonator configuration is theoretically optimized for achieving a high contrast ratio and an optical confinement factor by optimizing the intrinsic region width, applied bias, coupling coefficient between ring-bus waveguide and lifetime of carriers in the intrinsic region.

Parasitic photon process versus productive photon process: a theoretical study of free-carrier absorption in conventional and hot-carrier solar cells

The effects of free-carrier absorption on conventional and hot-carriers solar cells are theoretically investigated in this work. The common view that free-carrier absorption in solar cells is ‘parasitic’ is re-examined, with the assistance of a theoretical framework and formulation developed and verified for calculating free-carrier absorption coefficients. In the case of spatial partitioning with photon absorption selectivity (e.g. solar cells with embedded quantum structures), free-carrier-absorption can facilitate and enhance carrier escape processes and increases photocurrents, especially in deep potential wells. Carrier heating resulting from free-carrier absorption is shown to be extremely beneficial to hot-carrier solar cells, especially for heavily-doped wide-band-gap optical absorbers. The energy conversion processes from carrier heating of free-carrier absorption could potentially make ideal hot-carrier solar cells function like solar thermal converters. As a result, their energy conversion efficiency is closer to the thermodynamic limit, regardless of optical absorbers’ band gap energy. It is illustrated that, as an optical process which is not limited by band gap energy, free-carrier absorption could benefit possible materials of hot-carrier solar cells regardless of their band gap energy. From this perspective, free-carrier absorption is far from a ‘parasitic’ process. Its usefulness depends on how we turn it into productive work.

Optical modulation in Ge-rich SiGe waveguides in the mid-infrared wavelength range up to 11 \xb5m

Waveguide integrated optical modulators in the mid-infrared wavelength range are of significant interest for molecular spectroscopy. This is because on-chip synchronous detection can improve the performance of detection systems and can also be used for free-space communications where optical modulators working in atmospheric transparency windows are needed. Here we report optical modulation in a mid-infrared photonic circuit, reaching wavelengths larger than 8 µm. Optical modulation in the wavelength range from 5.5 to 11 µm is shown, relying on a broadband Ge-rich graded-SiGe platform. This demonstration experimentally confirms the free-carrier absorption effect modeling. These results pave the way towards efficient high-performance electrically-driven integrated optical modulators in the mid-infrared wavelength range.

The effect of doping and annealing on the nonlinear absorption characteristics in hydrothermally grown Al doped ZnO thin films

The effects of annealing and Al dopant on the nonlinear absorption characteristics of amorphous ZnO thin films were studied experimentally and theoretically. The structural patterns of AZO thin films grown at various growth temperatures (80ᵒC, 90ᵒC, 100ᵒC) and Al doping ratios (%5, %10, %20) were determined by using x-ray diffraction. The results revealed that the crystallinity increased with increasing growth temperature and Al doping ratio. The optical band gap decreased from 3.91 to 3.78 eV and the transmittance of AZO thin films decreased from 90% to 55% with increasing Al molar ratio. The open aperture Gaussian beam Z-scan measurements indicated that all studied films exhibited nonlinear absorption behavior due to the defect states inside energy bandgap. The nonlinear absorption coefficients increased from 1.34 × 10−4 to 6.12 × 10−4 cm/W for 4 ns pulse duration with increasing of doping concentration. The experimental curves were fitted to the theory of open aperture Gaussian beam Z-scan based on the Adomian decomposition method to obtain nonlinear absorption coefficients along with saturation intensity thresholds. In order to investigate the effect of free carrier absorption, the OA Z-scan experiments were also performed for 65 ps pulse duration. The nonlinear absorption coefficients were found to be 0.58 × 10−4, 1.81 × 10−4 and 3.29 × 10−4 cm/W for 65 ps pulse duration. For nanosecond pulse durations, the obtained nonlinear absorption coefficient values of thin films are bigger than the values of picosecond pulse durations due to the greater contribution of free carrier absorption. Increasing annealing temperature from 100ᵒC to 400ᵒC leads to decreasing nonlinear absorption coefficients owing to decreasing localized defect states.

Impact of phosphorus diffusion on n-type poly-Si based passivated contact silicon solar cells

This paper describes the impact of phosphorus dopant concentrations in n-type passivated contact structures. Free carrier absorption (FCA) of the phosphorus doped poly-Si layers was studied by using different phosphorus dopant concentrations, with a range of from 9 × 1019 to 2.3 × 1020 cm−3, and different poly-Si layer thicknesses, with a range of from 30 to 200 nm. In the worst case, the effect of FCA reduce the short circuit current density by 0.89 mA/cm2. The impacts of tunnel SiO2 thicknesses, poly-Si thicknesses and diffusion temperatures on the performances of the poly-Si passivated contacts were studied. Phosphorus concentrations induced silicon surface field passivation play an important role in passivated contact structure. On the planar n-type mono crystalline-silicon substrates, excellent J0 value of 6.5 fA/cm2 was achieved by optimizing phosphorus doping in poly-Si films and silicon substrate. Different poly-Si thickness and diffusion temperature were studied simultaneously to optimize the phosphorous dopants in the poly-Si passivated contacts for the specified poly-Si thickness. N-type silicon solar cell have been fabricated by using the optimized poly-Si passivated contacts, as addressed above. Average efficiency of 22.52%, and best cell efficiency of 23.04% were demonstrated.

A Novel Blocking State of Silicon Electro-Optical Switch Based on Free Carrier Absorption Effect

We propose and experimentally demonstrate a novel blocking state of a silicon electro-optical switch based on free carrier absorption effect. By injecting the holes and electrons into waveguides of a $2\times 2$ Mach-Zehnder interferometer electro-optical switch, both outputs of the switches can be blocked at the same time. The extinction ratios of two outputs of 24-dB and 28-dB are respectively obtained for the blocking state around 1550 nm.

Communication—Effect of Free-Carrier Absorption on an Anodized Silicon Surface for Producing Dense and Uniform Nanocrystals

The present study disclosed that free-carrier absorption (FCA) activated by 1310-nm laser irradiation caused a distinct effect on the formation of nanocrystals in the porous layer of p++ silicon with the anodization less than 10 minutes. Under ultraviolet excitation, the photoluminescence peak locations are quite different in the irradiated spot (∼535 nm) and in the normal porous silicon (∼640 nm). In the irradiated spot, the intensity of the photoluminescence is at least 10 times higher than that of the peripheral porous silicon layer. This shows that the FCA effect provides an alternative way to produce dense and uniform nanocrystals.

Free-Carrier Absorption Assisted Photoelectrochemistry of Silicon

Free-carrier absorption (FCA) in semiconductors is a process that carriers within the conduction or valence band absorb photons with energy less than bandgap to transfer themselves into a higher energy state without generating electron–hole pairs. The energy absorption activates phonon scattering and is generally considered to have an undesirable impact on the performance of electronic or optical devices. But in an electrochemical processing of silicon, the FCA has a significant impact on the formation of nanocrystals. By the effect of FCA, nanocrystals form directly on the bulk surface via a rate-limited photoelectrochemistry processing. To prove the effect, we employed a 0.9 W/cm2, 1310-nm laser that can penetrate through silicon but be absorbed by free carriers. After anodizing, a brilliant nonetching spot which was irradiated by the laser appeared on the black anodized surface. Under He–Cd laser photoexcitation, we found the peak location of the photoluminescence (PL) in the spot was shifted to 540 nm. The examination of transmission electron microscope showed 2.9 nm silicon nanocrystals embedded in an amorphousness-like layer as silicon quantum dots.

On the infrared absorption coefficient measurement of thick heavily Zn doped GaAs using spectrally resolved modulated photothermal infrared radiometry

In this paper, we report on measurements of the infrared absorption coefficient in the mid-infrared range of a heavily Zn-doped GaAs wafer using spectrally resolved modulated photothermal infrared radiometry (PTR). The method allows us to measure the infrared absorption coefficient of (i) much thicker samples as compared to the one used in Fourier Transform Infrared (FTIR) spectroscopy in transmission configuration and (ii) with non-mirror-like surfaces as would be required for measurements in the reflection configuration. From the best fits of the theoretical model to the PTR results, the values of the infrared absorption coefficient and thermal diffusivity of GaAs wafer are obtained. These values of infrared absorption coefficients are compared both with the literature values on very thin, similarly doped GaAs:Be sample and with infrared absorption coefficients calculated from FTIR specular reflectance measurements on the same sample. FTIR reflectance measurements demand additional assumptions for the evaluation of absorption coefficient and mirror-like surfaces. The results obtained from both experimental methods yield the same order of the infrared absorption coefficients. It is observed that the infrared absorption coefficient decreases with increasing wavelength because of inter-valence band transitions. However, only the infrared spectrum estimated using PTR exhibits free carrier absorption effect at a shorter wavelength as observed in previous works on very thin Be-doped GaAs samples. It is worth mentioning that the presented method is not limited to semiconductors, but can be used for other highly infrared absorbing samples. In addition, the spectrally resolved PTR measurements simultaneously provide the same values of thermal diffusivity of the GaAs wafer within estimation uncertainties thus demonstrating the reliability of the PTR method in the measurement of thermal diffusivity of such samples.

Electrodeless measurement method of conduction type based on the free carrier absorption effect in CdZnTe

In this paper, we review theoretical and experimental studies on the infrared attenuation spectrum between 400 and 4000cm−1, which is principally due to the free carrier absorption (FCA). The free carrier absorption is possible by means of inter-conduction band transitions which occur in n-CdZnTe, or by means of inter-valence band transitions which occur in p-CdZnTe. Based on the power law fit results of absorption spectra determined by FCA characteristics, we present an optical measurement technique, which can help us to determine the conduction type of CdZnTe in another way.

Free Carrier Absorption Assisted Photoelectrochemistry of Silicon

Free-carrier absorption (FCA) in semiconductor is a process which carriers within conduction or valence band absorbs near-bandgap photon energies to transfer themselves into a higher energy state without generating electron-hole pairs. The energy absorption activates phonon scattering and is generally considered to have undesirable impact on the performance of electronic or optical devices. Here we find, in an electrochemical processing of silicon, the FCA not only has a significate impact on chemical reaction but also contributed to the formation of nanocrystals. By the effect of FCA, nanocrystals form directly on the bulk surface via a rate-limited photo-electrochemistry processing. The formation mechanism of silicon quantum dots as follows: phonon scattering is generated at the photo-absorption site with its nearby atoms from absorbing laser as an hole-countercurrent to reduce redox reaction for dissolving silicon and caused amphoteric defects around the point. To proving the concept, we employed a 0.9 W/cm2, 1310 nm laser that can penetrate through silicon but be absorbed by free-carriers. After anodizing, a brilliant non-etching spot irradiated by the laser appeared on the black surface of P++ silicon substrate. Under He-Cd laser photoexcitation, we found the photoluminescence in the spot was enhanced to 450% than the around porous silicon forming in dark in the vicinity of the spot.

Two‐Photon Absorption‐Free Ultrafast Optical Switching in Carbon‐Rich SixC1−x Microring

Ultrafast Kerr effect switching in near‐infrared two‐photon absorption (TPA)‐free and free‐carrier‐absorption (FCA)‐free nonstoichiometric silicon carbide (SixC1−x) microring waveguides is performed and investigated. With pulsed return‐to‐zero on–off‐keying (PRZ‐OOK) stream for the Si‐rich SixC1−x‐based all‐optical switches, the inversely modulated probe reveals an asymmetric bit shape data extinction ratio (ER) of only 8.7 dB owing to the TPA + FCA effect. By eliminating the FCA effects in nearly stoichiometric SixC1−x microring waveguide, the high‐speed wavelength conversion of a data stream with an ER of 14 dB can be observed when coinciding the probe wavelength with the transmission dip. Slightly deviating the probe wavelength from the transmission dip, most probe power still remains in bus waveguide to cause a strong FCA effect to degrade the ER down to 7.4 dB. A symmetrically converted and inverted data stream with high on/off extinction can be observed in the C‐rich SixC1−x‐based microring waveguide with completely suppressed trailing‐edge response. To further approach the upper limitation on bandwidth, the wavelength‐converted and sign‐reversible PRZ‐OOK data switching at a bit rate up to 12 Gbit s−1 can be obtained with similar ER of better than 20 dB, as contributed mainly by the enhanced Kerr nonlinearity in the C‐rich SixC1−x.

All-Silicon Ultra-Broadband Infrared Light Absorbers.

Absorbing infrared radiation efficiently is important for critical applications such as thermal imaging and infrared spectroscopy. Common infrared absorbing materials are not standard in Si VLSI technology. We demonstrate ultra-broadband mid-infrared absorbers based purely on silicon. Broadband absorption is achieved by the combined effects of free carrier absorption, and vibrational and plasmonic absorption resonances. The absorbers, consisting of periodically arranged silicon gratings, can be fabricated using standard optical lithography and deep reactive ion etching techniques, allowing for cost-effective and wafer-scale fabrication of micro-structures. Absorption wavebands in excess of 15 micrometers (5–20 μm) are demonstrated with more than 90% average absorptivity. The structures also exhibit broadband absorption performance even at large angles of incidence (θ = 50°), and independent of polarization.

Carrier dynamics in femtosecond-laser-excited bismuth telluride

The carrier dynamics of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ is studied using the femtosecond pump-probe technique. Three distinct processes, including free carrier absorption, band filling, and electron-hole recombination, are found to contribute to the reflectivity changes. The two-temperature model is used to describe the intraband energy relaxation process of carriers, and the Drude contribution well explains the intensity dependence of the peak values of the nonoscillatory component in the reflectivity signal. The combined effects of free carrier absorption and band filling result in a reflection minimum at about 2 ps after laser excitation. The nonzero background signal increases linearly with the pump fluence, which is attributed to the electron-hole recombination. Finally, our results provide an illustration of investigating the carrier dynamics in semiconductors from the ultrafast reflectivity spectra.

Optically tunable full 360° microwave photonic phase shifter using three cascaded silicon-on-insulator microring resonators

A broadband optically tunable microwave phase shifter with a tunable phase shift covering the entire 360° range using three cascaded silicon-on-insulator (SOI) microring resonators (MRRs) that are optically pumped is proposed and experimentally demonstrated. The phase tuning is implemented based on the thermal nonlinear effect in the MRRs. By optically pumping the MRRs, the stored light in the MRRs is absorbed due to two photon absorption (TPA) to generate free carriers, which result in free carrier absorption (FCA). The FCA effect would lead to the heating of the MRRs and cause a redshift in the phase response, which is used to implement a microwave phase shifter with a tunable phase shift. The device is designated and fabricated on an SOI platform, which is experimentally evaluated. The experimental results show that by optically pumping the MRRs, a broadband microwave photonic phase shifter with a bandwidth of 7GHz from 16 to 23GHz with a tunable phase shift covering the entire 360° phase shift range is achieved.