Intraband Carrier Research Articles

Nonlinear Optical Response in Natural van der Waals Heterostructures

AbstractVan der Waals heterostructures (vdWH) composed from individual 2D crystals offer a platform to obtain unprecedented functionalities that are not accessible in other heterostructures. The research to date has been largely limited to exfoliated and restacked flakes, and the controlled growth of such heterostructures remains a significant challenge. Here, an experimental study of the broadband nonlinear optical performance of a naturally occurring vdWH franckeite is presented, which is composed of alternating SnS2‐like and PbS‐like layers stacked on top of each other. Few‐layer franckeite is prepared via liquid‐phase exfoliation method and its broadband ultrafast third‐order nonlinear optical behavior is characterized experimentally. It is found that the layered franckeite exhibits broadband nonlinear optical response and the ultrafast carrier dynamics with the intraband carrier recovery time of ≈16 ps and the interband carrier recovery time of ≈300 ps. With the large optical nonlinearity of layered franckeite, the all‐optical modulator and switcher have been demonstrated based on the spatial phase modulation effect. The experimental results provide a fundamental understanding of the ultrafast nonlinear optical response in a complex naturally occurring vdWH, and may pave an avenue toward developing novel broadband optoelectronic devices.

Excitation Energy Dependence of Photoluminescence Quantum Yields in Semiconductor Nanomaterials with Varying Dimensionalities.

The excitation energy dependence (EED) of the photoluminescence quantum yield (ΦPL) of semiconductor nanoparticles with varying dimensionalities is reported. Specifically, the EEDs of CdSe quantum dots, CdSe quantum platelets, CdSe quantum belts, and CdTe quantum wires were determined via measurements of individual ΦPL values and photoluminescence efficiency (PLEff(E)) spectra. There is a general trend of overall decreasing efficiency for radiative recombination with increasing excitation energy. In addition, there are often local minima in the PLEff(E) spectra that are most often at energies between quantum-confinement transitions. The average PL lifetimes of the samples do not depend on the excitation energy, suggesting that the EED of ΦPL arises from charge carrier trapping that competes efficiently with intraband carrier relaxation to the band edge. The local minima in the PLEff(E) spectra are attributed to excitation into optically coupled states that results in the loss of carriers in the semiconductor. The EED data suggest that the PLEff(E) spectra depend on the sample synthesis, preparation, surface passivation, and environment.

A Crest Factor Reduction Scheme with Optimum Spacing Peak Cancellation for Intra-band Non-contiguous Carrier Aggregated OFDM Signals

This paper proposes a new crest factor reduction scheme to reduce instantaneous-to-average power ratio (IAR) resulting decent peak reduction, and adjacent channel leakage ratio (ACLR) reduction, though the error vector magnitude (EVM) is significantly enhanced. In the conventional peak cancellation (CPC) technique, peak regrowth occurs as a result of finite impulse response (FIR) filter windows overlapping. The peak regrowth problem in CPC technique can be compensated, though this will lead to increased computational complexity and degradation in EVM. The proposed algorithm allocates spacing to contiguous peaks to avoid FIR filter windows overlapping. Two types of overlapping are discussed, incremental and decremental. The optimum spacing can be determined from the FIR filter impulse response characteristic. Simulations are carried out with different intra-band non-contiguous carrier aggregated OFDM signals. The results of simulations show between 3 and 10% improvement in EVM performance while maintaining equivalent IAR and ACLR performance. Applying the proposed optimum spacing peak cancellation scheme compared to the conventional peak cancellation technique.

Optical properties and dynamics of excitons in Ga(Sb, Bi)/GaSb quantum wells: evidence for a regular alloy behavior

Optical properties and carrier dynamics in 6.6, 10.4, and 14.4 nm wide Ga(Sb, Bi)/GaSb quantum wells (QWs) with ∼10%–11% of Bi were studied by photoluminescence (PL), time-resolved PL, and transient reflectivity. Experiments revealed that low temperature emission is strongly governed by the decay of excitonic population that undergoes weak localization on the QW potential fluctuations rather than the strong defect-like localization typically found for highly mismatched alloys. This statement is supported first by the nearly linear increase of the PL intensity with the excitation power, second, by the lack of the S-shape signature in the temperature-dependent PL studies, and third, the absence of a strong lifetime dispersion for excitons. The low-temperature intraband carrier relaxation time is established in the range of 14–19 ps, nearly independent on the well width, while the exciton lifetime exhibits a well width dependence, i.e. this time decreases from ∼265 ps, through ∼206 ps, to ∼147 ps with the increase of the QW width from 6.6 to 14.4 nm. Our results demonstrate that in contrast to other dilute bismide alloys, GaSbBi behaves as a regular alloy rather than as a highly-mismatched material.

Ultrafast THz Conductivity Dynamics of a Novel Fe-Doped InGaAs Quantum Photoconductor

We propose an Fe-doped, InGaAs-based semiconductor heterostructure optimized for the application as an ultrafast photoconductor and investigate its optical as well as THz properties. A sample series with varying doping concentration is grown by molecular-beam epitaxy. After examination of its static optical properties, the pump-induced intraband carrier dynamics are investigated via optical pump-THz probe measurements. Here, we observe conductivity decay times as low as 0.23 ps which are attributed to electron capture into Fe-related defects. These results are corroborated by monitoring the corresponding interband dynamics via all-optical pump-probe measurements. In addition, the competitiveness of THz detectors fabricated from a subset of these samples is demonstrated by integrating them into a standard time-domain spectrometer.

Hot Phonon and Auger Heating Mediated Slow Intraband Carrier Relaxation in Mixed Halide Perovskite

A clear understanding of excited carrier relaxation to their equilibrium state via carrier-phonon, carrier-carrier interaction etc. is always required for the advancement of perovskite based device performance. Organic/inorganic lead halide perovskite have shown remarkable slow hot carrier cooling properties. Herein, we investigate the effect of hot phonon and Auger heating on hot carrier relaxation dynamics in Methyl-ammonium lead iodide-chloride (CH3NH3PbI3-XClX or mixed halide) perovskite using transient absorption spectroscopy. The reduced energy-loss rate on sub-picosecond time scale clearly shows the non-equilibrium build-up of longitudinal optical and acoustic phonon modes (carrier-phonon interaction) at low pump excitation (1018 cm-3). However at high pump excitation (1019 cm-3), dominating Auger heating process (carrier-carrier interaction) further reduces the energy-loss rate by more than two order of magnitude and delays the carrier relaxation process to tens of picoseconds which makes perovskite as a promising candidates in hot carrier photovoltaic.

Kinetic modelling of intraband carrier relaxation in bulk and nanocrystalline lead-halide perovskites.

The relaxation of high-energy "hot" carriers in semiconductors is known to involve the redistribution of energy between hot and cold carriers, as well as the transfer of energy from hot carriers to phonons. Over the past few years, these two processes have been identified in lead-halide perovskites (LHPs) using ultrafast pump-probe experiments, but their interplay is not fully understood. Here we present a practical and intuitive kinetic model that accounts for the effects of both hot and cold carriers on carrier relaxation in LHPs. We apply this model to describe the dynamics of hot carriers in bulk and nanocrystalline CsPbBr3 as observed by multi-pulse "pump-push-probe" spectroscopy. The model captures the slowing of the relaxation dynamics in the materials as the number of hot carriers increases, which has previously been explained by a "hot-phonon bottleneck" mechanism. The model also correctly predicts an acceleration of the relaxation kinetics as the number of cold carriers in the samples is increased. Using a series of natural approximations, we reduce our model to a simple form containing terms for the carrier-carrier and carrier-phonon interactions. The model can be instrumental for evaluating the details of carrier relaxation and carrier-phonon couplings in LHPs and other soft optoelectronic materials.

Bandwidth‐switchable cellular CMOS RF receiver front‐end for intra‐band carrier aggregation

AbstractA bandwidth‐switchable cellular complementary metal‐oxide‐semiconductor radio frequency receiver front‐end design employing a current‐reusing technique is presented in a 65‐nm RF CMOS technology. The proposed single‐ended receiver front‐end can fully support both narrowband and wideband multiple‐channel RF signals for up to three intraband carrier aggregations with high performance and low direct current consumption. To achieve different bandwidth‐handling capability, a narrowband‐ and a wideband‐switchable low noise amplifier is provided to realize appropriate matching conditions and transconductance, while maintaining high performance in multiple component carrier (CC) modes. The design operates at frequency bands of 700‐2700 MHz with 1.2 V. The design achieves conversion gains of more than 40 and 46 dB and noise figures (NFs) of less than 4.9 and 3.5 dB in wideband and narrowband modes, respectively. NF degradation in multiple CC modes is less than 0.5 dB for both bandwidth conditions.

Impact of one step alloying on the carrier relaxation and charge separation dynamics of CdxZn1-xSe graded nanocrystals

Slow carrier relaxation and extent of charge separation in semiconductor nanocrystals (NCs) are the desired parameters, which control the power conversion efficiency in photovoltaic. Although several synthesis approaches have explored, the spectroscopic analysis of the fundamental understanding of charge carrier dynamics in the quantum confined states of cadmium zinc selenide (CdxZn1-xSe) graded alloy NCs remain relatively unexplored. Here, we study the charge carrier dynamics of a series of CdxZn1-xSe alloy NCs by changing the metal (Cd and Zn; M) to selenide ratio adopting one-step synthesis. Our findings underline that both excitonic absorption and band edge photoluminescence (PL) of all the alloys are red shifted concerning the binary analogs (CdSe and ZnSe), which implies the formation of gradient alloy structure where the core is CdSe rich that leads to increasing light absorption cross-section in the solar spectrum. Femtosecond transient absorption (fs-TA) measurements of the alloy NCs reveal that the CdxZn1-xSe alloy show three distinct bleaches due to the different electronic transition like 1S (1Se-1S3/2), 2S (1Se-2S3/2), and 1P (1Pe-1P3/2) while only CdSe shows two bleaches due to 1S, and 1P. The distinctive band structure (quasi type-II) of the alloy leads to slow down the intraband carrier cooling time and prolonging the charge separation due to specially decoupled photo-excited electron and hole. This finding offers new insights to understand the photophysical behavior and suggest that CdxZn1-xSe graded NCs can be used as efficient light harvesting materials in quantum dot solar cell.

A Carrier Aggregation Transmitter Front End for 5-GHz WLAN 802.11ax Application in 40-nm CMOS

Carrier aggregation will be a key feature of the emerging 802.11ax system to boost data rate. Voltage-controlled oscillator (VCO) pulling and crosstalk between RF channels are two main problems related to carrier aggregation, which introduce severe impacts on adjacent channel leakage ratio (ACLR) and error vector magnitude (EVM). To address these two problems, high isolation between transmitter channels is required, but may not be realizable in some practical cases due to the constraint of the layout. This article presents a transmitter front end that can support two-carrier aggregation for 802.11ax without high on-chip isolation. The proposed transmitter front end employs a parallel direct conversion and double conversion configuration, and two carriers are combined before the power amplifier (PA). In this way, the intrinsic problems of VCO pulling and crosstalk can be mitigated. The proposed transmitter front end was implemented in a 40-nm CMOS technology. For contiguous intraband carrier aggregation with VHT80, MCS9 signals [80-MHz bandwidth, 256-quadrature amplitude modulation (QAM), and 11.25-dB peak-to-average power ratio (PAPR)], the transmitter front end delivers an average output power of 5.3 + 4.8 dBm for two carriers with an EVM of ≤−32 dB, and the EVM can reach −36.1 dB.

Auger Suppression in n-Type HgSe Colloidal Quantum Dots.

Transient infrared photoluminescence upconversion is used to study the exciton dynamics in small-gap HgSe colloidal quantum dots in the 2000-6500 cm-1 (0.25-0.80 eV) range. The intraband mid-infrared photoluminescence decays show absent or greatly reduced Auger relaxation of biexcitons, proposed as a generic feature of weakly n-type quantum dots due to the sparse density of states in the conduction band. The nonradiative relaxation of the intraband carriers is instead consistent with near-field energy transfer to molecular vibrations of the surface ligands. In contrast, the interband near-infrared photoluminescence decays exhibit the typical distinct exciton and biexciton lifetimes with Auger coefficients comparable to other similarly sized quantum dots. Also observed are spectral and dynamical evidence of fine structure in the intraband transitions consistent with spin-orbit splitting of the electron P levels, and the emergence of plasmonic resonances in large particles.

Carrier heating and its effects on the current-voltage relations of conventional and hot-carrier solar cells: A physical model incorporating energy transfer between carriers, photons, and phonons

Theoretical models and frameworks are developed to investigate the effects of carrier heating on conventional and hot-carrier solar cells. They incorporate intraband carrier energy relaxation into the Shockley-Queisser model for conventional solar cells and the Ross-Nozik model for hot-carrier solar cells. Explicit formulas are presented to calculate intraband carrier energy relaxation rates via the interactions of polar longitudinal optical phonons and deformation-potential longitudinal and transverse optical phonons. These formulas incorporate the decay processes and phonon bath sharing of optical phonons. The effects of carrier heating on the current-voltage characteristics of conventional and hot-carrier solar cells are investigated and discussed for GaAs, InP, GaSb, and InAs materials. This paper will particularly discuss the differences between extraction cooling in hot-carrier solar cells and recombination heating from stimulated photon emissions in semiconductor lasers. It will prove that carriers in hot-carrier solar cells need to be extracted with a selective-contact energy larger than their quasi-Fermi-level separation in order to make devices work with extraction cooling, rather than recombination heating.

Dual-Band 3-Way Doherty Power Amplifier With Extended Back-Off Power and Bandwidth

This brief presents an energy-efficient dual-band 3-way Doherty power amplifier (DPA) suitable for wireless infrastructure that transmits intra-band and inter-band carrier aggregated communication signals with high peak-to-average power ratios. This brief begins with a bandwidth analysis of dual-band impedance inverting networks, especially when used to absorb the transistors' output capacitances. This analysis produced a set of equations which directed the synthesis of a dual-band 3-way DPA combining network capable of producing the proper load modulation over an extended output back-off while maintaining acceptable fractional bandwidth (FBW). A 60-watt dual-band 3-way DPA was designed as a proof-of-concept demonstrator to operate in two bands, 2.0-2.2 GHz and 3.3-3.6 GHz. Under continuous wave stimuli, drain efficiencies of about 48% and 43% were recorded at an output back-off of 9dB in the two bands. These efficiencies were maintained over a FBW of about 10%. The linearizability of the fabricated DPA using digital pre-distortion techniques was also confirmed under intra-band and inter-band carrier-aggregated signals.

A Wideband Inductorless LNA for Inter-band and Intra-band Carrier Aggregation in LTE-Advanced and 5G

A Wideband Inductorless LNA for Inter-band and Intra-band Carrier Aggregation in LTE-Advanced and 5G

Observation of crossover from intraband to interband nonlinear terahertz optics.

In this Letter, we investigate the nonlinear effects of extremely intense few-cycle terahertz (THz) pulses (generated from the organic crystal 4-NN, NN-dimethylamino-4'4'-N'N'-methyl-stilbazolium 2, 4, 6 trimethylbenzenesulfonate, with peak electrical fields of a few MV/cm) on the carrier dynamics in n-doped semiconductor thin film In0.53Ga0.47As. By performing open-aperture Z-scan measurements and recording the transmitted THz energy through semiconductor sample, we observed a strong THz absorption bleaching effect at high fields, followed by an absorption enhancement at even higher fields. We attribute our observations to a crossover from pure intraband carrier dynamics to an interplay between intraband carrier heating and interband carrier generations.

Effects of longitudinal optical phonon lifetimes on hot-carrier solar cells: a theoretical study

Theoretical models and explicit formulas with the decay processes of optical phonons are presented for calculating intraband carrier energy relaxation rates. They are incorporated into the Ross-Nozik model for hot-carrier solar cells. Effects of LO phonon lifetimes on hot-carrier solar cells are investigated. The results demonstrate the potential of phononic engineering for producing optical-absorber materials with prolonged LO phonon lifetimes and slowed intraband carrier energy relaxation. They also suggest that using such materials can potentially provide a direct method of realizing hot-carrier solar cells.

Carrier Lifetimes and Polaronic Mass Enhancement in the Hybrid Halide Perovskite CH3NH3PbI3 from Multiphonon Fröhlich Coupling

We elucidate the nature of the electron-phonon interaction in the archetypal hybrid perovskite CH_{3}NH_{3}PbI_{3} using abinitio many-body calculations and an exactly solvable model. We demonstrate that electrons and holes near the band edges primarily interact with three distinct groups of longitudinal-optical vibrations, in order of importance: the stretching of the Pb-I bond, the bending of the Pb-I-Pb bonds, and the libration of the organic cations. These polar phonons induce ultrafast intraband carrier relaxation over timescales of 6-30fs and yield polaron effective masses 28% heavier than the bare band masses. These findings allow us to rationalize previous experimental observations and provide a key to understanding carrier dynamics in halide perovskites.

Femtosecond pulse generation beyond photon lifetime limit in gain-switched semiconductor lasers

Femtosecond semiconductor lasers are ideal devices to provide the ultrashort pulses for industrial and biomedical use because of their robustness, stability, compactness and potential low cost. In particular, gain-switched semiconductor lasers have significant advantages of flexible pulse shaping and repetition rate with the robustness. Here we first demonstrate our laser, which is initiated by very strong pumping of 100 times the lasing threshold density, can surpass the photon lifetime limit that has restricted the pulse width to picoseconds for the past four decades and produce an unprecedented ultrashort pulse of 670 fs with a peak power of 7.5 W on autocorrelation measurement. The measured phenomena are reproduced effectively by our numerical calculation based on rate equations including the non-equilibrium intraband carrier distribution, which reveal that the pulse width is limited by the carrier–carrier scattering time, instead of the photon lifetime.

Probing the free-carrier absorption in multi-layer black phosphorus

We study the carrier relaxation dynamics in thin black phosphorus (bP) using time-resolved differential transmission measurements. The inter-band and intra-band transitions, relaxation, and carrier recombination lifetimes are revealed by tuning the mid-infrared probe wavelength above and below the bandgap of black phosphorus. When the probe energy exceeds the bandgap, Pauli blocked inter-band transitions are observed. The differential transmission signal changes sign from positive to negative when the probe energy is below the bandgap, due to the absence of inter-band transitions and enhancement in the free-carrier absorption. The minority carrier lifetime and radiative recombination coefficient are estimated to be 1.3 ns and 5.9 × 10−10 cm3/s, respectively. The overall recombination lifetime of bP is limited by radiative recombination for excess carrier densities larger than 5 × 1019 cm−3.

Photoexcited states in correlated band insulators

We study the photo-excitation dynamics of correlated band insulators, using non-equilibrium dynamical mean-field theory for the ionic Hubbard model. We find two distinct behaviors, depending on the ratio of the on-site interaction $U$ and the bare band gap $\Delta$. For small interactions, the relaxation is characterized by intra-band carrier scattering in relatively rigid bands, leading to a non-thermal intermediate state with separate thermal distributions of electrons and holes. This behavior can be viewed as typical for a band insulator with weak interactions. For larger interaction, on the other hand, we observe a strong modification of the electronic spectrum and a filling-in of the gap after photo-excitation, along with a rapid thermalization of the system. The two behaviors therefore provide a dynamical distinction of a correlated band insulator and a band-insulator, which can differ even when the spectra of the two systems are similar in equilibrium. The crossover happens when the interaction $U$ is comparable to $\Delta$.