High Bias Currents Research Articles

Improvement of optical noise in optical-injection-locked quantum dot lasers epitaxially grown on silicon by reducing external carrier noise

Abstract This study theoretically investigates the impact of external carrier noise from pumping sources on the optical noise of epitaxial quantum dot (QD) lasers on silicon. The findings indicate that the spectral linewidth and relative intensity noise (RIN) of silicon-based QD lasers using a quiet pump are significantly reduced. At 5.5 times the threshold current, the spectral linewidth decreases from 337.2 kHz to 213.2 kHz, and the RIN decreases from −141.3 dB Hz−1 to −168.8 dB Hz−1. This reduction is attributed to the lower external carrier noise level of the quiet pump, which also suppresses the spectral linewidth rebroadening effect at a high bias current. Moreover, when external optical injection locking is applied, the spectral linewidth further decreases to 24 kHz at an injection ratio of −70 dB and to 1.8 mHz at 0 dB. The RIN also slightly decreases to −172.4 dB Hz−1 with an injection ratio of 10. These results demonstrate that using a quiet pump is an effective and manageable strategy for significantly reducing both the spectral linewidth and RIN of QD lasers, thereby facilitating their application in next-generation photonic integrated circuits, continuous-variable quantum key distribution, quantum computing and ultra-precise quantum sensing.

Improvement in CPP-GMR read head sensor performance using [001]-oriented polycrystalline half-metallic Heusler alloy Co2FeGa0.5Ge0.5 and CoFe bilayer electrode

ABSTRACT A current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) device with a half-metallic electrode is one of the most promising candidates of next-generation read head for hard disk drive. In this study, we fabricate [001]-oriented polycrystalline CPP-GMR devices with the normal ferromagnet (NFM) CoFe/half-metallic ferromagnet (HMFM) Co2FeGa0.5Ge0.5 (CFGG) bilayer electrodes to enhance the magnetoresistance (MR) ratio by large interfacial spin-dependent scattering at the NFM/HMFM interface. The CoFe/CFGG bilayer electrode provides the additional large interfacial spin-dependent scattering and achieves high MR ratio of 22.7% with the CoFe(4.5 nm)/CFGG(2.5 nm) bilayer electrodes, which is almost three(two) times larger than the MR ratio with the single CoFe(CFGG) (7 nm) electrodes. The bias voltage dependent study revealed an additional advantage of increasing the output voltage |ΔV| by using the CoFe/CFGG bilayer due to the improvement of the endurance against spin-transfer torque under high bias current. A maximum output voltage Δ V max of 6.5 mV was obtained with the CoFe(5.5 nm)/CFGG(1.5 nm) electrodes, which is the highest ever reported in the CPP-GMR devices with a uniform metallic spacer including high-quality epitaxial devices.

Suppressing the Thermal Quenching Effect via a Cluster Conformer in Copper(I)-Iodide Coordination Polymeric Phosphors for High-Power White LED Lighting.

High-power LED lighting is a crucial challenge due to the notorious thermal quenching (TQ) effect of traditional phosphors at high operating currents, which would result in poor device performance and hamper practical optoelectronic application. Herein, we demonstrate ligand engineering of a cubane- versus staircase-like [Cu4I4] conformer as a node in coordination polymers, which remarkably suppresses the TQ effect of cluster-based photoluminescence. For complex 1 (the formula [Cu4I4(bbimb)2]n) with the cubane-like [Cu4I4] conformer as a node, the metallophilicity interaction enables ultrabright triplet emission with a photoluminescence quantum yield over 82%, and the phonon-assisted detrapping process of excitons effectively suppresses the TQ effect in the wide temperature range. In contrast, the staircase-like [Cu4I4] conformer as a node in complex 2 (the formula [Cu4I4(bbtmb)2]n) exhibits a serious TQ effect over the investigated temperature. Phosphor-converted white LEDs (pc-wLEDs) were fabricated by integrating the cluster-based coordination polymers as a color converter, and their electroluminescence performances were investigated under high bias currents. The prototype pc-wLED device by incorporating the phosphor with the suppressed TQ effect exhibits a continuous rise in brightness under a high bias current of 300 mA. The results demonstrate that ligand engineering of the cluster conformer via suppressing the TQ effect proves efficient in designing an ideal color converter for high-power pc-wLED lighting.

Cascaded optically injection-locked semiconductor laser, rate equations analysis, frequency response, and its application for complex optical signal generation.

In this paper, we introduce the concept of the cascaded optically injection-locked (OIL) semiconductor laser and present its novel rate equations. Then, the new locking range for this configuration has been obtained by mathematical demonstration. Subsequently, we modified a new adjustment for the detuning frequencies (Δ f i n j ) of the cascaded OIL system as well as the linewidth enhancement factors (α) values. Utilizing these tunings, improvements in the steady-state photon number and the phase modulation (PM) range become possible. Afterward, we define the generation of the complex optical signal area and extract the transfer function for investigating the frequency response of the cascaded system. The simulations have been performed once with identical α values and once with the various α values in the slave laser (SL) stages. We conclude that these novel proposed adjustments, combined with a strong injection ratio (Rinj) of 15 dB and a high bias current, can significantly broaden the bandwidth near 700 GHz while maintaining the fair gain available up to 180 GHz. Further, the generation of complex optical signal areas has been boosted for high-quality complex modulation applications. Eventually, we exhibit a novel approach for generating different α values in the SL stages by applying managed temperature variations in the experimental setup of the cascaded system, regardless of employing similar SLs.

17.1: SRAM‐based LED CMOS driver circuit for a 512x512 GaN microdisplay

GaN microLED technology has the potential to offer displays with high brightness, bandwidth, and long lifetime with a very low energy consumption. Moreover, GaN‐on‐Si hybrid interconnection technology allows the development of displays with a very high pixel density integration. In this work we present an in‐pixel driving circuit designed in a 0.18µm CMOS technology to be integrated on a 512x512 microLED array by using the KlettWelding hybrid interconnection technique forming a microLED display of 1411 ppi with 10kfps working speed. The pixel driver is able to achieve switching times of 1MHz and can be operated at high bias currents of 120µA. The individual driver consists of 4 SRAMs that apply a weighted current and allows to avoid the current stability problems associated to CMOS backplanes in hybridly interconnected displays.

Experimental demonstration of an 8-Gbit/s free-space secure optical communication link using all-optical chaos modulation.

For the first time, to the best of our knowledge, we experimentally demonstrate a high-speed free-space secure optical communication system based on all-optical chaos modulation. The effect of atmospheric turbulence on optical chaos synchronization is experimentally investigated via a hot air convection atmospheric turbulence simulator. It is shown that, even under moderately strong turbulent conditions, high-quality chaos synchronization could be obtained by increasing the transmission power. Moreover, a secure encryption transmission experiment using a high bias current induced chaotic carrier for 8-Gbit/s on-off-keying data over a ∼10-m free-space optical link is successfully demonstrated, with a bit-error rate below the FEC threshold of 3.8 × 10-3. This work favorably shows the feasibility of optical chaotic encryption for the free-space optical transmission system.

Enhanced reverse inter-system crossing process of charge-transfer stated induced by carrier balance in exciplex-type OLEDs

The reverse inter-system crossing (RISC, CT<sup>3</sup> → CT<sup>1</sup>) process in charge transfer (CT<sup>1</sup> and CT<sup>3</sup>) states is an effective approach to improving the energy utilization rate of excited states, and precise control and full use of the RISC process have important scientific significance and application prospect for fabricating and realizing the efficient exciplex-type organic light-emitting diodes (OLEDs). The conventional exciplex-type OLEDs based on m-MTDATA: Bphen have received extensive attention among researchers owing to the fact that the energy difference between CT<sup>1</sup> and CT<sup>3</sup> around zero promotes the efficient occurrence of RISC process. But up to now, only transient photoluminescence can infer the existence of RISC process in experiment, which is quite unfavorable for the comprehensive understanding and application of this process to design high-performance OLEDs. Fortunately, in this paper, a series of balanced and unbalanced exciplex-based devices are prepared by changing the donor-acceptor blending ratio in the emitting layer (<i>x</i>% <i>m</i>-MTDATA:<i>y</i>% Bphen; <i>x</i>%, <i>y</i>% is the weight percent) and the carrier density flowing through the device. The RISC process of CT states is directly observed via analyzing fingerprint magneto-conductance (MC) traces of the balanced device at room temperature, and the balanced device has higher electroluminescence (EL) efficiency than the unbalanced device. Specifically, the low-field MC curves of unbalanced device only show an inter-system crossing (ISC) line shape, whereas those from the balanced exciplex device present an RISC line shape at low bias-current and the conversion into an ISC line shape with the further increase of bias current. The line shape transition from RISC to ISC is attributed to the triplet-charge annihilation (TQA) process caused by excessive charge carries under high bias current. Combining the physical microscopic mechanism of device, the above-mentioned MC curves of various exciplex devices can be explained as follows: under the same bias current, extra holes or electrons are generated in the emitter layer of unbalanced devices due to the mismatch of donor-acceptor molecular concentrations. These superfluous holes or electrons will react with the CT<sup>3</sup> state, which aggravates the TQA process in the device and weakens the RISC process in which the CT<sup>3</sup> state participates. That is to say, there are strong TQA process and weak RISC process in unbalanced exciplex device. Contrarily, the strong RISC process and weak TQA process in the balanced exciplex device are beneficial to the occurrence of delayed fluorescence, resulting in its EL efficiency higher than that of the unbalanced device. This work not only deepens the physical understanding of the influence of donor-acceptor blending ratio on the carrier balance in exciplex devices, but also paves the way for designing highly efficient OLED by fully employing the RISC process of balanced device.

A Copper Iodide Cluster-Based Coordination Polymer as an Unconventional Zero-Thermal-Quenching Phosphor.

Phosphor-converted white light-emitting diodes (pc-wLEDs) are promising candidates for next-generation solid-state lighting and display technologies. However, most of the conventional phosphors in pc-wLED devices suffer from serious thermal quenching (TQ) at high temperatures. Herein, we investigate an unconventional high-efficiency metal-halide cluster-based phosphor with dynamic Cu-Cu interactions that can resist the TQ effect of photoluminescence. The temperature-dependent structure and solid-state and in situ NMR spectroscopy reveal that the weakening of the Cu-Cu interaction in such a phosphor system enables the electronic structural transition from a bonding to a nonbonding state and hence sustains the PL efficiency at high temperatures (up to 100 °C). The pc-wLEDs incorporating the zero-TQ phosphor show a rapid brightness rise even at a high bias current (1000 mA) with a color rendering index as high as 90, comparable to the commercial phosphor-based prototype LEDs (e.g., YAG:Ce3+). This work establishes a novel prototype of a cluster-based phosphor featuring dynamic intermetallic interactions, which paves the way for the exploration of pc-wLEDs against thermal quenching.

A Red-Emitting Cu(I)–Halide Cluster Phosphor with Near-Unity Photoluminescence Efficiency for High-Power wLED Applications

Solid-state lighting technology, where light-emitting diodes (LEDs) are used for energy conversion from electricity to light, is considered a next-generation lighting technology. One of the significant challenges in the field is the synthesis of high-efficiency phosphors for designing phosphor-converted white LEDs under high flux operating currents. Here, we reported the synthesis, structure, and photophysical properties of a tetranuclear Cu(I)–halide cluster phosphor, [bppmCu2I2]2 (bppm = bisdiphenylphosphinemethane), for the fabrication of high-performance white LEDs. The PL investigations demonstrated that the red emission exhibits a near-unity photoluminescence quantum yield at room temperature and unusual spectral broadening with increasing temperature in the crystalline state. Considering the excellent photophysical properties, the crystalline sample of [bppmCu2I2]2 was successfully applied for the fabrication of phosphor-converted white LEDs. The prototype white LED device exhibited a continuous rise in brightness in the range of a high bias current (100–1000 mA) with CRI as high as 84 and CCT of 5828 K, implying great potential for high-quality white LEDs.

Modeling of Relative Intensity Noise in QD-VCSEL

In this paper, the dynamics and statics properties of a 1.3 m InAs/InGaAs Quantum-Dot (QD) vertical-cavity surface-emitting laser (VCSEL) have been investigated using a self-consistent coupled rate-thermal equations model. Results demonstrate that self-heating caused by current transition through the Bragg mirrors and active area of the laser produces temperature rise and deteriorates the performance of the laser where it is most apparent at higher temperatures. Although the minimum value of Relative intensity noise (RIN) and the turn-on delay and maximum value of the output power occurs at the same bias current, the maximum achievable 3-dB modulation bandwidth is obtained at lower bias currents. In addition, the larger aperture size and the greater number of QD layers modify the self-heating and enhance the efficiency and output characteristics of the laser where the rolled-off of the laser happens at higher bias currents.

Relative Intensity Noise and Linewidth for Hybrid-Cavity Semiconductor Lasers

Low relative intensity noise (RIN) was demonstrated experimentally for hybrid square-rectangular lasers (HSRLs). In this paper, RIN of the HSRL is numerically simulated and the laser linewidth is investigated theoretically and experimentally. The non-zero delayed self-heterodyne method is utilized to measure the linewidth, and the laser linewidth of 2.9 MHz is obtained experimentally. A two-section single-mode rate equation model including Langevin noise sources is established to study the noise characteristics theoretically. The time-varying mode photon density and frequency are simulated, then the intensity and frequency noise (FN) spectra are calculated. With the rising of bias currents, RIN decreases and approaches the standard quantum limit. Large mode <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor, high bias current, and passive square microcavity are effective methods to reduce FN and linewidth of the HSRLs. By inducing a passive square microcavity, we can expect a low linewidth of 37 kHz for the hybrid-cavity laser with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> = 1.17 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> .

On the Dynamic Range and Resolution of Thermal-Piezoresistive Resonant Mass Sensors

Thermal-actuation and piezoresistive-detection effects have been employed to pump the effective quality factor of MEMS resonators, targeting better mass sensing performance in air. In this paper, frequency resolution (bias instability) of a thermal-piezoresistive resonator operating in air at room temperature is experimentally investigated. It is found that the dynamic range decreases when increasing the bias direct current whereas the effective quality factor rises. The measurement results indicate a maximum effective quality factor of 169k with a dynamic range of 47.8 dB for a bias current of 6.25 mA, and a minimum effective quality factor of 11.3k with a dynamic range of 70.1 dB for a bias current is 5.8 mA. Our work also shows that the frequency and amplitude bias instabilities are significantly lower due to the dynamic range decrease for a high bias current. [2021-0250]

Intra-device gating effect in graphene electrode-based organic diodes

Current–voltage characteristics of graphene electrode-based thin-film organic diodes show non-saturating high reverse bias currents, unlike standard indium tin oxide-based devices. We show that this deviation is controlled by the intra-device self-gating electric field on graphene anode from the counter cathode through the organic thin-film device layer. This effect contributes to additional charge dynamics due to the quantum capacitance presented by the semi-metallic graphene in series to the semiconductor capacitance of the organic device layer. This leads to a dynamic workfunction change of the graphene electrode as a function of the applied voltage across the diode. The proposed self-gating phenomenon is captured using an analytical model established using the combination of current–voltage and capacitance–voltage relations in these devices based on drift and diffusion equations. The results of the analytical model agree well with the experimental measurements carried out with fabricated P3HT:PCBM based organic thin-film diodes with graphene anode and aluminum cathode. We further support these results with elaborate self-consistent numerical simulations, all showing an increased reverse bias current contributed by the self-gating effects in graphene devices. Our results, being obtained from fundamental device physics, are applicable to a broad range of other devices like light-emitting diodes, perovskite-based devices, and thin-film a-Si devices. • The work function of graphene changes when subject to an external electric field. • The field effects, when employed as an electrode in a thin-film organic device, can impart changes to the behavior of these devices. • A combinational approach contributed by the device layer and the graphene stack can map this behavioral change. • The results showed a non-saturating higher reverse currents in graphene electrode-based organic diodes.

Enhanced spin accumulation in nano-pillar-based lateral spin valve using spin reservoir effect

Lateral spin valves are ideal nanostructures for investigating spin-transport physics phenomena and promoting the development of future spintronic devices owing to dissipation-less pure spin current. The magnitude of the spin accumulation signal is well understood as a barometer for characterizing spin current devices. Here, we develop a novel fabrication method for lateral spin valves based on ferromagnetic nanopillar structures using a multi-angle deposition technique. We demonstrate that the spin-accumulation signal is effectively enhanced by reducing the lateral dimension of the nonmagnetic spin channel. The obtained results can be quantitatively explained by the confinement of the spin reservoir by considering spin diffusion into the leads. The temperature dependence of the spin accumulation signal and the influence of the thermal spin injection under a high bias current are also discussed.

Impact of base bias current and incident optical power on the InP/InGaAs heterojunction phototransistor performances

In this article, the optical and electrical properties of NPN InP/InGaAs heterojunction bipolar phototransistor (HPT) are studied by two dimensional semiconductor simulation. The phototransistor is modelled using a numerical method, and the results are presented and discussed. This model is based on heterojunction bipolar transistor (HBT) model with the consideration of the optical beam effect. Responsivity of 12.62 A/W for 1100 nm light is achieved when the thickness of base layer is 50 nm. A good qualitative agreement of the numerical and analytical simulated value of both collector current and responsivity as a function of the wavelength with the existing experimental data was achieved. We present a comparative analysis of our obtained solutions at IB = 5 μA with that simulated at IB = 100 μA and we found that the value of optical power (Popt) giving high injection into a phototransistor at IB = 5 μA is three times higher than that at IB = 100 μA. The effects of both the base bias current and the incident optical power on the device performance are investigated. The simulation results, which are presented and compared at low, ideal and high base bias current, show that the highest photocurrent and optical gain are obtained when the base bias current reaches the ideal region. The HPT optical characteristics such as the electrical characteristics of HBT are affected by the high injection effect (Kirk effect).

Unidirectional emission and detection of Lamb waves based on a powerful and compact coils-only EMAT

This study investigated a compact noncontact electromagnetic acoustic transducer (EMAT) without bulky magnets or separate electromagnets for unidirectional transmission and reception of ultrasonic Lamb waves in 1 mm thick steel plates. The transducer head consists of only two, closely adjacent and elongated flat coils. The powerful sensor technology bridged air gaps up to 1.5 mm. Two phase-adjusted 1 MHz pulses at 1 kV amplitude enhanced the sideways-projected ultrasound field in one direction; others were attenuated by destructive interference. A 5 mm ultrasound wavelength with 1 mm thick steel plate was used for testing; wave field calculations were in good agreement with results. As a possible application, an inaccessible edge of a steel plate was scanned, where the difficulty was increased by an additional edge at a similar distance representing competing interfering reflections. More general progress beyond this specific non-contact application is the demonstration of phase-accurate ultrasound transmission and sensitive echo reception with a compact system of mutually coupling inductors at high power and continuous high-bias currents (1 kA), allowing compact and powerful EMAT phased arrays based on single-layer densely arranged coils.

Generation of laser chaos with wide-band flat power spectrum in a circular-side hexagonal resonator microlaser with optical feedback.

We numerically demonstrate the generation of wide-band laser chaos with flat power spectrum in a 2D circular-side hexagonal resonator (CSHR) microlaser subject to long-cavity optical feedback. The bandwidth and flatness of the chaotic power spectrum are investigated under different bias currents and optical feedback rates. Under low bias current, the bandwidth under an optimized optical feedback rate increases obviously as raising bias current and the power spectrum flatten simultaneously. Under high bias current, the optimized bandwidth gradually tends toward stabilization, with corresponding flatness less than 5 dB. We compare the chaotic power spectra with small-signal modulation response (SSR) curves under different bias currents. It can be concluded that wide-band and flat SSR indicates wide-band and flat chaotic power spectrum. This work argues that we can enhance laser chaos by using a laser device with wide-band and flat SSR and simple optical feedback configuration, which is significantly beneficial to synchronization-based applications including chaos communication and key distribution.

The Effect of Self-Heating on the Modulation Characteristics of a Microdisk Laser

The operation speed of microdisk lasers with quantum dots working at room temperature without thermal stabilization has been experimentally examined, and the widest modulation bandwidth of microdisks with various diameters has been calculated. It was shown that taking into account the effect of self-heating of a microlaser at high bias currents, which is manifested in a decrease of the ultimate operation speed and in an increase in the current at which the widest modulation bandwidth is reached, enables a good description of the experimental data. The self-heating most strongly affects microlasers with a small diameter (less than 20 μm).

A Study of Ultra-Thin Superconducting Films at High Bias Currents in Different Cooling Environments

The influence of the cooling method on thermal stability is a non-trivial aspect for superconducting material to be used in several applications, for example, in photon detection. Indeed, a wasteful cooling can induce quenches in the device that can lead to false counts. So far, the efficient cooling has been realized by immersion of the superconducting device in a liquid He bath. However, cheaper cryogen-free (CF) cooling techniques are now commercially available, as well as cryocoolers are becoming the only way to obtain a cooling environment in the liquid He shortage. Here we consider three different cooling methods: one is by liquid He in a standard cryostat and the other two are a dynamic or a static He gas cooling in a CF cryostat. Then, we are able to evaluate the performance of the cooling method by the impact on current-voltage curves at very high bias currents. In particular, we acquire current-voltage characteristics on ultra-thin microbridges made by two different superconducting materials commonly used in detectors fabrication, which are NbN and NbTiN. Here, the flux-flow instability (FFI) phenomenon is used as a tool to determine the influence of the three different cooling techniques on the voltage stability of the devices under current biasing. It results that the CF cooling method has performance comparable to liquid He bath. This finding supports the spread of the CF technique for applications, and it validates the FFI as a tool to test superconducting materials.

Design, Simulation and Fabrication of Highly Sensitive Cooled Silicon Bolometers for Millimeter-Wave Detection

This paper reports our results on the electrothermal modeling of cryogenic silicon bolometers with pixel pitches of 500 and 1200 µm designed for cosmic microwave background polarimetric observation in 0.6 mm and 1.5 mm bands. These detectors should provide a high responsivity, typically around 10$^{11}$ V/W, and a very low noise equivalent power (NEP) of 10$^{−18}$ W/Hz$^{1/2}$ between 50 and 100 mK. They are based on doped silicon thermometers, which exhibit a nonohmic behavior described by the “hot electron model” (HEM) at very low temperature under high bias currents. We compare this model to the experimental characterization of these thermometers at cryogenic temperatures to confirm that the HEM is governing their electrical characteristics and their sensitivity at very low temperature. Finally, this model is used to derive the simulated responsivity and NEP performances of the pixels under weak and moderate optical power illumination.