Fine Tunability Research Articles

An Area-Efficient and Tunable Bandwidth- Extension Technique for a Wideband CMOS Amplifier Handling 50+ Gb/s Signaling

This paper reports an area-efficient and tunable bandwidth (BW)-extension technique for a wideband CMOS amplifier to handle very high rate (50+ Gb/s) signaling while keeping a low jitter penalty. We identify its architectural advantages by correlating the performances with the frequency domain (magnitude and group delay (GD) responses) and time domain (impulse and step responses) and comparing them with the existing solutions. Specifically, our technique enables a flexible ac characteristic by introducing a tunable grounded active inductor in the bridged-shunt peaking topology, offering: 1) a high BW enhancement ratio (BWER = $2.65\times$ ); 2) BW-power scalability with small in-band gain variation; and 3) fine tunability of the passband gain without affecting the BW, GD, and power. The experimental prototype is a 65-nm CMOS four-stage differential amplifier occupying just 0.0077 mm2. It delivers a 15-dB gain over a 43-GHz BW with 45-mW power consumption. Small in-band gain variation (0.58 dB) and ripple (1.53 dB) are concurrently achieved with low in-band GD variation (17 to 35.3 ps) and ripple (18.3 ps). The achieved figure of merit of 5.48 [(dc Gain $\times$ BW)/Power] compares favorably with the prior art.

A Designed Ladder‐Type Heteroarene Benzodi(Thienopyran) for High‐Performance Fullerene‐Free Organic Solar Cells

To meet the imminent requirement of fullerene‐free electron acceptors, a new ladder‐type heteroarene benzodi(thienopyran) (BDTP) is designed and synthesized by embedding pyran rings in [2,2‐(2,5‐dialkyloxy‐1,4‐phenylene)dithiophene] (PDT) for organic photovoltaic (OPV) applications. Originating from the strong p–π conjugation of the oxygen atom in the pyran rings, electron‐rich BDTP results in the BDTP‐cored molecular acceptor, NBDTP, with strong intramolecular charge transfer effect. With sp3 carbons in pyran rings, NBDTP have the flexibility of targeted modification to resolve the solubility as well as the morphological tuning by handy alkyl chain attachment. As a result, the fusion of pyran rings in BDTP means that the acceptor exhibits a narrow optical band gap of 1.58 eV and a high LUMO energy level of −3.79 eV. Meanwhile, blend morphology of NBDTP‐based OPVs shows fine tunability via thermal annealing. Owing to the beneficial optical absorption, suitable energy‐level alignment, favorable molecule packing, and proper morphology, the optimized NBDTP‐based OPVs achieve a power conversion efficiency of up to 10.1%, which exemplifies the potential of utilizing molecular acceptors featuring ladder‐type heteroarenes for high‐performance OPVs.

Unsupervised Learning Using Charge-Trap Transistors

Unsupervised learning is demonstrated using a device ubiquitously found in today's technology: a transistor with high-k -metal gate. Specifically, the charge-trapping phenomenon in the high-k gate dielectric is leveraged so that the device can be used as a non-volatile analog memory. Experimental data from 22-nm silicon-on-insulator devices reveal that a charge-trap transistor possesses promising characteristics for implementing synapses in neural networks, such as very fine tunability, weight-dependent plasticity, and low power consumption. A proof-of-concept winner-takes-all neural network is simulated based on experimental data and perfect clustering is achieved within tens of training cycles. This means that the network can be trained for multiple times, and a larger system can be built. The robustness of the procedure to the device variation is also discussed.

CaMoO4:RE3+,Yb3+,M+ phosphor with controlled morphology and color tunable upconversion

CaMoO4:RE3+,Yb3+ (RE=Er, Ho, Tm) phosphors were successfully synthesized by a facile hydrothermal method. XRD patterns confirmed tetragonal structure under different RE3+ and M+ ions doping conditions. Particles shapes and sizes were confirmed by SEM and TEM analyses. Particles shape and size were well tuned by control of solution pH; spherical balls consisting of nano-grains at low pH of ∼2, rice grain shapes at moderate pH of ∼6, and thin flakes at higher pH of ∼12, were observed. Fine tunability of upconversion (UC) emission color was achieved by doping multiple RE3+ ions within a single CaMoO4 host. Blue, green and orange upconverted emission were observed by doping Tm3+, Er3+ and Ho3+ in the CaMoO4, respectively. Further, the emission colors were well tuned by the combination of Tm, Er and Ho ions and their concentrations. CaMoO4:Tm3+,Ho3+,Yb3+ exhibited perfect white emission with well tunability from cool white to warm white colors. Substitution of part of Ca ions by M+ (M=Li, Na, K, Rb) ions affected the crystal field symmetry around RE3+ ions and hence changed the transition probabilities between their f–f transition levels, consequently intensified the UC intensities. The blue (Tm3+), green (Er3+), and orange (Ho3+) upconversion intensities of CaMoO4:RE3+,Yb3+,0.10K+ phosphors increased by 60, 50 and 40 folds compared to the unsubstituted analogues, respectively. The K substituted CaMoO4:RE3+,Yb3+,K+ phosphors exhibited intense UC emissions visible by naked eye even pumped by less than 1mW laser power and can have potential application in displays and variety of other applications.

Tuning Equilibrium Compositions in Colloidal Cd1-xMnxSe Nanocrystals Using Diffusion Doping and Cation Exchange.

The physical properties of semiconductor nanocrystals can be tuned dramatically via composition control. Here, we report a detailed investigation of the synthesis of high-quality colloidal Cd1-xMnxSe nanocrystals by diffusion doping of preformed CdSe nanocrystals. Until recently, Cd1-xMnxSe nanocrystals proved elusive because of kinetic incompatibilities between Mn(2+) and Cd(2+) chemistries. Diffusion doping allows Cd1-xMnxSe nanocrystals to be prepared under thermodynamic rather than kinetic control, allowing access to broader composition ranges. We now investigate this chemistry as a model system for understanding the characteristics of nanocrystal diffusion doping more deeply. From the present work, a Se(2-)-limited reaction regime is identified, in which Mn(2+) diffusion into CdSe nanocrystals is gated by added Se(2-), and equilibrium compositions are proportional to the amount of added Se(2-). At large added Se(2-) concentrations, a solubility-limited regime is also identified, in which x = xmax = ∼0.31, independent of the amount of added Se(2-). We further demonstrate that Mn(2+) in-diffusion can be reversed by cation exchange with Cd(2+) under exactly the same reaction conditions, purifying Cd1-xMnxSe nanocrystals back to CdSe nanocrystals with fine tunability. These chemistries offer exceptional composition control in Cd1-xMnxSe NCs, providing opportunities for fundamental studies of impurity diffusion in nanocrystals and for development of compositionally tuned nanocrystals with diverse applications ranging from solar energy conversion to spin-based photonics.

Widely tunable LP11 cladding-mode resonance in a twisted mechanically induced long-period fiber grating.

A record tunability of 35 nm for the LP(11) cladding-mode resonance in a twisted mechanically induced long-period fiber grating using standard single-mode communication fiber is demonstrated. By forming the LP(11) resonance far away from its cut-off wavelength and modifying the grooves of the grating in the form of smooth semicircular humps, a high twist sensitivity of 8.75 nm/(rad/cm) and a controlled tunability of 35 nm is achieved. The fiber with its lacquer coating is not broken even at a severe twist rate of 5.44 rad/cm. The present design can be used as a novel variable optical selective wavelength attenuator since the bandwidth, rejection efficiency, and center wavelength can be controlled by changing the grating length, pressure over the grating, and fiber twist, respectively. Using the results, a cost-effective tunable variable optical attenuator for selective channel-blanking applications is also demonstrated. A fine tunability of 1.5 nm is achieved for a twist rate change of 0.1 rad/cm.

Temporal, thermal, and light stability of continuously tunable cholesteric liquid crystal laser array.

Fine-structured polymerized cholesteric liquid crystal (PCLC) wedge laser devices have been realized, with high fine spatial tunability of the lasing wavelength. With resolution less than 0.3 nm in a broad spectral range, more than one hundred laser lines could be obtained in a PCLC cell without extra devices. For practical device application, we studied the stability of the device in detail over time, and in response to strong external light sources, and thermal perturbation. The PCLC wedge cells had good temporal stability for 1 year and showed good stability for strong perturbations, with the lasing wavelength shifting less than 1 nm, while the laser peak intensities decreased by up to 34%, and the high energy band edge of the photonic band gap (PBG) was red shifted 3 nm by temperature perturbation. However, when we consider the entire lasing spectrum for the PCLC cell, the 1-nm wavelength shift may not matter. Although the laser peak intensities were decreased by up to 34% in total for all of the perturbation cases, the remaining 34% laser peak intensity is considerable extent to make use. This good stability of the PCLC laser device is due to the polymerization of the CLC by UV curing. This study will be helpful for practical CLC laser device development.

Fabrication of Ultrafine Soft-Matter Arrays by Selective Contact Thermochemical Reaction

Patterning of functional soft matters at different length scales is important for diverse research fields including cell biology, tissue engineering and medicinal science and the development of optics and electronics. Here we have further improved a simple but very efficient method, selective contact thermochemical reaction (SCTR), for patterning soft matters over large area with a sub-100 nm resolution. By selecting contact between different precursors through a topographically patterned PDMS stamp and subsequently any heating way for thermalchemical reaction, thermal-related soft matters can be patterned to form controllable micro or nano structures, even three-dimensional structures. The fine tunability and controllability of as-prepared micro and nano structures demonstrate this versatile approach a far wide range of uses than the merely academic.

Extremely large bandwidth and ultralow-dispersion slow light in photonic crystal waveguides with magnetically controllability

A line-defect waveguide within a two-dimensional magnetic-fluid-based photonic crystal with 45o-rotated square lattice is presented to have excellent slow light properties. The bandwidth centered at \( \lambda_{0} \) = 1,550 nm of our designed W1 waveguide is around 66 nm, which is very large than that of the conventional W1 waveguide as well as the corresponding optimized structures based on photonic crystal with triangular lattice. The obtained group velocity dispersion \( \beta_{2} \) within the bandwidth is ultralow and varies from −1,191\( a/(2\pi c^{2} ) \) to 855\( a/(2\pi c^{2} ) \) (a and c are the period of the lattice and the light speed in vacuum, respectively). Simultaneously, the normalized delay-bandwidth product is relatively large and almost invariant with magnetic field strength. It is indicated that using magnetic fluid as one of the constitutive materials of the photonic crystal structures can enable the magnetically fine tunability of the slow light in online mode. The concept and results of this work may give a guideline for studying and realizing tunable slow light based on the external-stimulus-responsive materials.

Highly bright multicolor tunable ultrasmall β-Na(Y,Gd)F4:Ce,Tb,Eu/β-NaYF4 core/shell nanocrystals

Herein, we report highly bright multicolor-emitting β-Na(Y,Gd)F₄:Ce,Tb,Eu/β-NaYF₄ nanoparticles (NPs) with precise color tunability. First, highly bright sub-20 nm β-Na(Y,Gd)F₄:Ce,Tb,Eu NPs were synthesized via a heating-up method. By controlling the ratio of Eu(3+) to Tb(3+), we generated green, yellow-green, greenish yellow, yellow, orange, reddish orange, and red emissions from the NP solutions via energy transfer of Ce(3+)→ Gd(3+)→ Tb(3+) (green) and Ce(3+)→ Gd(3+)→ Tb(3+)→ Eu(3+) (red) ions under ultraviolet light illumination (254 nm). Because of Ce(3+) and Gd(3+) sensitization, Tb(3+) ions exhibited strong green emission. The decay time of Tb(3+) emission decreased from 4.0 to 1.4 ms as the Eu(3+) concentration was increased, suggesting that energy was transferred from Tb(3+) to Eu(3+). As a result, Eu(3+) emission peaks were generated and the emission color was transformed from green to red. Monodisperse sub-6 nm β-Na(Y,Gd)F₄:Ce,Tb,Eu NPs were synthesized through a simple reduction of the reaction temperature. Although fine color tunability was retained, their brightness was considerably decreased owing to an increase in the surface-to-volume ratio. The formation of a β-NaYF₄ shell on top of the sub-6 nm NP core to produce β-Na(Y,Gd)F₄:Ce,Tb,Eu/β-NaYF₄ significantly increased the emission intensity, while maintaining the sub-10 nm sizes (8.7-9.5 nm). Quantum yields of the ultrasmall NPs increased from 1.1-6.9% for the core NPs to 6.7-44.4% for the core/shell NPs. Moreover, highly transparent core/shell NP-polydimethylsiloxane (PDMS) composites featuring a variety of colors, excellent color tunability, and high brightness were also prepared.

Tuning the properties of graphene using a reversible gas-phase reaction

Graphene, because of its remarkable electronic and structural properties, has attracted considerable attention in both the scientific and technological communities. However, a major roadblock to the realization of graphene-based field-effect transistors is that large-area graphene behaves as a semimetal with zero bandgap, making it unsuitable for real applications in sensing, detecting and switching systems. Surface functionalization could result in the construction of periodic micro/nanostructures by breaking sp2 bonds and forming sp3 bonds. Therefore, direct chemical grafting might provide a useful way to covalently modify graphene to tailor its properties. Owing to the inert reactivity of its surface, however, only a few chemical reactions have been used to modify its atomic structure. Here, we demonstrate a controllable and efficient means of mild plasma methylation to manipulate the reversible interconversion of two distinct species of graphene (one crystalline and the other methylated). The strategy of incorporating diverse functional substituents (that is, methyl groups and hydrogen atoms) into graphene, instead of a single type of chemical group, could provide a useful route for the development of different applications, such as chemical/biosensors and multifunctional electrical circuits. Moreover, this finely tunable, methylated graphene is stable at room temperature, which suggests that it has intrinsic potential for novel applications in graphene-based optoelectronic devices, inviting further studies. The discovery of graphene — a sheet of graphite that is only one atom thick — has sparked much enthusiasm in the science and technology communities. Dubbed ‘wonder material’ owing to its intriguing and promising properties, graphene is poised to play a crucial role in applications that range from electronic to biomedical devices; however, tuning its properties remains challenging. Grafting methyl (–CH3) groups on its surface, for example, can open a band gap in its electronic structure, but too many functional groups will damage the original sheet. Xuefeng Guo and co-workers from Peking University have now described a route that circumvents previous issues. Through a mild gas-phase reaction between graphene and methane plasma, the researchers incorporated both methyl groups and hydrogen atoms to the sheet in a controlled, and reversible, manner. This process, which introduces two functional groups, may enable in future the preparation of advanced materials for applications in sensing or optoelectronic devices. Graphene, owing to its remarkable electronic and structural properties, has attracted considerable attention in both science and technology communities. However, a major roadblock to the realization of graphene-based field-effect transistors is the fact that large-area graphene behaves like a semimetal with zero bandgap, making it unsuitable for real applications in sensing, detecting and switching systems. Surface functionalization could result in the construction of periodic micro/nanostructures by breaking sp2 bonds and forming sp3 bonds. Therefore, direct chemical grafting might provide a useful way to covalently modify graphene for tailoring its properties. Owing to the inert reactivity of its surface, however, up to date only few chemical reactions were used to modify its atomic structure. Here, we demonstrate a controllable and efficient means of mild plasma methylation to manipulate the reversible interconversion of two distinct species of graphene (one crystalline and the other methylated). The strategy of incorporating diverse functional substituents (methyl group and hydrogen atoms here) into graphene instead of a single type of chemical groups could provide a useful route for the development of different applications, such as chemical/biosensors and multifunctional electrical circuits. Moreover, the methylated graphene with fine tunability is stable at room temperature, which suggests the intrinsic potential of novel applications in graphene-based optoelectronic devices that invites further studies.

Control of optical bandgap energy and optical absorption coefficient by geometric parameters in sub-10 nm silicon-nanodisc array structure

A sub-10 nm, high-density, periodic silicon-nanodisc (Si-ND) array has been fabricated using a new top-down process, which involves a 2D array bio-template etching mask made of Listeria-Dps with a 4.5 nm diameter iron oxide core and damage-free neutral-beam etching (Si-ND diameter: 6.4 nm). An Si-ND array with an SiO2 matrix demonstrated more controllable optical bandgap energy due to the fine tunability of the Si-ND thickness and diameter. Unlike the case of shrinking Si-ND thickness, the case of shrinking Si-ND diameter simultaneously increased the optical absorption coefficient and the optical bandgap energy. The optical absorption coefficient became higher due to the decrease in the center-to-center distance of NDs to enhance wavefunction coupling. This means that our 6 nm diameter Si-ND structure can satisfy the strict requirements of optical bandgap energy control and high absorption coefficient for achieving realistic Si quantum dot solar cells.

Discrete and Fine Wavelength Tunable Thermo-Optic WSS for Low Power Consumption ${\rm C}{+}{\rm L}$ Band Tunability

Broadband wavelength tunability is demonstrated in a microring resonator (MRR)-based thermo-optic wavelength selective switch (TO-WSS) with low power consumption and low thermal crosstalk. Instead of direct tuning a single MRR spectrum, the discrete and fine wavelength tunability in a Vernier configured TO-WSS has been used to achieve single wavelength selection over a high free spectral range (FSR). With the observed discrete tunability (7.3 mW/FSR) and fine tunability (5 mW/nm) of the WSS, the power requirement for full band tuning can be reduced to 32.7% of that of a single ring tuning.

Fine tuning of plasmonic properties of monolayers of weakly interacting silver nanocubes on thin silicon films

Plasmonic properties, such as refractive index sensitivity (RIS), surface enhancement of the Raman signal (SERS), fluorescence quenching, and photocatalytic activity, of monolayers of weakly interacting monodisperse silver nanocubes were qualitatively modified in a very well controlled manner by supporting them on thin silicon films with varying thickness. Such fine tunability is made possible by the strong dependence of the nanocube dipolar (D) and quadrupolar (Q) plasmon mode hybridization on the refractive index of the supporting substrate. By increasing the Si film thickness from zero to ~25 nm we were able to "shift" the D resonance mode by up to 200 nm for ~80 nm cubes without significantly affecting the Q mode. The silicon supported nanocubes showed a significant improvement in RIS via the Q mode with a figure of merit greater than 6.5 and about an order of magnitude enhancement of the SERS signal due to the stronger electric field created by the D mode. Such substrates also showed a ~10 times decrease in rhodamine 6G fluorescence as well as the rates of amorphous carbon formation. The study proposes a new way to design and engineer plasmonic nanostructures.

Quantum dot states and optical excitations of edge-modulated graphene nanoribbons

We investigate from first principles the electronic and optical properties of edge-modulated armchair graphene nanoribbons, including both quasiparticle corrections and excitonic effects. Exploiting the oscillating behavior of the ribbon energy gap, we show that minimal width-modulations are sufficient to obtain confinement of both electrons and holes, thus forming optically active quantum dots with unique properties, such as the coexistence of dotlike and extended excitations and the fine tunability of optical spectra, with great potential for optoelectronic applications.

Frequency-tunable superconducting cloaking

We have theoretically suggested a temperature-dependent superconducting cloak whose cloaking frequency can be externally controlled, since the dielectric constant of the superconductor is well tuned with the operating temperature. The results show that this tunable cloak exhibits a fine tunability in the low-temperature range 0–0.3 Tc and a fast tunability in the high-temperature range 0.3Tc–Tc, together with a large reduction in scattering (<−50 dB). It may provide a potential way to design a tunable cloak with considerable flexibility.

Two‐dimensional air‐bridged silicon photonic crystal slab devices

AbstractAs silicon has a large refractive index and low loss in infrared wavelengths, it becomes an important optical material that has been widely used for integrated photonics applications. In this paper, we present some of our recent experimental works on infrared two‐dimensional air‐bridged silicon photonic crystal (PhC) slab devices that are based on both the band gap and band structure engineering. We have designed, fabricated, and characterized a series of PhC waveguides with novel geometries, resonant microcavities with fine tunability, and channel drop filters utilizing resonant coupling between waveguide and cavity. These devices are aimed to construct a more flexible network of transport channels for infrared light at micrometer/nanometer scale. We have also explored the remarkable dispersion properties of PhCs by engineering the band structures to achieve negative refraction, self‐collimation, superprism, and other anomalous dispersion behaviors of infrared light beams. We have designed and fabricated a PhC structure with negative refraction effect and used scanning near‐field optical microscopy to observe the negative refraction beam.

Silicon coupled-ring resonator structures for slow light applications: potential, impairmentsand ultimate limits

Coupled-ring resonator-based slow light structures are reported and discussed. Bycombining the advantages of high index contrast silicon-on-insulator technology withan efficient thermo-optical activation, they provide an on-chip solution with abandwidth of up to 100 GHz and a slowdown factor of up to 16, as well as a continuousreconfiguration scheme and a fine tunability. The performance of these devices isinvestigated in detail for both static and dynamic operation, in order to evaluate theirpotential in optical signal processing applications at high bit rate. The mainimpairments imposed by fabrication imperfections are also discussed in relation to theslowdown factor. In particular, the analysis of the impact of backscatter, disorderand two-photon absorption on the device transfer function reveals the ultimatelimits of these structures and provides valuable design rules for their optimization.

Phase-locked arrays of surface-emitting terahertz quantum-cascade lasers

We report the demonstration of phase-locked arrays of surface-emitting distributed-feedback (DFB) terahertz quantum-cascade lasers with single-mode operations. Carefully designed “phase sector” locks several surface-emitting DFB laser ridges in-phase, creating tighter beam-patterns along the phased-array direction with full width at half maximum (FWHM)≈10°. In addition, the phase sector can be individually biased to provide a mechanism of frequency tuning through gain-induced optical index change, without significantly affecting the output power levels. A tuning range of 1.5 GHz around 3.9 THz was achieved. This fine tunability could be utilized to frequency- or phase-lock the DFB array to an external reference.

All-fiber birefringent filter with fine tunability and changeable spacing

A novel all-fiber birefringent filter has been proposed and demonstrated. Simulation results show that the birefringence fiber filter with one section of polarization-maintenance fiber (PMF) can be finely tuned with its free spectrum range, and the filter with two sections of PMFs can generate changeable wave-length spacing. Experiment agrees well with simulation. This filter has potential applications for flexible multiwavelength fiber laser.