Realization of Harmonic Oscillator Arrays with Graded Semiconductor Quantum Wells.

The harmonic oscillator is a foundational concept in both theoretical and experimental quantum mechanics. Here, we demonstrate harmonic oscillators in a semiconductor platform by faithfully implementing continuously graded alloy semiconductor quantum wells. Unlike current technology, this technique avoids interfaces that can hamper the system and allows for the production of multiwell stacks several micrometers thick. The experimentally measured system oscillations are at 3 THz for two structures containing 18 and 54 parabolic quantum wells. Absorption at room temperature is achieved: this is as expected from a parabolic potential and is unlike square quantum wells that require cryogenic operation. Linewidths below 11% of the central frequency are obtained up to 150 K, with a 5.6% linewidth obtained at 10 K. Furthermore, we show that the system correctly displays an absence of nonlinearity despite electron-electron interactions-analogous to the Kohn theorem. These high-quality structures already open up several new experimental vistas.

An {Fe60} tetrahedral cage: building nanoscopic molecular assemblies through cyanometallate and alkoxo linkers.

A nanoscopic {Fe60} coordination cage (approximately 3 nm) was prepared by the self assembly of a partially blocked tricyanidoferrate(iii) complex and tris(alkoxo)-based iron(iii) coordination motifs. This cage is a rare example of a mixed cyanido/alkoxo-bridged high nuclearity complex and it exemplifies the great potential of this new synthetic route to generate uncommon molecular architectures using cyanometallates as metalloligands versus alkoxo-based polynuclear entities.

Elastic coupling between spin-crossover particles and cellulose fibers.

Composite materials made of cellulose fibers and spin crossover micro-particles were investigated by magnetic measurements and dynamic mechanical analysis (DMA). The storage modulus of the cellulose handsheet (0.6 GPa at room T) is significantly enhanced in the composite (1.7 GPa). The latter also displays a reversible increase of ca. 10% when switching the magnetic spin state of the particles from the low spin (LS) to the high spin (HS) form. Around the spin transition temperature a loss modulus peak is also observed, highlighting the strong viscoelastic coupling between the particles and the cellulose matrix. These results pave the way for the development of a novel family of actuator materials based on spin crossover-polymer composites.

Enhanced luminescence stability with a Tb-spin crossover nanocomposite for spin state monitoring.

[Fe(Htrz)2(trz)]BF4@SiO2 nanorods were synthesized using a reverse emulsion technique and a Tb(3+) complex was chemically bound to the silica surface. This Tb-spin crossover nanocomposite demonstrates high luminescence stability under continuous thermal cycling. A reabsorption mechanism is determined to be responsible for the luminescence intensity variation during the spin state switching of the iron complex.

Nanocrystals of Fe(phen)2(NCS)2 and the size-dependent spin-crossover characteristics.

We report on the size reduction of the neutral Fe(phen)2(NCS)2 prototypical compound exhibiting a cooperative spin-crossover associated with a first-order phase transition (at ca. 176 K). We use the [Fe(phen)3](NCS)2 ionic precursor and the solvent-assisted precipitation technique to prepare an array of crystalline objects with sizes varying over two orders of magnitude (from 15 up to 1400 nm). TEM, X-ray diffraction and IR measurements provide evidences for the formation of particles of neutral and ionic species, which results from the interplay between the relevant chemical equilibrium and the reaction kinetics (ligand extraction, complex precipitation), and the modulation of the latter by physico-chemical parameters. A thermal transformation of diamagnetic nanocrystals of [Fe(phen)3](NCS)2 leads to spin-crossover particles of Fe(phen)2(NCS)2 of a comparable size. Powders of nano-, micro- and polycrystals of Fe(phen)2(NCS)2 present X-ray diffractograms typical of the so-called polymorph II. The importance of size effects on the cooperative spin-crossover process was probed with magnetic, Mössbauer, Raman and IR spectroscopic measurements. Each sample exhibits spin-state switching of the Fe(ii) ions. The salient features are: a cooperativity preserved at the micrometric scale, a very limited downshift of the transition temperature and an asymmetric spreading of the thermal process (over ca. 100 K) with the size reduction. At temperatures close to room temperature, the process appears to be quasi complete whatever the size of the samples. This result, extracted from Raman data, was confirmed by Mössbauer measurements in the case of the largest objects (LS residue <5-10% for bulk and microparticles). Below 150 K, a very efficient low-spin to high-spin photoexcitation was induced by the Raman laser beam in all the samples which prevents the extraction of the high-spin fraction in this temperature range. However variable temperature IR spectra of the 29 nm particles indicate that the HS residue, that is close to zero in the case of microparticles, does not drastically increase (<30%) for the smallest particles. The processing of a number of related spin-crossover compounds in the form of nanoparticles may be achieved with this general approach.

Near-field analysis of metallic DFB lasers at telecom wavelengths.

We image in near-field the transverse modes of semiconductor distributed feedback (DFB) lasers operating at λ ≈ 1.3 µm and employing metallic gratings. The active region is based on tensile-strained InGaAlAs quantum wells emitting transverse magnetic polarized light and is coupled via an extremely thin cladding to a nano-patterned gold grating integrated on the device surface. Single mode emission is achieved, which tunes with the grating periodicity. The near-field measurements confirm laser operation on the fundamental transverse mode. Furthermore--together with a laser threshold reduction observed in the DFB lasers--it suggests that the patterning of the top metal contact can be a strategy to reduce the high plasmonic losses in this kind of systems.

In situ generation of surface plasmon polaritons using a near-infrared laser diode.

We demonstrate a semiconductor laser-based approach which enables plasmonic active devices in the telecom wavelength range. We show that optimized laser structures based on tensile-strained InGaAlAs quantum wells-coupled to integrated metallic patternings-enable surface plasmon generation in an electrically driven compact device. Experimental evidence of surface plasmon generation is obtained with the slit-doublet experiment in the near-field, using near-field scanning optical microscopy measurements.

Sub-wavelength energy concentration with electrically generated mid-infrared surface plasmons.

While freely propagating photons cannot be focused below their diffraction limit, surface-plasmon polaritons follow the metallic surface to which they are bound, and can lead to extremely sub-wavelength energy volumes. These properties are lost at long mid-infrared and THz wavelengths where metals behave as quasi-perfect conductors, but can in principle be recovered by artificially tailoring the surface-plasmon dispersion. We demonstrate - in the important mid-infrared range of the electromagnetic spectrum - the generation onto a semiconductor chip of plasmonic excitations which can travel along long distances, on bent paths, to be finally focused into a sub-wavelength volume. The demonstration of these advanced functionalities is supported by full near-field characterizations of the electromagnetic field distribution on the surface of the active plasmonic device.

Electrical modulation of the complex refractive index in mid-infrared quantum cascade lasers.

We have demonstrated an integrated three terminal device for the modulation of the complex refractive index of a distributed feedback quantum cascade laser (QCL). The device comprises an active region to produce optical gain vertically stacked with a control region made of asymmetric coupled quantum wells (ACQW). The optical mode, centered on the gain region, has a small overlap also with the control region. Owing to the three terminals an electrical bias can be applied independently on both regions: on the laser for producing optical gain and on the ACQW for tuning the energy of the intersubband transition. This allows the control of the optical losses at the laser frequency as the absorption peak associated to the intersubband transition can be electrically brought in and out the laser transition. By using this function a laser modulation depth of about 400 mW can be achieved by injecting less than 1 mW in the control region. This is four orders of magnitude less than the electrical power needed using direct current modulation and set the basis for the realisation of electrical to optical transducers.

Design of an integrated coupler for the electrical generation of surface plasmon polaritons.

Recently a surface plasmon polariton (SPP) source based on an electrically operated semiconductor laser has been demonstrated. Here we present a numerical investigation of the light-SPP coupling process involved in the device. The problem consists in the coupling via a diffraction grating between a dielectric waveguide mode--the laser mode--and a SPP mode. The issue of the coupling efficiency is discussed, and the dependence on various geometrical parameters of both the grating and the dielectric waveguide is studied in detail. A maximum coupling efficiency of ≈24% is obtained at telecom wavelengths, which could lead to a high-power integrated SPP source when combined to a laser medium.

Semiconductor surface plasmon sources.

Surface-plasmon polaritons (SPPs) are propagating electromagnetic modes bound at a metal-dielectric interface. We report on electrical generation of SPPs by reproducing the analogue in the near field of the slit-doublet experiment, in a device which includes all the building blocks required for a fully integrated plasmonic active source: an electrical generator of SPPs, a coupler, and a passive metallic waveguide. SPPs are generated upon injection of electrical current, and they are then launched at the edges of a passive metallic strip. The interference fringes arising from the plasmonic standing wave on the surface of the metallic strip are unambiguously detected with apertureless near-field scanning optical microscopy.

A semiconductor laser device for the generation of surface-plasmons upon electrical injection.

Surface plasmons are electromagnetic waves originating from electrons and light oscillations at metallic surfaces. Since freely propagating light cannot be coupled directly into surface-plasmon modes, a compact, semiconductor electrical device capable of generating SPs on the device top metallic surface would represent an advantage: not only SP manipulation would become easier, but Au-metalized surfaces can be easily functionalized for applications. Here, we report a demonstration of such a device. The direct proof of surface-plasmon generation is obtained with apertureless near-field scanning optical microscopy, which detects the presence of an intense, evanescent electric field above the device metallic surface upon electrical injection.

Selective photoswitching of the binuclear spin crossover compound {[Fe(bt)(NCS)2]2(bpm)} into two distinct macroscopic phases.

The low-spin (LS-LS, S = 0) diamagnetic form of the binuclear spin crossover complex {[Fe(bt)(NCS)(2)](2)(bpm)} was selectively photoconverted into two distinct macroscopic phases at different excitation wavelengths (1342 or 647.1 nm). These long-lived metastable phases have been identified, respectively, as the symmetry-broken paramagnetic form (HS-LS, S = 2) and the antiferromagnetically coupled (HS-HS, S = 0) high-spin form of the compound. The selectivity may be explained by the strong coupling of the primary excited states to the paramagnetic state.

Spin crossover in a catenane supramolecular system.

The compound [Fe(tvp)(2)(NCS)(2)] . CH(3)OH, where tvp is 1,2-di-(4-pyridyl)-ethylene, has been synthesized and characterized by x-ray single-crystal diffraction. It consists of two perpendicular, two-dimensional networks organized in parallel stacks of sheets made up of edge-shared [Fe(II)](4) rhombuses. The fully interlocked networks define large square channels in the [001] direction. Variable-temperature magnetic susceptibility measurements and Mössbauer studies reveal that this compound shows low-spin to high-spin crossover behavior in the temperature range from 100 to 250 kelvin. The combined structural and magnetic characterization of this kind of compound is fundamental for the interpretation of the mechanism leading to the spin crossover, which is important in the development of electronic devices such as molecular switches.