Ultra-narrow Quantum Research Articles

Calculating the width of an ultranarrow quantum resonance

Calculating the width of an ultranarrow quantum resonance

High-precision methanol spectroscopy with a widely tunable SI-traceable frequency-comb-based mid-infrared QCL

There is an increasing demand for precise molecular spectroscopy, in particular in the mid-infrared fingerprint window that hosts a considerable number of vibrational signatures, whether it be for modeling our atmosphere, interpreting astrophysical spectra or testing fundamental physics. We present a high-resolution mid-infrared spectrometer traceable to primary frequency standards. It combines a widely tunable ultra-narrow Quantum Cascade Laser (QCL), an optical frequency comb and a compact multipass cell. The QCL frequency is stabilized onto a comb controlled with a remote near-infrared ultra-stable laser, transferred through a fiber link. The resulting QCL frequency stability is below 10-15 from 0.1 to 10s and its frequency uncertainty of 4x10-14 is given by the remote frequency standards. Continuous tuning over ~400 MHz is reported. We use the apparatus to perform saturated absorption spectroscopy of methanol in the low-pressure multipass cell and demonstrate a statistical uncertainty at the kHz level on transition center frequencies, confirming its potential for driving the next generation technology required for precise spectroscopic measurements.

Quantum conductance staircase of holes in silicon nanosandwiches

The results of studying the quantum conductance staircase of holes in one-dimensional channels obtained by the split-gate method inside silicon nanosandwiches that are the ultra-narrow quantum well confined by the delta barriers heavily doped with boron on the n-type Si (100) surface are reported. Since the silicon quantum wells studied are ultra-narrow (~2nm) and confined by the delta barriers that consist of the negative-U dipole boron centers, the quantized conductance of one-dimensional channels is observed at relatively high temperatures (T>77K). Further, the current-voltage characteristic of the quantum conductance staircase is studied in relation to the kinetic energy of holes and their sheet density in the quantum wells. The results show that the quantum conductance staircase of holes in p-Si quantum wires is caused by independent contributions of the one-dimensional (1D) subbands of the heavy and light holes. In addition, the field-related inhibition of the quantum conductance staircase is demonstrated in the situation when the energy of the field-induced heating of the carriers become comparable to the energy gap between the 1D subbands. The use of the split-gate method made it possible to detect the effect of a drastic increase in the height of the quantum conductance steps when the kinetic energy of holes is increased; this effect is most profound for quantum wires of finite length, which are not described under conditions of a quantum point contact. In the concluding section of this paper we present the findings for the quantum conductance staircase of holes that is caused by the edge channels in the silicon nanosandwiches prepared within frameworks of the Hall geometry. This longitudinal quantum conductance staircase, Gxx, is revealed by the voltage applied to the Hall contacts, with the plateaus and steps that bring into correlation respectively with the odd and even fractional values.

Green Emitting Cubic GaInN/GaN Quantum Well Stripes on Micropatterned Si(001) and Their Strain Analysis

GaInN/GaN heterostructures in the cubic lattice variant have the potential to overcome the limitations of wurtzite structures as commonly used for light emitting and laser diodes. Wurtzite GaInN (0001), suffers from large internal polarization fields, which force design compromises toward ultranarrow quantum wells and reduce recombination volume and efficiency, particularly in the green, yellow, and red visible spectral regions. Cubic GaInN microstripes on micropatterned Si(001), with {111} V‐grooves oriented along Si , offer a system free of internal polarization fields, wider quantum wells, and a smaller bandgap energy. 6 and 9 nm Ga1‐xInxN/GaN single quantum well structures are prepared and their emission spectra found to be dominated by the recombination in the cubic wells. The peak wavelength ranges from 520 to 570 nm with a polarization predominately perpendicular to the grooves. These values are about 26 nm longer in wavelength than the equivalent wurtzite sample portions and 40 nm longer than the wurtzite (0001) oriented portions. An alloy composition range of 0.2 < x < 0.3 has been estimated in those cubic portions and quantum efficiency comparable to planar wurtzite structures. The stripe geometry and photon polarization may be suitable for optical mode confinement and reduced threshold stimulated emission.

Quantum spin Hall effect in nanostructures based on cadmium fluoride

Tunneling current-voltage (I-V) characteristics and temperature dependences of static magnetic susceptibility and specific heat of the CdBxF 2- x/pCdF 2-QW/CdBxF 2- x planar sandwich structures formed on the surface of an nCdF 2 crystal have been studied in order to identify superconducting properties of the CdBxF 2- x δ barriers confining the ptype CdF 2 ultranarrow quantum well. Comparative analysis of current-voltage (I-V) characteristics and conductance-voltage dependences (measured at the tempera� tures, respectively, below and above the critical temperature of superconducting transition) indicates that there is an interrelation between quantization of supercurrent and dimensional quantization of holes in the pCdF 2 ultranarrow quantum well. It is noteworthy that detection of the Josephson peak of current in each hole subband is accompanied by the appearance of the spectrum of the multiple Andreev reflection (MAR). A high degree of spin polarization of holes in the edge channels along the perimeter of the pCdF 2 ultranarrow quantum well appears as a result of MAR and makes it possible to identify the quantum spin Hall effect I-V characteristics; this effect becomes pronounced in the case of detection of nonzero conductance at the zero voltage applied to the vertical gate in the Hall geometry of the experiment. Within the energy range of super� conducting gap, the I-V characteristics of the spin transistor and quantum spin Hall effect are controlled by the MAR spectrum appearing as the voltage applied to the vertical gate is varied. Beyond the range of the superconducting gap, the observed I-V characteristic of the quantum spin Hall effect is represented by a quantum conductance staircase with a height of the steps equal to e 2 /h; this height is interrelated with the Aharonov-Casher oscillations of longitudinal and depends on the voltage applied to the vertical gate.

Minimum of oscillator strength of excitons in ultra-narrow GaAs/AlGaAs quantum wells:: Theory and experiment

A theoretical and experimental study of the exciton in ultra-narrow quantum wells (QWs) is performed. A crossover from strong (separate localization of electron and hole levels) to weak confinement (with localization of excitonic center of mass) is predicted to occur for decreasing thickness, and is characterized by a minimum of the oscillator strength per unit area. Cathodoluminescence measurements performed on a series of GaAs/Al 0.35Ga 0.65As QWs with thicknesses from one to eight monolayers show a minimum of the oscillator strength, in agreement with theory, and indicate that the crossover from strong to weak confinement occurs at a thickness of about three monolayers for this composition.

Transport in ultra-narrow quantum well wires

We have fabricated the narrowest conducting channels reported to date using hihg mobility GaAs/AlGaAs heterojunction material. In our narrowest wires at low temperatures and for small magnetic fields, where the lD subband splitting exceed both κ BT and h ̵ ω C , we observe striking departures from the 2D Hall effect. An unexpected Hall plateau at low magnetic field is observed which systematically moves to higher field and higher Hall resistance, R H, as the wire width is decreased. In samples having widths less than ≈ 200 nm, this is acompanied by a precipitous, complete suppression of R H within a region about zero field. We believe these to be unambiguous manifestations of one-dimensional electrical transport; they appear to provide a direct measure of the number of quantum conduction channels that participate [1].