Effective nonlinear GaSe crystal. Optical properties and applications

We present an overview of the current state of the literature and research performed by the authors of the present paper on the experimental and theoretical results on the structural-, optical-, nonlinear optical (NLO)-properties (including two-photon absorption (TPA) and the terahertz (THz) range of spectra) and practical applications of a highly anisotropic Gallium Selenide (GaSe) semiconductor with emphasis on the ?-GaSe. Physical properties of ?-GaSe are important to researchers and designers developing different devices by using this material. This crystal possesses an outstanding NLO properties: high optical birefringence Δn ～ 0.3 at 700 nm; high transparency range (0.7?18.0 μm) with low absorption coefficient (α ≤ 0.3 cm^?1); very high nonlinear susceptibility χ⁽²⁾ (d ₂₂ ≈ 86 ± 17 pm/V, corresponding to (2.0 ± 0.4) × 10^?7 esu) that is used for phase matched second harmonic generation (SHG) in a wide transparency range; high power threshold for optical damage; possibility to perform optical frequency conversion under phase-matching conditions in the near- to mid-IR and THz range of spectra, etc. The domain structure of crystal in connection with the NLO properties is discussed as studied by confocal Raman microscopy experiments. Perspectives for future research of GaSe are considered in the present article, which does not pretend to be one reflecting all existing papers on GaSe crystal and discussed subjects.     

THz near-field measurements of metal structures

We report on electro-optic measurements of THz electric fields in the near-field of metal structures lying on GaP electro-optic sampling crystals. With (110) oriented crystals, the x or y component of the THz electric field is measured, whereas with a (001) oriented crystal, the z-component is measured. With an estimated spatial resolution of 20 μm, the technique allows us to map the field on a scale, orders of magnitude smaller than the spatial dimensions of these structures. We illustrate the technique by showing results of measurements of the THz electric field in a plane underneath a metal sphere and a single square hole in a metal foil. The technique provides us with a wealth of information on the time-evolution of light fields near metal structures. To cite this article: A.J.L. Adam et al., C. R. Physique 9 (2008).     

Sources semiconductrices de photons uniques ou de photons jumeaux pour l'information quantique

A large number of scientific proposals in recent years are based on transport and manipulation of information using single quantum objects. Although very impressive theoretical perspectives have been envisaged, experimental demonstrations are still limited due to technological difficulties with present state-of-the-art devices. This paper presents various approaches aiming at efficient single or twin photons semiconductor sources. The emergence of these devices will be an important technological breakthrough in the field of quantum information. To cite this article: V. Berger, J.-M. Gérard, C. R. Physique 4 (2003).     

Cyclotron masses in n-GaAs/Ga 1−xAl xAs heterojunctions

Electron cyclotron resonance is studied in single layer GaAs/Ga _1−xAl _xAs heterostructures. The cyclotron mass is measured over a wide range of the two-dimensional electron density N _s and the magnetic field strength B to enable detailed comparison with self-consistent theoretical results that take into account the effects of band nonparabolicity. The calculations are performed using an effective 2 × 2 subband Hamiltonian, which is derived from a five-level k·p-model by fourth order perturbation theory and includes remote band contributions. Close agreement between experimental and theoretical cyclotron masses is achieved and the importance of band nonparabolicity in these systems is demonstrated.     

Terahertz wave narrow bandpass filter based on photonic crystal

We designed a narrow bandpass terahertz wave filter using photonic crystals with a line defect. An inserted linear defect in one-dimensional photonic crystal structures for a channeled filtering in the terahertz range are studied and designed theoretically. By using transfer matrix method, we examined the transmittance spectra for the proposed terahertz wave filter has a 3 dB transmission loss bandwidth of 20 MHz ranging from 0.29998 THz to 0.30001 THz. The simulated results show that a very narrow transmission band and high transmission (higher than 99.99%) centered at λ₀, and very sharp edges can be achieved.     

Resonant-tunneling devices for millimeter-wave generation

Oscillations from resonant-tunneling diodes have been observed up to 200 GHz, and theoretical estimates predict that device performance should extend into the THz range. This paper addresses the issue of the ultimate frequency response and power generation capability of these devices. Techniques recently developed to solve the time-dependent Schrödinger equation are used to predict the rf power vs. frequency obtainable from resonant-tunneling diode oscillators, based on the calculated small-signal response. Factors limiting the rf power output from these devices are presented. Also, recently obtained dc experimental results for the In_.53Ga_.47As-In_xAl_1-xAs heterostructure material system grown on InP are presented. Using a quasi-static approximation, the rf power available from these devices under large-signal conditions is estimated.     

Efficient generation of terahertz pulses in lithium niobate crystal with integrated matching prism

We present results of experimental studies of Cherenkov-type generation of a terahertz (THz)-pulse during propagation of a femtosecond laser pulse in LiNbO₃ crystal with a Si-prism output coupler. Application of the Si-prism allows reducing essentially the damping of THz radiation because of shortening of the path of THz pulse in the nonlinear crystal. Results of experimental studies agree sufficiently well with theoretical calculations.     

Collective excitations in the 1-D-ferromagnet CsFeCl3 with singlet ground state

By means of inalastic neutron scattering we have determined the dispersion relation of the magnetic excitations in CsFeCl₃ at different temperatures.The dispersion in c-direction, along the Fe-chains is typically ferromagnetic and in the hexagonal plane antiferromagnetic. Due to the lack of an applicable theory the data were parametrized by the simple heuristic formula:?ω = [2J[1 - cos πq_c] [A + 2J(1 - cos π)q_c)] + [C + J' (1.5 + γ(q))]²]^{$\text{1}\text{2}$}The gap was found to be C = 0.148 THz, the easy plane anisotropy A = 0.308 THz, the ferromagnetic interaction J = 0.148 THz and the antiferromagnetic interaction J' = -0.04 THz. At 1.25 K all excitations had a width smaller than the instrumental resolution ΔE = 0.025 THz. These results can be interpreted as follows: CsFeCl₃ is a singlet ground state system with strong ferromagnetic interaction J along the crystallographic c-axis and weak antiferromagnetic interaction J' in the plane perpendicular to c.In addition we have measured the influence of a magnetic field along the hexagonal c-axis. The splitting found agrees with the assumed level scheme yielding g = 2.5 for the first excited level.     

First-principles investigations on elastic, phonon and thermodynamic properties of SrB6 under pressure

The elastic, phonon and thermodynamic properties of the divalent alkaline-earth hexaboride SrB₆ are investigated by using plane-wave pseudopotential density functional theory method. The calculated structure parameters and bulk modulus are well consistent with the available experiment and theoretical data. The pressure dependences of elastic constants C_ij, bulk modulus B₀, shear modulus G, Young's modulus E and Poisson's ratio σ are also presented. With these elastic parameters, we investigate the mechanical stability and compressibility of SrB₆. For the thermodynamic properties, both phonon and quasi-harmonic Debye model methods are adopted. Through the comparison with experimental and other theoretical results, we found the method of quasi-harmonic Debye model is a little better. Moreover, the phonon dispersion relations are also obtained. It is found that there are two LO/TO splitting around 5 THz and 26 THz, respectively.     

Design of Hilbert transformers with tunable THz bandwidths using a reconfigurable integrated optical FIR filter

We present the design and analysis of a wideband and tunable optical Hilbert transformer (OHT) using a tunable waveguide-based finite-impulse response (FIR) filter structure by using the digital filter design method and the Remez algorithm. The tunable Nth-order waveguide-based FIR filter, which simply consists of N delay lines, N tunable couplers, N tunable phase shifters and a combiner, can be tuned, by thermally adjusting the tunable couplers and tunable phase shifters, to tune the bandwidth of an OHT using silica-based planar lightwave circuit (PLC) technology. To demonstrate the effectiveness of the method, the simulation results have an excellent agreement with the theoretical predictions. The tunable OHT can function as a wideband and tunable 90° phase shifter and thus has many potential applications. The two unique features of wideband characteristic (up to ~ 2 THz) and tunable bandwidth (THz tuning range) of the proposed OHT cannot be obtained from the existing OHTs.     

Modified grating-based external cavity diode laser for simultaneous dual-wavelengths operation

We have reported a modified V-shaped external cavity, which is constructed around a commercial diode laser operating at a center wavelength of λ=785 nm by adding a new coated glass plate with about 50% reflectivity to the cavity. This allows simultaneous dual-wavelengths operation in the vicinity of Δν_min=0.18 THz to Δν_max=0.22 THz, which can be used as laser source for terahertz generation either for semiconductor devices or nonlinear schemes.     

Terahertz acoustic-phonon signal generated by heated electrons in AlGaAs/GaAs-based quantum wires

In this paper, we explore theoretically the possibility of applying AlGaAs/GaAs-based quantum wire systems as a terahertz (THz) ultrasonic generator. For structures such as Al_xGa_1-xAs/GaAs-based low-dimensional semiconductor systems and semiconductor nanostructures, electrons are confined within the nanometer distance scale so that energies (e.g. electronic subband energy, electron kinetic energy, Fermi energy, etc.) are in the meV scale, which consequently results in the acoustic-phonons generated by heated electrons from these novel systems to be around the THz frequency range. Our theoretical results indicate that: (i) Al_xGa_-xAs/GaAs-based quantum wires are suitable for generating THz acoustic-phonon signals; (ii) both longitudinal and transverse acoustic-phonon modes contribute to the detected phonon signals; (iii) the THz ultrasound wave can be generated through both intra- and inter-subband scattering processes; and (iv) the strong dependence of the acoustic-phonon emission from a quantum wire on phonon frequency and phonon emission angle can be observed.     

Terahertz spectroscopic investigations of explosives

Experimental measurement and theoretical analysis of THz spectrum for five different explosives and related compounds are introduced. The refractive index and absorption coefficient of the samples are measured in the region of 0.2–2.6 terahertz by time-domain spectroscopy. The simulated spectrum of γ-HNIW is in agreement with the experimental data.     

Design of type-II interband cascade lasers for terahertz emission

We report the design of type-II interband cascade (IC) lasers emitting at terahertz (THz) range. The band structures are obtained using an eight-band k · p finite difference method with strain effects, which accounts for the coupling between the conduction and valence bands. Also, a detailed analysis of the interband transition energy in the type-II IC laser structures as a function of layer thickness is carried out using the self-consistent Schrödinger–Poisson calculations. The calculations show that the promising type-II IC lasers operating at 2.3 THz can be reached.     

Electro-optic polymer based devices and technology for optical telecommunication

Electro-optic polymer waveguide devices are very attractive for optical communication systems, because of their potentially simple and low-cost fabrication procedure. High bandwidth devices are enabled by the low dielectric constant of polymers with negligible dispersion from DC to optical frequencies. We first detail the modelization steps relating to the optical and electrical aspects of devices. We then outline the different steps of the fabrication process of electro-optic polymer based devices. By way of illustrating these considerations, we present some original realizations namely polarization insensitive modulators, switching devices using an asymetric X coupler and optic to RF converters based on difference frequency mixing. To cite this article: P. Labbé et al., C. R. Physique 3 (2002) 543–554.     

用短谐振腔结构优化THz电磁波参量振荡器的输出特性

以MgO：LiNbO₃为非线性光学介质，通过采用一种高性能腔反射镜实现了一种85mm短腔长的法布里-珀罗式的光学参量振荡器，产生THz电磁波的实验结果.这种短腔长THz参量振荡器比传统的160mm腔长的振荡阈值降低了22.3％；峰值能量提高了170％；频率调谐范围从0.5—2.4THz提高到0.8—3.1THz.还报道了一种基于平面金属丝网的法布里-珀罗干涉仪测量THz波线宽的实验结果. 关键词： THz电磁波 OPO 非线性光学 3')" href="#">LiNbO₃     

Quantum Cascade Lasers: the quantum technology for semiconductor lasers in the mid-far-infrared

The quantum cascade laser is a new light source based on resonant tunnelling and optical transitions between quantised conduction band states. In these semiconductor devices the principles of operation arise from the quantum engineering of electronic energy levels and tailoring of their wavefunctions. In recent years the performance of these devices has improved markedly and this semiconductor technology is now an attractive choice for the fabrication of mid-far infrared lasers in a very wide spectral range (3–80 μm). At present, quantum cascade lasers are capable of continuous-wave room temperature operation and can deliver 200–300 mW of average power (at λ∼9 μm) operating on a Peltier cooler. To cite this article: C. Sirtori, J. Nagle, C. R. Physique 4 (2003).     

A new approach to modelling Kelvin probe force microscopy of hetero-structures in the dark and under illumination

A numerical method is proposed to model Kelvin probe force microscopy of hetero-structures in the dark and under illumination. It is applied to FTO/TiO₂ and FTO/TiO₂/MAPbI₃ structures. The presence of surface states on the top of the TiO₂ layers are revealed by combining theoretical computation and experimental results. Basic features of Kelvin probe force microscopy under illumination, namely surface photovoltage, are simulated as well. The method paves the way toward further investigations of more complicated optoelectronic devices.     

Enhanced harmonic generation efficiency using focussing devices based on Fresnel diffraction

We propose the use of focusing devices based on Fresnel diffraction (such as a Fresnel zoneplate) as a means to enhance the efficiency of nonlinear-optical processes starting from radiation in the ultraviolet region, where ordinary lenses cannot be used. We present experimental results of a proof-of-concept experiment involving the efficient generation of second-and third-harmonic laser radiation in the visible range. An enhancement of a factor 6 in the third-harmonic yield is obtained by simply limiting the laser beam with a circular aperture. Higher enhancement factors (up to 16) are obtained using a Fresnel zoneplate. The experimental findings are in good agreement with the results of a simple theoretical analysis.