Kerr nonlinearity and optical multi-stability in a four-level Y-type atomic system

The nonlinear response to applied fields of a four-level Y-type atomic system is investigated. The effect of laser intensity and quantum interference induced by spontaneous emission on optical bistability, optical multi-stability and Kerr nonlinearity is then discussed. It is found that the threshold of the optical bistability can substantially be reduced by the quantum interference. So, an enhanced Kerr nonlinearity with reduced absorption can be achieved.     

Quantum interference in double photon emission

We study Fano-like interference in double photon absorption, describing quantum beats in the second-order correlation function for both a free Λ-V system and a driven four-level scheme which approximately reproduces the first one in the dressed state picture. Because of interference, both amplitude and frequency of the beat oscillations are modified and (in some case) suppressed. In the case of the driven system, a partial coherent control of these features can be obtained, even in the presence of spontaneous emission for the driven transition.     

Optical nonlinearity and optical limiting of CdSeS/ZnS quantum dots

CdSeS/ZnS quantum dots (QDs) were synthesized through the chemical route. The optical limiting behavior of these QDs was observed. The quantum dots’ nonlinear absorption and nonlinear refraction were investigated by the Z-scan technique using a Nd:YAG laser second-harmonic radiation (λ = 532 nm, t = 35 ps). Based on the absorption and fluorescence spectra, it is reasonable for us to infer that the nonlinear absorption arises from free carrier absorption (FCA). These QDs have average absorption cross-section of 1.26 × 10^?16cm² and nonlinear refractive index in the order of 10^?8esu. The large nonlinear absorption perhaps allows them to be candidate material for the optical limiting devices.     

Quantum Jump Studies Using the 5d2

Abstract

Quantum jumps arising from excitation on the magnetic dipole allowed 5d²D_3/2–5d²D_5/2 transition of a single Ba⁺ ion have been observed from the changes in the ion's strong 493 nm fluorescence. Studies of the quantum jump dark and bright periods were performed for variations in the weak- and strong-transition laser intensities. The observations made on this four-level system were found to be well described by the available three-level quantum jump theory.     

Linear and nonlinear optical properties of plasma-enhanced chemical-vapour deposition grown silicon nanocrystals

We provide a systematic study on the linear and nonlinear optical properties of silicon nanocrystals (Si-nc) grown by plasma-enhanced chemical vapour deposition (PECVD). Linear optical properties, namely absorption, emission and refractive indices are reported. The sign and magnitude of both real and imaginary parts of third-order nonlinear susceptibility X(³) of Si-nc are measured by the Z-scan method. Closed aperture Z-scan reveals a positive nonlinearity for all the samples. From the open aperture measurements, nonlinear absorption coefficients are evaluated and attributed to two-photon absorption. Absolute values of X(³) are in the order of 10^-9 esu and show systematic correlation with the Si-nc size, due to quantum confinement related effects. A correlation has been made between X(³), nanocrystalline size, linear refractive index and optical band gap.     

Lasing without or with inversion in an open four-level system with a phase-fluctuation driving field

The effect of exit rate and the ratio of atomic injection rate on gain behaviour has been investigated, and the effects of phase fluctuation on absorption, dispersion and population difference in an open four-level system have been analysed by using numerical simulation from the steady linear, analytical solution. The variation of the linewidth, Rabi frequency of the driving field, the exit rate or the ratio of atomic injection rate can change the lasing properties in the open system. The presence of finite linewidth due to driving-field phase fluctuation prevents the open four-level atomic system from obtaining a high refractive index along with zero absorption.     

Enhanced optical limiting performance in phthalocyanine-quantum dot nanocomposites by free-carrier absorption mechanism

Enhanced nonlinear optical properties (in dimethyl sulphoxide) is observed for 2(3),9(10),16(17),23(24)-tetrakis-(4-aminophenoxy)phthalocyaninato indium(III) chloride (InPc) when covalently linked to CdSe/ZnS or CdSe quantum dots (QDs). The experimental nonlinear optical parameters were obtained from Z-Scan measurements. Contributions from two-photon absorption (2PA) due to the InPc, and free-carrier absorption (FCA) by QDS have been identified as the main factors responsible for the enhanced optical limiting. The effective nonlinear absorption coefficient for InPc-CdSe/ZnS was found to be 700.0 cm/GW. The FCA cross-sections for InPc-CdSe/ZnS and InPc-CdSe composites were found to be 1.52 × 10⁻¹⁹ and 6.00 × 10⁻²⁰ cm² respectively. A much lower limiting threshold of 92 mJ cm⁻² was observed for InPc-CdSe/ZnS nanocomposite, hence, making it suitable for use as optical limiting material. Density Functional Theory (DFT) calculations on similar phthalocyanine-quantum dots system was modeled in order to explain the enhancement in the observed nonlinear optical properties of the Pc in the presence of the QDs. The experimentally determined nonlinear optical properties are well within the range of the DFT calculated properties.     

Squeezed vacuum,coherent and dc field effects on optical multistability via decay-induced quantum beats

Effect of the relative control phase as well as the squeezed phase parameter on the optical switching process of the optical bi-multistable behaviour are investigated in detail for a four-level Y-type atomic system; in the presence of a control field (coherent + dc fields) with squeezed vacuum field and quantum interference from spontaneous emission. The results indicated that (a) the quantum interference assists to decrease the rate of switching via the relative phase of the control field and makes the switching on/off points more rapid in squeezed vacuum compared to a normal vacuum case, and (b) the phase switching effects between the output field and the relative control phase increases the switching on/off processes in normal vacuum more than the squeezed vacuum case. In addition, we have demonstrated the possibility of displaying quantum beats with a novel type of bi-multistable behaviour in the cooperative switching diagram.     

Switching from negative to positive nonlinear absorption in p type 0.5 at% Sn doped GaSe semiconductor crystal

The nonlinear absorption properties of p type 0.5 at% Sn doped GaSe crystal have been studied by using open-aperture Z-scan technique under 1064 nm wavelength and 4 ns or 65 ps pulse duration. A switching from negative nonlinear absorption (saturated absorption) to positive nonlinear absorption (two photon absorption) has been observed as the input laser irradiance increases from 0.049 GW/cm² to 0.106 GW/cm². The nonlinear absorption coefficient increases monotonically with the increase of pulse duration from 65 ps to 4 ns.     

Coherent superposition states of light and their interference in three-photon absorption process

Abstract

For the process of three-photon absorption in the case of a cubic parametric perturbation a possibility to obtain quantum superposition states of three coherent components is shown. The one-photon and two-photon absorption processes are shown to destroy the interference between the state components: the quantum superposition state decays into the classical mixture of its components. It is shown that the interference between different three-component coherent superposition states formed in the system can, depending on the initial state of the field, result in almost full localization of the optical system in a two-component state, or in destruction of the interference between different coherent components. The Wigner functions and quantum entropy of the system are calculated for a variety of initial states.     

Transparency and parametric generation in a four-level system † Reviewing of this paper was handled by a member of the Editorial Board.

We analyse two distinct nonlinear phenomena in a four-level quantum system having a tripod configuration. First, we show that the system can become transparent at two different frequencies of a probe laser field. The potential for parametric generation in this medium is also investigated. We show that, if the system is prepared in a coherent superposition of two or three of the lower levels, one or two new fields can be generated.     

Control of spontaneous emission in a four-level system

Spontaneous emission in a four-level atomic system coupled by two coherent driven fields is investigated. Using a wave function technique, an explicit expression of the spontaneous emission spectrum is derived. The influence of driven fields on spontaneous emission spectra of k and q photons are investigated. The interesting phenomena including spectral line narrowing, enhancement, suppression and quenching, are observed by adjusting driven fields and detunings.     

硫硒化镉微晶玻璃制备和非线性光学性质研究

具有明显量子尺寸效应和较大三阶非线性光学效应的CdSxSe1-x(O相似文献   

Group velocity control of a light pulse using giant nonlinearities

In this paper, we propose and demonstrate a scheme to enhance nonlinearities of a probe pulse at both cross-phase modulation (XPM) and self-phase modulation (SPM) in a four-level system. Based on standing wave grating generated by counter-propagating resonant signal fields and an additional off-resonant coupling field, a giant nonlinear refractive index of the resonant probe field is obtained with absorption suppressed. Group velocity of the probe pulse can be controlled by both XPM and SPM nonlinearities.     

Synthesis,surface profile,nonlinear reflective index and photophysical properties of curcumin compound

Curcumin and other three curcuminoids (bisdemethoxycurcumin, α-chlorocurcumin and α-methylcurcumin) were synthesized. Fourier transform infrared spectroscopy, Fluorescence quantum yields, AFM analysis and image surface profiles were characterized. All compounds possessed electron donor moieties at both ends of the conjugated π-system and an electron acceptor moiety in the middle of the molecules (D-A-D system) and should exhibit different optical properties depending on substituents on the benzene rings. The third order nonlinear optical properties of the curcuminoids have been investigated by z-scan technique. The optical response was characterized by measuring the refractive index (n₂) of the derivatives of curcumin using the Z-scan technique. The compounds showed negative and large nonlinear refractive index values of the order of 10^?7 cm²/W and reverse saturable absorption with high values of the nonlinear absorption coefficient of the order of 10^?4 cm/W. The nonlinear refractive index was found to vary with the different compound. The optical constants of the different compound films were studied and the dispersion of the refractive index was discussed in terms of the Wemple-DiDomenico single oscillator model. The photo-physical properties of these compounds are compared to those of native curcumin, in order to provide a rationale to the design of samples with molecular structures optimized for a photosensitizer. These types of materials may be considering new photonic applications.     

Quantum-interference and the concentration of Er3+ ion effect on left-handedness with zero absorption and large negative refractive index in Er3+:YAG crystal

The electromagnetically induced left-handedness with zero absorption and large negative refractive index was investigated in a solid Er³⁺:YAG crystal with a four-level system proposed for an atomic medium. It was found that the frequency region with simultaneous negative permittivity and negative permeability, the zero absorption intervals, and the maximum values of the negative refractive index can be adjusted by changing the signal field, the coherent field, as well as the concentration of Er³⁺ ion in crystal. It is shown that wider zero absorption intervals with a higher index of refraction can be easily obtained when the signal field is only off resonance. The slab fabricated by the left-handed solid medium Er³⁺:YAG crystal with zero absorption may be a practical candidate for designing perfect lenses.     

Nonlinear optical characterization and measurement of optical limiting threshold of CdSe quantum dots prepared by a microemulsion technique

CdSe quantum dots prepared by micro emulsion technique shows quantum confinement effect and broad emission at 532 nm. These quantum dots have about 4.35 nm size, and they exhibit good nonlinear effects which are measured using z-scan technique. The samples have a reverse saturation in the nonlinear absorption as nonlinear optical absorption coefficient β is 2.545 × 10⁻¹⁰ W m⁻¹ and nonlinear optical refraction coefficient n ₂ is −1.77 × 10⁻¹⁰ esu. The third-order nonlinear optical susceptibility is found to be 4.646 × 10⁻¹¹ esu and also the figure of merit is 2.01 × 10⁻¹² esu m. The optical limiting threshold which is found to be 0.346 GW/cm² makes it a good candidate for device fabrication.     

Switching enhancement and suppression of four-wave mixing via a dressing field

The switch between the enhancement and suppression of the four-wave mixing (FWM) process in an electromagnetically induced transparency (EIT) window via quantum interference has been demonstrated experimentally. The enhancement and suppression of the FWM signal, as well as the EIT window, switch with a change in the probe detuning. Also, by controlling the powers of the dressing and probe fields, the enhancement and suppression of the FWM signal can be switched and an optimized enhancement can be obtained. Such a switch of the enhancement and suppression of the FWM signal intensity can find potential applications in optical switches, optical communication and quantum information processes.     

Thin tin oxide films of low conductivity prepared by chemical vapour deposition

Homogeneous SnO₂ films were produced from dibutylin diacetate by chemical vapour deposition at rates of 10–30 nm min^?1. The films obtained had conductiveties between 3 and 25Ω^?1 cm^?1 and they showed a high quantum efficiency for dye sensitization. Under various conditions a linear dependence of the thickness and resistivity of the films on deposition time was observed. The transmission of most of the films was greater than 90%. SnO₂ films with a thickness of more than 500 nm exhibit absorption. The refractive index of these films, as determined by UV-visible interference measurements in reflection, is between 1.8 and 2.2 in the wavelength range 300–860 nm and agrees well with the refractive index determined from interference microscope measurements. Heat treatment of the films exposed to air lowers their conductivity and refractive indices.