Asymmetric multimode interference isolator based on nonreciprocal phase shift

This paper presents an asymmetric multimode interference-based (MMI) optical isolator, by utilizing the magneto-optical nonreciprocal phase shift (NPS). Equivalent beam propagation method (BPM) simulation of symmetric and asymmetric isolators are performed using configuration of Air/Ce:YIG/SiO₂ on silicon substrate for integration. The asymmetric isolator is found to be much more compact in size and efficient in isolation compared with symmetric isolator. Simulation results show that the isolation of asymmetric structure is 23.8 dB higher than that of symmetric one. It is mainly because both symmetric mode and anti-symmetric mode are excited in asymmetric structure and hence can interfere destructively. The proposed device may play an important role in the optical communication systems and photonic integrated devices.     

Geometric Phase in a Two Energy Level <Emphasis Type="Italic">k</Emphasis>-Photon Jaynes-Cummings Model with Imaginary Photon Process

By using the Lewis-Riesenfeld invariant theory, we have studied the dynamical phase and the geometric phase in a two energy level k-photon Jaynes-Cummings model with imaginary photon process. We find that the geometric phase in a cycle case is independent of the frequency of the photon field, the coupling coefficient between photons and atoms, and the atom transition frequency. We predict the physical effect of the geometric phase in the imaginary photon process may be measured.     

Study of theoretical tensile strength of Fe by afirst-principles computational tensile test

This paper employs a first-principles total-energy method to investigate the theoretical tensile strengths of bcc and fcc Fe systemically. It indicates that the theoretical tensile strengths are shown to be 12.4, 32.7, 27.5~GPa for bcc Fe, and 48.1, 34.6, 51.2~GPa for fcc Fe in the [001], [110] and [111] directions, respectively. For bcc Fe, the [001] direction is shown to be the weakest direction due to the occurrence of a phase transition from ferromagnetic bcc Fe to high spin ferromagnetic fcc Fe. For fcc Fe, the [110] direction is the weakest direction due to the formation of an instable saddle-point `bct structure' in the tensile process. Furthermore, it demonstrates that a magnetic instability will occur under a tensile strain of 14%, characterized by the transition of ferromagnetic bcc Fe to paramagnetic fcc Fe. The results provide a good reference to understand the intrinsic mechanical properties of Fe as a potential structural material in the nuclear fusion Tokamak.     

Structural phase transitions in KMnF3

The structural transitions of the perovskite KMnF₃ are studied with an energy-dispersive X-ray diffractometer. It is found that the 83 K transition is of first order, though the transition related to the condensation of a M₃-soft phonon mode is considerably affected by crystallographic domain-walls occurring below the 186 K transition. The latter transition is observed at 88.0 K (first-order), and 92.0 K (second-order) in different single crystals, respectively. The difference of the transition temperature and the transition order is interpreted in terms of inner strains appeared in the domain walls.     

Phase transitions and modulated structure of the incommensurate phase in Mg[H2O]6SiF6 crystal

The temperature and angular dependences of the EPR spectra of Mg[H₂O]₆SiF₆:Mn²⁺ crystal were investigated in order to clarify the successive phase transitions and existence of the incommensurate phase. Five successive phase transitions were found to occur, and phase II was found to be incommensurately modulated. The modulated structure is caused mainly by the vibrational displacement of the Mg[H₂O]²⁺ ₆ ion along the c-axis. The soliton density of this phase is almost independent of temperature and remains equal to unity.     

Phase transition in polymer gels due to local heating by illumination of light

The volume phase transition in gels induced by visible light and its related properties are presented, the mechanism of which is based on local heating of a polymer network by illumination of light. The gels consist of a covalently cross-linked copolymer network of thermosensitive N-isopropylacrylamide and a chromophore. Without light illumination, the gel volume changes sharply, but continuously at approximately 34°C when the temperature is varied. At a fixed temperature of an appropriate value, a discontinuous volume transition is observed when the light intensity is gradually changed. The phase transitions can be understood in terms of the temperature increment at the immediate vicinity of polymer chains due to the local heating via light absorption and subsequent thermal dissipation of light by the chromophore. The results can be qualitatively described by the Flory-Huggins mean-field equation of state of gels. In order to make clear the mechanism of the light-induced phase transition in the present system, we measured the light transmitting properties and the swelling as well as shrinking kinetics. These preliminary results are described semi-qualitatively by making use of a simple phenomenological model.     

LARGE MAGNETIC ENTROPY CHANGE NEAR CHARGE-ORDERED TRANSITION TEMPERATURE IN PEROVSKITE-TYPE MANGANITE

The magnetocaloric effect in polycrystalline of Pr_1-xSr_xMnO₃ (x=0.33, 0.43, 0.50) has been investigated. A large magnetic entropy change (7.1J/kgK) was discovered in Pr_0.5Sr_0.5MnO₃ under a low magnetic field of 1T at charge-ordered state transition temperature (161K). The physical mechanism is related to a drastic magnetization change at a temperature where the field-induced magnetic, electron and structural phase transitions occur (from the antiferromagnetic charge-ordered state to the ferromagnetic charge-disordered state).     

Entangling atoms in photonic crystals

We propose a method for entangling a system of two-level atoms in photonic crystals. The atoms are assumed to move in void regions of a photonic crystal. The interaction between the atoms is mediated either via a defect mode or via a resonant dipole-dipole interaction. We show that these interactions can produce pure entangled atomic states. We analyze the problem with parameters typical for currently existing photonic crystals and Rydberg atoms and we show that the atoms can emerge from photonic crystals in entangled states. Depending on the linear dimensions of the crystal we estimate that a pair of atoms entangled in a photonic crystal can be separated by tens of centimeters. Receive 11 June 1999 and Received in final form 4 October 1999     

Triplet-triplet transfer of electronic energy upon nonequilibrium vibrational excitation of triplet molecules

The specific features of the triplet-triplet (T-T) transfer of electronic excitation energy in a gas phase upon nonequilibrium vibrational excitation of the triplet molecules of a donor were studied for an anthraquinone-diacetyl donor-acceptor pair using the time-resolved slow fluoresence of anthraquinone and sensitized phosphorescence of diacetyl. It is shown that in the gas phase, which allows regular control of the number of collisions, competition between the processes of T-T transfer and intermolecular vibrational relaxation is observed for nanosecond time resolution. The T-T transfer rate for the molecular system investigated exceeded the rate of intermolecular vibrational relaxation k_V in the triplet state T₁ of the donor. The effectiveness of the T-T transfer of energy by vibrationally excited molecules turned out to be higher than the effectiveness of transfer by thermalized ones, but even the highest of them was much less than unity. An increase in the equilibrium temperature of vapors led to a decrease in the effectiveness of transfer for both vibrationally excited and thermalized triplet molecules, thus indicating the importance of the collisional complex in the intermolecular process studied. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 474–479, July–August, 2000.