We report scanning tunneling spectroscopy measurements of the threshold energy for injecting electrons or holes into thin, conjugated polymer films deposited on Au(111) substrates. Combining these results with optical absorption measurements, we estimate an exciton binding energy of E b 0.36 6 0.10 eV for poly[(2-methoxy-5-dodecyloxy)-1,4-phenylenevinylene-co-1,4-phenylenevinylene] and E b 0.30 6 0.10 eV for poly (9,9'-dioctylfluorene). In addition, we determine the alignment of the electronic levels of the polymers relative to the substrate. [S0031-9007(98)06767-2]

Gallium phosphide (GaP) is an indirect bandgap semiconductor used widely in solid-state lighting. Despite numerous intriguing optical properties-including large χ (2) and χ (3) coefficients, a high refractive index (> 3), and transparency from visible to long-infrared wavelengths (0.55 − 11 µm)its application as an integrated photonics material has been little studied. Here we introduce GaPon-insulator as a platform for nonlinear photonics, exploiting a direct wafer bonding approach to realize integrated waveguides with 1.2 dB/cm loss in the telecommunications C-band (on par with Si-on-insulator). High quality (Q > 10 5 ), grating-coupled ring resonators are fabricated and studied. Employing a modulation transfer approach, we obtain a direct experimental estimate of the nonlinear index of GaP at telecommunication wavelengths: n2 = 1.2(5) × 10 −17 m 2 /W. We also observe Kerr frequency comb generation in resonators with engineered dispersion. Parametric threshold powers as low as 3 mW are realized, followed by broadband (> 100 nm) frequency combs with sub-THz spacing, frequency-doubled combs and, in a separate device, efficient Raman lasing. These results signal the emergence of GaP-on-insulator as a novel platform for integrated nonlinear photonics.

No abstract

The electronic structure of Alq3 is investigated using density functional theory-based calculations, photoemission and near-edge x-ray absorption fine structure. The distinct features of the observed spectra are understood in terms of contributions from the different atoms and molecular orbitals. Fingerprints of the molecular bonding and of the individual atoms are identified. These results are meant to be a reference for the monitoring of chemical processes that Alq3 may undergo during fabrication or degradation of light-emitting devices, and for the understanding of the effects of ligand or metal substitution.

Transient electroluminescence ͑EL͒ from single-and multilayer organic light-emitting diodes ͑OLEDs͒ was investigated by driving the devices with short, rectangular voltage pulses. The single-layer devices consist of indium-tin oxide ͑ITO͒/tris͑8-hydroxy-quinoline͒aluminum ͑Alq 3 ͒/magnesium ͑Mg͒:silver ͑Ag͒, whereas the structure of the multilayer OLEDs are ITO/copper phthalocyanine ͑CuPc͒/N,NЈ-di͑naphthalene-1-yl͒-N,NЈ-diphenyl-benzidine ͑NPB͒/Alq 3 /Mg:Ag. Apparent model-dependent values of the electron mobility ( e ) in Alq 3 have been calculated from the onset of EL for both device structures upon invoking different internal electric field distributions. For the single-layer OLEDs, transient experiments with different dc bias voltages indicated that the EL delay time is determined by the accumulation of charge carriers inside the device rather than by transport of the latter. This interpretation is supported by the observation of delayed EL after the voltage pulse is turned off. In the multilayer OLED the EL onset-dependent on the electric field-is governed by accumulated charges ͑holes͒ at the internal organic-organic interface (NPB/Alq 3 ) or is transport limited. Time-of-flight measurements on 150-nm-thin Alq 3 layers yield weak field-dependent e values of the order of 1ϫ10 Ϫ5 cm 2 /Vs at electrical fields between 3.9ϫ10 5 and 1.3ϫ10 6 V/cm.

No abstract

We have performed electric-field and temperature-dependent electron injection studies in an aluminum/ tris͑8-hydroxy-quinolinolato͒aluminum/magnesium:silver single-layer organic light-emitting diode. Analysis of the observed injection currents in terms of the classic Fowler-Nordheim ͑FN͒ tunneling or RichardsonSchottky ͑RS͒ thermionic emission proved to be inadequate. Whereas, the FN-type behavior at high-electric fields must be considered accidental, the injection currents qualitatively resemble those of the RS concept. However, quantitative differences are observed concerning the RS coefficient, the prefactor current, and the temperature dependence. On the other hand, the experimental data are in excellent agreement with a recently presented Monte Carlo simulation ͓U. Wolf et al., Phys. Rev. B 59, 7507 ͑1999͔͒ of carrier injection from a metal to an organic dielectric with random hopping sites. ͓S0163-1829͑99͒14535-1͔

Optomechanical cavities in the well-resolved-sideband regime are ideally suited for the study of a myriad of quantum phenomena with mechanical systems, including backaction-evading measurements, mechanical squeezing, and generation of non-classical states. For these experiments, the mechanical oscillator should be prepared in its ground state; residual motion beyond the zero-point motion must be negligible. The requisite cooling of the mechanical motion can be achieved using the radiation pressure of light in the cavity by selectively driving the anti-Stokes optomechanical transition. To date, however, laser-absorption heating of optical systems far into the resolved-sideband regime has prohibited strong driving. For deep ground-state cooling, previous studies have therefore resorted to passive cooling in dilution refrigerators. Here, we employ a highly sideband-resolved silicon optomechanical crystal in a 3 He buffer gas environment at ∼ 2 K to demonstrate laser sideband cooling to a mean thermal occupancy of 0.09 +0.02 −0.01 quantum (self-calibrated using motional sideband asymmetry), which is −7.4 dB of the oscillator's zero-point energy and corresponds to 92% ground state probability. Achieving such low occupancy by laser cooling opens the door to a wide range of quantum-optomechanical experiments in the optical domain.Laser cooling techniques developed several decades ago [1][2][3][4] have revolutionized many areas of science and technology, with systems ranging from atoms, ions and molecules [5-11] to solid-state structures and macroscopic objects [12][13][14]. Among these systems, mechanical oscillators play a unique role given their macroscopic nature and their ability to couple to diverse physical quantities [15]. Laser cooling of mechanical systems occurs via coupling of mechanical and electromagnetic degrees of freedom (optomechanical coupling) and has been demonstrated with a wide range of structures [12,[16][17][18][19][20][21][22][23][24][25]. It has led to the observation of radiation pressure shot noise [26], ponderomotive squeezing of light [27,28], and motional sideband asymmetry [16,[29][30][31][32].Many optomechanical protocols, including mechanical squeezing [33][34][35][36], entanglement [37], state swaps [38], generation of non-classical states [39][40][41][42], and back-action evading (BAE) measurements below the standard quantum limit (SQL) [43][44][45], require ground state preparation of a wellsideband-resolved system, where Stokes and anti-Stokes motional transitions can be driven selectively. In this case, driving of anti-Stokes transitions can be efficiently applied to damp the motion and sideband cool the system. The cooling limit is set by laser noise (classical or quantum) or by technical limitations, such as absorption heating, and determines the residual thermal noise. For the case of squeezing or BAE measurements, the amount of cooling beyond half quantum (equivalent to the zero point energy) determines the amount of squeezing or the amount to which the SQL on resonance is surp...