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2 Nov 2009

Volume 95, Issue 18, Articles (18xxxx)

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Appl. Phys. Lett. 95, 183504 (2009); http://dx.doi.org/10.1063/1.3256223 (3 pages)

Hoon-Sik Kim, Sang Min Won, Young-Geun Ha, Jong-Hyun Ahn, Antonio Facchetti, Tobin J. Marks, and John A. Rogers
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Electrically tunable terahertz quantum-cascade laser with a heterogeneous active region

Suraj P. Khanna, Mohammed Salih, Paul Dean, A. Giles Davies, and Edmund H. Linfield

Appl. Phys. Lett. 95, 181101 (2009); http://dx.doi.org/10.1063/1.3253714 (3 pages) | Cited 9 times

Online Publication Date: 2 November 2009

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We demonstrate experimentally a terahertz frequency quantum-cascade laser which can be tuned electronically, in a step-wise manner from typically 3.07 to 3.40 THz, by changing the applied electric field across the device. To achieve this, the gallium growth rate was successively reduced during growth of the active region from +6% to −4% of its nominal 1 μm/h operating value. This produced a heterogeneous active region, comprising 23 sections of different lasing units stacked together. With a single-metal surface-plasmon waveguide configuration, the devices operated in pulsed mode, up to a temperature of 81 K, and gave peak powers of ∼ 8 mW.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Fc Modulation, tuning, and mode locking
42.79.Gn Optical waveguides and couplers
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

Thermal-induced refractive-index planar waveguide laser

Hongxiang Kang (康宏向), Haitao Zhang (张海涛), and Dongsheng Wang (王东生)

Appl. Phys. Lett. 95, 181102 (2009); http://dx.doi.org/10.1063/1.3258068 (3 pages) | Cited 1 time

Online Publication Date: 2 November 2009

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The feasibility of a thermal-induced refractive-index planar waveguide laser was demonstrated. The condition for the guided modes to be confined in the gain region was calculated. A thermal-induced refractive-index planar waveguide laser was made simply by pumping a thin center layer of a slab crystal and its modes can be controlled by the thermal input unit area and the pumped region width. An output power of 23.1 W with an optical to optical conversion efficiency of 46% was obtained. A thermal-induced refractive-index planar waveguide laser is very promising for efficient and bright laser sources due to its simple waveguide structure.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.72.-g Optical sources and standards
42.82.Et Waveguides, couplers, and arrays

Near-field imaging of femtosecond laser ablated sub-λ/4 holes in lithium niobate

Airán Ródenas, Jorge Lamela, Daniel Jaque, Ginés Lifante, Francisco Jaque, Antonio García-Martín, Guangyong Zhou, and Min Gu

Appl. Phys. Lett. 95, 181103 (2009); http://dx.doi.org/10.1063/1.3256219 (3 pages) | Cited 3 times

Online Publication Date: 2 November 2009

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We report on the direct femtosecond laser ablation of sub-λ/4 (80–250 nm) holes in LiNbO3 crystals and on its local near-field imaging. We show that the near-field transmission of holes can feature an attenuation of ∼ 75% at hole central position, and a ∼ 20% transmission enhancement at its sides. This high-contrast ring-shaped near-field distribution is found to be in agreement with simulations, suggesting the surface relief as the main contrast mechanism.
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52.38.Mf Laser ablation
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Optical position clamping with predictive control

Heikki Ojala, Anders Korsbäck, Anders E. Wallin, and Edward Hæggström

Appl. Phys. Lett. 95, 181104 (2009); http://dx.doi.org/10.1063/1.3257693 (3 pages) | Cited 2 times

Online Publication Date: 4 November 2009

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We increase the effective stiffness of optical tweezers by position clamping a polystyrene bead with a predictive feedback control algorithm. This algorithm mitigates the effect of feedback loop delay. Hence, higher gain than with proportional control can be employed, which results in higher effective trap stiffness, without trap instability. In experiments (initial trap stiffness 0.056 pN/nm with a 1.78 μm diameter polystyrene bead), predictive control increased the effective trap stiffness by 55% relative to proportional control. We also derive theoretical expressions for the power spectra of the bead position controlled by our algorithm.
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07.05.Dz Control systems

Flexible metamaterials for wireless strain sensing

Rohat Melik, Emre Unal, Nihan Kosku Perkgoz, Christian Puttlitz, and Hilmi Volkan Demir

Appl. Phys. Lett. 95, 181105 (2009); http://dx.doi.org/10.1063/1.3250175 (3 pages) | Cited 14 times

Online Publication Date: 4 November 2009

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We proposed and demonstrated flexible metamaterial-based wireless strain sensors that include arrays of split ring resonators (SRRs) to telemetrically measure strain. For these metamaterial sensors, we showed that a flexible substrate (e.g., Kapton tape) delivers greater sensitivity and a more linear response as compared to using silicon substrates. Specifically, these tape-based flexible SRR sensors exhibit a significantly improved sensitivity level of 0.292 MHz/kgf with a substantially reduced nonlinearity error of 3% for externally applied mechanical loads up to 250 kgf. These data represent a sixfold increase in sensitivity and a 16-fold reduction in error percentage.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.10.Pz Instruments for strain, force, and torque

Controlling the spectrum of x-rays generated in a laser-plasma accelerator by tailoring the laser wavefront

S. P. D. Mangles, G. Genoud, S. Kneip, M. Burza, K. Cassou, B. Cros, N. P. Dover, C. Kamperidis, Z. Najmudin, A. Persson, J. Schreiber, F. Wojda, and C.-G. Wahlström

Appl. Phys. Lett. 95, 181106 (2009); http://dx.doi.org/10.1063/1.3258022 (3 pages) | Cited 10 times

Online Publication Date: 4 November 2009

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By tailoring the wavefront of the laser pulse used in a laser-wakefield accelerator, we show that the properties of the x-rays produced due to the electron beam’s betatron oscillations in the plasma can be controlled. By creating a wavefront with coma, we find that the critical energy of the synchrotronlike x-ray spectrum can be significantly increased. The coma does not substantially change the energy of the electron beam, but does increase its divergence and produces an energy-dependent exit angle, indicating that changes in the x-ray spectrum are due to an increase in the electron beam's oscillation amplitude within the wakefield.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.59.Px Hard X-ray sources
52.38.Kd Laser-plasma acceleration of electrons and ions

Bistable twisted-bend and twisted-nematic liquid crystal display

Y. W. Li and H. S. Kwok

Appl. Phys. Lett. 95, 181107 (2009); http://dx.doi.org/10.1063/1.3254212 (3 pages) | Cited 1 time

Online Publication Date: 5 November 2009

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Bistability of twisted-bend and twisted configuration is found. It is based on the elastic and topological inequivalency of director orientation. Multiple states of minimum local energy exist, with appropriate boundary conditions. The effects of elastic constants, d/P ratio and pretilt angle are also investigated. Fast electrical switching time <500 μs and high average contrast of 45:1 are obtained experimentally.
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42.79.Kr Display devices, liquid-crystal devices
85.60.Pg Display systems

Ultrafast imaging of photoelectron packets generated from graphite surface

Ramani K. Raman, Zhensheng Tao, Tzong-Ru Han, and Chong-Yu Ruan

Appl. Phys. Lett. 95, 181108 (2009); http://dx.doi.org/10.1063/1.3259779 (3 pages) | Cited 5 times

Online Publication Date: 5 November 2009

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We present an electron projection imaging method to study the ultrafast evolution of photoelectron density distribution and transient fields near the surface. The dynamical profile of the photoelectrons from graphite reveals an origin of a thermionic emission, followed by an adiabatic process leading to electron acceleration and cooling before a freely expanding cloud is established. The hot electron emission is found to couple with a surface charge dipole layer formation, with a sheet density several orders of magnitude higher than that of the vacuum emitted cloud.
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78.47.J- Ultrafast spectroscopy (<1 psec)
79.60.Bm Clean metal, semiconductor, and insulator surfaces
72.20.Ht High-field and nonlinear effects
79.40.+z Thermionic emission
72.80.Sk Insulators

Engineering single photon emitters by ion implantation in diamond

B. Naydenov, R. Kolesov, A. Batalov, J. Meijer, S. Pezzagna, D. Rogalla, F. Jelezko, and J. Wrachtrup

Appl. Phys. Lett. 95, 181109 (2009); http://dx.doi.org/10.1063/1.3257976 (3 pages) | Cited 9 times

Online Publication Date: 5 November 2009

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Diamond provides unique technological platform for quantum technologies including quantum computing and communication. Controlled fabrication of optically active defects is a key element for such quantum toolkit. Here we report the production of single color centers emitting in the blue spectral region by high energy implantation of carbon ions. We demonstrate that single implanted defects show sub-poissonian statistics of the emitted photons and can be explored as single photon source in quantum cryptography. Strong zero phonon line at 470.5 nm allows unambiguous identification of this defect as interstitial-related TR12 color center.
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61.72.jn Color centers
61.72.up Other materials
63.20.-e Phonons in crystal lattices
61.72.jj Interstitials

Polarization entanglement with graded-index lenses

Giuseppe Vallone, Gaia Donati, Francesco De Martini, and Paolo Mataloni

Appl. Phys. Lett. 95, 181110 (2009); http://dx.doi.org/10.1063/1.3257376 (3 pages) | Cited 2 times

Online Publication Date: 6 November 2009

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By using an optical device based on the integration of a graded-index (GRIN) rod lens and a single-mode optical fiber we efficiently collected photon pairs generated by spontaneous parametric down-conversion. We show that this system preserves the polarization entanglement of the two-photon states. Hence this device, characterized by a remarkable easiness of alignment and allowing for high coupling efficiency of single-mode radiation, can be used with photons entangled in various degrees of freedom, such as polarization and spatial momentum and to interconnect different sides of complex optical circuits.
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42.79.Bh Lenses, prisms and mirrors
42.79.Ry Gradient-index (GRIN) devices

Birefringence-induced mode-dependent tuning of liquid crystal infiltrated InGaAsP photonic crystal nanocavities

M. A. Dündar, H. H. J. E. Kicken, A. Yu. Silov, R. Nötzel, F. Karouta, H. W. M. Salemink, and R. W. van der Heijden

Appl. Phys. Lett. 95, 181111 (2009); http://dx.doi.org/10.1063/1.3259814 (3 pages) | Cited 8 times

Online Publication Date: 6 November 2009

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Mode-dependent shifts of resonant frequencies of cavities in liquid crystal (LC) infiltrated planar photonic crystals (PhC) are experimentally observed when the temperature is varied across the LC ordering transition. The shifts can be in opposite directions, even for two very similar nearly degenerate modes. The behavior is attributed to the different interactions of the modes with the two components of the refractive index of the LC infill and directly demonstrates that at least a substantial amount of the LC is oriented perpendicular to the PhC-hole axis.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.20.Fm Birefringence
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.70.Qs Photonic bandgap materials

Effect of polarization on spectral switches in the diffraction of stochastic electromagnetic beams

Liuzhan Pan, Zhiguo Zhao, Chaoliang Ding, and Baida Lü

Appl. Phys. Lett. 95, 181112 (2009); http://dx.doi.org/10.1063/1.3259824 (3 pages) | Cited 4 times

Online Publication Date: 6 November 2009

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We consider the stochastic electromagnetic Gaussian Schell-model (EGSM) beam and show that the polarization of stochastic electromagnetic EGSM beams in the source plane affect spectral switches of diffracted stochastic electromagnetic EGSM beams in the far field. For two cases of fully coherent and partially coherent δxx = δyy the behavior of spectral switches are same in different degrees of polarization of the source.
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42.65.Pc Optical bistability, multistability, and switching, including local field effects

Microbubble-enhanced ultrasound-modulated fluorescence in a turbid medium

Baohong Yuan, Yuan Liu, Patrick M. Mehl, and Joseph Vignola

Appl. Phys. Lett. 95, 181113 (2009); http://dx.doi.org/10.1063/1.3262959 (3 pages) | Cited 9 times

Online Publication Date: 6 November 2009

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The feasibility of using ultrasound to modulate fluorescence in a turbid medium is still in debate due to the difficulty of detecting the modulated signal. We have demonstrated a system that could detect the weak signals of ultrasound-modulated fluorescence (UMF) by using a broadband lock-in amplifier and microbubbles as enhancement agents. By detecting the microbubble-enhanced UMF signal, a sub-millimeter fluorescent tube submerged in a turbid medium with a depth of 2 cm has been clearly observed with an ultrasonic spatial resolution. The modulation efficiency was significantly improved by using microbubbles, and was found to linearly increase with the drive voltage applied to the ultrasound transducer and the fluorophore concentration within the range adopted in this study. Possible modulation mechanisms are discussed.
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87.63.dh Ultrasonographic imaging
87.64.kv Fluorescence
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