Bong Je Park

Polymer‐Waveguide‐Based Flexible Tactile Sensor Array for Dynamic Response

A polymer-waveguide-based transparent and flexible force sensor array is proposed, which satisfies the principal requirements for a tactile sensor working on curvilinear surfaces, such as thinfilm architecture (thickness < 150 μm), localized force sensing (ca. 0-3 N), multiple-point re cognition (27 points), bending robustness (10.8% degradation at R = 1.5 mm), and fast response (bandwidth > 16 Hz).

Ho3+: (5S2,5F4)→5I5 transition in fluoride glasses

Abstract We propose that Ho 3+ and Ho 3+ /Yb 3+ -doped fluoride glasses are a good candidate material for fiber amplifiers operating at 1340–1400 nm band. Upconversion excitation in the wavelengths of 880–920 and 970–990 nm is achieved for Ho 3+ -single-doped glasses, while wavelength region of 910–1000 nm for Ho 3+ /Yb 3+ -codoped glasses. Ho 3+ concentration is preferably less than 0.5 mol% to avoid lifetime decrease of ( 5 S 2 , 5 F 4 ) manifold. Yb 3+ codoping enhances the upconversion excitation efficiency, however as Yb 3+ concentration increases, back energy transfer from holmium to ytterbium slightly diminishes the fluorescing level lifetime. Some transition-metal ions and rare-earths ions doped in the clad part of a fluoride fiber waveguide, for example, may quench unwanted amplified spontaneous emissions at ∼540 nm and ∼750 nm.

Controlling fluorescence lifetime of rare-earth element in amorphous inorganic solids via very small compositional adjustments

Fluorescence lifetime of hypersensitive 4f-4f transitions of rare-earth elements embedded in amorphous inorganic solids can be dramatically modified by compositional adjustment of the hosts tantamount to not more than 1 mol % without any elaborated thermal treatments. It is possible to modify a spontaneous emission rate of Dy3+:(F11∕26,H9∕26)→H15∕26 transition in chalcogenide Ge–As–S glasses through selective addition of low levels of Ga and CsBr. Along with the change of the spontaneous emission rate, multiphonon relaxation rate involved in the (F11∕26,H9∕26) state also significantly varies upon the minute compositional adjustment. The combination of these effects results in the measured lifetime of the fluorescing (F11∕26,H9∕26) level being greatly enhanced. Such behaviors are attributed to the hypersensitive nature associated with the H15∕26↔F11∕26 transition and preferential coordination of bromine in the nearest-neighboring shell of the Dy3+ ions, which is formed spontaneously during the vitrificatio...

Emission properties of Ho3+∕Tb3+ Co-doped in Ge30Ga2As8S60 glass

Emission properties of Ho3+ and Ho3+∕Tb3+ doped Ge30Ga2As8S60 glasses were investigated. Population densities of the I55 and I75 levels and the gain coefficients for the 1.6μm emission were calculated. Upon the addition of more than 0.05mol% Tb3+, population inversion between the I55 and I75 levels was achieved while it was not possible in the glass doped with Ho3+ only. The positive optical gain in the 1620–1750nm wavelength region was obtained with the addition of 0.3mol% Tb3+ ions.

Crossrelaxations between and multiphonon relaxation of near-infrared excited states of Pr3+ ions in selenide glasses

Abstract We have investigated crossrelaxations and multiphonon relaxation in Pr3+-doped Ge–As–Ga–Se glasses with a special interest in 1.6 μm emission from ( 3 F 3 , 3 F 4 )→ 3 H 4 transition. Crossrelaxation in the route of ( 3 F 3 , 3 F 4 ): 3 H 4 → 3 H 5 :( 3 F 2 , 3 H 6 ) together with ( 3 F 3 , 3 F 4 ): 3 H 5 →( 3 F 2 , 3 H 6 ):( 3 F 2 , 3 H 6 ) and ( 3 F 3 , 3 F 4 ): 3 H 4 →( 3 F 2 , 3 H 6 ): 3 H 5 is responsible for concentration quenching of the 1.6 μm emission lifetimes, and the nonradiative energy transfers intervening in the ( 3 F 3 , 3 F 4 ) state arise from the electric dipole–dipole interactions. Multiphonon relaxation rate in our selenide glasses is the lowest among the conventional glasses even including sulfide glasses. Such a minimized multiphonon relaxation in selenide glasses provides unique opportunity for observing many radiative emissions that would otherwise be seemingly forbidden in the conventional glass hosts.

A thin film active-lens with translational control for dynamically programmable optical zoom

We demonstrate a thin film active-lens for rapidly and dynamically controllable optical zoom. The active-lens is composed of a convex hemispherical polydimethylsiloxane (PDMS) lens structure working as an aperture and a dielectric elastomer (DE) membrane actuator, which is a combination of a thin DE layer made with PDMS and a compliant electrode pattern using silver-nanowires. The active-lens is capable of dynamically changing focal point of the soft aperture as high as 18.4% through its translational movement in vertical direction responding to electrically induced bulged-up deformation of the DE membrane actuator. Under operation with various sinusoidal voltage signals, the movement responses are fairly consistent with those estimated from numerical simulation. The responses are not only fast, fairly reversible, and highly durable during continuous cyclic operations, but also large enough to impart dynamic focus tunability for optical zoom in microscopic imaging devices with a light-weight and ultra-slim configuration.

Characterization of an ASE reflector-based gain-clamped erbium-doped fiber amplifier

We develop a simulation tool for an all-optical gain-clamped erbium-doped fiber amplifier (GC-EDFA) based on an amplified spontaneous emission (ASE) reflector and thoroughly verify its validity by comparing simulation data with experimental ones. We carry out simulation work as changing conditions like reflection ratio and bandwidth of the ASE reflector, EDF length, and pump power. From this work, we have an exact understanding about the gain clamping principle that a reflected ASE acts like an intensity reservoir against input signal intensity variation. In general, as a reflected ASE power becomes higher, both a dynamic range and a noise figure (NF) increase; on the other hand, a clamped gain value decreases. The ASE reflector-based gain clamping scheme can be used for EDFAs with low NF characteristics at small input signal range in case a reflected ASE power is set at a level much lower than powers required for normal gain clamping function.

Structure modulated electrostatic deformable mirror for focus and geometry control

We suggest a way to electrostatically control deformed geometry of an electrostatic deformable mirror (EDM) based on geometric modulation of a basement. The EDM is composed of a metal coated elastomeric membrane (active mirror) and a polymeric basement with electrode (ground). When an electrical voltage is applied across the components, the active mirror deforms toward the stationary basement responding to electrostatic attraction force in an air gap. Since the differentiated gap distance can induce change in electrostatic force distribution between the active mirror and the basement, the EDMs are capable of controlling deformed geometry of the active mirror with different basement structures (concave, flat, and protrusive). The modulation of the deformed geometry leads to significant change in the range of the focal length of the EDMs. Even under dynamic operations, the EDM shows fairly consistent and large deformation enough to change focal length in a wide frequency range (1~175 Hz). The geometric modulation of the active mirror with dynamic focus tunability can allow the EDM to be an active mirror lens for optical zoom devices as well as an optical component controlling field of view.

An electro-active polymer based lens module for dynamically varying focal system

We demonstrate a polymer-based active-lens module allowing a dynamic focus controllable optical system with a wide tunable range. The active-lens module is composed of parallelized two active-lenses with a convex and a concave shaped hemispherical lens structure, respectively. Under operation with dynamic input voltage signals, each active-lens produces translational movement bi-directionally responding to a hybrid driving force that is a combination of an electro-active response of a thin dielectric elastomer membrane and an electro-static attraction force. Since the proposed active lens module widely modulates a gap-distance between lens-elements, an optical system based on the active-lens module provides widely-variable focusing for selective imaging of objects in arbitrary position.

Flexible transparent displays based on core/shell upconversion nanophosphor-incorporated polymer waveguides

Core/shell (C/S)-structured upconversion nanophosphor (UCNP)-incorporated polymer waveguide-based flexible transparent displays are demonstrated. Bright green- and blue-emitting Li(Gd,Y)F4:Yb,Er and Li(Gd,Y)F4:Yb,Tm UCNPs are synthesized via solution chemical route. Their upconversion luminescence (UCL) intensities are enhanced by the formation of C/S structure with LiYF4 shell. The Li(Gd,Y)F4:Yb,Er/LiYF4 and Li(Gd,Y)F4:Yb,Tm/LiYF4 C/S UCNPs exhibit 3.3 and 2.0 times higher UCL intensities than core counterparts, respectively. In addition, NaGdF4:Yb,Tm/NaGdF4:Eu C/S UCNPs are synthesized and they show red emission via energy transfer and migration of Yb3+ → Tm3+ → Gd3+ → Eu3+. The C/S UCNPs are incorporated into bisphenol A ethoxylate diacrylate which is used as a core material of polymer waveguides. The fabricated stripe-type polymer waveguides are highly flexible and transparent (transmittance > 90% in spectral range of 443–900 nm). The polymer waveguides exhibit bright blue, green, and red luminescence, depending on the incorporated UCNPs into the polymer core, under coupling with a near infrared (NIR) laser. Moreover, patterned polymer waveguide-based display devices are fabricated by reactive ion etching process and they realize bright blue-, green-, and red-colored characters under coupling with an NIR laser.