Ultrafast All-optical Modulation Research Articles

Ultrafast all-optical light modulation in the near infrared region by phase sensitive polymer guided wave mode geometry containing porphyrin tapes

A phase sensitive detection was introduced into reflection type all-optical light modulation based on guided wave mode (GWM) geometry. A polymer film containing a copper porphyrin tape and a low refractive polymer film formed GWM at specific incident angle and wavelength. Sensitive changes of output intensity were observed upon femtosecond laser excitation depending on azimuthal angles of the analyzer, which was caused by phase changes at the GWM by Δk alone. The present system has a great advantage as compared with the conventional GWM or simple transmission methods for all-optical light modulation especially for materials showing the ground state absorption.

Ultrafast all-optical modulation in silicon-based nanoplasmonic devices

A five-layer silicon-based nanoplasmonic waveguiding structure is proposed for ultrafast all-optical modulation and switching applications. Ultrafast nonlinear phase and amplitude modulation is achieved via photo-generated free carrier dynamics in ion-implanted silicon using above-bandgap femtosecond pump pulses. Both an analytical model and rigorous numerical simulations of the structures have shown that a switching time of 5 ps and an on-off contrast of 35 dB can be achieved in these devices.

Enhancement of All-Optical Cross Phase Modulation in InGaAs/AlAsSb Coupled Quantum Wells Using InAlAs Coupling Barriers

Ultrafast all-optical cross phase modulation (XPM) was enhanced in the InGaAs/AlAsSb coupled quantum wells using new InAlAs coupling barrier. Furthermore, a high XPM efficiency of 0.15 rad/pJ, which is approximately three times as large as that of a previous sample with the AlAs coupling barrier, was obtained by realizing the strong optical confinement in a narrower waveguide. From the analysis of the quantum levels and the measurement of the absorption spectra, the enhanced XPM efficiency was suggested to be contributed by the refractive index dispersion of the interband transition that was modulated by the intersubband transitions (ISBTs). This large XPM efficiency is expected to give a higher performance in Mach-Zehnder interferometer-type ultrafast all-optical switch.

Ultrafast femtosecond all-optical modulation through nondegenerate two-photon absorption in silicon-on-insulator waveguides

We present numerical investigations of ultrafast femtosecond (with time duration of 100 fs at 1/e intensity point) all-optical modulation of a pump-probe wave arrangement by using nondegenerate two-photon absorption (TPA), namely cross absorption, inside silicon-on-insulator (SOI) optical waveguides. Our results show that when a pump pulse with femtosecond duration and a continuous probe wave are co-propagating along the SOI, the probe wave can be modulated inversely by the ultrafast pump pulse, whose modulation depth depends strongly on the system parameters such as the waveguide length, the peak power and initial chirp of the pump wave, the group velocity dispersion (GVD), etc.; this means that the modulation depth can be improved by an appropriate increase of the waveguide length, the pump peak, and the initial chirp, in addition, which has a larger value for the probe wavelength in the normal dispersion regime compared with the case of abnormal dispersion when the center wavelength of the pump wave is located at the zero-dispersion wavelength.

Excited State Relaxation Processes of Naphthalocyanatovanadium(IV) oxide in Polymer Films for Ultrafast All-optical Modulation in the Near-infrared Region

Abstract Excited state relaxation process of naphthalocyanatovanadium(IV) oxide (VONc) in solutions and in polymer films was studied by transient absorption measurements. The transient absorption decay of VONc in polymer films is faster than that in toluene solution. The excitation power dependence and weight fraction dependence of the excited state dynamics and absorption spectrum suggests that the aggregation of VONcs contributes to faster decay from singlet excited state. The aggregation of VONc is concluded to be useful to ultrafast all-optical light modulation in the near-infrared region.

Ultrafast all-optical light modulation with high wavelength resolution in a near-infrared region by composite polymer guided wave mode

A tricyclodecane polymer film with vanadyloxophthalocyanine was prepared on a low refractive index polymer film by spin coating. It showed a guided wave mode (GWM) dependent on the refractive index and thickness of each layer as minimum reflectance in near infrared spectrum. The width of GWMs was found to considerably decrease with increasing thickness of the low refractive index polymer. The reflectance at the GWM position was optically modulated by ultrafast complex refractive index changes upon femtosecond laser excitation. The present results will be used for ultrafast all-optical light modulation or switching with high wavelength resolution in photonic network systems.

Ultrafast all-optical modulation on a silicon chip

We experimentally demonstrate ultrafast all-optical modulation using a micrometer-sized silicon photonic integrated device. The device transmission is strongly modulated by photoexcited carriers generated by low-energy pump pulses. A p-i-n junction is integrated on the structure to permit control of the generated carrier lifetimes. When the junction is reverse biased, carriers are extracted from the device in a time as short as 50 ps, permitting greater than 5 Gbit/s modulation of optical signals on a silicon chip.

Highly sensitive ultrafast all-optical light modulation by complex refractive-index changes in guided mode geometry composed of a photoresponsive polymer and a low-refractive-index polymer

Highly sensitive ultrafast all-optical light modulation is proposed on the basis of photoinduced complex refractive-index changes in composite guided wave mode (GWM) geometry composed of a low-refractive-index polymer and a photoresponsive polymer. Both calculated and observed results indicated that reflectance, which was unity at an extinction coefficient k=0, decreased rapidly with increasing k until a certain value (kc), and increased again after that. The kc-value and the GWM dip width decreased with increasing the thickness of both polymer layers. The real-part changes shifted the GWM dip very sensitively. The low-refractive polymer contributed great improvements, such as high sensitivity and durability against a repeated femtosecond laser. The present device can give highly enhanced changes of probe light as compared with simple transmission-type devices or the GWM geometry on a metal thin film. A photoelectrochromic polymer was employed in this geometry to demonstrate ultrafast reflectance control upon femtosecond laser excitation.

Raman-enhanced regenerative ultrafast all-optical fiber XPM wavelength converter

The Raman gain enhancement of a regenerative ultrafast all-optical cross-phase modulation (XPM) wavelength converter (WC) is quantitatively investigated and experimentally demonstrated to operate error free at 40 and 80 Gb/s. The regenerative nature of the converter is shown by experimentally demonstrating a negative 2-dB power penalty at 80 Gb/s. It is also shown that the Raman gain greatly enhances the wavelength conversion efficiency at 80 Gb/s by 21 dB at a Raman pump power of 600 mW using 1 km of highly nonlinear fiber. An analytical theory based on nonlinear phase-shift enhancement of the fiber-effective length is presented and shows the relationship between a nonlinear enhancement and Raman gain as a function of pump power and fiber design parameters. Measured parameters are used in the analytical model, and a good fit between experiment and theory is shown for two different types of fiber: one dispersion-shifted and one highly nonlinear fiber. The ultrafast response time of Raman gain makes this technique applicable to fiber-based ultrafast WCs. In addition, the applicability to other nonlinear fiber wavelength conversion techniques is discussed.

Ultrafast all-optical modulation via two-photon absorption in silicon-on-insulator waveguides

Ultrafast all-optical modulation in a silicon-on-insulator waveguide via simultaneous two-photon absorption of a CW signal from 1545–1570 nm and a 2 ps control pulse at 1530 nm is demonstrated. A modulation depth >7 dB and a response time <15 ps, limited by the detection bandwidth, is achieved.

Ultrafast all-optical spatial light modulation based on complex refractive index changes of copper phthalocyanine-doped polymer film upon photoexcitation in guided mode geometry

Ultrafast all-optical reflectance control was achieved by photoinduced complex refractive index changes in thin poly(vinyl alcohol) (PVA) films containing water-soluble phthalocyanine at the incident angle corresponding to a sharp guided mode resonance upon excitation with a ns pulsed laser as a writing beam. The rise time of reflectance was less than 20 ns and the fall time was controlled by the lifetime of the excited triplet phthalocyanine, 31 ns for copper phthalocyanine (CuPcS). The spatial resolution was determined to be better than about 3 μm by using a photochromic dye. The present system based on photoexcitation of CuPcS will allow all-optical spatial light modulation upto about 30 MHz.

Ultrafast all-optical modulation by near-infrared intersubband transition in n-doped InGaAs/AlAsSb quantum wells

We demonstrate the ultrafast modulation of interband (IB)-resonant light (1.0–1.1 μm) by near-infrared intersubband (ISB)-resonant light (1.55–1.9 μm) in n-doped InGaAs/AlAsSb multiple quantum wells (QWs) using non-degenerate pump–probe spectroscopy. A modulation with an absorption recovery time of 1.0–2.0 ps has been observed in a planer-type modulation device due to ultrafast ISB relaxation of the carriers. The IB carrier relaxation process in the absence of an ISB-resonant light has also been investigated by time-resolved photoluminescence (PL) measurement. The modulation speed is determined by the inter- and intra-subband relaxation of the carriers in the conduction subband. The modulation speed at 1.1 μm due to an ISB-resonant pump light at 1.95 μm has been observed to be 1.4 ps at excitation energy of 500 fJ/μm2.

Intersubband transitions and ultrafast all-optical modulation using multiple InGaAs-AlAsSb-InP coupled double-quantum-well structures

InP-based InGaAs-AlAs-AlAsSb coupled double-quantum-well structures have been optimized using wave-function engineering techniques to achieve near-infrared intersubband (ISB) transitions. Intersubband transitions at communication wavelengths of 1.3 and 1.55 /spl mu/m can be achieved in both near-symmetric and asymmetric coupled quantum-well structures by tailoring the well width, the central barrier width, and the carrier population in the conduction subband states by controlling either the doping level or the carrier temperature. We demonstrate ultrafast all-optical modulation of interband-resonant light at 1.3 /spl mu/m using intersubband-resonant light pulses at 1.55 /spl mu/m. An ultrafast absorption recovery time of 1.3 ps has been observed at 1.3 /spl mu/m, which can be reduced to 800 fs by probing at a higher energy above the Fermi level in the conduction band.

Pump-probe measurement of ultrafast all-optical modulation based on intersubband transition in n-doped quantum wells

Modulation of interband-resonant light (∼800 nm) by intersubband-resonant light (5–7 μm) was investigated in n-doped AlGaAs/GaAs multiple quantum wells by a two-color femtosecond pump-probe technique. Modulation with a recovery time of ∼1 ps is observed in a plainer-type modulation device at room temperature. The modulation of interband absorption coefficient is ∼1000 cm−1 when the energy density of the intersubband light pulse is ∼4 fJ/μm2. The modulation efficiency indicates that 99% modulation can be achieved with a control pulse energy of ∼1 pJ when a conventional waveguide-type device structure is utilized. The mechanism which determines the modulation speed is discussed in terms of carrier relaxation process. It is shown that the modulation speed is mainly determined by the inter- and intrasubband relaxation times, where the latter is influenced by hot phonon effects.

Femtosecond pump and probe measurement of all-optical modulation based on intersubband transition in n-doped quantum wells

An all-optical modulation of interband-resonant light (near-infrared signal light: 800 nm) by intersubband-resonant light (mid-infrared control light: 4–7μm) in n-doped AlGaAs/GaAs multiple quantum wells is investigated by two-color femtosecond pump–probe experiments at room temperature. The modulation of the near-infrared signal light with an ultrafast recovery as short as ∼1 ps is successfully observed when the quantum wells are pumped by the mid-infrared control light pulse (∼4fJ/μm2). The dependence of the modulation depth on the wavelength of the control light is also measured, which is shown to be consistent with the intersubband absorption spectrum of the quantum wells. The results indicate that the utilization of the intersubband transition is promising for the ultrafast all-optical modulation and switching.

Ultrashort-pulse-controlled all-optical modulation by interband and intersubband transitions in doped quantum wells

The optimum condition for achieving highly efficient ultrafast all-optical pulse modulation by modification of the absorption of a pulsed interband light field by ultrasubband control-light pulses in a doped quantum well is investigated. The modulation efficiency in the femtosecond domain can be maximized by an intersubband control light with a pulse width (200 fs) that is comparable with the phase-relaxation time of the system and an intensity that is close to the intersubband saturation intensity.

Observation of ultrafast all-optical modulation based on intersubband transition in n-doped quantum wells by using free electron laser

A picosecond-range all-optical modulation based on the nonlinear interaction between interband- and intersubband-resonant light in GaAs/AlGaAs multiple quantum wells was demonstrated. A free electron laser (FEL) with a very short pulse width (&amp;lt;10 ps) was used for the intersubband excitation. The dependence of the modulation depth on the wavelength of the intersubband-resonant light was investigated by utilizing the wide tunability of the FEL. The result is in good agreement with the intersubband absorption spectrum of the multiple quantum wells measured by FT-IR spectroscopy, which indicates that the observed modulation is indeed based on the intersubband transition induced by the FEL.

Chiral media for ultrafast modulation of light by light

Ultrafast all-optical light modulation by an isotropic sample containing chiral molecules is discussed. The light modulation derives from a reduction of the sample transmission as well as the appearance of a light wave travelling in the direction opposite to the incident signal beam. The modulation effect is proportional to the nonlinear susceptibility of the second order and, therefore, depends only linearly on the field strength of the switching field. For a 0.1 M solution of ruthenium-tris-phenanthrolin and a sample length of 1 cm a reduction of the signal light transmission to 75% is evaluated for a switching intensity of 4.8x10 7 W/cm 2. By optimizing the molecular optical properties of the chiral medium a further improvement of the efficiency of the effect is conceivable.