Atsushi Sugita

High-dispersive mirrors for femtosecond lasers

We report on the development of highly dispersive mirrors for chirped-pulse oscillators (CPO) and amplifiers (CPA). In this proof-of-concept study, we demonstrate the usability of highly dispersive multilayer mirrors for high-energy femtosecond oscillators, namely for i) a chirped-pulse Ti:Sa oscillator and ii) an Yb:YAG disk oscillator. In both cases a group delay dispersion (GDD) of the order of 2x10(4) fs(2) was introduced, accompanied with an overall transmission loss as low as approximately 2 per cent. This unprecedented combination of high dispersion and low loss over a sizeable bandwidth with multilayer structures opens the prospects for femtosecond CPA systems equipped with a compact, alignment-insensitive all-mirror compressors providing compensation of GDD as well as higher-order dispersion.

Pump-seed synchronization for MHz repetition rate, high-power optical parametric chirped pulse amplification

We report on an active synchronization between two independent mode-locked lasers using a combined electronic-optical feedback. With this scheme, seed pulses at MHz repetition rate were amplified in a non-collinear optical parametric chirped pulse amplifier (OPCPA). The amplifier was seeded with stretched 1.5 nJ pulses from a femtosecond Ti:Sapphire oscillator, while pumped with the 1 ps, 2.9 µJ frequency-doubled output of an Yb:YAG thin-disk oscillator. The residual timing jitter between the two oscillators was suppressed to 120 fs (RMS), allowing for an efficient and broadband amplification at 11.5 MHz to a pulse energy of 700 nJ and an average power of 8 W. First compression experiment with 240 nJ amplified pulse energy resulted in a pulse duration of ~10 fs.

Second-Order Nonlinear Optical Susceptibilities of Nonelectrically Poled DR1–PMMA Guest–Host Polymers

Guest-host nonlinear optical polymers have attracted considerable interest due to their applications in fast electro-optical modulators and wavelength converters. In general, the electrical poling procedures, for which high DC external fields are applied, are necessary for aligning guest chromophores in polar order and activating the second-order nonlinearity. We present the nonelectrical poling behaviors for guest-host polymers: DR1 (4-[ethyl (2-hydroxyethyl) amino]-4-nitroazobenzene) is the guest, and PMMA (poly (methyl methacrylate)) is the host. Second-order nonlinear optical susceptibility was induced in the conventional guest-host polymers after annealing at temperatures above the glass transition points of the host polymer even without applying the external fields. This phenomenon did not occur in the side-chain polymers, where the guests were directly bonded to the host chains. The guest polar alignments were most likely generated from the guest hydroxyl groups chemisorbing on the substrates. The polar alignments of the guest formed not only near the surface of the substrate, but also inside the host polymers. The optimized conditions for the SHG conversion were examined in the context of the polymer film thickness and guest concentration. The nonelectrical poling techniques described in this study are useful for enhancing the surface nonlinearity in the several materials, and they will be useful for further developments in nanophotonics and plasmonics.

Second order nonlinear optical susceptibility of nonelectrically poled DR1-doped PMMA host-guest polymers

Nonlinear optical (NLO) polymers and their applications have been studied for long decades. The fundamental structures of the NLO polymers consist of the host-guest structures; the guest chromophores play a role of nonlinear mixing of multiple waves, while the host polymers fix the positions of the guest. The symmetry of the materials is closely associated with the nonlinear optical susceptibilities. The second order nonlinearity requires the materials to break the centro-symmetry. In general, the NLO polymers exhibit the centro-symmetry in as-prepared conditions, because the guest chromophores are randomly distributed in the host. So-called poling procedure, the procedure applying the DC electric fields, is conducted so as to align the chromophores in the polar order and break the symmetry. In our previous study, we successfully obtain the second order nonlinear susceptibility by using the NLO polymers with the amorphous ferroelectric polymers as host without the conventional poling procedure [1]. Taking advantage of the polarization self-organization behaviors specific to the amorphous ferroelectric polymers, the noncentro-symmetry of the guest chromophores was induced just by annealing the materials at the temperatures higher than the glass transition points (Tg) of the polymers. In the present study, we will report that the nonelectrical poling is available even for the host-guest polymers with poly (methyl methacrylate) (PMMA), one of the most popular host materials for the NLO polymers.

Second harmonic generation in non-electrically-poled NLO polymers excited by surface plasmon enhanced electric field

We will report second harmonics generations (SHG) in nonlinear optical (NLO) polymers excited by Surface Plasmon (SP) enhanced fields. The surface plasmon polariton was excited in an attenuated total reflection geometry having the Kretchmann configuration. Here, the NLO interactions occurred in the thin films of the NLO polymers, consisting of Disperse Red 1 as guest chromophores and PMMA as host, coated on Ag thin films. Our experimental results indicated that NLO polymers emitted strong SHG signals in the SP resonance conditions. The SHG intensities from NLO polymer coated Ag films were more than 10 times higher than those of the uncoated Ag films. The measurements were conducted for the polymer films with different thickness between 10 and 100 nm. All of the samples with different polymer thickness exhibited the SHG signals at its SP resonance condition that depends on the film thicknesses. The highest SHG conversion efficiency was recorded from the sample with 40 nm film thicknesses. The p-polarized pump beams gave the highest SHG conversion efficiency while the s-polarized ones gave almost no signals. On the other hand, the polarizations of the SH signals were highly oriented in the direction of the p-polarizations. The SP-enhanced field attracts a lot of interests as light sources for sub-diffraction-limit imaging. The SP can be excited only in the regions lower than surface plasma frequencies peculiar to the metals. The present frequency conversion technologies for the SP enhanced fields in the NLO polymers will extend the frequency regions available for the sub-diffraction-limit imaging.

PMMA copolymerized with polyacrylonitrile as novel host material for host-guest type second-order NLO polymers

We will present second-order NLO susceptibility of non-electrically poled NLO chromophore-doped PMMA copolymerized with polyacrylonitrile. The second-order nonlinear coefficient of 2 pm/V was obtained without conventional poling procedure.

Poly (cyanophenylene sulfide) as new host materials of second order nonlinear optical polymers

Here, we will propose poly (cyano phenylene sulfide) as novel host materials of nonlinear optical (NLO) polymers. Our experimental results proved that NLO chromophore-doped PCPS thin films deposited on the metal layers exhibited second order NLO susceptibilities just by annealing at the temperatures higher than a glass transition point even without conventional poling procedures. We determined the optimized annealing temperatures and the film thickness for the nonelectrical poling procedure. The nonelectrical poling procedure was applicable for the films as thick as a few μm. The second order nonlinear coefficient of was 0.5 pm/V for the PCPS doped with 10w% of 4-[Ethyl(2- hydroxyethyl)amino]-4-nitroazobenzene. Taking advantage of the unique polarization self-organization procedure, we may prepare one- or two-dimensionally periodically poled structures in these materials for the second-order NLO active photonic crystals pretty easily.

Acrylonitrile-containing methyl methacrylate polymers as new host materials for second-order NLO polymers

Polymer-based nonlinear optical (NLO) materials have been studied for long decades because of their unique properties such as light weightiness and easiness in fabrications. Conventional NLO polymers consist of guest chromophores with large second order hyperpolarizabilities in host amorphous polymers. These NLO polymers did not exhibit second order NLO activities in as-prepared conditions. So-called poling procedures, the procedures applying high electric field to the materials, were necessary to induce NLO activities. However, dielectric breakdown always occurred during the poling procedures and it was quite difficult to prepare the NLO polymer devices with large volumes and wide areas.

Second-order NLO of non-electrically-poled choromophore-doped amorphous ferroelectric polymers

We succeed in obtaining second-order NLO susceptibilities in host-guest NLO polymers with thickness as wide as a few ten micrometer with by non-electrical poling method, taking advantage of polarization self-organization properties of amorphous ferroelectric polymers.

Noise properties of mode-locked microjoule thin-disk oscillators

Oscillators generating tens up to hundreds of µJ femtosecond pulses at the MHz repetition rate [1] nowadays allow experiments such as direct gas ionization [2,3], where the level of intensity reaches and exceeds 1014 W/cm2. One can expect soon the pump-probe experiments with short electron pulses and direct high-harmonic generation in gases, parametric mixing of the femtosecond pulses from different oscillators, coherent enhancement of the femtosecond pulses in an empty cavity [4] etc. All these experiments will benefit from low pulse timing jitter.