Makoto Furuki

Terahertz demultiplexing by a single-shot time-to-space conversion using a film of squarylium dye J aggregates

We applied time-to-space conversion using femtosecond nonlinear-optical response of squarylium-dye (SQ) J-aggregates film. A pump pulse and a train of four probe pulses were illuminated on the same area (10 mm φ) of the film in direction of oblique and normal to the film plane, respectively. Due to the oblique illumination, the pump pulse met probe pulses (interval time: 1 ps) at separate places. The film picked out part of each probe pulse by its transmittance change, which was observed for a transmitted image of spatially separated four lines. Response time of the SQ J aggregates is enough for the single-shot 1 THz demultiplexing.

Observation of sub-100-fs optical response from spin-coated films of squarylium dye J aggregates

For spin-coated films of squarylium dye J aggregates, ultrafast nonlinear optical responses were investigated by pump–probe measurements. By using a broadband mode-locked titanium:sapphire laser, we succeeded in observing the optical response with a time resolution of better than 60 fs. Time-resolved transmission data are shown for different excitation wavelengths, resonant to the excitonic absorption band and off-resonant. Relaxation times of the absorption saturation were evaluated to be 140 fs (fast component) and 950 fs (slow component) in the case of resonant excitation and 98 fs in the case of off-resonant excitation.

Monomolecular layer of squarylium dye J aggregates exhibiting a femtosecond optical response of delocalized excitons

We report on the demonstration of the femtosecond nonlinear optical response from a two-dimensional monomolecular layer of squarylium dye J aggregate at 5 °C. The formation of a monomolecular layer Langmuir film was achieved by spreading squarylium dye modified by two propyl and two hexyl groups at the air–water interface, which resulted in a very strong J band (o.d.=0.3) at 777 nm. The transient absorption spectra in a resonant pump-probe measurement showed a low absorption saturation power (9.7×106 W/cm2) and an ultrafast response (300 fs), which are indicative of exciton delocalization over 18 molecules in this J aggregate, even at 5 °C.

Nonlinear optical properties of an organic film formed of dye aggregates with absorption in the telecommunication wavelength range

An organic film formed of newly synthesized dye aggregates with an absorption peak in telecommunication wavelength range exhibited large third-order nonlinear optical susceptibility χ(3) and fast recovery of bleached absorption. A derivative with a di(benzofuranonyl)methanolate structure forms stable aggregates in a spin-coated film and shows an absorption maximum at 1.24 μm. An imaginary part χI(3) of the film assumes a maximum at 1.30 μm, which is −0.83×10−7 esu. The time evolution of differential transmission spectra indicates that the time constant of the recovery of bleached absorption is 80–240 fs for a fast component and 2.2–6.5 ps for a slow one. This organic film has considerable advantages for future planar optical devices.

Ultrafast all-optical switching at 1.55μm using an organic multilayer device

We report ultrafast all-optical switching at optical communication wavelength using a device with a layered structure containing organic films. Spin-coated layers of di(benzofuranonyl)methanolate (BM) derivative are formed alternately with vacuum evaporated layers of germanium (II) oxide. An optical Kerr shutter is constructed using this BM multilayer with 1.55μm signal and 1.63μm gate pulses of 100fs time durations. As a result, optical switching with signal-to-noise ratio over 20dB is attained at gate-pulse intensity above 30pJ∕μm2 and a response time comparable to pulse width is observed. The BM multilayer could be an efficient optical communication device for parallel data processing.

Highly sensitive NO2 optical detector with squarylium dye Langmuir-Blodgett film containing J aggregate

Abstract A highly sensitive optical gas detector based on the fluorescence quenching of a squarylium dye Langmuir-Blodgett film is reported. A squarylium dye Langmuir-Blodgett film containing J aggregate deposited on a substrate emits fluorescence from the J aggregate by excitations of He-Ne laser (632.8 nm) and laser diode (750 nm). This fluorescence was observed to be quenched quickly and reversibly by the presence of NO2 in an air atmosphere. The change in fluorescence intensity can be easily detected by an NO2 level of only parts per billion. High sensitivity was discussed in relation to the high mobility of excited states in the J aggregate.

Improvement of on/off ratio in single-shot multichannel demultiplexing by using an optical Kerr gate of a squarylium dye J aggregate film

We demonstrated ultrafast single-shot multichannel demultiplexing by using a squarylium dye J aggregate film as an optical Kerr medium. High efficiency and fast recovery of the optical Kerr responses were achieved when a signal-pulse wavelength was close to the absorption peak of the J aggregate film with off-resonant excitation. The on/off ratio in demultiplexing of 1 Tb/s signals was improved to be approximately 5. By introducing time delay to both horizontal and vertical directions, we succeeded in directly observing the conversion of 1 Tb/s serial signals into two-dimensionally arranged parallel signals.

Multipiled monolayer assembly of J-aggregates of squarylium dye with short alkyl chains☆

The first observation of J-aggregates of squarylium dyes with short alkyl chains has been made in the monolayer. As the temperature of the aqueous subphase increases, two types of J-aggregates of squarylium dyes with short alkyl chains are formed on the surface and J-aggregates in the monolayer reversibly exchange with each other, depending on the surface pressures and temperatures. In a mixed monolayer with a fatty acid, either an ordinary structure or a multipiled structure is formed by controlling surface pressure. The monolayer assembly with a multipiled structure of dyes is formed under higher surface pressure through phase transition and successively transferred to solid substrates by the Langmuir-Blodgett technique. The multipiled structure of the mixed monolayer is characterized as a triple layer of J-aggregates of squarylium dyes stabilized with a fatty acid monolayer by π-A isotherms, spectroscopic measurements and X-ray diffraction.

Synthesis of new squaraine dyes for optical switches

This work describes the synthesis of new, highly soluble squaraine dyes, which have absorption maxima over 900 nm in solution. The absorption spectra of their spin-coated films show broad absorption bands, which are caused by aggregation phenomena. The absorption bands in the solid phase are red-shifted compared to those in solution. Because of these characteristics and nonlinear optical data which were obtained by Z-scan measurements, the new substances are principally useful for optical switches.

Hybrid gas detector of squarylium dye Langmuir-Blodgett film deposited on a quartz oscillator

A hybrid gas detector combined optical detector with a weighing system for absorbed gas in Langmuir-Blodgett films deposited on a quartz oscillator is reported. With this detector, the fluorescence intensity change from a squarylium dye LB film containing J-aggregates and the amount of adsorbed gas in the dye LB film are measured simultaneously. Responses of this detector for NO2, trichloroethylene, NH3 were evaluated. On exposure to NO2, the fluorescence was sharply quenched in correspondence with a decrement of oscillation frequency. The ratio of fluorescence quenching to the amount of adsorbed NO2 gas molecules was estimated with two different signals from this hybrid gas detector. On the other hand, a slight increment of fluorescence was observed with a decrement of oscillation frequency after introduction of NH3 gas. In the case of an organic solvent such as trichloroethylene, a decrement of oscillation frequency was observed without change of fluorescence intensity. trichloroethylene, a decrement of oscillation frequency was observed without change of fluorescence intensity.