Lyong Sun Pu

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.

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.

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.

Spin-coated Films of Squarylium Dye J-Aggregates Exhibiting Ultrafast Optical Responses

The formation of J-aggregates of squarylium dye derivatives in spin-coated films is reported. Squarylium dye derivatives with dipropylamino bases are found to spontaneously aggregate in a spin-coated film. Aggregation is promoted when dye molecules are dispersed in a poly(vinyl alcohol) film, and when a spin-coated film of dye molecules is heated in the presence of acid vapor. In particular, J-aggregates formed by exposure to acetic acid vapor show the narrowest spectral width. J-aggregates formed by the acid treatment method are stable at room temperature and the spectral full-width at half maximum of the J-band is 20 nm. Optical response of the acid-treated film is confirmed to exhibit a short relaxation time of bleached absorption of 300 fs.

Large χ(3) of squarylium dye J aggregates measured using the Z-scan technique

Third-order nonlinear optical coefficients χ(3) were measured for the J aggregates of two types of squarylium dye derivatives at resonant and near-resonant wavelengths by using the Z-scan technique. The maximum χ(3) value evaluated at one-photon resonance was 2.9×10−6 e.s.u., which was greater than that of phthalocyanines by 4 orders of magnitude. χ(3) for one squarylium derivative was approximately two times as large as that of the other. This can be attributed to the difference of the number of molecules contributing to a coherent state in each J aggregate.

Gas Detection by a Multi-Hybrid Sensor with Dye Langmuir-Blodgett Films Deposited on a Quartz Oscillator

Abstract A hybrid gas sensor combined an optical detector with a weighing system for adsorbed gas is realized by dye LB-films deposited on a quartz oscillator. With this sensor, the change of fluorescence intensity from dye LB-film is simultaneously measured with the change of oscillation frequency of the quartz oscillator which depends on the amount of adsorbed gas in the LB-film. Squarylium dye LB-film and perylene dye LB-film were applied for this hybrid gas sensor. The quartz oscillators with LB-film commonly showed decrement of their oscillation frequency corresponding to the adsorption of gases. On the other hands, changes of fluorescence intensity depend on the type of gases, the dye molecules and its aggregation forms. These results suggest advantage of a multi-hybrid gas sensor using multiple quartz oscillators with different dye molecules. In this report, we discuss the responses of hybrid gas sensor in connection with the semiconductivity of the dye and its aggregate in LB-films.