Izumi Iwasa

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.

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.

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.

Cadmium telluride bulk crystal as an ultrafast nonlinear optical switch

A semiconductor bulk crystal has several advantages as a nonlinear optical material, such as low processing cost, long interaction length, and alleviation of the free-carrier absorption caused by two-photon absorption (TPA). We examine optical properties of semiconductor bulk crystals with different orientations at 1.55μm and find that CdTe[111] has favorable capabilities, such as nonlinear refractive index of 5.23×10−17(m2W), TPA coefficient of 18.3(mmGW), and consequent figure of merit of 0.54. Optical three-dimensional measurements of reflective material are carried out using CdTe[111] as ultrafast optical Kerr shutter and clear images corresponding to surface steps are obtained.

Terabit all-optical logic based on ultrafast two-dimensional transmission gating

Terabit all-optical complementary logic is proposed using two successive time slots to represent a unique logical status. An organic molecular thin film is used as an array of optically controlled optical switches. By utilizing the planar structure of the film and its ultrafast optical response, proof-of-principle fully optical NOT and AND logic operations were demonstrated with 400-fs interval pulses.

Femtosecond timing measurement and control using ultrafast organic thin films

We show a femtosecond timing measurement and control technique using a squarylium dye J-aggregate film, which is an organic thin film that acts as an ultrafast two-dimensional optical switch. Optical pulse timing is directly mapped to space-domain position on the film, and the large area and ultrafast response offer a femtosecond-resolved, large dynamic range, real-time, multichannel timing measurement capability. A timing fluctuation (jitter, wander, and skew) reduction architecture is presented and experimentally demonstrated.

Microstructure of squarylium dye J aggregate films examined on the basis of optical behavior at low temperature

The microstructure of a spin-coated film of squarylium dye J aggregates is examined on the basis of the measurement of the optical properties and the third-order nonlinear optical susceptibility χ(3) at low temperature. The absorption maximum of J aggregates shifted to lower energies as the film temperature decreased, while χ(3) was independent of the temperature. The latter finding indicates that the coherent length of J aggregates is confined by a structural boundary rather than by phonons; consequently, the observed peak energy shift can be due to temperature-dependent conformational change of the aggregates. The small aggregation size may contribute to the ultrahigh-speed optical response of squarylium dye J aggregates.

Femtosecond two-dimensional serial-to-parallel pulse converter using a squarylium dye J-aggregate film

Based on a time-to-space conversion technique, a two-dimensional serial-to-parallel pulse converter using a squarylium dye J-aggregate film is proposed. The squarylium dye J-aggregate film, which exhibits an ultrafast recovery of bleached absorption, operates as a large-area femtosecond all-optical shutter. The system is composed of a few simple optical components and enables conversion of temporally serial signals into multiple parallel outputs in one- or two-dimensional space. The serial-to-parallel conversion of 1 Tb/s optical pulses was demonstrated and different 4-bit signal patterns were successfully discriminated in one dimension. Also, 2 × 2 outputs were clearly observed as two-dimensional outputs using this converter.

GaAs (001) surface balanced with arsine partial pressure in metalorganic chemical vapor deposition reactor observed by surface photoabsorption

Abstract We measured surface photoabsorption signals from As adsorbed surfaces of GaAs (001) substrates in a reactor of metalorganic chemical vapor deposition (MOCVD) under various partial pressures of AsH3. Desorption of As atoms from the surface was balanced with adsorption of As atoms under AsH3 partial pressure. We found that the surface conditions varied as a function of the partial pressure and the temperature of the substrate. Comparing the experimental results with a multi-layer Langmuir adsorption model, we determined the phase boundaries of the surface structures under MOCVD conditions at temperatures between 500 and 700°C. One of the boundaries was assigned to the boundary between (2 × 4) and c(4 × 4), and the other to that between c(4 × 4) and a surface of excess As.