paper singleshotdemultiplexingof1thzlightpulsesby time-to

974IEICE TRANS. ELECTRON., VOL.E83–C, NO.6 JUNE 2000

PAPER Special Issue on Advanced Optical Devices for Next Generation Photonic Networks

Single Shot Demultiplexing of 1THz Light Pulses by

Time-to-Space Conversion Using a Film of Organic

Dye J-Aggregates

Makoto FURUKI†a), Satoshi TATSUURA††, Nonmembers, Osamu WADA††, Regular Member,Minquan TIAN†, Yasuhiro SATO†, and Lyong Sun PU†, Nonmembers

SUMMARY Principle of a single shot demultiplextion bymeans of time-to-space conversion was investigated using fem-tosecond nonlinear optical response of absorption bleachingof squarylium dye (SQ) J-aggregates. Spincoated films ofsquarylium dye J-aggregates on glass substrates exhibit efficientand ultrafast transmittance change, which recovers 73% of itsinitial level (0 fs) within 1 ps. A simple method for time-to-spaceconversion was applied for this film. We took our attention toone of the characteristics of femtosecond pulse, which is the spa-tial thinness in its propagation direction. Femtosecond pulsesof a single pump pulse and train of four probe pulses were il-luminated to the same area (diameter of 10mm) of the surfaceof the SQ J-aggregates film. Direction of the probe beam wasnormal to the surface of the film and that of the pump beamwas oblique angle in horizontal plane. Caused by spatial delayof a pump pulse due to the illumination in oblique angle to thefilm, four probe pulses with interval time of 1 ps (1THz) meetseparate places on the film. Because of the fast response of theSQ J-aggregates, the film picked out part of each probe pulse,which has narrower shapes in horizontal direction compared tothe initial circular one by transmittance change of the film. Thespatially separated four lines were observed by a CCD camerafor an image of the transmitted probe pulse train. These resultssuggest that the response time of SQ J-aggregate film, whichdetermines the horizontal width of each line, to be enough fordemultiplexing of 1THz optical signals.key words: single shot demultiplexing, squarylium dye, J-

aggregate, time-to-space conversion, absorption bleaching

1. Introduction

Optical switching devices with driving speeds of fem-tosecond time-scale are strongly desired for future Tbpsinformation networks. Some attractive devices for Tbpsdemultiplexing using inorganic semiconductors havebeen introduced [1], [2], although there are no proposalof devices picking-out multi-signals by a single shot de-multiplexing operation which should be important ininterconnection between a Tbps network and Gbps net-works.

For single shot multiplexing, the ideas of time-to-

Manuscript received October 15, 1999.Manuscript revised December 27, 1999.

†The authors are with Advanced Research Lab., Corpo-rate Research Center, Fuji Xerox Co., Ltd., Kanagawa-ken,259-0157 Japan.

††The authors are with The Femtosecond Technology Re-search Association, Tsukuba-shi, 300-2635 Japan.a) E-mail: [email protected]

space conversion must be one of the powerful solutions[3]–[6]. Numbers of interesting methods have been in-troduced in this field of time-to-space conversion, whichenables pulse shaping [3], making pulse trains [4], [5]and also spatial imaging of pulse trains [6]. However,these three methods use correlation of the femtosecondpulse between time-domain and spectrum-domain, andthe fundamental femtosecond pulse is required to betransform-limited. This requirement should reduce theability for practical applications especially in the futuresystems using OTDM with WDM.

One of the most important characteristics of fem-tosecond light pulses is the spatial thinness in theirpropagation direction between the wave front and theend. For a 100 fs pulse, the thinness is 30 µm. If we il-luminate some plane by a femtosecond laser beam witha large diameter in an oblique angle, there is a spa-tial delay of illumination (reaching and passing) tim-ing of a femtosecond pulse. We also can measure thedelay of illumination timing with another femtosecondpulse like the fs pump-probe method. This principleis also applied in measurement of a pulse width in thetime domain using SHG crystals called single-shot auto-correlation [7] and also in a visualization of femtosec-ond time-resolved nonlinear polarigraphy with a two-dimensional detector [8]. On this point of view, singleshot demultiplexing using time-to-space conversion canbe available, if we have a film with large area, which canswitch transmittance or reflectance of the light pulsesin femtosecond time-scales.

Figure 1 shows the schematic viewgraph for a newsingle shot demultiplexing method with an ultrafasttwo-dimensional optical switch. The train of signalpulses is illuminated normally to the two-dimensionaloptical switching film in a beam diameter of severalmm. Single read-out pulse with a diameter of also sev-eral mm is illuminated to the same area of the film inan oblique angle. Optical switching of transmittancechange starts and finishes from the near side to thefar side of the film within time duration of several ps,which is required for passing the read-out pulse throughthe film. This time duration can be tuned by changingthe oblique angle and the diameter of the illuminatedarea. Because of this spatial delay of transmittance

FURUKI et al.: SINGLE SHOT DEMULTIPLEXING USING DYE J-AGGREGATES975

Fig. 1 Schematic viewgraph showing principle of single shotdemultiplexing by means of time-to-space conversion.

change, each pulse in the signal pulse train can trans-mit in spatially separated narrow area, nearer for earlierone. So, each signal pulse can be separated spatially ifthe switching-off time of the film is faster than the in-terval times in the train of signal pulses.

For the materials of this optical switch, not onlyfast recovery of switching actions but also the abilityin the formation of a large film such as several tensmm2 are strongly required. We have studied on for-mation of films with squarylium dye J-aggregates andobserved highly efficient and ultrafast nonlinear-opticalresponse of this film at room temperature [9]–[12]. Spe-cial features of these films are the fast recovery of ab-sorption saturation in 200–300 fs, with low energy ofpumping pulses such as several tens fJ/µm2 requiredfor absorption saturation. A quite large absorptioncoefficient more than 106/cm at resonant wavelength[10], [11] is also important in constructing a thin filmdevice. The group velocity dispersion of the femtosec-ond light pulses passing through the film is expectedto be ignored because of this large absorption coeffi-cient which related with the thinness (in the order ofhundreds nm) of the optical pass in a device. The filmof this J-aggregates is expected to be one of the mostpromising materials for this application of the singleshot demultiplexing. In this paper, first observation oftime-to-space conversion of THz pulse train using fem-tosecond transmittance change of an organic thin filmsis investigated.

2. Experimental

For the two-dimensional optical switching material,squarylium dye (SQ) with a chemical formula as shownin Fig. 2 is used. Films of SQ J-aggregates were formedon glass substrates by spincoating method from a so-lution of 1wt% SQ in 1,2-dichloloethane [12]. Fem-tosecond optical responses of this SQ J-aggregateswere characterized by pump-probe method using a1 kHz Ti:sapphier regenerative amplifier system (Spec-

Fig. 2 Chemical formula of squarylium dye substituted withfour butyl-groups (SQ).

Fig. 3 Experimental setup for imaging of single shotdemultiplexing.

tra Physics, Spitfire) with an optical parametric am-plifier (Spectra Physics, OPA-800). In the pump-probe measurement, SHG (776 nm) of OPA signal beam(1.55 µm) was used for the pump pulses and femtosec-ond pulse of white-light continuum made by focusingOPA idler beam to a sapphire plate (thickness of 2 mm)was used for the probe pulses [9]. The cross-correlationwidth of the pump and the probe pulses evaluated bysum frequency generation using a BBO crystal wasabout 200 fs in the measured wavelength range. Tran-sient spectra of transmitted probe beam were measuredby an array of photodiode coupled with a grating.

In the evaluation of switching ratio and the confir-mation of the principle of single shot demultiplexing, fspump-probe method with two wavelengths monochro-matic beams was applied. SHG of OPA signal beam atwavelength of 776 nm and regenerative amplified beamat wavelength of 795 nm were used for the signal (probe)pulses and the read out (pump) pulses respectively. Op-tical setup as shown in Fig. 3 was used for the singleshot demultiplexing. The trains of four probe pulseswith repetition rate of 1 THz (interval time of 1 ps)were made by using beam splitters and retro-reflectors[2]. Differences in lengths between each four availablepass of light were aligned to be 300 µm so that the in-terval time of illuminations on the film by each probepulse to be 1 ps. The probe beam illuminates the op-tical switching film in normal direction which meanseach probe pulse arrive to the illumination area at thesame time. On the other hand, the oblique angle ofthe pump illumination was set to be 17.5 deg. whichresulting the time delay for 1 ps corresponds to spatialdistance of 1 mm at the film plane in horizontal direc-tion. Diameter of illumination area by both the probeand the pump beams were 10 mm. Beam pattern of


Fig. 4 Absorption spectra of SQ dye J-aggregates film(solid line) with that of its solution (broken line).

picked out (transmitted) probe pulse trains was takenby a CCD camera facing normal to the probe beamwithout using any expanding or reducing optics. NDfilters were used to inhibit saturation of the detectors.For observation of the demultiplexing image, differen-tial light intensity calculated by subtracting an imageby illumination of probe only from that of probe withpump was used. Exposure time for each image was 1 s(corresponding to 1,000 pulses).

3. Results and Discussion

3.1 Formation and Femtosecond Optical Response ofSQ J-Aggregates Films

Absorption spectrum of an SQ J-aggregates film ona glass substrate is shown in Fig. 4 with absorptionspectrum of the SQ solution in an organic solvent.For the spincoated film, peak of absorption band (J-band) red-shifted from its solution (at 640 nm) was ob-served at 776 nm. Typical thickness of the spincoatedfilms measured by a stylus instrument was approxi-mately 100 nm. Transient absorption change spectrawere measured by the fs pump-probe method with thepump beam of resonant excitation on this J-band (cen-ter wavelength of 776 nm) and with the probe beamof white light continuum. The pump beam was illu-minated to an area of 0.6 mm2 with pulse energy of200 fJ/µm2. In the absorption change spectra as shownin Fig. 5, clear bleaching of absorption appeared uponpump illumination (0 ps) and it soon recovered in pstime scale. The valley of absorption bleaching corre-sponds to the peak of J-band. Temporal absorbancechanges at this valley wavelength of 776 nm on the de-lay time of the probe pulse illumination with respectto that of the pump pulse are plotted in Fig. 6. Twoexponential-recovery time constant (half-recovery time)

Fig. 5 Transient absorption change spectra of a SQ J-aggregates film observed by pump-probe (white light continuum)measurement. The spectra have been shifted vertically.

Fig. 6 Temporal change of absorbance at 776 nm on the delayof probe illumination compared to that of pump pulses. Thesolid line is the fitted result of a double-exponential curve withhalf decay time constants of 250 fs and 5 ps.

of 250 fs and 5 ps with intensity ratios of 80% and 20%,respectively, fit the recovery curve of the absorbancechange. Because these two time constants are similarto those observed for J-aggregates in a Langmuir film [9]and a Langmuir-Blodgett film [10]. Relaxation processoccurring on these time scale can be attributed to thesame process of coherent response in imaginary part ofthe complex refractive index (fast) and population de-cay of excited states by spontaneous emission (slow).These results suggest that films of the SQ J-aggregateshave ability for optical switching in the order of THzrepetition rate.


Fig. 7 Temporal change of transmittance at the center wave-length of monochromatic fs probe pulse. Transmittance changeobserved at 0 fs was relaxed in 73% after 1 ps.

3.2 Evaluation of Switching Ratio Using Monochro-matic fs Probe Pulses

In the transient absorption spectra, fast recovery ofabsorption saturation was observed at resonant wave-length in probe pulses of white light continuum. Toevaluate the optical switching performance of this SQJ-aggregates film, transmittance change of monochro-matic fs signal (probe) pulses should be measured, be-cause we do not use white light continuum and alsomonochrometer in optical information networks. Onlyintensity change of the signal pulses should be impor-tant. To inhibit optical noises such as interference ofpump and probe pulses, each wavelength is requiredto be different. SHG of OPA signal beam with centerwavelength of 776 nm and FWHM of 11 nm was used forthe probe pulses and fundamental beam of Ti:sapphierregenerative amplifier picked out from OPA (centerwavelength of 795 nm, FWHM of 10 nm) was used forthe pump pulses in this monochromatic fs pump-probemeasurement. Energy of the single pump pulse was800 fJ/µm2 which was 4 times larger than previous ex-periment, because the wavelength of this pump beamwas slightly off-resonant (at 795 nm) for the J-band,where excitation efficiency must be lower than that byresonant one. Figure 7 shows the result of responsetime in the transmittance change of the film at the peakwavelength of the probe pulses. Ratio of transmittancechange reached to 37% of its static (linear) transmit-tance. This transmittance change recovers 73% of thepeak value after 1 ps from the pump illumination. Thisratio of the transmittance change is almost the same asthat observed in the previous section using white lightcontinuum for the probe pulses.

On the other hand, we used the optical setupshown in Fig. 3 which using a CCD camera withoutany monochrometer to detect demultiplexing of the

Fig. 8 Transmitted light spectra in the direction of probe beamon illumination of probe + pump (solid line), probe only (brokenline), pump only (dotted line). Numbers written in the figure aretheintegrated light intensity as a function of photon energy.

probe pulse trains; therefore, the ratio of transmittancechange in full wavelength range (sensitive for the Si de-tectors) must be more important. Integrated intensi-ties of transmitted light as a function of photon energywere calculated on the case of probe-only, probe withpump and pump-only illuminations as shown in Fig. 8.The calculated value of transmitted light intensity forprobe with pump is almost 30% larger than that forthe probe-only. This ratio of photon-energy integratedtransmittance change is a little smaller than the dif-ference at the center wavelength (Fig. 7) because thetransmittance change is most distinguished at the peakof the J-band. However, this ratio of difference (30%)is almost enough for detection by a CCD camera. Asthe value of integrated light intensity for the pump-onlyis 33 times smaller than that of the probe with pump,optical noise due to scattered pump beam at the filmof SQ J-aggregates must be negligible.

3.3 Imaging of Time-to-Space Converted Probe PulseTrain

Large intensity change of transmitted probe beam upto 30% by the pump illumination was confirmed bythe monochromatic fs pump-probe measurement with-out using any spectral separations, which must be anenough value of S/N ratio for detection by a CCD cam-era. For the purpose of using spatial delay of several pson illumination of single pump pulse, diameter of boththe pump and the probe beam were expanded and col-limated to be 10 mm. Pump pulses of the same energydensity with the previous section was used in this exper-iment. As shown Fig. 3, the train of four probe-pulsesilluminates the surface plane of the SQ J-aggregate filmand the CCD camera through the film normally. Onthe other hand, the pump beam illuminates the same


Fig. 9 (a) Image of picked out four probe pulses by single shotdemultiplexing. The image was calculated by subtracting beamprofile of probe beam from that of probe with pump illumination.Exposure time of each image was 1 s (1000 pulses). (b) Typicalprofile of light intensity on the above image, written in the samescale.

area of the film in horizontally oblique angle. The an-gle of 17.5 deg. was chosen because travel length ofthe pump beam in the plane of the film surface to be1 mm in 1 ps. So the picked out four pulses of probebeam with time interval with 1 ps should make imageof vertical four lines aligning with interval distance of1 mm on the CCD camera. The demultiplexing im-age of probe beam taken by the CCD camera is shownin Fig. 9(a). For the purpose of clarifying the pickedout pulses, this demultiplexing image was calculated bysubtracting an image under illumination of only-probebeam from that under probe-with-pump beam of thesame exposure time. This image clearly shows thateach femtosecond probe pulse is picked out at differentand separated rectangular area in the SQ J-aggregatesfilm as a pump pulse passes at the speed of 1 mm/pson the surface plane. Typical cross-section of the im-age is shown in Fig. 9(b) for light intensity as a functionof distance in horizontal direction of the CCD camera.Although tailing of the last picked out pulse at mostleft beam was observed in Fig. 9(b), the response speedof J-aggregates is ready to the repetition rate of 1 THz,because the heights of picked out four pulse intensityare almost similar. High frequency noises observed foreven narrower vertical lines in Fig. 9(a) and vibrationsin (b) may be caused by low quality in field patternof the probe beam containing diffraction due to someoptical instruments. A little fluctuation of distance be-tween lines of picked out pulses in Fig. 9(a) should becaused by slight difference of incident angle among thefour light passes of the probe beam. More adjustments

of optical setup and the use of optical parts with largerareas can improve the demultiplexing image. The SQ J-aggregate film is stable enough for these demultiplexingexperiments, because there were no clear decrease of itsoptical density after these experiments. Also more de-tailed of stability characterization should be necessaryfor practical applications.

4. Conclusion

Films of SQ J-aggregates were confirmed to exhibithigh efficiency and fast response of absorption bleach-ing. Ultrafast optical switching with transmittance in-crease up to 30% of the level before pump illuminationcan be achieved with the pump energy of several hun-dreds fJ/µm2 for spincoated film of SQ J-aggregates.By a preliminal experiment using wide beam of probepulse trains and pump pulses, demultiplextion of time-to-space conversion was visualized for vertical lines ofpicked out probe pulses. These results suggest possibil-ity of a single shot demultiplexing at 1 THz using filmsof SQ J-aggregates. Much enhancement of signal tonoise ratio is still required for true single shot opera-tions, which will be available by improvement of opti-cal setup and application of some reflection structuresto the film [2]. However, importance of this work lieson highlighting the performance of the large-area or-ganic dye film which is attractive for simple single shotspace-to-time conversion between ps time domain andmm spatial domain. In this time, demultiplexing in onedimension was confirmed using the oblique illuminationof readout pulses and the extension to two dimensionaldemultiplexing can be easily achieved by applying an-other delay to perpendicular direction where the num-ber of separated pulses in a single shot must be muchlarger.

Acknowledgement

This work was supported by the New Energy and In-dustrial Technology Development Organization withinthe framework of Femtosecond Technology ResearchProject. We thank Dr. Toshiro Tani of Tokyo Uni-versity of Agriculture and Technology and Shyun-suke Kobayashi, Fumio Sasaki, Hitoshi Kawashimaand Tsuyoshi Kato of Electrotechnical Laboratory andmembers of the FESTA Laboratories and Yuk Lung(Elton) Chan from University of Toronto for stimulat-ing and fruitful discussions.

References

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[3] C. Froehly, B. Colombeau, and M. Vampouille, “Shapingand analysis of picosecond light pulses,” in Progress in Op-tics, ed. E. Wolf, vol.XX, pp.65–153, North Holland, Ams-terdam, 1983.

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[5] M.C. Nuss, M. Li, T.H. Chiu, A.M. Weiner, and A. Par-tovi, “Time-to-space mapping of femtosecond pulses,” Opt.Lett., vol.19, no.9, pp.664–666, May 1994.

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[7] F. Salin, P. Georeges, G. Roger, and A. Brun, “Single-shot measurment of 52-fs pulse,” Appl. Opt., vol.26, no.21,pp.4528–4531, Nov. 1987.

[8] M. Fujimoto, S. Aoshima, M. Hosoda, and Y. Tsuchiya,“Femtosecond time-resolved optical polarigraphy: Imagingof the propagation dynemics if intense light in a medium,”Opt. Lett., vol.24, no.12, pp.850–852, June 1999.

[9] M. Furuki, L.S. Pu, F. Sasaki, S. Kobayashi, and T. Tani,“Monomolecular layer of squarylium dye J-aggregates ex-hibiting a femtosecond optical response of delocalized exci-tons,” Appl. Phys. Lett., vol.72, no.21, pp.2648–2650, May1998.

[10] M. Furuki, O. Wada, L.S. Pu, Y. Sato, H. Kawashima,and T. Tani, “Fabrication and femtosecond optical re-sponse of Langmuir-Blodgett filmswith two dimensional J-aggregates,” J. Phys. Chem. B, vol.103, no.36, pp.7607–7612, June 1999.

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Makoto Furuki was born in To-yama on October 1, 1960. He receivedhis B.Eng., and M.Eng. degrees fromTsukuba University, Japan, in 1981, and1985 respectively. He joined Fundamen-tal Research Laboratory, Fuji Xerox Co.,Ltd. Since 1985 he has been engaged indevelopment of highly organized molec-ular films and the applications of filmsfor sensors and optical switching devices.During April 1997 to June 1999, he was

a member of FESTA Laboratory of the Femtosecond ResearchAssociation. Currently he is a principal researcher of CorporateResearch Center, Fuji Xerox Co., Ltd.

Satoshi Tatsuura received his B.Sci.degree from Kyoto University, Japan andjoined Fujitsu Laboratories Ltd. in 1987.He was involved in the research on electro-optic polymer materials for optical inter-connects from 1990 to 1993. From 1993to 1994, he was an associate researcher ofUniv. of California, Santa Barbara for theresearch of organic light emitting diodes(OLED). He moved to Fundamental Re-search Laboratory, Fuji Xerox Co., Ltd.

in 1995 and joined the Femtosecond Technology Research Asso-ciation (FESTA), Tsukuba in 1999. He is now engaged in thedevelopment of ultrafast optical switching devices.

Osamu Wada received his B.Eng.degree from Himeji Institute of Technol-ogy, Himeji, and M.Eng. from Kobe Uni-versity, Kobe and Ph.D. from Universityof Sheffield, Sheffield, UK, in 1969, 1971and 1980. He joined Fujitsu Laborato-ries Ltd. in 1971. During 1976–1978, hewas a SRC Independent Research Workerfor the research on InP at the Universityof Sheffield. He has been involved in re-searches on GaAs- and InP-based semi-

conductor materials and devices including LEDs, LDs, PIN-PDs,APDs, OEICs and ultrafast all-optical devices. From 1996, hehas become a Group Leader at the Femtosecond Technology Re-search Association (FESTA), Tsukuba. He is a Fellow of IEEEand of OSA.

Minquan Tian was born in Fu-jian Province, P.R. China on February17, 1969. He received his B.S. degree inapplied chemistry from University of Sci-ence and Technology of China, Hefei, P.R.China in 1990, and his M.S. degree in or-ganic chemistry from Shanghai Instituteof Organic Chemistry, Chinese Academyof Sciences, Shanghai, P.R. China in 1992.He then received the Ph.D. degree in or-ganic material engineering in Tokyo In-

stitute of Technology, Tokyo, Japan in 1998. During January toJuly 1993, He worked as an assistant researcher in Departmentof Advanced Materials, Shanghai Institute of Organic Chemistry,Chinese Academy of Sciences, Shanghai, P.R. China. Then hejoined Laboratory for Nano-Photonics Materials, Frontier Re-search Program, The Institute of Physical and Chemical Re-search (RIKEN), Saitama, Japan in August 1993. As a frontierresearcher there, he was engaged in research on syntheses andnonlinear optical properties of novel symmetrical and unsymmet-rical phthalocyanine derivatives during August 1993 to February1998. In February 1998, he joined Corporate Research Center,Fuji Xerox Co., Ltd., and since then he has been engaged in thedevelopment of advanced information-processing technology. Atpresent, he is a member of the Chemical Society of Japan.


Yasuhiro Sato was born in Sap-poro on December 8, 1966. He receivedhis B.Eng., M.Eng., and Ph.D. degreesat Faculty of Engineering from HokkaidoUniversity, Sapporo, Japan, in 1992, 1994and 1998 respectively. He joined the Cor-porate Research Center, Fuji Xerox Co.,Ltd., Japan in 1997, where he has beenengaged in the development of the ultra-fast optical switching devices.

Lyong Sun Pu was born in 1941 inJapan. He has received Dr. of Eng. in1969 from Tokyo Institute of Technology.After post-doctoral research activities atLiege University in Belgium and ReadingUniversity in England, he joined Fuji Xe-rox Co., Ltd., and has been engaged inresearch and development of organic pho-toconductor, 2nd nonlinear optical ma-terials and ultrafast optical properties ofsquarylium dye J-aggregates.

paper singleshotdemultiplexingof1thzlightpulsesby time-to

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