In chapter 2, I introduce our NOPA system for pump probe experiment in our lab. A regenerative chirped pulse amplifier (Legend-USP-HE; Coherent) seeded with a Ti:sapphire laser oscillator (Micra 10; Coherent) is used to pump and seed the NOPA. The amplifier
generates 40-fs pulses with a central wavelength of 800 nm, a repetition rate of 5 kHz, and an average power of 2.5 W. The beam from the regenerative amplifier is split into two beams by a beam sampler. One beam of 800 mW is used to generate a second harmonic of 400 nm to pump the NOPA. The other beam of 2.5 mW is focused on a sapphire plate to induce self-phase modulation to generate white light, which is used as the seed beam of the NOPA.
The NOPA generates a broad visible spectrum extending from 520 to 700 nm with a nearly constant phase. A beam splitter splits the visible laser pulse into pump and probe beams. The intensities of these beams are adjusted using a variable neutral density filter, and the ratio of the pump to the probe intensities is set to be about four for the weakest excitation. The duration of the broadband visible laser pulse is compressed to 6.5 fs using a pulse compression system, which consists of a diffraction-grating telescopic dispersion line, specially designed multilayer dielectric chirped mirrors, and a computer-controlled flexible mirror. The probe pulse is dispersed by a polychromator (SP2300i; Princeton Instruments) into a 128-branch fiber bundle, whose other end is separated into 128 fiber branches and connected to avalanche photodiodes (APDs). Therefore, the time-resolved transmittance differences ΔT at 128 probe wavelengths are simultaneously detected at the APDs. The signals detected at the APDs are sent to a multichannel lock-in amplifier developed by our group to obtain a signal with a high signal-to-noise ratio.
In chapter 3, a fast-scan method is introduced and the comparison with traditional step-scan method is also demonstrated in this part. We develop a fast-scan pump-probe spectroscopic system that can complete a set of measurements in less than 2 minutes.
Quantitative estimates of the signal reproducibility demonstrate that these fast-scan measurements provide much higher reproducibility and reliability than conventional measurements, and are thus suitable for applications involving chemical, physical, and biological materials.
In the chapter 4, a spectroscopic application of the sub-10-fs NOPA to pump-probe experiment of a poly(3-hexylthiophene) thin film is presented. The fast geometrical relaxation (GR) time was attributed to the transition from a free exciton (FE) to form a bound polaron pair (BPP), which is equivalent to a self-trapped exciton, and the time constant of the transition was estimated to be τGR = 90±2 fs. The relaxation time constant of BPP was determined as τBPP = 710±40 fs. The relaxation process corresponds to faster two channels among three parallel decay channels of “trapping by defects”, “direct recombination to the ground state”, and “dissociation into polarons”. Trapping process was distinguished from other two processes observing pump intensity dependence of the signal, which estimated the trapped BPPs and defect concentration in the P3HT thin film. Finally the chapter 5 summarizes this thesis.
References
[1] Ursula Keller, “Recent developments in compact ultrafast lasers”, Nature 424, 831, 2003.
[2] Spielmann Ch., Curley P. F., Brabec T., Krausz F., “Ultrabroadband femtosecond lasers”, IEEE J. Quantum Electron. 30, 1100, 1994.
[3] Spence D. E., Kean P. N., Sibbett W., “60 fsec pulse generation from a self-mode-locked Ti:sapphire laser”, Opt. Lett. 16, 42, 1991.
[4] Rolland C., Corkum P. B., “Compression of high-power optical pulses”, J. Opt. Soc. Am.
B, 5, 641, 1988.
[5] Dharmadhikari A. K., Rajgara F. A., Mathur D., “Systematic study of highly efficient white light generation in transparent materials using intense femtosecond laser pulses”, Appl.
Phys. B 80, 61, 2005.
[6] Baltuška A., Wei Z., Pshenichnikov M., Wiersma D. A., “Optical pulse compression to 5 fs at a 1-MHz repetition rate”, Opt. Lett. 22, 102, 1997.
[7] Nisoli M., De Silvestri S., Svelto O., “Generation of high energy 10 fs pulses by a new pulse compression technique”, Appl. Phys. Lett. 68, 2793, 1996.
[8] Nisoli M., Stagira S., De Silvestri S., Svelto O., Sartania S., Cheng Z., Lenzner M., Spielmann Ch., Krausz F., “A novel high-energy pulse compression system: generation of multigigawatt sub-5-fs pulses”, Appl. Phys. B 65, 189, 1997.
[9] Hauri C. P., Kornelis W., Helbing F. W., Heinrich A., Couairon A., Mysyrowicz A., Biegert J., Keller U., “Generation of intense, carrier-envelope phase-locked few-cycle laser pulses through filamentation”, Appl. Phys. B 79, 673, 2004.
[10] Trushin S. A., Panja S., Kosma K., Schmid W. E., Fuss W., “Supercontinuum extending from >1000 to 250 nm, generated by focusing ten-fs laser pulses at 805 nm into Ar”, Appl.
Phys. B 80, 399, 2005.
[11] Wilhelm T., Piel J., Riedle E., “Sub-20-fs pulses tunable across the visible from a blue-pumped single-pass noncollinear parametric converter”, Opt. Lett. 22, 1494, 1997
[12] Cerullo G., Nisoli M., De Silvestri S., “Generation of 11 fs pulses tunable across the visible by optical parametric amplification”, Appl. Phys. Lett. 71, 3616, 1997.
[13] Baltuška A., Fuji T., Kobayashi T., “Self-referencing of the carrier-envelope slip in a 6-fs visible parametric amplifier”, Opt. Lett. 27, 306, 2002.
[14] Wilhelm T., Piel J., Riedle E. “Sub-20-fs pulses tunable across the visible from a blue-pumped single-pass noncollinear parametric converter”, Opt. Lett. 22, 1494, 1997.
[15] Cerullo G., Nisoli M., De Silvestri S. “Generation of 11 fs pulses tunable across the visible by optical parametric amplification”, Appl. Phys. Lett. 71, 3616, 1997
[16] Shirakawa A., Sakane I., Kobayashi T. “Pulse-front-matched optical parametric amplification for sub-10-fs pulse generation tunable in the visible and near infrared”, Opt.
Lett. 23, 1292, 1998
[17] Cerullo G., Nisoli M., Stagira S., De Silvestri S., Tempea G., Krausz F., Ferencz K.
“Mirror-dispersion-controlled sub-10-fs optical parametric amplifier in the visible”, Opt. Lett.
24, 1529, 1999.
[18] Shirakawa A., Sakane I., Takasaka M., Kobayashi T., “Sub-5-fs visible pulse generation by pulse-front-matched noncollinear optical parametric amplification”, Appl. Phys. Lett. 74, 2268, 1999.
[19] Kobayashi T., Shirakawa A., “Tunable visible and near-infrared pulse generator in a 5 fs regime”, Appl. Phys. B 70, S239, 2000.
[20] Baltuska A., Kobayashi T., “Adaptive shaping of two-cycle visible pulses using a flexible mirror”, Appl. Phys. B 75, 427, 2002.
[21] Baltuska A., Fuji T., Kobayashi T., “Visible pulse compression to 4 fs by optical parametric amplification and programmable dispersion control”, Opt. Lett. 27, 306, 2002.
[22] Baum P., Lochbrunner S., Riedle E., “Generation of tunable 7-fs ultraviolet pulses:
achromatic phase matching and chirp management”, Appl. Phys. B 79, 1027, 2004.
[23] Pollard W. T., Dexheimer S. L., Wang Q., Peteanu L. A., Shank C. V. Mathies R. A.,
“Theory of dynamic absorption-spectroscopy of nonstationary states .4. application to 12-fs resonant impulsive raman-spectroscopy of bacteriorhodopsin”, J. Phys. Chem. 96, 6147, 1992.
[24] Orlandi G., Siebrand W., “Theory of vibronic intensity borrowing - comparison of Herzberg-Teller and Born-Oppenheimer coupling”, J. Chem. Phys. 58, 4513, 1973.
[25] Lin S. H., Erying H., “Study of franck-condon and herzberg-teller approximations”, Proc.
Natl Acad. Sci. USA 71, 3802, 1974.
[26] Laubereau A., Stockburger M., Time-Resolved Vibrational Spectroscopy, Berlin:
Springer, 1985
[27] Schreiber E., Femtosecond Real-Time Spectroscopy of Small Molecules and Clusters, Springer Tracts in Modern Physics vol 143 Berlin: Springer, 1998
[28] De Schryver F., De Feyter S., Schwertzer G., Femtochemistry, New York: Wiley-VCH, 2001.
[29] Martin M. M., Hynes J. T., Femtochemistry and Femtobiology, Ultrafast Events in Molecular Science Amsterdam: Elsevier, 2004.
[30] Hannaford P., Femtosecond Laser Spectroscopy Berlin: Springer, 2005
[31] Castleman A. W., Kimble M. L., Jr., Femtochemistry VII, Fundamental Ultrafast Processes in Chemistry, Physics, and Biology Amsterdam: Elsevier, 2006.
[32] Vierheilig A., Chen T., Waltner P., Kiefer W., Materny A., Zewail A. H., “Femtosecond dynamics of ground-state vibrational motion and energy flow: polymers of diacetylene”, Chem. Phys. Lett. 312, 349, 1999.
[33] Zhu L. Y., Widon A., Champion P. M., “A multidimensional Landau-Zener description of
chemical reaction dynamics and vibrational coherence”, J. Chem. Phys. 107, 2859, 1997.
[34] Rosca F., Kumar A. T. N., Ionascu D., Sjodin T., Demidov A. A., Champion P. M.,
“Wavelength selective modulation in femtosecond pump-probe spectroscopy and its application to heme proteins”, J. Chem. Phys. 114, 10884, 2001.
[35] Heritage J. P., Bergman J. G., Pinczuk A., Worlock J. M. “Surface picosecond raman gain spectroscopy of a cyanide monolayer on silver”, Chem. Phys. Lett. 67, 229, 1979.
[36] Pollard W. T., Lee Soo-Y., Mathies R. A. “Wave packet theory of dynamic absorption spectra in femtosecond pump–probe experiments”, J. Chem. Phys. 92, 4012, 1990.
[37] Book L. D., Arnett D. C., Hu H., Scherer N. F. “Ultrafast pump−probe studies of excited-state charge-transfer dynamics in blue copper proteins”, J. Phys. Chem. A 102, 4350 1998.
[38] Pollard W. T., Dexheimer S. L., Wang Q., Peteanu L. A., Shank C. V., Mathies R. A.,
“Theory of dynamic absorption spectroscopy of nonstationary states. 4. Application to 12-fs resonant impulsive Raman spectroscopy of bacteriorhodopsin”, J. Phys. Chem. 96, 6147, 1992.
Figure
Fig. 1. Evolution of the pulse duration as a function of year from 1960. The shortest pulse durations of dye-laser oscillators, Ti:sapphire oscillators, and amplifiers in each year are
shown.
Chapter 2 Noncollinear Optical Parametric Amplification (NOPA) system and pump-probe spectroscopy
2.1 Noncollinear optical parametric amplifier
A NOPA, the design of which is described in this thesis, consists of three stages. The first stage is the parametric amplifier itself, which has a sufficiently high gain bandwidth to support sub-10-fs operation. The second stage is a grating-chirped-mirror compressor, which is used to approximately compensate for group delay. It has a flexible mirror, which is used to finely adjust the spectral phase. The third stage is used to diagnose the pulses and it is based on second-harmonic-generation (SHG) frequency-resolved optical gating (FROG) with feedback to a personal computer (which also controls the actuators of the flexible mirror).
Figure 1 shows an overview of the amplifier. The source laser is a regenerative chirped pulse amplifier (Coherent Legend-USP) seeded with a Ti:sapphire laser oscillator (Micra 10;
Coherent). The amplifier generates femtosecond pulses whose duration, central wavelength, repetition rate, and average power are 40 fs, 800 nm, 5 kHz, and 2.5 W, respectively. The NOPA uses a phase-matching condition to generate amplified visible pulses with a broad spectral width of 200 THz. Below, we consider the design and function of each stage of this setup.
2.2 Extension of phase-matching bandwidth
The discovery of “magic” phase-matching conditions in a type-I β-barium borate (BBO) crystal pumped with 400-nm light [1,2] facilitated the generation of amplified visible pulses that have a bandwidth of nearly 200 THz [3,4]. The pumping arrangement utilized to achieve
these phase-matching conditions is unique since the angle of the pump beam relative to the seed beam (about 3.7°) is generally equal to the birefringent walk-off angle between ordinary and extraordinary waves inside a crystal. Consequently, BBO crystals as long as 1–2 mm can be utilized in 5-fs NOPAs. Noncollinear phase-matching conditions are well understood and have been characterized in numerous studies [2–11].
Shirakawa et al. [3,12] investigated the subtleties in parametric amplification using a noncollinear configuration. They considered the effect of tilting a signal pulse front on the ability to compress the signal pulse into a sub-5-fs pulse. They recommended using a pump beam with a tilted wavefront to prevent signal pulse tilting in space, which causes angular dispersion of the amplified pulse. This configuration (known as pulse-front matching) was implemented by sending a pump beam through a prism and adjusting the pulse-front tilt using a telescope with two convex lenses [13].
The angular dispersion of the pump beam of a sub-5-fs NOPA is important when increasing phase-matching bandwidth. When considering the mechanism responsible for this broadening, one must recall that the pump is not monochromatic and its spectral extent is determined by the duration of input fundamental pulses and frequency-doubling conditions.
Even with relatively thick SHG crystals (12 mm lithium triborate or BBO) and comparatively long (120–150 fs) pulses generated using a conventional regenerative amplifier to pump the NOPA [3,4], the resulting second-harmonic (SH) radiation has a bandwidth of several nanometers. Figure 2 schematically depicts how this can improve phase-matching conditions by focusing the incidence angles of individual pump wavelengths onto the NOPA crystal.
The noncollinearity angle, α, can be optimized to minimize the solid angle over which broadband superfluorescence is emitted. Wide-bandwidth parametric amplification can subsequently be achieved by aligning a seed beam in that direction. In other words, each
pump wavelength has a different optimal noncollinearity angle α. Pump beam dispersion for phase-matching optimization is adjusted by selecting the apex angle of the prism in the SH pathway (Fig. 1) and the distance between the prism and focusing optic l. The required SH dispersion can be calculated using the following simplified equation:
atan tan ,
where λ is the pump (SH) wavelength, λ0 is the central component of the SH spectrum, f is focal distance, and γ(λ) is exit angle of the SH beam after the prism, which is given by
atan tan ,
where γ0 is the angle between the SH beam and normal of the prism input face (i.e., the incidence angle on the prism), β is the prism apex angle, and n(λ) is the glass refractive index.
2.3 Time window for broadband parametric amplification
To achieve ultrashort pulses, one must consider time-domain shaping of amplified radiation. Despite the extremely broad bandwidth of parametric gain, only the spectral components of a seed that temporally overlap the pump pulse are amplified. Two methods exist that increase the amplification time window—pump pulse elongation and seed compression. The proposed system uses both methods.
To stretch the pump pulse, a 10-cm quartz crystal was installed in the SH beam, which pumps the NOPA. As a result of SH pulse stretching, a confocal focusing arrangement for the pump beam can be utilized (Fig. 1), which produced a significantly cleaner mode structure of the output beam than the nonconfocal pump arrangement, which was used in previous studies [3,12].
The white-light seed does not require meticulous precompression to achieve maximum (1)
(2)
continuum produced in a 2-mm-thick sapphire window passing through a pair of chirp mirrors. By changing reflection times between two chirp mirrors, the temporal window of parametric amplification and the output spectral shape were optimized.
We demonstrated that a high level of seed precompression is not required to achieve broadband amplification. Instead, flexible spectral shaping of an amplified signal is attained by balancing pump-seed delay for each of the two passes through the NOPA crystal.
2.4 Pulse compressor design and the FROG trace of NOPA pulse
The spectral width of the output pulses is large enough to generate sub-10-fs laser pulses.
For time-resolved spectroscopy, it is necessary to use ultrashort laser pulses to resolve real-time molecular vibrations. Consequently the broadband visible laser pulses were compressed using two dielectric broadband chirped mirrors, a 300-lines/mm diffraction grating in –1st order, and a 200-mm-radius spherical mirror with a flexible mirror positioned in its focal plane (see Fig. 1). A horizontal grating angle was used for crude compression of the pulse width. To fine tune the pulse compression, the voltages applied to the piezoelectric array behind the flexible mirror were adjusted, over a tuning range of 6 μm. During the tuning process, the character of the ultrashort pulses is monitored to guide the alignment procedure, using the second harmonic generation (SHG) frequency-resolved optical gating (FROG) technique. Feedback is provided to the piezoelectric array behind the flexible mirror. A translation stage (SIGMA TECH model STC-1020X), with a built-in interferometer and active position stabilization, is used to obtain a position accuracy of 10 nm. It was used in the FROG apparatus for both wide- and narrow-range delay scans with adequate accuracy. Two identical 2-μm-thick pellicle beamsplitters, which have flat reflectivity across the visible spectral range, were used to balance the dispersion in the two beams of the FROG apparatus.
Both beams are focused onto a wedged ultra-thin -BaB2O4 crystal for broadband SHG and are collimated behind it using a 200-mm off-axis parabolic mirror. The thickness of the wedge varies across the face of the crystal between 5 and 20 μm. By observing the spectral phase in the SHG FROG measurement, chirp compensation was performed by adjusting the 38 actuators of the flexible mirror (lined up in two rows that provide a clear aperture of ~39x11 mm2).
The broadband visible laser pulses were compressed enough to resolve real-time molecular vibrations whose frequencies are lower than 3000 cm-1. The setup of the SHG FROG measurement system is identical to that of the time-resolved pump-probe system used for the molecular vibrational analysis. Therefore, we also used the SHG FROG setup for the measurement of the time-resolved pump-probe signals. The thin beamsplitters used for the SHG FROG measurement divide the compressed ultrashort visible laser pulses into pump and probe beams. A typical FROG trace of the NOPA signal at the optimal compression at 600 nm is shown in Fig. 3(c). Fig. 3(a) and Fig. 3(b) give the corresponding spectrum and its transform-limited intensity profile with constant phase, respectively. The spectrum is intensity-calibrated and the pulse width is as short as 6.5 fs.
2.5 Adaptive pulse shaping
A robust adaptive algorithm based on simple spectral measurements improved pulse quality rapidly (i.e., within several minutes). The inability to remove the remaining discrepancy of the spectral phase is due only to the limited SHG bandwidth. Adaptive phase correction is essential to achieving high-quality compression of extremely broadband laser pulses due to the complexity of phase distortion and the bandwidth and tunability limits of conventional pulse compressors.
2.6 Development of a broadband detector
To detect weak pump-probe signals at multiple probe wavelengths, this work used a multichannel lock-in amplifier (Fig. 4) for time-resolved spectroscopy. The multichannel lock-in amplifier was designed to detect low-intensity signals at multiple probe wavelengths simultaneously over the entire probe spectrum. Although multichannel detection of pump-probe signals has been realized using photodiode arrays or charge-coupled devices, these detectors are less sensitive to low-level signals submerged in a high background, resulting in a lower signal-to-noise ratio than that for lock-in detection. Multichannel lock-in detection is the preferred solution for extracting all probe information and for avoiding various experimental instabilities such as sample degradation and laser instability. In this experiment, signals were spectrally resolved by a monochromator (SpectraPro 2300i; ACTON, country) at 128 wavelengths in the range of 540–740 nm and they were detected using avalanche photodiodes and a lock-in amplifier with a reference from an optical chopper that modulated pump pulse at 2.5 KHz.
References
[1] Driscoll T. J., Gale G. M., Hache F., “Ti:sapphire 2nd-harmonic-pumped visible range femtosecond optical parametric oscillator”, Opt. Commun. 110, 638, 1994.
[2] Gale G. M., Cavallari M., Driscoll T. J., Hache F., “Sub-20-fs tunable pulses in the visible from an 82-MHz optical parametric oscillator”, Opt. Lett. 20, 1562, 1995.
[3] Shirakawa A., Sakane I., Takasaka M., Kobayashi T., “Sub-5-fs visible pulse generation by pulse-front-matched noncollinear optical parametric amplification.”, Appl. Phys. Lett. 19, 2268, 1999.
[4] Cerullo G., Nisoli M., Stagira S,, De Silvestri S., “Sub-8-fs pulses from an ultrabroadband optical parametric amplifier in the visible”, Opt. Lett. 23, 1283, 1998.
[5] Di Trapani P., Andreoni A., Solcia C., Foggi P., Danielis R., Dubietis A., Piskarskas A.
“Matching of group velocities in three-wave parametric interaction with femtosecond pulses and application to traveling-wave generators”, J. Opt. Soc. Am. B 12, 2237, 1995.
[6] Gale G. M., Hache F., Cavallari M., “Broad-bandwidth parametric amplification in the visible: femtosecond experiments and simulations”, IEEE J. Sel. Topics Quantum. Electron. 4, 224, 1998.
[7] Riedle E., Beutter M., Lochbrunner S., Piel J., Schenkl S., Spoerlein S., Zinth W.,
“Generation of 10 to 50 fs pulses tunable through all of the visible and the NIR”, Appl. Phys.
B. 71, 457, 2000.
[8] Cerullo G., Nisoli M., Stagira S., De Silvestri S., Tempea G., Krausz F., Ferencz K.,
“Mirror-dispersion-controlled sub-10-fs optical parametric amplifier in the visible”, Opt. Lett.
24, 1529, 1999.
[9] Cerullo G., Nisoli M., Stagira S., De Silvestri S., Tempea G., Krausz F., Ferencz K.,
“Mirror-dispersion-controlled OPA: a compact tool for sub-10-fs spectroscopy in the visible”, Appl. Phys. B S70, S253, 2000.
[10] Shirakawa A., Kobayashi T., “Noncollinearly phase-matched femtosecond optical parametric amplification with a 2000 cm−1 bandwidth”, Appl. Phys. Lett. 72, 147, 1998.
[11] Shirakawa A., Sakane I., Kobayashi T., “Pulse-front-matched optical parametric amplification for sub-10-fs pulse generation tunable in the visible and near infrared”, Opt.
Lett. 23, 1292, 1998.
[12] Kovacs A. P., Osvay K., Bor Z., Szipöcs R., “Group-delay measurement on laser mirrors by spectrally resolved white-light interferometry”, Opt. Lett. 20, 788, 1995.
[13] Danielius R., Piskarskas A., Di Trapani P., Andreoni A., Solcia C., Foggi P., “Matching of group velocities by spatial walk-off in collinear three-wave interaction with tilted pulses”, Opt.
Lett. 21, 973, 1996.
Figures
Fig. 1. Schematic plot of NOPA setup.
Fig. 2. Schematic representation of noncollinear pumping geometry.
Fig. 3. FROG characterization of NOPA pulse. (a) NOPA spectrum. (b) The transform-limited intensity profile with 6.5 fs FWHM. (c) Retrieved FROG trace.
Fig. 4. Schematic of the multichannel lock-in amplifier with a pump-probe measurement system.
Chapter 3 Development of a multiplex fast-scan system for ultrafast time-resolved spectroscopy
3.1 Introduction to the measurements of time-resolved study
Ultrafast spectroscopy is a powerful method for investigating photochemical, photophysical, and photobiological processes. Photochemistry of various reactions has been studied, including isomerization, proton transfer, and electron transfer. Photophysical processes have been measured for carrier dynamics and nonlinear excitations. Photobiological processes have been clarified in vision and photosynthesis [1–4]. Ultrafast spectroscopy also has various important applications to photosensors [5–7], ultrafast optical switches [8–10], and ultrafast optical memories [11–13].
Following the development of femtosecond lasers, ultrafast spectroscopy blossomed [14–17] and expanded into areas such as plasma diagnosis and in situ probes for laser manufacturing. Subsequent development of sub-10-fs laser pulses [18–23] elucidated the ultrafast dynamics [24–28] of electronic relaxations and molecular vibrations. After the development of a NOPA [29–30], ultrafast spectroscopy could be performed across wide spectral regions because its structure is broadband and smooth. The resulting observation of real-time molecular vibrations has provided information about changes in molecular structure
Following the development of femtosecond lasers, ultrafast spectroscopy blossomed [14–17] and expanded into areas such as plasma diagnosis and in situ probes for laser manufacturing. Subsequent development of sub-10-fs laser pulses [18–23] elucidated the ultrafast dynamics [24–28] of electronic relaxations and molecular vibrations. After the development of a NOPA [29–30], ultrafast spectroscopy could be performed across wide spectral regions because its structure is broadband and smooth. The resulting observation of real-time molecular vibrations has provided information about changes in molecular structure