Yoichi Kawada

Real-time measurement of temporal waveforms of a terahertz pulse using a probe pulse with a tilted pulse front

We propose a novel method that uses a femtosecond pulse with a tilted pulse front as a probe pulse to perform real-time measurement of temporal waveforms of a terahertz pulse. This method can operate on a single shot in principle. Real-time measurement of the terahertz pulse was achieved using the dynamic subtraction method with a synchronized high-speed complimentary metal-oxide semiconductor sensor. A time window of 2.6 ps was achieved. This value agrees well with results that were calculated theoretically and measured by an interferometric inverted field autocorrelator.

Evaluation of Drug Crystallinity in Aqueous Suspension Using Terahertz Time‐Domain Attenuated Total Reflection Spectroscopy

Terahertz pulsed spectroscopy has recently been demonstrated to be a novel technique for the investigation of the solid-state properties of pharmaceutical materials. In this study, we directly measured the crystallinity of a drug suspended in water, using a terahertz pulsed attenuated total reflection (ATR) method. The dihydropyridine calcium channel blocker nifedipine is classified as a poorly soluble drug; its most stable crystalline form is known as form I. Transmission spectra, collected from 0.2 to 2.0 THz (6.6 to 66 cm(-1) ), of nifedipine crystals had a strong absorption peak at 1.2 THz (40 cm(-1) ) at room temperature. When the nifedipine crystals were mixed with poloxamer 188 and suspended in water, the resulting spectra measured using the ATR method had a peak at the same frequency as in the spectra obtained in transmission mode. Furthermore, the peak area was proportional to the amount of crystals. The upward sloping baseline in the spectra, corresponding to water absorption, decreased stepwise with increasing amounts of crystalline particles. We confirmed that the spectra gave excellent quantitative results, using partial least-squares regression analysis. The results suggest the possibility of using this method for qualitative and quantitative assessments of crystalline drugs in suspension.

Terahertz movies of internal transmission images

Recently terahertz imaging using two-dimensional E-O sampling has attracted much interest because it can acquire real-time terahertz images unlike a conventional raster scan method. We are applying this technique to the non-destructive measurement of opaque materials in a visible range. We acquired 10-fps consecutive terahertz transmission images: dripping water in a plastic pipe and metal included in a piece of gum. Since the obtained images were confirmed to be proportional to the electric field of the terahertz waves, the images in the present paper are useful for quantitative analysis. We also showed the signal-to-noise ratio of the terahertz images.

Single-shot terahertz spectroscopy using pulse-front tilting of an ultra-short probe pulse

We developed a single-shot terahertz pulse measurement technique using pulse-front tilting of an ultra-short probe pulse and demonstrated single-shot terahertz spectroscopy. A transmission grating was used to introduce a sufficiently large pulse-front tilt angle. A measuring time range of 23.8 ps was achieved. The measured temporal waveforms were corrected in consideration of the nonlinearity arising from the crossed-Nicols arrangement employed and the beam profiles of the probe and terahertz pulses. The characteristic spectrum of lactose was measured with a single terahertz pulse, and the effectiveness of our single-shot technique was confirmed by comparison with a conventional sampling method.

Repetitive production of positron emitters using deuterons accelerated by multiterawatt laser pulses

Positron emitters (11)C, (13)N, and (15)O, which can be used in positron emission tomography, were produced using deuterons accelerated by irradiation of laser pulses approximately 70 TW in peak power and approximately 30 fs in duration with a repetition of 10 Hz during a period of as long as 200 s. Every laser pulse irradiates the fresh surface of a long strip of a solid-state thin film. Deuterons contained in the film are accelerated in the relativistic plasma induced by the pulse. The deuterons are repetitively incident on solid plates, which are placed near the film, to produce positron emitters by nuclear reactions. The radioactivities of the activated plates are measured after the termination of laser irradiation. In activation of graphite, boron-nitride, and melamine plates, the products had total activities of 64, 46, and 153 Bq, respectively. Contamination in the setup was negligible even after several thousands of laser shots. Our apparatus is expected to greatly contribute to the construction of a compact PET diagnostic system in the future.

Terahertz time domain attenuated total reflection spectroscopy with an integrated prism system

We demonstrated attenuated total reflection (ATR) spectroscopy with an integrated prism system that included a terahertz emitter, a terahertz receiver, and an ATR prism. The ATR prism had two internal off-axis parabolic mirrors for, respectively, collimating and focusing the terahertz waves. The Fresnel loss at each interface was reduced, and the total propagation efficiency was 3.36 times larger than when using a non-integrated prism system. The refractive index of water samples calculated from the experimental data showed good agreement with values reported in the literature.

Single-shot measurement of terahertz temporal waveform using pulse-front tilting by a direct vision dispersion prism

We demonstrated a single-shot measurement of terahertz temporal waveform using pulse-front tilting by a direct vision dispersion prism (DVDP). An advantage of this technique is the simplicity with which an electro-optic terahertz imaging optical system can be changed into a single-shot measurement system by inserting a DVDP in the probe beam path. In this technique, control of the angle of pulse-front tilting is very important. We precisely designed DVDP and measured the angle of the pulse-front tilting by interference measurement. We obtained a terahertz temporal waveform with a single shot with a time window of 3.2 ps.

Diagnosis of laser-induced relativistic plasma by positron imaging technique

Energetic deuterons emitted from laser-induced relativistic plasma are measured with a spatial resolution using a plate activated by them. Laser pulses of 33 TW in peak power and 34 fs in duration generate a plasma on a solid target, which is a microporous polytetrafluoroethylene film loaded with deuterated polystyrene, to accelerate deuterons. The deuterons are incident on a melamine plate, causing nuclear reactions of C12(d,n)N13 and N14(d,n)O15. The spatial distributions of N13 and O15 created in the plate are visualized by a diagnosis using two-dimensional positron-emission tomography. The obtained images are further converted to temperature distribution of the emitted deuterons. The results show that the temperature of deuterons varies depending on the direction of deuteron emission. The present study provides knowledge of thermal equilibrium in laser-induced relativistic plasma in an ultrashort period of time.

Carrier envelope phase shifter for broadband terahertz pulses

We demonstrated controlled shifting of the internal phase of broadband terahertz (THz) pulses. The internal phase of an ultrashort pulse is called the carrier envelope phase (CEP), which is an important parameter in the interaction of few-cycle light pulses and matter. Our CEP shifter utilizes the ultra-broadband feature of prism wave plates. We analytically derived the amount of CEP shift achievable by the CEP shifter using Jones matrixes. THz time-domain measurements clearly showed the shift of the CEP, and the results agreed well with the calculated values. The CEP shift was as high as 2π, indicating that any CEP values can be chosen using our CEP shifter.

Fast frequency-resolved terahertz imaging.

We propose a fast, frequency-resolved, real-time, terahertz imaging method. With our method, images at two specific terahertz frequencies can be acquired in 1 min. Three kinds of drugs (L-histidine, maltose, and CBZ3), which have absorption peaks in the terahertz region, were distinguished in 3 min by using our method. This technique can be used in industrial applications, such as nondestructive testing.