Details of a Researcher - Hasegawa, Taro

Papers - Hasegawa, Taro

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Twisted radio frequency ion traps for all-metal string ion traps

Taro Hasegawa

Review of Scientific Instruments (American Institute of Physics) 96 ( 10 ) 103201-1 - 103201-7 2025.10

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

Twisted radio frequency (rf) ion traps, which have electrode configurations that are twisted variants of conventional linear rf ion traps, have been proposed and successfully demonstrated using charged alumina dust particles. When constructed from thin metal wire electrodes, the twisted traps offer excellent optical accessibility to the trapped ions, making them suitable for applications in quantum information technology. In the experimental demonstration, 14 μm-diameter alumina dust particles were charged via discharge and successfully confined in the twisted trap. A linear chain of trapped particles was observed within the trap.

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Improvement in the Sagnac-loop laser frequency stabilization

Taro Hasegawa

Journal of Optical Society of America B (Optica) 42 ( 2 ) 285 - 291 2025.01

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

The laser frequency stability locked to the D2 transition of 85Rb at 780 nm using a Doppler-free Sagnac-loop stabilization scheme has been improved. In a previous work [J. Opt. Soc. Am. B 40, 667 (2023)], changes in birefringence at the absorption cell windows, influenced by the cell temperature, and in the rubidium vapor, affected by magnetic field fluctuations, significantly influenced the laser frequency stability. By aligning the polarization, the short-term stability (<10 s) of the locked laser frequency was improved; however, the long-term stability remained unaffected. We attribute this long-term stability to temperature-induced changes in the optical path length of the wave plates. By removing the wave plates from the setup, the long-termstability has been successfully improved.

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Phase structure of harmonically mode-locked optical frequency combs

Taro Hasegawa

Optics Express (Optica) 32 ( 5 ) 8092 - 8100 2024.02

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

Carrier phase of a harmonically mode-locked optical frequency comb (OFC) is investigated in detail. While harmonically mode-locked OFCs show promise for high-repetition-rate applications, their mode spacing is not the same as the pulse repetition rate, unlike fundamentally mode-locked OFCs. Consequently, harmonically mode-locked OFCs are unsuitable for applications requiring OFCs with wide mode spacing. This study examines the pulse-to-pulse carrier phase evolution of 4th- and 5th-order harmonically mode-locked OFCs, revealing uneven carrier phase evolution responsible for the narrow mode spacing. The possibility of achieving harmonically mode-locked OFCs with wide mode spacing is suggested by implementing pulse-to-pulse phase modulation to ensure even phase evolution.

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Evaluation of the Sagnac-loop laser frequency stabilization to sub-Doppler spectral lines

Taro Hasegawa

Journal of Optical Society of America B (Optica) 40 ( 3 ) 667 - 672 2023.03

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted, ISSN 07403224

Laser frequency stability locked to D2 transition of 85Rb at 780 nm by the Sagnac-loop stabilization scheme was evaluated using the heterodyne measurement with an optical frequency comb. This scheme does not require modulation and magnetic field to generate the feedback error signal. Therefore, the setup for the laser frequency stabilization is simple. Because of the mechanical stability of the Sagnac interferometer, the dispersion of atom transition, which is used as an error signal for feedback loop, can be observed with negligibly small effects of mechanical fluctuations. In the experiment, the Allan variance for relative frequency uncertainty was 2.6×10-12 over 64 s averaging.

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Mode spacing multiplication of optical frequency combs without power loss

Taiki Kageyama, Taro Hasegawa

Optics Express (Optica) 30 ( 11 ) 19090 - 19099 2022.05

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted

We demonstrate the mode spacing multiplication of optical frequency combs (OFCs) using interleaving technique. The pulse train of an OFC after interleaving is phase modulated, and phase demodulation is necessary for mode spacing multiplication. This scheme enables mode spacing multiplication without power loss in principle, which is in contrast to the conventional mode filtering technique. During demonstration, the mode spacing of the OFC of a mode-locked Er-doped fiber laser (repetition rate of 97 MHz) is quadrupled by the interleaving process and successive phase demodulation.

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Fast polarization control for optical frequency combs

Taiki Kageyama, Taro Hasegawa

Optics Express (Optica) 29 ( 23 ) 38477 - 38487 2021.11

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted

Polarization of an optical frequency comb is electrically controlled using a waveguide electro-optic phase modulator (WG-EOM). Owing to the low operation voltage and wide electric bandwidth of the WG-EOM, fast polarization control is possible. It is found that birefringence of the WG-EOM and polarization-maintaining optical fibers causes polarization-dependent pulse separation, which makes polarization control of the optical frequency comb impossible. Therefore, compensation of the birefringence is required for polarization control. In the experiment, a delay line in free space is used for birefringence compensation, and pulse-to-pulse polarization control of an optical frequency comb (with a repetition rate of 100 MHz) is demonstrated.

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260 kHz mode-spacing optical frequency combs for scan-free high-resolution direct-comb spectroscopy

Taro Hasegawa

Optics Express (The Optical Society) 28 ( 22 ) 32837 - 32846 2020.10

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

For scan-free high-resolution direct-comb spectroscopy, mode spacing of an optical
frequency comb is reduced down to 260 kHz by phase modulation. It turns out that timedomain signal is degraded by averaging because of slow optical path length fluctuations and
fast optical pulse timing jitter. In this study, compensation of these effects is introduced, and
signal degradation by averaging is avoided. With demonstrations of direct-comb spectroscopy
with the narrow-mode-spacing optical frequency comb, Doppler-limited absorption spectrum of
methane and reflection spectrum from an optical ring cavity are observed. As a result, detailed
resonance spectral line profile of 8 MHz linewidth for the optical ring cavity is obtained in 50 ms
measurement time.

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Robust offset locking of laser frequency with electronically tunable LC circuits for sub-millihertz uncertainty

Yuhei Seishu, Taro Hasegawa

Applied Physics B (Springer) 125 ( 8 ) 142(1) - 142(8) 2019.07

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted

A simple scheme for laser frequency offset locking introduced in Cheng et al. (Opt Express 25:2752, 2017) has been improved by the use of a variable-capacitance diode. By employing this scheme with the optical phase lock loop, robust and precise laser frequency stabilization is achieved, and long-term laser frequency drift is expected to be substantially suppressed even in noisy circumstances. The optical frequency uncertainty becomes less than 10 μ Hz for 1000 s averaging.

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Mode density multiplication of an optical frequency comb by N<sup>2</sup> with phase modulation

Hasegawa T.

Optics Express (The Optical Society) 26 ( 19 ) 24551 - 24563 2018.09

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

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A radio-frequency ion trap with string electrodes

Daisuke Kanai, Taro Hasegawa

Review of Scientific Instruments (American Institute of Physics) 89 ( 2 ) 023106-1 - 023106-5 2018.02

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted, ISSN 00346748

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Optical bistability induced by Gouy phase shift

Taro Hasegawa

Journal of Optical Society of America B (The Optical Society) 34 ( 7 ) 1319 - 1326 2017.07

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

Optical bistability in a passive ring cavity with no designed nonlinear materials is experimentally observed in the near-infrared region. The nonlinear material that induces the optical bistability is considered to be water molecules in atmosphere. In the model calculation, it is found that the optical bistability is induced by transversal distribution of optical power-dependent refractive index of water in the Gaussian beam, which causes the power-dependent Gouy phase shift in the cavity. The model calculation agrees qualitatively with the experimental results. This result suggests that the Gouy phase shift has an effect on cavity-enhanced molecular spectroscopy.

Direct-comb molecular spectroscopy by heterodyne detection with continuous-wave laser for high sensitivity

Taro Hasegawa, Hiroyuki Sasada

Optics Express (The Optical Society) 25 ( 16 ) A680 - A688 2017.06

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted

We have demonstrated direct-comb molecular spectroscopy in which an individualcomb mode is resolved by heterodyne detection with a continuous-wave (cw) laser. This simple, fast, and sensitive technique was demonstrated for atomic spectroscopy in [Appl. Phys. Lett. 101, 051105 (2012)], but is more suitable for molecular spectroscopy. Four rotation-vibration transitions of methane have simultaneously been recorded in a Doppler-limited resolution using a spectrally broadened comb based on an Er-doped fiber laser and a cw 1651-nm distributedfeedback laser diode. Even though the power level of the comb modes and the sample absorption are weaker than those of the previous studies, in this work the cw laser frequency is stabilized to one of the comb modes, and the data are thereby averaged for extended periods of time, resulting an improvement in sensitivity.

Broadened optical spectrum of repetitionrate-multiplied erbium-doped fiber comb

Taro Hasegawa

Journal of Optical Society of America B (The Optical Society) 33 ( 10 ) 2057 - 2061 2016.10

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

The repetition rate of an erbium-doped fiber comb is multiplied by an optical cavity for filtering, and the optical spectral distribution is broadened in a highly nonlinear fiber. For erbium-doped fiber combs, filtering cavities are commonly used to obtain the high-repetition-rate combs by the repetition rate multiplication. After the multiplication, optical spectral broadening by a highly nonlinear fiber is necessary for applications to optical frequency measurements in broad bandwidth and astronomy. In this study, the broadened spectrum covers over the range from 1050 to 2250 nm, which is over an octave so that the determination of the filtered mode numbers is feasible. The optical power of the broadened beam within a narrow optical bandwidth is also measured in the time domain and in the frequency domain to examine the broadening property.

Nonlinear absorption and phase shift in coupled optical cavities

Taro Hasegawa

Physical Review A (American Physical Society) 92 ( 4 ) 043815-1 - 043815-5 2015.10

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

The nonlinear absorption process and associated phase shift in coupled optical cavities are studied experimentally by observing optical resonance properties. In the coupled cavity configuration, two optical cavities, one for a fundamental beam and the other for a second-harmonic (SH) beam, are coupled by a nonlinear crystal for the second-harmonic generation (SHG). The cavity for the SH beam effectively extends the nonlinear crystal length so that the frequency-conversion efficiency of the SHG, which is proportional to the square of the crystal length, is enhanced. In the observation, power reduction of the fundamental beam at resonance is observed as a small dip in the resonance curve. According to a model calculation, it is found that this power reduction is caused by blue-induced infrared absorption.

Theoretical analysis of mid-infrared molecular spectral line profiles with transversal relaxation in an intense optical field

Taro Hasegawa

Journal of Optical Society of America B (The Optical Society) 31 ( 8 ) 1879 - 1884 2014.08

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

Nonlinear effects by intense optical field on mid-infrared spectral lines (∼3 μm) are discussed. Recent development of intense mid-infrared coherent radiation makes nonlinear mid-infrared spectroscopy possible. In the midinfrared region, the transversal relaxation process is important compared to the visible or near-infrared region. In the analysis, a two-energy-state system is discussed. The intense pump beam scattering spectral lines (Mollow triplet) and the absorption spectral lines of the probe beam with the intense pump beam are calculated.

Stabilization of the coupled resonant cavity without dither

Taro Hasegawa

Optics Communications (Elsevier) 308 270 - 273 2013.11

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

A simple scheme without dither to obtain an error signal for the stabilization of the coupled-cavity configuration for efficient fourth harmonic generation of continuous-wave lasers is proposed and demonstrated. This scheme is ananalogue of the noise immune cavity enhanced optical heterodyne molecular spectroscopy (NICEOHMS) technique. In the proposed scheme, the robust stabilization is expected in comparison with the dither lock technique. In the demonstration, the stabilization of the coupled resonant cavity at the resonance is successfully achieved without the noise caused by the dither.

Fourth harmonic generation of continuous-wave lasers in coupled-cavity configuration

Taro Hasegawa

Optics Communications (Elsevier) 285 ( 24 ) 5368 - 5371 2012.11

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

Frequency quadrupling of a continuous-wave laser is carried out in coupled cavities with the stabilization to the resonance. The fourth harmonic generation in the coupled cavities is expected to be more efficient than in the two build-up cavity configuration in tandem, because input couplers, which increase the optical loss of the cavity, are not required. The cavity length is stabilized by the feedback of the error signal obtained by the cavity length modulation. In the present demonstration, the ultraviolet beam of 6 mW at 238 nm is obtained by the fourth harmonic generation of the near infrared laser of 180 mW at 952 nm. This output power is expected to become higher (~ 1.7 mW) when the cavity setup is optimized.

Modulation-free laser frequency stabilization to a saturated sub-Doppler spectral line in a transversal magnetic field

Sho Okubo, Kana Iwakuni, Taro Hasegawa

Optics Communications (Elsevier) 285 ( 20 ) 4107 - 4111 2012.09

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted

We demonstrate frequency stabilization of a modulation-free laser to a saturated absorption spectral line of atoms in a transversal magnetic field. This stabilization scheme has been proposed for wide capture range in comparison with the dichroic atomic vapor laser lock (DAVLL) scheme and demonstrated for a Doppler-broadened spectral line in J. Opt. Soc. Am. B, 26, 1216 (2009). In this paper, a 1083-nm external-cavity laser diode is frequency-stabilized to the sub-Doppler spectral line of helium transition (2^3 S_1, m_J = 0 <-> 2 ^3P_0). Even though the error signal shape strongly depends on the pump beam polarization, the stabilized frequency is expected to be insensitive to the pump beam polarization.

Efficient photo-ionization for barium ion trapping using a dipole-allowed resonant two-photon transition

Guenther. Leschhorn, Taro Hasegawa, Tobias Schaetz

Applied Physics B (Springer) 108 ( 1 ) 159 - 165 2012.07

Research paper (scientific journal), Joint Work, Accepted

Two efficient and isotope-selective resonant twophoton ionization techniques for loading barium ions into radio-frequency (RF)-traps are demonstrated. The scheme of using the strong dipole-allowed transition 6s2 1S0 → 6s6p 1P1 at λ = 553 nm as a first step towards ionization is compared to the established technique of using a weak inter-combination line (6s2 1S0 →5d6p 3D1,λ = 413 nm). An increase of two orders of magnitude in the ionization efficiency is found favoring the transition at 553 nm. This technique can be implemented using commercial all-solidstate laser systems and is expected to be advantageous compared to other narrowband photo-ionization schemes of barium in cases where highest efficiency and isotope-selectivity are required.

Sympathetic cooling of a mass-mismatched two-ion chain in a double-well trap potential

HASEGAWA TARO

Physical Review A (American Physical Society) 83 ( 5 ) 053407-1 - 053407-6 2011.05

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

Sympathetic cooling of two-ion system, in which one is laser-cooled and the other is sympathetically cooled and their masses are mismatched, in a linear rf trap with a double-well potential is proposed. The double-well potential consists of two wells, and there is one ion in each well. The axial frequencies of the two wells and the spatial interval between them are experimentally controllable. By theoretical analysis, the normal modes of the small oscillations around the equilibrium are derived, and a measure of the sympathetic cooling rate is obtained. As a result, it is found that the sympathetic cooling rate is fast when the frequency of the axial motion of the sympathetically cooled ion is close to that of the laser-cooled ion. In the double-well potential, the sympathetic cooling rate of the ion species whose mass is much heavier or lighter than that of the laser-cooled ion can be fast. The sympathetic cooling rate of C60 + by the laser-cooled Ba+ in the double-well potential is estimated to be about 80 times faster than in the conventional setup. The double-well potential may be made by the microfabricated electrode configuration or by the optical dipole force trap.

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