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論文

Simultaneous measurements of superradiance at multiple wavelength from helium excited states, 2; Analysis

大饗 千彰*; Harries, J.; 岩山 洋士*; 川口 健太郎*; 久間 晋*; 宮本 祐樹*; 永園 充*; 中嶋 享*; 中野 逸夫*; 繁政 英治*; et al.

Journal of the Physical Society of Japan, 85(3), p.034301_1 - 034301_10, 2016/03

 被引用回数:7 パーセンタイル:56.08(Physics, Multidisciplinary)

Previous experimental studies of superradiance (SR) in multi-level systems have been explainable using the predictions of the well-known simple two-level SR model. However our recent study using EUV free-electron laser excitation of helium atoms, where SR was observed at wavelengths of 502 nm, 668 nm, and 728 nm, revealed behaviour which necessitates a full multi-level treatment of the SR development. In this paper, we report simulations of the initial excitation by the FEL pulses, and the subsequent development of multi-level SR. The results of the simulation reproduce the experimental findings, and reveal that competitive SR on two transitions with a common upper level plays an important role in the development of the system.

論文

A Streak camera study of superfluorescence at multiple wavelengths from helium atoms excited using free electron laser pulses

Harries, J.; 岩山 洋士*; 永園 充*; 富樫 格*; 矢橋 牧名*; 久間 晋*; 中嶋 享*; 宮本 祐樹*; 大饗 千彰*; 笹尾 登*; et al.

Journal of Physics B; Atomic, Molecular and Optical Physics, 48(10), p.105002_1 - 105002_9, 2015/05

 被引用回数:7 パーセンタイル:45.32(Optics)

We report the observation of superfluorescence at wavelengths (in air) of 501.6 nm, 667.8 nm, and 728.1 nm following the excitation of helium atoms with free-electron laser pulses at wavelengths of 53.7 nm ($$n$$=3 excitation) and 52.2 nm ($$n$$=4 excitation). The observed wavelengths of the superfluorescence pulses correspond to 1s3p-1s2s, 1s3d-1s2p, and 1s3s-2p transitions. Observation of superfluorescence on these transitions implies either competing cascade decays, or direct excitation of non-dipole allowed transitions. We have studied the time structure of the emitted pulses using a streak camera, and the results cannot be explained by straightforward considerations using the usual model for two-level superfluorescence.

論文

Simultaneous measurements of super-radiance at multiple wavelengths from helium excited states, 1; Experiment

中嶋 享*; Harries, J.; 岩山 洋士*; 久間 晋*; 宮本 祐樹*; 永園 充*; 大饗 千彰*; 富樫 格*; 矢橋 牧名*; 繁政 英治*; et al.

Journal of the Physical Society of Japan, 84(5), p.054301_1 - 054301_7, 2015/05

 被引用回数:6 パーセンタイル:50.68(Physics, Multidisciplinary)

In this paper, we report the results of measurements of the intensities and delays of super-radiance decays from excited helium atoms at multiple wavelengths. The experiment was performed using extreme ultraviolet radiation produced by the free electron laser at the SPring-8 Compact SASE Source test accelerator facility as an excitation source. We observed superradiant transitions on the 1s3p$$rightarrow$$1s2s ($$lambda$$=502 nm), 1s3d$$rightarrow$$1s2p ($$lambda$$=668 nm), and 1s3s$$rightarrow$$1s2p ($$lambda$$=728 nm) transitions. The pulse energy of each transition and its delay time were measured as a function of the target helium gas density. Several interesting features of the data, some of which appear to contradict with the predictions of the simple two-level super-radiance theory, are pointed out.

論文

Transient charge dynamics in argon-cluster nanoplasmas created by intense extreme-ultraviolet free-electron-laser irradiation

岩山 洋士*; Harries, J.; 繁政 英治*

Physical Review A, 91(2), p.021402_1 - 021402_5, 2015/02

 被引用回数:14 パーセンタイル:68.48(Optics)

We present extreme-ultraviolet (EUV) fluorescence spectra of Ar clusters irradiated by intense EUV free-electron laser (FEL) pulses focused to intensities of up to 3$$times$$10$$^{13}$$ W/cm$$^2$$ at a wavelength of 51 nm. The spectra reveal fluorescence at wavelengths shorter than that of the incident radiation, which can be assigned to EUV fluorescence lines from excited multiply-charged ions Ar$$^{z+ast}$$ with $$z$$ as high as 6. This demonstrates that charge states as high as 7+ are produced by the FEL irradiation. The cluster size and FEL power dependences of the spectral features suggest that an inhomogeneous distribution of charge within the clusters reduces ionization thresholds at the cluster surface.

論文

Photon-in, photon-out gas-phase experiments at the SPring-8 compact SASE source SCSS

Harries, J.; 岩山 洋士*; 沖原 理沙*; 永園 充*; 繁政 英治*

JPS Conference Proceedings (Internet), 1, p.013083_1 - 013083_6, 2014/03

In this article we describe four recent results from "photon-in, photon-out" fluorescence spectroscopy studies using the SPring-8 compact SASE source SCSS. Fluorescence spectroscopy avoids problems due to space charge, and is particularly suited to the highly-intense FEL pulses, since multiple excitations/ionisations occur for each laser shot.

論文

極端紫外自由電子レーザーによる超蛍光の観測

Harries, J.; 繁政 英治*

放射光, 26(6), p.327 - 334, 2013/11

極端紫外(EUV)自由電子レーザー(FEL)光と高密度ヘリウムガスセルを利用して、EUV-FEL由来の超蛍光(集合的自然放出)の観測に世界に先駆けて成功した。超蛍光は、「量子光学」現象の一つであり、新世代光源の利用によって初めて観測が可能となった。より短波長域での観測の可能性を含めて、今後の展望についても議論する。

論文

Resonance-enhanced three-photon single ionization of Ne by ultrashort extreme-ultraviolet pulses

繁政 英治*; 永園 充*; 岩山 洋士*; Harries, J.; 石川 理沙*

Journal of Physics B; Atomic, Molecular and Optical Physics, 46(16), p.164020_1 - 164020_5, 2013/08

 被引用回数:0 パーセンタイル:0.01(Optics)

Time-resolved extreme-ultraviolet (EUV) fluorescence spectroscopy has been applied to study the multi-photon, single ionization of Ne irradiated by intense EUV-free-electron laser (FEL) pulses at a wavelength of 51 nm. A broad, intense peak at a wavelength of around 46 nm is observed, which is shorter than the incident FEL wavelength. The time-dependence of the fluorescence reveals that the peak has two un-resolved components, which we attribute to the decay of the excited ion states Ne$$^+$$2s$$^{-1}$$($$^2$$S) and 2p$$^{-2}$$($$^3$$P$$rm ^e$$)3s($$^2$$P$$rm ^e$$) at 46.0 nm and 44.6 nm. From the observed intensity ratios and fluorescence lifetimes we conclude that the Ne$$^+$$ 2p$$^{-2}$$ ($$^3$$P$$rm ^e$$)3s($$^2$$P$$rm ^e$$) state is populated by two-photon resonance-enhancement, via a 2p$$^4$$3p$$^2$$ doubly-excited state of Ne.

論文

Towards coherent control of SASE pulses using propagation through helium gas at wavelengths corresponding to double excitation

Harries, J.; 永園 充*; 岩山 洋士*; 繁政 英治*

Journal of Physics B; Atomic, Molecular and Optical Physics, 46(16), p.164021_1 - 164021_5, 2013/08

 被引用回数:0 パーセンタイル:0.01(Optics)

Using the third harmonic of the FEL radiation from the SPring-8 compact SASE source SCSS we have studied the effects on SASE (self-amplified stimulated emission) pulses with central wavelengths near 20 nm due to passage through a helium gas cell. The positions of zero ionisation cross-section close to wavelengths corresponding to double-excitations allow operation as an efficient wavelength filter, with effectively 100% transmitted peak intensity until the Doppler-broadening limit is reached. We discuss how the time profile of the SASE pulses is affected, and discuss potential applications.

論文

Compact XFEL and AMO sciences; SACLA and SCSS

矢橋 牧名*; 田中 均*; 田中 隆次*; 冨澤 宏光*; 富樫 格*; 永園 充*; 石川 哲也*; Harries, J.; 彦坂 泰正*; 菱川 明栄*; et al.

Journal of Physics B; Atomic, Molecular and Optical Physics, 46(16), p.164001_1 - 164001_19, 2013/08

 被引用回数:68 パーセンタイル:96.33(Optics)

The concept, design, and performance of Japan's compact FEL facilities, the SPring-8 Compact SASE Source test accelerator (SCSS) and SPring-8 Angstrom Compact free electron LAser (SACLA), and their applications are reviewed. At SCSS, intense, ultrafast FEL pulses at extreme ultraviolet (EUV) wavelengths have been utilized for investigating various multiphoton processes in atoms, molecules and clusters by means of ion and electron spectroscopy. The quantum optical effect superfluorescence has been observed with EUV excitation. A pump-probe technique combining FEL pulses with near infrared laser pulses has been realized to study the ultrafast dynamics of atoms, molecules and clusters in the sub-picosecond regime. At SACLA, deep inner-shell multiphoton ionization by intense X-ray free-electron laser pulses has been investigated. The development of seeded FEL sources for producing transversely and temporally coherent light, as well as the expected impact on advanced science are discussed.

論文

Demonstration of up-conversion fluorescence from Ar clusters in intense free-electron-laser fields

岩山 洋士*; 永園 充*; Harries, J.; 繁政 英治*

Optics Express (Internet), 20(21), p.23174 - 23179, 2012/10

 被引用回数:15 パーセンタイル:63.55(Optics)

Extreme ultraviolet (EUV) fluorescence emitted from Ar clusters irradiated by intense EUV free electron laser (FEL) pulses has been investigated. The EUV fluorescence spectra display rich structure at wavelengths shorter than the incident FEL wavelength of 51 nm. The results suggest that multiply-charged ions are produced following the ion-electron recombination processes which occur in the nanoplasma created by multi-photon excitation during the intense EUV-FEL pulses.

論文

Observation of free-electron-laser-induced collective spontaneous emission (superfluorescence)

永園 充*; Harries, J.; 岩山 洋士*; 富樫 格*; 登野 健介*; 矢橋 牧名*; 仙波 泰徳*; 大橋 治彦*; 石川 哲也*; 繁政 英治*

Physical Review Letters, 107(19), p.193603_1 - 193603_5, 2011/11

 被引用回数:33 パーセンタイル:82.13(Physics, Multidisciplinary)

We have observed and characterized 501.6 nm collective spontaneous emission (superfluorescence) following 1$$s$$$$^2rightarrow$$1$$s$$3$$p$$ excitation of helium atoms by 53.7 nm free-electron laser radiation. Emitted pulse energies of up to 100 nJ ($$sim$$10$$^{{11}}$$ photons) are observed, corresponding to a photon number conversion efficiency of up to 10%. We observe the peak intensity to scale as $$rho^2$$, and the pulse width and delay with respect to the pump pulse to scale as $$rho^{-1}$$, where $$rho$$ is the atom number density. Emitted pulses as short as 1 ps are observed, which corresponds to a rate around 75000 times faster than the spontaneous 1$$s$$3$$p$$$$rightarrow$$1$$s$$2$$s$$ decay rate. To our knowledge this is the first observation of superfluorescence following pumping in the extreme ultraviolet wavelength region, and extension of the technique to the generation of EUV and X-ray superfluorescence pulses should be straightforward by using suitable atomic systems and pump wavelengths.

口頭

Multi-atom, multi-photon collective decay (superfluorescence) following EUV-FEL pumping of He

永園 充*; Harries, J.; 岩山 洋士*; 富樫 格*; 登野 健介*; 矢橋 牧名*; 仙波 泰徳*; 大橋 治彦*; 石川 哲也*; 繁政 英治*

no journal, , 

Superfluorescence is a collective, multi-atom, multi-photon decay process which can occur when a population inversion is created in a large number of atoms with sufficient number density. We create indirectly such an inversion on the helium 3p$$rightarrow$$2s transition by pumping using 53.7 nm EUV Free-electron laser (FEL) radiation at the SPring-8 Compact SASE Source (SCSS) test accelerator beamline, producing swept inversion in a 44 mm cube gas cell. Intense, highly-directional (divergence $$<$$ 20 mrad), linearly-polarised radiation at 501.7 nm is emitted in the direction of the incident beam. We have studied the characteristics of the emitted pulses using a streak camera and a calibrated photodiode.

口頭

Coherent spontaneous decay following multi-atom excitationwith FEL pulses; Prospects for X-ray superradiance from clusters

Harries, J.; 永園 充*; 岩山 洋士*; 富樫 格*; 登野 健介*; 矢橋 牧名*; 仙波 泰徳*; 大橋 治彦*; 石川 哲也*; 繁政 英治*

no journal, , 

Recently we have observed superfluorescence following 1s$$^2 rightarrow$$ 1s np excitation in helium using 100 fs pulses from the SPring-8 Compact SASE Source (SCSS) test accelerator beamline. One requirement for collective decay (as opposed to the competing natural decay and amplified spontaneous emission) is that the inter-atom spacing must be less than the wavelength of the emitted radiation. For visible radiation this is easily achievable in the gas phase, but for shorter wavelengths, clusters and other nano-scale particles present an ideal opportunity for observing and exploiting this mode of decay, which competes with all other decay mechanisms, including Auger. The potential for observing X-ray superradiance from multiphoton excited clusters has been described previously, but to our knowledge never been observed experimentally. We present details of our study in atomic helium gas, and discuss prospects for the extension to clusters and other nano-sized particles.

口頭

極紫外自由電子レーザー誘起による超蛍光の観測

永園 充*; Harries, J.; 岩山 洋士*; 登野 健介*; 富樫 格*; 仙波 泰徳*; 大橋 治彦*; 矢橋 牧名*; 石川 哲也*; 繁政 英治*

no journal, , 

実験はSCSS試験加速器施設で行われた。He原子を試料とし、SCSSからの極紫外自由電子レーザー(EUV-FEL)の波長を1s$$^2$$$$rightarrow$$1s3p共鳴励起相当に合わせ、高濃度のHe原子に照射し、その1s3p$$rightarrow$$1s2s脱励起過程に伴う蛍光(波長=501nm)の時間発展をガス濃度の関数としてストリークカメラにより計測した。He原子濃度が高くなるに従い、遅延時間が短くなり、パルス幅も狭くなっている様子が明確に見いだせた。観測結果の詳細な解析により、EUV-FEL光により励起されたHe原子が、原子集団として蛍光を放出する、超蛍光現象が起きていることが確認された。なお、半導体検出器による強度測定から、入射EUV-FEL光強度に対する超蛍光の変換効率は約10%と見積もられた。超蛍光の観測は、FELの利用研究としては現時点までで唯一のものである。EUV励起による超蛍光は、今回のような可視発光のみならず、1s3p$$rightarrow$$1s2脱励起によるEUV発光においても同様の振る舞いが観測されている。超蛍光は、特殊な環境下で起こる集団的発光現象であり、条件さえ整えば、XFEL励起によるX線超蛍光の観測も可能であると考えられる。SACLAの利用により、研究が大きく進展することが期待される。

口頭

EUV自由電子レーザーを用いたAr$$_2$$エキシマー発光の計測

西 亮祐*; 南 佑輝*; 山ノ井 航平*; 中里 智治*; 清水 俊彦*; 猿倉 信彦*; 岩山 洋士*; 繁政 英治*; Harries, J.; 永園 充*

no journal, , 

本研究ではSCSS自由電子レーザーを利用してArのエキシマー発光の測定を行った。実験として、高濃度のArガスに波長51nmの高強度な極端紫外レーザーを照射し、Arエキシマーを発生させた。このときの脱励起過程を真空紫外領域に対応した分光器とストリークカメラで計測したところ波長126nmのArエキシマーの発光が観測できた。この波長126nmの発光はこれまでArエキシマーの発光として広く知られているが、極端紫外レーザーを用いて励起させた例はない。このArエキシマーの発光が、極端紫外レーザーを用いても観測できたことで、極端紫外領域においてもArの光応答が見られることがわかった。今後極端紫外レーザーの強度やArガスの濃度などの条件を変化させ、この現象をより詳細に解析したい。この結果は今後の極端紫外レーザー並びにX線自由電子レーザー技術の応用・展開に寄与するものである。

口頭

Superfluorescence observed following EUV-FEL excitation of helium gas

Harries, J.; 永園 充*; 岩山 洋士*; 富樫 格*; 登野 健介*; 矢橋 牧名*; 仙波 泰徳*; 大橋 治彦*; 石川 哲也*; 繁政 英治*

no journal, , 

We are currently witnessing rapid progress in the extension of effects now commonplace with visible light lasers towards shorter and shorter wavelengths. Highlights include XFEL-pumped lasing, and electromagnetically-induced transparency. Superfluorescence shares similarities with ASE lasing, but occurs only if several specific conditions are met. We have observed and characterised superfluorescence at a wavelength of 501.7 nm following 1s1s$$rightarrow$$1s3p excitation of helium atoms by 53.7 nm free-electron laser radiation. Emitted pulse energies of up to 100 nJ (10$$^{11}$$) photons were observed, corresponding to a photon number conversion efficiency of up to 10%. We observed the peak intensity to scale as $$rho^2$$, and the pulse width to scale as $$rho^{-1}$$, where $$rho$$ is the atom number density. Emitted pulses as short as 1 ps were observed.

口頭

The Interaction of SASE EUV-FEL pulses with high density atomic gas

永園 充*; Harries, J.; 岩山 洋士*; 富樫 格*; 登野 健介*; 矢橋 牧名*; 仙波 泰徳*; 大橋 治彦*; 田中 均*; 石川 哲也*; et al.

no journal, , 

Mirroring the development of research using optical wavelength lasers following their invention, we are currently witnessing rapid progress in studies of non-linear optical phenomena and applications at shorter and shorter wavelengths, as short wavelength FEL facilities reach maturity. Much of the recent work done at the Japanese SCSS test accelerator facility showcases non-linear phenomena. Here we present recent research exploiting the characteristics of the SCSS to observe superfluorescence, "Fano profile" wavelength filtering, and plasma up-conversion.

口頭

ヘリウム2電子励起状態を使ったSASE光の単色化

Harries, J.; 岩山 洋士*; 永園 充*; 繁政 英治*

no journal, , 

Using an idea suggested for use in a self-seeding scheme, we have demonstrated an efficient filter for SASE pulses at wavelengths near 20 nm using a helium gas cell. In this wavelength region absorption occurs due to double-excitation. The cross-sections for absorption show Fano profiles, with a region of transparency. For the 2lnl' doubly-excited states the cross-section drops to exactly zero, and absorption only occurs due to Doppler broadening, which is around 0.4 meV at 298 K. Using a gas cell inserted in the beamline of the SPring-8 compact SASE source SCSS, we have demonstrated efficient monochromatisation of the SASE pulses at wavelengths corresponding to the 2lnl' states, using the 3rd FEL harmonic. Due to the absorption there is also a profound effect on the time profile of the SASE pulses, and this has also been investigated. Multi-colour filtering has also been demonstrated.

口頭

EUV-FEL励起He(1snp)からの超蛍光; ストリークカメラを用いた観測

Harries, J.; 岩山 洋士*; 永園 充*; 富樫 格*; 久間 晋*; 中嶋 享*; 繁政 英治*

no journal, , 

Following excitation of 1s3p atoms in a sample of pressures around 3000 Pa the dominant mode of decay was found to be superfluorescence at 501.6 nm. At higher densities superfluorescence on other transitions, 3d-2p at 667.8 nm and 3s-2p at 728.1 nm was also observed. Using filters in front of a streak camera, we have characterized this competitive process. Further, we have studied excitation at 52.2 nm, corresponding to 1s4p excitation. Emission at 667.8 nm emission was found to be dominant, indicating two-step cascade superfluorescence 4p-3d-2p.

口頭

Cascade and competitive superfluorescence in He studied at the EUV free-electron laser SCSS using a streak camera

Harries, J.; 岩山 洋士*; 永園 充*; 富樫 格*; 久間 晋*; 中嶋 享*; 繁政 英治*

no journal, , 

Following excitation of 1s3p atoms in a sample of pressures around 3000 Pa the dominant mode of decay was found to be superfluorescence at 501.6 nm. At higher densities superfluorescence on other transitions, 3d-2p at 667.8 nm and 3s-2p at 728.1 nm was also observed. Using filters in front of a streak camera, we have characterized this competitive process. Further, we have studied excitation at 52.2 nm, corresponding to 1s4p excitation. Emission at 667.8 nm emission was found to be dominant, indicating two-step cascade superfluorescence 4p-3d-2p.

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