Research Technology

Research Technology

Attosecond sources

Attosecond pulses

The primary focus of ELI-ALPS is the generation of the best quality XUV attosecond pulses to set new working standards in terms of pulse energy, repetition rate and photon energy. This goal is only achievable with the highest quality primary sources and expertly designed, innovative high-harmonic beamlines. ELI-ALPS will focus on two methods for generating high fluxes of attosecond pulse trains (APT) and isolated attosecond pulses (IAP) using Gas or Solid target High Harmonic Generation, GHHG and SHHG respectively. Table 5 shows the predicted performance of the ELI-ALPS attosecond secondary sources and compares them with current competitive methods.

Table 5: Comparison of the main working parameters of current XUV pulse sources and the ELI-ALPS attosecond beam lines
  Rep. rate (Hz)

Pulse Duration (fs)

Pulse energy (µJ)

Peak power (GW)

Tuning range (eV)

Synchrotrons

≥ 106

> 102

≈ 10-9*

≤ 10-9*

10-3 - 105

SASE FEL #

1-5000

30 - 300

1 - 500

0.03 - 16

28 - 295

Seeded FEL *

10

≈ 100

20 - 100

0.2 – 1

12 - 60

GHHG

103 - 105

0.07 - 0.5

< 0.01

≤ 10-3

10 - 120

 

10 - 100

0.07 - 0.5

0.1 - 10

10-3 - 1

10 - 120

GHHG HR †

105

≤ 0.5

≤ 10-3

≈ 0.002

17 - 90

GHHG SYLOS †

103

≤ 0.5

≈ 1

≥ 2

10 - 70

SHHG SYLOS †

103

≤ 1

≤ 3

≤ 3

8 - 60

 

* Estimated from peak brilliance relative to FELs. ##Values for FLASH, *Values for FERMI at ELETTRA. † Guaranteed values, improved operating values expected

Gas High Harmonic Generation – GHHG

Gas High Harmonic Generation is a well-established method to produce attosecond XUV pulses in which an IR laser pulse is focused into a gas-cell or gas-jet, filled with noble gases. However, there are numerous experimental and technical challenges which need to be overcome in order to fully harness the outstanding pulse energy and repetition rates of ELI-ALPS primary lasers. One of the major problems is the ionization of the gas target which limits the permissible peak intensity of the driving pulses.

At ELI-ALPS, four beamlines will be dedicated to GHHG beamlines: Two driven by the SYLOS laser; the others by the HR laser. All four GHHG beamlines will be available for user experiments in various studies and will also serve attosecond source research aims to accommodate to user needs and to maintain the the cutting-edge property of the technology. Both SYLOS beamlines will use loose focusing to maximize the efficiency of attosecond pulse generation resulting in beamline lengths of several tens of meters, based on optimization studies. The LONG beamline will use an extremely loose laser focusing arrangement in conjunction with a long gas cell running at a very low gas pressure whilst the COMPACT beamline also uses loose focusing but instead a higher pressure, short medium to ensure phase matching by the short interaction length. Both systems utilize multiple generation regions to increase the output power by quasi phase matching. Unconverted IR laser light in these systems will be removed by using long propagation lengths (LONG) and radial structuring of the generating laser beam (COMPACT) as well as XUV reflecting and IR transmitting silica plates.

The Phase 1 SYLOS laser configuration results in pulses which are too long for IAP generation and require a gating technique to confine the XUV generation process to a single half cycle of the driving field. However, this measure may become redundant in the second implementation phase of SYLOS (Phase 2) when the pulse duration falls below 2 optical cycles. Table 6 shows the main characteristics of the attosecond pulses generated from SYLOS.

Schematic design layout of the GHHG SYLOS COMPACT beam line.

 

The HR beamlines have modest pulse energies but the high repetition rate results in very high average power levels on the optical components. Custom optics with cooling elements have to be used which avoid CEP fluctuations as well as group delay dispersion (GDD).

Table 6: Predicted output pulse parameters for the GHHG SYLOS LONG beamline

 

 

Phase 1

Phase 2

Attosecond

 pulse

trains

Isolated 

attosecond

 pulses

Attosecond

 pulse

trains

Isolated 

attosecond 

pulses

Spectral range (eV)

17-30 eV (generating gas: xenon or krypton, aluminum filter)

Output energy at the

end station interaction 

point (pJ)

15-50

5-15

85-250

25-90

Spectral range (eV)

25-55 eV (generating gas: argon, aluminum filter)

Output energy at the

end station interaction 

point (pJ)

5-25

3-8

35-125

10-35

Spectral range (eV)

70-90 eV (generating gas: neon, zirconium filter)

Output energy at the

end station interaction 

point (pJ)

3-10

1-3

15-45

4-15

 

 

 

Surface Plasma High Harmonic Generation – SHHG

The high harmonic generation during the relativistic interaction of an intense ultrashort, high temporal contrast laser pulse with surface plasma provides a pathway to generate attosecond pulses in the reflection mode. The rising edge of an intense (I > 1017 Wcm -2) ultrashort laser pulse can form a thin and highly reflective plasma layer (Plasma mirror, PM) when interacting with an optically polished solid target. If the incident pulse is polarized in the plane of incidence, the main part of the pulse periodically drives and nonlinearly interacts with the plasma mirror. The PM induces periodic temporal spikes in the reflected light field due to periodic relativistic electronic dynamics.

Surface Plasma High Harmonic Generation – SHHG

There are currently two well-understood competing mechanisms for SHHG where their relative predominance is dependent on the driving field intensity and the sharpness of the driven plasma mirror. Coherent Wake Emission (CWE) involves surface electron bunches being pulled into the vacuum by the combined laser/plasma field then pulled back into the overdense plasma. This generates excited charge density waves across the plasma density gradient in the wake of the returning electron bunches. These plasma oscillations emit the subcycle light pulse once every optical cycle and thus both the odd and even harmonics of the incident laser beam are generated. At higher laser intensities (Iλ2 ≳ 1018 Wcm-2μm2), the driving laser field causes the electronic motion to become relativistic. The relativistic oscillation of the reflecting plasma mirror (the Relativistic Oscillating Mirror or ROM) surface leads to periodic Doppler shifts in the local reflected light field leading to the generation of phase locked ROM harmonics. Both the mechanisms have their relative advantages, leading to attopulses with complementary nature to those generated from gaseous media at substantially lower light intensities.

The complementary nature of SHHG and GHHG are shown in Table 7.

Table 7: Summary of the complementary nature of high-order harmonics generated from gaseous medium and via relativistic excitation of a plasma mirror, in the case of linearly polarized single color laser field

Properties

Gas HHG

Surface HHG

HHG spectrum

Only odd harmonics

Both odd and even harmonics

Attosecond bursts per cycle

Twice in every cycle and Ï€ out of phase

Once in every cycle and in phase

HHG Cut-off & intensity

Limited by saturation intensity. Harmonic intensity and cut-off drops after saturation intensity 

No such limit. Harmonic intensity scales up with laser intensity & harmonic cut-off either depends on the material (CWE) or increases with laser intensity (ROM).

Phase matching

GHHG occurs in transmission mode. Proper phase matching during propagation is needed for intense harmonics. Harmonic properties depends on phase matching condition 

SHHG takes place in the reflection mode. Phase matching condition is intrinsically satisfied 

Spatio-temporal coupling (STC)

HHG emission is fundamentally space-time coupled irrespective of the phase matching conditions 

There is no fundamental STC in SHHG emission 

HHG as a probe of the source

GHHG are being used to extract information about atomic, molecular and condensed matter structure and dynamics

SHHG have been used to extract information on attosecond electron dynamics in Î¼-scale plasma

 

At ELI-ALPS, there will be two beamlines developed to tap in the potential of SHHG: One driven by the SYLOS laser and the other by the HF laser. Both the beamlines would be development beamlines pushing the frontiers of state of the art technology in high intensity laser matter interaction and XUV science at relativistic intensities.

The SHHG SYLOS development beamline will be able to explore both CWE and ROM SHHG regimes by changing only the focusing and plasma conditions. A “low contrast mode” uses soft focusing to ensure that the incident laser intensity is below the ROM threshold (1018 Wcm-2) whilst a “high contrast mode” uses an additional plasma mirror to enhance the pulse contrast and this “clean” pulse is then tightly focused onto the target to reach intensities ~ 10­19 – 1020 Wcm-2. Figure 3 shows the proposed SYLOS SHHG beamline.

Figure 6: SYLOS SHHG beamline

The capability to finely tune the plasma mirror density profile and the few cycle laser phase, allows for sub cycle control of the generated attopulses. The SHHG SYLOS would also be the first SHHG beamline working at kHz repetition rate, while working at relativistic intensities.

Table 8

Attosecond pulse specs

SYLOS Phase 1

SYLOS Phase 2

APT

SAP

APT

SAP

Spectral range*

8–40 eV

6–20 eV

8–60 eV

6–40 eV

Pulse energy at the SHHG source*

1–10 µJ

0.3–3 µJ

3–30 µJ

1–10 µJ

Pulse energy at the end station interaction region*

0.3–3 µJ

0.1–1 µJ

1–10 µJ

0.3–3 µJ

Pulse duration at the end station interaction region*

< 10 fs

< 1 fs

< 5 fs

< 1 fs

Spot size at the end station interaction region

< 10 µm

< 10 µm

< 10 µm

< 10 µm

Beam divergence emitted from SHHG source

< 1 rad

Polarization state

linear, horizontal

 

* Estimations based on single-shot long-pulse high-energy laser experiments

The SHHG HF development beamline will be able to operate at on target laser intensity of 10­21 – 1022 Wcm-2 exploring the extremely relativistic domain of laser plasma interaction where the efficiency of ROM SHHG process and its spectral window scales up, potentially giving access to more intense and shorter attopulses.

The combination of subtle control of the plasma mirror density profile, state of the art feedback stabilized high repetition rate targetry and a temporally and spatially high contrast on focus PW laser allows for opportunities of unique user experiments after the development phase. The SHHG SYLOS would also be the first SHHG beamline working at 10 Hz repetition rate, while working with a PW class ultrashort laser.

Finally the facility is equipped with its own electrical-, mechanical- and optical workshop in order to manufacture adequate custom components supplying the wide variety of experiments needs.

May

25

Friday