Angle-resolved photoemission spectroscopy (ARPES) reveals band structures [1], and its time-resolved variant (TARPES) enables observation of nonequilibrium states using femtosecond lasers [2]. While $\sim$ 6 eV light from BBO crystals limits energy access, high harmonic generation (HHG) extends reach up to 70 eV, allowing studies of large momentum space as well as valence and core levels. Yet, NIR pump pulses cause broad excitations and heating, obscuring pure electronic dynamics. To address this, we developed a wavelength-tunable TARPES system as shown in Fig. 1.

Our setup employs twin Ti:sapphire amplifiers (800 nm, 35 fs, 0.7 mJ, 10 kHz) seeded by a shared oscillator to minimize timing jitter. One drives the pump, the other the probe. SHG at 3.1 eV is used to generate HHG in Ar gas (4 mm, 3-5 Torr), with a stabilizer maintaining beam alignment. The 7th harmonic (21.7 eV) is filtered and directed via MLM mirrors with differential pumping (10^-3 Torr → 10^-8 Torr), yielding $\sim$10⁹ photons/s. A He lamp checks sample quality.

The pump beam is wavelength-tunable (1160-2580 nm) via OPA, with pulse profile monitored by autocorrelator and fluence/polarization adjusted using waveplates and attenuator. An optional 800 nm source is also available. Photoelectrons are analyzed with a Scienta Omicron R4000 under 2$\times$10^-11 Torr vacuum, and temperature controlled down to 7 K via cryostat.

We validated our system using highly oriented pyrolytic graphite crystals with a fixed probe energy of 21.7 eV and pump photons ranging from 0.52 eV (2400 nm) to 1.03 eV (1200 nm), plus an 800 nm reference.

Figures 2(a) and 2(b) show the differential TARPES images excited by 0.52 and 1.03 eV pulses, respectively, by subtracting the image before the arrival of the pump. The delay time is set to be the largest change, corresponding to 160 and 80 fs for Figs. 2(a) and 2(b), respectively. Since the Dirac band has a linear dispersion symmetric about the Dirac point ($E = E_\mathrm{D}$), the electrons are excited from $E_{\mathrm{D}} - 1/2 h\omega$ to $E_{\mathrm{D}} + 1/2 h\omega$ by a pump with energy $h\omega$. To track the initial dynamics after photoexcitation, we highlight the photoexcited electrons at $E = E_\mathrm{D} + 1/2 h\omega$, which correspond to the $E - E_\mathrm{F} =$ 0.25 and 0.5 eV for 0.52 and 1.03 eV pumps, respectively.

By employing wavelength-tunable excitation, it becomes possible to achieve resonant and non-resonant excitations in intriguing quantum materials. Consequently, our developed system opens up the potential for discovering novel non-equilibrium phenomena in the future

References

• [1] A. Damascelli, A. Hussain, and Z. -X. When, Rev. Mod. Phys. 75, 473 (2003).
• [2] T. Suzuki, S. Shin, and K. Okazaki, J. Electron Spectrosc. Relat. Phenom. 251, 1474105 (2021).

Authors

• T. Susuzki, Y. Zhong, K. Liu, T. Kanai, J. Itatani, and K. Okazaki