U LTRAFAST PUMP PROBE SPECTROSCOPY OF GRAPHENE

3. RELAXATION DYNAMICS OF GRAPHENE ON DIFFERENT SUBSTRATES

3.2. U LTRAFAST PUMP PROBE SPECTROSCOPY OF GRAPHENE

Since 2007, many different pump probe studies have been reported on single and multi- layer epitaxial, exfoliated and CVD graphene which sometimes resulted in contradictory arguments. It is understood that upon photoexcitation, the interaction of photons with hot carriers in graphene create an out of equilibrium distribution. The strong Coulomb induced interactions and the subsequent electron-electron collisions, result in an evolution towards a hot Fermi-Dirac distribution on a time window of tens of fs.[96][97][98][99] This quasi- thermal distribution subsequently relaxes to lower energies (mainly through phonon coupled channels), on a longer picosecond time scale.

The less explored, sub 100 𝑓𝑠 non- equilibrium dynamics of graphene was experimentally studied by Brida et al. [96] where the role of Auger recombination and carrier Figure 3.1. Time-evolution of the electron population per unit cell (in units of eV ^-1) in sub picosecond time scale.[96]

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multiplication in the thermalization process was highlighted. Due to the very fast Auger processes that bridge the conduction and valance bands on a very short time scale, the separation of chemical potentials for electrons and holes is considered insignificant in our measurements. [96][100]

Figure 3.1 shows the thermalization process for an extremely hot electron distribution (red curve) and the shifting of the electron population to lower energies as time evolves. One can see that the initial narrow hot distribution, broadens into a non-thermal distribution by 10 𝑓𝑠 (black curve), which then thermalizes into a Fermi Dirac distribution (green curve) and by 500 𝑓𝑠 starts to cool further down by coupling to phonons.

A uniform positive differential transient transmission (DTT) or a uniform decrease in absorption was first seen in epitaxial graphene by Dawlaty et al.[101] and ever since, has been reported by a number of different groups. [102][103][104] This positive peak in transmission, is attributed to the Pauli Blocking (PB) or bleaching of the interband transitions, where the time evolving hot carrier distribution discussed above, inhibits the absorption of the probe beam due to the band filling effects. As the distribution cools down, the peak of the distribution moves to lower energies and the absorption of the probe beam increases. [101][102][96]. Monitoring the femtosecond time evolution of the transmission response of graphene, enables us to directly measure the carrier density and carrier distribution as a function of time.

The cooling mechanisms for the carriers of graphene after photoexcitation, have been studied by many different groups. Wang et al.[102] studied the hot optical phonon dynamics and the different carrier cooling channels for multi-layer graphene grown on silicon face of SiC, carbon face of SiC and CVD graphene grown on nickel and transferred

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on quartz substrate. A biexponential decay was observed for all different samples, where the dynamics were explained based on the interband transition of the carriers and the subsequent decay of hot optical phonons into two acoustic phonons. As discussed earlier, during the rapid electron-electron scattering events, photo generated carriers thermalize within themselves on a very short timescale. [96][105] The thermalized carrier distribution then cools down through generation of optical phonons. This thermalized electron-optical phonon bath, subsequently cools down through lower energy acoustic phonons which presents a bottle neck for the energy relaxation and is apparent in the slower decay component of the dynamics.

The optical phonon emission rates: 𝑅_𝛤𝑒(𝑅_𝐾𝑒) due to the intraband intravalley (intraband intervalley) electron-optical phonon scattering at 𝛤 (𝐾, 𝐾’ ) points can be defined as: [106]

𝑅_𝛤e ≈ 9 (𝑑𝑡/𝑑𝑏)²

𝜋𝜌𝜔_𝛤ℏ⁴𝑣⁴∫ 𝑑𝐸 𝐸(𝐸 − ℏ𝜔_𝛤)

∞ ℏ𝜔𝛤

× {𝑓_c(𝐸)[1 − 𝑓_c(𝐸 − ℏ𝜔_𝛤)](1 + 𝑛_𝛤) − 𝑓_c(𝐸

− ℏ𝜔_𝛤)[1 − 𝑓_c(𝐸)]𝑛_𝛤}.

Where , ρ ∼ 7.6 × 10⁻⁷ kg/m² is the density of graphene 𝜔_𝛤 and 𝑛_𝛤 are the frequency and occupation number of the 𝛤 point phonons, 𝑑𝑡/𝑑𝑏 ~ 45eV/nm, [106] 𝜐 is the Fermi velocity and 𝑓_c(𝐸) is the Fermi Dirac distribution for electrons. The carrier temperature and the optical phonon occupation numbers can be described as a coupled rate equation system:

𝑑𝑇_c

𝑑𝑡 = −(𝑅_𝛤e+ 𝑅_𝛤h)ℏ𝜔_𝛤+ (𝑅_Ke+ 𝑅_Kh)ℏ𝜔_K 𝐶_e+ 𝐶_h , 𝑑𝑛_𝛤

𝑑𝑡 = 𝑅_𝛤e+ 𝑅_𝛤h

𝑀_𝛤 −𝑛_𝛤− 𝑛_𝛤^𝑜 𝜏_ph ,

Eq 3.1

Eq 3.2

63 𝑑𝑛_K

𝑑𝑡 =𝑅_Ke+ 𝑅_Kh

𝑀_K −𝑛_K− 𝑛_K^𝑜 𝜏_ph .

Where 𝐶_𝑒 and 𝐶_ℎ are heat capacities of electrons and holes, 𝜏_𝑝ℎ is the average lifetime of the optical phonons, 𝑛_K^𝑜 and 𝑛_Γ^𝑜 are phonon occupation numbers in equilibrium and 𝑀_𝐾 and 𝑀_𝛤 are the number of phonon modes per unit area that are involved in carrier-phonon scattering that can be estimated by methods presented in [102][107].

Knowing the carrier distribution and the temperature as a function of time, one can find the transient transmission response of graphene using the expression derived in [101]. Fitting the data to the described quantitative model, with two fitting parameters , 𝜏_𝑝ℎand 𝑛₀ ,

Wang et al. [102] find a fluence and substrate independent optical phonon life time

~ 2.5 𝑝𝑠. This substrate and growth independence is contrary to our observation which we will present shortly. This was also contrary to the authors’ expectations where they expected that the surface optical phonons of quartz and SiC would affect the optical phonon lifetimes and carrier relaxation times of graphene. They conjecture that this independence might be due to the fact that the samples used in this study were not single layer graphene.

Furthermore, we believe that the lack of attention to the surface roughness as a critical and non-negligible parameter in the any pump probe experiment, can have a substantial impact on one’s ability to resolve substrate coupled energy relaxation pathways.

Until 2011, most pump probe studies of graphene reported a bleaching of interband transitions presented as a uniform positive DTT or a uniform negative DTT (mostly seen in Terahertz studies).[102], [108]–[110] In a study by Malard et al., [111] it was shown that at relatively lower fluences, both a negative and a positive peak can appear. This

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fluence dependent study highlights the fact that the dynamics of graphene can only be described accurately with consideration of both inter and intraband contributions.

The optical response of graphene is known to originate from two distinct processes: inter band transitions and intra band transitions. [27][31][95] The previously discussed bleaching of interband transitions due to the band filling dynamics or Pauli blocking, results in an increase in transmission at very high electronic temperatures where a substantial filling of states can be induced. The intraband contribution induces a decrease in transmission through free carrier absorption and can be more dominant at lower carrier temperatures. The optical response of graphene arises from both inter and intraband contributions to the optical conductivity:

∆𝜎 _{𝑡𝑜𝑡𝑎𝑙} = ∆𝜎 _{𝑖𝑛𝑡𝑟𝑎} + ∆𝜎 _{𝑖𝑛𝑡𝑒𝑟}

Where,[112]

Re(𝜎_{𝑖𝑛𝑡𝑟𝑎}(𝜔)) =4𝑘_𝐵𝑇𝜎₀

𝜋ℏ [ln (1 + 𝑒^{𝐸𝐹ℎ/𝑘𝐵𝑇}) + ln(1 + 𝑒^{𝐸𝐹𝑒/𝑘𝐵𝑇})] 𝛤 𝜔² + 𝛤²

Re(𝜎_{𝑖𝑛𝑡𝑒𝑟}(𝜔)) =^𝜎0

2 [tanh (^{ℏ𝜔−2𝐸𝐹ℎ}

4𝑘𝐵𝑇 ) + tanh (^{ℏ𝜔−2𝐸𝐹𝑒}

4𝑘𝐵𝑇 )]

With the universal conductivity 𝜎₀ = 𝑒²/(4ℏ) and 𝛤 being the Drude scattering rate and 𝐸_𝐹 the Fermi energy. The separation of chemical potentials for electrons and holes is considered to be insignificant due to the very fast Auger processes that connect the conduction and valance bands on a very short time scale.[96][100]

The photo induced change in 𝜎_{𝑖𝑛𝑡𝑟𝑎} is expected to be positive and scale linearly with T, this is attributable to the free carrier absorption due to the presence of additional carriers Eq 3.3

Eq 3.4

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excited to the conduction band of graphene by the pump beam. Whereas, the change in 𝜎_{𝑖𝑛𝑡𝑒𝑟} will be negative due to Pauli blocking and the induced band filling dynamics. As seen in Figure 3.2 below, at lower fluences (carrier temperatures), the optical response of graphene, reflects the changes in the intraband dynamics and is positive in sign, whereas at higher fluences, it is overwhelmed by the interband contribution and has a negative sign.

In the next section, we will present the fluence and excitation energy dependent dynamics of graphene transferred on quartz and discuss the inter vs intraband contribution strength.

Figure 3.2. Simulated change in the transient transmission of graphene as a function of electronic temperature for a pumping energy of 1.6 eV and an intrinsic carrier density of 1x10¹² /cm². The blue and red curves show the expected contributions from intraband and interband respectively and the black curve shows the total interband+intraband response.

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