FIG 2 - uploaded by Jean Benoit Heroux
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(a) Plot of the experimental maximum time-domain terahertz signal amplitude for InAs as a function of the photon energy. (b) Simulated result with equal hole and Γ electron kinetic energy (blue) and equal momentum (yellow). (c) Calculated Γ-band scattering rate for equal kinetic energy. The legend refers to the band to which carriers are scattered. The dashed lines positioned at the phonon-absorption thresholds show the alignment of features in (a), (b), and (c).
Source publication
The efficiency of terahertz emission by the photo-Dember effect from bare InAs, GaAs, and InSb samples as a function of the excitation-light wavelength is investigated to demonstrate that an electron-hole kinetic-energy equilibrium is rapidly reached, before the radiation peak. Numerical and experimental results show clear features that match the o...
Citations
... 1,2 Gunn diodes are already well known, and recently, energy, momentum, and time resolved studies of hot electrons have resulted in a huge wealth of information. [3][4][5][6] Further with the rise of spintronics, it is of immense interest to explore the spin resolved phenomena associated with hot electrons. However, spin dynamics of electrons with high kinetic energy is still unexplored, although immense spin-orbit coupling is predicted for high momentum (k) values of conduction band (CB) due to large Dresselhaus spin splitting. ...
Realization of Inverse Spin Hall Effect (ISHE) with hot electrons in direct bandgap semiconductors is an intriguing puzzle. Here, we report the influence of steady state carrier accumulation in satellite L valley on the establishment of ISHE in GaAs epilayer. Steady state carrier accumulation is calculated by analytically solving the rate equations where the whole conduction band is divided into four distinct regions based on the energy and momentum. Electron-hole generation, energy and spin relaxation, and various recombination mechanisms are considered in the theoretical framework where carrier accumulation in the L valley of GaAs is driven by inter-valley scattering. This is then followed by a set of experiments to measure the photo induced ISHE at excitation energy (Eex) of 1.65, 1.94 and 2.33 eV where significant differences are theoretically predicted. The measured values of ISHE signal are thereafter compared with the numerically calculated ones which establishes the validity of proposed formalism. Further, the physical origin of ISHE signal is investigated for different regimes which leads to the observation of inter-valley scattering induced ISHE at Eex =2.33 eV.
... The dielectric properties of InAs were determined by the Drude formula, which is extensively used in probing semiconductors with s-SNOM technique [4,[29][30][31]. The effect of optical pumping is modeled by the rise of carrier density N d to ~10 19 cm −3 [32]. The parameters for the Drude formula are the following: ...
In this letter, we report optical pump terahertz (THz) near-field probe (n-OPTP) and optical pump THz near-field emission (n-OPTE) experiments of graphene/InAs heterostructures. Near-field imaging contrasts between graphene and InAs using these newly developed techniques as well as spectrally integrated THz nano-imaging (THz s-SNOM) are systematically studied. We demonstrate that in the near-field regime ( λ / 6000 ), a single layer of graphene is transparent to near-IR (800 nm) optical excitation and completely “screens” the photo-induced far-infrared (THz) dynamics in its substrate (InAs). Our work reveals unique frequency-selective ultrafast dynamics probed at the near field. It also provides strong evidence that n-OPTE nanoscopy yields contrast that distinguishes single-layer graphene from its substrate.
... Most importantly, in low bandgap semiconductors, THz generation mechanisms related to carrier dynamics (surge current generation and photo-Dember emission) are known to saturate quickly as the excitation increases. A recent study [63] modelling photo-Dember carrier dynamics has attributed its saturation to the Coulomb attraction. For excitation fluences of about tens of µJ cm 2 , the THz emission is ruled by SOR. ...
... This also allowed us to exclude any relevant contribution of the photo-Dember effect, which is unaffected by a change of polarisation [76] and was not detectable for angles of minimum generation by optical rectification. In contrast to [63], where the sample was rotated to suppress the SOR, we oriented the surface and the generating pump polarisation to maximise the nonlinear conversion efficiency. ...
With the massive advantages of THz radiation and the current technical di�culties in
mind, I have chosen to undertake research into terahertz surface phenomena, which is the
focal point of my thesis. Ultrathin surface terahertz emitters have many advantages as
they have an extremely thin active region, typically hundreds of atomic layers. In this
framework, III-V semiconductors, such as InAs and InSb, have record-breaking conversion
e�ciencies per unit thickness. In addition, the phase mismatch, which commonly limits
the generation of terahertz from optical crystal, is negligible and so there is an opportunity
for enhancing the emitted bandwidth.
My thesis is born as the core of many research interests of my research lab (Emergent
Photonics), which enabled the appropriate availability of resources that made my results
possible. It also created several spin-out research lines. All the work presented is my work
(with the exception of the background research). Parts of chapters have been published
in journals and publications which see me as the �rst author.
The structure of this thesis is as follows. First I discuss optical pump recti�cation
emission, and the saturation of InAs terahertz emissions. Then I introduce my work on
terahertz enhancement emission through graphene. Finally, I present my work on an
exotic terahertz emission mechanism, namely the all-optical surface optical recti�cation
and I place my concluding remarks.
... A significant fraction of the pioneering works on THz surface emission focuses on narrow band-gap III-V semiconductors, such as InAs and InSb, which exhibit surface terahertz (THz) emission when excited with ultrashort optical pulses. In most scenarios, the generation is driven by the kinetic carrier dynamics [8][9][10] and surface field induced Optical Rectification (OR) [11][12][13], upon excitation with photon energies well above the energy bandgap [14]. ...
The interest in surface terahertz emitters lies in their extremely thin active region, typically hundreds of atomic layers, and the agile surface scalability. The ultimate limit in the achievable emission is determined by the saturation of the several different mechanisms concurring to the THz frequency conversion. Although there is a very prolific debate about the contribution of each process, surface optical rectification has been highlighted as the dominant process at high excitation, but the effective limits in the conversion are largely unknown. The current state of the art suggests that in field-induced optical rectification a maximum limit of the emission may exist and it is ruled by the photocarrier induced neutralisation of the medium's surface field. This would represent the most important impediment to the application of surface optical rectification in high-energy THz emitters. We experimentally unveil novel physical insights in the THz conversion at high excitation energies mediated by the ultrafast surface optical rectification process. The main finding is that the expected total saturation of the Terahertz emission vs pump energy does not actually occur. At high energy, the surface field region contracts towards the surface. We argue that this mechanism weakens the main saturation process, re-establishing a clearly observable quadratic dependence between the emitted THz energy and the excitation. This is relevant in enabling access to intense generation at high fluences.
Numerical simulations are carried out to estimate the Inverse spin Hall voltage (VISHE)
as a function of applied electric field, dopant density and excitation energy (Eex) for n-GaAs
based opto-spintronic devices. Adopting a three valley rate equation model, an expression is
derived for the density of spin polarized electrons (nS) accumulated in different valleys of
conduction band. It is noted that an external electric field can be used to enhance the magnitude
of VISHE significantly, however the shape of curve depends upon the choice of excitation
energy. A significant rise of VISHE is noted beyond a critical value of electric field when the
carriers are injected into Γ-valley of GaAs. On the other hand, a peak like behaviour is observed
when hot electrons are injected into Γ-valley. A dual slope behaviour of VISHE with applied
electric field is noticed when carriers are injected directly into L-valley of GaAs, where a
reasonable value of VISHE can be predicted even for a modest value of electric field. Further, a
peak like behaviour of VISHE with dopant density is predicted irrespective of the choice of
excitation energy. The optimum dopant density of n-GaAs based Inverse spin Hall devices is
found to be ~4×1016 cm-3. Theoretical predictions made in this work are critically important
for the realization of next generation Inverse spin Hall devices involving L-valley electrons.
Time jitter of GaAs photoconductive semiconductor switches (PCSS) is investigated at an optical excitation of 1053 nm wavelength and 500 ps pulse duration. The experimental results indicate that the time jitter of the GaAs PCSS exhibits a nonmonotonic variation in negative differential mobility (NDM) region. All of these time jitters are lower than the 4% of the rise time of the switching waveform. The optimum time jitter of ~15 ps is achieved at the onset of the NDM region. The theoretical relationship between the optical excitation parameters, the bias electric field and the time jitter of the GaAs PCSS are built up. The nonmonotonic behavior of the time jitter with electric field is attributed to the instability of the relative fluctuation of drift velocity caused by inter-valley transition of carriers in GaAs.
We introduce a method for diagnosing the electric surface potential of a semiconductor based on THz surface generation. In our scheme, that we name Optical Pump Rectification Emission, a THz field is generated directly on the surface via surface optical rectification of an ultrashort pulse after which the DC surface potential is screened with a second optical pump pulse. As the THz generation directly relates to the surface potential arising from the surface states, we can then observe the temporal dynamics of the static surface field induced by the screening effect of the photo-carriers. Such an approach is potentially insensitive to bulk carrier dynamics and does not require special illumination geometries.
