Jessica Afalla

University of Tsukuba · Graduate School of Pure and Applied Sciences

Terahertz (THz) time-domain emission spectroscopy was performed on layered 2H-MoSe2 and 2H-WSe2. The THz emission shows an initial cycle attributed to surge currents and is followed by oscillations attributed to coherent interlayer phonon modes. To obtain the frequencies of the interlayer vibrations, analysis of the THz emission waveforms were perf...

Terahertz (THz) time-domain emission spectroscopy was performed on layered 2H-MoSe 2 and 2H-WSe 2. The THz emission shows an initial cycle attributed to surge currents and is followed by oscillations attributed to coherent interlayer phonon modes. To obtain the frequencies of the interlayer vibrations, an analysis of the THz emission waveforms was...

Terahertz emission from ferromagnetic/non-magnetic spintronic heterostructures had been demonstrated as pump wavelength-independent. We report, however, the pump wavelength dependence of terahertz emission from an optimized Fe/Pt spintronic bilayer on MgO substrate. Maximum terahertz generation per total pump power was observed in the 1200- to 1800...

We explore non-equilibrium states in a two-dimensional transition-metal dichalcogenide under periodical phonon driving. A time-frequency analysis reveals strong coupling between electrons and a long-lived coherent phonon, leading to phonon-combination modes in the early time region.

Terahertz (THz) time domain emission spectroscopy was performed on bulk single crystal MoSe 2 and WSe 2 . Results show THz emission signals which comprise of a single cycle transient current-driven signal and damped oscillatory signals arising from coherent phonon modes. The damped oscillations have frequencies below 1 THz and are attributed to int...

We demonstrate that terahertz radiation can be generated from titanium-diffused magnesium oxide-doped lithium niobate optical strip waveguides when pumped by a single laser source with a wavelength of 1.55 μm. Titanium-in-diffusion was performed on a magnesium oxide-doped lithium niobate substrate by the deposition of titanium in a 40-µm wide strip...

Epitaxial low temperature grown GaAs (LT-GaAs) on silicon (LT-GaAs/Si) has the potential for terahertz (THz) photoconductive antenna applications. However, crystalline, optical and electrical properties of heteroepitaxial grown LT-GaAs/Si can be very different from those grown on semi-insulating GaAs substrates (reference). In this study, we invest...

Spintronic THz emission from Ni/Pt bilayer grown on MgO is reported based on the novel THz emitter using metallic structures. The Ni metal was deposited first on a MgO substrate and capped with a thin Pt metal via electron beam deposition. The THz emission data was obtained using a standard terahertz time-domain spectroscopy setup using a Ti: sapph...

Epitaxial low temperature grown GaAs (LT-GaAs) on silicon (LT-GaAs/Si) has the potential for terahertz (THz) photoconductive antenna applications. However, crystalline, optical and electrical properties of heteroepitaxial grown LT-GaAs/Si can be very different from those grown on semi-insulating GaAs substrates ("reference"). In this study, we inves...

We report on the continuing efforts of the National Institute of Physics, University of the Philippines, in the design, growth, and fabrication of novel structures for THz photoconductive antennas (PCA’s) [1-3]. In particular, the talk presents recent works on the molecular beam epitaxial growth of gallium arsenide-based semiconductor trilayer film...

We present the implementation of an efficient terahertz (THz) photoconductive antenna (PCA) emitter design that utilizes high mobility carriers in the two-dimensional electron gas (2DEG) of a modulation-doped heterostructure (MDH). The PCA design is fabricated with recessed metal electrodes in direct contact with the 2DEG region of the MDH. We comp...

We present threefold enhancement of terahertz emission from silicon nanowire (SiNW)-coated gallium-arsenide photoconductive antenna over its uncoated counterpart. The enhancement is attributed to the increased photoabsorption, and possibly additional photoconductive pathways induced by the SiNWs.

***This is a peer-reviewed paper published in the JOURNAL OF THE JAPAN SOCIETY OF INFRARED SCIENCE AND TECHNOLOGY, Vol. 29, Issue No. 2, pp. 57-62 (February 2020).*** A spintronic bilayer structure, which consists of 2-nm Fe and 3-nm Pt epitaxial grown on 500-µm MgO substrate, was investigated as a terahertz (THz) source for different optical pump...

Illuminating metallic spintronic heterostructures with ultrafast laser pulses can generate terahertz (THz) radiation due to spin current-to-charge current conversion. Such metallic spintronic heterostructures are essentially ultrathin ferromagnetic and non-ferromagnetic metallic layers stacked on appropriate substrates. They are particularly attrac...

Carrier dynamics and photoconductivity in epitaxial-grown low-temperature GaAs on nominal and vicinal Si(1 0 0) substrates ('LT-GaAs/Si') were studied to predict their actual performance as THz photoconductive antenna (PCA) detectors. An optical-pump terahertz-probe technique was used to obtain the transmittance, carrier lifetime and photoconductiv...

Spintronic heterostructures are emerging and interesting emitters of terahertz (THz) radiation. In the past, THz emission of spintronic layers is by an antenna-free approach. In this paper, we report on THz wave generation of a diabolo-shaped Fe/Pt spintronic antenna using a 780-nm wavelength pump beam from a femtosecond laser. THz emission amplitu...

Terahertz (THz) generation of spintronic heterostructures is becoming popular due to broader THz bandwidth with tunable emission amplitude and polarization [1-2]. These heterostructures typically consist of stacks of ferromagnetic (FM) and nonmagnetic (NM) thin films which generate THz radiation either by the inverse spin-Hall [3] or the interface...

Metallic spintronic heterostructures consisting of ultrathin ferromagnetic (FM) and non-ferromagnetic (NM) layers emit THz radiation through a distinct and efficient THz generation mechanism, which arises from the conversion of spin current generated in the FM layers into transverse charge current in the NM layers [1, 2]. We have previously demonst...

We present concurrent measurement of sample and reference terahertz waveforms for an optical-pump terahertz-probe spectrometer, using a controlled optical diaphragm shutter for the optical pump line. When waveforms are taken consecutively, laser power fluctuations and other experimental conditions can introduce spectral artefacts, thus a concurrent...

Spintronic heterostructures are emerging and promising THz sources due to their considerable advantages over other conventional THz emitters. In a recent study [6], an Fe/Pt spintronic bilayer structure was observed to be an efficient THz source when using either 800-nm or 1550-nm pump wavelength. In this paper, we performed terahertz (THz)-time do...

An externally applied magnetic field was used to induce increased photocarrier transport along the high mobility channel in GaAs/AlGaAs modulation-doped heterostructures (MDH). The terahertz (THz) emission from GaAs/AlGaAs MDH increases with increasing magnetic field, applied parallel to the heterojunction. The THz emission enhancement factors due...

Spintronic heterostructures consisting of layers of ferromagnetic (FM) and nonmagnetic (NM) metals, are promising THz sources. In a previous study [5], an Fe/Pt spintronic bilayer structure was observed to be an effective THz radiation source when using either 800-nm or 1.55-μm excitation wavelengths. In this paper, excitation wavelengths of 400-nm...

The carrier lifetimes and electron mobility values were estimated for 2-μm-thick GaAs films grown on Si (100) substrates by means of optical pump terahertz probe (OPTP) technique. The GaAs/Si films measured were epitaxial grown at different substrate temperatures (TS = 520˚C or TS = 630˚C). From X-ray diffraction measurements and Raman spectroscopy...

This study presents the terahertz (THz) emission of molecular beam epitaxy (MBE)-grown Gallium Arsenide (GaAs) on surface textured p-type Silicon (p- Si) (100) substrates. Surface texturing was achieved by anisotropic wet chemical etching using 5% wt Potassium Hydroxide (KOH): Isopropyl alcohol (IPA) (50:1) solution for 15, 30, 45, and 60 minutes....

In this work, we will demonstrate that an optimized spintronic bilayer of iron and platinum thin films is a versatile and convenient source of terahertz (THz) radiation. We will discuss the mechanism for spintronic THz emission and report efficient THz generation using 1550-nm (communication wavelength) optical pump pulses from a femtosecond fiber...

The terahertz time-domain spectroscopy (THz-TDS) imaging is inherently the hyperspectral technique with sliced imaging in frequency/time domains. It has a promising potential for non-destructive testing (NDT) of various materials. However, the associated large image data volumes and complex image contrasts could make the analysis and interpretation...

Spintronic heterostructures consisting of ferromagnetic and non-magnetic metal thin films have been demonstrated as novel and highly promising sources of broadband THz radiation. Such spintronic THz emitters had only been intensively investigated so far at 800-nm excitation wavelength using femtosecond Ti:sapphire lasers. In this work, we demonstra...

Spintronic heterostructures consisting of ferromagnetic (FM) and non-magnetic (NM) metal thin films have been demonstrated as promising sources of THz radiation. However, such spintronic THz emitters had only been investigated so far at 800-nm excitation wavelength using femtosecond Ti:sapphire lasers. In this work, we report experimental proofs of...

In the previous report [1], we have successfully demonstrated that the electro-optic (EO) sampling signal, ΔI/I (optical intensity modulation due to EO effect normalized by the optical probe beam intensity), can be enhanced by “polarization filtering”.

As a results of collaborative work, this poster gives details on spintronic metal bilayers as efficient terahertz sources. Two different wavelengths were used, showing that regardless of excitation wavelength, spintronic emitters still operate at higher efficiency than normal PCAs.

Simultaneous molecular beam epitaxial growth of GaAs/AlGaAs multiple quantum wells on two different substrates (one on GaAs (100) and another on a GaAs substrate misoriented by 4° in the (111) direction) resulted in samples of similar structure, but having different defect profiles. The on-axis sample had a higher defect density and more types of e...

Two asymmetric double quantum wells of different coupling strengths (barrier widths) were grown via molecular beam epitaxy, both samples allowing tunneling. Photoluminescence was measured at 10 and 300 K to provide evidence of tunneling, barrier dependence, and structural uniformity. Carrier dynamics at room temperature was investigated by optical...

A typical photoconductive antenna (PCA)-based terahertz time-domain spectroscopy (THz-TDS) system has a mode-locked Ti: sapphire laser with a 100-MHz repetition rate as the pulsed excitation source. When an average probe beam power of 10 mW is focused to a 10-μm diameter on a semiconductor substrate, it yields an estimated fluence of around 130 μJ/...

The principle of coherent Raman spectroscopy in terahertz (THz) frequency region using frequency-chirped optical pulses is explained. The examples of THz Raman spectroscopy for solid state (-GaSe crystal) and liquid state materials (CCl4 , DMSO/water mixture, aqueous solution of ZnBr2 ) are demonstrated. The advantage of this technique over the co...

We have demonstrated experimental evidence of non-resonant tunneling at room temperature in GaAs/AlGaAs asymmetric coupled double quantum wells (ACDQW’s) using time resolved photoluminescence (TRPL) spectroscopy at 300 K. Two ACDQW samples (A and B) with a barrier thickness of 25Å were grown via molecular beam epitaxy. The energy separation (ΔE) be...

In this work, we have studied the temperature-dependent conductivity of graphene oxide (GO) and graphene oxide-polyaniline (GO-PANI) nanocomposites by terahertz time-domain spectroscopy (THz-TDS) from 78 K to 293 K. The refractive index and absorption coefficient are related to the conductivity, and it has been found that in the THz region, the rea...

A one-order-of-magnitude terahertz (THz) emission enhancement in the transmission geometry, over a 0.7-THz broadband range, was observed in semi-insulating gallium arsenide (SI-GaAs) integrated with a subwavelength one-dimensional metal line array (1DMLA). THz emission of the 1DMLA samples showed an intensity increase and exhibited a full-width-at-...

GaAs/Al 0.1 Ga 0.9 As core-shell nanowires (CSNWs), with average lateral size of 125 nm, were grown on gold nanoparticle-activated Si (100) and Si (111) substrates via molecular beam epitaxy. Room temperature-photoluminescence (RT-PL) from the samples showed bulk-like GaAs and Al 0.1 Ga 0.9 As bandgap emission peaks at 1.43 and 1.56 eV, respectivel...

The Cardona equation describing the temperature behavior of a bulk semiconductor is utilized to fit experimental photoluminescence data for both strained and unstrained quantum wells (QWs). To account for confinement energy and strain energy, the Cardona equation was modified to include the respective energy offsets. Results of modeling experimenta...

Increased terahertz (THz) emission of surface modified semi-insulating gallium arsenide (SI-GaAs) is reported. A metal line array with sub-wavelength spacing was fabricated via e-beam deposition and THz measurements were done in the transmission excitation geometry. Results show an order of magnitude THz broadband emission enhancement. Pump-power d...

We present experimental demonstration of photocarrier dynamics in InAs quantum dots (QDs) via terahertz (THz) time-domain spectroscopy (TDS) using two excitation wavelengths and observing the magnetic field polarity characteristics of the THz signal. The InAs QDs was grown using standard Stranski-Krastanow technique on semi-insulating GaAs substrat...

We report on the shell-to-core carrier-transfer in GaAs/Al0.1Ga0.9As core-shell nanowires grown on Si(1 0 0) substrates via molecular beam epitaxy. The nanowires are dominantly zincblende and are tilted with respect to the substrate surface. Photoluminescence (PL) excitation spectrosocopy at 77 K revealed an abrupt increase in the GaAs PL intensity...

Tensile and compressive strains via epitaxial lift‐off (ELO) techniques were applied on single‐layer InAs/GaAs quantum dots (QDs). At low temperatures, due to the difference in thermal expansion coefficients of the ELO film and host substrate, the ELO QDs film bonded to Si and MgO substrates experienced tensile and compressive strain, respectively....

Indium oxide (In2O3) films grown by thermal oxidation on MgO substrates were optically excited by femtosecond laser pulses having photon energy lower than the In2O3 bandgap. Terahertz (THz) pulse emission was observed using time domain spectroscopy. Results show that THz emission saturates at an excitation fluence of ~400 nJ/cm2. Even as two-photon...

A low field AC magnetic susceptibility has been measured for a superconducting bulk YBa2Cu3O7-δ sample with an AC excitation field superimposed with a DC field. The effects on the susceptibility due to either type of field have been interpreted without any assumption regarding the presence of vortices in the material. From the in-phase susceptibili...