Maria Herminia Balgos | University of the Philippines (original) (raw)

Papers by Maria Herminia Balgos

Applied Optics, Oct 25, 2023

Review of Scientific Instruments, Apr 1, 2023

We report a fast pump modulation scheme in a terahertz time domain spectroscopy (THz-TDS) system ... more We report a fast pump modulation scheme in a terahertz time domain spectroscopy (THz-TDS) system by utilizing the intensity modulation from the megahertz repetition rate of the pump pulse for lock-in detection. In conventional THz-TDS, the modulation required for the high signal-to-noise ratio lock-in detection is achieved through the use of an optical chopper or an AC bias. Here, we propose the use of an electrooptic modulator (EOM), operated as a pulse picker, to vary the repetition rate of the pump pulse, relative to the probe pulse, allowing us to directly use the megahertz laser repetition rate as the reference modulation frequency for lock-in detection. Our proposed scheme is applicable to all types of pulsed THz emitters, including those that cannot be electronically biased. Since the maximum allowable modulation frequency is limited only by the laser repetition rate and/or by the bandwidth of the EOM, megahertz modulation rates, and, consequently, rapid data acquisition times, become possible. Using our technique, we were able to detect an oscillating signal with frequencies up to 10 kHz, using ∼1 μs integration time per point, ∼100× faster than previously reported values for THz-TDS systems.

Proceedings of the Samahang Pisika ng Pilipinas, Oct 22, 2012

Terahertz (THz) emission was demonstrated from silicon (Si) wafers deposited with zinc oxide (ZnO... more Terahertz (THz) emission was demonstrated from silicon (Si) wafers deposited with zinc oxide (ZnO) films for the first time. The ZnO films were prepared by the oxidation of pulsed laser deposited zinc (Zn) thin films. The THz emission of Si was enhanced by the underlying ZnO layer regardless of the oxidation time and film microstructure. We offer an explanation of the enhanced emission mechanism in the basis of charge carrier acceleration due to large surface fields at the ZnO/Si interface.

Using molecular beam epitaxy, we study self-assembled InAs quantum dots (QDs) grown on GaAs (001)... more Using molecular beam epitaxy, we study self-assembled InAs quantum dots (QDs) grown on GaAs (001) substrates by lowtemperature scanning tunneling microscopy. We compare two different growth processes, the well-known Stranski-Krastanow mode and the more recently studied droplet epitaxy mode. We show that, for the same amount of deposited indium, the shape of the dots shows similar anisotropic tendency, but the InAs coverage dependence of the density shows that the growth mechanisms are different at the initial stage of the QD formation. By scanning tunneling spectroscopy at low temperature, we succeed in mapping the local density of states (LDOS) of a single quantum dot grown by the droplet epitaxy. Maps of LDOS are consistent with those previously reported for the Stranski-Krastanow mode; however, energy spectrum of our QDs shows different peak structures.

Journal of Applied Physics, Dec 19, 2019

Probing nanoscale defects and wrinkles in MoS 2 by tip-enhanced Raman spectroscopic imaging

Applied Surface Science, May 1, 2020

Abstract We reveal the As-antisite (AsGa) defect close to the surface of GaAs(1 1 0) with bulk ch... more Abstract We reveal the As-antisite (AsGa) defect close to the surface of GaAs(1 1 0) with bulk characteristics using first-principles methods with experimental verifications. We found that the AsGa in the third-layer mimics the geometry, partial charge density and more importantly, the density of states of AsGa in bulk GaAs. Notably, the mid-gap state induced by AsGa in bulk GaAs is well-reproduced by the AsGa in the third layer of GaAs(1 1 0). Simulated and experimental STM images show an “asymmetric two-lobe” feature in the region around the defect. Using local density of states (LDOS) and STS spectra, we propose three prominent peaks with characteristic energy levels corresponding to the third layer AsGa. The above results present the first report of surface electronic signatures of true bulk defect near the surface of GaAs(1 1 0).

Science of Advanced Materials, Feb 1, 2017

Proceedings of the Samahang Pisika ng Pilipinas, Oct 24, 2011

We modify the rate equations model for the time resolved spectra of GaAs/AlGaAs asymmetric couple... more We modify the rate equations model for the time resolved spectra of GaAs/AlGaAs asymmetric coupled quantum wells (ACQW) to include tunneling effects. The modification takes into account the stoppage of tunneling after a certain maximum number of carriers tunnel from the narrow well to the wide well, which is a direct consequence of effective mass filtering. To check the validity of the modification, we apply the model to the TRPL spectra of GaAs/AlGaAs ACQW. Results show a good fit between the experimental data and the model. The model also gives values of the tunneling and recombination time constants which are of the same order as what is expected for the structures investigated.

Journal of Physics and Its Applications, Dec 14, 2018

This study presents the terahertz (THz) emission of molecular beam epitaxy (MBE)-grown Gallium Ar... more 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. Reflectivity measurements for the textured p-Si(100) substrates indicated that the overall texturing increases with longer etching times. Raman spectroscopy of the post-growth samples confirmed GaAs growth. The THz emission intensities were the same order of magnitude. The GaAs grown on p-Si(100) textured for 60 minutes exhibited the most intense THz emission attributed to the increased absorption from a larger surface-to-volume ratio due to surface texturing. All GaAs on textured p-Si(100) samples had frequency bandwidth of ~2.5 THz.

Superlattices and Microstructures, Sep 1, 2017

We have demonstrated experimental evidence of non-resonant tunneling at room temperature in GaAs/... more 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 (∆E) between the ground state of the conduction band of the wide well and that of the narrow well are 42.7 meV and 19.5 meV, for samples A and B respectively. The TRPL measurement revealed a double decay rate in sample A whose ∆E is greater than one GaAs LO phonon energy (36meV), suggesting a phonon assisted tunneling mechanism. The evidence of tunneling was supported by measuring the relative intensity of the PL contributions from the narrow and wide well at 10K.

Proceedings of the Samahang Pisika ng Pilipinas, Oct 24, 2011

The temperature dependence of the photoluminescence (PL) spectra of an MBE- grown GaAs/AlGaAs sin... more The temperature dependence of the photoluminescence (PL) spectra of an MBE- grown GaAs/AlGaAs single quantum well system is presented in this study. The sample consists of three independent single wells with well widths 34 A, 51 A and 103A grown on a single sample. The behavior of transition energies with temperature follows the Varshni equation for semiconductor bandgaps with an additional offset contributed by the confinement energies of the quantum wells. To reveal recombination efficiency for the wells, the electron-hole wavefunction overlap is computed, revealing that as well width is increased, the recombination efficiency consequently decreases. The PL spectra also shows defect-related transitions which disappear above 50K.

PHYSICS OF SEMICONDUCTORS: 30th International Conference on the Physics of Semiconductors, 2011

ABSTRACT Gold (Au) nanoparticle‐catalyzed GaAs‐AlGaAs core‐shell nanowires were grown by molecula... more ABSTRACT Gold (Au) nanoparticle‐catalyzed GaAs‐AlGaAs core‐shell nanowires were grown by molecular beam epitaxy on Si (111) subtrates. The Au nanoparticles were pre‐patterned using an anodized aluminum oxide template. Scanning electron microscopy reveals nanowire diameters of 130–220 nm, lengths of approximately 6.5 μm and density of 2×108 cm−2. Photoluminescence measurement of the core‐shell nanowires reveals that they are of good optical quality.

Physica Status Solidi B-basic Solid State Physics, Mar 31, 2022

Journal of Materials Science: Materials in Electronics, May 28, 2018

The deposition of n-GaAs cap on low-temperature GaAs (LT-GaAs) improved the THz emission of LT-Ga... more The deposition of n-GaAs cap on low-temperature GaAs (LT-GaAs) improved the THz emission of LT-GaAs grown at a much lower temperature (< 300 • C), where the defect density is high, without compromising the spectral bandwidth and carrier lifetimes necessary for ultrafast THz detection. The LT-GaAs grown at 220 and 270 • C showed a 192 and 10% enhancement THz emission peak-to-peak intensity, respectively, while the sample grown at 310 • C showed a 49% reduction. The n-GaAs cap reduced the As-related defects density in the LT-GaAs resulting to improved THz emission. The THz emission from the sample grown at 310 • C with already low defect density suffered possibly due to the free carrier absorption by the n-GaAs cap. The results are relevant in future material design of LT-GaAs based photoconductive antenna.

Proceedings of the Samahang Pisika ng Pilipinas, Oct 25, 2010

The temperature dependence of photoluminescence (PL) spectra of Er-Tm codoped calcium boroalumina... more The temperature dependence of photoluminescence (PL) spectra of Er-Tm codoped calcium boroaluminate (CABAL) glasses with different dopant concentrations was investigated under 15-298 K, by pumping at 795 nm. The intensities of three band emissions located at 1.46, 1.53, and 1.80 μm decreased monotonically when increasing the temperature from at lower concentrations. However, the emissions peaked at 1.80 μm increase with the increasing temperature at higher concentrations. This was attributed to the increasing of cross relaxation (CR) resulting from the high doping concentration of Tm ions. This was evidenced by the much shorter fluorescence lifetime of 56 μs for the 3 F 4 emission due to 3 F 4 → 3 H 6 transition for the CABAL glass codoped with 2.00 mol.% Tm 2 O 3 , in comparison with 185 μs for that of 0.2 mol.% Tm 2 O 3. The energy transfer (ET) and CR processes between Er 3 and Tm 3 ions have been discussed at different doping concentrations and operating temperatures. The nonexponential character of the decays of 4 I 13∕2 and 3 H 4 with the increasing concentration indicated the occurring of a dipole-dipole quenching processes in the framework of a diffusion-limited regime. The average critical distances of CR between Tm 3 ions and ET between Er 3 and Tm 3 ions were approximately 1 nm.

Optical and Quantum Electronics, May 19, 2015

ABSTRACT The Cardona equation describing the temperature behavior of a bulk semiconductor is util... more ABSTRACT 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 experimentally obtained PL of GaAs/AlGaAs QWs for the unstrained case, and PL of InGaAs/GaAs QWs for the strained case, shows that the modified Cardona equation gives excellent fits to the temperature behavior even for QW bandgaps. Thus, the equation is utilized to extract QW eigenenergies, which are verifiable using the usual effective mass approximation method in solving Schrodinger’s equation for a finite potential well.

Review of Scientific Instruments

We report a fast pump modulation scheme in a terahertz time domain spectroscopy (THz-TDS) system ... more We report a fast pump modulation scheme in a terahertz time domain spectroscopy (THz-TDS) system by utilizing the intensity modulation from the megahertz repetition rate of the pump pulse for lock-in detection. In conventional THz-TDS, the modulation required for the high signal-to-noise ratio lock-in detection is achieved through the use of an optical chopper or an AC bias. Here, we propose the use of an electro-optic modulator (EOM), operated as a pulse picker, to vary the repetition rate of the pump pulse, relative to the probe pulse, allowing us to directly use the megahertz laser repetition rate as the reference modulation frequency for lock-in detection. Our proposed scheme is applicable to all types of pulsed THz emitters, including those that cannot be electronically biased. Since the maximum allowable modulation frequency is limited only by the laser repetition rate and/or by the bandwidth of the EOM, megahertz modulation rates, and, consequently, rapid data acquisition tim...

Proceedings of the Samahang Pisika ng Pilipinas, Oct 25, 2010

Bulletin of the American Physical Society, Mar 16, 2021

Optical Materials Express, 2018

Intense terahertz (THz) emission in high quality GaAs film upon ultrafast excitation is demonstra... more Intense terahertz (THz) emission in high quality GaAs film upon ultrafast excitation is demonstrated. Results showed that high quality GaAs grown by molecular beam epitaxy with a thin n-doped buffer can rival existing intense bare semiconductor THz surface emitters. The incorporation of a 0.2 µm n-GaAs buffer proved effective in enhancing the THz emission of GaAs by 281% and 295% in reflection and transmission THz time-domain excitation geometries, respectively. The GaAs film was of high crystallinity with or without the n-doped buffer layer as confirmed from X-ray diffraction and Raman scattering. The similar crystalline quality of the two samples was further exemplified by their comparable built-in field strength as measured by photoreflectance spectroscopy. The distinguishable difference in GaAs with and without the doped buffer was observed via low temperature photoluminescence (PL) spectroscopy. The GaAs film with the n-doped buffer exhibited intense GaAs PL while the GaAs film without the n-doped buffer exhibited prominent carbon impurity-related PL. THz enhancement was inferred to be due to the decrease in shallow defects in GaAs with n-doped buffer.

Applied Optics, Oct 25, 2023

Review of Scientific Instruments, Apr 1, 2023

We report a fast pump modulation scheme in a terahertz time domain spectroscopy (THz-TDS) system ... more We report a fast pump modulation scheme in a terahertz time domain spectroscopy (THz-TDS) system by utilizing the intensity modulation from the megahertz repetition rate of the pump pulse for lock-in detection. In conventional THz-TDS, the modulation required for the high signal-to-noise ratio lock-in detection is achieved through the use of an optical chopper or an AC bias. Here, we propose the use of an electrooptic modulator (EOM), operated as a pulse picker, to vary the repetition rate of the pump pulse, relative to the probe pulse, allowing us to directly use the megahertz laser repetition rate as the reference modulation frequency for lock-in detection. Our proposed scheme is applicable to all types of pulsed THz emitters, including those that cannot be electronically biased. Since the maximum allowable modulation frequency is limited only by the laser repetition rate and/or by the bandwidth of the EOM, megahertz modulation rates, and, consequently, rapid data acquisition times, become possible. Using our technique, we were able to detect an oscillating signal with frequencies up to 10 kHz, using ∼1 μs integration time per point, ∼100× faster than previously reported values for THz-TDS systems.

Proceedings of the Samahang Pisika ng Pilipinas, Oct 22, 2012

Terahertz (THz) emission was demonstrated from silicon (Si) wafers deposited with zinc oxide (ZnO... more Terahertz (THz) emission was demonstrated from silicon (Si) wafers deposited with zinc oxide (ZnO) films for the first time. The ZnO films were prepared by the oxidation of pulsed laser deposited zinc (Zn) thin films. The THz emission of Si was enhanced by the underlying ZnO layer regardless of the oxidation time and film microstructure. We offer an explanation of the enhanced emission mechanism in the basis of charge carrier acceleration due to large surface fields at the ZnO/Si interface.

Using molecular beam epitaxy, we study self-assembled InAs quantum dots (QDs) grown on GaAs (001)... more Using molecular beam epitaxy, we study self-assembled InAs quantum dots (QDs) grown on GaAs (001) substrates by lowtemperature scanning tunneling microscopy. We compare two different growth processes, the well-known Stranski-Krastanow mode and the more recently studied droplet epitaxy mode. We show that, for the same amount of deposited indium, the shape of the dots shows similar anisotropic tendency, but the InAs coverage dependence of the density shows that the growth mechanisms are different at the initial stage of the QD formation. By scanning tunneling spectroscopy at low temperature, we succeed in mapping the local density of states (LDOS) of a single quantum dot grown by the droplet epitaxy. Maps of LDOS are consistent with those previously reported for the Stranski-Krastanow mode; however, energy spectrum of our QDs shows different peak structures.

Journal of Applied Physics, Dec 19, 2019

Probing nanoscale defects and wrinkles in MoS 2 by tip-enhanced Raman spectroscopic imaging

Applied Surface Science, May 1, 2020

Abstract We reveal the As-antisite (AsGa) defect close to the surface of GaAs(1 1 0) with bulk ch... more Abstract We reveal the As-antisite (AsGa) defect close to the surface of GaAs(1 1 0) with bulk characteristics using first-principles methods with experimental verifications. We found that the AsGa in the third-layer mimics the geometry, partial charge density and more importantly, the density of states of AsGa in bulk GaAs. Notably, the mid-gap state induced by AsGa in bulk GaAs is well-reproduced by the AsGa in the third layer of GaAs(1 1 0). Simulated and experimental STM images show an “asymmetric two-lobe” feature in the region around the defect. Using local density of states (LDOS) and STS spectra, we propose three prominent peaks with characteristic energy levels corresponding to the third layer AsGa. The above results present the first report of surface electronic signatures of true bulk defect near the surface of GaAs(1 1 0).

Science of Advanced Materials, Feb 1, 2017

Proceedings of the Samahang Pisika ng Pilipinas, Oct 24, 2011

We modify the rate equations model for the time resolved spectra of GaAs/AlGaAs asymmetric couple... more We modify the rate equations model for the time resolved spectra of GaAs/AlGaAs asymmetric coupled quantum wells (ACQW) to include tunneling effects. The modification takes into account the stoppage of tunneling after a certain maximum number of carriers tunnel from the narrow well to the wide well, which is a direct consequence of effective mass filtering. To check the validity of the modification, we apply the model to the TRPL spectra of GaAs/AlGaAs ACQW. Results show a good fit between the experimental data and the model. The model also gives values of the tunneling and recombination time constants which are of the same order as what is expected for the structures investigated.

Journal of Physics and Its Applications, Dec 14, 2018

This study presents the terahertz (THz) emission of molecular beam epitaxy (MBE)-grown Gallium Ar... more 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. Reflectivity measurements for the textured p-Si(100) substrates indicated that the overall texturing increases with longer etching times. Raman spectroscopy of the post-growth samples confirmed GaAs growth. The THz emission intensities were the same order of magnitude. The GaAs grown on p-Si(100) textured for 60 minutes exhibited the most intense THz emission attributed to the increased absorption from a larger surface-to-volume ratio due to surface texturing. All GaAs on textured p-Si(100) samples had frequency bandwidth of ~2.5 THz.

Superlattices and Microstructures, Sep 1, 2017

We have demonstrated experimental evidence of non-resonant tunneling at room temperature in GaAs/... more 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 (∆E) between the ground state of the conduction band of the wide well and that of the narrow well are 42.7 meV and 19.5 meV, for samples A and B respectively. The TRPL measurement revealed a double decay rate in sample A whose ∆E is greater than one GaAs LO phonon energy (36meV), suggesting a phonon assisted tunneling mechanism. The evidence of tunneling was supported by measuring the relative intensity of the PL contributions from the narrow and wide well at 10K.

Proceedings of the Samahang Pisika ng Pilipinas, Oct 24, 2011

The temperature dependence of the photoluminescence (PL) spectra of an MBE- grown GaAs/AlGaAs sin... more The temperature dependence of the photoluminescence (PL) spectra of an MBE- grown GaAs/AlGaAs single quantum well system is presented in this study. The sample consists of three independent single wells with well widths 34 A, 51 A and 103A grown on a single sample. The behavior of transition energies with temperature follows the Varshni equation for semiconductor bandgaps with an additional offset contributed by the confinement energies of the quantum wells. To reveal recombination efficiency for the wells, the electron-hole wavefunction overlap is computed, revealing that as well width is increased, the recombination efficiency consequently decreases. The PL spectra also shows defect-related transitions which disappear above 50K.

PHYSICS OF SEMICONDUCTORS: 30th International Conference on the Physics of Semiconductors, 2011

ABSTRACT Gold (Au) nanoparticle‐catalyzed GaAs‐AlGaAs core‐shell nanowires were grown by molecula... more ABSTRACT Gold (Au) nanoparticle‐catalyzed GaAs‐AlGaAs core‐shell nanowires were grown by molecular beam epitaxy on Si (111) subtrates. The Au nanoparticles were pre‐patterned using an anodized aluminum oxide template. Scanning electron microscopy reveals nanowire diameters of 130–220 nm, lengths of approximately 6.5 μm and density of 2×108 cm−2. Photoluminescence measurement of the core‐shell nanowires reveals that they are of good optical quality.

Physica Status Solidi B-basic Solid State Physics, Mar 31, 2022

Journal of Materials Science: Materials in Electronics, May 28, 2018

The deposition of n-GaAs cap on low-temperature GaAs (LT-GaAs) improved the THz emission of LT-Ga... more The deposition of n-GaAs cap on low-temperature GaAs (LT-GaAs) improved the THz emission of LT-GaAs grown at a much lower temperature (< 300 • C), where the defect density is high, without compromising the spectral bandwidth and carrier lifetimes necessary for ultrafast THz detection. The LT-GaAs grown at 220 and 270 • C showed a 192 and 10% enhancement THz emission peak-to-peak intensity, respectively, while the sample grown at 310 • C showed a 49% reduction. The n-GaAs cap reduced the As-related defects density in the LT-GaAs resulting to improved THz emission. The THz emission from the sample grown at 310 • C with already low defect density suffered possibly due to the free carrier absorption by the n-GaAs cap. The results are relevant in future material design of LT-GaAs based photoconductive antenna.

Proceedings of the Samahang Pisika ng Pilipinas, Oct 25, 2010

The temperature dependence of photoluminescence (PL) spectra of Er-Tm codoped calcium boroalumina... more The temperature dependence of photoluminescence (PL) spectra of Er-Tm codoped calcium boroaluminate (CABAL) glasses with different dopant concentrations was investigated under 15-298 K, by pumping at 795 nm. The intensities of three band emissions located at 1.46, 1.53, and 1.80 μm decreased monotonically when increasing the temperature from at lower concentrations. However, the emissions peaked at 1.80 μm increase with the increasing temperature at higher concentrations. This was attributed to the increasing of cross relaxation (CR) resulting from the high doping concentration of Tm ions. This was evidenced by the much shorter fluorescence lifetime of 56 μs for the 3 F 4 emission due to 3 F 4 → 3 H 6 transition for the CABAL glass codoped with 2.00 mol.% Tm 2 O 3 , in comparison with 185 μs for that of 0.2 mol.% Tm 2 O 3. The energy transfer (ET) and CR processes between Er 3 and Tm 3 ions have been discussed at different doping concentrations and operating temperatures. The nonexponential character of the decays of 4 I 13∕2 and 3 H 4 with the increasing concentration indicated the occurring of a dipole-dipole quenching processes in the framework of a diffusion-limited regime. The average critical distances of CR between Tm 3 ions and ET between Er 3 and Tm 3 ions were approximately 1 nm.

Optical and Quantum Electronics, May 19, 2015

ABSTRACT The Cardona equation describing the temperature behavior of a bulk semiconductor is util... more ABSTRACT 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 experimentally obtained PL of GaAs/AlGaAs QWs for the unstrained case, and PL of InGaAs/GaAs QWs for the strained case, shows that the modified Cardona equation gives excellent fits to the temperature behavior even for QW bandgaps. Thus, the equation is utilized to extract QW eigenenergies, which are verifiable using the usual effective mass approximation method in solving Schrodinger’s equation for a finite potential well.

Review of Scientific Instruments

We report a fast pump modulation scheme in a terahertz time domain spectroscopy (THz-TDS) system ... more We report a fast pump modulation scheme in a terahertz time domain spectroscopy (THz-TDS) system by utilizing the intensity modulation from the megahertz repetition rate of the pump pulse for lock-in detection. In conventional THz-TDS, the modulation required for the high signal-to-noise ratio lock-in detection is achieved through the use of an optical chopper or an AC bias. Here, we propose the use of an electro-optic modulator (EOM), operated as a pulse picker, to vary the repetition rate of the pump pulse, relative to the probe pulse, allowing us to directly use the megahertz laser repetition rate as the reference modulation frequency for lock-in detection. Our proposed scheme is applicable to all types of pulsed THz emitters, including those that cannot be electronically biased. Since the maximum allowable modulation frequency is limited only by the laser repetition rate and/or by the bandwidth of the EOM, megahertz modulation rates, and, consequently, rapid data acquisition tim...

Proceedings of the Samahang Pisika ng Pilipinas, Oct 25, 2010

Bulletin of the American Physical Society, Mar 16, 2021

Optical Materials Express, 2018

Intense terahertz (THz) emission in high quality GaAs film upon ultrafast excitation is demonstra... more Intense terahertz (THz) emission in high quality GaAs film upon ultrafast excitation is demonstrated. Results showed that high quality GaAs grown by molecular beam epitaxy with a thin n-doped buffer can rival existing intense bare semiconductor THz surface emitters. The incorporation of a 0.2 µm n-GaAs buffer proved effective in enhancing the THz emission of GaAs by 281% and 295% in reflection and transmission THz time-domain excitation geometries, respectively. The GaAs film was of high crystallinity with or without the n-doped buffer layer as confirmed from X-ray diffraction and Raman scattering. The similar crystalline quality of the two samples was further exemplified by their comparable built-in field strength as measured by photoreflectance spectroscopy. The distinguishable difference in GaAs with and without the doped buffer was observed via low temperature photoluminescence (PL) spectroscopy. The GaAs film with the n-doped buffer exhibited intense GaAs PL while the GaAs film without the n-doped buffer exhibited prominent carbon impurity-related PL. THz enhancement was inferred to be due to the decrease in shallow defects in GaAs with n-doped buffer.