Time-resolved spectroscopy of non-thermal carrier dynamics in GaN

K. Kyhm,R. Lota,R.A. Taylor,J.F. Ryan,N.J. Cain 한국물리학회 2006 Current Applied Physics Vol.6 No.5

The dynamics of carriers in GaN epilayers is investigated using femtosecond pump-probe spectroscopy. After the residual chirpon the continuum probe is removed, the normalized dierence spectra (NDS) for dierent probe energies are synchronized, recov-ering the full time resolution of our laser pulse. Our Monte-Carlo simulation agrees well with the unchirped NDS spectrum, whichshows the development of the carrier distribution at early times, where phonon satellites are seen, together with a strong non-ther-mal electron distribution in the region of the LO-phonon energy arising from the remarkably strong electronLO phonon interac-tion. Employing a new technique which involves the integration of the normalized NDS multiplied by the corresponding energy, ameasure of the mean energy of the carriers in non-thermal states is obtained. By comparing the time-dependent energy loss with thetheoretical energy loss rate, we estimate the eective temperature of the phonon modes as well as the population of phonons.

Electron-Hole Plasma Mott Transition and Stimulated Emission in GaN

K. KYHM,B. BEAUMONT,J. F. RYAN,P. GIBART,R. A. TAYLOR 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.45 No.3

We present femtosecond pump-probe re ectance measurements of excitons in GaN for various pump intensities. Saturation of the excitonic absorption with increasing carrier density has been measured in the case of resonant and non-resonant excitations during the rising times, and the exciton bleaching densities for various excitation energies have also been measured. We found that the resonant excitons are bleached at higher densities (10 times) than the non-resonant excitons due to the absence of free carriers. The stimulated emission mechanism is investigated by measuring simultaneously the photoluminescence and the time-resolved re ectance near the band edge, over a range of excitation densities. The onset of the stimulated emission coincides with the bleaching density of the non-resonant excitons as well as a theoretical gain threshold density. These results suggest that the stimulated emission in GaN is due to the electron-hole plasma.

Time-resolved gain dynamics in InGaN MQW structures

K. Kyhm,J.D. Smith,R.A. Taylor 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.2

Transient gain spectra were measured for an In0.02Ga0.98N / In0.16Ga0.84N multiple quantum well by using the variable-stripe-length method (VSLM) in combination with the ultrafast optical Kerrgate (OKG) technique. Gain dynamics were measured for a range of excitation lengths from short (50 μm) to long (350 μm) stripes with the sample under femtosecond photoexcitation. Analysis ofthe temporal behaviour of gain and chemical potential suggests that stimulated emission originates from a photoexcited electron-hole plasma at early times; at later times, localized states dominate as the electron-hole plasma becomes exhausted. Gain reduction at early times is attributable to coupling of the electron-hole plasma with photons along the stripe, whilst localized states are less susceptible to gain saturation.

Hot carrier dynamics and carrier-phonon interaction in GaN

K. Kyhm,N.J. Cain,R.A. Taylor 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.2

The dynamics of carriers in GaN epilayers is investigated by using femtosecond pump-probe spectroscopy. After the residual chirp on the continuum probe is removed, the normalized difference spectra (NDS) for different probe energies are synchronized, recovering the full time resolution of our laser pulse. Our Monte-Carlo simulation agrees well with the unchirped NDS spectrum, which shows the development of the carrier distribution at early times, where phonon satellites are seen, together with a strong non-thermal electron distribution in the region of the LO-phonon energy arising from the remarkably strong electron-LO phonon interaction. By employing a new technique which involves the integration of the normalized NDS multiplied by the corresponding energy, ameasure of the mean energy of the carriers in non-thermal states is obtained. By comparing the timedependent energy loss with the theoretical energy loss rate, we estimate the effective temperature of the phonon modes as well as the population of phonons.

Low gain threshold density of a single InGaP quantum well sandwiched by digital alloy

Kim, B.,Kyhm, K.,Je, K.C.,Song, J.D.,Kim, S.Y.,Le, E.H.,Taylor, R.A. Elsevier 2014 Current Applied Physics Vol.14 No.9

A single In<SUB>0.49</SUB>Ga<SUB>0.51</SUB>P quantum well sandwiched by In<SUB>0.49</SUB>Ga<SUB>0.51</SUB>P/In<SUB>0.49</SUB>(Ga<SUB>0.6</SUB>Al<SUB>0.4</SUB>)<SUB>0.51</SUB>P digital alloy structures was investigated in terms of optical modal gain, where gain saturation effects were also considered for both changes in wavelength and stripe length by using a modal gain contour map analysis. We found the gain threshold density is considerably lower by an order of magnitude when compared to the Mott density (~2 x 10<SUP>12</SUP> cm<SUP>-2</SUP>), which can be attributed to a carrier-harvesting effect through the mini-band of the digital alloy.

Theoretical Study for the ITO/Si based High Contrast Grating Structure with Focusing Capability and its Fabrication

Kim, J.Y.,Yeon, K.H.,Kyhm, J.,Cho, W.J.,Kim, T.J.,Kim, Y.D.,Song, J.D. The Korean Vacuum Society 2015 Applied Science and Convergence Technology Vol.24 No.6

High contrast grating (HCG) is the structure made up of the sub-wavelength grating of high-index and the surrounding layer of low-index, which reveals high contrast between two materials. Its advantages include high reflectivity over a broad bandwidth, polarization and wavelength selectivity, optical high-Q resonator, and phase modulation. In this work, the HCG structure comprising of indium tin oxide (ITO) and Silicon (Si), for the surrounding layer and the grating layer respectively, was studied. Its theoretical model was established, and transmittance, phase and optical behavior were calculated by rigorous coupled-wave analysis and finite element method. Furthermore, the established structure was fabricated to validate its feasibility. The fabricated structure shows the focusing capability whose length is about $10{\mu}m$, and the feasibility of the structure was demonstrated. It is also meaningful that ITO layer can contribute to the fabrication of the HCG structure, leading to enable the structure to be electrical-driven.

Time-Resolved Free Exciton Trapping and Thermal Activation in Bound-Exciton in GaN

Na, S. H.,Kyhm, K. 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.2

Dynamics and Bleaching of Ground State in CdSe/ZnS Quantum Dots

Kim, J.H.,Kyhm, K. Optical Society of Korea 2006 Current Optics and Photonics Vol.10 No.4

For resonant excitation of the ground state $1s^e-1S^h_{3/2}$, dynamics of 'the electron-hole pair in a CdSe quantum dot was investigated by degenerate pump-probe measurement. At low e-h pair densities, the decay of $1s^e-1S^h_{3/2}$ state is dominated by radiative recombination. As the number of the electron-hole pairs increases, new decay features become significant. Theoretical comparison suggests this is attributed to the bi-molecular and Auger-type scattering.

Theoretical Study for the ITO/Si based High Contrast Grating Structure with Focusing Capability and its Fabrication

J. Y. Kim,K. H. Yeon,J. Kyhm,W. J. Cho,T. J. Kim,Y. D. Kim,J. D. Song 한국진공학회(ASCT) 2015 Applied Science and Convergence Technology Vol.24 No.6

High contrast grating (HCG) is the structure made up of the sub-wavelength grating of high-index and the surrounding layer of low-index, which reveals high contrast between two materials. Its advantages include high reflectivity over a broad bandwidth, polarization and wavelength selectivity, optical high-Q resonator, and phase modulation. In this work, the HCG structure comprising of indium tin oxide (ITO) and Silicon (Si), for the surrounding layer and the grating layer respectively, was studied. Its theoretical model was established, and transmittance, phase and optical behavior were calculated by rigorous coupled-wave analysis and finite element method. Furthermore, the established structure was fabricated to validate its feasibility. The fabricated structure shows the focusing capability whose length is about 10 μm, and the feasibility of the structure was demonstrated. It is also meaningful that ITO layer can contribute to the fabrication of the HCG structure, leading to enable the structure to be electrical-driven.

Upconversion luminescence enhancement in plasmonic architecture with random assembly of metal nanodomes

Lee, G.,Jung, K.,Jang, H.,Kyhm, J.,Han, I.,Park, B.,Ju, H.,Kwon, S. J.,Ko, H. Royal Society of Chemistry 2016 Nanoscale Vol.8 No.4

<P>We report an experimental study on the highly enhanced upconversion luminescence (UCL) of beta-NaYF4: Yb3+/Er3+ nanocrystals (NCs) in a plasmonic architecture. For the architecture, we designed a thin film device composed of a thin layer of NCs capped with an upper layer of a plasmonic nanodome array (pNDA) and lower substrate of a back reflector (BR). Compared to the UCL intensity observed in a glass reference substrate, the designed plasmonic architecture exhibits distinctively strong luminescence enhanced by up to 800-fold. The intensity considerably exceeds the previously reported luminescence intensity regardless of the excitation power. We elucidated a mechanism explaining the large UCL enhancement, which quantitatively analyzes the combination of plasmonic effects as well as multiple large scattering. More importantly, we provided a detailed analysis of the Ag NDA-derived and BR-assisted plasmonic effects that contribute to an increase in the radiative decay rate and an enhancement of the absorption of incident light. The present study is expected to be beneficial for designing a thin film-based plasmonic structure with a randomized metal nanostructure for high-efficiency photovoltaic devices and infrared detectors.</P>