Showing 1–8 of 8 results for author: Ulhaq, A

Helicity-selective Raman scattering from in-plane anisotropic α-MoO$_3$

Authors: Shahzad Akhtar Ali, Abdullah Irfan, Aishani Mazumder, Sivacarendran Balendhran, Taimur Ahmed, Sumeet Walia, Ata Ulhaq

Abstract: Hyperbolic crystals like α-MoO$_3$ can support large wavevectors and photon density as compared to the commonly used dielectric crystals, which makes them a highly desirable platform for compact photonic devices. The extreme anisotropy of the dielectric constant in these crystals is intricately linked with the anisotropic character of the phonons, which along with photon confinement leads to the r… ▽ More Hyperbolic crystals like α-MoO$_3$ can support large wavevectors and photon density as compared to the commonly used dielectric crystals, which makes them a highly desirable platform for compact photonic devices. The extreme anisotropy of the dielectric constant in these crystals is intricately linked with the anisotropic character of the phonons, which along with photon confinement leads to the rich physics of phonon polaritons. However, the chiral nature of phonons in these hyperbolic crystals have not been studied in detail. In this study, we report our observations of helicity selective Raman scattering from flakes of α-MoO$_3$. Both helicity-preserving and helicity-reversing Raman scattering are observed. We observe that helical selectivity is largely governed by the underlying crystal symmetry. This study shed light on the chiral character of the high symmetry phonons in these hyperbolic crystals. It paves the way for exploiting proposed schemes of coupling chiral phonon modes into propagating surface plasmon polaritons and for compact photonic circuits based on helical polarized light. △ Less

Submitted 14 October, 2021; v1 submitted 26 July, 2021; originally announced July 2021.

Comments: The following article has been submitted to Applied Physics Letters

Measurement of the spin temperature of optically cooled nuclei and GaAs hyperfine constants in GaAs/AlGaAs quantum dots

Authors: E. A. Chekhovich, A. Ulhaq, E. Zallo, F. Ding, O. G. Schmidt, M. S. Skolnick

Abstract: Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid state quantum information technologies. While polarization of individual nuclear spins in diamond and SiC reaches 99% and beyond, it has been limited to 60-65% for the nuclei in quantum dots. Theoretical models have attributed this limit to formation of coheren… ▽ More Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid state quantum information technologies. While polarization of individual nuclear spins in diamond and SiC reaches 99% and beyond, it has been limited to 60-65% for the nuclei in quantum dots. Theoretical models have attributed this limit to formation of coherent "dark" nuclear spin states but experimental verification is lacking, especially due to the poor accuracy of polarization degree measurements. Here we measure the nuclear polarization in GaAs/AlGaAs quantum dots with high accuracy using a new approach enabled by manipulation of the nuclear spin states with radiofrequency pulses. Polarizations up to 80% are observed - the highest reported so far for optical cooling in quantum dots. This value is still not limited by nuclear coherence effects. Instead we find that optically cooled nuclei are well described within a classical spin temperature framework. Our findings unlock a route for further progress towards quantum dot electron spin qubits where deep cooling of the mesoscopic nuclear spin ensemble is used to achieve long qubit coherence. Moreover, GaAs hyperfine material constants are measured here experimentally for the first time. △ Less

Submitted 10 January, 2017; originally announced January 2017.

Journal ref: Nature Materials 16, 982-986 (2017)

Electron and nuclear spin properties of the nanohole-filled GaAs/AlGaAs quantum dots

Authors: Ata Ulhaq, Qingqing Duan, Fei Ding, Eugenio Zallo, Oliver G. Schmidt, Maurice S. Skolnick, Alexander I. Tartakovskii, Evgeny A. Chekhovich

Abstract: GaAs/AlGaAs quantum dots grown by in-situ droplet etching and nanohole infilling offer a combination of strong charge confinement, optical efficiency, and spatial symmetry required for polarization entanglement and spin-photon interface. Here we study spin properties of such dots. We find nearly vanishing electron $g$-factor ($g_e<0.05$), providing a route for electrically driven spin control sche… ▽ More GaAs/AlGaAs quantum dots grown by in-situ droplet etching and nanohole infilling offer a combination of strong charge confinement, optical efficiency, and spatial symmetry required for polarization entanglement and spin-photon interface. Here we study spin properties of such dots. We find nearly vanishing electron $g$-factor ($g_e<0.05$), providing a route for electrically driven spin control schemes. Optical manipulation of the nuclear spin environment is demonstrated with nuclear spin polarization up to $60\%$ achieved. NMR spectroscopy reveals the structure of two types of quantum dots and yields the small magnitude of residual strain $ε_b<0.02\%$ which nevertheless leads to long nuclear spin lifetimes exceeding 1000 s. The stability of the nuclear spin environment is advantageous for applications in quantum information processing. △ Less

Submitted 23 July, 2015; originally announced July 2015.

Journal ref: Phys. Rev. B 93, 165306 (2016)

Mollow "quintuplets" from coherently-excited quantum dots

Authors: Rong-Chun Ge, S. Weiler, A. Ulhaq, S. M. Ulrich, M. Jetter, P. Michler, S. Hughes

Abstract: Charge-neutral excitons in semiconductor quantum dots have a small finite energy separation caused by the anisotropic exchange splitting. Coherent excitation of neutral excitons will generally excite both exciton components, unless the excitation is parallel to one of the dipole axes. We present a polaron master equation model to describe two-exciton pumping using a coherent continuous wave pump f… ▽ More Charge-neutral excitons in semiconductor quantum dots have a small finite energy separation caused by the anisotropic exchange splitting. Coherent excitation of neutral excitons will generally excite both exciton components, unless the excitation is parallel to one of the dipole axes. We present a polaron master equation model to describe two-exciton pumping using a coherent continuous wave pump field in the presence of a realistic anisotropic exchange splitting. We predict a five-peak incoherent spectrum, thus generalizing the Mollow triplet to become a Mollow quintuplet. We experimentally confirm such spectral quintuplets for In(Ga)As quantum dots and obtain very good agreement with theory. △ Less

Submitted 14 February, 2013; originally announced February 2013.

Journal ref: Optics Letters, Vol. 38, Issue 10, pp. 1691-1693 (2013)

Detuning-Dependent Mollow Triplet of a Coherently-Driven Single Quantum Dot

Authors: Ata Ulhaq, Stefanie Weiler, Chiranjeeb Roy, Sven Marcus Ulrich, Michael Jetter, Stephen Hughes, Peter Michler

Abstract: We present both experimental and theoretical investigations of a laser-driven quantum dot (QD) in the dressed-state regime of resonance fluorescence. We explore the role of phonon scattering and pure dephasing on the detuning-dependence of the Mollow triplet and show that the triplet sidebands may spectrally broaden or narrow with increasing detuning. Based on a polaron master equation approach wh… ▽ More We present both experimental and theoretical investigations of a laser-driven quantum dot (QD) in the dressed-state regime of resonance fluorescence. We explore the role of phonon scattering and pure dephasing on the detuning-dependence of the Mollow triplet and show that the triplet sidebands may spectrally broaden or narrow with increasing detuning. Based on a polaron master equation approach which includes electron-phonon interaction nonperturbatively, we derive a fully analytical expression for the spectrum. With respect to detuning dependence, we identify a crossover between the regimes of spectral sideband narrowing or broadening. A comparison of the theoretical predictions to detailed experimental studies on the laser detuning-dependence of Mollow triplet resonance emission from single In(Ga)As QDs reveals excellent agreement. △ Less

Submitted 2 October, 2012; originally announced October 2012.

Phonon-Assisted Incoherent Excitation of a Quantum Dot and its Emission Properties

Authors: S. Weiler, A. Ulhaq, C. Roy, S. M. Ulrich, D. Richter, M. Jetter, S. Hughes, P. Michler

Abstract: We present a detailed study of a phonon-assisted incoherent excitation mechanism of single quantum dots. A spectrally-detuned laser couples to a quantum dot transition by mediation of acoustic phonons, whereby excitation efficiencies up to 20 % with respect to strictly resonant excitation can be achieved at T = 9 K. Laser frequency-dependent analysis of the quantum dot intensity distinctly maps th… ▽ More We present a detailed study of a phonon-assisted incoherent excitation mechanism of single quantum dots. A spectrally-detuned laser couples to a quantum dot transition by mediation of acoustic phonons, whereby excitation efficiencies up to 20 % with respect to strictly resonant excitation can be achieved at T = 9 K. Laser frequency-dependent analysis of the quantum dot intensity distinctly maps the underlying acoustic phonon bath and shows good agreement with our polaron master equation theory. An analytical solution for the photoluminescence is introduced which predicts a broadband incoherent coupling process when electron-phonon scattering is in the strong phonon coupling (polaronic) regime. Additionally, we investigate the coherence properties of the emitted light and study the impact of the relevant pump and phonon bath parameters. △ Less

Submitted 20 July, 2012; originally announced July 2012.

Heralded Single-Photon Emission from the Mollow Triplet Sidebands of a Quantum Dot

Authors: Ata Ulhaq, Stefanie Weiler, Sven M. Ulrich, Robert Roßbach, Michael Jetter, Peter Michler

Abstract: Emission from a resonantly excited quantum emitter is a fascinating research topic within quantum optics and a useful source for different types of quantum light fields. The resonance spectrum consists of a single spectral line below saturation of a quantum emitter which develops into a triplet at powers above saturation of the emitter. The spectral properties of the triplet strongly depends on pu… ▽ More Emission from a resonantly excited quantum emitter is a fascinating research topic within quantum optics and a useful source for different types of quantum light fields. The resonance spectrum consists of a single spectral line below saturation of a quantum emitter which develops into a triplet at powers above saturation of the emitter. The spectral properties of the triplet strongly depends on pump power and detuning of the excitation laser. The three closely spaced photon channels from the resonance fluorescence have different photon statistical signatures. We present a detailed photon-statistics analysis of the resonance fluorescence emission triplet from a solid state-based artificial atom, i.e. a semiconductor quantum dot. The photon correlation measurements demonstrate both 'single' and 'heralded' photon emission from the Mollow triplet sidebands. The ultra-bright and narrowband emission (5.9 MHz into the first lens) can be conveniently frequency-tuned by laser detuning over 15 times its linewidth ({Δν} \approx 1.0 GHz). These unique properties make the Mollow triplet sideband emission a valuable light source for, e.g.quantum light spectroscopy and quantum information applications. △ Less

Submitted 16 August, 2011; originally announced August 2011.

{\it In-situ} Laser Microprocessing at the Quantum Level

Authors: Armando Rastelli, Ata Ulhaq, Suwit Kiravittaya, Lijuan Wang, Artur Zrenner, Oliver G. Schmidt

Abstract: One of the biggest challenges of nanotechnology is the fabrication of nano-objects with perfectly controlled properties. Here we employ a focused laser beam both to characterize and to {\it in-situ} modify single semiconductor structures by heating them from cryogenic to high temperatures. The heat treatment allows us to blue-shift, in a broad range and with resolution-limited accuracy, the quan… ▽ More One of the biggest challenges of nanotechnology is the fabrication of nano-objects with perfectly controlled properties. Here we employ a focused laser beam both to characterize and to {\it in-situ} modify single semiconductor structures by heating them from cryogenic to high temperatures. The heat treatment allows us to blue-shift, in a broad range and with resolution-limited accuracy, the quantized energy levels of light and charge carriers confined in optical microcavities and self-assembled quantum dots (QDs). We demonstrate the approach by tuning an optical mode into resonance with the emission of a single QD and by bringing different QDs in mutual resonance. This processing method may open the way to a full control of nanostructures at the quantum level. △ Less

Submitted 3 October, 2006; originally announced October 2006.

Comments: 3 figures

Journal ref: Appl. Phys. Lett. 90, 073120 (2007)