arXiv subjects

Sangeeth Kallatt

Publications and source records attributed to Sangeeth Kallatt.

15 recordsLinked to original sources

arXiv2026

Epitaxial single T centres in silicon-on-insulator

Spin-photon interfaces based on silicon quantum emitters offer a scalable platform for quantum computing and networking. However, achieving coherent photon emission remains a primary challenge due to stringent material quality requirements. To overcome this, we utilise high-purity molecular-beam epitaxy (MBE) to epitaxially incorporate single T centres in silicon-on-insulator (SOI) wafers. We demonstrate single T-centre emission coupled to a nanophotonic waveguide and observe significant suppression of homogeneous broadening, yielding optical linewidths as narrow as 30 MHz using natural silicon for crystal growth. These results establish epitaxial T centres as a robust foundation for coherent spin-photon interfaces in silicon quantum photonics.

quant-ph↗

arXiv2026

Millisecond-Scale Calibration and Benchmarking of Superconducting Qubits

Superconducting qubit parameters drift on sub-second timescales, motivating calibration and benchmarking techniques that can be executed on millisecond timescales. We demonstrate an on-FPGA workflow that co-locates pulse generation, data acquisition, analysis, and feed-forward, eliminating CPU round trips. Within this workflow, we introduce sparse-sampling and on-FPGA inference tools, including computationally efficient methods for estimation of exponential and sine-like response functions, as well as on-FPGA implementations of Nelder-Mead optimization and golden-section search. These methods enable low-latency primitives for readout calibration, spectroscopy, pulse-amplitude calibration, coherence estimation, and benchmarking. We deploy this toolset to estimate $T_1$ in 10 ms, optimize readout parameters in 100 ms, optimize pulse amplitudes in 1 ms, and perform Clifford randomized gate benchmarking in 107 ms on a flux-tunable superconducting transmon qubit. Running a closed-loop on-FPGA recalibration protocol continuously for 6 hours enables more than 74,000 consecutive recalibrations and yields gate errors that consistently retain better performance than the baseline initial calibration. Correlation analysis shows that recalibration suppresses coupling of gate error to control-parameter drift while preserving a coherence-linked performance. Finally, we quantify uncertainty versus time-to-decision under our sparse sampling approaches and identify optimal parameter regimes for efficient estimation of qubit and pulse parameters.

quant-ph↗

arXiv2026

Colour Centre Formation in Silicon-On-Insulator for On-Chip Photonic Integration

Colour centres in silicon have great potential as single photon sources for quantum technologies. Some of them - like the T centre - also possess optically-active spins that enable spin-photon interfaces for generating entangled photons and multi-spin registers. This paper explores the generation of several types of colour centres in silicon for mass-manufacturable silicon-on-insulator quantum devices. We investigate how different processes in the device development affect the presence of the quantum emitters, including thermal annealing and fabrication steps for optical nanostructures. The study reveals coupled formation dynamics between different colour centres, identifies optimal parameters for annealing processes, and reports on the sensitivity to annealing duration and nanofabrication procedures for photonic integrated circuits. Furthermore, we discern stable optical signals from colour centres in silicon which have not been identified before.

quant-ph↗

arXiv2020

Highly tunable layered exciton in bilayer WS$_2$: linear quantum confined Stark effect versus electrostatic doping

In 1H monolayer transition metal dichalcogenide, the inversion symmetry is broken, while the reflection symmetry is maintained. On the contrary, in the bilayer, the inversion symmetry is restored, but the reflection symmetry is broken. As a consequence of these contrasting symmetries, here we show that bilayer WS$_2$ exhibits a quantum confined Stark effect (QCSE) that is linear with the applied out-of-plane electric field, in contrary to a quadratic one for a monolayer. The interplay between the unique layer degree of freedom in the bilayer and the field driven partial inter-conversion between intra-layer and inter-layer excitons generates a giant tunability of the exciton oscillator strength. This makes bilayer WS$_2$ a promising candidate for an atomically thin, tunable electro-absorption modulator at the exciton resonance, particularly when stacked on top of a graphene layer that provides an ultra-fast non-radiative relaxation channel. By tweaking the biasing configuration, we further show that the excitonic response can be largely tuned through electrostatic doping, by efficiently transferring the oscillator strength from neutral to charged exciton. The findings are prospective towards highly tunable, atomically thin, compact and light, on chip, reconfigurable components for next generation optoelectronics.

cond-mat.mes-hall↗

arXiv2020

Gate-tunable trion switch for excitonic device applications

Trions are excitonic species with a positive or negative charge, and thus, unlike neutral excitons, the flow of trions can generate a net detectable charge current. Trions under favourable doping conditions can be created in a coherent manner using resonant excitation. In this work, we exploit these properties to demonstrate a gate controlled trion switch in a few-layer graphene/monolayer WS2/monolayer graphene vertical heterojunction. By using a high resolution spectral scan through a temperature controlled variation of the bandgap of the WS2 sandwich layer, we obtain a gate voltage dependent vertical photocurrent strongly relying on the spectral position of the excitation, and the photocurrent maximizes when the excitation energy is resonant with the trion peak position. Further, the resonant photocurrent thus generated can be effectively controlled by a back gate voltage applied through the incomplete screening of the bottom monolayer graphene, and the photocurrent strongly correlates with the gate dependent trion intensity, while the non-resonant photocurrent exhibits only a weak gate dependence -unambiguously proving a trion driven photocurrent generation under resonance. We estimate a sub-100 fs switching time of the device. The findings are useful towards demonstration of ultra-fast excitonic devices in layered materials.

physics.app-ph↗

arXiv2019

Highly Sensitive, Fast Graphene Photodetector with Responsivity $>10^6$ A/W Using Floating Quantum Well Gate

Graphene, owing to its zero bandgap electronic structure, is promising as an absorption material for ultra-wideband photodetection applications. However, graphene-absorption based detectors inherently suffer from poor responsivity due to weak absorption and fast photocarrier recombination, limiting their viability for low intensity light detection. Here we use a graphene/WS$_2$/MoS$_2$ vertical heterojunction to demonstrate a highly sensitive photodetector, where the graphene layer serves dual purpose, namely as the light absorption layer, and also as the carrier conduction channel, thus maintaining the broadband nature of the photodetector. A fraction of the photoelectrons in graphene encounter ultra-fast inter-layer transfer to a floating monolayer MoS$_2$ quantum well providing strong quantum confined photogating effect. The photodetector shows a responsivity of $4.4\times 10^6$ A/W at 30 fW incident power, outperforming photodetectors reported till date where graphene is used as light absorption material by several orders. In addition, the proposed photodetector exhibits an extremely low noise equivalent power ($N\!E\!P$) of $<4$ fW/$\sqrt{Hz}$ and a fast response ($\sim$ milliseconds) with zero reminiscent photocurrent. The findings are attractive towards the demonstration of graphene-based highly sensitive, fast, broadband photodetection technology.

physics.app-ph↗

arXiv2019

Light emission from the layered metal 2H-TaSe$_2$ and its potential applications

Conventional metals, in general, do not exhibit strong photoluminescence. 2H-TaSe$_2$ is a layered transition metal dichalcogenide that possesses metallic property with charge density wave characteristics. Here we show that 2H-TaSe$_2$ exhibits a surprisingly strong optical absorption and photoluminescence resulting from inter-band transitions. We use this perfect combination of electrical and optical properties in several optoelectronic applications. We show a seven-fold enhancement in the photoluminescence intensity of otherwise weakly luminescent multi-layer MoS$_2$ through non-radiative resonant energy transfer from TaSe$_2$ transition dipoles. Using a combination of scanning photocurrent and time-resolved photoluminescence measurements, we also show that the hot electrons generated by light absorption in TaSe$_2$ have a rather long lifetime unlike conventional metals, making TaSe$_2$ an excellent hot electron injector. Finally, we show a vertical TaSe$_2$/MoS$_2$/graphene photodetector demonstrating a responsivity of $>10$ AW$^{-1}$ at $0.1$ MHz - one of the fastest reported photodetectors using MoS$_2$.

cond-mat.mes-hall↗

arXiv2019

Strong Single- and Two-Photon Luminescence Enhancement by Nonradiative Energy Transfer across Layered Heterostructure

The strong light-matter interaction in monolayer transition metal dichalcogenides (TMDs) is promising for nanoscale optoelectronics with their direct band gap nature and the ultra-fast radiative decay of the strongly bound excitons these materials host. However, the impeded amount of light absorption imposed by the ultra-thin nature of the monolayers impairs their viability in photonic applications. Using a layered heterostructure of a monolayer TMD stacked on top of strongly absorbing, non-luminescent, multi-layer SnSe2, we show that both single-photon and two-photon luminescence from the TMD monolayer can be enhanced by a factor of 14 and 7.5, respectively. This is enabled through inter-layer dipole-dipole coupling induced non-radiative Forster resonance energy transfer (FRET) from SnSe2 underneath which acts as a scavenger of the light unabsorbed by the monolayer TMD. The design strategy exploits the near-resonance between the direct energy gap of SnSe2 and the excitonic gap of monolayer TMD, the smallest possible separation between donor and acceptor facilitated by van der Waals heterojunction, and the in-plane orientation of dipoles in these layered materials. The FRET driven uniform single- and twophoton luminescence enhancement over the entire junction area is advantageous over the local enhancement in quantum dot or plasmonic structure integrated 2D layers, and is promising for improving quantum efficiency in imaging, optoelectronic, and photonic applications.

cond-mat.mes-hall↗

arXiv2019

Inter-layer charge transport controlled by exciton-trion coherent coupling

The possibility of electrical manipulation and detection of charged exciton (trion) before its radiative recombination makes it promising for excitonic devices. Using a few-layer graphene/monolayer WS$_{2}$/monolayer graphene vertical heterojunction, we report inter-layer charge transport from top few-layer graphene to bottom monolayer graphene, mediated by coherently formed trion state. This is achieved by using a resonant excitation and varying the sample temperature, the resulting change in the WS$_{2}$ bandgap allows us to scan the excitation around the exciton-trion spectral overlap with high spectral resolution. By correlating the vertical photocurrent and in situ photoluminescence features at the heterojunction as a function of the spectral position of the excitation, we show that (1) trions are anomalously stable at the junction even up to 463 K due to enhanced doping, and (2) the photocurrent results from the ultra-fast formation of trion through exciton-trion coherent coupling, followed by its fast inter-layer transport. The demonstration of coherent formation, high stabilization, vertical transportation and electrical detection of trions marks a step towards room temperature trionics.

cond-mat.mes-hall↗

arXiv2017

Substrate effects in high gain, low operating voltage SnSe2 photoconductor

High gain photoconductive devices find wide spread applications in low intensity light detection. Ultra-thin layered materials have recently attracted a lot of attention from researchers in this regard. However, in general, a large operating voltage is required to obtain large responsivity in these devices. In addition, the characteristics are often confounded by substrate induced trap effects. Here we report multi-layer SnSe2 based photoconductive devices using two different structures: (1) SiO2 substrate supported interdigitated electrode (IDE), and (2) suspended channel. The IDE device exhibits a responsivity of ~ 10^3 A/W and 8.66x10^4 A/W at operating voltages of 1 mV and 100 mV, respectively - a superior low voltage performance over existing literature on planar 2D structures. However, the responsivity reduces by more than two orders of magnitude, while the transient response improves for the suspended device - providing insights into the critical role played by the channel-substrate interface in the gain mechanism. The results, on one hand, are promising for highly sensitive photoconductive applications consuming ultra-low power, and on the other hand, show a generic methodology that could be applied to other layered material based photoconductive devices as well for extracting the intrinsic behavior.

cond-mat.mes-hall↗

arXiv2017

Nature of carrier injection in metal/2D semiconductor interface and its implications to the limits of contact resistance

Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent electronic and optical properties. However, the performance of these two-dimensional (2D) devices are often limited by the large resistance offered by the metal contact interface. Till date, the carrier injection mechanism from metal to 2D TMDC layers remains unclear, with widely varying reports of Schottky barrier height (SBH) and contact resistance (Rc), particularly in the monolayer limit. In this work, we use a combination of theory and experiments in Au and Ni contacted monolayer MoS2 device to conclude the following points: (i) the carriers are injected at the source contact through a cascade of two potential barriers - the barrier heights being determined by the degree of interaction between the metal and the TMDC layer; (ii) the conventional Richardson equation becomes invalid due to the multi-dimensional nature of the injection barriers, and using Bardeen-Tersoff theory, we derive the appropriate form of the Richardson equation that describes such composite barrier; (iii) we propose a novel transfer length method (TLM) based SBH extraction methodology, to reliably extract SBH by eliminating any confounding effect of temperature dependent channel resistance variation; (iv) we derive the Landauer limit of the contact resistance achievable in such devices. A comparison of the limits with the experimentally achieved contact resistance reveals plenty of room for technological improvements.

cond-mat.mes-hall↗

arXiv2017

Asymmetrically Encapsulated vertical ITO/MoS2/Cu2O photodetector with ultra-high sensitivity

Strong light absorption, coupled with moderate carrier transport properties, makes two-dimensional (2-D) layered transition metal dichalcogenide (TMD) semiconductors promising candidates for low intensity photodetection applications. However, the performance of these devices is severely bottlenecked by slow response with persistent photocurrent due to long lived charge trapping, and nonreliable characteristics due to undesirable ambience and substrate effects. Here we demonstrate ultra-high specific detectivity (D*) of 3.2x10^14 Jones and responsivity (R) of 5.77x10^4 AW-1 at an optical power density (P_op) of 0.26 Wm-2 and external bias (V_ext) of -0.5 V in an indium tin oxide (ITO)/MoS2/copper oxide (Cu2O)/Au vertical multi-heterojunction photodetector exhibiting small carrier transit time. The active MoS2 layer being encapsulated by carrier collection layers allows us to achieve negligible trap assisted persistent photocurrent and repeatable characteristics over large number of cycles. We also achieved a large D*>10^14 Jones at zero external bias due to the built-in field of the asymmetric photodetector. Benchmarking the performance against existing reports in literature shows a pathway for achieving reliable and highly sensitive photodetectors for ultra-low intensity photodetection applications.

cond-mat.mes-hall↗

arXiv2017

Direct observation of giant binding energy modulation of exciton complexes in monolayer MoSe$_2$

Screening due to surrounding dielectric medium reshapes the electron-hole interaction potential and plays a pivotal role in deciding the binding energies of strongly bound exciton complexes in quantum confined monolayers of transition metal dichalcogenides (TMDs). However, owing to strong quasi-particle bandgap renormalization in such systems, a direct quantification of estimated shifts in binding energy in different dielectric media remains elusive using optical studies. In this work, by changing the dielectric environment, we show a conspicuous photoluminescence (PL) peak shift at low temperature for higher energy excitons (2s, 3s, 4s, 5s) in monolayer MoSe$_2$, while the 1s exciton peak position remains unaltered - a direct evidence of varying compensation between screening induced exciton binding energy modulation and quasi-particle bandgap renormalization. The estimated modulation of binding energy for the 1s exciton is found to be 58.6% (70.5% for 2s, 78.9% for 3s, 85% for 4s) by coating an Al$_2$O$_3$ layer on top, while the corresponding reduction in quasi-particle bandgap is estimated to be 248 meV. Such a direct evidence of large tunability of the binding energy of exciton complexes as well as the bandgap in monolayer TMDs holds promise of novel device applications.

cond-mat.mes-hall↗

arXiv2016

Photoresponse of atomically thin MoS2 layers and their planar heterojunctions

MoS2 monolayers exhibit excellent light absorption and large thermoelectric power, which are, however, accompanied with very strong exciton binding energy - resulting in complex photoresponse characteristics. We study the electrical response to scanning photo-excitation on MoS2 monolayer (1L) and bilayer (2L) devices, and also on monolayer/bilayer (1L/2L) planar heterojunction and monolayer/few-layer/multi-layer (1L/FL/ML) planar double heterojunction devices to unveil the intrinsic mechanisms responsible for photocurrent generation in these materials and junctions. Strong photoresponse modulation is obtained by scanning the position of the laser spot, as a consequence of controlling the relative dominance of a number of layer dependent properties, including (i) photoelectric effect (PE), (ii) photothermoelectric effect (PTE), (iii) excitonic effect, (iv) hot photo-electron injection from metal, and (v) carrier recombination. The monolayer and bilayer devices show peak photoresponse when the laser is focused at the source junction, while the peak position shifts to the monolayer/multi-layer junction in the heterostructure devices. The photoresponse is found to be dependent on the incoming light polarization when the source junction is illuminated, although the polarization sensitivity drastically reduces at the monolayer/multi-layer heterojunction. Finally, we investigate laser position dependent transient response of photocurrent to reveal trapping of carriers in SiO2 at the source junction is the critical factor to determine the transient response in 2D photodetectors, and also show that, by systematic device design, such trapping can be avoided in the heterojunction devices, resulting in fast transient response. The insights obtained will play an important role in designing fast 2D TMDs based photodetector and related optoelectronic and thermoelectric devices.

cond-mat.mes-hall↗

arXiv2016

Valley Coherent Hot Carriers and Thermal Relaxation in Monolayer Transition Metal Dichalcogenides

We show room temperature valley coherence with in MoS2, MoSe2, WS2 and WSe2 monolayers using linear polarization resolved hot photoluminescence (PL), at energies close to the excitation - demonstrating preservation of valley coherence before sufficient scattering events. The features of the co-polarized hot luminescence allow us to extract the lower bound of the binding energy of the A exciton in monolayer MoS2 as 0.42 (+/- 0.02) eV. The broadening of the PL peak is found to be dominated by Boltzmann-type hot luminescence tail, and using the slope of the exponential decay, the carrier temperature is extracted in-situ at different stages of energy relaxation. The temperature of the emitted optical phonons during the relaxation process are probed by exploiting the corresponding broadening of the Raman peaks due to temperature induced anharmonic effects. The findings provide a physical picture of photo-generation of valley coherent hot carriers, and their subsequent energy relaxation path ways.

cond-mat.mes-hall↗