Layer-dependent spin properties of charge carriers in vertically coupled telecom quantum dots

This is the authors' abstract. We don't add key points for this paper.

We investigate the spin properties of charge carriers in vertically coupled InAs/InAlGaAs quantum dots grown by molecular beam epitaxy, emitting at telecom C-band wavelengths, with a silicon <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>δ</a:mi> </a:math> -doped layer. Using time-resolved pump-probe Faraday ellipticity measurements, we systematically study single-, two-, and four-layer quantum dot (QD) configurations to quantify how vertical coupling affects key spin-coherence parameters. Our measurements reveal distinct layer-dependent effects: (i) Adding a second QD layer flips the resident charge from electrons to holes, consistent with optically induced electron tunneling into lower-energy dots and resultant hole charging. (ii) Starting from the four-layer sample, the pump-probe signal develops an additional nonoscillating, decaying component absent in single- and two-layer samples, attributed to multiple layer growth changing the strain environment, which reduces heavy-hole and light-hole mixing. (iii) With four layers or more, hole spin mode locking (SML) can be observed, enabling quantitative extraction of the hole coherence time <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:msub> <b:mi>T</b:mi> <b:mn>2</b:mn> </b:msub> <b:mo>≈</b:mo> <b:mn>13</b:mn> </b:mrow> </b:math> ns from SML amplitude saturation. We also extract longitudinal spin relaxation ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msub> <c:mi>T</c:mi> <c:mn>1</c:mn> </c:msub> </c:math> ) and transverse ( <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msubsup> <d:mi>T</d:mi> <d:mn>2</d:mn> <d:mo>*</d:mo> </d:msubsup> </d:math> ) spin dephasing times, g-factors, and inhomogeneous dephasing parameters for both electrons and holes across all layer configurations. The hole spin dephasing times <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:msubsup> <e:mi>T</e:mi> <e:mn>2</e:mn> <e:mo>*</e:mo> </e:msubsup> </e:math> remain relatively constant (2.3–2.7 ns) across layer counts, while longitudinal relaxation times <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:msub> <f:mi>T</f:mi> <f:mn>1</f:mn> </f:msub> </f:math> decrease with increasing layers (from <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:mn>0.9</g:mn> <g:mspace width="0.28em"/> <g:mi>µ</g:mi> <g:mi mathvariant="normal">s</g:mi> </g:mrow> </g:math> for single-layer to <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:mrow> <j:mn>0.32</j:mn> <j:mspace width="0.28em"/> <j:mi>µ</j:mi> <j:mi mathvariant="normal">s</j:mi> </j:mrow> </j:math> for four-layer samples). These findings provide potential design guidelines for engineering spin coherence in telecom-band QDs for quantum information applications.