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Quantum Optoelectronics

Squeezed Light Generation in Semiconductors

A. M. Fox, J.J. Baumberg, Maurizio Dabbicco, B. Hüttner, J.F. Ryan

Published Jan 1, 1995• 13 citations
Publisher

Abstract

The signal to noise ratio of optical systems is ultimately limited by the shot noise due to the Poissonian statistics of the light field. In recent years there has been a considerable effort to find ways to overcome this limitation by generating non-classical light with sub-shot noise statistics. Several groups have demonstrated non-classical light generation in diode lasers or light emitting diodes, and the amplitude fluctuations have been reduced substantially below the shot noise level [1,2]. However, for phase-sensitive measurements it is necessary to generate the more general quadrature squeezed states. Such quadrature squeezed light can be generated either by second-order or third-order nonlinear processes provided the nonlinear material has very low losses at the optical wavelength. To date, the successful generation schemes for quadrature squeezed light have been restricted to atomic vapours, optical fibres, or second-order nonlinear crystals [3]. The possibility of using the large χ(3) nonlinearities of semiconductors at frequencies below the band gap where the optical absorption is small has been discussed in the literature, but so far there have been no successful experiments [4-6]. In this paper we report measurements on GaAs/Al0.3Ga0.7As multiple quantum well waveguides and bulk ZnS in the wavelength range 750 - 890nm. Although we measure much larger nonlinearities in the quantum wells, the strong two-photon absorption is detrimental for squeezed light generation. However, in the ZnS where the laser frequency is below the two- photon absorption edge, we have succeeded in generating quadrature squeezed light in a semiconductor for the first time. On the basis of our results, we would expect that squeezed light could be generated in many other semiconductor systems provided the photon energy is less than half the band-gap energy.

DOI
10.1364/qo.1995.qfa2

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The signal to noise ratio of optical systems is ultimately limited by the shot noise due to the Poissonian statistics of the light field. In recent years there has been a considerable effort to find ways to overcome this limitation by generating non-classical light with sub-shot noise statistics. Several groups have demonstrated non-classical light generation in diode lasers or light emitting diodes, and the amplitude fluctuations have been reduced substantially below the shot noise level [1,2]. However, for phase-sensitive measurements it is necessary to generate the more general quadrature squeezed states. Such quadrature squeezed light can be generated either by second-order or third-order nonlinear processes provided the nonlinear material has very low losses at the optical wavelength. To date, the successful generation schemes for quadrature squeezed light have been restricted to atomic vapours, optical fibres, or second-order nonlinear crystals [3]. The possibility of using the large χ(3) nonlinearities of semiconductors at frequencies below the band gap where the optical absorption is small has been discussed in the literature, but so far there have been no successful experiments [4-6]. In this paper we report measurements on GaAs/Al0.3Ga0.7As multiple quantum well waveguides and bulk ZnS in the wavelength range 750 - 890nm. Although we measure much larger nonlinearities in the quantum wells, the strong two-photon absorption is detrimental for squeezed light generation. However, in the ZnS where the laser frequency is below the two- photon absorption edge, we have succeeded in generating quadrature squeezed light in a semiconductor for the first time. On the basis of our results, we would expect that squeezed light could be generated in many other semiconductor systems provided the photon energy is less than half the band-gap energy.
We have generated pulsed squeezed light using the third-order nonlinear susceptibility of the semiconductor ZnS at room temperature. The photon energy was chosen to be below midgap in order to minimize nonlinear absorption. Efficient quadrature squeezing of 2.2 dB (40%) was obtained using 125 fs pulses at a center wavelength of 780 nm. The measured noise is suppressed below the quantum limit over the entire range of our detection bandwidth (30–50 MHz). The scheme employed can generally be applied to semiconductors, and opens the way for squeezed light generation over a wide range of wavelengths.
We have generated pulsed squeezed light using the third-order nonlinear susceptibility of the semiconductor ZnS at room temperature. The photon energy was chosen to be below midgap in order to minimize nonlinear absorption. Efficient quadrature squeezing of 2.2 dB (40%) was obtained using 125 fs pulses at a center wavelength of 780 nm. The measured noise is suppressed below the quantum limit over the entire range of our detection bandwidth (30-50 MHz). The scheme employed can generally be applied to semiconductors, and opens the way for squeezed light generation over a wide range of wavelengths.
We present experimental studies based on all three methods by which the generation of squeezed light in semiconductors has thus far been demonstrated experimentally: Fourwave mixing, multi-photon absorption and direct generation at the source. Four-wave mixing was used to generate femtosecond-pulsed quadrature squeezed light by cross-phase modulation in single-crystal hexagonal CdSe at wavelengths between 1.42 and 1.55 μm. We measured 0.4 dB squeezing (1.1 dB is inferred at the crystal) using 100 fs pulses. The wavelength and the intensity dependence, as well as variations in the local oscillator configuration were investigated. At higher intensities squeezing was shown to deteriorate owing to competing nonlinear processes. We also characterised the nonlinear optical properties of CdSe in this wavelengths range using an interferometric autocorrelator. In addition, we studied the feasibility of extending this technique to AlGaAs waveguides. The key problems are addressed and solutions are proposed. In a different experiment we used an AlGaAs waveguide to demonstrate for the first time photon-number squeezing by multi-photon absorption. By tuning the pump energy through the half bandgap energy we could effectively select two- or three-photon absorption as the dominant mechanism. Squeezing by these two mechanisms could be clearly distinguished and was found to be in good agreement with longstanding theoretical predictions. We also established the generality of the effect, by demonstrating the same mechanism in organic semiconductors, where it led to the first ever observation of squeezed light in an organic material. Finally, we present our measurements of photon-number squeezing in high-efficiency double heterojunction AlGaAs light-emitting diodes. We measured squeezing of up to 2.0 dB. In addition, we observed quantum noise correlations when several of these devices were connected in series.
Squeezed states of light with reduced quantum noise in one quadrature of the light field can be generated by parametric processes in atomic or solid systems. The solid systems, in principle, have the advantage that the ratio of the nonlinear (lossless) interaction to the losses in the solid required to generate the squeezed state can be much larger than the corresponding ratio for an atomic system. In the atomic system, losses are dominated by the Lorentzian absorption tail, decreasing as the inverse square of the frequency shift from resonance. Absorption losses in solids can decrease exponentially (or even faster) at frequencies below an absorption band or an exciton feature while the nonresonant nonlinear response decreases slowly, often with a power-law dependence. We concentrate on the third-order susceptibility for a broad class of semiconductors (e.g., GaAs) and organic materials (e.g., polydiactylene) where losses can be very low. The ratio of nonlinearity to loss required for significant squeezing of light noise is in the same range as that required for a good optical switch. A nonlinear phase shift of ir is required while losses are reduced so that only a small fraction of the incident light is absorbed. Several solid systems show promise for obtaining large squeezing for photon energies just below the exciton band-edge feature. Enhanced nonlinearity is also obtained at the threshold energy for two-photon absorption. Present crystal growth technologies can decrease losses due to impurities into a very optimistic regime for squeezed state generation.
In this dissertation measurements are presented that extend our understanding of the important noise processes involved in the semiconductor laser generation of photon-number squeezed light. Strong sub-shot noise correlations between photon-number fluctuations on orthogonal polarization axes are observed in the output of a low temperature injection-locked quantum-well laser. Such polarization dependent effects are accounted for by intrinsic correlations generated in the laser gain medium and polarization mixing due to birefringence. Using polarization preserving balanced homodyne detection, up to 4.5 dB of photon-number squeezing is reported, which is within 95% of the expected maximum squeezing based on the device quantum efficiency. The work in this dissertation demonstrates the application of such nonclassical fields to practical measurements that achieve sub-shot noise sensitivity. Coherent nonlinear optical spectroscopy with a photon-number squeezed probe field is used to measure the Urbach tail and exciton resonances in a GaAs/AlGaAs multiple quantum well with sensitivity below the semiclassical shot-noise limit. Direct drive current modulation of an injection-locked room temperature laser results in a nonclassical amplitude modulated field with a signal to noise ratio that exceeds the maximum ratio permitted for a classical source under equivalent modulation conditions.