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Journal of the Optical Society of America B

Single-beam squeezed-state generation in semiconductor waveguides with X^(3) nonlinearity at below half-band gap

Seng‐Tiong Ho, Xiaolong Zhang, M. K. Udo

Published Sep 1, 1995• 10 citations
Publisher

Abstract

We analyze a new scheme for generating squeezed states in a short semiconductor AlxGa1−xAs waveguide with χ(3) nonlinearity at below half the band-gap energy. We find that for a Gaussian pulse the amount of squeezing achievable is limited by the squeezed-state detection phase mismatch caused by the pump self-phase modulation and is also degraded slightly by the pump–probe phase mismatch that is due to the different nonlinear refractive indices experienced by the pump and the probe beams. We show theoretically that the amount of squeezing observed can be increased by use of either a short pulse or a pulse with matched phase variation as the local oscillator. In a centimeter-long AlxGa1−xAs waveguide more than 85% (8.2 dB) of squeezing potentially can be obtained, limited mainly by two-photon absorption.

DOI
10.1364/josab.12.001537

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J.J. Baumberg, A. M. Fox, M. Dabbiccco +2 more

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.

•Aug 24, 2005•23 citations
We analyze a new scheme for generating squeezed states in a short semiconductor AlxGa1−xAs waveguide with χ(3) nonlinearity at below half the band-gap energy. We find that for a Gaussian pulse the amount of squeezing achievable is limited by the squeezed-state detection phase mismatch caused by the pump self-phase modulation and is also degraded slightly by the pump–probe phase mismatch that is due to the different nonlinear refractive indices experienced by the pump and the probe beams. We show theoretically that the amount of squeezing observed can be increased by use of either a short pulse or a pulse with matched phase variation as the local oscillator. In a centimeter-long AlxGa1−xAs waveguide more than 85% (8.2 dB) of squeezing potentially can be obtained, limited mainly by two-photon absorption.
The realization of a compact squeezed-state-generation device at 1.5 µm may have potential applications in optical communication. The squeezed state has been generated in both χ(2) and χ(3) media. In χ(2) media the frequency of the squeezed vacuum is half that of the pump. Therefore frequency doubling is necessary for generating a local oscillator (LO). Such frequency doubling is not needed in χ(3) media. However, squeezing in y(3) media has not been as successful as in X(2) media, for example, in atomic vapor,1 self-focusing seriously limits the amount of squeezing, whereas in optical fibers,2 guided-acoustic-wave Brillouin-scattering (GAWBS) noise causes difficulties that can be circumvented through low- temperature cooling of the fiber or the use of soli ton pulses.
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.
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 apply our quantum theory of nondegenerate multiwave mixing [ Phys. Rev. A37, 2017 ( 1988)] to squeezed-state generation experiments with two-level atoms. Our main interest is to predict the amount of squeezing achievable with a Doppler-broadened two-level medium. We are particularly interested in the single-beam configuration, in which all four interacting beams are spatially degenerate. We analytically solve the coupled-mode quantum Langevin equations for nondegenerate four-wave mixing. The solutions are used to compute the amount of squeezing. In the computation the effects of pump–probe phase mismatch, collisions, Doppler broadening, and Gaussian-intensity variation are comprehensively taken into account for the first time to our knowledge. Simple rules of thumb as to where one can see squeezing in both degenerate- and nondegenerate-frequency cases are derived by examining the limit of a short medium. We then present the case of an infinitely long medium, in which maximum squeezing is achieved when there is no pump–probe phase mismatch. With the inclusion of pump–probe phase mismatch, however, the maximum amount of squeezing is obtained with a finite-length medium instead. This prompts us to investigate in detail the finite-length medium case. Our results show that the effects of Doppler broadening and Gaussian-intensity variation can be largely circumvented by detuning the pump frequency more than three Doppler half-widths from resonance and that good broadband squeezing can be achieved even with a Doppler-broadened medium that has a moderate amount of collision broadening. Under these circumstances it is found that the effect of pump self-focusing or defocusing will be the major factor that limits the amount of achievable squeezing. In particular, the spatially varying nonlinear refractive indices seen by the pump and the probe modes are quite different, which causes the former to become spatially mismatched with the latter in the region in which strong squeezing is otherwise expected.
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.