## 1 Introduction  
The development of sixth-generation (6G) communication imposes a thousand-fold increase in the traffic volume for wireless networks in the foreseeable future. Visible light communication (VLC) is a significant supplement technology used to release the pressure of the crowded radio frequency resource due to its advantages, including high transmission speed, license-free spectrum, high physical layer security, ubiquitous light-emitting diodes (LEDs) transmitters, and free radio frequency (RF) interference [1]. However, VLC's high dependency on the line-of-sight (LoS) channel and illumination demand limit its further deployment and industrialization. Optical intelligent reflecting surfaces (IRSs) have recently emerged as a revolutionary technology to enhance the performance of VLC by providing additional non-line-of-sight (NLoS) communication links [2, 3]. In particular, an IRS is a novel planar material that is able to improve the wireless channel gain by interacting with incident electromagnetic waves [4]. Plenty of studies have been conducted to improve the performance of RF wireless communication systems by introducing an IRS [5, 6, 7]. It has been shown that the data rate performance of IRS-aided multiple-input multiple-output (MIMO) systems was comparable to that of massive MIMO systems [5]. Moreover, a channel acquisition approach for IRS-aided multiuser MIMO systems was proposed in [6]. And the IRS was deployed to improve the dirty-paper coding (DPC) sum rate in the IRS-assisted broadcast channels [7]. Mirror arrays and metasurfaces are two mainstream IRS units for VLC systems [8]. Mirror arrays control the lightwave propagation by rotating the planar micro-mirrors based on [PERSON]'s law of reflection [2]. Since the reflectance of the micro-mirror cannot be changed, most IRS-aided VLC systems with mirror arrays focus on the micro-mirrors' associationswith users or LEDs. For example, the micro-mirror's associations and LED's transmit power are jointly optimized to improve the spectral efficiency in multiuser IRS-aided VLC systems [1]. The mean square error is minimized by jointly designing the IRS configuration and the transceiver signal processing in IRS-aided MIMO VLC systems [9]. The sum rates of the IRS-aided VLC system were maximized by optimizing the micro-mirror's associations with users and the mirrors' orientation in [10] and [11], respectively. The mirror-based optical IRS unit is a kind of passive reflecting surface that can change the direction of light rays of the VLC signal only, while an active IRS unit can change the signal amplitude or power with equipped amplifiers [12]. Optical metasurfaces are artificial planar structures capable of manipulating the wavelength, polarization, and phase of the incident wave by electrical management, and thus can realize anomalous reflection based on the generalized [PERSON]'s law of reflection [8]. In [13], a newly developed polarization-insensitive electro-tunable metasurface was deployed for optical intensity modulation, and a modulation bit rate of up to 900 Mbps can be achieved by changing the reflectances of the IRS units in terms of its electro-optic properties. This newly proposed metasurface can be regarded as a kind of active optical IRS unit and adds design degrees of freedom to the IRS-aided VLC channels employing the intensity modulation and direct detection (IM/DD) scheme. Motivated by this tunable reflective materials, we investigate the bit error rate (BER) performance improvement by jointly optimizing the pulse amplitude modulation (PAM) constellation and the coherent IRS reflectances in IRS-aided single-input single-output (SISO) VLC systems in this paper. The main contributions of this paper are listed as follows:  
* A newly IRS-aided SISO VLC system under dimming constraints is investigated with a metasurface-based IRS for which its reflectance can be synchronously tuned with the LED's emitting signals.